(6a,b) Basanite, McMurdo Volcanic Group, Antarctica (6a); nepheline hawaiite, Dunedin Volcano, New Zealand. (6b) (Kyle and Price, 1975). (4 EPMA analyses).
Mineralogical Magazine, August 2003, Vol. 67(4), pp. 639–651
Rho«nite paragenesis in pyroxenite xenoliths, Mount Sidley volcano, Marie Byrd Land, West Antarctica R. H. GRAPES1,*, R. J. WYSOCZANSKI2
AND
P. W. O. HOSKIN1
1
Institut fu¨r Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universita¨t Freiburg, D-79104 Freiburg, Germany 2 Department of Mineral Sciences, National Museum of Natural History, MRC-0119, Smithsonian Institution, Washington, D.C., 20013-7012, USA
ABSTR ACT
Rho¨nite occurs in lower crustal pyroxenite xenoliths erupted in phonolite from the Mount Sidley composite volcano, Marie Byrd Land, Antarctica, as a localized breakdown product, with plagioclase, clinopyroxene, ± olivine ± Ti-magnetite + melt, of kaersutite, and as microphenocrysts (with olivine, plagioclase, clinopyroxene) in pockets of basanitic melt. Rho¨nite after kaersutite has a more NaSi-rich/ CaAl-poor composition, lower Ti, and formed at higher oxidation (~NNO) conditions than rho¨nite occurring as microphenocrysts in basanite. Comparison with experimentally determined rho¨nite stability in understaturated alkali basalt and as a reaction product after Ti-amphibole indicates that the Mount Sidley rho¨nite (and associated minerals) formed between 1090 and 1190ºC at 10%, and ~±5 –10% for less abundant elements.
(0.5 – 1.0 Ma) phonolite lavas erupted from the Mount Sidley composite volcano, southern Executive Committee Range, Antarctica (Panter et al., 1994). In the xenoliths, rho¨nite occurs: (1) as a reaction product together with plagioclase (An 40) ± clinopyroxene (Ca 49.7Mg 38.7Fe 11.6; 8.1%Al2O3; 1.7% TiO2; 0.73% Na2O) ± olivine (Fo81) and glass, replacing both interstitial kaersutite and kaersutite after clinopyroxene, typically along grain boundaries, cracks, cleavage planes and in marginal parts of kaersutite grains fragmented by basanitic melt (Fig. 1a); and (2) as microphenocrysts together with plagioclase (An 60 ), olivine (Fo 76 ) and clinopyroxene (Ca 48.0 – 48.1 Mg 44.5 –40.9 Fe 7.5 – 11. 0 ; 8.2 –7.2% Al2O3; 1.32 – 3.79% TiO2; 1.06 – 0.20% Na2O) in larger ‘pools’ of what was originally basanitic melt that have partly crystallized to a ne groundmass of plagioclase (An45), clinopyroxene (Ca49.0Mg36.2Fe14.8; 9.2%Al2O3; 3.65% TiO2; 1.9% Na2O), Ti-magnetite and apatite (Fig. 1b). Electron probe microanalyses and structural formulae, calculated on the basis of 14 cations 641
and 20 (O) to give Fe 3+ and Fe 2+, of 22 compositionally homogeneous rho¨nite crystals from the associations described above are listed in Table 1. The analyses are plotted in terms of IV Si – VIIINa vs. VITi4+ in relation to the rho¨nite subgroup of the aenigmatite-rho¨nite group nomenclature recently proposed by Kunzmann (1999). They straddle the rho¨nite – ‘no name’/ (baikovite) compositional subvolumes, and, overall, have subequal amounts of 2Al + 2M3+ > 2Si + 2M2+ and M2+ + Ti > 2M3+ substitutions (Fig. 2). The average composition of rho¨nite after kaersutite (samples C, J, S, H in Table 1) plots close to the ideal Ca2(Mg3Fe2+Fe3+Ti)(Si3Al3)O20 composition. A Ca + VIAl vs. Na + Si plot, which is a measure of the VIIICaIVAlNa-1Si -1 exchange component in rho¨nite is shown in Fig. 3. Deviation of analysis points below the line S = 8.0 is a measure of additional IV Fe3+VIFe3+Si – 1(Fe2+Mg) – 1 and VIIIFe2+Ca – 1 exchange components present (Table 1). Figure 3 highlights the main compositional difference between the occurrences of rho¨nite in
R. H.GRAPES ETAL.
FIG. 1. BSE images of rho¨ nite occurrences in pyroxenite xenoliths from Mount Sidley, Antarctica. C = clinopyroxene, G = glass, K = kaersutite, Ol = olivine, P = plagioclase, R = rho¨nite. (a) (Sample 90033J) Marginal part of a fractured kaersutite grain invaded by basanitic melt showing replacement of kaersutite by rho¨nite, clinopyroxene and plagioclase. Anastomosing black areas (glass + plagiolcase) contain ne-grained Ti magnetite (white) and clinopyroxne (grey) and represent partly crystallized basanitic melt. Note the sieved margin (glass- lled areas) of the large olivine grain at the bottom of the photo indicating presumed uptemperature growth of a more Mg-rich rim (darker grey tone) by reaction with the basanite melt (compare with Fig. 3a of Gamble and Kyle, 1987). Scale bar = 50 mm. (b) (Sample 90033Q1) Rho¨nite, plagioclase, olivine and clinopyroxene microphenocrysts within a groundmass of plagioclase, clinopyroxene, titanomagnetite, apatite and basanitic glass. Scale bar = 100 mm.
the xenoliths. Except for two analyses (21, 22; Table 1), rho¨nite microphenocrysts have lower NaSi/higher CaAl than those replacing kaersutite. Rho¨nite after kaersutite that replaces clinopyroxene is the most NaSi-rich (analyses 1 – 4; Table 1). With respect to the ‘ideal rho¨nite’ composition, all but the latter rho¨nite occurrences
have more IVAl than can be compensated by VIII Ca, necessitating charge-balancingsubstitution of Al and Fe3+ in the octahedral sites by way of the coupled substitutions IVAl + VIAl and IVAl + VI Fe3+ for Si + (Fe2+,Mg). Johnson and Stout (1985) pointed out that substitution of octahedral Fe3+ (and also Al) involves replacement of Ti according to the substitution 2(Fe3+,Al) > Ti + (Fe2+,Mg) which is plotted in Fig. 4. Analyses that deviate from the S = 6.0 line contain appreciable VI Al and/or VI Ca (Table 1). Noteworthy is the observation that rho¨nite microphenocrysts in basanite have excess Ti (i.e. HM buffer (Johnston and Stout, 1984). Undersaturated alkali basalts of the McMurdo Volcanic Group, Antarctica, containing rho¨nite microphenocrysts (average Fe2+/[Fe2+ + Fe3+] = 0.62) (No. 6a in Fig. 9) coexisting with ilmenite and magnetite crystallized under quartzforsterite-magnetite (QFM) buffer conditions at 970±15ºC (Price and Kyle, 1975). Rho¨nites in basanites (Soellner, 1907; Gru¨nhagen and Seck, 1972; Magonthier and Velde, 1976; Olsson, 1984; Bonaccorsi et al., 1990) (Nos. 1, 7, 11 in Fig. 9) coexist with titanomagnetite and have relatively high Fe3+ (Fe2+/[Fe2+ + Fe3+] = 0.39 –0.58). From these data, it is evident that rho¨nite replacing kaersutite in the Mount Sidley xenoliths (Field I in Fig. 9) formed under more oxidized conditions (approximately at the NNO buffer) than rho¨nite in basanite (Field II in Fig. 9). The absence of
RHO«NITE IN PYROXENITE XENOLITHS, WESTANTARCTICA
FIG. 7. Solidus-liquidus relations of Ti-hornblende up to 1 kbar, NNO buffer conditions (after Kunzmann, 1989) (a) and kaersutite at atmospheric pressure, IW and NNO buffer conditions (after Boivin, 1980) (b). In a, stability eld of rho¨nite (delineated by hatched line) coexisting with plagioclase and olivine (light shaded eld), and with plagioclase, clinopyroxene and olivine (dark shaded eld) is shown. Note that the highest temperature appearance of rho¨nite in both amphibole starting compositions under NNO buffer conditions is similar.
magnetite and/or spinel in the Mount Sidley microphenocryst assemblage implies fO2 below that of the NNO buffer (e.g. Fig. 8b), and probably approximating QFM buffer conditions.
Low-pressure formation of rhÎnite Evidence of low-pressure formation of rho¨nite after Ti-amphibole and crystallizing from a basanitic melt, is provided by the instability of kaersutite in the xenoliths and the magmatic evolution of the Mount Sidley volcano.
FIG. 8. Solidus-liquidus phases relations in basanites showing the stability eld of rho¨ nite (hatched line) coexisting with plagioclase, olivine and clinopyroxene under NNO buffer conditions in the Mount Sidley xenoliths (shaded eld) in (a) (after Kunzmann, 1989), compared with those in (b) at atmospheric pressure under IW and NNO buffer conditions (after Boivin, 1980). Note the difference for the highest temperature appearance of rho¨ nite under NNO buffer conditions in both experiments that may re ect differences in bulk compositions of the basanite starting material.
647
R. H.GRAPES ETAL.
FIG. 9. Plot of VIFe3+ vs. VITi + Mg + Fe2+ contents of published analyses of terrestrial rho¨ nites (all EPMA analyses recalculated on the basis of 20 oxygens and 14 cations) compared with those in the Mount Sidley xenoliths (shaded elds); Field I = rho¨nite after kaersutite and kaersutite replacing clinopyroxene; Field II = rho¨ nite as microphenocrysts in basanitic melt. Rho¨nite analyses sources as in Fig. 5. Analyses that deviate from the S = 6.00 line contain appreciable VIAl (Cr and Mn). In this respect note the plots of rho¨nites 2a,b with V1Al of 0.72 and 0.17 respectively (Lacroix, 1909; Gru¨nhagen and Seck, 1972). Inset shows trend of rho¨ nite compositions in nepheline basanite synthesized under IQF, NNO buffer conditions and in air at 1060ºC/0.3 kbar after Kunzmann (1989). The natural rho¨nite in the nepheline basanite starting material has the composition: VIFe3+ = 1.00 and VITi + Fe2+ + Mg = 4.88, i.e. close to the NNO rho¨ nite composition. It should be noted that while rho¨ nite compositions will change with T, P and bulk composition, greater Fe3+ equates with higher fO2 conditions of crystallization.
Mantle-derived kaersutite typically contains less Ti than kaersutite of shallow crustal origin (Best, 1974; Cawthorn, 1976; Vinx and Jung, 1977) and decreasing Ti in kaersutite with increasing pressure has been demonstrated experimentally by Oba et al. (1986) over a temperature range of 950 – 1100ºC using synthetic and natural kaersutite starting materials. Kaersutite in the Mount Sidley pyroxenite xenoliths has an average Ti of 0.69 cations per 23 (O) implying a pressure of ~9 kbar from experimental work on kaersutite 648
from basanite (Oba et al., 1986). This is in broad agreement with two-pyroxene pressure estimates obtained on orthopyroxene-bearing pyroxenite xenoliths of ~8±1.5 kbar at 950±100ºC from elsewhere in the Executive Committee Range (Wysoczanski et al., 1995). The steep dT/dP stability curve for Ti-amphibole (Yagi et al., 1975; Kunzmann, 1989; Fig. 7a) implies that the breakdown of kaersutite to form rho¨nite in the Mount Sidley xenoliths was not due to decompression, but rather an increase in temperature
RHO«NITE IN PYROXENITE XENOLITHS, WESTANTARCTICA
(4) A plot of VIFe3+ vs. (Ti + Fe2+ + Mg) in the Mount Sidley rho¨nites compared with global occurrences, indicates that rho¨nite replacing kaersutite formed under more oxidizing conditions (~NNO buffer) compared with rho¨nite crystallizing from basanitic melt (~QFM buffer). (5) Experimentally determined stability relations of Ti-amphibole (Boivin, 1980; Kunzmann, 1989) indicate that a rho¨nite + plagioclase + clinopyroxene ± olivine assemblage after kaersutite is stable between 1090 and 1160ºC at pressures 0.46) suitable for the primary crystallization of rho¨nite. The rho¨nite-bearing xenoliths occur in phonolite that evolved by fractional crystallization from basanite in the inferred magma chamber.
caused by their entrainment in, and in ltration by, a basanitic melt. Reconstruction of the volcanic history and petrogenesis of the Mount Sidley volcano by Panter et al. (1997) indicates that the presence of mantle xenoliths and lack of crustal contamination in the basanites of the complex is evidence for their rapid ascent, and that they underwent differentiation in a magma reservoir within the volcanic edi ce to produce phonolite. The rho¨nitebearing xenoliths that contain basanitic melt were erupted in phonolite. Wysoczanski (1993) also records rho¨nite-bearing pyroxenite xenoliths in basanitic scoria cones at Mount Sidley. The groundmass of the host basanite is characterized by microphenocrysts of rho¨nite as in basanites of the McMurdo volcanics (Kyle and Price, 1975). In the xenoliths, rho¨nite exhibits the same paragenesis as in those in phonolite. Thus, evidence for rapid ascent of basanite magma containing lower crustal kaersutite-bearingpyroxenite xenoliths and its near surface crystallization and/or shallow depth differentiation to form phonolite, supports the experimental data for rho¨nite formation by reaction of kaersutite and crystallization from basanitic melt at pressures 4.30 a.p.f.u.). (3) Rho¨ nite microphenocrysts in areas of former basanitic melt have excess Ti (
