Primary magmatic carbon in ureilites: Evidence from cohenite-bearing metallic spherules

Gporhrmrca et Cosmochrmrca Aao D Pergamon

PR?S

@I%-?03?/86/f3.00 t .DD

Vol. 50. pp. 681-691

Ltd.1986.Printed in U.S.A.

Primary magmatic carbon in ureilites: Evidence from cohenite-bearing metallic spherules C&RENA ANNE GOODRICH Institute of Meteoritics and Department of Geology. University of New Mexico, Albuquerque. NM 8713 1. U.S.A.

and JOHN L. BERKLEY Department of Geology, State University of New York, College at Fredonia. Fredonia, NY 14063, U.S.A. (Received October 3, 1985; accepted in revised.form January 2 1, 1986)

Abstract-Metallic spherules containing cohenite, Fe-Ni metal, sulfides, and rare phosphorus-bearing minerals occur as inclusions within olivine and pigeonite grains in five low-shock ureilites. The bulk compositions. mineralogy, and textures of these spherules indicate that they represent Fe-Ni-C-S alloys that existed as droplets of immiscible metallic liquid in the magma(s) from which ureilites crystallized. At magmatic temperatures these alloys were carbon-saturated and probably coexisted with graphite in the magma. This is consistent with the occurrence of graphite crystals as inclusions in olivine in some uteilites and supports the view that graphite in ureilites was a primary magmatic component. Graphite could have been a residue from the solid source materials that melted to produce the ureilite parent magma(s) or could have crystallized from the magma(s). The metallic liquids separated into immiscible Fe-C liquids and Fe-S liquids after they were trapped within silicates. The Fe-C liquids contained 4-5 wt.% C and 3-7 wt.% Ni. Cohenite was the liquidus phase and started to crystallize at 1200-1225”C, forming euhedral and subhedral crystals. At the eutectic (- 1175°C) r-iron containing 7-9% Ni and - 1.8% C crvstallized along with cohenite. At lower temperatures the y-iron exsolved cohenite, and possibly a-iron. _ INTRODUCMON

THE ORIGINOFthe carbon in ureihtes has been a subject of major controversy. Carbon occurs in ureihtes in the form of interstitial graphite and diamond. The absence of diamond in the low-shock ureilite ALHA demonstrates that the diamond is not a primary phase but was formed from graphite by shock (WACKER, 1984). BERKLEYet al. (1980) argued on the basis of petrologic and textural evidence that graphite is a primary phase that was present in the ureilite parent magma(s) and was trapped between accumulating olivine crystals. BERKLEYand JONES( 1982) emphasized the close association of graphite and intergranular metal, and suggested that both formed from a primary, carbon-rich metallic phase. However, BOYNTONet al. (1976) and WASSON ef al. (1976) suggested that the graphite was injected into the solid silicate assemblage in a secondary (nonmagmatic) process such as a shock event. WLOTZKA ( 1972) argued that the existence of reduced rims on the silicates, formed by reaction with intergranular graphite, demonstrates that the silicate cores were not in equilibrium with graphite and therefore that the graphite was introduced late. However, BERKLEY and JONES ( 1982) and GOODRICH and BERKLEY( 1985a,b) pointed out that the silicate cores could have been in equilibrium with graphite at depth. A drop in pressure caused by excavation, would have led to reduction of the silicates (GOODRICHand BERKLEY, 1985a,b). Metal-sulfide spherules occurring as inclusions in olivine were reported in the ureihtes ALHA78019,

ALHA78262, and ALHA (BERKLEYet al., 1980: BERKLEYand JONES, 1982). Carbon was tentatively identified in the metal. The purpose of the work presented in this paper was to determine the compositions and minem~ogy of these spherules, with particular attention to the possible presence of C-rich phases. These spherules must have been trapped within crystallizing olivine, and therefore represent a primary component of the ureilite parent magma(s). We examined 8 recently discovered ureilites (GOODRICHand BERKLEY, 1985a,b). Shock effects in these are limited to minor fracturing, undulatory extinction, and kink bands in olivine. X-ray diffraction studies indicate that diamonds are not present in ALHA 19 (WACKER, 1984). We have searched for fluorescing phases in intergranular areas of the others and have found none, which suggests that diamonds are absent. The low-shock state of these meteorites gives us confidence that we are seeing primary features. These ureilites span the entire range of mg numbers known for ureilites and appear to be related by progressive reduction (GOODRICHand BERKLEY,1985a,b). We found metallic spherules as inclusions in the cores of silicate crystals in ALH82 130 (which is paired with ALH82 106), and PCA82506, as well as ALHA 19, ALHA78262, and ALHA77257. The mg numbers of these ureilites are: ALHA78019-76.2; ALHA7826277.7; PCA82506-78.3; ALHA77257-84.9; and ALH82 130-94.9 (GOODRICH and BERKLEY,1985b). The spherules are assemblages of cohenite ([Fe, Ni]Q, Fe-Ni metal, troilite, and rare phosphorusbearing minerals. We have interpreted the crystalli-

681

C. A. Goodrich and J. L. Betkley

682

zation history of the spherules in terms of the Fe-NiS-C system. The compositions, mineralogy, and textures of the spherules indicate that they represent metallic alloys that were carbon-saturated at magmatic temperatures and existed as immiscible liquid droplets in the silicate magma from which ureilite olivine and pigeonite crystallized. Because the metallic liquid was C-saturated, the magma probably was also, and probably contained graphite. This result supports the view that carbon was a primary magmatic component in ureilites (BERKLEYet al.. 1980). rather than the view that carbon was injected late into the solid ureilite silicate assemblage (BOYNTONef ai., 1976; WASSON et al., 1976). We discuss the implications of this result. The ureilite parent material m&t have been carbonrich, unlike that of any other achondrites. In addition, ureilite noble gases, which are associated with the carbon (WEBER et al., 1971, 1976; BEGEMANNand OTT. 1983; OTT et al., 1985; WACKER, 1985), might also be primary. ANALYTICAL PROCEDURES Our electron microprobe is a JEOL 733 equipped with a hack-scatten%lelectron detector. For analysis of carbon-bearing metallic phases we u+

an accelerating potential of 10 KeV

and a beam current of 30 na; for analysis of sulfides we used I5 KeV and 20 na. As standards we used pure metals for Fe. Ni, Co, Cr, and Si; apatite for P; pyrite for S; and Disko Island

cohenite for C. A ZAF tion procedure was used. Unless otherwise indicated (Table 1) S was not measured in metallic phases, and C and P were not measured in sulfides. When analyzing metallic phases we used counting times of 200 seconds at peaks and 100 seconds at each background position for all eiements except Fe. This yielded theoretical standard deviations in ~untins statistics of i-SO% for Ni, Co, Si. 530%for Cr and P. and 3% for C. A standard deviation ofO.Y%

was achieved for Fe using counting times of 30 seconds at the peak and 15seconds at background positions. Analysis of carbon by electron microprobe is difficult for several reasons (GOLDSTEIN ef ui..198 1). 1) The long wavelength carbon X-rays are easily absorbed in the sample. Ab sorption is minimized, however. by using 10 KeV so that Xrays are generated only at shallow depths in the sample. Ii Carbon is commonly used as a conductive coatmg material on probe samples. Heavy metal coatings cannot be used btcause they strongiy absorb X-rays. To minimize problems caused by carbon coating, we coated our carbon standard (cohenite) and the samples at the same time. This assures thicknesses of carbon coat that are as nearly as possible iden., tical. Therefore, the contribution (in absolute counts) of the carbon coat to the measured carbon was as nearly as possible the same for the standard and the samples. In the case of analyzing cohenite this contribution would cancel out. Nevertheless, carbon coating may not be uniform even over the standard, and so probably contributes to imprecision in our cohenite analyses. In analyzing carbon-bearing metal with lower carbon contents than that of the cohenite standard. the presence of carbon coat on the samples leads to an apparently high carbon content. The lower the carbon content the higher the relative error. Analysis of carbon-coated pure Fe and purr Ni standards showed up to 0.5 wt.% C. This represents the maximum amount by which our measured carbon contents for carbon-bearing metal could be too high. 31 Over long counting times. carbon contamination can build up under the electron beam. Fortunately, with our instrument, which is pumped with a molecular pump, we observed no significant increase in carbon X-ray production over the counting times used. 4) Mass absorption coefficients for carbon are not well known. The mass absorption coefficients used in our ZAF program are those of HENKE and EBISU(1974). For analysis of cohenite by comparison with a cohenite standard. however. uncertainties in the carbon absorption coefficients are of nepligible importance. We analyzed our cohenite standard as if it were an unknown after using it to calibrate carbon. The measured carbon contents showed deviations of 3-41 retative from the theoretical carbon content (6.67%) of cohenite. This va~atjon is not due to inhom~eneity of carbon in the standard because cohenitr

683

Primary magmatic C in ureihtes

ALw2130

(g

94.9)

2+

1*

pr

0)

mtt

PC mat

lv*

bulk

(2)

Mt.1

cob

*“If

is always stoichiometric (PETCH,1944), and so is an indication of the actual precision of our carbon analyses. Measurements of carbon in cohenite in our samples showed similar deviations from the value for stoichiometric cohenite. The following thin sections were used in this study: ALHA77257,lSa; ALHA77257,15b; PCA82506.26; ALHA78262,lS; ALHA78019,19; and ALH82130.9. PETROGRAPHY We studied 5 spherules from ALHA78262, 7 from PCA82506, 8 from ALH82130, and 1 from ALHA78019. These include most of the spherules we observed in our thin sections of these ureilites. Others were either too pitted, too altered (to iron oxides), or too small for identification or analysis of phases. Spherules are especially abundant in ALHA77257. We surveyed 26 in ALHA77257,lSa and 32 in ALHA77257,iSb. These represent about half the number observed in each of these sections. In thin section the spherules appear round or ovoid with diameters ranging from 5-150 p (Fig. 1). The majority are in the IO-60 r~ range. They are enclosed in the homogeneous cores of olivine crystals (see BERKLEYet al.. 1980 for a general description of the petrography of ureilites). In ALH82 130 they also occur in pigeonite. The following description applies to the majority of the spherules studied. Each spherule contains several large (up to 20 PC)euhedral or subhedral cohenite crystals intergrown with what we call the “mottled phase” (Fig. I A-E). In reflected light cohenite is easily distinguished from homogeneous Fe-Ni metal or a cubic carbide (SCOTT, 197 I) by its high relief and anisotropy. The mottled phase appears to be a line-grained intergrowth of at least two phases that differ in relief, and one of which is lamellar, In back-scattered electron images (BEI), the cohenite crystals are darker than the average mottled phase (therefore, of lower average atomic number). In BEI, the mottled areas (Fig IA-E) appear to contain tiny lamellae of a phase that has the same average

4

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Pt mott

bulk war*,

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pr

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atomic number as the cohenite crystals. As explained below, electron microprobe analyses indicate that the mottled areas contain cohenite and Fe-Ni metal with variable Ni contents, possibly y and a phases. Sulfide occurs as rims or partial rims around the metallic phases (therefore forming whole or partial shells around the spherules) or as small round inclusions (Fig. IA-E). Point-counting of back-scattered electron images of 17 representative spherules from all the ureilites yields an apparent range of 28-82 vol.% cohenite crystals in the metal part (sulfide excluded) of the spherules. The variation is probably a result of sampling error. Most spherules have values close to the average, 53 vol.% (52 wt.%), so we take this to be representative of all spherules of this type. In addition, there are some spherules that do not appear to contain large cohenite crystals. These spherules are most abundant in ALHA77257. Of 58 spherules that we surveyed in this ureilite, 15 show only apparently homogeneous Fe-Ni metal (with or without sulfide). In a few cases, they have narrow cohenite rims (Fig. 1F). We will argue below that the apparent absence of large cohenite crystals in these spherules is not a result of sampling error (un~p~~ntative sectioning). Of 74 spherules surveyed, 19 appear to contain no sulfide. Point-counting of 11 representative spherules that do show sulfide yields a range of 12-50 vol.%. This variation, and the apparent lack of sulfide in some spherules, is probably a result of sampling error (two possible exceptions will be discussed below). Among the spherules that do show sulfide, apparent sulfide contents are correlated with spherule sizes: the most sulfide occurs in the smallest spherules. This isconsistent with our interpretation that the sulfide forms shells around the spherules. Therefore, those spherules that appear largest are most likely to have been cut through their centers and to give the most representative bulk sulfide contents. Under the assumption, then. that all sphenties have approximately the same bulk com~sition, we use a bulk sulfide content of 12 vol.% (8 wt.%) for all spherules,

684

C. A. Goodrich and J. L. Berkiey

I%. 1. Back-scattered electron images of metallic spherules in olivine in unilites. Bar s&es = 10 microns. Samples are ALHA in A, B, C; KA82506 in D and F; and ALHA in E. Sphendes in A-E contain hypereutectic cohenite crystals (grey) and the mottled phase (lighter &zy with dark lamellae), In A and C the contfirst is set so that sulfxies are black and s&de rims are indisting~~b~e from surrounding &vine. In B, D, and E sulfide rims and inclusions are dark grcy. The spherule in F does not show large cohenite crystals, but has a cohenite rim.

Primary magmatic C in ureilites ALH82 130. No spherules in ALHA enough for quantitative analysis.

68.s 19 were large

Cohenite Carbon contents measured for the large cohenite crystals range from 6.4-6.9 wt.%, within 4% relative of the carbon content of stoichiometric cohenite (6.67 wt.%). No other carbide has a similar carbon content. Haxonite, (Fe&), which is the most commonly occuring carbide besides cohenite, contains 5.4 wt.% C. Also, haxonite is cubic, unlike the carbide observed here, and has higher Ni contents (SCOTT, 197 1). Thus, we are confident that the carbide observed here is cohenite. The large cohenite crystals contain 1.1-2.5 wt.% Ni, 0.3-0.5s Co, and minor amounts (in order ofdecreasing abundance) of Cr, Si, and P (Table 1). These abundances are similar to those previously reported for meteoritic and terrestrial cohenite (BREM, 1967; LovERING, 1964; GOODRICH and BIRD, 1985). Nickel contents of cohenite appear to decrease with increasing mg of the ureilite, from -2.3 in ALHA (mg 77.7) to - 1.4 in ALHA 130 (mg94.9). In two spherules in ALH82 130 (numbers 1 and 3 in Table 1) the Cr contents of the cohenite (1.4 and 0.7%) are significantly higher than in other spherules. Mottled phase

FIG. 1. (Continued)

PHASE COMPOSITIONS

Table I gives electron microprobe analyses of cohenite, the mottled phase, and sulfides from 17 spherules in ALHA78262, PCA82506, ALHA77257, and

Focused (point) beam microprobe analyses of the mottled areas generally give variable results as the beam is moved from one spot to the next, which suggests that they are polyphase intergrowths. Carbon contents shown by 14 point analyses vary from 0.6 to 4.6 wt.%, and average 1.8 wt.% (Table 1). This variation is consistent with our interpretation that the submicron lamellar phase that appears darker in BE1 of the mottled phase is cohenite, and that the areas which appear brighter are low-C metal. Variations in C content could result from different ratios of cohenite to metal in the analyzed area. Broad beam analyses of the mottled phase show less variation and give C contents of l-2% (Table I). Nickel contents shown by the 14 point analyses of the mottled phase vary from 5-l 1 wt.%. ALH82130 shows lower values (6-7s) than the other ureilites (generally 8-l l%, average 9%). Variations in Ni content of the mottled phase (within each spherule) could result from variations in the amount of cohenite in the analyzed area (in general, cohenite will have a lower Ni content than coexisting Fe-Ni metal). However, in some cases (see spherule number 1 in ALHA77257; Table 1) analyses from one spherule show Ni variation as great as 5- 11% without any variation in C content. Such variation could result from zoning of Ni in the Fe-Ni phase, or from the presence of distinct high-Ni (y) and low-Ni (CI)phases. Broad beam analyses show less variation and Ni contents around the average of the point analyses (Table 1).

686

C.

A. Goodrich and J. L. Berkley

Phosphorus, Si, and Co contents of the mottled phase are higher than in cohenite, whereas the Cr content is lower. This is similar to partitioning of these elements between cohenite and coexisting y-iron observed in natural terrestrial occurrences of cohenite (GOODRICH and BIRD, 1985) and in commercial FeC alloys (BAIN, 1939). Occasional high values of P in analyses of the mottled phase or cohenite may be due to small inclusions of Fe3P (~hrei~~ite) or Fe*P, but are not resolvable with the electron beam or distinguishable in BEI or optical images. These analyses were left out of averages reported in Table 1. Microprobe analyses of the spherules that do not contain large cohenite crystals (for example, spherule number 3 in PCA82506 and spherules number 4 and number 6 in ALHA77257; Table 1) show that they are not homogeneous metal. Point and broad beam analyses show variation in C content from OS-3.0%. These variations may be due to the presence of tiny f
This composition represents over 75% of the spherules studied. However, the spherules that do not contain large cohenite crystals have lower bulk carbon contents. Broad beam analyses of the metal in these Sphentles show carbon contents of 0.5-30/o, which are representative of the bulk compositions. In the few cases where there are narrow cohenite rims (Fig. IF) the bulk C contents are higher than those ofthe metal. The fact that the Ni contents of metal in these spheruies (5-6%) are the same as the bulk Ni contents (rather than those of the mottled phase) for spherules with large cohenite crystals, is consistent with the true (rather than just apparent) absence of large cohenite crystals. It seems likely that all spherules have similar bulk Ni contents. Two spherules in ALH82 130 (number 1and 3, Table 1) probably contain no sulfide (that is, the apparent absence of sulfide is not just a result of unrepresentative sectioning). The metal (cohenite pius mottled phase) in these spherules has higher bulk Cr contents (0.5 and 1%) than does the metal in snide-~ng spherules in ALH82130 (0.2%). This is consistent with the absence of S in these two spherules, because Cr is preferentially partitioned into sulfide over metal,

Sulfides

Phase relations of cohenite-bearing spherurulescrystallizationof Fe-C liquids

The sulfide in the spherules is mainly troilite, with 0.1-0.26 Ni and variable Cr contents, up to 0.7%. One spherule (number 3 in PCA82506; see Table I) has sulfide with 2% Ni and 2% Cr. The sulfides in spheruies in ALH82 130 are Cr-rich. In some cases only one suifide, with up to 7% Cr occurs; in others, high-Q (25%) and low-Cr (4%) sulfides coexist fspheruIe number 2 in ALH82 130; Table 1). The distribution of minor elements between the bulk metal (cohenite plus mottled phase) and coexisting sulfide (Table 1) is consistent with known partitioning behavior: Ni, Co, Si, and P are preferentially partitioned into metal, and Cr is preferentially partitioned into sulfide. Bulk compositions By combining the results of lint-coun~ng (above) with microprobe analyses we calcuiated bulk compositions for the metal part of eleven of the spherules that contain large cohenite crystals (Table I). They show Ni contents of 3-7% and C contents of 4-S%. There is a decrease in Ni content with increasing mg. ALHA (mg 77.7) has 6-7% Ni and ALH82 130 (mg 94.9) has 3-54 Ni. In addition, the bulk Cr content increases with increasing mg, from 0.03-0.05 in ALHA to 0.2- 1% in ALH82 130. Our best estimate of the average bulk composition of the cohenitebearing spherules (metal plus sulfide) in the Fe-S-C system (Ni included with Fe, other elements ignored) is 4.1 wt.% C and 2.9 wt.% S (or 61 wt.% Fe& 8 wt.% FeS).

DlSCUSSlON

Cohenite has not previously been recognized in ureilites. Cohenite occurs in iron meteorites (BRETT. 1967; Scorr, 197 1, 1972; B~CHWALD, 1975) and less commonly in chondrites (TAYLOR et al., 1981). It has also been identified in lunar soils (GOLDSTEINet ai, 1976) and in rare terrestrial occurrences such as the basalts of K.assel, Germany (IRMER, 1927) and Disko Island, Greenland (CLARKE and PEDERSON, 1976: GOODRICHand BIRD, 1985). Cohenite is also a wellknown phase in commercial iron-carbon alloys where it is known as cementite. This is the first reported occurrence of cohenite in achondrites. In the Fe-C system cohenite is thermodynamically me&stable relative to the assemblage iron + graphite at low pressure, but is stable at higher pressures (RINGWOOD, 1960). At high tem~ratures its me&stability is slight. At 1027°C the free energy of formation of cohenite from C-saturated iron pIus graphite is +54 cal/mole. At both higher and lower temperatures this number increases, for example to +66 Cal/mole at 1127V.Z and +88 Cal/mole at 927°C (DARKEN and GURRY, 195 I). Lower limits calculated for the pressures required to stabilize cohenite are 664 bars ai 1027’C and 810 bars at 1127’C (GOODRICH, 1983), These are consistent with experimental data that show that in the eutectic region (- 115O’C) cohenite is stable at 1 kbar (KORSUNSKAYAef al., 1975). It is not iikely that pressures during ureilite formation were high enough to stabilize cohenite. If / O2 was controlled by C redox equilibria, which are strongly pressure depen-

681

Primary magmatic C in ureilites dent (SATO et al., 1973), then the maximum

pressure during ureilite formation was only - 100 bars (GOOD RICH and BERKLEY, 1985b). Similar pressures were estimated by BERKLEY et al. ( 1980) and BERKLEY and JONES (1982). Nevertheless, even in the presence of graphite nuclei, cohenite commonly nucleates in preference to graphite at low pressures because of more favorable nucleation kinetics, both in iron-carbon liquids (HILLERT and RAO, 1968) and during subsolidus exsolution (SCOTT, 1972). Therefore, cohenite is the C-rich phase expected to form in an Fe-C alloy and its presence is not a pressure indicator. The bulk composition of the spherules can be adequately represented in the system Fe-Ni-S-C. Using the subsystems Fe-S-C (Fig. 2) and Fe-Ni-C (Fig. 3) we can predict phase relations for alloys with this bulk composition. The minor elements Co, Si, and Cr are present in such small amounts that they would have had no effect on phase relations and only minor effects on transformation temperatures. Phosphorus appears to have been present in sufficient quantity to produce phosphide in at least some of the spherules. However, even in these, the bulk P content is sufficiently low (-0.2%) that it would not have affected liquidus phase relations in the Fe-C system (BENEDICKS and LOFQUIST, 193 l), although it probably lowers transformation temperatures somewhat. Figure 2 showsliquidus relations in the system Fe-S-C. The diagram is from VOGELand RITZAU( 193 1) and differs only slightly from that of BENEDICKSand LOFQUIS~(I 93 1). The

minimum temperature tieline for two coexisting liquids is given as I 100°C by VOGELand RITZAUand 1125’C by BENEDICKS and L~FQUIST.The bulk composition of the spherules is shown on this diagram (asterisk). This composition plots within the primary phase field of cohenite. Unfortunately, transformation temperatures are poorly known for this system, especially for C-rich compositions such as this one. Nevertheless, we can see that this bulk composition would have been completely molten above some temperature, TX, probably not much greater than 1100 or 1125X and at TXwould have produced two immiscible liquids: a dominantly Fe-C liquid and a dominantly Fe-S liquid. This is consistent with textural relations between sulfide and the metal-cohenite assemblage in the spherules: the rounded boundaries between sulfide rims and the metal-cohenite assemblages are typical of boundaries between two liquids. The two liquids coexisting at TXare indicated by 4 and b in Fig. 2. With decreasing temperature k moves toward F, along &F, , and Ls moves toward the FeS-FeSC cotectic line, both precipitating cohenite. F, is the eutectic point for the Fe-C liquid: at this point cohenite, yiron, and Fe-S liquid coexist. When the composition of b reaches the FeSFe9C cotectic, troilite begins to crystallize along with cohenite (from the Fe-S liquid) and the composition of the remaining liquid moves to E (975°C). at which point yiron crystallizes along with cohenite and troilite in a ternary eutectic. The small amount of cohenite that precipitates from the Fe-S liquid could easily segregate from the remaining liquid to join cohenite precipitated from the Fe-C liquid, and the small amount of additional Fe-S liquid exsolved from the FeC liquid at point F, would join the remainder of the original Fe-S liquid, so that Fe-C phases would remain separated from Fe-S phases. The two liquids wouid tend to separate because of the difference in their densities. The eutectic temperature of the Fe-C liquid (1100 or 1125’C) in the Fe-S-C system is slightly lower than in the pure Fe-C system (- 1150°C; CHIPMAN,1973). However, the presence of Ni raises all temperatures. Figure 3 shows a liquidus

Fe& (6.67 %C)

Fe

/ /

97&C

988X

:Z&%SI

FIG. 2. Fe-rich portion of the Fe-S-C liquidus diagram, after VOGEL and RITZAU(193l), showing field of 24iquid immiscibility with tielines (dashed) giving compositions of coexisting liquids. The minimum temperature tieline for two liquids (F, - F2) is given as 1100°C by VOGELand RITZAU(1931) and as 1125°C by BENEDICKSand L~FQUIST(193 1). Asterisk indicates the estimated hulk composition for large cohenitebearing spherules in ureilites. At some temperature TX, above 1100 or 1125”C, this composition would consist of immiscible Fe-C and Fe-S liquids, represented by & and L.s.

C. A. Goodrich and J. L. BerWey

688

5-7

1600

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wt%Ni

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1400

I

1 I 1200 1 L-i I

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8 3 wt

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FIG. 3. Fe-C liquidus diagram for bulk compositions containing 5-7 wt.% Ni, constructed from the 1-e. Ni-C liquidus surface diagram of EU_lOT’r(1973). The heavy lines represent the y-iron and cohenite liquid]. The dashed line is the graphite liquidus. The cohenite liquidus was estimated by comparison with the pure Fe-C system (CHIPMAN, 1973).Bulk compositions of the majority of the spherules are indicated by the shaded area. They are hypereutectic alloys that crystallize cohenite on the liquidus.

section of the system Fe-Ni-C for bulk Ni contents (5-7 wt.%) appropriate to the spherules. This diagram was constructed from the Fe-N&C ~q~dus surface diagram of ELtIO’rr f 1973). The cohenite Iiquidus was estimated by comparison with the pure Fe-C system (CHIPMAN, 1973). The eutectic tempemture for a bulk composition containing S-7% Ni is about 25OC higher than in the pure Fe-C system, so the melting point of cohenite for this bulk composition was estimated to be about 25°C higher than that given by CHIPMAN (1973) for the pure Fe-C system (that temperature is in itself an estimate; see CHIPMAN, 1972). In any case, it is unlikely that the very flat shape of the cohenite liquidus would change much with the addition of 5-7% Ni to the system. At such low Ni contents there is probably complete solid solution of N&C in Fe&. We can use this diagram (Fig. 3) to follow the cry&&i&on history of the Fe-C liquid coexisting with Fe-S liquid. The calculated bulk carbon contents ofthe metal part ofthe spherules are represented by the shaded region. Ailoys of this composition are hy~reut~c. with a liquidus temperature of 1225’C. Cohenite is the proeutectic or Iiquidus phase. At - il75”C y-iron crystallizes with cohenite at the eutectic. The cohenite-mottled phase assemblage of the spherules is consistent with this crystallization sequence, with the large cohenite crystals being proeutectic. In some spherules (Fig. 1E) the cohenite shows a branching structure which is typical of proeutectic cohenite (HILLERT and STEINHAUSER, 1960). Cohenite that formed at the eutectic may have plated onto proeutectic cohenite plates so that it is not texturally distinguishable from proeutectic cohenite. This is common in commercial hypereutectic Fe-C alloys (HILLERT and STEINHAUSER, 1960). The mottled phase would then represent primarily the y-iron that coexisted with cohenite at the eutectic. The complete phase diagram for this system is not known, so it is not possible to predict the Ni and C contents of the y-iron, nor the Ni content of the proeutectic or eutectic cohenite. However, data for the solubifity of graphite in Fe-Ni y-iron (ELLIOTT, 1973) shows that if the -y-iron contained 7-9% Ni (the bulk composition of the mottled phase). at the eutectic temperature (- 1175“C), it would have contained 1.7-1.8% C, which is consistent with the average C content measured for the mottled phase (- I .8%). Further phase transformations occur at subsolidus temperatures. Figure 4 (from ROMIG and GOLDSIWN, 1978) shows the Fe-rich comer of the Fe-N&C system at 730°C and 600°C. by 730°C r-iron that formed at the eutectic has exsolved into a two-phase assemblage: r-iron + cohenite. By 600°C the two-phase assembiage has been replaced by a three-phase assemblage: y-iron ( - 11%Ni) + a-iron (- 5.5% Ni) + cohenite. The mottled are.as, which cleariy contain cohenite lamellae, and also show variation in Ni content f&m 5- 11J, seem likely to be this assemblage. Schreibersite would have exsolved from

y-iron (into which most of the bulk P had been partitioned) at some temperature. probably below 700°C. The maximum ~lubility of P in carbon-saturated y-iron is 0.25% at 700900°C (BENEDICKSand LOFQUIST. 193 t f. If discrete LYand y-iron phases are present, they provide information on coobog rate. NARAYAN and GOLDSTEIN (1985) produced intragranular r-iron precipitates in Fe-N&P alloys (P contents similar to those of the spherules) cooled at S”C/day through 800575’C. The presence of P significantly raises the diffusion

(Fe.Nt)& '\ 6\,

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2

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i

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wt % N , FIG. 4. Fe-rich portion of the Fe-Ni-C phase diagram at 730°C and 6OO”C, from ROMIG and GOLDSTEINf 1978). Asterisk represents the average composition of the mottled phase in the Iarge cohenite-bearing spherules in ureilites. At 730°C this com~tion consists of y-iron + Cohen&e. At 600°C its com~ition consists of the three phase assemblage r-iron + a-iron + cohenite.

Primary magmatic C in ureilites rates of Ni in -r-iron, which is the limiting factor in growth of a-iron. It is possible that C would also increase Ni diffusion rates, so that exsolution of d-iron might have occurred in the spherules at higher cooling rates than Sac/day. This rate is nevertheless much slower than the lO”C/hour estimated by MWAMOTO etai. (1985)for cooling of ureilites through 12W 900°C. based on the width of reduction rims, and the 3-2O”C/ hour estimated by MORI and TAKEDA (1983)for the same temperature range, based on rni~~st~ctur~ features of pigeonite. However, a lowering of the cooling rate is not inconsistent with excavation followedby shallow burial in an ejecta

blanket. The phase assemblage and textures of most of the spherules (those with large cohenite crystals) are thus those expected for crystallization of an Fe-Ni-S-C liquid that separated into an Fe(Ni)-S liquid and a hypereutectic Fe(Ni)-C liquid. If C had diffused late into solid spherules of Fe-Ni metal and sulfide, the observed hypereutectic and eutectic textures would not have resulted. The C must have been a primary component of the metal. The presence of the spherules as round or ovoid inclusions within olivine suggests that they were droplets of immiscible metallic liquid within the silicate magma from which the olivine crystallized. The fact that the Fe-C assemblage and the sulfide are contained together in spherules within silicates indicates that the metallic liquid was trapped before it separated into two liquids, that is, at a temperature above TX (Fig. 2); otherwise, the two liquids would have separated due to density differences and would have been trapped separately. This constrains TX to be less than the crystallization temperature of olivine. The greater abundance of spherules in olivine than in pyroxene indicates that pyroxene crystallized later than oiivine, after most of the metallic liquid droplets had been trapped in olivine. This supports the contention of BERKLEY (I985) that pigeonite is an intercumulus rather than cumulus phase. Only two of the spheruies observed probably contain no sulfide (spherules numbers 1 and 3, ALH82 130, described above). Because these spherules are inclusions in pigeonite, which crystallized later than olivine (BERKLEYef al., 1985), it is plausible that the sulfide separated from the spheruies before they were trapped. The textures of the low-C spherules (extremely finegrained cohenite-Mets inter~o~hs with occasional cohenite rims) are consistent with those expected for crystallization of hypoeutectic Fe-C alloys (containing ~3.8% C, the composition at the eutectic). The liquidus phase for such compositions is y-iron (Fig. 3) with C in solid solution. For bulk carbon contents between -2% and 3.8%, a eutectic iron-cohenite intergrowth forms. Below the eutectic temperature, cohenite exsolves from y-iron (see CHIPMAN, 1973), often forming rims around the liquidus y-iron grains (AARONSON, 1962). Graphite in the ureilite parent magma~s~ From Fig. 3 we can see that metallic liquids with 45 wt.% C are saturated with respect to graphite at tem-

689

peratures appropriate for a mafic silicate magma. 12001250°C. If the stable situation prevailed these metallic liquids would exsoive graphite. However, graphite does not exsolve readily from Fe-C liquids because of slow nucleation kinetics, and graphite-oversaturated metallic liquids can exist. Nevertheless, preexisting graphite would stably coexist at these temperatures with an Fe-C liquid containing 4-5% C. Such a liquid would then precipitate metastable cohenite at lower temperatures (see cohenite liquidus; Fig. 3). The presence of graphite nuclei would not necessarily prevent metastable formation ofcohenite (HILLERTand RAO, 1968). It is, therefore, likely that the Fe-C liquids represented by the spherules coexisted with graphite within silicate magma, and that the silicate magma was also C-saturated. This is consistent with the presence of graphite as inclusions in oiivine cores in many ureiiites (BERKLEY and JONES, 1982) and supports the contention that the graphite in ureilites was present during the magmatic stage (~ERIUEY et al., 1980; BERKLEYand JONES, 1982). In a carbon-~tumted magma, carbon must have been the dominant control on redox conditions (BERKLEYand JONES, 1982; BERKLEYef al.. 1980). GOODRICHand BERKLEY(1985a,b) argue that minor element trends in a suite of eight ureilites (including all those that contain cohenite-bearing spherules) are the product of progressive reduction of a silicate magma. according to the general reaction C (graphite) + Fe0 = Fe + CO. if the system remained carbonsaturated (that is, there was always excess graphite), then the Fe metal produced by this reaction would become C-saturated. This metal is represented by the spherules. The model of GOODRKH and BERKLEY (1985b) predicts that the Ni content of metal produced by the reduction reaction would have decreased as the reaction proceeded. The decrease in bulk Ni contents of the spherules with increasing mg of the host ureilite confirms this prediction. The attendant increase in bulk Cr of the spherules may also be consistent with a higher degree of reduction. The low-C spherules do not represent C-saturated alloys. Alloys such as these could have been produced if, locally, graphite was not present. Carbon could have been removed from the C-saturated metal by the reaction C (in metal) + Fe0 = Fe i- CO if thef02 remained such that the reduction of Fe0 continued, but graphite was not present to be the reductant. GOOERICH and BIRD (I 985) have proposed this as an expianation for the presence of both high-C and low-C metal in Disko Island basalts. WLOTZKA ( 1972) argued that the occurrence of reduced rims on the silicates is evidence that graphite was introduced late into the silicate assemblage, because they show that graphite was not in equilibrium with core silicate compositions. However, C-CO/CO2 equilibria are strongly pressure dependent, and buffer an JO, that is lower at lower pressures (SATO et al., 1973). Graphite would have been in ~uilib~um with olivine core compositions for ALHA 19 at pressures

690

C. A. Goodrich and J. L. BerMey

of - IO0 bars (BERKLEYand JONES, 1982; GWDRICH and BERKLEY,1985b). At lower pressures the olivine would be reduced. GOODRICH and BERKLEY(1985a.b) have therefore proposed that the reduction rims formed during sudden excavation of the ureilites by impact, and their presence does not require late introduction of carbon. ~ntergr~nul~r metal in ureiiiies

One of the questions that remains unanswered is the origin of the inte~anu~~ metal and its relation to the spherule metal in ureilites. The composition of intergranuiar metal and associated sulfides varies considerably both within and among ureilites, especialfy in Ni, Si, and Cr contents (see BERKLEY ef al.. 1980; BERKLEYand JONES, 1982; BERKLEY, 1985). If the intergranular metal was primary metal trapped between accumulating olivine crystals along with intercumulus silicate liquid, then it ought to be similar in composition to the spherules. However, the intergranular metal is not high-C metal and is not polyphase. There is also no systematic relation between minor element contents of the spherules and of intergranular metal in the same ureilite. BERKLEYand JONES( I982) suggested that intergranular graphite precipitated from a C-saturated metahic phase, now represented by the intergranular metal. However, the ratio of intergranular metal to graphite is variable, and in most ureilites is too low. A homogeneous metallic phase reconstructed from the graphite and metal would have far more C than the spherules and would have an extremely high melting point. Therefore, most of the graphite could not have exsolved from intergranular metal. In addition, carbon in the intergranular metallic phase should have exsolved as cohenite rather than graphite if the metallic phase. had evolved under the same conditions as the spherules. It seems more iikely that all the intergranular graphite existed as a solid phase in the silicate liquid and was trapped between accumulating silicate crystals, rather than exsolving from a metallic phase. The intergranular metal may once have been C-saturated and similar in composition to the spherules. Carbon may have been removed from the metal during the secondary reduction reaction that caused the formation of reduction rims on the silicates. Locally, where intergranular graphite was not present, carbon in intergranular meta could have reacted with the silicates and have been lost as CO. Spherules of C-saturated metal that were completely enclosed in olivine may not have reacted similarly because they were not in contact with the gas phase, The minor element content of the intergranular metal may aIso have been modified during the secondary reduction reaction. Silicon, Cr, and Ni in olivine would have been reduced and could have diffused into intergranular metal. The inhomogeneous distribution of these elements could be a result ofquenching during the reduction reaction. However, in the presence of intergranular graphite, metal should remain C-saturated (graphite would react with the sificates before C in the metal would), and it

appears that intergranular graphite was abundant. The origin of the intergranular metal, therefore. remains unresolved. Origin ofgraphite and noble gases in ureiilte,

It is difficult to reconcile the data presented here with late-stage introduction of graphite into a solid silicate assemblage (e.g. WASSON el al., 19% BOYNTON et al., 1976). The conclusion that the C was present before the shock event was aiso reached by WACKEK f 1985), who observed that degree of shock does not correlate with C content in ureihtes. The data presented here shows clearly that graphite was present when olivine crystallized. The graphite could be a refractory residue from the solid source material that melted to form the ureilite parent magma(s). This would require a large degree of partial melting. Alternatively, the graphite might have crystallized from a C-saturated magma. The solubility of elemental carbon in mafic magmas is quite low and Iittle graphite could be produced by crystallization. Therefore, accumulation of graphite would be required to produce the volume of graphite observed in ureilites. A third possibiIity is that graphite was assimiIated from country rock by ureilite parent magmas. In any case, the parent body must have been rich in carbon, unlike that of any other achondrites. The only known primitive materials with sufficient carbon to have been the precursors of ureihtes are carbonaceous chondrites. Our results have direct bearing on the question of whether noble gases in ureilites are primary and if so how they could have survived magmatic processes (BEGEMANN and OTT, 1983; OTT ei ai,, 1985: WACKER, 1985). Noble gases in ureilites are p~n~i~ly associated with carbon. BEGEMA~~ and OTT (1985) proposed that the gases were introduced along with graphite, in a shock event. However, our work has shown that the graphite is primary, and recently WACKER (1985) has shown that neither noble gas abundances nor C contents of ureilites are correlated with shock. It therefore seems clear that neither carbon nor noble gases in ureilites were introduced by shock, and that both were derived from the material that melted to produce the ureilite parent magma(s). Acknow~edge~e~f~-We wish to thank E. R. D. %ott, G. 9. Taylor, at&K. KeiI for helpful discussions,and GeorgeConrad

for mainlining the UNM micronrobe facilities. MelnfuI reviews by J. H.‘jones, C. Narayan, and J. F. Wacker.are ag preciated. This research was supported by NASA grants NAG 9-30 (to K. Keii) and NAG 9-82 (to J. L. Berkley). Editorial handling: E. J. Olsen

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