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The Blithfield meteorite and the origin of sulfide-rich, metal-poor clasts and inclusions in brecciated enstatite chondrites
Earth and Planetary Science Letters, 67 (1984) 273-283 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
273
[6]
The Blithfield meteorite and the origin of sulfide-rich, metal-poor clasts and inclusions in brecciated enstatite chondrites Alan E. Rubin * Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 (U.S.A.)
Received August 16, 1983 Revised version received December 9, 1983
Blithfield (EL6) is one of five known enstatite chondrite breccias. It consists of troilite-rich clasts (35 + 5 vol.%) embedded in an extremely metallic Fe,Ni-rich matrix (65 _+ 5 vol.%) that contains metal nodules up to 17 mm in size. Clasts and matrix agglomerated independently in the solar nebula under conditions of high and low p S 2 / p O 2 ratios, respectively. The matrix accreted to an EL chondrite planetesimal and was metamorphosed to - 1000°C, above the Fe-Ni-S eutectic; chondrule textures were obliterated. The S-rich eutectic melt was lost from the matrix. The matrix material was buried to a depth of at least 3 km; accreting troilite-rich material was incorporated into the matrix as clasts. The breccia cooled through ~ 500°C at 1000-10,000°C/Myr. After cooling below - 500°C, Blithfield was quenched, possibly by impact excavation from depth and deposition onto the surface. Clasts or inclusions that are enriched in sulfide and depleted in metallic Fe,Ni are common in brecciated enstatite chondrites. Variations in the p $2/p 02 ratio in the nebular regions where these materials formed may explain many of their petrologic properties. The silica-rich clasts in Adhi Kot (EH4) formed at very high pS2/pO 2 ratios (> 1027); niningerite, free silica and troilite were produced from the sulfurization of enstatite and metallic Fe. The troilite-rich clasts in Blithfield and Atlanta (EL6) as well as the troilite-rich regions of the Hvittis (EL6) matrix formed at somewhat lower pS2//pO2 ratios where sulfurization of metallic Fe produced troilite. The Ni content of the residual metal increased, forming some metal of martensitic composition. The dark inclusions in Abee (EH 4), which contain up to 9 wt.% oldhamite, formed at high PS2/pO 2 ratios in the presence of an additional Ca-rich component.
1. Introduction
c o n s t r a i n m o d e l s o f the r e g i o n o f the solar n e b u l a w h e r e the e n s t a t i t e c h o n d r i t e s a g g l o m e r a t e d .
A single s t o n e (1.9 kg) of t h e B l i t h f i e l d e n s t a t i t e c h o n d r i t e was f o u n d o n A u g u s t 13, 1 9 1 0 , in the t o w n s h i p o f Blithfield, O n t a r i o , C a n a d a [1]. V a n S c h m u s a n d W o o d [2] classified B l i t h f i e l d as p e t r o l o g i c t y p e 6; Sears et al. [3] listed it as an E L 6 c h o n d r i t e . B l i t h f i e l d is o n e of five k n o w n e n s t a t i t e c h o n d r i t e breccias, a l o n g w i t h A b e e ( E H 4 ) [4,5]. A d h i K o t ( E H 4 ) [6], H v i t t i s ( E L 6 ) [7,8] a n d A t l a n t a ( E L 6 ) [9]. All c o n t a i n clasts, i n c l u s i o n s o r m a t r i x r e g i o n s t h a t are r e l a t i v e l y e n r i c h e d in sulfide a n d d e p l e t e d in m e t a l . T h e i r i n t e r r e l a t i o n s c a n h e l p * Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, U.S.A. 0012-821X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
2. Analytical procedures S m i t h s o n i a n I n s t i t u t i o n s p e c i m e n U S N M 534 o f the B l i t h f i e l d m e t e o r i t e (120 g) was cut in h a l f and polished without water. The polished surfaces w e r e s t u d i e d m i c r o s c o p i c a l l y in r e f l e c t e d light. P o l i s h e d t h i n s e c t i o n U S N M 534 of the B l i t h f i e l d m a t r i x was s t u d i e d m i c r o s c o p i c a l l y in t r a n s m i t t e d a n d r e f l e c t e d light. M o d a l a n a l y s e s w e r e m a d e microscopically using an automated point counter; i n d i v i d u a l silicate a n d m e t a l l i c F e , N i m i n e r a l s w e r e p o i n t c o u n t e d o n the e l e c t r o n m i c r o p r o b e . M i n e r a l vol.% was c o n v e r t e d i n t o wt.% u s i n g e s t i m a t e d m i n e r a l densities. E l e c t r o n m i c r o p r o b e m i n e r a l
274 analyses were m a d e using crystal spectrometers, following s t a n d a r d Bence-Albee and Z A F correction procedures. M i n o r elements and oxides h a d the following detection limits (in wt.%): C a O (0.03), A1203, F e O a n d K 2 0 (0.04), P a n d M g (0.05), M n a n d Cr (0.07), Ti, T i O 2 a n d M n O (0.08) a n d C o (0.10). A n a l y s e s of metallic F e , N i and schreibersite were also corrected for C o interference from the K~ peak of Fe. Bulk c o m p o s i t i o n s and b u l k atomic ratios for the different c o m p o n e n t s were calculated
from mineral dances.
compositions
and
modal
abun-
3. Results
3.1. Petrography T h e Blithfield breccia (Fig. 1; T a b l e 1) consists of troilite-rich clasts e m b e d d e d in an extremely
BLITHFIELD CLJ
i~i i I Fig. 1. The Blithfield EL6 enstatite chondrite breccia with its metallic Fe,Ni-rich matrix and dark-colored troilite-rich clasts. The large metal nodules (white) in the matrix are visible. A 1.0-mm-diameter spherical bleb of troilite occurs in the round metal nodule closest to clast 1. This nodule appears darker in the photograph because it was intentionally overexposed to enhance the image of the troilite bleb.
275 TABLE 1 Mineral abundances (wt.%) in Blithfield
Enstatite Plagioclase Kamacite Martensite Troilite Ferromagnesian alabandite Daubr~ehte Graphite Schreibersite Limonite
Clast 1
Clast 2
55 3 8 0.4 29 0.8 2 0.2 < 0.1 1
62 4 5 0.7 25 0.1 3 0.1 < 0.1 1
Total
99.4
100.9
Points counted Area analyzed (cm 2)
2161 6.0
1064 0.70
a n.f. = not found.
Matrix 31 2 64 n.f. a 0.5 0.1 0.1 0.1 2 0.4 100.0 7683 10.4
metallic Fe,Ni-rich matrix. Orthoenstatite (MgSiO3), plagioclase (Na,Ca) (AI,Si)A1Si208, troilite (FeS), ferromagnesian alabandite (Mn,Fe, Mg)S, daubr6elite (FeCr2S4) , graphite (C), schreibersite (Fe,Ni)3P, and kamacite (Fe,Ni) occur in the matrix and troilite-rich clasts. Accessory martensite (Fe,Ni) occurs in the troilite-rich clasts, but not in the matrix. Minor limonite (FeO. OH • nH20 ), a terrestrial weathering product, occurs throughout the breccia. Binns [10] found accessory cristobalite (SiO2) in Blithfield, but I did not. Neither Keil [11] nor I encountered any oldhamite (CaS) or sinoite (Si2N20) in this meteorite. Enstatite grains are orthorhombic [12] and range from 50 to 500 /lm; most have slightly undulose extinction. Plagioclase grains (90-300 #m) exhibit polysynthetic twinning.
Fig. 2. Reflected light photomicrograph of a portion of clast 1, showing large troilite grain ( T ) with daubr~elite exsolution lamellae (D). Also present are metallic Fe,Ni (M), ferroan alabandite (A) and rounded enstatite grains ( E ) with intergranular plagioclase (P).
276 The troilite-rich clasts lack chondrules and consist of a recrystallized intergrowth of troilite with daubr~elite exsolution lamellae (1-15 /tin wide), ferromagnesian alabandite (primarily occurring as 15- to 150-ttm-sized patches inside troilite), rounded to euhedral enstatite with intergranular plagioclase, and minor metallic Fe,Ni (Fig. 2). Schreibersite occurs as elongated 10- to 25-/xmwide strips at metallic Fe,Ni grain boundaries with troilite or silicate. The boundaries between the matrix and troilite-rich clasts are extensively recrystallized; grains of kamacite, silicate and troilite have grown across the boundaries, rendering them diffuse. The matrix (65 + 5 vol.% of the breccia) contains 25 + 5 vol.% large metal nodules (1-17 mm in maximum dimension) and no chondrules. The nodules appear similar to some of the metal nodules in mesosiderites [13-16], especially Barea. The boundaries between the Blithfield metal nodules and other matrix components are recrystallized. The total metallic Fe,Ni content of the matrix (including the nodules) is 64 wt.%--much higher than that of any other enstatite chondrite (_< 28 wt.% metal [11]). Schreibersite occurs as elongated euhedral to subhedral grains (25-70 ~m x 200-1000 ~tm), spaced as closely as 100/~m apart, within the metal nodules. At the edges of the metal nodules, schreibersite occurs as elongated to massive grains (up to 1 mm) adjacent to matrix silicate grains. Additional schreibersites occur within smaller kamacite grains throughout the matrix. Although a few 100- to 200-/tm-sized euhedral enstatite grains occur within the inner edges of the metal nodules, only very rare euhedral enstatites occur deep within the metal nodules. One large metal nodule contains a spherical 1.0-mm-diameter bleb of troilite (Fig. 1). The troilite bleb appears to be a single crystal with completely uniform extinction. It contains parallel daubrrelite exsolution lamellae, up to 15 ~m wide; two grains of ferromagnesian alabandite (50 and 150 ~m) occur at opposite ends of the troilite bleb. Smaller sulfide blebs occur in some of the other metal nodules. Graphite occurs within small metal grains throughout the matrix, but not inside the metal nodules. Graphite grains (10-300 ~m) occur as
irregular aggregates to elongated laths. A few 8 x 60/.tm graphite laths in the matrix are completely surrounded by enstatite, as is one isolated graphite lath in Adhi Kot [6] and almost all of the graphite in Abee [5].
3.2. Mineralogy Mineral compositions (Table 2) agree closely with those of Keil [11], except for enstatite. Keil [11] examined part of a troilite-rich clast from Blithfield and found only 0.03 wt.% FeO in enstatite (Fs0Wol.5). However, my analysis of 30 grains from Keil's sample indicates 0.31 wt.% FeO, the same as for troilite-rich clasts 1 and 2 (0.30 wt.% FeO; Fs0.4Wo~6) (Table 2). Enstatite in the matrix is systematically slightly more ferroan (0.38 wt.% FeO; Fs0.sWOl.6). The very rare enstatite grains deep within the metal nodules in the matrix are significantly enriched in FeO (1.1 wt.% FeO; FS1.4WoI.6)relative to the rest of Blithfield enstatite. Plagioclase is of oligoclase composition: Abvs_79Or4 in the troilite-rich clasts and AbsoOr 4 in the matrix. Two grains in the matrix are more sodic: Ab87Or 5. Kamacite throughout the breccia averages - 6.7 wt.% Ni, but matrix kamacite on average contains less Si than kamacite in the troilite-rich clasts (Table 2). Different metal nodules in the matrix contain different amounts of Si, ranging from 1.0 to 1.8 wt.%. Two grains of martensite (8.4 and 15.0 wt.% Ni) were found in clast 1 and two in clast 2 (13.3 and 15.8 wt.% Ni). No martensite was found in the matrix or the metal nodules. Euhedral and subhedral schreibersites inside the metal nodules have shallow "M-shaped" Ni profiles; grain edges contain 0.5-1.5 wt.% more Ni than grain interiors. Smaller schreibersites contain more Ni than larger grains. The kamacite surrounding these schreibersites in the metal nodules averages 0.14 wt.% P and 6.8 wt.% Ni, but these concentrations gradually decrease with increasing proximity to the schreibersites. Kamacite at the schreibersite interface averages only 0.07 wt.% P and 5.3 wt.% Ni. Matrix troilite and daubrrelite contain less Ti (0.31 and < 0.08 wt.%, respectively) than troilite
277 TABLE 2
TABLE 2 (continued)
Mineral compositions (wt.%) in Blithfield Clast 1
Clast 2
Clast 1
31
25
Number of grains
65
SiO 2 A1203 FeO MgO CaO MnO TiO 2
60.2 0.12 0.30 40.2 0.89 < 0.08 < 0.08
59.9 0.16 0.30 39.8 0.89 < 0.08 < 0.08
59.6 0.15 0.38 39.8 0.90 < 0.08 < 0.08
Total
101.71
101.05
100.83
98.0 0.4 1.6
98.0 0.4 1.6
97.9 0.5 1.6
15
11
21
En Fs Wo
SiO 2 A1203 FeO CaO K20 Na20 Total An Ab Or
65.4 21.9 0.32 3.6 0.68 9.0 100.90
66.0 21.7 0.44 3.6 0.67 8.7 101.11
65.9 21.6 0.33 3.3 0.70 9.0 100.83
17 79 4
18 78 4
16 80 4
Number of grains
45
15
61
Si Fe Co Ni P
1.62 91.2 0.35 6.7 0.09
1.37 90.9 0.31 6.5 0.13
1.22 91.1 0.33 6.7 0.13
Kamacite
99.96
99.21
99.48
Martensite Number of grains
10
18
Fe Mg Cr Ti Mn S
61.7 0.08 0.93 0.71 < 0.07 36.8
61.3 0.05 0.93 0.55 < 0.07 37.4
61.4 0.05 0.87 0.31 < 0.07 37.1
Total
100.22
100.23
99.73
10
2
1
1
18.2 12.8 0.23 < 0.08 28.0 0.54 40.4
Number of grains
2 66.6 20.5 0.33 1.6 0.81 9.9 99.74 8 87 5
Fe Mg Cr Ti Mn Ca S
13.5 7.2 0.14 < 0.08 40.2 0.26 38.9
14.8 6.2 0.23 < 0.08 39.1 0.35 39.4
12.1 4.3 0.13 < 0.08 44.6 0.24 39.4
Total
100.20
100.08
100.77
Number of grains
15
5
6
Fe Mg Cr Ti Mn S
16.7 0.06 34.9 0.10 2.8 44.3
2
2
Si Fe Co Ni P
1.81 86.0 0.23 11.7 < 0.05
1.28 83.3 0.23 14.5 < 0.05
Total
99.74
99.31
0
100.17
Daubrbelite
Total
Total
25
Ferromagnesian alabandite
Plagioclase Number of grains
Matrix
Troilite
Enstatite Number of grains
Clast 2
Matrix
98.86
16.7 0.06 35.2 0.11 2.6 46.0
17.3 < 0.05 34.6 < 0.08 2.1 45.8
100.67
99.80
8
2
17
Si Fe Co Ni P
0.15 55.3 < 0.09 29.3 14.6
0.06 60.9 < 0.09 23.7 15.0
Total
99.35
99.66
Schreibersite Number of grains
0.10 62.4 0.12 22.5 14.7 99.82
and daubr6elite in the troilite-rich clasts (0.66 and 0.10 wt.%). Although Keil [11] encountered no troilite grains in any enstatite chondrite with less than 0.20 wt.% Ti, a few troilite grains in the Blithfield matrix contain no detectable Ti at all.
278 TABLE 3 Bulk composition (wt.%) of Blithfield Bulk ~
Bulk b
Clast 2
Matrix
SiO 2 MgO A1203 CaO Na 2° K 2° H 2° Fe Ni Co S Cr Mn Ti P C
42.51 24.64 2.08 0.87 0.92 0.09 1.30 20.71 1.11 0.08 4.44 0.24 0.12 0.04 0.06 0.17
26.7 15.9 0.59 0.44 0.22 0.01 48.1 3.3 0.15 4.0 0.38 0.11 0.07 0.25 0.08
Clast 1 35.4 22.2 0.73 0.60 0.27 0.02 26.1 0.60 0.03 11.9 0.97 0.38 0,21 0.02 0,20
40.0 24.7 0.97 0.69 0.35 0.03 21.1 0.42 0.02 10,8 1.3 0.12 0,14 0.01 0.10
21.5 12.3 0.48 0.35 0.18 0.01 60.0 4.7 0.21 0.25 0.03 0.01 < 0.01 0.37 0.04
Total
99.38
100.30
99.63
100.75
100.43
" Johnston and Connor [1]. b Determined by combining bulk compositions and modal abundances of clasts and matrix.
M o s t o f t h e few f e r r o m a g n e s i a n a l a b a n d i t e g r a i n s i n t h e m a t r i x a r e s i m i l a r i n c o m p o s i t i o n to t h o s e i n t h e c l a s t s ( T a b l e 2), b u t o n e l a r g e g r a i n , a t t h e o u t s i d e e d g e o f a m e t a l n o d u l e , is s i g n i f i c a n t l y e n r i c h e d i n F e a n d M g a n d d e p l e t e d i n M n . I t is s i m i l a r i n c o m p o s i t i o n t o all s u c h g r a i n s i n A l l a n Hills A81021 (E6) (A.E. Rubin, unpublished data) a n d t h e N o r t o n C o u n t y e n s t a t i t e a c h o n d r i t e [17].
3.3. Bulk composition The bulk composition of the matrix, troilite-rich clasts and bulk meteorite, determined from modal abundances and mineral compositions, are given i n T a b l e 3, a l o n g w i t h t h e b u l k c o m p o s i t i o n o f 20 g o f B l i t h f i e l d d e t e r m i n e d b y w e t c h e m i s t r y [1]. The bulk composition of the meteorite determined
TABLE 4 Bulk atomic ratios of Blithfield
Clast 1 Clast 2 Matrix Bulk a Bulk b E-chondrites c CI chondrites d a b c d
AI/Si
Mg/Si
Ca/Si
Fe/Si
S/Si
0.025 0.028 0.026 0.058 0.026 0.047-0.059 0.085
0.93 0.92 0.85 0.86 0.89 0.74-0.86 1.05
0.018 0.018 0.017 0.022 0.018 0.032-0.038 0.062
0.79 0.57 3.00 0.52 1.94 0.34-1.10 0.87
0.63 0.51 0.02 0.20 0.28 0.12-0.33 0.49
Johnston and Connor [1]. Determined by combining bulk compositions and modal abundances of clasts and matrix. Wasson [33], Sears [34]. G.W. Kallemeyn, personal communication, 1983.
279 from the mode is significantly different from that determined by wet chemistry. Because the SiO 2, MgO and Fe contents of the wet chemical determination is most similar to that of clast 2, it is possible that Johnston and Connor [1] in large part sampled a troilite-rich clast. Thus, the bulk composition determined from the mode is probably more accurate. Bulk atomic ratios are given in Table 4. The matrix and troilite-rich clasts have significantly lower C a / S i and A1/Si ratios than average enstatite chondrites. The clasts have slightly higher M g / S i ratios and much higher S/Si ratios than normal enstatite chondrites. The matrix has a much higher F e / S i ratio and much lower S/Si ratio than normal enstatite chondrites.
4. Discussion
4.1. Thermal history Blithfield's highly recrystallized texture, the presence of large grains of plagioclase and absence of significant clinoenstatite are consistent with extensive metamorphism and with the breccia's classification as petrologic type 6. Easton [18] found that Blithfield contains the largest metallic Fe,Ni grains (in the matrix) of any enstatite chondrite. The occurrence of such large metal grains, the absence of recognizable chondrules in Blithfield and their presence in other EL6 chondrites suggest that Blithfield was heated to higher temperatures than any other enstatite chondrite [18]. Larimer and Buseck [19] calculated equilibration temperatures for enstatite chondrites based on equilibrium constants for the following chemical reactions: Si + 02 = SIO2; 2CaSiO 3 + S2 = 2CaS + 2SiO 2 + O2; and 2Fe + S2 = 2FeS, and microprobe analyses of CaO in enstatite and Si in metal. They also found that Blithfield equilibrated at higher temperatures than other enstatite chondrites. However, their calculated temperature (840°C) was estimated to be uncertain by more than 100°C. The CaO content of enstatite in the Blithfield matrix (0.90 wt.%; Table 2) is highest among the enstatite chondrites [11]. From the low-pressure enstatite-diopside solvus [20], this CaO content
suggests equilibrium at temperatures of - 980°C. However, the lack of excess diopside makes this temperature a minimum [21]. Such high temperatures obliterated the (inferred) pre-existing chondrule textures in the matrix. The occurrence in one metal nodule of a single crystal of troilite with a spherical outline (which presumably formed from a melt) suggests that the Fe-Ni-S eutectic temperature of 950-1000°C was reached. In view of the evidence for extreme heating of the matrix, it is likely that the metal nodules melted in situ and did not accrete to the matrix as clasts. (An in situ molten origin for the metal nodules in mesosiderites has been advocated by Delaney et al. [16], but not Floran [15].) The few enstatite grains deep within the metal nodules were probably incorporated from the matrix before the nodules solidified. The higher FeO in these enstatites suggests the possibility that some Fe in the nodules was subsequently oxidized and FeO diffused from the nodules into the enstatites. At the Fe-Ni-S eutectic, a S-rich liquid would form [22]. Loss of this liquid from the matrix accounts, in part, for the matrix's extremely low S / F e atomic ratio (0.01) compared to average EL chondrites (0.21), and indicates that Blithfield suffered open-system metamorphism. The absence of metal nodules or significant metal in the troilite-rich clasts indicates that these clasts were probably not heated to the Fe-Ni-S eutectic temperature, and thus were incorporated into the breccia after the matrix had cooled below about 950°C. If these clasts had been adjacent to the matrix at such high temperatures, they undoubtedly would have been invaded by the eutectic melt and subsequent more metal-rich liquids. However, the recrystallized texture of the clasts, the recrystallized clast-matrix boundaries, and the apparent absence of chondrules suggest that the clasts were incorporated into the breccia at temperatures > 870°C (the probable minimum temperature experienced by EL6 chondrites [21]). Larimer and Buseck [19] derived equations for calculating partial pressures of sulfur and oxygen during metamorphism. Assuming that the Blithfield matrix experienced a maximum metamorphic temperature of - 1000°C and using the equations of Larimer and Buseck [19], I find that pS 2 = 4 ×
280 10 - 7 atm and that p O 2 = 1 × 10 -23 atm for the matrix. This results in a p S 2 / p O 2 ratio of 4 × ]016, by far the lowest among enstatite chondrites (which otherwise range from - ] 0 20 t o 10 27 [6]). This very low p S 2 / p O 2 ratio is probably responsible for the extremely low total sulfide (0.6 wt.%; Table 1) and consequently, the extremely low bulk atomic S/Si ratio (0.02; Table 4) in the matrix. The low p S z / p O 2 ratio (a) allowed effective oxidation of Fe and Ti in the matrix, accounting for the higher amount of FeO in matrix enstatite (0.38 wt.% vs. 0.30 wt.% in the clasts; Table 2) and (b) inhibited sulfurization of Ti in matrix troilite (matrix troilite contains less Ti (0.31 wt.%; Table 2) than any other EL6 chondrite [11]). Matrix daubraelite also contains less Ti than daubr+elite in the troilite-rich clasts, i.e., < 0.08 wt.% vs. 0.10 and 0.11 wt.% (Table 2). Although matrix Ti apparently was oxidized, TiO 2 in matrix enstatite is still below the detection limit. After the matrix cooled below the Fe-Ni-S eutectic temperature, schreibersite nucleated inside and along the edges of the large metal nodules. The schreibersites inside the nodules have " M shaped" Ni profiles; smaller schreibersite grains are richer in Ni. Hewins and Goldstein [23] studied lunar phosphide-metal grains and found that as long as no taenite is present, bulk composition is not a "significant variable" in the determination of cooling rates. The bulk composition of the metal nodules in the Blithfield matrix, including the schreibersites, is - 7 wt.% Ni and - 0.18 wt.% P (with major Fe and minor Si and Co). Neither taenite nor martensite occurs in the matrix. Theoretical cooling rate curves were given in fig. 6 of Randich and Goldstein [24] for phosphide Ni content vs. phosphide width in a hexahedrite containing 5.5 wt.% Ni and 0.5 wt.% P. These curves were used to determine an approximate cooling rate of the metal nodules in the Blithfield matrix (Fig. 3). Fig. 3 indicates that the matrix cooled through - 5 0 0 ° C at a rate of 1000-10,000°C/Myr. (The occurrence of a few grains of martensite in the troilite-rich clasts--which may possibly have been incorporated into the breccia prior to cooling to - 5 0 0 ° C - - p r o b a b l y do not substantially affect the equilibria or this cooling rate.) The shallow M-shaped profiles of schreibersites
40--i
I
I
+i
r- ~
I
l
I
l
r
I
BLITHFIELD MATRIX METAL NODULES
35 ~ 30
.00
K/M_yr
20(
~25
--ir-jOOO
15 10
• i
1 ~3
~
2~0
i
31b
• ~
4~0
i
• 510
,
610
PHOSPHIDE WIDTH (pro) Fig. 3. Central Ni concentrations of schreibersite grains in the metal nodules of the Blithfield matrix plotted against phosphide width. Data lie subparallel to the cooling rate curves o! Randich and Goldstein [24] and indicate an approximate cooling rate of 1000-10,000°C/Myr.
in the metal nodules and the gradual decrease in the P content of the surrounding kamacite with increasing proximity to the schreibersites are consistent with final schreibersite growth occurring at roughly 500°C [23]. The Fe-Ni-P phase diagram [25] shows that at temperatures of 550_+ 50°C, kamacite, taenite and schreibersite with respective compositions of 6.1-7.5 wt.% Ni, 15.7-27.3 wt.% Ni, and 12.9-31.0 wt.% Ni, are in equilibrium. These compositions are similar to those in Blithfield (including the troilite-rich clasts): - 6.7 wt.% Ni in kamacite, ~< 16 wt.% Ni in martensite, and - 2 5 wt.% Ni in schreibersite, suggesting a final equilibrium temperature between 500 and 600°C. However, the 0.07 wt.% P in kamacite at the schreibersite interface in the nodules indicates that schreibersite growth ceased at a temperature in the interval 417-424°C [26]. This discrepancy in the final equilibration temperature may be due to the different equilibria involved in metal with more than 1.2 wt.% Si. The cooling rate of the Blithfield breccia (1000-10,000°C/Myr) can be used to estimate its burial depth beneath the surface of its parent body [27,28]. If a thermal diffusivity at 500°C (approximately the final equilibration temperature) of 0.05 cm2/s is assumed for Blithfield, then a burial
281 depth between 3.2 and 10 km is indicated. This calculation assumes that little regolith was present on the parent body. If an extremely low diffusivity (10 -4 cm2/s), comparable to that of the lunar regolith [29], is assumed for the material overlying Blithfield, then burial depths of 0.15-0.46 km are indicated. However, the existence of a thick, lunar-like regolith on a small, metal-rich, rheologically strong asteroid is very unlikely [30]. Thus, a burial depth of at least 3 km seems probable. After cooling below 500°C or so, the breccia was quenched (probably at cooling rates exceeding a few hundred degrees per second), transforming the small amount of taenite in the troilite-rich clasts into martensite. It is possible that quenching took place by radiative cooling when the breccia was impact-excavated from depth and deposited onto the parent body surface. 4.2. Origin of sulfide-rich clasts in brecciated enstatite chondrites
Clasts or inclusions that are relatively enriched in sulfide, depleted in metallic Fe,Ni and contain martensite are common in brecciated enstatite chondrites. The Blithfield clasts contain 25-29 wt.% troilite and 5.7-8.4 wt.% metallic Fe,Ni, including some martensite (Table 1). Atlanta (EL6) contains a clast with 11 wt.% troilite and 0.2 wt.% metallic Fe,Ni, including some martensite [9]. Hvittis (EL6) has impact melt-rock clasts embedded in a heterogeneous matrix; this matrix contains a few grains of martensite and some centimeter-sized areas with 15-20 wt.% total sulfides (mostly troilite) and 3-5 wt.% metallic Fe,Ni [8]. The silica-rich clasts in Adhi Kot (EH4) contain - 3 0 wt.% troilite plus niningerite and 9-24 wt.% metallic Fe,Ni (much of it martensite) [6]; these clasts also contain up to 28 wt.% cristobalite and only 12-14 wt.% enstatite. Abee (EH4) contains - 0.3 vol.% dark inclusions that contain 19-39 wt.% troilite, up to 9 wt.% oldhamite and 2 - 6 wt.% metallic Fe,Ni (primarily martensite) [5]. Although isolated equilibrium reactions with mareaction ( 0 can be written for forming all the different kinds of sulfide-rich, metal-poor clasts (e.g. [6]), in the absence of complete equilibrium condensation calculations it is not certain that the
indicated species would actually coexist or that even more favorable reactions would preclude the written reactions. Therefore, the remainder of this section is qualitative. Variations in the p S 2 / p O 2 ratio in the regions of the nebula where the sulfide-rich, metal-poor clasts formed may explain many of their petrologic properties. Rubin [6] found that the silica-rich clasts in Adhi Kot were formed at very high p S a / p O 2 ratios ( > 10 27) and proposed that they agglomerated in a region of the solar nebula containing a gas of non-cosmic composition. At high p S 2 / p O 2 ratios niningerite, free silica and troilite may have been produced at the expense of enstatite and metallic Fe. The Ni content of the residual metal increased, forming metal of martensitic composition. If the ambient nebular p S 2 / p O 2 ratios were somewhat lower than those experienced by the silica-rich clasts in Adhi Kot (although still several orders of magnitude greater than those experienced by the majority of the enstatite chondrites), then sulfurization of metallic Fe, but not enstatite may have occurred. The Ni content of the residual metal would have increased, forming some metal of martensitic composition. Such reactions could have formed the troilite-rich clasts in Blithfield and Atlanta, as well as the centimeter-sized troilite-rich, metal-poor regions of the Hvittis matrix. The metal-rich matrix regions of these breccias must have agglomerated at lower p S 2 / p O 2 ratios. The S / F e atomic ratios of the Blithfield clasts (0.80 and 0.89) are significantly greater than the CI ratio (0.56). This suggests that such large clasts decoupled from the surrounding gas and settled toward the nebula midplane where they may have had access to additional S (J.A. Wood, personal communication, 1983), probably by encountering gases with higher p S 2 / p O 2 ratios. Matrix regions may have consisted of finer grains which were carried along with the surrounding gases. These grains would have effectively been in a closed system and could have acquired the system's entire complement of S, resulting in a cosmic S / F e ratio (J.A. Wood, personal communication, 1983). However, the probable loss of a S-rich liquid (formed at the Fe-Ni-S eutectic) from the matrix during subsequent open-system metamorphism, precludes
282 the d e t e r m i n a t i o n of the original S / F e ratio of the matrix. The origin of the dark inclusions in Abee has b e e n the subject of considerable controversy; suggestions have ranged from high-temperature impact-heating [31] to a complex, multi-stage process involving mixing a n d either c o n d e n s a t i o n or partial melting [32]. The Abee dark inclusions may have formed at a moderately high pS2/pO 2 ratio such that a b u n d a n t troilite formed from sulfurization of metallic Fe,Ni a n d schreibersite. The Ni c o n t e n t s of these phases increased, resulting in a b u n d a n t martensite a n d nickeliferous schreibersite. The dark inclusions must also have acquired a n additional Ca-rich c o m p o n e n t . This is evident because of their great e n r i c h m e n t in oldhamite (CaS) a n d their relatively calcic compositions of enstatite a n d plagioclase [5]. Because A1 a n d Ti are n o t enriched in the dark inclusions [32,5], refractory Ca-bearing species such as hibonite, melilite, anorthite a n d perovskite are ruled out. A possible c a n d i d a t e is diopside, b u t this phase has not been observed in enstatite chondrites. The c o m m o n occurrence of sulfide-rich, metalpoor clasts and inclusions in brecciated enstatite chondrites (from both the E H a n d EL groups) indicates that in the region of the n e b u l a where enstatite chondrites formed it was not u n u s u a l for a m b i e n t c o n d i t i o n s to include high p $2/p 02 ratios. Therefore, individual enstatite chondrites may exist that are composed entirely of materials similar to the different kinds of inclusions in the enstatite chondrite breccias,
Acknowledgements I thank D.W.G. Sears, R.S. Clarke, Jr. a n d R.H. Hewins for helpful discussions, K. Keil for l o a n i n g his specimen of Blithfield to me a n d J.A. W o o d and a n a n o n y m o u s reviewer for useful comments.
References 1 R.A.A. Johnston and M.F. Connor, The Blithfieldmeteorite, Trans. R. Soc. Can., Sect. IV, 16, 187-194, 1922.
2 W.R. Van Schmus and J.A. Wood, A chemical-petrologic classification for the chondritic meteorites, Geochim. Cosmochim. Acta 31,747-765, 1967. 3 D.W. Sears, G.W. Kallemeyn and J.T. Wasson, The compositional classification of chondrites, II. The enstatite chondrite chondrite groups, Geochim. Cosmochim.Acta 46, 597-608, 1982. 4 K.R. Dawson, J.A. Maxwell and D.E. Parsons, A Description of the meteorite which fell near Abee, Alberta, Canada, Geochim. Cosmochim. Acta 21, 127-144, 1960. 5 A.E. Rubin and K. Keil, Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions, Earth Planet. Sci. Lett. 62, 118-131, 1983. 6 A.E. Rubin, The Adhi Kot breccia and implications for the origin of chondrules and silica-rich clasts in enstatite chondrites, Earth Planet. Sci. Lett. 64, 201-212, 1983. 7 L.H. Borgstr0m, Die Meteoriten yon Hvittis und Marjalahti, Bull. Comm. Geol. Finl. 14, 1-80, 1903. 8 A.E. Rubin, Impact melt-rock clasts in the Hvittis enstatite chondrite breccia: implications for a genetic relationship between EL chondrites and aubrites, Proc. 14th Lunar Planet. Sci. Conf. (in press). 9 A.E. Rubin, The Atlanta enstatite chondrite breccia, Meteoritics 18, 113-121, 1983. 10 R.A. Binns, Olivine in enstatite chondrites, Am. Mineral. 52, 1549-1554, 1967. 11 K. Keil, Mineralogical and chemical relationships among enstatite chondrites, J. Geophys. Res. 73, 6945-6976, 1968. 12 S.S. Pollack, X-ray detection of disordered orthopyroxene in meteorites, Semi-Annu. Rep., Mellon Inst., Pittsburgh, Pa., 1966. 13 B.N. Powell, Petrology and chemistry of mesosiderites, I. Textures and composition of nickel iron, Geochim. Cosmochim. Acta 33, 789-810, 1969. 14 W.E. Wilson, Jr., The Bondoc mesosiderite: mineralogy and petrology of the metal nodules, 74 pp., M.Sc. Thesis, Arizona State University, 1972. 15 R.J. Floran, Silicate petrography, classification, and origin of the mesosiderites: reviewand new observations, Proc. 9th Lunar Planet. Sci. Conf., pp. 1053-1081, 1978. 16 J,S. Delaney, C.E. Nehru, M. Prinz and G.E. Harlow, Metamorphism in mesosiderites, Proc. 12th Lunar Planet. Sci. Conf., 12B, pp. 1315-1342, 1981. 17 K. Keil and K. Fredriksson, Electron microprobe analysis of some rare minerals in the Norton County achondrite, Geochim. Cosmochim. Acta 27, 939-947, 1963. 18 A.J. Easton, Grain-size distribution and morphology of metal in E-chondrites, Meteoritics 18, 19-27, 1983. 19 J.W. Larimer and P.R. Buseck, Equilibration temperatures in enstatite chondrites, Geochim. Cosmochim. Acta 38, 471-477, 1974. 20 C.E. Nehru, Pressure dependence of the enstatite limb of the enstatite-diopside solvus, Am. Mineral. 61, 578-581, 1976. 21 R.T. Dodd, Meteorites--A Petrologic-ChemicalSynthesis, 368 pp., Cambridge University Press, 1981. 22 G. Kullerud, The Fe-Ni-S system, Carnegie Inst. Washington Yearb. 62, 175-189, 1963.
283 23 R.H. Hewins and J.I. Goldstein, Cooling rates for lunar samples determined with a diffusion model for phosphide exsolution, Proc. 8th Lunar Sci. Conf., pp. 1625-1638, 1977. 24 E. Randich and J.l. Goldstein, Cooling rates of seven hexahedrites, Geochim. Cosmochim. Acta 42, 221-233, 1978. 25 A.D. Romig, Jr. and J.I. Goldstein, Determination of the Fe-Ni and the Fe-Ni-P phase diagrams at low temperatures (700-300°C), Met. Trans. llA, 1151-1159, 1980. 26 R.S. Clarke, Jr. and J.l. Goldstein, Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites, Smithsonian Contrib. Earth Sci. 21, 1-80, 1978. 27 G.J. Taylor, K. Keil, J.L. Berkley, D.E. Lange, R.V. Fodor and R.M. Fruland, The Shaw meteorite: history of a chondrite consisting of impact-melted and metamorphic lithologies, Geochim. Cosmochim. Acta 43, 323-337, 1979. 28 A.E. Rubin, K. Keil, G.J. Taylor, M.-S. Ma, R.A. Schmitt and D.D. Bogard, Derivation of a heterogeneous lithic
29
30 31
32
33 34
fragment in the Bovedy L-group chondrite from impactmelted porphyritic chondrules, Geochim. Cosmochim. Acta 45, 2213-2228, 1981. M.G. Langseth, S.J. Keihm and K. Peters, Revised lunar heat-flow values, Proc. 7th Lunar Sci. Conf., pp. 3143-3171, 1976. K.R. Housen, L.L. Wilkening, C.R. Chapman and R. Greenberg, Asteroidal regoliths, Icarus 39, 317-351, 1979. J.F. Wacker and L.L. Wilkening, Structure of the Abee E-cbondrite breccia and implications for its origin (abstract), Meteoritics 17, 291-292, 1982. D.W. Sears, G.W. Kallemeyn and J.T. Wasson, Composition and origin of clasts and inclusions in the Abee enstatite chondrite breccia, Earth Planet. Sci. Lett. 62, 180-192, 1983. J.T. Wasson, Meteorites--Classificationand Properties, 316 pp., Springer-Verlag, Berlin, 1974. D.W. Sears, Formation of E chondrites and aubrites--a thermodynamic model, Icarus 43, 184-202, 1980.
273
[6]
The Blithfield meteorite and the origin of sulfide-rich, metal-poor clasts and inclusions in brecciated enstatite chondrites Alan E. Rubin * Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 (U.S.A.)
Received August 16, 1983 Revised version received December 9, 1983
Blithfield (EL6) is one of five known enstatite chondrite breccias. It consists of troilite-rich clasts (35 + 5 vol.%) embedded in an extremely metallic Fe,Ni-rich matrix (65 _+ 5 vol.%) that contains metal nodules up to 17 mm in size. Clasts and matrix agglomerated independently in the solar nebula under conditions of high and low p S 2 / p O 2 ratios, respectively. The matrix accreted to an EL chondrite planetesimal and was metamorphosed to - 1000°C, above the Fe-Ni-S eutectic; chondrule textures were obliterated. The S-rich eutectic melt was lost from the matrix. The matrix material was buried to a depth of at least 3 km; accreting troilite-rich material was incorporated into the matrix as clasts. The breccia cooled through ~ 500°C at 1000-10,000°C/Myr. After cooling below - 500°C, Blithfield was quenched, possibly by impact excavation from depth and deposition onto the surface. Clasts or inclusions that are enriched in sulfide and depleted in metallic Fe,Ni are common in brecciated enstatite chondrites. Variations in the p $2/p 02 ratio in the nebular regions where these materials formed may explain many of their petrologic properties. The silica-rich clasts in Adhi Kot (EH4) formed at very high pS2/pO 2 ratios (> 1027); niningerite, free silica and troilite were produced from the sulfurization of enstatite and metallic Fe. The troilite-rich clasts in Blithfield and Atlanta (EL6) as well as the troilite-rich regions of the Hvittis (EL6) matrix formed at somewhat lower pS2//pO2 ratios where sulfurization of metallic Fe produced troilite. The Ni content of the residual metal increased, forming some metal of martensitic composition. The dark inclusions in Abee (EH 4), which contain up to 9 wt.% oldhamite, formed at high PS2/pO 2 ratios in the presence of an additional Ca-rich component.
1. Introduction
c o n s t r a i n m o d e l s o f the r e g i o n o f the solar n e b u l a w h e r e the e n s t a t i t e c h o n d r i t e s a g g l o m e r a t e d .
A single s t o n e (1.9 kg) of t h e B l i t h f i e l d e n s t a t i t e c h o n d r i t e was f o u n d o n A u g u s t 13, 1 9 1 0 , in the t o w n s h i p o f Blithfield, O n t a r i o , C a n a d a [1]. V a n S c h m u s a n d W o o d [2] classified B l i t h f i e l d as p e t r o l o g i c t y p e 6; Sears et al. [3] listed it as an E L 6 c h o n d r i t e . B l i t h f i e l d is o n e of five k n o w n e n s t a t i t e c h o n d r i t e breccias, a l o n g w i t h A b e e ( E H 4 ) [4,5]. A d h i K o t ( E H 4 ) [6], H v i t t i s ( E L 6 ) [7,8] a n d A t l a n t a ( E L 6 ) [9]. All c o n t a i n clasts, i n c l u s i o n s o r m a t r i x r e g i o n s t h a t are r e l a t i v e l y e n r i c h e d in sulfide a n d d e p l e t e d in m e t a l . T h e i r i n t e r r e l a t i o n s c a n h e l p * Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, U.S.A. 0012-821X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
2. Analytical procedures S m i t h s o n i a n I n s t i t u t i o n s p e c i m e n U S N M 534 o f the B l i t h f i e l d m e t e o r i t e (120 g) was cut in h a l f and polished without water. The polished surfaces w e r e s t u d i e d m i c r o s c o p i c a l l y in r e f l e c t e d light. P o l i s h e d t h i n s e c t i o n U S N M 534 of the B l i t h f i e l d m a t r i x was s t u d i e d m i c r o s c o p i c a l l y in t r a n s m i t t e d a n d r e f l e c t e d light. M o d a l a n a l y s e s w e r e m a d e microscopically using an automated point counter; i n d i v i d u a l silicate a n d m e t a l l i c F e , N i m i n e r a l s w e r e p o i n t c o u n t e d o n the e l e c t r o n m i c r o p r o b e . M i n e r a l vol.% was c o n v e r t e d i n t o wt.% u s i n g e s t i m a t e d m i n e r a l densities. E l e c t r o n m i c r o p r o b e m i n e r a l
274 analyses were m a d e using crystal spectrometers, following s t a n d a r d Bence-Albee and Z A F correction procedures. M i n o r elements and oxides h a d the following detection limits (in wt.%): C a O (0.03), A1203, F e O a n d K 2 0 (0.04), P a n d M g (0.05), M n a n d Cr (0.07), Ti, T i O 2 a n d M n O (0.08) a n d C o (0.10). A n a l y s e s of metallic F e , N i and schreibersite were also corrected for C o interference from the K~ peak of Fe. Bulk c o m p o s i t i o n s and b u l k atomic ratios for the different c o m p o n e n t s were calculated
from mineral dances.
compositions
and
modal
abun-
3. Results
3.1. Petrography T h e Blithfield breccia (Fig. 1; T a b l e 1) consists of troilite-rich clasts e m b e d d e d in an extremely
BLITHFIELD CLJ
i~i i I Fig. 1. The Blithfield EL6 enstatite chondrite breccia with its metallic Fe,Ni-rich matrix and dark-colored troilite-rich clasts. The large metal nodules (white) in the matrix are visible. A 1.0-mm-diameter spherical bleb of troilite occurs in the round metal nodule closest to clast 1. This nodule appears darker in the photograph because it was intentionally overexposed to enhance the image of the troilite bleb.
275 TABLE 1 Mineral abundances (wt.%) in Blithfield
Enstatite Plagioclase Kamacite Martensite Troilite Ferromagnesian alabandite Daubr~ehte Graphite Schreibersite Limonite
Clast 1
Clast 2
55 3 8 0.4 29 0.8 2 0.2 < 0.1 1
62 4 5 0.7 25 0.1 3 0.1 < 0.1 1
Total
99.4
100.9
Points counted Area analyzed (cm 2)
2161 6.0
1064 0.70
a n.f. = not found.
Matrix 31 2 64 n.f. a 0.5 0.1 0.1 0.1 2 0.4 100.0 7683 10.4
metallic Fe,Ni-rich matrix. Orthoenstatite (MgSiO3), plagioclase (Na,Ca) (AI,Si)A1Si208, troilite (FeS), ferromagnesian alabandite (Mn,Fe, Mg)S, daubr6elite (FeCr2S4) , graphite (C), schreibersite (Fe,Ni)3P, and kamacite (Fe,Ni) occur in the matrix and troilite-rich clasts. Accessory martensite (Fe,Ni) occurs in the troilite-rich clasts, but not in the matrix. Minor limonite (FeO. OH • nH20 ), a terrestrial weathering product, occurs throughout the breccia. Binns [10] found accessory cristobalite (SiO2) in Blithfield, but I did not. Neither Keil [11] nor I encountered any oldhamite (CaS) or sinoite (Si2N20) in this meteorite. Enstatite grains are orthorhombic [12] and range from 50 to 500 /lm; most have slightly undulose extinction. Plagioclase grains (90-300 #m) exhibit polysynthetic twinning.
Fig. 2. Reflected light photomicrograph of a portion of clast 1, showing large troilite grain ( T ) with daubr~elite exsolution lamellae (D). Also present are metallic Fe,Ni (M), ferroan alabandite (A) and rounded enstatite grains ( E ) with intergranular plagioclase (P).
276 The troilite-rich clasts lack chondrules and consist of a recrystallized intergrowth of troilite with daubr~elite exsolution lamellae (1-15 /tin wide), ferromagnesian alabandite (primarily occurring as 15- to 150-ttm-sized patches inside troilite), rounded to euhedral enstatite with intergranular plagioclase, and minor metallic Fe,Ni (Fig. 2). Schreibersite occurs as elongated 10- to 25-/xmwide strips at metallic Fe,Ni grain boundaries with troilite or silicate. The boundaries between the matrix and troilite-rich clasts are extensively recrystallized; grains of kamacite, silicate and troilite have grown across the boundaries, rendering them diffuse. The matrix (65 + 5 vol.% of the breccia) contains 25 + 5 vol.% large metal nodules (1-17 mm in maximum dimension) and no chondrules. The nodules appear similar to some of the metal nodules in mesosiderites [13-16], especially Barea. The boundaries between the Blithfield metal nodules and other matrix components are recrystallized. The total metallic Fe,Ni content of the matrix (including the nodules) is 64 wt.%--much higher than that of any other enstatite chondrite (_< 28 wt.% metal [11]). Schreibersite occurs as elongated euhedral to subhedral grains (25-70 ~m x 200-1000 ~tm), spaced as closely as 100/~m apart, within the metal nodules. At the edges of the metal nodules, schreibersite occurs as elongated to massive grains (up to 1 mm) adjacent to matrix silicate grains. Additional schreibersites occur within smaller kamacite grains throughout the matrix. Although a few 100- to 200-/tm-sized euhedral enstatite grains occur within the inner edges of the metal nodules, only very rare euhedral enstatites occur deep within the metal nodules. One large metal nodule contains a spherical 1.0-mm-diameter bleb of troilite (Fig. 1). The troilite bleb appears to be a single crystal with completely uniform extinction. It contains parallel daubrrelite exsolution lamellae, up to 15 ~m wide; two grains of ferromagnesian alabandite (50 and 150 ~m) occur at opposite ends of the troilite bleb. Smaller sulfide blebs occur in some of the other metal nodules. Graphite occurs within small metal grains throughout the matrix, but not inside the metal nodules. Graphite grains (10-300 ~m) occur as
irregular aggregates to elongated laths. A few 8 x 60/.tm graphite laths in the matrix are completely surrounded by enstatite, as is one isolated graphite lath in Adhi Kot [6] and almost all of the graphite in Abee [5].
3.2. Mineralogy Mineral compositions (Table 2) agree closely with those of Keil [11], except for enstatite. Keil [11] examined part of a troilite-rich clast from Blithfield and found only 0.03 wt.% FeO in enstatite (Fs0Wol.5). However, my analysis of 30 grains from Keil's sample indicates 0.31 wt.% FeO, the same as for troilite-rich clasts 1 and 2 (0.30 wt.% FeO; Fs0.4Wo~6) (Table 2). Enstatite in the matrix is systematically slightly more ferroan (0.38 wt.% FeO; Fs0.sWOl.6). The very rare enstatite grains deep within the metal nodules in the matrix are significantly enriched in FeO (1.1 wt.% FeO; FS1.4WoI.6)relative to the rest of Blithfield enstatite. Plagioclase is of oligoclase composition: Abvs_79Or4 in the troilite-rich clasts and AbsoOr 4 in the matrix. Two grains in the matrix are more sodic: Ab87Or 5. Kamacite throughout the breccia averages - 6.7 wt.% Ni, but matrix kamacite on average contains less Si than kamacite in the troilite-rich clasts (Table 2). Different metal nodules in the matrix contain different amounts of Si, ranging from 1.0 to 1.8 wt.%. Two grains of martensite (8.4 and 15.0 wt.% Ni) were found in clast 1 and two in clast 2 (13.3 and 15.8 wt.% Ni). No martensite was found in the matrix or the metal nodules. Euhedral and subhedral schreibersites inside the metal nodules have shallow "M-shaped" Ni profiles; grain edges contain 0.5-1.5 wt.% more Ni than grain interiors. Smaller schreibersites contain more Ni than larger grains. The kamacite surrounding these schreibersites in the metal nodules averages 0.14 wt.% P and 6.8 wt.% Ni, but these concentrations gradually decrease with increasing proximity to the schreibersites. Kamacite at the schreibersite interface averages only 0.07 wt.% P and 5.3 wt.% Ni. Matrix troilite and daubrrelite contain less Ti (0.31 and < 0.08 wt.%, respectively) than troilite
277 TABLE 2
TABLE 2 (continued)
Mineral compositions (wt.%) in Blithfield Clast 1
Clast 2
Clast 1
31
25
Number of grains
65
SiO 2 A1203 FeO MgO CaO MnO TiO 2
60.2 0.12 0.30 40.2 0.89 < 0.08 < 0.08
59.9 0.16 0.30 39.8 0.89 < 0.08 < 0.08
59.6 0.15 0.38 39.8 0.90 < 0.08 < 0.08
Total
101.71
101.05
100.83
98.0 0.4 1.6
98.0 0.4 1.6
97.9 0.5 1.6
15
11
21
En Fs Wo
SiO 2 A1203 FeO CaO K20 Na20 Total An Ab Or
65.4 21.9 0.32 3.6 0.68 9.0 100.90
66.0 21.7 0.44 3.6 0.67 8.7 101.11
65.9 21.6 0.33 3.3 0.70 9.0 100.83
17 79 4
18 78 4
16 80 4
Number of grains
45
15
61
Si Fe Co Ni P
1.62 91.2 0.35 6.7 0.09
1.37 90.9 0.31 6.5 0.13
1.22 91.1 0.33 6.7 0.13
Kamacite
99.96
99.21
99.48
Martensite Number of grains
10
18
Fe Mg Cr Ti Mn S
61.7 0.08 0.93 0.71 < 0.07 36.8
61.3 0.05 0.93 0.55 < 0.07 37.4
61.4 0.05 0.87 0.31 < 0.07 37.1
Total
100.22
100.23
99.73
10
2
1
1
18.2 12.8 0.23 < 0.08 28.0 0.54 40.4
Number of grains
2 66.6 20.5 0.33 1.6 0.81 9.9 99.74 8 87 5
Fe Mg Cr Ti Mn Ca S
13.5 7.2 0.14 < 0.08 40.2 0.26 38.9
14.8 6.2 0.23 < 0.08 39.1 0.35 39.4
12.1 4.3 0.13 < 0.08 44.6 0.24 39.4
Total
100.20
100.08
100.77
Number of grains
15
5
6
Fe Mg Cr Ti Mn S
16.7 0.06 34.9 0.10 2.8 44.3
2
2
Si Fe Co Ni P
1.81 86.0 0.23 11.7 < 0.05
1.28 83.3 0.23 14.5 < 0.05
Total
99.74
99.31
0
100.17
Daubrbelite
Total
Total
25
Ferromagnesian alabandite
Plagioclase Number of grains
Matrix
Troilite
Enstatite Number of grains
Clast 2
Matrix
98.86
16.7 0.06 35.2 0.11 2.6 46.0
17.3 < 0.05 34.6 < 0.08 2.1 45.8
100.67
99.80
8
2
17
Si Fe Co Ni P
0.15 55.3 < 0.09 29.3 14.6
0.06 60.9 < 0.09 23.7 15.0
Total
99.35
99.66
Schreibersite Number of grains
0.10 62.4 0.12 22.5 14.7 99.82
and daubr6elite in the troilite-rich clasts (0.66 and 0.10 wt.%). Although Keil [11] encountered no troilite grains in any enstatite chondrite with less than 0.20 wt.% Ti, a few troilite grains in the Blithfield matrix contain no detectable Ti at all.
278 TABLE 3 Bulk composition (wt.%) of Blithfield Bulk ~
Bulk b
Clast 2
Matrix
SiO 2 MgO A1203 CaO Na 2° K 2° H 2° Fe Ni Co S Cr Mn Ti P C
42.51 24.64 2.08 0.87 0.92 0.09 1.30 20.71 1.11 0.08 4.44 0.24 0.12 0.04 0.06 0.17
26.7 15.9 0.59 0.44 0.22 0.01 48.1 3.3 0.15 4.0 0.38 0.11 0.07 0.25 0.08
Clast 1 35.4 22.2 0.73 0.60 0.27 0.02 26.1 0.60 0.03 11.9 0.97 0.38 0,21 0.02 0,20
40.0 24.7 0.97 0.69 0.35 0.03 21.1 0.42 0.02 10,8 1.3 0.12 0,14 0.01 0.10
21.5 12.3 0.48 0.35 0.18 0.01 60.0 4.7 0.21 0.25 0.03 0.01 < 0.01 0.37 0.04
Total
99.38
100.30
99.63
100.75
100.43
" Johnston and Connor [1]. b Determined by combining bulk compositions and modal abundances of clasts and matrix.
M o s t o f t h e few f e r r o m a g n e s i a n a l a b a n d i t e g r a i n s i n t h e m a t r i x a r e s i m i l a r i n c o m p o s i t i o n to t h o s e i n t h e c l a s t s ( T a b l e 2), b u t o n e l a r g e g r a i n , a t t h e o u t s i d e e d g e o f a m e t a l n o d u l e , is s i g n i f i c a n t l y e n r i c h e d i n F e a n d M g a n d d e p l e t e d i n M n . I t is s i m i l a r i n c o m p o s i t i o n t o all s u c h g r a i n s i n A l l a n Hills A81021 (E6) (A.E. Rubin, unpublished data) a n d t h e N o r t o n C o u n t y e n s t a t i t e a c h o n d r i t e [17].
3.3. Bulk composition The bulk composition of the matrix, troilite-rich clasts and bulk meteorite, determined from modal abundances and mineral compositions, are given i n T a b l e 3, a l o n g w i t h t h e b u l k c o m p o s i t i o n o f 20 g o f B l i t h f i e l d d e t e r m i n e d b y w e t c h e m i s t r y [1]. The bulk composition of the meteorite determined
TABLE 4 Bulk atomic ratios of Blithfield
Clast 1 Clast 2 Matrix Bulk a Bulk b E-chondrites c CI chondrites d a b c d
AI/Si
Mg/Si
Ca/Si
Fe/Si
S/Si
0.025 0.028 0.026 0.058 0.026 0.047-0.059 0.085
0.93 0.92 0.85 0.86 0.89 0.74-0.86 1.05
0.018 0.018 0.017 0.022 0.018 0.032-0.038 0.062
0.79 0.57 3.00 0.52 1.94 0.34-1.10 0.87
0.63 0.51 0.02 0.20 0.28 0.12-0.33 0.49
Johnston and Connor [1]. Determined by combining bulk compositions and modal abundances of clasts and matrix. Wasson [33], Sears [34]. G.W. Kallemeyn, personal communication, 1983.
279 from the mode is significantly different from that determined by wet chemistry. Because the SiO 2, MgO and Fe contents of the wet chemical determination is most similar to that of clast 2, it is possible that Johnston and Connor [1] in large part sampled a troilite-rich clast. Thus, the bulk composition determined from the mode is probably more accurate. Bulk atomic ratios are given in Table 4. The matrix and troilite-rich clasts have significantly lower C a / S i and A1/Si ratios than average enstatite chondrites. The clasts have slightly higher M g / S i ratios and much higher S/Si ratios than normal enstatite chondrites. The matrix has a much higher F e / S i ratio and much lower S/Si ratio than normal enstatite chondrites.
4. Discussion
4.1. Thermal history Blithfield's highly recrystallized texture, the presence of large grains of plagioclase and absence of significant clinoenstatite are consistent with extensive metamorphism and with the breccia's classification as petrologic type 6. Easton [18] found that Blithfield contains the largest metallic Fe,Ni grains (in the matrix) of any enstatite chondrite. The occurrence of such large metal grains, the absence of recognizable chondrules in Blithfield and their presence in other EL6 chondrites suggest that Blithfield was heated to higher temperatures than any other enstatite chondrite [18]. Larimer and Buseck [19] calculated equilibration temperatures for enstatite chondrites based on equilibrium constants for the following chemical reactions: Si + 02 = SIO2; 2CaSiO 3 + S2 = 2CaS + 2SiO 2 + O2; and 2Fe + S2 = 2FeS, and microprobe analyses of CaO in enstatite and Si in metal. They also found that Blithfield equilibrated at higher temperatures than other enstatite chondrites. However, their calculated temperature (840°C) was estimated to be uncertain by more than 100°C. The CaO content of enstatite in the Blithfield matrix (0.90 wt.%; Table 2) is highest among the enstatite chondrites [11]. From the low-pressure enstatite-diopside solvus [20], this CaO content
suggests equilibrium at temperatures of - 980°C. However, the lack of excess diopside makes this temperature a minimum [21]. Such high temperatures obliterated the (inferred) pre-existing chondrule textures in the matrix. The occurrence in one metal nodule of a single crystal of troilite with a spherical outline (which presumably formed from a melt) suggests that the Fe-Ni-S eutectic temperature of 950-1000°C was reached. In view of the evidence for extreme heating of the matrix, it is likely that the metal nodules melted in situ and did not accrete to the matrix as clasts. (An in situ molten origin for the metal nodules in mesosiderites has been advocated by Delaney et al. [16], but not Floran [15].) The few enstatite grains deep within the metal nodules were probably incorporated from the matrix before the nodules solidified. The higher FeO in these enstatites suggests the possibility that some Fe in the nodules was subsequently oxidized and FeO diffused from the nodules into the enstatites. At the Fe-Ni-S eutectic, a S-rich liquid would form [22]. Loss of this liquid from the matrix accounts, in part, for the matrix's extremely low S / F e atomic ratio (0.01) compared to average EL chondrites (0.21), and indicates that Blithfield suffered open-system metamorphism. The absence of metal nodules or significant metal in the troilite-rich clasts indicates that these clasts were probably not heated to the Fe-Ni-S eutectic temperature, and thus were incorporated into the breccia after the matrix had cooled below about 950°C. If these clasts had been adjacent to the matrix at such high temperatures, they undoubtedly would have been invaded by the eutectic melt and subsequent more metal-rich liquids. However, the recrystallized texture of the clasts, the recrystallized clast-matrix boundaries, and the apparent absence of chondrules suggest that the clasts were incorporated into the breccia at temperatures > 870°C (the probable minimum temperature experienced by EL6 chondrites [21]). Larimer and Buseck [19] derived equations for calculating partial pressures of sulfur and oxygen during metamorphism. Assuming that the Blithfield matrix experienced a maximum metamorphic temperature of - 1000°C and using the equations of Larimer and Buseck [19], I find that pS 2 = 4 ×
280 10 - 7 atm and that p O 2 = 1 × 10 -23 atm for the matrix. This results in a p S 2 / p O 2 ratio of 4 × ]016, by far the lowest among enstatite chondrites (which otherwise range from - ] 0 20 t o 10 27 [6]). This very low p S 2 / p O 2 ratio is probably responsible for the extremely low total sulfide (0.6 wt.%; Table 1) and consequently, the extremely low bulk atomic S/Si ratio (0.02; Table 4) in the matrix. The low p S z / p O 2 ratio (a) allowed effective oxidation of Fe and Ti in the matrix, accounting for the higher amount of FeO in matrix enstatite (0.38 wt.% vs. 0.30 wt.% in the clasts; Table 2) and (b) inhibited sulfurization of Ti in matrix troilite (matrix troilite contains less Ti (0.31 wt.%; Table 2) than any other EL6 chondrite [11]). Matrix daubraelite also contains less Ti than daubr+elite in the troilite-rich clasts, i.e., < 0.08 wt.% vs. 0.10 and 0.11 wt.% (Table 2). Although matrix Ti apparently was oxidized, TiO 2 in matrix enstatite is still below the detection limit. After the matrix cooled below the Fe-Ni-S eutectic temperature, schreibersite nucleated inside and along the edges of the large metal nodules. The schreibersites inside the nodules have " M shaped" Ni profiles; smaller schreibersite grains are richer in Ni. Hewins and Goldstein [23] studied lunar phosphide-metal grains and found that as long as no taenite is present, bulk composition is not a "significant variable" in the determination of cooling rates. The bulk composition of the metal nodules in the Blithfield matrix, including the schreibersites, is - 7 wt.% Ni and - 0.18 wt.% P (with major Fe and minor Si and Co). Neither taenite nor martensite occurs in the matrix. Theoretical cooling rate curves were given in fig. 6 of Randich and Goldstein [24] for phosphide Ni content vs. phosphide width in a hexahedrite containing 5.5 wt.% Ni and 0.5 wt.% P. These curves were used to determine an approximate cooling rate of the metal nodules in the Blithfield matrix (Fig. 3). Fig. 3 indicates that the matrix cooled through - 5 0 0 ° C at a rate of 1000-10,000°C/Myr. (The occurrence of a few grains of martensite in the troilite-rich clasts--which may possibly have been incorporated into the breccia prior to cooling to - 5 0 0 ° C - - p r o b a b l y do not substantially affect the equilibria or this cooling rate.) The shallow M-shaped profiles of schreibersites
40--i
I
I
+i
r- ~
I
l
I
l
r
I
BLITHFIELD MATRIX METAL NODULES
35 ~ 30
.00
K/M_yr
20(
~25
--ir-jOOO
15 10
• i
1 ~3
~
2~0
i
31b
• ~
4~0
i
• 510
,
610
PHOSPHIDE WIDTH (pro) Fig. 3. Central Ni concentrations of schreibersite grains in the metal nodules of the Blithfield matrix plotted against phosphide width. Data lie subparallel to the cooling rate curves o! Randich and Goldstein [24] and indicate an approximate cooling rate of 1000-10,000°C/Myr.
in the metal nodules and the gradual decrease in the P content of the surrounding kamacite with increasing proximity to the schreibersites are consistent with final schreibersite growth occurring at roughly 500°C [23]. The Fe-Ni-P phase diagram [25] shows that at temperatures of 550_+ 50°C, kamacite, taenite and schreibersite with respective compositions of 6.1-7.5 wt.% Ni, 15.7-27.3 wt.% Ni, and 12.9-31.0 wt.% Ni, are in equilibrium. These compositions are similar to those in Blithfield (including the troilite-rich clasts): - 6.7 wt.% Ni in kamacite, ~< 16 wt.% Ni in martensite, and - 2 5 wt.% Ni in schreibersite, suggesting a final equilibrium temperature between 500 and 600°C. However, the 0.07 wt.% P in kamacite at the schreibersite interface in the nodules indicates that schreibersite growth ceased at a temperature in the interval 417-424°C [26]. This discrepancy in the final equilibration temperature may be due to the different equilibria involved in metal with more than 1.2 wt.% Si. The cooling rate of the Blithfield breccia (1000-10,000°C/Myr) can be used to estimate its burial depth beneath the surface of its parent body [27,28]. If a thermal diffusivity at 500°C (approximately the final equilibration temperature) of 0.05 cm2/s is assumed for Blithfield, then a burial
281 depth between 3.2 and 10 km is indicated. This calculation assumes that little regolith was present on the parent body. If an extremely low diffusivity (10 -4 cm2/s), comparable to that of the lunar regolith [29], is assumed for the material overlying Blithfield, then burial depths of 0.15-0.46 km are indicated. However, the existence of a thick, lunar-like regolith on a small, metal-rich, rheologically strong asteroid is very unlikely [30]. Thus, a burial depth of at least 3 km seems probable. After cooling below 500°C or so, the breccia was quenched (probably at cooling rates exceeding a few hundred degrees per second), transforming the small amount of taenite in the troilite-rich clasts into martensite. It is possible that quenching took place by radiative cooling when the breccia was impact-excavated from depth and deposited onto the parent body surface. 4.2. Origin of sulfide-rich clasts in brecciated enstatite chondrites
Clasts or inclusions that are relatively enriched in sulfide, depleted in metallic Fe,Ni and contain martensite are common in brecciated enstatite chondrites. The Blithfield clasts contain 25-29 wt.% troilite and 5.7-8.4 wt.% metallic Fe,Ni, including some martensite (Table 1). Atlanta (EL6) contains a clast with 11 wt.% troilite and 0.2 wt.% metallic Fe,Ni, including some martensite [9]. Hvittis (EL6) has impact melt-rock clasts embedded in a heterogeneous matrix; this matrix contains a few grains of martensite and some centimeter-sized areas with 15-20 wt.% total sulfides (mostly troilite) and 3-5 wt.% metallic Fe,Ni [8]. The silica-rich clasts in Adhi Kot (EH4) contain - 3 0 wt.% troilite plus niningerite and 9-24 wt.% metallic Fe,Ni (much of it martensite) [6]; these clasts also contain up to 28 wt.% cristobalite and only 12-14 wt.% enstatite. Abee (EH4) contains - 0.3 vol.% dark inclusions that contain 19-39 wt.% troilite, up to 9 wt.% oldhamite and 2 - 6 wt.% metallic Fe,Ni (primarily martensite) [5]. Although isolated equilibrium reactions with mareaction ( 0 can be written for forming all the different kinds of sulfide-rich, metal-poor clasts (e.g. [6]), in the absence of complete equilibrium condensation calculations it is not certain that the
indicated species would actually coexist or that even more favorable reactions would preclude the written reactions. Therefore, the remainder of this section is qualitative. Variations in the p S 2 / p O 2 ratio in the regions of the nebula where the sulfide-rich, metal-poor clasts formed may explain many of their petrologic properties. Rubin [6] found that the silica-rich clasts in Adhi Kot were formed at very high p S a / p O 2 ratios ( > 10 27) and proposed that they agglomerated in a region of the solar nebula containing a gas of non-cosmic composition. At high p S 2 / p O 2 ratios niningerite, free silica and troilite may have been produced at the expense of enstatite and metallic Fe. The Ni content of the residual metal increased, forming metal of martensitic composition. If the ambient nebular p S 2 / p O 2 ratios were somewhat lower than those experienced by the silica-rich clasts in Adhi Kot (although still several orders of magnitude greater than those experienced by the majority of the enstatite chondrites), then sulfurization of metallic Fe, but not enstatite may have occurred. The Ni content of the residual metal would have increased, forming some metal of martensitic composition. Such reactions could have formed the troilite-rich clasts in Blithfield and Atlanta, as well as the centimeter-sized troilite-rich, metal-poor regions of the Hvittis matrix. The metal-rich matrix regions of these breccias must have agglomerated at lower p S 2 / p O 2 ratios. The S / F e atomic ratios of the Blithfield clasts (0.80 and 0.89) are significantly greater than the CI ratio (0.56). This suggests that such large clasts decoupled from the surrounding gas and settled toward the nebula midplane where they may have had access to additional S (J.A. Wood, personal communication, 1983), probably by encountering gases with higher p S 2 / p O 2 ratios. Matrix regions may have consisted of finer grains which were carried along with the surrounding gases. These grains would have effectively been in a closed system and could have acquired the system's entire complement of S, resulting in a cosmic S / F e ratio (J.A. Wood, personal communication, 1983). However, the probable loss of a S-rich liquid (formed at the Fe-Ni-S eutectic) from the matrix during subsequent open-system metamorphism, precludes
282 the d e t e r m i n a t i o n of the original S / F e ratio of the matrix. The origin of the dark inclusions in Abee has b e e n the subject of considerable controversy; suggestions have ranged from high-temperature impact-heating [31] to a complex, multi-stage process involving mixing a n d either c o n d e n s a t i o n or partial melting [32]. The Abee dark inclusions may have formed at a moderately high pS2/pO 2 ratio such that a b u n d a n t troilite formed from sulfurization of metallic Fe,Ni a n d schreibersite. The Ni c o n t e n t s of these phases increased, resulting in a b u n d a n t martensite a n d nickeliferous schreibersite. The dark inclusions must also have acquired a n additional Ca-rich c o m p o n e n t . This is evident because of their great e n r i c h m e n t in oldhamite (CaS) a n d their relatively calcic compositions of enstatite a n d plagioclase [5]. Because A1 a n d Ti are n o t enriched in the dark inclusions [32,5], refractory Ca-bearing species such as hibonite, melilite, anorthite a n d perovskite are ruled out. A possible c a n d i d a t e is diopside, b u t this phase has not been observed in enstatite chondrites. The c o m m o n occurrence of sulfide-rich, metalpoor clasts and inclusions in brecciated enstatite chondrites (from both the E H a n d EL groups) indicates that in the region of the n e b u l a where enstatite chondrites formed it was not u n u s u a l for a m b i e n t c o n d i t i o n s to include high p $2/p 02 ratios. Therefore, individual enstatite chondrites may exist that are composed entirely of materials similar to the different kinds of inclusions in the enstatite chondrite breccias,
Acknowledgements I thank D.W.G. Sears, R.S. Clarke, Jr. a n d R.H. Hewins for helpful discussions, K. Keil for l o a n i n g his specimen of Blithfield to me a n d J.A. W o o d and a n a n o n y m o u s reviewer for useful comments.
References 1 R.A.A. Johnston and M.F. Connor, The Blithfieldmeteorite, Trans. R. Soc. Can., Sect. IV, 16, 187-194, 1922.
2 W.R. Van Schmus and J.A. Wood, A chemical-petrologic classification for the chondritic meteorites, Geochim. Cosmochim. Acta 31,747-765, 1967. 3 D.W. Sears, G.W. Kallemeyn and J.T. Wasson, The compositional classification of chondrites, II. The enstatite chondrite chondrite groups, Geochim. Cosmochim.Acta 46, 597-608, 1982. 4 K.R. Dawson, J.A. Maxwell and D.E. Parsons, A Description of the meteorite which fell near Abee, Alberta, Canada, Geochim. Cosmochim. Acta 21, 127-144, 1960. 5 A.E. Rubin and K. Keil, Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions, Earth Planet. Sci. Lett. 62, 118-131, 1983. 6 A.E. Rubin, The Adhi Kot breccia and implications for the origin of chondrules and silica-rich clasts in enstatite chondrites, Earth Planet. Sci. Lett. 64, 201-212, 1983. 7 L.H. Borgstr0m, Die Meteoriten yon Hvittis und Marjalahti, Bull. Comm. Geol. Finl. 14, 1-80, 1903. 8 A.E. Rubin, Impact melt-rock clasts in the Hvittis enstatite chondrite breccia: implications for a genetic relationship between EL chondrites and aubrites, Proc. 14th Lunar Planet. Sci. Conf. (in press). 9 A.E. Rubin, The Atlanta enstatite chondrite breccia, Meteoritics 18, 113-121, 1983. 10 R.A. Binns, Olivine in enstatite chondrites, Am. Mineral. 52, 1549-1554, 1967. 11 K. Keil, Mineralogical and chemical relationships among enstatite chondrites, J. Geophys. Res. 73, 6945-6976, 1968. 12 S.S. Pollack, X-ray detection of disordered orthopyroxene in meteorites, Semi-Annu. Rep., Mellon Inst., Pittsburgh, Pa., 1966. 13 B.N. Powell, Petrology and chemistry of mesosiderites, I. Textures and composition of nickel iron, Geochim. Cosmochim. Acta 33, 789-810, 1969. 14 W.E. Wilson, Jr., The Bondoc mesosiderite: mineralogy and petrology of the metal nodules, 74 pp., M.Sc. Thesis, Arizona State University, 1972. 15 R.J. Floran, Silicate petrography, classification, and origin of the mesosiderites: reviewand new observations, Proc. 9th Lunar Planet. Sci. Conf., pp. 1053-1081, 1978. 16 J,S. Delaney, C.E. Nehru, M. Prinz and G.E. Harlow, Metamorphism in mesosiderites, Proc. 12th Lunar Planet. Sci. Conf., 12B, pp. 1315-1342, 1981. 17 K. Keil and K. Fredriksson, Electron microprobe analysis of some rare minerals in the Norton County achondrite, Geochim. Cosmochim. Acta 27, 939-947, 1963. 18 A.J. Easton, Grain-size distribution and morphology of metal in E-chondrites, Meteoritics 18, 19-27, 1983. 19 J.W. Larimer and P.R. Buseck, Equilibration temperatures in enstatite chondrites, Geochim. Cosmochim. Acta 38, 471-477, 1974. 20 C.E. Nehru, Pressure dependence of the enstatite limb of the enstatite-diopside solvus, Am. Mineral. 61, 578-581, 1976. 21 R.T. Dodd, Meteorites--A Petrologic-ChemicalSynthesis, 368 pp., Cambridge University Press, 1981. 22 G. Kullerud, The Fe-Ni-S system, Carnegie Inst. Washington Yearb. 62, 175-189, 1963.
283 23 R.H. Hewins and J.I. Goldstein, Cooling rates for lunar samples determined with a diffusion model for phosphide exsolution, Proc. 8th Lunar Sci. Conf., pp. 1625-1638, 1977. 24 E. Randich and J.l. Goldstein, Cooling rates of seven hexahedrites, Geochim. Cosmochim. Acta 42, 221-233, 1978. 25 A.D. Romig, Jr. and J.I. Goldstein, Determination of the Fe-Ni and the Fe-Ni-P phase diagrams at low temperatures (700-300°C), Met. Trans. llA, 1151-1159, 1980. 26 R.S. Clarke, Jr. and J.l. Goldstein, Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites, Smithsonian Contrib. Earth Sci. 21, 1-80, 1978. 27 G.J. Taylor, K. Keil, J.L. Berkley, D.E. Lange, R.V. Fodor and R.M. Fruland, The Shaw meteorite: history of a chondrite consisting of impact-melted and metamorphic lithologies, Geochim. Cosmochim. Acta 43, 323-337, 1979. 28 A.E. Rubin, K. Keil, G.J. Taylor, M.-S. Ma, R.A. Schmitt and D.D. Bogard, Derivation of a heterogeneous lithic
29
30 31
32
33 34
fragment in the Bovedy L-group chondrite from impactmelted porphyritic chondrules, Geochim. Cosmochim. Acta 45, 2213-2228, 1981. M.G. Langseth, S.J. Keihm and K. Peters, Revised lunar heat-flow values, Proc. 7th Lunar Sci. Conf., pp. 3143-3171, 1976. K.R. Housen, L.L. Wilkening, C.R. Chapman and R. Greenberg, Asteroidal regoliths, Icarus 39, 317-351, 1979. J.F. Wacker and L.L. Wilkening, Structure of the Abee E-cbondrite breccia and implications for its origin (abstract), Meteoritics 17, 291-292, 1982. D.W. Sears, G.W. Kallemeyn and J.T. Wasson, Composition and origin of clasts and inclusions in the Abee enstatite chondrite breccia, Earth Planet. Sci. Lett. 62, 180-192, 1983. J.T. Wasson, Meteorites--Classificationand Properties, 316 pp., Springer-Verlag, Berlin, 1974. D.W. Sears, Formation of E chondrites and aubrites--a thermodynamic model, Icarus 43, 184-202, 1980.