sepiolite (Jones, 1986; Chameley, 1989). This obser- vation led Bodine and Madsen (1987) to postulate that minerals they interpreted as C/S (with varying pro-.
Clays and Clay Minerals, Vol. 41, No. 2, 240-259, 1993.
ORIGIN, DIAGENESIS, AND MINERALOGY OF CHLORITE MINERALS IN D E V O N I A N LACUSTRINE MUDROCKS, ORCADIAN BASIN, SCOTLAND S. HILLIER l
Department of Geology, University of Southampton, Southampton SO9, 5BP, U.K. Laboratoire de Grologie, Ecole Normale Superieur, 24 rue Lhomond, 75231 Paris, France Abstract--Chlorite and corrensite are common clay minerals in lacustrine mudrocks from the Devonian Orcadian Basin, Scotland. The relationship of their occurrence to vitrinite reflectance data demonstrate that they are authigenic minerals, formed during burial diagenesis/metamorphism at temperatures of ->120"C. Whole rock mineralogical and chemical analyses show that chlorite authigenesis occurred by reactions between the detrital dioctahedral clay mineral assemblage and dolomite that was formed under early evaporitic conditions in the lacustrine environment. XRD and electron microprobe analyses indicate that phases intermediate between corrensite and chlorite are probably mixed-layer chlorite/corrensite with a tendency towards segregation of layer types. Chemically, the conversion of corrensite to chlorite involves an increase in A1 for Si substitution in tetrahedral sites, but there is no change in the Fe/Mg ratio of octahedral cations. There is also no relationship of mixed-layer proportions to paleotemperature; only a general paleotemperature interval of approximately 120~ to 260~ in which a range of phases between corrensite and chlorite occurs. Chlorite polytypes are exclusively IIb, indicating the formation of this polytype at diagenetic temperatures. The occurrence of corrensite and Mg-rich chlorite in evaporite and carbonate successions is probably a reliable indicator ofdiagenetic alteration at temperatures of >-100~ Burial diagenetic reactions between dioctahedral clay minerals and Mg-rich carbonates may possibly explain many occurrences of corrensite and Mg-rich chlorite in such rocks. Key Words--Chlorite, Corrensite, Diagenesis, Dolomite, Vitrinite Reflectance. amples are interpreted as a more or less continuous series of mixed-layer chlorite/smectite (C/S) minerals (Helmold and van de Kamp, 1984; Chang et aL, 1986), analogous to the classic model for the dioctahedral smectite to illite reaction. Other examples are interpreted as prograde sequences of the three phases smectite, corrensite, and chlorite, but in zones that may be grossly overlapping (Inoue, 1987; Inoue and Utada, 1991) and with various interstratifications and/or intergrowths of these minerals being the norm. The emerging consensus seems to be with the latter interpretation, wherein corrensite is considered a discrete phase, rather than an interstratification of smectite and chlorite layers (Reynolds, 1988). In contrast, for the "evaporite-carbonate" association, the origin of chlorite and corrensite is classically related to formation in an evaporitic e n v i r o n m e n t either during, or shortly after, deposition. The proposed mechanisms for corrensite and chlorite formation include neoformation from concentrated hypersaline brines or, alternatively, transformation of detrital clay minerals (Lucas, 1962; Jeans, 1978; Hauff, 1981; Fisher, 1988; L i p p m a n n and Pankau, 1988). The evidence for such interpretations is basically two-fold: 1) Mgrich chlorite and corrensite occur in association with evaporitic facies from m a n y different sedimentary basins worldwide, yet they are comparatively rare in other facies deposited under normal marine or fresh
INTRODUCTION
Within the realm ofdiagenesis and low temperature metamorphism the geological occurrences of Mg-rich chlorite and corrensite (1:1 regularly interstratified chlorite/smectite) can be most simply divided into two types. The first includes those associated with volcaniclastic sediments (Almon et aL, 1976; H e l m o n d and van de Kamp, 1984; Chang et aL, 1986; Inoue, 1987; Inoue and Utada, 1991) or various types of altered igneous rocks (Bettison and Schiffman, 1988; Schiffm a n and Fridleifsson, 1991). The second, those that are associated with ancient marine evaporites (Droste, 1963; Jeans, 1978; Bodine, 1985; Bodine and Masden, 1987; L i p p m a n n and Pankau, 1988), or carbonates (Peterson, 1961; Fraser et al., 1973; Rao and Bhattacharya, 1973), or lacustrine facies (April, 1981). For the first or 'marie' association, the origin of corrensite and chlorite is related to diagenesis or low temperature hydrothermal alteration. Essentially, this appears as a sequence of minerals from trioctahedral smectite in the lowest temperature zone to chlorite in the highest. Within this sequence, the first occurrence of the intermediate mineral corrensite is typically at about 100~ (e.g., Inoue and Utada, 1991). Some exPresent address: Geologisches Institut Universit~it Bern, Baltzerstrasse 1, CH3012, Bern, Switzerland. Copyright 9 1993, The Clay Minerals Society
240
Vol. 41, No. 2, 1993
Chlorite minerals in Devonian mudrocks
0.7-1.5%RoSC 6-7
(oil window...+) ,,/,/~'r
1,6=3.0%RoSC 8-9 (dry gas zone) ,'~ ~,x~ ~> 3.0%R0SC 10-11(rnetamorpho~ed)~ < ~"~ 1 ~'
,•
~
I
241
Orkney
50km 4~
380
478
I
50kin
r
856,858 824
S_." =r
,132
th
Figure 1. Location of samples sites in the Devonian, OrC~u~ad B:~nb~eOr~hen~ Sc~ samples of mixed layer chlorite minerals of Figures 4, 5, and 6, and Table 4. All samples are grey-green lacustrine mudrocks. water conditions; 2) the distribution o f clay minerals is often zoned, passing from dioctahedral clays at the basin margin, through an intermediate zone characterised by corrensite, and eventually to a chlorite zone in the basin centre. This zonation generally follows that o f the evaporites and other facies and, hence, the changing chemistry o f the depositional a n d / o r early diagenetic environment. These studies o f the 'evaporite-carbonate' association imply that corrensite m a y form at surface temperatures. However, as pointed out by Bodine and Madsen (1987), there is no evidence from any modern evaporitic environment to support this contention. The Mg-rich clay minerals which form at surface temperatures, in both alkaline saline lakes and restricted marine basins, are invariably varieties o f trioctahedral smectite or the fibrous clay minerals palygorskite and sepiolite (Jones, 1986; Chameley, 1989). This observation led Bodine and Madsen (1987) to postulate that minerals they interpreted as C/S (with varying proportions o f expandable layers) from a Pennsylvanian evaporite cycle in the Paradox Basin, Utah, m a y have formed from precursor authigenic Mg-rich smectites
Figure 2. Generalized organic maturation map for the Orcadian Basin, based on spore color (SC) and vitrinite reflectance (Ro) data from Hillier and Marshall (1992). Actual vitrinite reflectance values for the samples described are included in Tables 1 and 2.
during burial diagenesis. They did, however, stress that there was no direct evidence for the former presence o f smectite. Such a 'uniformitarian' explanation suggests immediate parallels, and greater analogy, with the smectite-corrensite-chlorite sequence o f the 'mafic' association. It further implies that the occurrence o f corrensite m a y always be an indicator o f diagenetic grade, as originally proposed by Kiibler (1973). This paper documents the occurrence, structure, chemistry, and origin o f corrensite and chlorite and what appear to be intermediate mixed-layer minerals, in Devonian lacustrine mudrocks from the Orcadian Basin, Scotland (Figure 1). The Orcadian lacustrine sediments were deposited in lakes that were frequently desiccated and show evidence for the former presence o f evaporites (Parnell, 1985; Astin and Rogers, 1991; Rogers and Astin, 1991). As such, the occurrence o f corrensite and chlorite in these rocks falls into the "evaporite-carbonate" category.
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Table 1. Whole rock mineral composition, vitrinite reflectance and clay mineralogy of the < 2 um fraction of Orcadian mudrocks. No.
Quartz
Albite
333 354 1013 409 353 339 340 342 345 346 348 385 389 478 464 465 471 928 315 870 5 380 535 448 449 856 858 968 883 969 795 876 740 494 794 483 432 426 824 866
33 33 28 25 28 30 32 18 35 41 27 16 34 22 18 15 26 29 40 35 14 15 26 14 12 20 18 27 13 25 25 13 23 25 17 22 17 11 25 29
4 11 7 10 11 9 9 17 8 6 10 14 3 6 3 5 3 13 10 13 10 10 11 8 17 19 13 10 20 11 12 8 12 9 14 18 14
Kfeldspar Dolomite Calcite
7 8 9 5 5 8 7 5 8 7 5 8 9 4 8 8 4 5 4 5 8 10 12 7 5 6 4 10 17 14 15 14 8 17 4 16 14 22 14 13
11 18 3
11 1 8 3 10 10 15
14 22 24 19 15 20 18 17 19 28 1
3
9 5 4 3 7 4 4
3 1 5 6 4 3 8 1 3
15 8 ll 2 4 10 8 6
Siderite
8
7 2
5 l 6 11 1 2
2
Dioet
Trioct
40 35 32 40 40 30 28 49 31 31 41 46 40 53 32 58 52 38 48 57 36 43 30 41 37 45 41 30 27 30 26 24 32 19 32 20 28 25 20 27
3 I0 5 12 16 17 12 10
Pyrite
1 3 2 2
8
2 13 12 12 18 20 19 18 10 14 15 13 13 13 15 16 17 17 24 17 21
2
1 1 2 4 3
Tolal
Ro%
Ulite
Chlorite
109 98 100 95 110 109 105 91 108 111 103 107 112 102 97 103 106 103 95 97 96 98 102 99 95 108 101 95 108 97 102 97 95 93 98 98 97 96 99 104
0.9 e 0.9 a 0.9 a 0.9 a 0.9 a 1.0 e 1.0 e 1.0 e 1.0 e 1.0e 1.0 e 1.0 e 1.0 e 1.0 a 1.1 a 1.2 a 1.2 a 1.2 a 1.3 a 1.3 a 1.4 e 1.4 a 1.5 a 1.8 a 1.9 a 1.9 a 1.9 a 2.0 a 2.3 a 2.4 a 2.9 a 2.9 a 3.1 a 3.7 a 3.7 a 4.3 a 4.7 a 5.0 e 6.1 a 7.2 a
94 73 88 68 72 60 62 93 96 93 95 92 93 99 82 100 100 100 87 85 48 55 50 60 52 55 63 76 50 49 50 54 49 40 56 28 40 10 28 48
3 15 5 32 11 17 12 7
Correrlsite Kaolinite
12
17 26 26
1 18
13 15 52 45 50 40 48 45 37 24 27 51 50 46 51 60 44 72 60 90 72 52
23
Dioct = total dioctahedral clay. Trioct = total trioctahedral clay. Ro % = mean vitrinite reflectance, a = actual, e = estimate.
MATERIALS AND METHODS All s a m p l e s d i s c u s s e d are grey-green m u d r o c k s collected at o u t c r o p i n t h e O r c a d i a n B a s i n a n d are p a r t o f a m u c h larger suite o f 444 s a m p l e s o f b o t h m u d r o c k s a n d s a n d s t o n e s s t u d i e d b y Hillier (1989), w h e r e i n t h e full details o f localities a n d m e t h o d s are given. T h e grey-green m u d r o c k s d i s c u s s e d h e r e r e p r e s e n t a facies w h i c h w a s s t u d i e d in t h e m o s t detail. T h e y are r i c h e r in clay m i n e r a l s a n d p o o r e r in c a r b o n a t e t h a n t h e ass o c i a t e d c a r b o n a t e l a m i n i t e facies ( D o n o v a n , 1980). T h e selection o f as c o n s t a n t a litho-facies as p o s s i b l e was d e e m e d essential t o facilitate s u b s e q u e n t m i n e r alogical c o m p a r i s o n . I n a d d i t i o n , t h e p a r t i c u l a r s a m ples w e r e selected b e c a u s e t h e i r d i a g e n e t i c g r a d e w a s well c o n s t r a i n e d , m o s t l y b y m e a s u r e m e n t o f v i t r i n i t e
reflectance o n t h e actual s a m p l e itself, o r b y a n e s t i m a t e b a s e d o n v i t r i n i t e reflectance d a t a f r o m o t h e r s a m p l e s at t h e s a m e , o r a n e i g h b o r i n g , locality (Tables 1 a n d 2). D e t a i l e d v i t r i n i t e reflectance a n d s p o r e c o l o r m a p s for t h e O r c a d i a n B a s i n are g i v e n in Hillier a n d M a r s h a l l (1992) a n d Figure 2 is a s u m m a r y m a p . Crushed samples were dispersed in de-ionised water using a n u l t r a s o n i c p r o b e a n d t h e < 2 # m f r a c t i o n w a s s e p a r a t e d b y gravity settling. T h e s a m p l e s w e r e M g s a t u r a t e d , w a s h e d free o f c h l o r i d e a n d s e d i m e n t e d b y centrifuge. O r i e n t a t e d s a m p l e s w e r e p r e p a r e d b y t h e s m e a r m e t h o d a n d X - r a y diffraction ( X R D ) p a t t e r n s were r e c o r d e d in the a i r - d r i e d state, after g l y c o l a t i o n ( v a p o r p r e s s u r e m e t h o d ) , a n d after h e a t i n g t o 375 ~ a n d 550~ for 1 hr. E s t i m a t e s o f t h e r e l a t i v e a b u n d a n c e o f clay m i n e r a l s in t h e < 2 ttm f r a c t i o n w e r e m a d e b y a
Vol. 41, No. 2, 1993
Chlorite minerals in Devonian mudrocks
243
Table 2. Major element X R F analyses and vitrinite reflectance of Orcadian mudrocks. No
SIO2
TIO2
A1203
b2E203
MNO
MGO
CAO
NA20
K20
P205
S
LOI
Ro%
333 354 1013 409 339 340 342 345 346 348 353 385 389 478 464 465 471 928 315 870 5 380 535 448 449 586 858 968 883 969 795 876 740 494 794 483 432 426 824 866
53.98 63.51 48.55 55.36 57.99 57.46 57.04 55.43 55.90 49.75 55.01 47.36 59.20 53.47 48.08 49.47 54.39 52,89 65.79 63.93 52.32 52.56 56.09 49.28 47.73 51.71 49.57 61.01 52.72 59.23 57.50 52.17 54.11 59.30 44.60 57.15 53.78 50.00 57.73 60.30
0.63 0.75 0.59 0.71 0.52 0.51 0.92 0.54 0.55 0.57 0.70 0.87 0.73 0.71 0.68 0.80 0.73 0.58 0.68 0.82 0.81 0.76 0.77 0.75 0.73 0.79 0.73 0.79 0.68 1.05 0.73 0.69 0.67 0.75 0.59 0.71 0.78 0.93 0.71 0.67
14.24 15.99 12.89 15.79 12.60 12.25 19.43 11.94 11.57 12.96 14.74 17.45 15.00 16.75 14.42 19.59 16.96 12.69 17.31 18.87 17.57 18.16 16.01 18.60 17.62 18.11 17.64 15.87 15.15 18.82 15.02 15.32 16.19 16.50 14.97 15.61 16.79 19.28 15.67 16.84
3.68 5.38 4.11 6.06 4.20 4.37 6.06 2.98 3.78 4.87 5.74 6.49 3.95 4.41 5.18 3.29 4.41 3.10 2.42 4.00 6.21 8.22 5.89 8.70 9.27 8.07 7.45 4.89 5.44 6.55 5.24 5.82 6.60 6.05 6.91 6.14 7.62 10.36 6.93 6.59
0.08 0.04 0.09 0.07 0.05 0.06 0.05 0.05 0.05 0.07 0.07 0.06 0.04 0.08 0.09 0.07 0.07 0.06 0.02 0.03 0.07 0.06 0.06 0.08 0.09 0.07 0.08 0.06 0.09 0.07 0.06 0.07 0.09 0.04 0.14 0.07 0.08 0.05 0.05 0.04
3.40 4.02 4.92 4.82 4.73 4.96 3.56 3.22 3.92 4.80 4.44 3.91 2.57 4.65 4.49 4.06 3.85 4.95 1.12 1.24 5.34 4.45 4.10 5.24 5.49 5.05 4.76 3.65 4.60 2.41 4.13 4.65 4.42 4.09 6.47 4.69 4.54 7.14 4.71 4.63
8.39 0.85 9.70 3.91 6.19 6.96 0.69 9.09 6.52 8.45 6.16 4.74 4.19 6.23 8.68 5.47 5.76 8.06 1.20 0.49 2.98 2.12 4.69 4.16 4.97 2.98 5.75 1.32 5.81 0.38 4.07 5.21 4.53 1.32 9.73 3.01 2.73 0.32 1.96 0.32
0.83 2.22 0.09 1.46 1.75 1.56 1.99 1.36 1.50 1.35 1.86 1.68 1.86 0.71 1.11 0.60 0.87 0.59 0.14 0.14 2.80 1.90 2.28 1.79 1.87 1.63 1.66 2.18 3.28 1.81 1.62 3.13 1.84 1.58 1.34 2.02 1.80 1.86 2.65 2.16
3.96 4.04 3.48 4.33 3.16 3.01 5.30 3.26 2.94 3.48 3.44 5.34 4.62 5.47 5.20 7.22 5.54 4.41 5.18 5.67 4.87 5.65 5.23 4.60 4.39 4.28 4.24 4.07 5.17 7.18 6.19 5.21 4.44 6.70 3.43 5.54 6.28 5.38 4.93 4.89
0.11 0.12 0.09 0.13 0.18 0.12 0.21 0.11 0.11 0.12 0.11 0.12 0.10 0.11 0.12 0.13 0.13 0.11 0.16 0.17 0.13 0.13 0.18 0.11 0.14 0.12 0.12 0.15 0.12 0.15 0.14 0.10 0.11 0.12 0.11 0.12 0.11 0.18 0.13 0.13
0.86 0.00 0.20 0.15 1.10 1.07 0.00 0.24 0.99 0.00 0.44 0.00 0.00 0.00 0.40 0.00 0.00 0.41 1.16 0.05 0.00 0.29 0.00 0.00 0.00 0.00 0.00 0.26 0.16 0.19 0.00 0.79 0.40 0.26 0.00 0.68 0.84 0.06 0.22 0.00
9.93 3.07 15.30 7.23 7.60 7.76 4.75 11.80 12.29 13.57 7.33 11.98 7.74 7.40 11.60 9.30 7.30 12.20 4.80 4.80 6.89 5.73 4.70 6.70 7.70 7.20 8.00 5.80 6.80 2.10 5.30 6.90 6.62 3.30 11.70 4.30 4.70 4.71 4.30 3,43
0.9 e 0.9 a 0.9 a 0.9 a 1.0 a 1.0e 1.0 e 1.0 e 1.0 e 1.0 e 1.0 e 1.0 e 1.0 e 1.0a 1.1 a 1.2 a 1.2 a 1.2 a 1.3 a 1.3 a 1.4 e 1.4 a 1.5 a 1.8 a 1.9 a 1.9 a 1.9 a 2.0 a 2.3 a 2.4 a 2.9 a 2.9a 3.1 a 3.7 a 3.7 a 4.3 a 4.7 a 5.0 e 6.1 a 7.2 a
Mean Standard deviation
54.59
0.72
15.98
5.69
0.07
4.30
4.50
1.62
4.79
0.13
0.28
7.37
4.86
0.11
2.18
1.79
0.02
1.13
2.83
0.73
1.09
0.03
0.37
3.15
LOI = Loss on ignition. % Ro = vitrinite reflectance, a = actual, e = estimate.
m o d i f i e d v e r s i o n (Hillier, 1989) o f t h e m e t h o d des c r i b e d b y J o h n s et al. (1954). T h e s e e s t i m a t e s s e r v e s i m p l y to illustrate t r e n d s in a b u n d a n c e b e t w e e n s a m pies, n o t a b s o l u t e a m o u n t s . Experimental XRD patterns of mixed-layered minerals w e r e c o m p a r e d to p a t t e r n s c a l c u l a t e d w i t h N E W M O D ( R e y n o l d s , 1985). T h e p a r a m e t e r s u s e d were: Soller slits 6.6 a n d 2, d i v e r g e n c e slit 1, e x c h a n g e c a t i o n Mg, g o n i o m e t e r r a d i u s 17.3 c m , s a m p l e l e n g t h 3 c m , m a s s a b s o r p t i o n coefficient ( m u s t a r ) 45, a n d o r i e n t a t i o n f u n c t i o n (sigma star) 12. T h e p r o g r a m w a s also m o d i f i e d to i n c l u d e a 31.1 ~ c o r r e n s i t e layer to e n a b l e calculation o f v a r i o u s t y p e s o f i n t e r s t r a t i f i e d c h l o r i t e / corrensite minerals.
S e m i q u a n t i t a t i v e analyses o f t h e w h o l e r o c k m i n eralogy were m a d e b y X R D o f r a n d o m p o w d e r s u s i n g a l u m i n u m p o w d e r as a n i n t e r n a l s t a n d a r d . T h e w e i g h t p e r c e n t a g e o f each m i n e r a l p r e s e n t w a s d e t e r m i n e d from the intensity ratio o f selected peaks o f the mineral to t h o s e o f t h e s t a n d a r d , c o r r e c t e d b y a c o n s t a n t o f proportionality determined from mixtures of pure m i n e r a l s a n d the s t a n d a r d . F o r m i n e r a l s p r e s e n t i n a m o u n t s greater t h a n 15%, t h e p r e c i s i o n a n d a c c u r a c y o f t h e d e t e r m i n a t i o n is generally • 10% r e l a t i v e t o t h e a m o u n t p r e s e n t . C h e m i c a l analyses o f m u d r o c k s w e r e m a d e b y X - r a y fluorescence s p e c t r o m e t r y ( X R F ) o n f u s e d glass b e a d s using a P h i l i p s P W 1400 X R F . E l e c t r o n m i c r o p r o b e a n a l y s e s o f clay m i n e r a l s w e r e
244
Clays and Clay Minerals
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made using a wavelength dispersive Cameca Camebax machine with an accelerating voltage o f 15 kV, b e a m current o f 10 n A and a spot size o f 2 #m. Chlorite analyses were selected using an arbitrary upper limit o f 0.5 wt. % for total CaO + N a 2 0 + K20, and structural formulae calculated assuming an ideal anionic framework ofOzo(OH)l 6. All Fe is assumed to be Fe 2§
IL IL+Q IL
928
RESULTS
Whole rock mineralogy and chemistry C
Mineralogically, the samples are typical of m a n y mudrocks, containing an average o f about 25% quartz and 50% clay minerals (phyllosilicates) with the remainder divided principally between feldspars and carbonates (Table 1). Carbonate content, mostly dolomite, varies widely and some o f the samples border on the compositional field o f marls (35%-65% carbonate: Pettijohn, 1957). Notably, the most calcareous samples are low organic maturity samples which typically contain about 20% dolomite with little i f any calcite. Amongst the clay minerals, illite, and illite/smectite are dominant, followed by chlorite minerals and kaolinite. The most obvious trend in the clay mineral composition o f the whole rock is the frequent absence o f chlorite from low organic maturity samples, particularly those samples which are rich in dolomite (Table 1). Chemically, all the samples appear to be similar (Table 2); the most obvious variation is CaO content (Table 2). The standard deviation for CaO is over two times that o f MgO for approximately the same mean concentration.
X-ray diffraction of the 35)
~
IL+O
IL C
3-29)
6
10
C
C
IL-I-Q
I
I
I
~
I
I
14
18
22
26
30
34
2 Theta Cu K alpha
Figure 3. Five examples of diffraction patterns of the glycolated
