set down in Table 1, and the chemical analyses in Table 5. Analytical .... Only one rock, AC5, a pitchstone from Scotland has no co-existing ferro- magnesian ...
Contr. Mineral. and Petrol. 14, 36--64 (1967)
The Iron-Titanium Oxides of Salic Volcanic Rocks and their Associated Ferromagnesian Silicates h~
S. E. CARMICItAEL
Department of Geology and Geophysics, University of California, Berkeley Eeceived August 8, 1966
Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analytical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Iron-Titaninm Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Iron-Titanium Oxide Equilibration Temperatures . . . . . . . . . . . . . . . . Biotite Phenocrysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biotite and Pera]uminous Liquids . . . . . . . . . . . . . . . . . . . . . . . . . Biotite and the Fugacity of Water . . . . . . . . . . . . . . . . . . . . . . . . Pyroxene Phenocrysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olivine Phenocrysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amphibole Phenoerysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron-Ratios of the Ferromagnesian Phases . . . . . . . . . . . . . . . . . . . . . Oxygen Fugacities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key to specimen localities . . . . . . . . . . . . . . . . . . . . . . . . . . . . Petrographic Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36 36 37 38 46 49 51 53 54 56 57 58 60 62 62 62 63 64
Abstract. The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of BUDDINGTONand LI~1)sL~Y (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900~ which may represent liquidus temperatures for those with sparse phenoerysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenoorysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.
Introduetion T h e i r o n - t i t a n i u m o x i d e m i n e r a l s h a v e b e e n r a t h e r n e g l e c t e d in i g n e o u s p e t r o l o g y p a r t l y as a r e s u l t of t h e d i f f i c u l t y of t h e i r s e p a r a t i o n , p a r t i c u l a r l y in g l a s s y rocks, a n d p a r t l y as a r e s u l t of t h e i n v o l v e d a n a l y t i c a l c h e m i s t r y r e q u i r e d for t h e i r comp l e t e analysis. T o a l a r g e e x t e n t t h e e l e c t r o n - p r o b e has p r o v i d e d t h e m e a n s to a v o i d b o t h difficulties a n d w i t h t h e w o r k of BUDDINGTOI~ a n d LI17DSLEY (1964) t h e i r o n - t i t a n i u m o x i d e s can n o l o n g e r be d i s m i s s e d as m e r e o p a q u e accessories. T h e c o m p o s i t i o n s of c o - e x i s t i n g m a g n e t i t e a n d i l m e n i t e c a n be u s e d n o w t o g i v e
Iron-TRanium Oxides of Salic Rocks
37
an e s t i m a t e of t h e t e m p e r a t u r e of e q u i l i b r a t i o n a n d t h e f u g a c i t y of o x y g e n a t t e n d a n t on t h e i r equilibration. More r e c e n t l y WONES a n d E u c s ~ (1965) h a v e sugg e s t e d t h a t t h e c o m p o s i t i o n of b i o t i t e m a y be used t o give a v a l u e for t h e f u g a c i t y of h y d r o g e n in c e r t a i n assemblages; t h e c o m b i n a t i o n of t h e i r o n - t i t a n i u m oxide d a t a w i t h those of EUGSTER a n d WONES m a y f u r t h e r be used to o b t a i n a v a l u e of t h e w a t e r f u g a c i t y a t t h e t i m e of e q u i l i b r a t i o n . A n a t t e m p t has been m a d e earlier to a n a l y s e t h e i r o n - t i t a n i u m oxides in glassy rocks (CAR~IC~AEL, 1963) a n d to e x a m i n e t h e i r influence on t h e l a t e r crystallizat i o n of p y r o x e n e . H o w e v e r , ilmenite was t o o scarce, being a l m o s t always subordin a t e in a m o u n t t o t h e spinel phase, t o c o n c e n t r a t e sufficiently for a n a l y s i s ; moreover, i t p r o v e d impossible t o o b t a i n clean s e p a r a t i o n s of t h e spinel p h a s e for analysis. These a n a l y s e s (op. cir.) are quite clearly in error, a n d all of t h e m h a v e been r e d o n e in this s t u d y . A c c o r d i n g l y t h e i r o n - t i t a n i u m oxides occuring as m i c r o p h e n o c r y s t s in a v a r i e t y of g l a s s y salic voleanics h a v e b e e n i n v e s t i g a t e d t o g e t h e r w i t h t h e i r a s s o c i a t e d f e r r o m a g e n s i a n p h e n o c r y s t s . M a n y of t h e rocks h a v e been described before a n d are p i t e h s t o n e s a n d obsidians from I c e l a n d a n d S c o t l a n d (CA~MICHAEL, 1960), P a n t e l l e r i a (CAR~IICHA~L, 1962), California (Mono Craters, Medicine Lake, Clear Lake), dacites from L a s s e n P a r k , California, a n d v a r i o u s sub-siliceous volcanies from F r a n c e , Scotland, t h e A n t a r c t i c a n d Tenerife. This l a s t group includes t w o o v e r s a t u r a t e d t r a c h y t e p i t c h s t o n e s (CA~[IC~AEL, 1960, 1965), a k e n y t e a n d a p h o n o l i t i c o b s i d i a n (CARMICHAEL, 1964). A brief p e t r o g r a p h i c d e s c r i p t i o n is given in t h e a p p e n d i x of t h e rocks i n v e s t i g a t e d from California. The m o d a l analyses are set d o w n in T a b l e 1, a n d t h e chemical analyses in T a b l e 5. A n a l y t i c a l Techniques As the amount of ferromagnesian minerals occuring as phenocrysts in most of these rocks is small (Table 1), they were separated by heavy liquids before mounting and polishing for the electron-probe. Almost all the oxide minerals and many of the silicate phases retained a sheath of glass or silicate mineral so that the direction and extent of zoning if present could be determined. For the Lassen daeites, the rocks themselves were polished as thin sections and mounted in the probe; this preserved the rather more complex zoning and reaction relationships in these rocks. Only those specimens which contained a one-phase spinel were investigated; oxidised or multiphase spinels and rhombohedral phases were of no use, as the electron-probe is unable to obtain a value for the bulk composition of multi-phase grains. Although none of the minerals was examined by X-ray techniques to establish the absence of a second-phase, reflected light microscopy and the microprobe electron-beam indicated that they are one phase, or within the resolution of the techniques. Mineral analyses were carried out with an ARL electron-probe using a variety of reference standards for the iron-titanium oxides. For total iron as FeO, TiO 2 and MnO, flmenite and haematite standards were used. For the minor elements Si02, A12Oa, Cr2Oa, V~O3, MgO, CaO and ZnO, aenigmatite, chromite, hedenbergite, and franklinite standards were used. The estimation of V20 a in Ti-bcaring compounds is difficult due to the near coincidence of TiK B with VKa; however the influence of Ti can be minimized by measuring the backround of VKa on a variety of Ti-bearing oxides in which V is absent. The values for V20a given in the tables are liable to far greater errors than the other elements due to the interference of TiK~, the high atomic number of the standard (vanadinite) and the uncertainty of the V content of the standard. For the other elements, corrections for background, atomic number, fluorescence and mass absorption were made where necessary. The precision of the microprobe data is approximately • % of the amount present; however this figure increases for some of the minor 3b Contr.Mineral.and Petrol., Vol. 1~,
IAI~ S. E . CARMICHAEL:
38
T a b l e 1.
Modal analyses (volume percent)
9
1 2 3 4 5 6 7 8 9
( G 151) (Oraefi) (P. 186) (AC. 7) ( C a m 73) (Cam99) (1910--199) (5748) (3112)
lO (3114) 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
( A C . 5) (Cam49) (Cam 55) ( C a m 66) (Cam95) (Cam 103) (Cam 104) (Cam 108) (Cam 110) (EC. 20) (Cam 118) (Cam 81) ( C a m 86) ( C a m 93) (3811) ( T C . 19) (Cal. 13) (Cal. 19) (L. 1 1 8 )
89.0 8.1 -97.2 2.1 -89.6 8.9 -83.4 9.4 -94.8 0.4 2.4 94.2 0.5 2.7 78.0 -20.0 88.2 -10.7 80.6 -18.0 98.7 -1.2 99.8 --93.7 5.6 -91.3 5.1 -99.9 --99.7 0.1 0.1 99.9 --99.9 --99.9 --99.9 --79.3 trace 19.2 98.4 1.2 -99.6 0.1 0.1 97.2 1.0 1.0 95.3 1.3 1.1 84.0 12.0 0.2 98.0 -2.0 77.8 18.8 -82.8 11.9 -70.8 24.6 --
---4.8 2.1 2.3 --0.1 -----0.1 ------0.1 0.2 0.4 ---trace --
1.6 ( c p x ) 0.3 (cpx) 0.8 (cpx) 1.5 --0.8 (cpx) 0.5 (cpx) 0.3 (cpx) trace (cpx) . . . 0.6 1.7 trace trace (opx) trace(opx) trace(opx) trace (opx) trace(opx) 1.0 0.2 (opx) 0.1 0.1
0.7 0.3 0.2 0.6 trace 0.1 0.6 0.4 trace trace . ------------
0.1
--
0.8 trace(cpx) 0.6
------
1.0
0.8
--
---------.
.
----------trace 0.1 0.5 trace trace 0.5 1.4
1.0
----0.2 0.2 ---. -trace --------trace 0.1 1.0 1.3 -1.9 2.0 2.3
O
0.6 0.1 0.3 0.3 trace trace 0.6 0.2 trace
--0.2* ---t r a c e ** t r a c e *** 0.9***
0.2 0.1 0.1 trace trace trace trace trace trace 0.5 0.2 trace 0.3 0.3 0.6 trace 0.4 0.7 0.5
--1.8 + ----------
.
---
tr. sphene ----
--
* basaltic xenoliths; ** n e p h e l i n e ; *** e o s s y r i t e ; + p l a g i o c l a s e p h e n o c r y s t s with s i e v e t e x t u r e , p o s s i b l y x e n o c r y s t s ; e p x ~-- r e f e r s t o c a l c i u m - r i c h p y r o x e n e a s t h e o n l y p y r o x e n e phase; opx ~ refers to orthopyrocene as the only pyroxene phase. e l e m e n t s ( C a O , S i 0 2 ) a n d f o r V 2 0 a is p r o b a b l y o f t h e o r d e r o f 4 - 1 5 % . B e t w e e n 15 a n d 2 0 grains were used for the determination of the major elements; all the oxides were essentially unzoned chemically. The silicate minerals were analyzed using a variety of analyzed standards of similar composition to the unknowns; the amphiboles however were analyzed using pyroxene, biotite, aenigmatite and plagioclase standards, with all available corrections (atomic number, mass absorpt i o n , e t c . ) t h r o w n i n f o r g o o d m e a s u r e . F o r a l l t h e m i n e r a l a n a l y s e s t o t a l i r o n is r e p o r t e d a s FeO. The rock analyses and the FeO determinations in the biotites were made using standard wet chemical techniques. The
Iron-Titanium
Small
crystals
(flmenite
1963).
of a
spinel
S. L.) are found
investigated. is not
Oxides
Commonly
invariable, Either
but
magnetite
phase
(magnetite
as discrete the
spinel
is enclosed
it does illustrate or ilmenite
S.
L.)
microphenocrysts the or both
order
and
a rhombohedral
in almost
in a ferromagnesian of crystallization
are the
first
phases
all of the
phase rocks
silicate;
this
(CARMIC~AnL, to crystallize
in
Iron-Titanium Oxides of Salie Rocks
39
v i r t u a l l y every one of those rocks; however t h e absence of a n oxide phase e n d o . sed b y a silicate m i n e r a l does n o t preclude its early crystallization. I n some of the rocks only one oxide is found, n a m e l y ilmenite i n a pantellerite 5748 a n d a rhyolite Cam 118, whereas m a g n e t i t e only is f o u n d i n Cam 81, P186, the k e n y t e 1910--199 a n d the phonolitie obsidian TC 19. As severM p o u n d s of sample were crushed to separate the oxide minerals, these absences are accounted real. I n one of the rocks, 3811, there are two clearly contrasted size fractions of a spinel phase, the larger oxidized to a m u l t i - p h a s e assemblage, the smaller microphenocrysts being one-phase. As the ilmenite microphenocrysts a n d t h e magn e t i t e microphenoerysts are b o t h one-phase, a n d b o t h v e r y m u c h smaller t h a n the v e r y large oxidized phenocrysts, t h e y are t r e a t e d here as if t h e y were coevM i n their crystallization. In reflected light, all the magnetites arc anisotropic and variable in color and reflection pleochroism, but they are invariably paler than the associated ilmenite, which can be easily identified optically by its anisotropy, color, and pleochroism. The micro-probe analyses of the spinel and associated discrete rhombohedrM phases are given in Tables 2, 3 and 4: those given in Table 2 have in iron-rich olivine as a co-existing ferromagncsian phenocryst, those in Table 3 have orthopyroxcnc as a co-existing ferromagnesian phase, and those in Table 4 have biotite and amphibole as the initial silicate phases with pyroxene precipitating later. Only one rock, AC5, a pitchstone from Scotland has no co-existing ferromagnesian silicate phenoerysts. It is not possible to use the total of the analyses in order to assess the quality of the analyses in Tables 2, 3 and 4, as total iron has been determined only as FeO. As the oxidation state of the iron cannot be determined by the electron-probe, it is necessary to campute a value for F%Oa in the analyses both to assess their quality and to recast them into mineral molecules. It may be assumed that as they are all one-phase spinels, their bulk composition lies either on the magnetite (FeO. F%Oa)-ulvospincl (2FeO. Ti02) join in the composition triangle FeOFe20~-TiO2 or within the field magnetite-ulvospinel-ilmenite,the limit being the join magnetite-ilmenite (FeO- Ti02). Although AKZ~OTOet ah (1957) have been able to produce one-phase spinels by oxidation beyond the join magnetite-ilmenite,it is suggested that these do not commonly exist in nature (AKzsmTOand KATSVRA,1959). In essence, then, the check of the analysis is the derivation of an F%08 value with (a) all the bivalent elements combined with the TiO2 in the ratio 2:1, the excess iron being calculated to form FeO "F%Oa (the ulvospinel basis), or (b) the bivalent elements combined with Ti02 in the ratio 1 : 1 and the excess iron calculated to form FeO. F%0 a (the ilmenite basis). The procedure is simple; to the molecular proportions of Ti02-t- Si02 is allotted all the MnO, MgO, CaO and ZnO and sufficient FeO equivalent to twice the value of TiO~-t-SiO2(ulvospinelbasis); FeO is then allotted in the ratio 1 : 1 to the molecular proportions of A1208, Cr20a and V~O3, and the excess FeO calculated to equivalent magnetite. From this, the amount of Fe208 can be calculated, and a correction made to the total iron as FeO determined by the probe. In an analogous way, the calculation can be made on the ilmenite basis, and the amount of F%Oa computed. As most natural one-phase spinel analyses lie in the field magnetite-ilmenitculvospinel, it is to be expected that these samples do too ,and accordingly the totals derived by calculation should come close to 100% if the analysis is otherwise satisfactory. It may be seen from Tables 2--4 that the average totM computed on the ulvospinel basis for the one-phase spinels is 99.9 and on the ilmenite basis it is 100.9; this is considered to be satisfactory. From the calculation on the ulvospinel basis, the molecular percent of ulvospinel may be calculated and is given in Tables 2--4.1 1 Some confirmation of the values derived by this method of calculation may be obtained from the cell-sizes of which four are known (CAI~MIGHAEL,1962, 1963). Using the curves of AI{IMOTOet al. (1957), the following values of tool. percent ulvospinel are obtained together with the calculated values from Tables 2 and 3 for comparison: l(G151) 63.6, 60.6 (Table 2); (3112) 79.2, 76.4 (Table 2); 10 (3114) 68.8, 62.4 (Table 2) and 20 (EC.20) 32.3, 38.2 (Table 3).
IA~ S. E. CA~CHA~,L:
40
Table 2. Analyses o] iron-titanium oxides coex@ting with olivine and calcium-rich pyroxene
or amphibole 1 2 3 4 5 6 7 (G151) (oraefi) (P. 186) (kc. 7) (Cam73) (Cam99) (1910 199) -
9 l0 11 (3112) (3114) (kc. 5)
-
Spinel phase SiO2 Ti02 AI~Oa V20 a Cr~O3 FeO MnO MgO CaO ZnO Sum
0.13 21.5 1.15 * * 73.6 0.88 0.32 0.03 0.23 97.8
0.11 22.1 0.64 * 0.01 73.0 1.00 0.06 0.04 0.27
0.08 19.8 0.93 0.05 0.02 75.5 0.64 0.34 0.01 0.21
0.11 15.1 0.86 * 0.01 78.1 0.96 0.13 0.01 0.19
0.10 14.4 0.84 * * 79.4 0.93 0.12 0.01 0.19
0.04 26.3 1.81 0.09 * 65.3 1.62 3.15 * 0.09
0.06 27.2 0.41 * * 69.0 1.71 0.25 * 0.13
0.09 21.9 0.18 * * 72.4 1.77 0.44 * 0.22
0.07 19.8 0.99 0.30 0.03 74.0 0.76 0.40 0.01 0.22
96.7
97.6
95.5
96.0
98.4
98.8
97.0
96.6
Recalculated analyses (see text) Ilmenite basis F%0 a 40.8 40.3 43.3 FeO 36.9 36.7 36.0 Total 101.1 101.2 101.1
43.2 36.6 101.9
48.3 34.6 100.3
50.2 34.2 101.0
35.3 33.6 102.0
34.4 38.0 102.2
40.9 35.6 101.1
42.2 36.0 100.8
Ulvospinel basis Fe~O a 26.3 FeO 49.9 Total 100.4
25.1 50.4 99.7
30.3 47.7 99.8
30.0 48.5 100.6
38.1 43.8 99.3
40.0 43.4 100.0
17.7 49.4 100.2
16.2 54.4 100.4
26.2 48.8 99.6
29.6 47.4 99.6
1Viol.% Usp.
63.0
55.3
55.8
43.6
41.2
71.9
76.4
62.4
55.8
60.6
1 (G 151)
97.2
0.09 19.4 0.78 0.03 0.01 74.9 0.90 0.24 * 0.32
2 (oraefi)
4 (AC. 7)
5 (Cam 73)
6 (Cam99)
8 9 (5748) (3112)
11 (AC. 5)
0.03 49.9 0.03
0.04 49.6 0.06
0.02 49.8 0.04
0.01 49.7 0.04
0.05 50.1 *
0.03 48.9 0.06
Rhombohedral phase Si02 Ti02 AI~O3 V20~
Cry08 FeO MnO MgO CaO ZnO Sum Recalculated F%O 3 FeO Total Mol. % R~Oa Temp. ~ ]o,
0.05 49.8 0.04 *
*
*
*
*
*
0.04 50.6 * *
*
* 47.7 0.98 0.62 0.06 0.04
* 47.9 1.23 0.10 0.04 *
* 48.3 0.82 0.63 0.02 *
* 47.5 1.71 0.31 0.02 *
* 47.6 1.68 0.35 0.02 *
* 46.9 1.96 0.21 0.04 0.03
* 46.7 2.17 0.09 0.04 0.03
0.02 48.7 1.02 0.47 * 0.05
99.3
99.2
99.5
99.4
99.4
99.3
99.7
99.2
analyses (see text) 5.7 5.0 6.2 5.6 5.8 4.7 42.6 43.4 42.7 42.5 42.4 42.7 99.9 99.7 100.1 100.0 100.0 99.8 5.4 4.8 6.0 5.4 5.6 4.5 925 915 900 810 790 -10-12"5 10-12"9 1 0 - 1 ~ ' 9 1 0 - 1 * ' ~ 10-15'1 - -
4.0 43.1 100.1 4.0 1025 10-n'~
7.3 42.1 99.9 7.1 925 10--1~'2
Ilmenite has not been found in samples P. 186, 1910--199, 3114. A spinel phase has not been found in 5748. AC. 5 has no co-existing ferromagnesian silicate phenoerysts. * Below limit of sensitivity 0.01%.
I r o n - T i t a n i u m Oxides of Salie Rocks Table 3.
41
Analyses o/iron.titanium oxide8 coexisting with orthopyroxene • calcium-rich pyroxene 12 13 14 15 16 17 18 19 20 (Cam 49) (cam55) (cam66) (Cam95) (Cam103)(Cam104)(cam108)(Cam110)(EC. 20)
Spinel phase SiO~ TiO 2 Al20 a V20 a Cr203 FeO MnO MgO CaO ZnO
0.07 14.0 1.74 1.57 0.07 77.0 0.44 1.50 0.02 0.08
0.06 14.2 1.79 1.00 0.02 76.0 0.40 1.56 0.01 0.07
0.05 15.7 1.97 1.80 0.05 74.9 0.42 1.81 0.01 0.08
0.11 14.3 0.92 * 0.01 79.4 0.90 0.18 0.01 0.21
0.11 13.5 0.91 * * 79.7 0.85 0.18 0.01 0.16
0.11 13.6 0.92 * 0.01 80.0 0.89 0.18 0.01 0.22
0.13 12.6 0.94 * 0.01 81.0 0.86 0.18 0.02 0.17
0.1O 12.8 0.92 * * 80.8 0.85 0.16 0.01 0.18
0.08 13.5 1.42 0.14 0.01 76.7 1.16 2,12 0.02 0.19
Sum
96.5
95.1
96.8
96.0
95.4
95.9
95.9
95.8
95.3
47.9 32.9 99.9
46.0 33.4 101.1
49.8 34.6 101.0
50.5 34.3 100.5
50.7 34.4 101.1
52.0 34.2 101.2
51.7 34.2 100.9
50.8 31.0 100.4
39.1 41.8 100.4
38.4 41.4 98.9
35.6 42.9 100.4
40,1 43.3 100.0
41.4 42.4 99.5
41.5 42.6 100.0
43.5 41.9 100.3
43.1 42.0 100.1
41.7 39.2 99.5
39.3
40.3
43.7
41.0
38.9
39.0
36.2
36.7
38.2
Recalculated analyses (see text) Ilmenite basis F%O 3 FeO To~al
48.5 33.3 101.3
Ulvospinel basis
Fe203 FeO Total Mol. % Usp.
12 13 14 15 16 17 19 20 21 (Cam 49) (Cam55) (Cam66) (Cam95) (Caml03) (Caml04) (Camll0) (EC. 20) (Cam118)
Rhombohedral phase SiO 2 Ti02 A1208 V20 a Cr203 FeO MnO MgO CaO ZnO
* 47.0 0.15 0.22 0.06 47.8 0.52 2.64 * *
* 47.6 0.22 0.21 * 46.3 0.46 3.60 0.Ol *
0.03 46.7 0.15 0.20 0.04 47.5 0.55 3.15 0.02 *
0.02 48.3 0.03 * * 49.1 1.50 0.35 0.01 *
0.02 48.5 0.04 * * 48.8 1.53 0.40 0.02 *
0.02 48.4 0.04 * * 49.0 1.55 0.42 0.02 *
0.02 48.3 0.04 * * 49.1 1.58 0.40 0.02 *
* 45.6 0.13 * * 48.2 1.25 3.31 0.03 *
0.01 51.6 0,05 0.43 0.21 44.2 0.57 2.65 0.01 *
Sum
98.4
98.4
98.3
99.3
98.9
99.4
99.4
98.5
99.7
Recalculated analyses (see tex$) Fe20 a 12.0 FeO 37.0 Total 99.6 Mol.% R20 a 11.8 Temp. ~ 880 /O, 10-1~.2
11.6 13.0 8.7 8.3 8.7 8.9 16.0 3.4 35.9 35.8 41.3 41.3 41.2 41.1 33.8 41.1 99.6 99.6 100.2 99.7 100.3 100.3 100.1 10O.0 11.3 12.6 8.3 8.0 8.3 8.5 15.1 3.9 895 930 850 820 825 815 910 -10-12.1 10-11.4 10-13.~ 10-la.9 10-13.7 10-13.9 10-11.4 __
Ilmenite h a s n o t been f o u n d in sample Cam 108. A spinel phase h a s n o t been found i n Cam 118. * Below limit of sensitivity 0.01%.
42
I , ~ S. E. CARMIC~AnL:
Table 4.
Analyses o] iron-titanium oxides co-existing with biotite-4-amphibole • pyroxene 22 23A 23B 24A 24B 25 (cam 81)(Cam86) (Cam86) (Cam93) (Cam93) (3811)
26 27 28 29 (TC.19) (Cal. 13) (Cal.19) (L.118)
SiO~ TiO 2 A120 a V20 a CreO a l~eO MnO MgO CaO ZnO
0.09 ]5.8 1.26 * * 77.4 0.81 0.44 0.01 0.14
0.i0 14.3 2.60 0.15 * 76.1 0.52 2.31 0.01 0.09
0.09 15.7 1.32 * * 78.0 0.81 0.52 0.02 0.14
0.08 14.6 2.38 0.07 * 76.0 0.56 2.23 0.02 0.07
0.09 15.8 1.40 * * 77.7 0.85 0.52 0.02 0.15
0.12 11.9 3.08 0.45 0.01 75.5 1.18 2.73 0.03 0.15
0.05 19.5 0.97 0.12 * 70.6 2.18 1.98 * 0.23
0.05 7.33 1.65 0.81 0.09 82.3 0.49 1.60 0.01 0.10
0.05 5.42 1.41 0.51 0.07 84.8 0.66 1.11 0.01 0.11
0.11 5.74 1.50 0.59 0.06 84.5 0.58 1.30 0.03 0.14
Sum
96.0
96.2
96.6
96.0
96.5
95.2
95.6
94.4
94.2
94.6
Spinel phase
Recalculated analyses (see text) Ilmenite basis FeO 3 FeO Total
47.4 34.8 100.8
48.8 32.2 101.1
48.0 34.8 101.8
48.7 32.2 100.9
47.7 34.8 101.3
50.7 29.8 100.2
43.1 31.8 99.9
57.4 30.6 100.1
60.3 30.5 100.2
59.9 30.5 100.5
Ulvospinel basis F%O a 36.7 FeO 44.3 Total 99.6
39.2 40.8 100.1
37.4 44.3 100.3
38.9 41.0 99.9
37.1 44.3 100.2
42.7 37.1 99.5
30.1 43.5 98.6
52.5 35.0 99.6
56.6 33.8 99.8
56.0 34.1 100.2
39.9
44.5
40.8
44.7
33.4
55.2
20.9
15.6
16.6
Mol.% Usp.
43.6
23 (Cam 86)
24 (Cam 93)
25
27
(3811)
(Cal. 13)
28 (Cal. 19)
29 (L. 118)
Rhombohedral phase SiO 2 TiO 2 AI~O3 V~O3 Cr~O3 FeO MnO MgO CaO ZnO
0.02 44.8 0.10 * * 50.7 1.23 1.23 0.03 *
0.02 44.9 0.09 * * 50.5 1.54 1.37 0.02 *
0.04 40.9 0.36 0.07 0.01 51.4 0.72 3.79 0.04 *
0.06 37.6 0.28 0.20 0.03 56.4 0.43 2.12 0.02 *
0.03 39.4 0.19 * 0.02 54.7 0.80 2.10 0.02 *
0.08 39.0 0.20 0.15 0.02 54.7 0.61 2.24 0.04 *
Sum
98.1
98.4
97.3
97.1
97.3
97.0
Recalculated analyses (see text) F%0 a 15.4 15.7 FeO 36.8 36.4 Total 99.6 100.0 ~oI. % t~20 a 14.8 14.9 Temp. ~ 920, 960 930, 985 fO~ 10-11"4, 10-10"8 10-11'2, 10-1~
24.6 29.3 99.8 23.6 1010 10-9'2
29.7 29.6 100.0 28.7 980 10-9'1
Ilmenite has n o t been found in samples Cam 81 a n d TC. 19. * Below limit of sensitivity 0.01%.
26.5 30.9 100.0 25.4 865 10-10"9
26.9 30.5 99.7 25.9 880 10-1~
Iron-Titanium Oxides of Salic Rocks
43
The recalculation of the rhombohedral phase analyses arc even more simple, all the MnO, CaO, ZnO and MgO being combined with the molecular proportions of TiOeq-SiO2, sufficient FeO then being added to make the ratio 1 : 1. The excess FeO is calculated as F%0s, and the determined content of total iron as :FeO amended accordingly. The average total of the timenite analyses after this computation is 99.9, which is very satisfactory. The molecular percent of three-valent ions in the rhombohedral phase is given in Tables 2 ~ 4 and refers to F%OsdV~Osq~Cr~Osq-A1203 (R20~). Both the calculated molecular percentages of ulvospinel in the spinel phase, and R20 s in the rhombohedral phase are derived rather differently than the corresponding values given by BUDDINGTO~ and LI~DSLEY (1964). However, the method outlined above seems to be the
\ ~d R-.0
R'] Os
got
Fig. 1. The analyses of the co-existing iron-titanium oxides (Tables 2, 3, and 4) are plotted in terms of the molecular percent of ulvospinel in the magnetite and of t~O3 in the ilmenite. 1~110 represents FeO, MnO, MgO, CaO and ZnO when part of the ulvospinel molecule, but only FeO when part of the magnetite molecule, l~0a represents A1203, V~08, Cr~Oa and Fe203. TiO~ includes the small amount of SiO~ in the analysis. Fig. l a are those assemblages which co-exist with olivine; Fig. l b those which co-exist with orthopyroxenc and Fig. l c those which co-exist with biotite and amphibole. I n Fig. 1 c, the tie-line 23.24 is taken from Fig. l b to show the relative relationship of the two groups
only one that can be used to check the quality of the analysis, and rather than invert the computation to calculate the molecular percentages of ulvospinel in magnetite according to the method of BUDDINGTON and LINDSLEY, the above method has been slightly extended. Thie scheme of computation will to an extent vitiate direct comparison with the results of B~;DmNGS9 O~ and LINDSLEY, but for comparative purposes as used here, it is sufficient. T h e compositions of t h e co-existing oxide phases in t e r m s of t h e molecular percent of nlvospinel in t h e m a g n e t i t e a n d t h e m o l e c u l a r p e r c e n t of R 2 0 s in t h e i l m e n i t e h a v e been p l o t t e d in Fig. 1, t h e d a t a being t a k e n from Tables 2, 3 a n d 4. I n Fig. 1 a, t h e oxide assemblages co-existing w i t h a n iron-rich olivine in rhyolites a n d one p a n t e l l e r i t e are shown, t h e spinel phases show a r a n g e of 76 to 41 molecular p e r c e n t ulvospinel, b u t t h e associated ilmenite shows o n l y a small r a n g e in c o m p o s i t i o n from 4 to 6 % R~O s. As t h e r e is little or no zoning in all t h e oxide minerals, t h e b u l k analysis d e r i v e d from t h e p r o b e d a t a of b e t w e e n 15 a n d 20 grains should v e r y closely a p p r o x i m a t e to t h e i r b u l k composition. I n Fig. 1 b, where t h e oxides are a s s o c i a t e d w i t h o r t h o p y r o x e n e , t h e spinel is v e r y r e s t r i c t e d in c o m p o s i t i o n b u t t h e i l m e n i t e is m o r e enriched in 1~203 c o m p a r e d to those in Fig. l a a n d shows a g r e a t e r range of composition ( 8 - - 1 5 % ) . I n :Fig. l c where b i o t i t e a n d a m p h i b o l e co-exist, b o t h oxides are b e c o m i n g i m p o v e r i s h e d in TiOz
44
IA~ S. E. CA~MIC~AEL:
with respect to those in Figs. 1 a and b, the ilmenites contain up to 26 % R20 a and the magnetites contain as little as 16% ulvospinel molecule. Although these acid rocks are not identical in composition, and therefore their different mineralogical assemblages cannot arise solely in response to different crystallization histories, or different environments, a general trend seems apparent, namely that there is a response in the composition of the oxides to the species or composition of the co-existing ferrogmagnesian silicate phenocrysts. To some extent this may be the result of the silicate, particularly biotite and amphibole, competing for TiO 2 or for the other components, but it must be noted that almost invariably the oxide phases are the first to precipitate. The oxide phases are sensitive to environmental conditions, the main tenet of Buddington and Lindsley's work, and from Fig. 1 it seems that in a general way that there is also a correspondence between the composition of the oxide phases and the coexisting ferromagnesians. BUDDINGTON and LI~DSLV,Y (1964) have experimentally determined the composition of magnetite in equilibrium with ilmenite for a wide range of temperatures and oxygen fugacities. These curves (op. cir. Fig. 5) may be used with natural oxide assemblages to estimate these two parameters, although the co-precipitation of biotite and amphibole or of any other phase containing Ti, Fe'" and Fe" may modify to some extent the applicability of these curves. Some of the co-existing biotites contain as much as 6.9 % TiO~ (Table 9). However, except in a few cases noted below, where a biotite-amphibole assemblage is replaced by pyroxene in response to a changing environment, the oxide minerals are the first to crystallize and may therefore be unaffected by the ferromagnesian silicates. The values for the temperature ( • 30oC) of the magnetitc-ilmenite equilibration in these rocks is given in Tables 2, 3 and 4 together with the corresponding oxygen fugacities. The three Lassen dacites (Table 4) have magnetite-ilmenite compositions which fall off the curves of Buddington and Lindsley, so that the estimation of the temperature and oxygen fugacity of their equilibration has required considerable extrapolation of the curves. In Fig. 2, the temperature and oxygen fugacity of magnetite and ilmenite microphenocryst equilibration has been plotted for each of the investigated rocks. The solid circles are rocks which contain an iron-rich olivine, and they fall very near to or straddle the synthetic fayalite-magnetite-quartz buffer curve. Although many of these rocks do not contain phenocrysts of quartz, it is to be expected that an iron-rich olivine magnetite assemblage plus a siliceous liquid could act as an oxygen buffer assemblage. The oxide assemblages which co-exist with orthopyroxene fall above the olivine curve (Fig. 2), coineidentally very near the synthetic nickel-nickel oxide curve, but clearly displaced above the olivine curve. Above this orthopyroxene curve are the three Lassen dacites whose oxide assemblages co-exist with biotite and hornblende together with an oversaturated trachyte of similar mineralogy. There is then a clear indication of an interdependence in most of these rocks of the composition of the oxide phases with the nature and composition of the silicate minerals even if they are present in only very small amounts (Table 1). In some of these liquids the picture is not quite so simple as portrayed in Fig. 2, for with the exception of 3811, all those which at some stage in their crystallization
Iron-Titanium Oxides of Salic Rocks
45
history have precipitated biotite and hornblende have subsequently precipitated pyroxene, presumably in response to a change ii1 environment as they moved towards the surface. It is theoretically possible then to have an oxide pair indicating conditions of biotite-amphibole stability, and a later pair indicative of the pyroxene conditions, provided that the oxides cannot continuously equilibrate. In two rhyolitic obsidians, Cam 86 and 93 (Table 4), there are two spinel phases each clearly compositionally distinct from the other, but unfortunately there is only one rhombohedral phase. It is suggested that one of the spinels precipitated during or before the crystallization of biotite and hornblende, and the other I
I
I
I
]
i
I
HM --4 --5
i
~
i t
/
1
1
--0
z~ |
-10
~M
j 23 / ZOo o. {Tf,;,'
~
q3~i 11
-/3
./~ls J ~
,,,9
z
-18 /////~.i*i -18 I
I 000
I
I YO0
I
I iO00
J 7" ~
Fig. 2. The values of the temperature and oxygen fugacity (log~o) of the magnetite-ihnenite equilibration are plotted. Filled circles represent assemblages which co-exist with olivine (Table 2); open circles those which coexist with orthopyroxene (Table 3) and filled circles inside open circles those which co-exist with biotite and amphibole (Table 4). The curve QF]~[ represents the variation of the fugacity of oxygen with temperature for the buffer fayalite-magnetite-quartz at 1 atmosphere total pressure. The curve HM is the ct~rve for magnetitehaematite buffer at 1 atmosphere (EUGSTERand WONES, 1962) and the dashed curve H~O is the decomposition of the pure water curve at 1 atmosphere (VE]RHOOGEN,1962)
after conditions changed to allow pyroxene to crystallize. I t is subjective in the extreme to decide which of the two spinel phases (Table 4, 23A, 23B, 24A, 24B) crystMlized first, but the higher A1203 and MgO in 23A and 24A suggest by analogy with the other magnetite analyses that they were earlier. The composition of the ilmenite of 23 and 24 in conjunction with both the magnetites give points in l~ig. 2 which fall with the rocks that have orthopyroxene assemblages, and it would appear then that either ilmenite was missing at the time of biotite-amphibole crystallization, or that it continously equilibrated as the environmental conditioas changed; it is difficult to distinguish the criteria by which one of the magnetites could be identified as a xenocryst.
46
IA~ S.E. CARMICHAEL:
The three Lassen dacites, 27, 28 and 29 (Fig. 2), have oxide minerals which probably equilibrated at the time of biotite-amphibole formation, for if they equilibrated at the later orthopyroxene crystallization, it seems likely t h a t they would fall on the orthopyroxene curve in Fig. 2, especially as the dacite pyroxene compositions overlap some of those from rocks which do fall on this curve. If this suggestion is correct, then the iron-titanium oxide relationships are preserved, or quenched, and they do not rapidly re-equilibrate under new conditions, assuming of course that they are still stable phases. (A~,C~,V)~03
ll/.J ~
~gO
24~
W~%
\
H,O*ZnO
Fig. 3. The v a r i a t i o n in m i n o r elements in the co-existing iron-titanimn oxides are plotted, the d a t a being t a k e n f r o m Tables 2, 3 a n d 4. The sample n u m b e r s are placed a t the spinel phase composition, ilmenite (unnumbered) being a t the distal end of the tie-line
The distribution of minor elements between the co-existing spinel and rhombohedral phases is of interest. The ilmenite is richer in MgO and MnO than the co-existing spinel which is enriched in AI2Og, Cr20g , V~03, ZnO and SiO 2. This enrichment of SiO~ is real and is not solely the result of analytical error. In Fig. 3, the variations of some of the minor elements in the co-existing existing are shown, both rhombohedral and spinel phases becoming progressively poorer in the threevalent elements as they become improverished in MgO and correspondingly enriched in MnO. The magnetites from the kenyte and the phonolitie obsidian (Fig. 3, Nos 7 and 26), which have no co-existing ilmenite phase, have minor element proportions near the mean of each of the ilmenite-magnetite pairs.
The Iron-Titanium Oxide Equilibration Temperatures Evidence has been cited above which suggests that the iron-titanium oxides do not rapidly re-equilibrate to a change in environment, so t h a t any interaction between magnetite, ilmenite and silicate liquid m a y be assumed in these rocks to cease at the time of congealing or quenching of the liquid. Therefore the oxide
Iron-Titanium Oxides of Salic Rocks
47
equilibration temperatures will, under this assumption, correspond to the quenching temperatures of the rocks. If these silicate liquids have been quenched having precipitated only a small amount of crystalline phases, then the quenching temperature will correspond to all intents and purposes to the liquidus temperature. It may be seen from the data in Table 1, that there are only eight specimens (Nos 2, l l , 14, 15, 16, 17, 19, and 23) which contain less than five percent of crystals (an arbitrary chosen value) and for which oxide equilibration temperatures are known (Tables 2, 3 and 4). Quenching temperatures indicated by the oxides may also be used for rocks with any amount of crystals if the composition of the residual glass is known; the residual glass of four porphyritic salic volcanics (Nos 1,4, 20 and 25) have previously been analysed together with the residual glass of a pantellerite lava (No. 9); the oxide quenching temperatures indicate the liquidus temperatures of these residual glasses. The compositions of these rocks and residual glasses in terms of their normative salic constituents (less anorthite) have been plotted in Fig. 4 together with the liquidus temperatures deduced from the oxide data. Although there is only a small amount of data, there seems to be a lack of a consistent pattern to the indicated temperatures. In part this may be the result of differences in the environmental vapor pressure before the quenching of the liquids; this pressure is likely to be low in most cases (although see below) as ten of the thirteen specimens are lavas, the remaining three being dykes or sills. However, the main influence in the variation of the liquidus temperatures of these rocks and residual glasses is undoubtedly their normative anorthite content, the values of which are given beside each plotted point in Fig. 4; small amounts of anorthite added to the system NaA1SiaOs-KA1SiaOs-SiO2-H20 have the effect of raising the isobaric liquidus temperatures for those compositions which begin to crystallize in the stability field of feldspar (CAI~$IGHAEL, 1964). Even so, there are still inconsistencies which could result from analytical error of the compositions of the rocks or the oxides or the derived equilibration temperatures. It would seem, however, that many salic acid liquids as represented by these specimens have a ]iquidus temperature above 9000 C at a low but unknown pressure, whereas the acid extrusions of Mono Craters, California have temperatures consistently below this with an average value of about 825~ These rocks, despite their paucity of phenocrysts, did not quench to a clear glass, but frequently have a perlitic texture with convoluted whorls of dusty iron oxides. The glass is often feebly anisotropic, and may be very finely crystalline in patches ; in some there are myriads of tiny feldspar and quartz crystals in the groundmass (Cam 95) so that the oxide equilibration temperature may represent a temperature lower than that of the precipitation of the sparse phenocrysts. The highest quenching temperature of the rocks from Mono Craters (Cam 95) is 850~ and is obtained from a rock which has the largest amount of crystals in the groundmass. This would suggest that the quenching temperatures of the Mono Craters rhyolites do not represent liquidus temperatures. By way of contrast, the residual glass of a pantellerite lava (No. 913112]) was quenched at 1025~ after precipitating almost twenty percent of crystals (Table 1) and just reaching a quartz-feldspar boundary surface.
48
IA~ S. E. C~MICHA~L:
Several of the specimens which contain only a few crystals, and for which the oxide quenching temperature m a y be accounted equivalent to the liquidus temperature have compositions almost identical with rocks with a more extensive precipitate. Thus No. 14 (Cam 66) with a liquidus temperature of 930~ has an almost identical composition to No. 12 (Cam 49) (Table 5) which contains 6.3 percent crystals. As the quenching temperature of Cam 49 was 880~ these crystals represent a cooling interval of 500 C before congelation or eruption. Similarly with the extrusions of Mono Craters, the compositions of the three specimens Nos 16, 17 and 19 which have an average "liquidus" temperature of 820~ are almost QU0'FZe
Albh'e
~/t %
Gr/hoclase
~ig. 4. The salie normative constituents are plotted in the system NaAISi~Os-gAlSiaO~-SiO~-It~O for those rocks (filled circles) a n d residual glasses (open circles) for which the oxide quenching t e m p e r a t u r e s (Tables 2, 3 a n d 4) are considered to represent liquidus temperatures. The d a t a for the rocks are t a k e n f r o m Table 5 (Nos. 2, 11, 19, 23) a n d those for the residual glasses f r o m CARi~IOKAEL (1960, 1965). Beside the respective quenching t e m p e r a t u r e s are the values of n o r m a t i v e anorthite in the rock or residual glass. The b o u n d a r y curve a t 500 b a r s w a t e r v a p o r pressure a n d the m i n i m u m on this curve are t a k e n f r o m TUTT/~E a n d BOWEN (1958)
identical to Nos 5 and 6 (T~ble 5). The average quenching temperature of these two rhyolites (Table 2) is 800~ so t h a t ~ cooling interval of 20~ produced 5.2 percent crystals Table 1). The extrusive rhyolites of the Cascade volcanic province and of Mono Craters indicate that considerable volumes of acid m a g m a existed at some depth in the crust, and could occur as intrusive granites of granophyres. The temperatures at which this acid m a g m a was intruded could be very similar to the quenching temperatures of the rhyolites, provided t h a t the obsidians and rhyolites have not gained temperature ~s they rose to the surface, for which there is no petrographic evidence, and that the acid m a g m a at depth was not saturated with water. I t is probable therefore t h a t acid m a g m a intruded at depth and later to form granite, m a y have a relatively high temperature at the time of intrusion (ca. 900 e) and t h a t as it may be undersaturated with respect to water, ~ very much larger crystallization interval than t h a t found (20--300 C) by TUTTL~ and B o w ~ (1958) for water-saturated granites.
Iron-Titaninm Oxides of Salic Rocks
49
Biotite Phenoerysts Phenocrysts of biotite are found in eight of the specimens, Nos 22--29 (Table 1) ; they are usually intensely pleochroic euhedral crystals with no sign of resorption or a reaction relation. I n this way they are different from the amphibole phenocrysts, which in the Lassen dacites (27, 28 and 29) m a y have a reaction rim of orthopyroxene. However, all the rocks which contain biotite and amphibole, with the exception of TC 19, have subsequently precipitated pyroxene, indicating that biotite becomes unstable at some stage in the cooling history, in accord with its absence as microlites in the residual glass.
M~
AI +[e ""~-Ti
Afore %
Fe"+gn
Fig. 5. The composition of the biotite phenocrysts (Table 6) plotted in terms of the oetahedral cations. The shaded area is the field of granitic biotites (Fos~EIL 1960); A and B represent biotite phenocrysts from the San J u a n volcanics (op. cir.), and C, biotite phenocrysts from the Whakamaru ignirabrite (EWART, 1965)
Although most of the biotite phenocrysts show some degree of zoning, becoming Ti-poor and iron-rich towards the margins, the phenocrysts in the three Lassen dacites show reverse zoning, becoming more magnesian towards the rim. The details of the extent and direction of zoning have not been fully investigated, but as the range in composition is not great, the probe analyses of between 10 and 15 grains should represent a reliable bulk composition. The analyses and fomulae of the analyzed biotites are given in Table 6. It is not possible to use the totals as an indication of the quality of the analyses as no determination has been made of the fluor-hydroxy components, nor of FeO in three of the specimens. The addition of approximately 3 . 5 ~ . 0 weight percent of H~O to the totals in order to satisfy the ideal formula of 4 (OH) indicates that those analyses with both oxidation states of iron determined are satisfactory. Several aspects os the chemistry of the biotite phenocrysts are of interest; they are all unusually rich in Ti compared to the igneous biotites listed by DEEl~ et al. (1962) and they are all poor in Al~Os as there is insufficient A1 to make up the tetrahedral sites to eight with Si. If these sites are filled then some of the Ti or Fe z+ will be in four fold co-ordination. 4
Contr. Mineral. and Petrol., Vol. 14
50
IAN S. E . CA~MIC~AEL:
~able 5. Analyses and CIPW norms o/salic volcanics (/or key to Table, see page 62)
SiO~ Ti02 AI~O~ F%0, FeO MnO
~gO CaO BaO Na~0 KsO P~O~ H~O + tt~OTotal
Iceland
Arran, Scotland
Antarctic
69.8 71.52 72.1 0.34 0.26 0.25 12.9 13.35 12.3 0.74 0.9 0.7 2.4 2.72 1.4 0.08 0.11 0.06 0.21 0.05 0.13 1.12 1.0 1.7
71.9 73.5 0.37 0.17 11.7 11.2 0.6 0.60 2.1 0.94 0.07 0.03 0.38 0.09 1.4 0.75
36.00 1.22 :9.74 1.37 3.30 0.20 1.12 3.26
4.8 3.1 0.07 3.4 0.4 99.9
5.97 3.49 0.02 0.42 0.05
4.6 3.8 0.02 3.0 0.2
99.82 99.8
4.0 3.5 0.05 3.5 0.3
3.7 4.5 0.03 4.6 0.3
99.9 100.4
7.97 4.28 0.50 0.58 0.17 99.87*
Mono Craters, California 75.04 74.98 75.56 75.52 75.86 76.21 76.09 0.07 0.07 0.07 0.06 0.07 0.07 0.07 12.29 12.30 12.43 12.49 12.50 12.58 12.54 0.33 0.38 0.36 0.41 0.35 0.30 0.35 0.71 0.67 0.68 0.64 0.69 0.73 0.70 0.05 0.05 0.05 0.04 0.04 0.04 0.05 0.04 0.04 0.04 0.04 0.03 0.03 0.02 0.58 0.60 0.59 0.61 0.59 0.61 0.60 4.03 4.66 0.005 1.81 0.20
4.01 4.66 0.006 1.83 0.19
4.02 4.70 0.008 1.23 0.08
4.06 4.70 0.006 1.03 0.16
4.07 4.71 0.010 0.71 0.08
4.05 4.72 0.008 0.39 0.13
4.08 4.70 0.008 0.55 0.16
99.81 99.79 99.82 99.87 99.71 99.87 99.91
* Includes 0.20 C1.
Norms Qtz Or Ab An Ne Ae ns Wo En Fs 01 mt il ap Rest
25.6 18.4 40.4 4.4
Total
99.6
20.34 28.7 20.57 22.2 49.25 38.8 -2.0
30.9 20.6 34.1 3.6
33.2 26.7 31.4 0.6
25.58 9.30 5.84 4.48
--
1.39
1.6 0.5 3.4
2.32 0.10 4.49
1.3
1.4
0.3 1.4
1.0 2.8
1.4 0.2 0.8
0.9 0.6
0.23 0.61
1.4 0.5
0.9 0.8
0.9 0.5
0.49
3.2
3.8
4.9
2.37 2.09 2.28 1.34 1.10
99.9 100.6
100.16
3.8
99.79 99.8
5.78
32.04 32.04 32.58 32.40 32.58 33.06 32.81 27.80 27.80 27.80 27.80 27.80 27.80 27.80 34.06 34.06 34.06 34.58 34.58 34.06 34.58 1.67 1.67 1.95 1.95 1.95 2.50 1.95
0.58 0.10 0.92
0.58 0.10 0.79
0.46 0.10 0.92
0.46 0.10 0.53
0.46 0.07 0.79
0.23 0.07 0.92
0.46 0.05 0.79
0.46 0.15
0.70 0.15
0.70 0.15
0.70 0.15
0.70 0.15
0.46 0.15
0.70 0.15
2.01
1.92
1.32
1.20
0.80
0.53
0.72
99.79 99.81 100.04 99.87 99.88 99.78100.01
The analyses of the pantellerites (8, 9, a n d 10) have not been quoted here (C~mc~AEL, 1962). (California) are new analyses b y the writer. The remainder have been t a k e n from C~MIC]~AEL new analysis. A g a i n i n c o n t r a s t t o t h e i g n e o u s b i o t i t e a n a l y s e s t a b u l a t e d b y D E ~ e t al., t h e s e c o n t a i n o n l y a t r a c e of CaO, b u t n o t a b l e a m o u n t s of B a O , c e r t a i n l y v e r y s u g g e s t i v e t h a t t h e r e c o u l d b e a s y s t e m a t i c e r r o r i n t h e C a O d e t e r m i n a t i o n s of t h o s e l i s t e d b y D E ~ e t al., o r t h a t t h e s p e c i m e n s w e r e c o n t a m i n a t e d b y a p a t i t e , w h i c h is a c o m m o n a s s o c i a t e . A s m a y b e s e e n i n Fig. 5, t h e s e b i o t i t e p h e n o e r y s t s h a v e c o m p o s i t i o n s w h i c h f a l l o u t s i d e t h e f i e l d of g r a n i t i c b i o t i t e c o m p o s i t i o n s c o m p i l e d b y F O S T ~ (1960).
51
Iron-Titanium Oxides of Sa]ic Rocks Table 5 (Continuation)
Clear Lake, California
Inyo Craters, California
Lassen Park, California
Eigg, Scotland
73.53 67.73 73.44 0.29 0.50 0.31 13.71 15.44 13.93 0.45 0.69 0.49 1.25 2.40 1.24 0.04 0.06 0.04 0.30 1.30 0.33 1.24 3.35 1.22 0.11 0.10 0.11 4.21 3.85 4.19 4.39 3.25 4.34 0.05 0.15 0.05 0.36 0.95 0.41 0.05 0.10 0.08
4.82 0.26 3.05 0.19 1.05 0.03 0.22 1.09 0.09 3.52 4.82 0.03 0.59 0.07
72.88 71.25 69.75 0.15 0.26 0.38 13.92 14.60 14.98 0.54 0.65 0.84 1.08 1.35 1.42 0.06 0.06 0.07 0.17 0.27 0.40 0.76 1.06 1.30 0.05 0.12 0.16 4.31 4.57 4.67 5.20 5.08 4.91 0.02 0.06 0.10 0.65 0.56 0.86 0.07 0.07 0.07
69.16 70.68 69.46 0.38 0.37 0.44 14.81 14.39 14.84 1.01 1.24 1.25 1.52 1.13 1.37 0.06 0.06 0.07 1.41 1.05 1.26 3.00 2.54 2.92 0.10 0.12 0.11 4.30 4.22 4.44 2.73 2.85 2.72 0.10 0.10 0.13 1.18 1.16 0.75 0.12 0.08 0.07
99.98 99.87 100.18
99.83
99.86 99.94 99.91
99.88 99.99 99.83
32.94 28.36 29.34 5.56
25.38 22.08 19.92 30.58 30.02 28.91 36.15 38.77 39.82 3.34 4.17 5.28
25.08 16.12 36.15 13.07
Medicine Lake, California
i
Cantal, France
Tenerife
6.0 0.71 5.1 1.5 1.2 0.11 0.69 1.2
0.73 1.08 6.75 2.05 2.13 0.16 1.11 3.55
59.29 0.69 19.23 1.01 2.37 0.19 0.44 0.86
4.9 6.2 0.11 1.7 0.4
4.56 5.24 0.36 1.90 0.10
9.80 5.84 0.10 0.29 0.01
99.8
99.72
100.37'
11.3 36.7 41.4 0.8
6.84 30.58 38.77 10.01
34.47 36.68
* Includes 0.32 C1.
28.38 26.13 35.63 5.28
22.98 28.44 18.90 25.58 32.49 35.63 15.29 6.12
28.38 16.68 35.63 11.95
25.38 16.12 37.20 12.51
15.09 2.77 2.68 0.46 0.80 1.45
0.35 3.30 3.17
0.12 0.80 1.45
0.12 0.60 1.19
0.23 0.40 1.45
0.23 0.70 1.45
0.23 1.00 1.45
0.58 3.50 1.45
-2.60 0.40
0.58 3.10 0.92
2.1 1.7
0.70 0.61 -0.46
0.93 0.91 0.34 1.05
0.70 0.61 -0.47
0.23 0.61
0.70 0.30 -0.74
0.93 0.61 0.34 0.63
1.16 0.76 0.34 0.93
1.39 0.76 0.34 1.30
1.86 0.76 0.34 1.24
1.86 0.76 0.34 0.82
2.1 1.4
99.90 99.71 99.92
99.64
99.74 99.84 99.59
3.10
5.66
2.33
0.69
99.69 99.94 99.80
2.1
3.02 2.13 1.01 2.00
1.37 0.34 0.77
99.6
100.02
100.41
The analyses from Mono Craters, Medicine Lake, Clear Lake, Inyo Craters and Lassen P a r k (1960, Nos 1, 3, 4 a n d 20; 1962, No. 11; 1964, l~os. 7 a n d 26; 1965, No. 25). )7o. 2 is also a
Biotite a n d P e r a l u m i n o u s L i q u i d s
O n e of t h e i n t e r e s t i n g c o n t r a s t s b e t w e e n t h e c o m p o s i t i o n of v o l c a n i c a c i d r o c k s a n d p l u t o n i c a c i d r o c k s is t h e r e l a t i v e l y c o m m o n p e r a l u m i n o u s c h a r a c t e r of t h e l a t t e r i n c o m p a r i s o n t o i t s r a r i t y i n t h e l o r m e r g r o u p . P e r a l u m i n o u s c h a r a c t e r is a n e x c e s s of t h e m o l e c u l a r p r o p o r t i o n s of AI~O a o v e r t h e t o t a l of t h o s e of K~O, N a 2 0 , B a O a n d C a O a n d is c o m m o n l y g i v e n m i n e r o l o g i c a l e x p r e s s i o n b y m u s 4*
52
IAN S. E. CARiVlICHAEL:
Table 6. Analyses and/ormulae o] biotite phenocrysts
SiO~ TiO2 A1203 F%0 a FeO MaO MgO CaO BaO Na20 K20 Sum
21(Cam81) 22(Cam86) 23(Cam93) 24(3811)
25(TC.19) 26(Ca1.13) 27(Ca1.19) 28(L.118
35.3 5.75 12.7 -24.2 0.34 7.2 0.02 0.63 0.60 8.64 95.4
36.4 6.88 12.1 -14.8 0.40 13.4 0.02 0.16 1.11 8.54 93.8
35.3 5.99 13.0 11.3 10.7 0.33 10.0 0.02 0.75 0.82 8.75 97.0
36.0 5.39 12.7 9.7 12.4 0.35 10.6 0.02 0.42 0.72 8.87 97.2
Formulae on basis of 22 oxygens [Si 5.573 5.312 5.406 Z ~A1 2.364 2.306 2.248 (Ti 0.063 0.382 0.346 Ti Fe'" u
/ MM~ (
C~
X Ba
Z Y X Fe"/Oet.
36.1 5.94 13.2 7.4 9.1 0.35 14.0 0.02 0.64 0.78 8.77 96.3
36.9 4.50 13.0 -13.0 0.10 17.5 0.01 0.70 1.32 8.20 95.2
36.9 4.41 12.9 9.7 5.9 0.19 15.9 0.02 0.69 0,91 8.40 95.9
37.1 4.49 13.2 7.2 7.2 0.17 16.4 0.02 0.68 1.11 8.43 96.0
5.358 2.310 0.332
5.568 2.182 0.250
5.508 2.287 0.205
5.430 2.237 0.333
5.449 2.286 0.265
0.620 -3.196 0.046 1.694
0.296 1.281 1.347 0.042 2.243
0.262 1.095 1.557 0.044 2.372
0.331 0.826 1.129 0.044 3.097
0.540 -1.893 0.052 3.054
0.300 -1.622 0.012 3.893
0.155 1.073 0.726 0.024 3.486
0.231 0.796 0.884 0.021 3.589
0.004 0.039 0.184 1.742 8.00 5.56 1.97 --
0.004 0.044 0.239 1.680 8.00 5.21 1.97 0.26
0.004 0.024 0.209 1.700 8.00 5.33 1.94 0.29
0.004 0.037 0.225 1.661 8.00 5.43 1.93 0.21
0.004 0.009 0.329 1.665 8.00 5.54 2.01 --
0.002 0.041 0.382 1.561 8.00 5.83 1.99 --
0.004 0.040 0.260 1.577 8.00 5.46 1.88 0.13
0.004 0.039 0.316 1.580 8.00 5.52 1.94 0.16
covite 2, which is u n k n o w n as a p r i m a r y phase i n acid volcanic rocks. F o r the volcanics i t seems possible t h a t early-crystallizing biotite m a y reflect this peraluruinous characteristic if present, or b y fractionation, generate it. N o n e of the acid rocks whose analyses are set down i n Table 5 are peraluminous, as t h e y do n o t c o n t a i n n o r m a t i v e c o r u n d u m , b u t the biotite phenocrysts each c o n t a i n slightly more A1 t h a n t h a t required to combine with K, Na, Ba a n d Ca to form theoretical feldspar molecules. F r a c t i o n a t i o n of these biotite phenocrysts would impoverish the liquid i n A1 with respect to the alkalies a n d alkaline earths, so t h a t carried to extreme the residual liquid would become progressively peralkaline. F u r t h e r m o r e , the early crystallization of " p e r a l u m i n o u s " biotite i n the peralkaline phonolitic obsidian TC19 testifies t h a t biotite m a y p e r p e t u a t e a n d e n h a n c e this character b y fractionation. F r o m the evidence of these volcanic rocks, biotite m a y itself be " p e r a l u m i n o u s " , b u t it does n o t require this characteristic of the liquid i n which it crystallizes, a n d if these biotites are t y p i c a l of the generality of acid volcanic rocks, it c a n n o t b y f r a c t i o n a t i o n produce this characteristic i n the residual liquids. P e r a l u m i n o u s acid liquids therefore are u n l i k e l y to owe their origin to a f r a c t i o n a t i o n process i n v o l v i n g solely liquid-crystal equilibria.
The feldspars are assumed to be stoichiometrie.
Iron-Titanium Oxides of Salic Rocks
53
Biotite and the Fugacity of Water The experimental results of WoNEs and EUGSTEIr (1965) show that the composition of a biotite in equilibrium with sanidine, magnetite and gas is controlled by temperature, lugacity of H~O and fugacity of oxygen; an estimate of temperature and oxygen fugacity m a y readily be made from the iron-titanium oxide equilibration data discussed above. For a biotite co-existing in equilibrium with a sanidinc solid solution and a magnetite-ulvospinel solid solution, the expression relating fugacity of water to the composition of biotite is as follows (WoN~s and EUGSTE~, 1965, p. 1249) log/~20 =
3428--4212 (l--x) 2 -I-logX ~- 1/21og/O2 ~- 8.23 T --log aXAlSi3Os- - l o g a F e 3 0 4
where x refers to the mol. fraction of Fen in the octahedral layer of the biotite (Table 6), aKAlSisO8 to the activity of potassium feldspar, and a FEB04 to the activity of magnetite. For this paper, it has been assumed that the alkali feldspar and magnetite-ulvospinel solid solutions are ideal at the equilibration temperature so that the activities are equivalent to molecular fractions. Substitution of the relevant values in the above equation, with the assumption that the molecular fraction of the sanidine is 0.70, leads to the following values for the fugacity of water at the stage of biotite precipitation
23 (Cam 86) 24 (Cam 93) 25 (3811) 27 (Cal. 19) 28 (L. I18)
2700 6600 9700 265 520
bars bars bars bars bars
Some of these values seem rather high (to the writer) for a volcanic environment, so that an assessment of the errors involved is necessary. In the discussion of the iron-titanium oxides, it was stated that Nor 23 and 24 contained two compositionally distinct and discrete spinel phases, but only one ilmenite phase. It was suggested that one of the spinels crystallized coevally with the biotite, but the ilmenite in conjunction with either magnetite gave temperatures and oxygen fugacities which suggested that they equilibrated with the later pyroxenes (Fig. 2) ; the oxides are not believed to be in equilibrium with the biotite, and the derived water fugacities are accordingly likely to be erroneous. I n No. 24, there are two generations of magnetite, a larger oxidized multi-phase generation which m a y have crystallized in equilibrium with the sphene phenocrysts, and a small fraction of one-phase magnetite and discrete ilmenite. There is no petrographic evidence to indicate whether this later fraction was in equilibrium with biotite, and accordingly the data inserted in the above equation for this rock m a y not apply to the time of biotite precipitation. The oxide eqilibration data for the two Lassen dacites is derived from the extrapolation of the curves of Buddington and Lindsley, so that there is greater uncertainty of the values of temperature and oxygen fugacity for these rocks; however, Fig. 2 suggests that the oxides equilibrated with biotite rather than orthopyroxene. If the derived fugacity of water is in a rough way equated to partial pressure, and thence to total pressure, then these Lassen dacites m a y have started to precipitate biotite at a depth of less than a kilometer, a not unreasonable figure.
54
IA~ S. E. C~Mm~AEL:
Table 7. Analyses and /ormulae o/ pyroxene phenocrysts 2 (Oraefi)
7 12 (1910--199) (Cam 49)
13A (Cam 55)
13B (Cam 55)
(47.7) 0.60 0.55 -29.2 1.18 1.03 19.4 0.29
(52.3) 1.52 2.68 -9.1 0.52 12.5 20.5 0.92
(52.6) 0.66 2.31 0.04 8.5 0.29 15.5 19.8 0.32
(54.3) 0.27 1.02 0.08 17.2 0.50 25.5 1.16 tr.
(51.8) 0.79 2.57 -8.8 0.25 15.9 19.6 0.32
(53.2) 0.32 1.26 -20.5 0.47 23.2 1.06 tr.
(51.9) 0.24 0.34 0.08 29.0 0.73 16.7 0.98 tr.
Formulae on basis of 6 oxygens Si 1.970 1.947 1.999 Z A1 0.027 0.053 -Ti 0.003 . . .
1.944 0.056 .
1.975 0.025 .
1.920 0.080
1.966 0.034
2.000 --
/F~e - Ti 0.016 1.009 WXY 0.041 Mg 0.063 Ca 0.858 Na 0.023 Z 2.00 WXu 2.01
0.045 0.018 0.263 0.009 0.854 0.784 0.023 2.00 2.00
0.019 0.007 0.523 0.015 1.382 0.045 -2.00 1.99
0.021 0.009 0.634 0.015 1.278 0.042 -2.00 2.00
0.015 0.007 0.935 0.024 0.959 0.040 -2.00 1.98
SiO~ TiOe A]~Oa Cr~O~ FeO IV[nO MgO CaO Na20
Atom percent Ca 43.5 Mg 3.2 Fe -~ ~_n 53.3
0.065 0.042 0.283 0.016 0.693 0.818 0.066 2.00 1.98 45.2 38.3 16.5
(52.7) 0.15 0.44 -25.9 0.92 18.9 1.03 tr.
0.020 0.004 0.822 0.030 1.069 0.042 -2.00 1.99 2.1 54.5 43.4
41.1 44.7 14.2
2.3 70.3 27.4
14A (Cam 66)
.
14B (Cam 66)
21 (Cam 118)
. 0.032 0.022 0.272 0.008 0.878 0.778 0.023 2.00 2.01
40.2 45.3 14.5
2.1 64.9 33.7
42.1 49.0 48.9
Pyroxene Phenocrysts W i t h t h e exception of a pitchstone sill from A r r a n (AC. 5), p y r o x e n e is ubiquitous i n all these volcanics; calcium-rich pyroxene co-exists with either a n orthopyroxene or a n iron-rich olivine, a l t h o u g h o r t h o p y r o x e n e m a y be the sole pyroxene phase i n a few cases (Table 1). Pigeonite has n o t been identified i n a n y of t h e i n v e s t i g a t e d rocks. P y r o x e n e is n o r m a l l y f o u n d as small phenocrysts scattered t h r o u g h o u t the groundmass, a n d when petrographic criteria are p r e s e n t it d e m o n s t r a b l y crystallizes after either biotite or amphibole. The range of zoning is c o m m o n l y small, except i n the more iron-rich p y r o x e n e s ; no d a t a is given on the e x t e n t of this zoning. I n two rocks (Cam 93 a n d L l l S ) two compositionally distinct orthopyroxenes are found, the more m a g n e s i a n (Table 7, 24C) resulting from the resorption or reaction of amphibole, the more iron-rich being considered to be the stable phase. I n the k e n y t e (1910--199) a n d phonolitic obsidian (TC-19), a soda bearing augite occurs with olivine i n the former a n d biotite i n the l a t t e r specimen; no calciumpoor p y r o x e n e has been found. The analyses of the pyroxenes are given i n Table 7 a n d p l o t t e d i n Fig. 6. Silica has not been determined in any of the analyses and the values in brackets are computed by difference. As a check on the quality of the analyses, ~he formulae on the basis of 6 oxygens
55
I r o n - T i t a n i u m Oxides oi Sa]ic Rocks Table 7 (Continuation)
22A 22]~ 23A 23B 24A 24B 240 26 27~. 27]~ 29 (CamS1) (Cam81) (Cam86) (Cam86) (Cam93) (Cam93) (Cam93) (TC.19) (Ca1.13) (Cal.13) (L.118)
(50.6) 0.15 0.69
(49.1) 0.15 0.31
(50.9) 0.15 0.71
(49.2) 0.24 0.50
(50.4) 0.15 0.68
(49.0) 0.24 0.49
(52.7) 0.26 0.83
(53.2) 0.88 1.35
(53.1) 0.18 0.62
(53.9) 0.18 0.68
(53.4) 0.22 0.74
20.0 1.09 8.59 18.5 0.37
37.1 1.91 10.1 1.37 --
20.1 1.01 8.56 18.2 0.37
37.0 1.61 10.1 1.34 tr.
20.1 1.05 8.56 18.7 0.39
37.2 1.58 10.1 1.35 --
23.0 0.83 21.3 1.12 --
9.6 0.84 12.3 20.6 1.2
9.1 0.50 15.4 20.8 0.32
19.1 0.86 24.6 0.69 tr.
19.2 1.42 24.6 0.40 tr.
1.981 0.019
1.989 0.011
1.989 0.011
1.989 0.011
1.975 0.025
1.984 0.016
1.975 0,025
1.990 0.010
1.977 0.023
1.980 0.020
1.968 0.032
0.013 0.004 0,655 0.036 0.501 0.776 0.028
0.004 0.004 1.257 0.065 0.610 0.059 --
0,022 0.004 0.657 0.033 0.498 0.762 0,028
0.013 0.007 1.251 0.055 0.608 0.058 --
0.007 0.004 0.659 0.035 0.500 0.785 0.030
0,007 0.007 1.260 0.054 0.609 0.059 --
0.011 0.007 0.721 0.026 1.190 0,045 --
0.049 0.025 0.300 0.026 0.686 0.826 0.088
0.004 0.005 0.283 0.016 0.854 0.830 0.023
0.010 0.005 0.587 0.027 1.346 0.027 --
-0.006 0.592 0.044 1.351 0.016 --
2.00 2.01
2,00 2.00
2,00 2,00
2,00 1,99
2.00 2.02
2.00 2.00
2.00 2.00
2.00 2,00
2.00 2.02
2.00 2.00
2.00 2.01
39.4 25.4 35.2
3,0 30.6 66.4
39,1 25.5 35.4
3,0 30.8 66,2
39,7 25.3 35,0
7
3.0 30,7 66.3
6
2.2 60.1 37.7
1
44.9 37.3 17.8
41.8 43.1 15.1
1.4 67.8 30.8
0.8 67.4 31.8
3
"X \
Mg
29
--
s
Fe
Atom% Fig. 6. The composition of the pyroxene phenocrysts (Table 7) plotted in terms of Ca, 3ig and Fe -F Mn. Additional
data are taken from CAI~lVIICHA]~L(1960) for Nos. 1 and 3; and CARI~IICHA]~IJ(1963) for the unnumbered pair. The composition of the olivine phenocrysts (Table 8) are plotted on the base and joined by dashed tie-lines to their respective calcium-rich pyroxenes. The composition of the pyroxene pair 22 is virtually identical to the pairs 23 and 24 (Table 7) which have accordingly not been plotted. The oxide quenching temperatures are shown on the relevent tie-lines (Tables 2, 3 and 4). The Skaergaard crystallization trend (dashed lines) has been taken from BROWNand VINC]~NT(1963) h a v e been calculated, a n d t h e s u m of t h e a t o m s in t e t r a h e d r a l sites (Z) t o g e t h e r w i t h t h e s u m of the a t o m s in oetahedral sites ( W X Y ) indicates t h a t t h e analyses are satisfactory.
56
IA~ S.E. CAR~IC~AnL:
The trend of the tie-lines between co-existing pyroxenes indicates by comparison with the Skaergaard data (BROWn, 1957) that they are in equilibrium one with the other, the small divergence of some tie-lines probably being the result of insufficient microprobe analyses to obtain a reliable estimate of bulk composition. As may be seen in Fig. 6 both the calcium-rich and calcium-poor pyroxenes fall on smooth trend curves, representing on Muir's model (1954) the intersection of the solidus with the solvus, and displaced toward the Di-Hd join and Mg-Fe join with respect to the Skaergaard trend. This displacement has been suggested (CA~CHA~L, 1963) to result from the lower temperatures of crystallization vis a v i s the Skaergaard. The oxide equilibration temperatures are shown in Fig. 6, and with the exception of No. 271 they are all believed to represent pyroxene co-precipitation temperatures. As may be seen, there is no systematic relationship between these temperatures and the composition of the pyroxenes, indicating that it is the composition of the liquid which in turn is controlled by the amount and nature of the early crystallizing phases (e. g. Fe-Ti oxides) that controls the initial composition of the pyroxenes (CAR~IC~A~L, 1963). The distribution of minor elements between co-existing pyroxenes follows the familiar pattern established for the Skaergaard (BROWN, 1957). The orthopyroxenes have less A1, Aliv, Na (at limit of detection) and Ti than the associated calcium-rich pyroxenes. The two augites from the kenyte and the phonolitic obsidian (7 and 26) have higher Ca (of total Ca-~-Mg~-Fe-~-Mn) than the calcium-rich pyroxenes from the acid volcanics, and appropriately fall near the trend of the pyroxenes of a]kalibasalts (WILK~son, 1956).
Olivine Phenoerysts The olivine phenocrysts have been only partially analysed (Table 8) but the recalculation to olivine molecules suggests that the analyses are satisfactory. The analyses have been plotted in Fig. 6 as part the pyroxene diagram, and the tielines between pyroxene and olivine are again subparMlel to those of the Skaergaard (B~o~:~ and VINCENT, 1963). For nos. 5 and 6, the co-existing ferromagnesian silicate with olivine is amphibole rather than pyroxene, ~ rather more unusual assemblage than is found in the average obsidian or rhyolite. Table 8. Analyse8 o] olivine phenocryst8 2 (Oraefi)
5 (Cam 73)
6 (Cam 99)
7 (1910--199)
FeO MnO MgO CaO
65.5 3.2 0.74 0.49
63.0 4.0 2.47 0.05
63.1 4.0 2.61 0.05
34.2 2.18 27.1 0.41
Recalculated Fo Fa Tp La Total
analyses (see text): Wt. percent 1.3 4.3 4.6 92.9 89.3 89.5 4.6 5.7 5.7 0.8 0.1 0.1 99.6 99.4 99.9
Atom percent ~Ig: Mg + Fe + 1.9 6.2
6.5
47.3 48.5 3.1 0.6 99.5 57.0
Iron-Titanium Oxides of Salie l~oeks
57
Amphibole Phenoerysts E n h e d r a l p h e n o e r y s t s of a m p h i b o l e w i t h d i s t i n c t i v e yellow-brown, deep redb r o w n pleochroism are f o u n d in nine of t h e i n v e s t i g a t e d rocks (Table 1), a n d in some, p a r t i c u l a r l y t h e L a s s e n daeites, t h e y show evidence of resorption. The m i c r o p r o b e analyses of t h e a m p h i b o l e s are g i v e n in T a b l e 9 t o g e t h e r w i t h t h e i r f o r m u l a e on t h e basis of 23 oxygens. The anMyses are n o t v e r y s a t i s f a c t o r y as t h e y c a n n o t be t o t a l l e d , for t h e o x i d a t i o n s t a t e s of iron are u n k n o w n , as are also t h e f l u o r - h y d r o x y components. Table 9. Analyses and/ormulae o/amphibole phenoerysts 6 (Cam 99)
24 (Cam 93)
25 (381l)
27 (Cal. 19)
28 (Cal. 19)
29 (L. 118)
SiOz TiOe AI203 Fee M_nO MgO CaO Na20 K20
41.0 1.99 7.48 29.1 0.85 4.17 10.5 1.93 0.91
46.4 1.53 6.70 17.0 0.64 11.6 11.0 1.77 0.81
40.3 5.25 11.4 12.2 0.23 12.5 11.8 2.29 1.19
43.8 3.48 9.53 12.5 0.25 14.2 11.1 2.06 0.53
46.0 1.78 8.35 12.6 0.26 14.6 10.8 1.93 0.33
46.9 1.74 7.63 11.9 0.32 15.8 10.8 1.62 0.35
Sum
97.9
97.4
97.2
97.4
96.7
97.1
Formulae on the basis of 23 oxygens {~ 6.586 6,974 Z 1.414 1.026 A1vI 0.003 0.16I Ti 0.240 0.173 Y Fe 11 3.909 2.137 0.116 0.081 ( 0.998 2.598 C~ 1.807 1.771 X 0.600 0,517 0.187 0.155 Z 8.00 8.00 Y 5.27 5.20 X 2.59 2.44
6.047 1.953 0.063 0.592 1.531 0.029 2.800 1.897 0.665 0.227 8.00 5.02 2.79
6.468 1.532 0.127 0.387 1.544 0.03i 3.125 1.756 0.589 0.099 8.00 5.21 2.44
6.798 1.202 0.253 0.198 1.558 0.033 3.216 1.710 0.552 0.062 8.00 5.26 2.32
6.869 1.131 0.186 0.192 1.457 0.040 3.449 1.695 0.459 0.065 8.00 5.32 2.22
The a m p h i b o l e s are varieties of Ti-rich h o r n b l e n d e , e x c e p t for 25 (3811) which is a k a e r s u t i t e a n d for 6 (Cam 99) which is a ferro-edenitic h o r n b l e n d e (LEA~:~, 1966). The a m p h i b o l e s show some zo~ling of t h e c o n v e n t i o n a l Fe-Mg t y p e , b u t in t h e t h r e e L a s s e n dacites (27, 28, 29) t h e r e is strong zoning of Si, A1 a n d Ti. There is no consistent p a t t e r n t h r o u g h o u t one c r y s t a l or p h e n o c r y s t , a n d t h e e x t r e m e values of these t h r e e c o m p o n e n t s are t a b u l a t e d below. 27 (Cal. 13) 28 (Cal. 19) 29 (L. 118) This d a t a suggests t h a t Si a n d wide limits in one c r y s t a l w i t h
SiO z A1203 TiO 2 36.2--45.2 12.9--7.3 2.2--5.3 39.3--48.7 11.2--4.6 2.1--1.0 39.8--49.3 11.9--5.2 2.9--1.0 A1 in t e t r a h e d r M c o - o r d i n a t i o n can r a n g e over no a p p a r e n t p h y s i c a l d i s c o n t i n u i t y . B y w a y of
58
IA~ S. E. C~IC~AEL:
contrast, the kaersutite (25) shows great homogeneity with respect to Ti, A1 and Si. The fractionation of amphibole represented by the composition of these phenocrysts (Table 9) would not, unlike biotite, tend to make the residual liquid either peraluminous or peralkaline; they would maintain the critical AI~Os balance.
Iron-Ratios of the Ferromagnesian Phases In the ferromagnesian minerals, the substitution of Fe n + Mn for Mg is a dominant characteristic of the various ferromagnesian solid-solution series, and is considered to represent in a simple way the continous reaction of liquid with crystals under conditions of falling temperature. However, investigations of synthetic systems with oxygen as a component have shown that is not invariably the case, and that under certain conditions (e. g. increase in oxygen iugacity) some of the ferromagnesian minerals may become enriched in Mg as crystallization proceeds (MIyA~ and OSBO~, 1956 ; Wo~Es and EUGSTEI~, 1965). In a previous paper (C~IC~AEL, 1963) the effect of the amount and composition of an early crystallizing oxide phase in the initial composition of a later pyroxene phase was illustrated by using the function 100 (Fe 11 + Mn)/ Fe n ~- Mn + Mg, which may be called the iron ratio. The iron-ratios for all the ferromagnesian phases are given in Table 10 together with the values for the rock and the residual glass if this has been analysed. The value for the rock is, in each case, taken to represent the condition just prior to crystallization, and it is assumed that no oxidation or reduction of iron in the rock as a whole has taken place subsequently. In these volcanics, the oxide phases are the first ferromagnesian phases to precipitate, and they invariably have higher iron-ratios than the liquids from which they precipitated (Table 10); in Mmost every case, the spinel phase has a higher iron-ratio than the co-existing ilmenite. The effect of the oxides crystallizing alone would be to enrich the liquid in magnesium relative to ferrous iron, so that, depending upon their amount, the later crystallizing ferromagnesian silicates could be very magnesian, and indeed some of the pyroxenes are so (Fig. 6). This effect can be very marked in acid volcanics, as the concentration of ferromagnesian components in the rocks or initial liquids is quite small (Table 5) and therefore very susceptible to change by the continued precipitation of oxide phases alone. Of the ferromagnesian silicates, biotite has the lowest iron-ratio and is followed by amphibole, calcium-rich pyroxene and orthopyroxene in order of increasing iron-ratio. These phases do not all exist in equilibrium one with the other in any of the rocks, but in each case, all the above silicates are more magnesian (lower iron-ratio) than the rocks which contain them. Where the composition of the residual glass is known, it always has a higher iron-ratio than the rock, indicating that the dominant effect on the iron-ratio in the residuM liquid is provided by the silicates rather than the oxide minerals. Thus there may be a reversal in the trend of iron-enrichment at some stage in the cooling history of acid rocks; the precipitation of oxides without co-existing silicates (Table I, no. 11), will enrich the liquid ill magnesium ralative to ferrous iron, but the later crystallization of ferromagnesian silicates will reverse this trend, their total mass being more effective than that of the oxides.
86.9
89.8 97.5 98.9
Rock
R e s i d u a l glass Ilmenite Magnetite
.
90.5
. 98.3 99.3
Rock
R e s i d u a l glass Ilmenite Magnetite
.
. . . .
.
93.6 -99.1
86.4
. 87.6
. .
. 98.3 99.3
91.9
.
.
.
.
.
. -99.2
93.9
.
.
81.7 97.5 98.7
76.2
87.9
.
.
(Cam 108)
.
. .
--
.
4 (AC.7)
.
-98.8 99.5
91.4
93.8
.
.
98.3 99.3
95.4
.
. .
(Cam 110)
.
--
.
.
60.3 85.6 91.4
51.6
28.9 . . 34.5 .
(]~C. 20)
-98.6 99.4
90.9
93.5
80.1
--
.
. .
. 89.8 --
73.5
. 50.0 .
--
.
(Cal. 118)
66.2 -90.0
63.7
. 43.0
.
30.1
. .
5 6 7 (Cam73) (Cam99) (1910--199)
96.4 99.2 --
94.8
91.2
.
98.8 99.7 99.2
96.7
.
. .
94.8
.
9 (3112)
-98.3
.
79.0
68.4
58.0
.
. 94.6 98.0
74.5
68.2
58.1 38.2
.
(Cam 81) (Cam 86)
.
84.8 . .
8 (5748)
.
. .
.
.
.
.
. 98.1 98.5
86.0
--
94.0 98.0
67.5
.
55.9 81.6 88.7
53.7
-27.4 35.8 -.
.
.
11 (AC. 5)
(Cam 93) (3811)
90.9 -98.5
90.9
58.1 40.3 46.0 68.3
.
--
10 (3114)
.
.
. -92.8
76.6
32.2 ---.
.
(TC. 19)
88.9 94.0
70.9
. 44.4 .
-.
.
.
. 88.8 92.6
38.6
25.9 -33.5 31.3 .
(Cal. 13)
. 85.0 93.8
51.5
28.0 .
.
24.2
. 98.6 99.3
90.9
--
--
. 89.4 94.6
38.8
-17.7 33.1 --
88.6 93.8
39.2
-20.1 30.3 32.0
(Cal. 19) (L. 118)
86.6 93.1
68.7
33.7
24.2
12 13 14 15 (Cam 49) (Cam 55) (Cam 66) (Cam 95)
F o r t h e a m p h i b o l e s a n d p y r o x e n e s t h e v a l u e s f o r F e 11 a r e t h o s e of t o t a l i r o n ; f o r t h e s p i n e l s F e 11 r e f e r s t o F e O c a l c u l a t e d o n t h e u l v o s p i n e l b a s i s a n d f o r i l m e n i t e i t is a l s o t h e c a l c u l a t e d v a l u e f o r F e O . F o r b i o t i t e , t h e r o c k s a n d t h e r e s i d u a l g l a s s e s F e n is t h e d e t e r m i n e d v a l u e of F e O
. . . . .
Augite Biotite Amphibole Orthopyroxene Olivine
.
75.1 . .
3 (P186)
(Cam 104)
-99.6 99.8
97.0
. 98.1
94.3 .
(Cam 103)
68.7 . . . 86.7
1 2 (G151) (Oraefi)
Values/or (Fell@Mn) X lO0/Fe11-~Mn ~- Mg ]or the/erromagnesian phases
Augite Biotite Amphibole Orthopyroxene Olivine
T a b l e 10.
g-
o
N
9
60
IA~ S. E. C~_~MIC~A~L:
There is one rather curious anomaly in the data of Table 10; the iron-rich olivines in these acid rocks (l~os 1--6) typically have iron-ratios which are close to or greater than their rocks (initial liquids) despite the observed fact that the oxide minerals crystallize earlier. Although analytical errors in the determination of small amounts of ferrous iron and magnesium in the rocks cannot be discounted, it is suggested that this relationship is reM, namely that an iron-rich olivine can have a higher iron-ratio than the liquid from which it precipitates. The inclusion of :Fe111in the iron-ratio function of the rock does not altogether resolve the anomaly, and some Fe 111 must be present in the early crystallizing oxide minerals, so that it need not be considered as part of the iron-ratio. A very similar relationship is found in the Skaergaard intrusion (WAGn~, 1960) where liquid UZc is more magnesian relative to ferrous iron than the olivine which precipitated from it. If these data are correct, then the simple forsterite-fayaLite system (Bow]~N and SCHAIRE~, 1935) may develop a minimum near fayalite composition with high concentrations of alkali, alumina and silica ( B u ~ s and FYFE, 1964).
Oxygen Fugaeities Fu])ALI (1965) has determined the oxygen fugacities of a series of basalts and andesites at a temperature (1200~ near their respective liquidns temperatures and at one atmosphere total pressure. The determined oxygen fugacities are those required to preserve the determined contents of FeO and F%O a in each of the rocks, which however are not reported to have been examined optically to determine whether or not the iron-titanium oxide minerals have been oxidized after eruption. In the writer's experience this phenomenon is very common even in recent apparently fresh lavas, so that the oxygen fugacity results given by FuD~J~I are likely to be high with respect to the original pristine conditions. His results also suggest that the fugacity of ocxygen increases with increasing acidity of the rock, and the range for the whole group is 10 -6.4 to 10 -8.6 atmopsheres of oxygen. For those acid rocks which contain only a small number of phenocrysts, and hence for which the quenching temperature more or less represents the liquidus temperature, the oxygen fugacitics derived from the oxides are Listed below ( ~- 1) 2(Oraefi) 10-1~'9; 11(AC-5) 10-1~'~; 14(Cam 66) 10-11"~; 15(Cam 96) 10-i~.~; 16(Cam 103) 10-13"9; 17(Cam 104) 10-13"7; 19(Cam 110) 10 -13"~and 23 (Cam 86) 10 - n ' l These values are very much lower than those of FUDALI, but the range of his determined values falls close to the oxide equilibration curves of Fig. 2 if these are extrapolated to 1200~ however the presence of a magnesian olivine rather than an iron-rich olivine in basic liquids will restrict the equilibrium fugacity of oxygen to higher values. The data on oxygen fugacities of these acid rocks taken in conjunction with those of FuDAnI gives no indication that the fugacity of oxygen remains constant during ffactionation in the Cascade orogenic volcanic series as has been suggested by OSBORN (1959). This is perhaps because firstly the acid rocks are not derived from a basaltic parent, and secondly that the oxygen conditions have changed markedly from the time at which the acid liquids were generated. The requirements of OSBO~NS model will therefore presumably cease well before this stage, as within limits of temperature and composition, all volcanic acid liquids
Iron-Titanium Oxides oI Salic Rocks
61
of whatever origin tend to produce either orthopyroxene or fayalite-magnetite assemblages, and therefore crystallized or perhaps were generated under nearly the same oxygen conditions. Moreover, consider a mass of basaltic magma cooling slowly at depth, with the necessary water and hydrogen diffusion requirements to keep oxygen fugacity constant (Osso~s, 1959). At any given time, an equilibrium is taken to exist between the crystalline phases, liquid and gas, until the crystals have accumulated at the bottom of the mass of magma or have been withdrawn from the system; equilibrium will then be set up with another crop of crystals whose composition will vary in some degree from their predecessors. What then is the mineralogical evidence for crystallization with a constant fugacity of oxygen ? The data in Fig 2 would suggest that firstly olivine plus magnetite (S. L.) would precipitate plus other phases, then orthopyroxene (or pigeonite ?) plus magnetite plus other phases, and then biotite-amphibole plus magnetite plus other phases. This succession of olivine, orthopyroxene and biotite-amphibole is not common in strongly fractionated gabbros, so that this may represent an extreme set of conditions either because the cooling interval is too great, or because of insufficient water for biotite to become a stable phase. Within the stability field of each of these silicates, the composition of the silicate will change with cooling in response to a constant fugacity of oxygen; if this is kept constant, and it remains in equilibrium with each crop of crystals, then for a small cooling interval, it is to be expected that the crystals (for example, olivine or orthopyroxene) will become progressively more magnesium rich as crystallization proceeds (Wo~]~s and E~JGs~]~, 1965, Fig. 13). This increase in magnesium, either as outer zones on the crystal phases (page 49), or as a progressive upward sequence of layers of crystals becoming increasingly magnesian is not easily found in the literature. In view of the abundance of orogenic voleanics and associated plutonic basic rocks, it is surprising that it has not been described more frequently. There would seem to be no convincing mineralogical evidence that basic magma does crystallize with a constant fugacity of oxygen; or if it does, then the mineralogy shows far smaller variation in composition than the techniques used in the past have been able to identify. It is surprising that gas, probably of widely differing composition, almost invariably causes a liquid of basaltic composition to precipitate orthopyroxene or olivine plus magnetite plus other phases. Perhaps the gas is of very uniform composition, but more reasonably it would seem that its mass is such that the silicate-oxide assemblages have an oxygen buffer capacity. It is perhaps surprising that the investigated acid rocks which contain orthopyroxene all fall on a reasonably smooth curve (Fig. 2) as the range of composition of the pyroxene throughout all the investigated rocks is quite large (Fig. 6). However, it would take extreme variation in the Fe/Mg ratio of the orthopyroxene acting as a buffer with magnetite and liquid for the iron-titanium oxices to show this clearly with the analytical techniques employed here. I t is to be expected that a magnesian orthopyroxene-magnetite-liquid assemblage will equilibrate (if the pyroxene is unzoned, as in effect most of them are) at a higher oxygen fugaeity than an assemblage with an iron-rich orthopyroxene at the same temperature ( V ~ g o o G ~ , 1962; W o ~ s and E~rGSTSR, 1965); any effect of this type cannot be identified in these rocks.
62
IAN S. E. CA~IOnA~L:
Conclusions The analytical data on the ferromagnesian minerals crystallizing early in a group of salic volcanics have been used to estimate the temperature and fugacity of oxygen at the time of quenching. These data indicate that the liquidus temperatures, at an unknown total pressure, of acid volcanics is commonly around 9000 C; water-undersaturated granite m a y also be intruded at this temperature although with a considerable crystalhzation interval. The combination of temperature and oxygen fugacity data derived from the coexisting oxides show that at any given temperature, the fugacity of oxygen is higher if orthopyroxene is present as a phenocryst than if an iron-rich olivine is present. The interdependence of the composition and species of the ferromagnesian phases on one another is perhaps surprising as they are often present in extremely small amounts. The composition of the biotites taken in conjunction with the oxide data suggests in a rough way that Lassen dacites started to precipitate biotite at depth of about 1 kilometer, these being the only rocks where the oxide data are considered to be relevant to the biotite phenocrysts.
Acknowledgements. I am indebted to Jomr V~RHOOGElqand W. S. FYF]~for reading and criticising the manuscript. The National ScienceFoundation provided funds (GA-338and GA-480) to support this research which are gratefully acknowledged. The National Park Service kindly allowed the writer to collect specimens. Key to specimen localities Glassy margin of acid dyke, Thingmuli, Eastern Iceland 1 (G 151) 2 (Oraefi) Obsidian from Oraefajokull, Eastern Iceland Pitchstone margin of intrusion, Rondolfur, Eastern Iceland 3 (P 186) Pitehstone dyke, Glen Shurig, Arran, Scotland (AC 7) Rhyolite, north of Devils Punchbowl, Mono Craters, California 5 (Cam 73) Rhyolite, south of Devils Punehbowl, Mono Craters, California 6 (Cam 99) 7 (1910--199) Kenyte, Cape Royds Peninsula, South Victoria Land, Antarctica s (574s)] Pantellerites, Pantelleria 0 (3112)~ 10 (3114)J Pitchstone sill, Cluachland Shore, Arran, Scotland iI (AC. 5) Porphyritic Obsidian, Little Glass Mountain, Medicine Lake, California 12 (Cam 49) Dacite, Medicine Lake glass flow, Medicine Lake, California 13 (Cam 55) Obsidian, Glass Mountain, Medicine Lake, California 14 (Cam 66) Rhyolitic obsidian, Wilson Butte, Mono Craters, California 15 (Cam 95) Pumice, north of pumice mine, Mono Craters, California 16 (Cam 103) Obsidian from same locality as above 17 (Cam 104) Rhyolitic obsidian from flow south of Route 120, Mono Craters, California 18 (Cam 108) Rhyolitic obsidian from circular extrusion by Route 120, Mono Craters, 19 (Cam 110) California Porphyritic pitchstone, Isle of Eigg, Scotland 20 (EC. 20) Obsidian, Bottle Rock Road, Clear Lake, Lake County, California 21 (Cam 118) Porphyritic rhyolitic obsidian, Glass Mountain, Inyo Craters, California 22 (Cam 81) Porphyritic rhyolitic obsidian, extrusion south of above, Inyo Craters, 23 (Cam 86) California Porphyritic rhyolitic obsidian, extrusion in Deadmans Creek, Inyo Craters, 24 (Cam 93) California Porphyritic traehy~e pitehstone, Murat, Cantal, France 25 (38n) Phonolitic obsidian, Tenerife 26 (TC 19)
Iron-Titanium Oxides of Salic l~ocks 27 (Cal. 13) 28 (Cal. 19) 29 (L. 118)
63
Pre-Lassen dacite, Sunflower Flat, Lassen National Volcanic Park, California Raker Peark dacite, Lassen National Volcanic Park, California Dacite dome of White Montain, Lassen National Volcanic Park, California
Petrographic Appendix ~Iany of the investigated tocks have been described before, but for those that have not, a brief description is given below.
Extrusions ]tom Mono Craters, Cali/ornia (Cam 73, 95, 99, 103, 104, 108, 110) In the recent acid extrusions which form a chain of plug-domes south of Mono Lake, there is seldom much clean obsidian, but typically the rocks are semi-pumiceous rhyolites. The glassy or microcrystallhle groundmass is frequently riddled with small laths of feldspar, together with tiny crystals of elongate amphibole, pyroxene and iron-ore, the whole enclosing sparse or rare phenocrysts of plagioclase, sanidine and quartz. Spheru]itic patches of intergrown quartz and feldspar may be common, and there are all gradations between a flow-banded obsidian (Cam 104) and a rhyolite with a groundmass of myriads of alkali feldspar laths with interstitial glass (Cam 73, 95 and 99). The amount of phenocrysts is variable, but the salic phases predominate. Orthopyroxene and less biotite or amphibole have been collected in all (except Cam 73 and 99) the heavy mineral concentrates, but their concentration must be at the p.p.m, level. Fayalite and amphibole are relatively common large phenoerysts in Cam 73 and 99.
Dacites, Lassen National Volcanic Park (Cal. 13, 19 and L. 118) Large plagioclase phenocrysts with sieve texture are characteristic of all the Lassen dacites; more rarely almost completely corroded plagioclase xenocrysts (?) are present. Large amphibole and biotite phenocrysts are ubiquitous and tend to form glomeroporphyritie clusters with plagioclase and iron-ore; both the amphibole and the biotite of these clusters may be partially resorbed, with a reaction rim of orthopyroxene. The groundmass, which may either be a clear perlitic glass (Cal. 19, L. 118), or cloudy with myriads of tiny alkali feldspar laths (Cal. 13) also contains an abundant smaller phenocryst fraction of pyroxene, plagioclase, amphibole and iron-ore. No clearcut distinction has been found between the composition of the biotite and amphibole in the crystal clusters, and that of the same minerals found as phenocrysts. Both these minerals, together with pyroxene typically enclose small crystals of iron-ore. Apatite and ziron are common accessories, the former being found frequently enclosed in biotite phenoerysts.
Medicine Lake, Cali]ornia (Cam 49, 55 and 66) Two of these specimens are finely banded obsidians; in one (Cam 66) the flow-banding is extremely fine and delicate which produces an optical interference effect on a fresh surface; this specimen contains extremely rare phenocrysts of pyroxene and iron-ore. In the other obsidian (Cam 49) large plagioclase phenocrysts predominate, and together with pyroxene are enclosed in a colorless glass with myriads of feldspar microlites which define the flow-banding. The third rock, (Cam 55), is more dacitic, with large phenocrysts of plagioclase, corroded xenocrysts of p]agioelase, pyroxene and iron-ore set in a brown microcrystalline groundmass. Small crystals of partially resorbed hornblende are found sparsely scattered throughout the rock.
Inyo Craters, Cali/ornia (Cam 81, 86, 93) These specimens are form three petrographically heterogeneous extrusions south of the chain of Mono Craters. They are all rather similar, pumiceous rhyolites varying only in the amounts of ferromagnesian phenocrysts, which are predominately biotite and amphibole. Phenocrysts of sanidine, plagioclase and quartz are common, together with rare pyroxene and iron-ore phenocrysts, all being enclosed in a partially crystalline vesicular groundmass. The groundmasses are petrographically not unlike the rhyolites of Mono Craters, except that elongate crystals of amphibole are very much more common; alkali-feldspar laths, iron-ore and pyroxene granules together with glass make up the groundmass. Apatite and ziron are accessories.
Clear Lake, Cali]ornia (Cal. 118) This is a very well flow-banded slightly porphyritic obsidian, in which small phenocrysts of plagioclase are the dominant phase. Pyroxene and iron-ore phenocrysts are enclosed in a microlitie colorless glass, which may show extremely contorted banding, unlike those obsidians of Medicine Lake.
64
I ~ S. E. C~R~aZO~_~L: Iron-Titanium Oxides of Salie Rocks
References A~OTO, S., and T. KATSVRA:Magneto-chemical study of the generalized titanomagnetite in volcanic rocks. J. Geomag. and Geoelect. 10, 69--90 (1959). , and M. YOSmDA: Magnetic properties of TiF%Oa--F%Oa system and their change with oxidation. J. Geomag and Geoelect. 9, 165--178 (1957). Bow~N, N. L., and J. F. SCOrnER: The system MgO-FeO-SiOe. Am. J. Sci. 29, 151--217 (1935). BROWN, G. M. : Pyroxenes from the early and middle stages of ffactionation of the Skaergaard intrusion, East Greenland. Mineral. Mag. 31, 511--543 (1957). --, and E. A. VI~C]~T: Pyroxenes from the late stages of ffactionation of the Skaergaard intrusion, East Greenland. J. Petrology 4, 175--197 (1963). BUDDI~GTO~,A. F., and D. H. LINDSL]~r: Iron-titanium oxide minerals and synthetic equivalents. J. Petrology 5, 310--357 (1964). Bytes, R. G., and W. S. FYFE: Site preference energy and selective uptake of transition-metal ions during magmatie crystallization. Science 144, 1001--1003 (1964). C)mMICHAEL,I. S. E. : The pyroxenes and olivines from some Tertiary acid glasses, ft. Petrology 1, 309--336 (1960). A note on the composition of some natural acid glasses. Geol. Mag. 99, 253--264 (1962). - - The oecurence of magnesian pyroxenes and magnetite in porphyritic acid glasses. Mineral. Mag. 83, 3 9 4 ~ 0 3 (1963). Natural liquids and the phonolitic minimum. Geol. J. 4, 55--60 (1964). - - Trachytes and their feldspar phenocrysts. Mineral. Mag. 84, 107--125 (1965). D E ~ , W. A., R. A. HowlE, and J. Zvss~A~: Rock forming minerals vol. 3, Sheet silicates. New York: John Wiley & Sons (1962). EUGSTER, It. P., and D. R. WON]~S: Stability relation of the ferruginous biotite, annite. J. Petrology 8, 82--125 (1962). EWA~T, A. : Mineralogy and petrogenesis of the Whakamaru ignimbrite in the Maraetai area of the Taupo volcanic zone, New Zealand. New Zealand J. Geol. Geophys. 8, 611--677 (1965). ~OSTE~, M. D. : Interpretation of the composition of trioctahedral micas. U.S.G.S. Prof. Pap. 354-B, 1 1 ~ 9 (1960). FvD~I, R. F. : Oxygen fugacities of basaltic and andesitic magmas. Geochim. et Cosmochim. Acta 29, 1063--1075 (1965). L]~AKE,B. E. : Catalogue of analysed calciferous and subcalciferous amphiboles together with their nomenclature and associated minerals. Geol. Soc. Am. (in press) (1966). MvA~, A., and E. F. 0s]3o~: Phase equilibria at liquidus temperatures in the system MgO~eO-F%Oa-SiO2. J. Am. Ceram. Soc. 89, 121--140 (1956). ~ v m , I. D. : Crystallization of pyroxenes in an iron-rich diabase from Minnesota. Mineral. Mag. a0, 376--388 (1954). OSBO~, E. F. : Role of oxygen pressure in the crystallization and differentiation of basaltic magma. Am. J. Sei. 2~7, 609--647 (1959). TVTTLE, O. F., and ~. L. BowE~: Origin of granite in the light of experimental studies in the system ~aA1SiaOs-KALSiaOs-SiOe-HeO.Geol. Soe. Am., Mere. 74 (1958). V ~ n o o o ~ , J. : Oxidation of iron-titanium oxides in igneous rocks. J. Geol. 79, 168--181 (1962). W~r L. R. : The major element variation of the layered series of the Shaergaard intrusion and a re-estimation of the average composition of the hidden layered series and of the successive residual magmas. J. Petrology 1, 364--398 (1960). WILKInSOn, J. F. G.: Clinopyroxenes of ~lkali olivine-basalt magma. Am. Mineralogist 41, 724--743 (1956). Wo~v.s, D. R., and H. P. E u o s ~ : Stability of biotite: Experiment, theory and application. Am. Mineralogist ~0, 1228--1272 (1965). -
-
-
-
I ~ S. E. CA~ICnAV.L Department of Geology and Geophysics, University of California Berkeley 94720, US.k
