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Classification of and elemental fractionation among ureilites
Geochimica
et Cosmochimics.
Acta, 1976, Vol. 40, pp. 1449 to 1458. Pergamon
Press. Printed
in Great Britain
Classification of and elemental fractionation among ureilites JOHN T. WATSON,CHEN-LIN CHOU,* RICHARDW. P~~ILIPA. BAEDECKER$
BILD~
and
Institute of Geophysics and Planetary Physics, Departments of Chemistry and Geophysics and Space Physics, University of California, Los Angeles, California 90024, U.S.A. (Received 14 October 1975; accepted in revisedform 4 June 1976) Abstract-Concentrations of Ni, Zn, Ga, Ge, Cd, In, Ir and Au in five ureilites can be combined with petrographic evidence to yield a well-defined suite extending from Goalpara (heavily shocked, low Ir concentration, low Ir/Ni ratio) through Have@ Dyalpur, Novo-Urei to Kenna (moderately shocked, high Ir concentration, high Ir/Ni ratio). Arguments are presented indicating that this suite represents the sampling of a vertical section within the ureilitic parent body. The large range in Ir/Ni and Ir/Au ratios indicates greater efficiency of extraction of primitive, refractory metal in the Goalpara region than in the Kenna region, and implies that higher maximum temperatures prevailed in the former during the production of ureilitic ultramaIic silicates by a partial melting process. A major impact event injected a deposit of C-rich material into the ultramafic silicates. This C-rich material had a moderately high content of metal; there is no direct evidence that it contained volatiles other than the rare gases. High Ca contents of the ferromagnesian minerals indicate that the ultramafics were hot at the time the injection occurred; the zoning of these mineral grains also indicates high temperatures (ea. 1400 K) and low pressures (C 10 atm) such that reaction between C and Fe2Si04 could occur, but that cooling occurred too quickly to allow complete equilibration. The ureilitic C-rich material appears to represent an important type of primitive material. Two siderophile-rich components are necessary to explain the relative siderophile trends in ureilites. We interpret the high-Ir component to be a refractory nebular condensate or residue that was retained during the partial melting event. The low-Ir component, which roughly resembles El chondrite siderophiles, is attributed to metal injected together with the vein material.
INTRODUGTION
THE SEVEN ureilites are unique meteorites showing some features typical of primitive solar-system matter (e.g. abundant planetary-type rare gases) and others typical of igneously ~fferentiat~ matter (e.g. silicates consist mainly of olivine and have vanishingly small concentrations of feldspar). VMVYKIN(1970) reviewed ureilite properties, and in December 1972 a series of 11 papers dealing with the Haverij fall was published in Meteoritics. Figure 1 illustrates the features common to most ureilites. The most abundant phase is olivine, generally present in polyciystalline domains having dimensions of about 0.5-l mm. The only other abundant silicate is clinopyroxene; this is pigeonite in all ureilites except Havero. Long, oriented cracks and voids are present in all ureilites except Kenna. Dark regions cont~ning graphite, diamond and FeNi metal border veins except where the latter are adjacent to pyroxene, and also separate olivine domains where no cavities are resolvable. The cracks, diamonds and * Present address: Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 1Al. t Present address: Max-Planck Institut ftlr Kernphysik, 69 Heidelberg, Germany. 1 Present address: U.S. Geological Survey, Reston, VA 22092, U.S.A.
polycrystallinity of the olivine appear to have resulted from shock. A number of authors (e.g. MASON, 1962; ANDERS, 1964; MUELLW, 1969; VWVYKIN, 1970) have suggested a close link between the ureilites and the carbonaceous chondrites. WLOTZKA (1972) proposed a relationship with L-group chondrites on the basis of similar Fe/(Fe + Mg) ratios in the ferromagnesian silicates. Our studies.(WAssoN et al., 1973) were initiated to gather data on volatile and siderophile trace elements which could be used to search for genetic ties between ureilites and known chon~it~ and to aid in the inv~tigation of the fractionation processes that occurred during the formation of ureilites. MUELLER(1969) and VDOVYKIN(1970) divided the ureilites into two types (I, type specimen Novo-Urei and II, type specimen Goalpara). The first type is less strongly shocked and has coarser olivine grains, a more common twinning in the clinopyroxene, a metal distribution dominated by net-like structures rather than plates, and smaller diamond-graphite aggregates. Two type-1 ureilites contained substantially larger amounts of rare gases than type II Goalpara (MAZORet al., 1970), and data for siderophile Ni and Co QVmc, 1969; Swrrr et at., 1972) show a similar trend. Thus, our secondary goal was to investigate whether representative ureilites showed the same trends for other siderophile (Ge, Ir, Au) and highly
1449
J. T. WASSONet al.
1450 volatile (Cd, In) elements, petrologic classification.
thus testing the chemical-
EXPERIMENTAL
The sources of the ureilite specimens arc listed in Table 1. Three of the meteorites are observed falls, and the fresh appearance of Goalpara suggests that it also was recovered soon after an unrecorded fall. Kenna is slightly weathered (BERKLEY et al., 1976). Our Kenna sample had wire sawed surfaces which we cleaned with an Al*O, wheel followed by washing in acetone. Duplicate samples (triplicate for Have@ consisting of 0.124.33 g chips were crushed in a steel percussion mortar and analyzed in separate neutron activation runs. Radiochemical neutron-activation analysis (RNAA) data on Kenna were determined by a recently developed procedure which allowed Ru to be added to our former suite of 8 trace elements; a description of this procedure is in preparation. RNAA determinations of Ni, Zn, Ga, Ge, Cd, In, Ir and Au in the remaining ureilites were by the procedures described in BAEDECKER et al. (1973) and earlier papers cited there. The accuracy and precision for Ru has not been determined. Precision for the other RNAA-determined elements is discussed in BAEDECKER et al. (1974). Samples were dissolved for RNAA by fusion in Na,O,. Since such a fusion does not dissolve diamonds, our data must be treated as lower limits for elements having appreciable abundances in the diamonds. It is probable that this fraction is negligible, since the four elements determined by both INAA and RNAA in Kenna are essentially the same with the exception of Ni. The Ni difference may be spurious since different flux monitors were used for the two procedures. Kenna samples were analyzed by instrumental neutron activation analysis (INNA) prior to the RNAA determinations. A discussion of our INAA techniques and data precision is given in BOYNTONet al. (1975). A 4034 correction for Si interference in the Al determination results in an unusually high uncertainty of about *20%. A comparison of our data with those of BINZ et al. (1975), BOYNTON et al. (1976), GILLUM et al. (1972), SCHMITT et al. (1972), WKNKE.~~al. (1972) and WIIK (1972) shows generally good agreeement, i.e. to within about + 10%. Kenna INAA Ni and Ir values determined by BOYNTON
et al. (1976) are about 40”/, lower than our data; this probably indicates a systematic difference in the metal contents of our respective samples. If the BOYNTON et al. (1976) Au value in Kenna is increased 40%, one discovers that their Au values in three ureilites average about 507” greater than ours. The two research groups plan further joint investigations of this discrepancy. With these exceptions it appears that systematic errors in our data are generally less than 10%. Inadequate comparison data are available for As and Ru. RESULTS
Data on 8 elements in five ureilites are listed in Table 1. Mean concentrations of sixteen elements in Kenna and 9 elements in Haverii are listed in Table 2; four of these INAA-determined elements were also determined by RNAA. Good agreement among replicate RNAA and INAA determinations of Ni, Zn, Ir and Au indicates that the minerals in which these elements are concentrated were uniformly distributed in our O.lLO.3g samples. The other elements show more scatter. Gallium and Ge are elements we determine with high precision; the range observed in HaverB Ga and Ge and in Dyalpur Ga therefore appears to represent real sampling variations. The Cd value in the second Havera replicate is much higher than those found in all other ureilite samples: a recheck of the data offers no evidence that this result is of lower quality than the others, but we believe nonetheless that less weight should be attached to it. The range in In is only a factor of 3 if the first Haverij and Novo-Urei replicates are ignored. It is puzzling that individual Ga, Ge. In or Cd determinations rarely correlate. Either these elements are sited in different, heterogeneously distributed phases, or experimental errors have affected some of our determinations. In addition to the data listed in Tables 1 and 2 we can report the following INAA data on one
Table 1. Concentrations of volatile and siderophile elements in ureilitic meteorites. Underlined values are of lower quality and were assigned l/2 weight in the calculation of the mean Museum*
Spec. No. Dyalpur
Goalpara
BM 51185 Meall
1.39 1.40 1.40
277 265 271
1.2 2.5 1.9
a.6
8.4
41 31 fB
1.7 2.1 1.9
205 24? 226
3II 24 27
SI 5623 Mean
0.75 0.73 0.74
R6 83 84
1.2 1.1 1.2
2.8 3.0 2.9
13 15 14
0.85 1.59 1.2
55 56 56
i3 15 14
Meall
1.07 1.03 1.12 1.08
151 149 123 141
2.8 1.0 1.6 1.8
16.6 38 2.0 228 7.2 T? 8.6 68
5.0 CT0 CO.71 1.6
206 197 195 199
22 27 1!? 23
Meall
1.74 1.64 1.69
164 199 181
3.1 2.5 2.8
29.5 26.1 27.8
47 50 49
1.4 1.5 1.4
760 736 748
53 38 45
Meall
1.59 1.41 1.50
276 256 266
2.4 2.0 2.2
b.6 8.6 7.6
23 29 26
6.2 TX? 2.5
449 372 410
II ?" 36
Havert)
wur
KWllla
AML H159
Nova-Urei GML
8.2
* Sources of the samples abbreviated as follows: AML, American Meteorite Laboratory, Denver; BM, British Museum, London; GML, Mining Museum, Leningrad; SI, Smithsonian Institution, Washington; UTur, University of Turku, Finland.
:,
ye.:
z,
.>,::a; .%:._
:L:..;~_-:‘~o:-i .i
y,
_
--e
:-
,n
.~ *
Fig. 1. Transmitted light photograph of the Goalpara ureilite showing the characteristic ureilitic features: light gray polycrystalline olivine domains, medium gray pigeonite grains with sharply defined borders, C- and metal-rich opaque regions, and white cavities, most of which are oriented NE-SW in this section. The cavities often border or enclose pigeonite grains. British Museum thin section 51187; Smithsonian Institution photo.
1450
1451
Classification of and elemental fractionation among ureilites Table 2. Concentrations of fifteen elements determined by INAA and one (Ru) determined by RNAA in the Kenna ureilite and MAA con~tratio~ of 9 elements in one sample of HaverG Kenna Na (an/s) 438, 460
Haverti 240
K.%llM
Have&l
Fe (nvgfg)
178, 174
163 121
Mg Cmgfg)
202, 206
-
CO (w/e)
200,
A.1 (mgfg)
0.68:
_
Ni (mgfg)
1.92,
Ca (mgfg)
7.4,
_
2n (ugfg)
164, 1.52
-
SC (w/g)
8.7, 9.4
7.1
as &g/g)
0.56,
-
v
(UP/PI
82,
0
(mgfgl
5.6.
HII (mgfgl
0.70* 7.2
85
2.75,
5.8 2.68
190 1.81
1.27
0.44
RI2 iuglg1
L).Ee+
5.1
Ir (ugfg)
0.72, 0.69
2.84
Au (nU/k?) 48, 47
0.24 20
* Uncertainty _ + 20%. t Uncertainty _ f 15%.
sample of Dyalpur: (+0.3, -0.7) mg/g.
Na, 255clg/g;
and
Mn
3.5
CLASSIFICATION OF UREILITES We noted above that type-1 ureilites have appreciably higher rare gas and siderophile concentrations than type-II ureilites. Our siderophile data (Table 1) confirm these differences; Ni, Ge, Au and Ir concentrations are 2-7 x times greater in Novo-Urei and Dyalpur than in Goaipara. Volatile elements also tend to be higher in the former, but in some cases one replicate is an exception. MARVINand W~QD (1972) concluded that Havero was a type-11 ureilite on the basis of evidence for heavy shock. VDOVYKIN(1972) found the evidence ambiguous (e.g. metal shows a net-like distribution but occasional plates are also observed). NEUVONEN et al. (1972) found that Haverij “in many respects resembles Novo-Urei” but that the diamond graphite aggregates reach 2 mm in size, even larger than the 0.9mm observed in Goalpara. Our compositional data (Table 1) also lead to ambiguity; concentrations of siderophiles and volatiles except Ge (which has a larger error attached) are intermediate between those in Goalpara and Dyalpur or Novo-Urei, and fill the hiatus between the two types. These obse~ations suggest that we are not dealing with two distinct types, but with a suite of objects having monotonically varying properties. Since no hiatus exists, it appears necessary to discard the type designations, and define a ureilite’s relative position within the suite in terms of some other property. The petrographic properties of Kenna (~ERKLEY et al., 1976) are in many respects divergent from those in the suite defined by Goalpara and the 3 falls. Kenna is more compact; oriented fissures characteristic of the others are missing; it contains more pronounced metamorphic features (abundant triple junctions). It resembles Novo-Urei in that the olivine is coarse (up to 2mm) and diamond-graphite aggregates are small. Peak shock pressures apparently were rather low. Volatile elements concentrations in Kenna (Table
1) are generally comparable to those in Novo-Urei and Dyaipur; In and the rare gases are inte~~iate as in Have&i. Siderophilic element ~ncentrations (Table 1) are consistently high, indicating Kenna to be one extreme of the ureilitic spectrum. Thus chemically, as petrographically (BERKLEYet al., 1976), Kenna is an extreme member of the suite, suggesting that siderophile (and particularly Ir) concentrations are good parameters for locating an individual ureilite within the suite. Inspection of ureilite sections shows that r~domly chosen samples will occasionally have anomalous metal (and thus siderophilic element) concentrations. The metal content of the Kenna sample studied by BOYNTONet al. (1976) is lower than that of ours by - 1.4 x . Ureilitic metal varies greatly in composition, another source of inhomogeneity; Ni contents in metal in Haverij veins are typically -3”/ whereas those in fine metal in olivine are typically < ly/, (WLOTZKA,1972). As discussed below, there appear to be two different sources of primary metal. It seems likely that smaller variations will be observed in siderophile/siderophile ratios than in bulk concentrations of any one siderophile. Figure 2 is a plot of siderophile/Ni ratios relative to those in Cl chondrites, plotted in order of increasing mean element/Ni ratio. The relative constancy of Ir/Ni ratios is illustrate by the fact that ratios measured in Kenna by BOYNTON et ai. (1976) and by our group are nearly identical despite the difference in metal contents of our samples. A BRIEF REVIEW OF KNOWLEDGE
ABOUT
UREILITES
It will simplify the following discussion to briefly review the properties of the ureilites, and those c A Kenna I l Novo- Urei Dyalpur
l
I’
’
Au Fig. 2. ~iderop~le~i
I
I
I
I
,
Go Ge Co It ratios normalized to those in Cl
chondrites are generally within a factor of 2 of each other. The chief exception is the Ir/Ni ratio, which appears to be an excellent parameter for determining the placement of a ureilite within the petrographic-compositional suite. The sequence is continuous, and there is therefore no value in using type designations for ureilites. The large range in Ir/Ni ratios indicates two sources for the metal, one associated with the C-rich vein material, the other trapped in the ultramatic silicates.
1452
J. T. WASSONet ai
aspects of their formational history that are generally accepted. The dominant phases are olivine and clinopyroxene, generally pigeonite. Feldspar is rare. This mineralogy suggests that the ureilites represent an ultramafic layer from which a low-melting fraction has been removed by partial melting (RINGWOOD, (1960). The alternative that the ureilites might be a cumulate phase is inconsistent with observed rare earth patterns (BOYNTONet al., 1976). The dark, C-rich material occurring as veins in the ureilites is generally believed to have been introduced into the ultramafic silicates at a late stage. This conclusion is supported by observations showing that this material was transported along cracks ~LOTZKA, 1972). Since the C-rich material contains abundant planetary-type rare gas (WEBERet al., 1971; 1976) it cannot have been very hot for any extended period. Our evidence presented later agrees with earlier proposals (RINGWOOD,1960, ANDERS,1964) that there was appreciable metal in the material parental to the ultramafic silicates. and that some of this metal has been removed by partial melting. Since C is highly soluble in molten Fe-Ni, primary C would have been removed in such an extraction step. Although the C-rich material contains abundant noble gases, its concentrations of other volatiles are so different from those in carbonaceous chondrites that it cannot be related to the latter by any simple evolutionary process (BINZ et al., 1975; WXNKEet al., 1972). On the other hand, the O-isotope data of CLAYTONet ul. (1976) suggest that the ureilitic ultramafic silicates are somewhat more closely related to carbonaceous chondrites than to ordinary or enstatite chondrites.
As discussed in the previous section, the intensity of shock effects in ureilites decreases through the suite extending from Goalpara to Kenna. SI~EROPHILE F~~ONATIO~ UREILITES
AMONG
WKNKE et al. (1972) showed that the C-rich veins were enriched not only in the rare gases but in siderophiles such as Ga, As and Ir as well. WKNKE et al. (1972) and WLOTZKA(1972) noted that Ni-poor metal is found near the margins of olivine grains and Nirich metal is associated with the veins, and inferred that most of the siderophiles found in bulk Haverii were associated with the C-rich veins. Figure 2 shows that, with the exception of Ge and Ir, siderophile/Ni ratios are constant to within a factor of 1.5. The variation in the ratio of Ir to other siderophiles among the ureilites appears to be inconsistent with the hypothesis that the bulk of the siderophiles were introduced with one component of constant composition. Since ureilitic Ni is found in the olivine. we will henceforth use Au as our normalizing element. Figure 3 is a diagram of Ni/Au vs Ir/Au, both relative to the ratios in Cl meteorites. Our data span a range of 4.2 in h/Au ratio and are significantly correlated at the 98% confidence level. The linearity of the distribution indicates that the data can be adequately explained by two compoilents, and that the amount of oxidized Ni in the silicate portion is negligible~ as expected from the low (40pgjg) concentration in HaverG olivine reported by NEUVONENet al. (1972); even the somewhat higher levels (-230 pg/g) in
Fig. 3. A strong correlation si~ific~t at 98% level is observed between Ni/Au and k/Au ratios in the five ureilites. These indicate the presence of two dominant components. The low-k component is similar to the E4 chondrites; it is attributed to metal associated with the C-rich veins. The high-k component appears to be similar to that in interiors of coarse-grained Ailende Ca-Al-rich clasts; it is attributed to refractory siderophiles retained during the formation of the ultramafic silicates by partial melting.
1453
Classification of and elemental fractionation among ureilites Kenna (BERKLEY et al., 1976) account for only N 107: of the total Ni. The large amount of metal in the veins indicates that at least one of these components originated together with the vein material; the other component may be associated with the mafic silicates, or may also originate in the vein material. The three possible two~om~nent hypotheses are: (1) vein metal has a low Ir/Au ratio, mafic silicate metal a high Ir/Au ratio; (2) vein metal has a high Ir/Au ratio, mafic silicate metal a low Ir/Au ratio; or (3) both components originated in the vein material. We also show the locations of certain types of chondritic materials in Fig. 3. Bulk concentrations in E!4 chondrites (BAEDECKERand WASSON,1975) fall near an extension of the regression line to low Ir/Au ratios, whereas the Ni/Au ratio in CI chondrites (CHOU et al., 1976) lies about 20% higher than the line. The interiors of Ca-Al-rich clasts (CARCs, WARK and LOVERING,1976) in Allende inclusions have very high Ir/Au ratios and very low Ni/Au ratios; the interiors of two coarse-grained CARCs plot near the opposite extrapolation of the mixing line (W~~NKEet al., 1974; CHOU et al., 1976). The first hypothesis seems the most attractive. The O-isotope data of CLAYTONet al. (1976) suggest that the nebular formation location of the ureilites was near that of the carbonaceous chondrites, and it is possibfe that the chondritic parental material of the ureilites intone Ca-Al-rich clasts. If the ureilitic mafic silicates were produced by partial melting, it is reasonable that some siderophiles could have heen trapped in refractory minerals. BOYNTONet al. (1976) show that rare earth data indicate that the ureilitic mafics were formed by this mechanism rather than by fractional crystallization. In this scenario the loci of the ureilite whole-rock data in Fig. 3 can be understood if the siderophiles in Goalpara largely originated in the C-rich vein material, whereas the Kenna siderophiles reftect substantial ~ntributions from both components. By assuming reasonable concentrations regarding elemental concentrations in the end components we can estimate the amounts of these components in each ureilite. The least ad hoc assumptions are that ratios in the low Ir component should lie intermediate between the E4 and CI chondrites, and that those in the high-Ir component should be similar to the plotted CARC concentrations. The values chosen are marked with asterisks. As Au con~ntrations in the low& com~nents we took 350 rig/g,, identical to that in F!4 chondrites, and in the high-Ir component we took 180 p&/g, a value typical of Allende inclusions. Combining these with the ratios estimated from Fig. 3 yields Ni, 0.96 mg/g and Ir, 7.3 pg/g in the high-Ir component, and Ni, 19.7 mg/g and Ir, 0.70 pg/g in the low Ir component. Abundances of the two hypothetical components were estimated using only Au and Ir data. Two caveats are in order before proceeding. First, the listed mounts of the components are subject to sys-
tematic errors because of uncertainties in the elemental concentrations in the end components. If we are correct in our assignment of the elemental ratios, these uncertainties should be about +30x in the high-Ir component but as much as f a factor of 2 in the low-Ir component. Second, although we believe that our choice of the positions of the end component on the mixing line is the most reasonable, they are not in fact constrained by the available ureilitic data. Moving an end component nearer the observed ureilite range (Fig. 3) increases the observed range of that component. The best way to confirm the correctness of our component assignments would be to show that they could be isolated as separate fractions of ureilites. Separated fractions of Haverij analyzed by WKNKE et af. (1972) are shown in Fig. 3. Their whole rock data differ from ours chiefly in terms of Ni content, which is 15% lower than ours. It appears that the same difference is found in their separated phase data, which plot well below our mixing line. Their hand separated samples of vein material (13 and 18-l) lie down and to right of their whole rock, in seeming contradiction of our assignment of the high-Ir component to the mafic silicates. However, metal separated magnetically from the bulk meteorite plots still lower and farther to the right, despite the fact that WLOTZKA(1972) infers that the fine portion of this metal was formed by reduction of olivine. Fraction 19-1, a residue after treatment with aqua regia and HF, is clearly anomalous. Fractions with high Ni/Au and low Ir/Au ratios needed to account for the whole rock composition were not found. We can find no simple interpretation of these data. Similar experiments in which additional fractions are analyzed are needed. Bearing in mind the uncertainties and returning to Table 3, we see that there is an apparent variation of a factor of 24 in the high-h component, but only slightly more than a factor of two in the low-Ir component. Concentration of C (WIIK, 1969,1972; GIBBON and MOORE,1976) tends to correlate with the amount of the low-Ir component listed in Table 3. This observation and the fact that Ni-rich metal tends to be found in veins (e.g. WLOTZKA,1972) tend to support the first hypothesis. According to the second hypothesis, vein metal has a high-Ir content and residual metal a low Ir content. We find this less plausible for two reasons: (1) the low-Ir metal is com~sitionally highly reduced E4 chondrites,
similar to that in the suggesting a closer rela-
Table 3. Relative amounts of hypothetical siderophile components in ureilites based on Au and Ir data. Assumed concentrations in carriers: high-h, llOng/g Au, 7300 rig/g Ir; low-k, 350 rig/g Au, 700 rig/g Ir component
Dyalpur
Goalpara
HavcrU Kenna Nova-Urei
High-Ir (%)
2.5
0.45
2.2
9.4
4.9
Low-xr
6.4
3.8
5.4
8.0
7.5
(%f
J. T. WASSONet al.
1454
tionship to the highly reduced vein material than to the oxidized matic silicates. (2) The high-Ir metal resembles the Ca-Al-rich clasts of CV chondrites, but differs greatly in composition from bulk chondrites; the high planetary rare gas contents of ureilite vein material indicate a closer relationship to the latter than to refractory materials such as the CARCs. According to the third hypothesis, both types of metal originated in the veins. In support of this can be cited the fact that the amounts of the two components (Table 3) seem to be correlated. This correlation would disappear, however, if the compositions assumed for the components were somewhat nearer the observed ureilite range. Further, the BERKLEYet al. (1976) observation of indigenous, sausage-shaped metal inclusions inside olivine and pyroxene grains tends to rule out this model. In fact, if these inclusions are really trapped, they offer an excellent means for choosing between the first two hypotheses. However, it is necessary that they be completely encased in a single crystal of olivine or pyroxene, else metamorphic reheating may have resulted in equilibration with vein metal. This may be the explanation for the relatively constant Ni content found in the metal of Kenna by BERKLEYer al. (1976). On balance, we feel the evidence favors the interpretation that two components of constant composition are responsible for the siderophile distributions observed in ureilites, and that the low-Ir component is associated with the C-rich veins whereas the high-Ir component is indigenous metal trapped in the mafic silicates.
similar to those in CV chondrites (CHOU et al., 1976). Zinc is a useful classificational indicator, and if it is associated with the ultramafic portion of the ureilites, the high ureilite Zn levels indicate a closer relationship to carbonaceous than to ordinary chondrite groups. The low Na, Ga, and Ge abundance ratios are probably associated with loss of low-melting silicate and metal fractions. These latter elements show a systematic increase in abundance from Goalpara to Kenna, suggesting a systematic decrease in the efficiency of extraction of the low-melting liquids. The distributions of highly volatile elements (Fig. 4) show several striking features: (1) the range of abundances is small (factor of N 30 for the rare gases, 334 for Cd and In) compared to the range observed among the ordinary chondrites. (2) Rare gas abundances tend to increase (though not monotonically) through the suite from Goalpara to Kenna. Since the amount of vein material in the ureilites seems to be the same within a factor of 2, this suggests greater diffusive loss of rare gas at the Goalpara-Havero location than at the Kenna-Novo-Urei location, and that following the introduction of the C-rich material, the temperature was higher at the former than at the latter location. (3) The abundance ratios of Cd and In are much lower than those of the rare gases. (4) The more volatile the rare gas, the higher the abundance ratio. The third and fourth points may be related. Recalling that the abundances of heavy rare gases in CI chondrites are lower than the cosmic abundances by factors ranging from 2 x 10’ for Ar to 7 x lo3 for
VOLATILE FRACTIONATION AMONG UREILITES Figure 4 shows ureilite volatile-element abundances divided by the abundances in Cl chondrites; Si concentrations of 19.0 and 10.3% were assumed for ureilites and CI chondrites, respectively; CI concentrations of elements other than rare gases are from CHOU et al. (1976) and MACON (1971). Data on Na and Mn in ureilites are from SCHMITT et al. (1972), WXNKE et al. (1972) and Table 2. Rare-gas data for Dyalpur, Goalpara and Novo-Urei and CI chondrites are from MAZOR et al. (1970) those for Havero are from WEBEX et al. (1971), and those for Kenna from WILKENING and MARTI (1976). In the ordinary chondrites the first 5 elements in Fig. 4 are moderately volatile, the second 5 highly volatile (WASSON and CHOU, 1974). The elements increase in volatility (under ordinary chondrite formation conditions) from left to right. A horizontal dashed line indicates the abundances of the 5 moderately volatile elements in the L group, the ordinary chondrite group most similar to ureilites in degree of oxidation. The abundance of Mn in ureilites is similar to that in CI and ordinary chondrites. The ureilite Zn abundance is 3 x higher than in ordinary chondrites, and
i
Cd
Fig. 4. Ureilite abundances normalized to CI abundances for 5 moderately volatile and 5 highly volatile elements: left-to-right ordering is roughly in terms of increasing volatility. The highest abundances are generally in Kenna, Novo-Urei or Dyalpur, and the lowest abundances in Goalpara. Abundances of highly volatile elements Cd and In are surprisingly low considering the high rare-gas abundances. The high Zn and Mn abundances are attributed to the ultramafic component of the ureilites. There is no evidence that volatiles other than the rare-gases and C have high abundances in the C-rich component.
Classification of and elemental fractionation among ureilites Xe (SUESS,1949; see WASSON,1974, p. 78), the inverse correlation of rare gas abundance with volatility may indicate that the CI chondrite material lost more of its original gas than did the C-rich veins. In this case one would expect that the lighter gases would be lost to a greater degree. It is of interest that when nonfissiogenic and nonradiogenic average carbonaceous chondrite (AVCC) Kr and Xe abundances are divided by those in Kenna (WILRENINGand MARrI, 1976), a small mass-dependent fractionation of light from heavy isotopes is observed in AVCC Kr, with the hint of a similar fractionation in Xe. This appears to be in keeping with our ideas regarding gas loss in carbonaceous chondrites as well as the proposal of WILKENINGand MARTI (1976) that the isotopic composition of Kenna Xe approaches that of a primitive end component. It is not certain that Cd and In originated in the vein material, but if they did, their concentration in that material were similar to those in CI chondrites. LOCATIONS OF INDIVIDUAL UREILITF.23 WITHIN THE PARENT BODY The parallel systematic differences in the abundance of elements associated with the low-melting silicates (Na) and with the indigenous metal (Ir) indicate decreasing efficiency in separation of these phases through the suite from Goalpara to Kenna. The simplest explanation for this difference is that magmatic tem~ratures were higher in the region where Goalpara formed. That these regions were an appreciable distance from each other can also be inferred from the relative differences in shock intensity. Going one step further in our speculation, since the source of the shock energy must have been a collision on the exterior of the parent body, we can infer that Goalpara formed nearer to the surface than Kenna, and that the higher maximum temperatures inferred at the Goalpara region implies an external heat source. Although this is our preferred scenario, an internal heat source cannot be ruled out at this time. It would be useful to know the approximate size of the ureilitic region. The similarity in distribution and amount of C-rich vein material suggest that its thickness could not have been much greater than 1 km, and was probably less; the horizontal dimensions could have been substantially greater. OLIVINE COMPOSITION AND THE TEMPERATURE HISTORY OF THE UREILITE PARENT BODY
The fayalite content of olivine, the dominant mineral, is substantially lower at the borders than in the centers of grains. This is the opposite of the zoning expected during nebula condensation or igneous crystallization. ANDERS’(1964) suggestion that this resulted from reaction with the C-rich vein material is surely correct. However, this reaction can only
1455
occur at low CO pressures; for example, at 1400 and 1OOOK and an FeSi,,50z activity of 0.1 the equilibrium CO fugacity is only 13 and 0.05 atm, respectively (we are indebted to E. M. Stolper for pointing this out to us). Thus, the presence of the reaction rims demands both high tem~ratur~ and low confining pressures during the period that reaction was occurring. The Ca contents (- 0.4 wt %) of the centers of large olivine grains (WLOTZKA,1972; BERKLRYet al., 1976) are much higher than those found in equilibrated ordinary chondrites (KELLand FREDRMSSON,1964) or in slowIy cooled plutonic rocks (SIMKM and SMITH, 1970). High Ca contents are found in unequilibrated chondrites, in the highly metamorphosed Shaw ordinary chondrite (DODD, 1972), and in rapidly cooled basalts (SIMKWand SMITH,1970). It seems quite likely that high Ca contents reflect a fre~ng-in of a high temperature equilibrium state of the olivine, implying that the olivine was hot at the time the C-rich material and Fe2Si04 began to react, since the observed zoning of the olivine clearly indicates that re-equilibration of the centers of olivine grains did not occur after this event. During slow cooling the Ca would have formed diopside, or if sufficient Al were present, anorthite. The final equilibration temperature of the olivine was probably somewhere in the range of 135&1550K estimated by FREDRIKS~~Nand MA~CIN (1967) and ONUMAet al. (1972) for Shaw. The olivine data allow several conclusions: (1) the high temperatures needed to bring about the reaction between the C-rich material and the ohvine were not entirely supplied by the shock event, but may have mainly been residual magmatic temperatures. (2) If the heat source which produced the ultramafic silicates by partial melting was a Hayashi phase of the sun or solo-wind induced electrical currents (SoNETTer al., 1970) that was available only during the first 103-lo6 yr of solar system history, injection of the C-rich material occurred very early in the history of the solar system. (3) If the ultramafic materials of ureilites were initially formed under a thick insulating layer, much of this i~ulation was no longer present following the shock event, otherwise the observed compositional gradients in the olivine would not have formed, and extensive loss of rare gas from the C-rich material would probably have occurred. The voids found in ureilites other than Kenna also suggest that, following the shock event, the overburden resulted in relatively low pressures. The most serious problem in this scheme is that it appears necessary to store the C-rich material at appreciable distances from the ultramafic silicates prior to the shock event, and then, within an exceedingly brief interval, to transport the material to the silicates and divide it evenly throughout the moderately large region needed to account for the compositional suite extending from Goalpara to Kenna. This event seems required by the high rare-gas content of the C-rich material and from the fact that loss of
1456
J. T. WASSONet al.
a metal phase should extract most of the elemental c.
that it originated together with the enstatite chondrites in the innermost portion of the solar nebula.
ON THE ORIGIN OF THE CARBON-RICH MATERIAL
RELATIONSHIPS OF UREILITES TO OTHER GROUPS OF METEORITES
In the previous section we showed that the evidence indicated that the C-rich material was injected into the silicates while they were hot, and that cooling occurred soon thereafter. However, it is possible that the injection of the C-rich material and the shock event that produced the diamonds and other shock effects were separated in time (BERKLEYet al., 1976). It seems quite likely that a massive deposit of C-rich material was present before the mixing occurred. BERKLEYet al. (1976) estimate the volume abundance of the C-rich material in Kenna to be z 1l:/,. Some portion of this material must be indigeous ultramafic silicates incorporated during the injection event, but C alone accounts for 20-25x of the material. It seems unavoidable that the linear dimensions of the deposit were at least 10% the thickness (5 1 km) of the ultramafic silicates. Shock waves move faster than material can be transported. If the C-rich material was introduced at the time of the shock event, it follows that most diamonds were formed in or near the original C-rich deposit, not in situ. Since this implies formation of the diamonds at higher pressures than those experienced by the silicate phases, it helps explain the remarkably high efficiency of diamond formation: the abundance of diamond-lonsdalitegraphite aggregates (0.36%) reported by VDOVYKIN(1972) in Havero accounts for _ 16% of the C in this meteorite. The preferred orientation observed in the diamonds by LIPSCHUTZ (1964) and NEUVONEN et ul. (1972) is an inherent property of the aggregates and would still be observed even if these were transported following their production. Graphite is rare in primitive meteorites. It is an accessory in enstatite chondrites (KEIL, 1968). Figure 3 shows that the siderophiles in Goalpara are closely related to those in E4 chondrites: we believe that most of the siderophiles other than Ir in Goalpara is from vein metal. The moderately reduced, primitive group IAB irons contain graphite as large nodules; roughly equal amounts of C are found in this form and as cohenite, Fe& The cohenite is formed by low-temperature exsolution from Fe--Ni, and it is likely that the graphite formed by exsolution at higher temperatures, and is not a primitive material. What, then, is the source of the ureilitic C-rich material? The high abundance of rare gases indicates that it probably existed as small particles in the solar nebula or in some other environment conducive to the incorporation of these gases, and was never strongly heated thereafter. This and the inference that in at least one location it was present as a deposit with dimensions of _ l&100 m suggests that it was a distinct and significant type of primitive material. Our best guess is
The O-isotope data of CLAYTON et a!. (1976) the Zn abundance and Fe/(Fe + Mg) ratios suggest a relationship between the ureilites and the carbonaceous chondrites. As discussed above, there is evidence for appreciable amounts of reduced metal in the parental material from which the ultramafic silicates formed. Since the CI and CM chondrites are essentially metal free, the parent material must have been more similar to the metal-bearing CV or CO groups of carbonaceous chondrites, or perhaps intermediate between these and the ordinary chondrites. BILD and WASSON (1976) report that the Lodran meteorite shares several properties with ureilites (e.g. zoned olivine decreasing in Fe content toward its periphery, abundant planetary-type rare gases). However, olivine cores in Lodran are Fal5, substantially lower than the value of Fa21 typical of the ureilites and the two groups must be assigned to separate parent bodies. BINZ et cd. (1975) suggest that Chassigny might be related to the ultramafic component of the ureilites. However, since the olivine in Chassigny is Fa32 (Pamz et al., 1974), 1.5 x greater than that in the ureilites, separate parent bodies are indicated. If the proto-ureilitic parent body had a chondritic bulk composition, upon differentiation it should have had a core of Fe-Ni and FeS and a feldspar-rich crust. The O-isotope data indicate that the achondrites analyzed by CLAYTON et ~2. (1976) are not samples of its crust. It is not now possible to infer the trace-element composition of the core in order to relate it to a known group of iron meteorites.
A SCENARIO The above discussion leads to the following model of ureilite formation. Some aspects are more speculative than others; in the preceding discussion we have attempted to provide the reader with qualitative indications of the strength of our conclusions, and also reference to the sources of ideas not our own. In this
section we will limit ourselves to a list of the sequence of processes needed to form the ureilites, and of some properties produced by these processes. (1) A parent body formed consisting of material similar to the metal-bearing carbonaceous chondrites. (2) A heat source produced partial melting, and led to the sinking of a metal-troilite melt and the ascension of a feldspar-rich melt. As a result of higher temperatures, the extraction of melts was more efficient in the Goalpara region than in the Kenna region. (3) An impact event injected C-rich material into the ultrarnafic silicates residual from the partial melting process. Diamond formation occurred during this
Ciassification of and elemental fraetianation among ureilites
event or resulted from a later irhpact. Prior to the injection event the C-rich material was present as a massive deposit on the proto-ureilitic body or, more likely, on the projectile responsible for the injection. (4) High” &vine Ca contents show that the ultram&c silicates were hot before the injection event. Shock heating resulted in still higher temperatures. As a result of these high temperatures and a low confining pressure following the shock, C and the oxidied Fe in the silicates began to react to give CO and metallic Fe. (5) The impact event had removed most of the strata overlying the ureilitic region, and within a short period the material’ cooled to a temperature (- 1000 K) too low to permit further reaction between C and oxidized Fe. (6) Further low-velocity collisions broke up the ureilitic material into small bodies that were eventually captured by the Earth as meteorit& Acknowledgements-We are grateful to R. Si CLARKG,G: I. Huss, R. HUTCHISON, V. D. KOLOMENSKII and K. J. NEU-
VONENfor the provision of samples, and to W. V. BOYNN. CLAYTON,K. KEIL,M.E. LEPSCHUTZ,K.MART& R. A. Smrrr, E. M. STOLI’?~,L. L. W~~K~~~ and their
TON, R
coworkers for providing copies of their manuscripts prior to publication. We are indebted to J. WOOD and F. BEGEMANNfor critical reviews and to W. V. BOYNTONfor constructive arguments regarding sideophiles in ureilites. Laboratory and technical assistance was provided by W. V. BOYNTON, H. J. CHLJN,S. KASS,G. PUSAVAT,K. ROBINSOS, L. L. SIlNDBER&,and P. H. WARREN.This research was primarily supported by NSF grants GA-32084 and
DES74-22495.
1457
Chemical evidence for the genesis of the ureilites, the achondrite Chassigny and the nakhlites. Geochim. Cosmochim. Acta 40, 1439-1447. CHOU C.-L.,BAEDECKER P. A.
and WASSONJ. T. (1976) Allende inclusions: volatile-element distribution and evidence for incomplete ~ola~l~t~on of presolar solids. Geochim. Cosm&im Acta 40, 85-94.
CLAYTONR. N., MA~EDAT. K.%Om N, and &BARER 5. (1976) Qxygen isotopic composition of minerals in the Kenna ureilite. Geochim. Cosmochim. Acta 40, 1475-1476. DODD R. T. (1972) Calcium in chondritic olivine. Mem. Geol Sot, Amer. 132, 651-660. FREDR~~~SSONK. and MASO%B. (1967) The Sbaw meteorite. Geochim. Casrno~~~ Acta 31, 170~t709.
K., JR. (1976) Nature of the carbon and sulfur phases and inorganic gases in the Kenna ureilite. Geochim. Cosmochim. Acta 40, 1459-1464. GILLWMD. E., JANGHORBAN~ M., MILLERM. D., CHYI L. L. and E&MANN W. D. (1972) Elemental abundances in the HaverG meteorite. i&eteo&tics 7, 573-578. KEIL K. (1968) Mjner~ogi~~ and chemical relationships GIBSON E.
among enstatite chondrites. J. Geaphys. Res. 73, 6945-6976.
KEIL K. and FREDRIKS~CIN K. (1964) The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of cbondrites. J. Geophys. Res. 69, 3487--3515. Ln%mTZ M. E. (1964) origin of diamonds in the ureilites. Science 143, 1431-1434. MARVIN W. 3. and WOOD.I. A. (1972) The Haverii ureilite: petrographic notes. Meteorit& 7, 601610. MASON B. (1962) Meteorites, 274 pp. Wiley. MASON B. (editor) (1971) Rundbook of Elemental Abundances in Meteorites. 555 pp. Gordon & Breach. MEZORE., HFYMANND. and ANDERSE. (1970) Noble gases in carbonaceous chondrites. Geochim. ~os~~hirn. Acta 34, 781-824.
MUELLERG. (1969) Geneticai interrelations between ureilites and carbonaceous chondrites. In Meteorite Research, (editor P. M. Millman), pp. 505-517,535-537. REFERENCES Reidel. ANDERSE. (1964) Origin, age and composition of meteorNEUVON~?~ K. J., OHLSONB., P~PUNEN H., H&LI T. A. ites. &X%XSE/, Rev. 3, 583-714. and RA~V~D~HR P. (1932) The Haverij ureilite. Meteor&s BAEDECKER P. A., &Ou C.-L, GR~~~~ E. B. and WAS7, 515-531. SONJ. T. (1973) Volatile and siderophik trace elements ONL?V~ N., CLAYTONR. N. and Mnmn T. K. (1972) Oxygen isotope temperatures of equilibrated ordinary in Apollo 15 samples: geocbemical implications and characterization of the long-lived and short-lived extrachondrites. Geochim. Cosmochim. Acta 36, 157-168. lunar materials. Proc. 4th Lunw Sci. Co@, Geochim. CosPRINZM., HLAVAP. H. and Kerr K. (1974) The Chassigny mochim. Acta Suppl. 4, pp. 1177.-1195. Pergaman Press. meteorite: a relatively iron-rich cumulate du& BIIEDECKEK P. A., CHOU C.-L., SUNSJBERG L_ L. and WAS(Abstract). Meteor&s 9. 393-394. SONJ. T. (1974) Volatile and siderophilic trace elements R&wooQ’A. E. (1960) %be Novo Urei meteorite. Geein the soils and rocks ofTalus-Li~~row. Pram. 5th Lunar chim. Cosmochim. Acta 20, l-4. S&i. Con& Geochim. Cosmochim Acta Suppf. 5, Scmrrr R. A., GOLESG. G., SMITHR. H. and Osaoa~ pp. 1625-1643. Pergamon Press. T. W. (1972) Elemental abundances in stone meteorites. BERKLEYJ. L.. BROWNIV H. G.. KBIL K.. CARTERN. L.. Meteor&s 7, 131-213. MERCIERJ-6. C. and Huss G. (i976) The Kenna yeilitei SIMKINT. and SMITHJ. V. (1970) Minor element distribuan ultramafic rock with evidence for igneous, metamortion in olivine. JI. Geol. 78, 304-325. phic, and shack origin. Geochim. Cosmochim. Acta 40, %%TT c. P., COLBURND. S., %~#‘pJlTz K. and KEft 1429-1437. K. (1970) The melting of asteroidal-sized bodies by uniBHB R. W, and WASSONJ. T. (1976)The Ladran meteorite polar dynamo induction from a primordial T Tauri sun. and its relationship to the ureilites. ~~~e~~~.Msg. 4@+ Astrophys. Space Sei 7, 446-488. 721-735. VDOVYKING. P. (1970) Ureilites. Space Sci. Reu. 10, BINZ C. M., IKRAMUDDIN M. and LIPSCHUTZM. E, (1975) 483-510. Contents of eleven trace elements in ureilite achondrites. VDOVYK~NC. P. (1972) Forms of carbon in the new Geochim. Cosmochim. Acta 39, 1576-1579. Haverij ureilite of Finland. Meteoritics 7, 547-552. BOYNTON W. V., BAE~ECKER P. A., CHOWC.-L., ROBINSON W&=XE HI., 3~~~~~~ H., SPETTEL.B., TPSCX-ZKE F, K. L. and W~sso~ J. T. (13%) Mixing and transport QUIJANO-RI~ M., Dmmn~s G. and PALHE H. (1972) The uf lunar surface materials: evident obtained by the chemistry of Haverti ureilite. Meteoritics 7(S), 579-5QU. determination of lithophile, siderophile and volatile ele- W.$KE IX, BADDENHAUSEN H., PALMEH. and SP~L B. ments. Proc. 6th Lunar Sci. Cc@., Geochim. Gosmochim. (1974) On the chemistry of the Allende inclusions and Acta Suppl, 6, pp. 2241-2259. Pergamon Press. their origin aa high temperature condensates. Earth BOYNTONW. V., STARZYKP. M. and SCHMITTR. A, (1976) Planet, Sci. Lett. 23, 1-7.
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D. A. and LOVERINGJ. F. (1976) Refractory/plaPrimordial gases in graphite-diamond-kamacite inclutinum metal grains in Allende CARC’s (Abstract). Lunar sions from the Haverii ureilite. Earth Planet. Sci. Lett. 29, 81-90. Science MI, pp. 913-914. Lunar Science Institute. WASSONJ. T. and CHOU C.-L. (1974) Fractionation of WIIK H. B. (1969) On regular discontinuities in the compomoderately volatile elements in ordinary chondrites. sition of meteorites. Comm. Phys.-Math. (Helsinki) 34, Meteoritics 9, 69-84. 135-145. WASSONJ. T., CHOU C.-L., BILD R. W. and BAEDECKER WIIK H. S. (1972) The chemical composition of the HaverG P. A. (1973) Volatile elements in anomalous planetarymeteorite and the genesis of the ureilites. Meteoritics 7, rare-gas-rich meteorites (Abstract). Trans. Amer. Geophys. 553-557. Union 54, 351. WILKENINCL. L. and MARTI K. (1976) Rare gases and WEBERH. W., HINTENBERGER H. and BEGEMANN F. (1971) fossil particle tracks in the Kenna ureilite, Geochim. Noble gases in the HaverG ureilite. Earth Planet. Sci. Cosmochim. Acta 40, 1465-1473. Lett. 13, 205-209. WLOTZKAF. (1972) Haverii ureilite: evidence for recrystalWEBERH. W.. BEGEMANN F. and HINTENBERGER H. (1976) lization and partial reduction. Meteoritics 7, 591-600.
et Cosmochimics.
Acta, 1976, Vol. 40, pp. 1449 to 1458. Pergamon
Press. Printed
in Great Britain
Classification of and elemental fractionation among ureilites JOHN T. WATSON,CHEN-LIN CHOU,* RICHARDW. P~~ILIPA. BAEDECKER$
BILD~
and
Institute of Geophysics and Planetary Physics, Departments of Chemistry and Geophysics and Space Physics, University of California, Los Angeles, California 90024, U.S.A. (Received 14 October 1975; accepted in revisedform 4 June 1976) Abstract-Concentrations of Ni, Zn, Ga, Ge, Cd, In, Ir and Au in five ureilites can be combined with petrographic evidence to yield a well-defined suite extending from Goalpara (heavily shocked, low Ir concentration, low Ir/Ni ratio) through Have@ Dyalpur, Novo-Urei to Kenna (moderately shocked, high Ir concentration, high Ir/Ni ratio). Arguments are presented indicating that this suite represents the sampling of a vertical section within the ureilitic parent body. The large range in Ir/Ni and Ir/Au ratios indicates greater efficiency of extraction of primitive, refractory metal in the Goalpara region than in the Kenna region, and implies that higher maximum temperatures prevailed in the former during the production of ureilitic ultramaIic silicates by a partial melting process. A major impact event injected a deposit of C-rich material into the ultramafic silicates. This C-rich material had a moderately high content of metal; there is no direct evidence that it contained volatiles other than the rare gases. High Ca contents of the ferromagnesian minerals indicate that the ultramafics were hot at the time the injection occurred; the zoning of these mineral grains also indicates high temperatures (ea. 1400 K) and low pressures (C 10 atm) such that reaction between C and Fe2Si04 could occur, but that cooling occurred too quickly to allow complete equilibration. The ureilitic C-rich material appears to represent an important type of primitive material. Two siderophile-rich components are necessary to explain the relative siderophile trends in ureilites. We interpret the high-Ir component to be a refractory nebular condensate or residue that was retained during the partial melting event. The low-Ir component, which roughly resembles El chondrite siderophiles, is attributed to metal injected together with the vein material.
INTRODUGTION
THE SEVEN ureilites are unique meteorites showing some features typical of primitive solar-system matter (e.g. abundant planetary-type rare gases) and others typical of igneously ~fferentiat~ matter (e.g. silicates consist mainly of olivine and have vanishingly small concentrations of feldspar). VMVYKIN(1970) reviewed ureilite properties, and in December 1972 a series of 11 papers dealing with the Haverij fall was published in Meteoritics. Figure 1 illustrates the features common to most ureilites. The most abundant phase is olivine, generally present in polyciystalline domains having dimensions of about 0.5-l mm. The only other abundant silicate is clinopyroxene; this is pigeonite in all ureilites except Havero. Long, oriented cracks and voids are present in all ureilites except Kenna. Dark regions cont~ning graphite, diamond and FeNi metal border veins except where the latter are adjacent to pyroxene, and also separate olivine domains where no cavities are resolvable. The cracks, diamonds and * Present address: Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 1Al. t Present address: Max-Planck Institut ftlr Kernphysik, 69 Heidelberg, Germany. 1 Present address: U.S. Geological Survey, Reston, VA 22092, U.S.A.
polycrystallinity of the olivine appear to have resulted from shock. A number of authors (e.g. MASON, 1962; ANDERS, 1964; MUELLW, 1969; VWVYKIN, 1970) have suggested a close link between the ureilites and the carbonaceous chondrites. WLOTZKA (1972) proposed a relationship with L-group chondrites on the basis of similar Fe/(Fe + Mg) ratios in the ferromagnesian silicates. Our studies.(WAssoN et al., 1973) were initiated to gather data on volatile and siderophile trace elements which could be used to search for genetic ties between ureilites and known chon~it~ and to aid in the inv~tigation of the fractionation processes that occurred during the formation of ureilites. MUELLER(1969) and VDOVYKIN(1970) divided the ureilites into two types (I, type specimen Novo-Urei and II, type specimen Goalpara). The first type is less strongly shocked and has coarser olivine grains, a more common twinning in the clinopyroxene, a metal distribution dominated by net-like structures rather than plates, and smaller diamond-graphite aggregates. Two type-1 ureilites contained substantially larger amounts of rare gases than type II Goalpara (MAZORet al., 1970), and data for siderophile Ni and Co QVmc, 1969; Swrrr et at., 1972) show a similar trend. Thus, our secondary goal was to investigate whether representative ureilites showed the same trends for other siderophile (Ge, Ir, Au) and highly
1449
J. T. WASSONet al.
1450 volatile (Cd, In) elements, petrologic classification.
thus testing the chemical-
EXPERIMENTAL
The sources of the ureilite specimens arc listed in Table 1. Three of the meteorites are observed falls, and the fresh appearance of Goalpara suggests that it also was recovered soon after an unrecorded fall. Kenna is slightly weathered (BERKLEY et al., 1976). Our Kenna sample had wire sawed surfaces which we cleaned with an Al*O, wheel followed by washing in acetone. Duplicate samples (triplicate for Have@ consisting of 0.124.33 g chips were crushed in a steel percussion mortar and analyzed in separate neutron activation runs. Radiochemical neutron-activation analysis (RNAA) data on Kenna were determined by a recently developed procedure which allowed Ru to be added to our former suite of 8 trace elements; a description of this procedure is in preparation. RNAA determinations of Ni, Zn, Ga, Ge, Cd, In, Ir and Au in the remaining ureilites were by the procedures described in BAEDECKER et al. (1973) and earlier papers cited there. The accuracy and precision for Ru has not been determined. Precision for the other RNAA-determined elements is discussed in BAEDECKER et al. (1974). Samples were dissolved for RNAA by fusion in Na,O,. Since such a fusion does not dissolve diamonds, our data must be treated as lower limits for elements having appreciable abundances in the diamonds. It is probable that this fraction is negligible, since the four elements determined by both INAA and RNAA in Kenna are essentially the same with the exception of Ni. The Ni difference may be spurious since different flux monitors were used for the two procedures. Kenna samples were analyzed by instrumental neutron activation analysis (INNA) prior to the RNAA determinations. A discussion of our INAA techniques and data precision is given in BOYNTONet al. (1975). A 4034 correction for Si interference in the Al determination results in an unusually high uncertainty of about *20%. A comparison of our data with those of BINZ et al. (1975), BOYNTON et al. (1976), GILLUM et al. (1972), SCHMITT et al. (1972), WKNKE.~~al. (1972) and WIIK (1972) shows generally good agreeement, i.e. to within about + 10%. Kenna INAA Ni and Ir values determined by BOYNTON
et al. (1976) are about 40”/, lower than our data; this probably indicates a systematic difference in the metal contents of our respective samples. If the BOYNTON et al. (1976) Au value in Kenna is increased 40%, one discovers that their Au values in three ureilites average about 507” greater than ours. The two research groups plan further joint investigations of this discrepancy. With these exceptions it appears that systematic errors in our data are generally less than 10%. Inadequate comparison data are available for As and Ru. RESULTS
Data on 8 elements in five ureilites are listed in Table 1. Mean concentrations of sixteen elements in Kenna and 9 elements in Haverii are listed in Table 2; four of these INAA-determined elements were also determined by RNAA. Good agreement among replicate RNAA and INAA determinations of Ni, Zn, Ir and Au indicates that the minerals in which these elements are concentrated were uniformly distributed in our O.lLO.3g samples. The other elements show more scatter. Gallium and Ge are elements we determine with high precision; the range observed in HaverB Ga and Ge and in Dyalpur Ga therefore appears to represent real sampling variations. The Cd value in the second Havera replicate is much higher than those found in all other ureilite samples: a recheck of the data offers no evidence that this result is of lower quality than the others, but we believe nonetheless that less weight should be attached to it. The range in In is only a factor of 3 if the first Haverij and Novo-Urei replicates are ignored. It is puzzling that individual Ga, Ge. In or Cd determinations rarely correlate. Either these elements are sited in different, heterogeneously distributed phases, or experimental errors have affected some of our determinations. In addition to the data listed in Tables 1 and 2 we can report the following INAA data on one
Table 1. Concentrations of volatile and siderophile elements in ureilitic meteorites. Underlined values are of lower quality and were assigned l/2 weight in the calculation of the mean Museum*
Spec. No. Dyalpur
Goalpara
BM 51185 Meall
1.39 1.40 1.40
277 265 271
1.2 2.5 1.9
a.6
8.4
41 31 fB
1.7 2.1 1.9
205 24? 226
3II 24 27
SI 5623 Mean
0.75 0.73 0.74
R6 83 84
1.2 1.1 1.2
2.8 3.0 2.9
13 15 14
0.85 1.59 1.2
55 56 56
i3 15 14
Meall
1.07 1.03 1.12 1.08
151 149 123 141
2.8 1.0 1.6 1.8
16.6 38 2.0 228 7.2 T? 8.6 68
5.0 CT0 CO.71 1.6
206 197 195 199
22 27 1!? 23
Meall
1.74 1.64 1.69
164 199 181
3.1 2.5 2.8
29.5 26.1 27.8
47 50 49
1.4 1.5 1.4
760 736 748
53 38 45
Meall
1.59 1.41 1.50
276 256 266
2.4 2.0 2.2
b.6 8.6 7.6
23 29 26
6.2 TX? 2.5
449 372 410
II ?" 36
Havert)
wur
KWllla
AML H159
Nova-Urei GML
8.2
* Sources of the samples abbreviated as follows: AML, American Meteorite Laboratory, Denver; BM, British Museum, London; GML, Mining Museum, Leningrad; SI, Smithsonian Institution, Washington; UTur, University of Turku, Finland.
:,
ye.:
z,
.>,::a; .%:._
:L:..;~_-:‘~o:-i .i
y,
_
--e
:-
,n
.~ *
Fig. 1. Transmitted light photograph of the Goalpara ureilite showing the characteristic ureilitic features: light gray polycrystalline olivine domains, medium gray pigeonite grains with sharply defined borders, C- and metal-rich opaque regions, and white cavities, most of which are oriented NE-SW in this section. The cavities often border or enclose pigeonite grains. British Museum thin section 51187; Smithsonian Institution photo.
1450
1451
Classification of and elemental fractionation among ureilites Table 2. Concentrations of fifteen elements determined by INAA and one (Ru) determined by RNAA in the Kenna ureilite and MAA con~tratio~ of 9 elements in one sample of HaverG Kenna Na (an/s) 438, 460
Haverti 240
K.%llM
Have&l
Fe (nvgfg)
178, 174
163 121
Mg Cmgfg)
202, 206
-
CO (w/e)
200,
A.1 (mgfg)
0.68:
_
Ni (mgfg)
1.92,
Ca (mgfg)
7.4,
_
2n (ugfg)
164, 1.52
-
SC (w/g)
8.7, 9.4
7.1
as &g/g)
0.56,
-
v
(UP/PI
82,
0
(mgfgl
5.6.
HII (mgfgl
0.70* 7.2
85
2.75,
5.8 2.68
190 1.81
1.27
0.44
RI2 iuglg1
L).Ee+
5.1
Ir (ugfg)
0.72, 0.69
2.84
Au (nU/k?) 48, 47
0.24 20
* Uncertainty _ + 20%. t Uncertainty _ f 15%.
sample of Dyalpur: (+0.3, -0.7) mg/g.
Na, 255clg/g;
and
Mn
3.5
CLASSIFICATION OF UREILITES We noted above that type-1 ureilites have appreciably higher rare gas and siderophile concentrations than type-II ureilites. Our siderophile data (Table 1) confirm these differences; Ni, Ge, Au and Ir concentrations are 2-7 x times greater in Novo-Urei and Dyalpur than in Goaipara. Volatile elements also tend to be higher in the former, but in some cases one replicate is an exception. MARVINand W~QD (1972) concluded that Havero was a type-11 ureilite on the basis of evidence for heavy shock. VDOVYKIN(1972) found the evidence ambiguous (e.g. metal shows a net-like distribution but occasional plates are also observed). NEUVONEN et al. (1972) found that Haverij “in many respects resembles Novo-Urei” but that the diamond graphite aggregates reach 2 mm in size, even larger than the 0.9mm observed in Goalpara. Our compositional data (Table 1) also lead to ambiguity; concentrations of siderophiles and volatiles except Ge (which has a larger error attached) are intermediate between those in Goalpara and Dyalpur or Novo-Urei, and fill the hiatus between the two types. These obse~ations suggest that we are not dealing with two distinct types, but with a suite of objects having monotonically varying properties. Since no hiatus exists, it appears necessary to discard the type designations, and define a ureilite’s relative position within the suite in terms of some other property. The petrographic properties of Kenna (~ERKLEY et al., 1976) are in many respects divergent from those in the suite defined by Goalpara and the 3 falls. Kenna is more compact; oriented fissures characteristic of the others are missing; it contains more pronounced metamorphic features (abundant triple junctions). It resembles Novo-Urei in that the olivine is coarse (up to 2mm) and diamond-graphite aggregates are small. Peak shock pressures apparently were rather low. Volatile elements concentrations in Kenna (Table
1) are generally comparable to those in Novo-Urei and Dyaipur; In and the rare gases are inte~~iate as in Have&i. Siderophilic element ~ncentrations (Table 1) are consistently high, indicating Kenna to be one extreme of the ureilitic spectrum. Thus chemically, as petrographically (BERKLEYet al., 1976), Kenna is an extreme member of the suite, suggesting that siderophile (and particularly Ir) concentrations are good parameters for locating an individual ureilite within the suite. Inspection of ureilite sections shows that r~domly chosen samples will occasionally have anomalous metal (and thus siderophilic element) concentrations. The metal content of the Kenna sample studied by BOYNTONet al. (1976) is lower than that of ours by - 1.4 x . Ureilitic metal varies greatly in composition, another source of inhomogeneity; Ni contents in metal in Haverij veins are typically -3”/ whereas those in fine metal in olivine are typically < ly/, (WLOTZKA,1972). As discussed below, there appear to be two different sources of primary metal. It seems likely that smaller variations will be observed in siderophile/siderophile ratios than in bulk concentrations of any one siderophile. Figure 2 is a plot of siderophile/Ni ratios relative to those in Cl chondrites, plotted in order of increasing mean element/Ni ratio. The relative constancy of Ir/Ni ratios is illustrate by the fact that ratios measured in Kenna by BOYNTON et ai. (1976) and by our group are nearly identical despite the difference in metal contents of our samples. A BRIEF REVIEW OF KNOWLEDGE
ABOUT
UREILITES
It will simplify the following discussion to briefly review the properties of the ureilites, and those c A Kenna I l Novo- Urei Dyalpur
l
I’
’
Au Fig. 2. ~iderop~le~i
I
I
I
I
,
Go Ge Co It ratios normalized to those in Cl
chondrites are generally within a factor of 2 of each other. The chief exception is the Ir/Ni ratio, which appears to be an excellent parameter for determining the placement of a ureilite within the petrographic-compositional suite. The sequence is continuous, and there is therefore no value in using type designations for ureilites. The large range in Ir/Ni ratios indicates two sources for the metal, one associated with the C-rich vein material, the other trapped in the ultramatic silicates.
1452
J. T. WASSONet ai
aspects of their formational history that are generally accepted. The dominant phases are olivine and clinopyroxene, generally pigeonite. Feldspar is rare. This mineralogy suggests that the ureilites represent an ultramafic layer from which a low-melting fraction has been removed by partial melting (RINGWOOD, (1960). The alternative that the ureilites might be a cumulate phase is inconsistent with observed rare earth patterns (BOYNTONet al., 1976). The dark, C-rich material occurring as veins in the ureilites is generally believed to have been introduced into the ultramafic silicates at a late stage. This conclusion is supported by observations showing that this material was transported along cracks ~LOTZKA, 1972). Since the C-rich material contains abundant planetary-type rare gas (WEBERet al., 1971; 1976) it cannot have been very hot for any extended period. Our evidence presented later agrees with earlier proposals (RINGWOOD,1960, ANDERS,1964) that there was appreciable metal in the material parental to the ultramafic silicates. and that some of this metal has been removed by partial melting. Since C is highly soluble in molten Fe-Ni, primary C would have been removed in such an extraction step. Although the C-rich material contains abundant noble gases, its concentrations of other volatiles are so different from those in carbonaceous chondrites that it cannot be related to the latter by any simple evolutionary process (BINZ et al., 1975; WXNKEet al., 1972). On the other hand, the O-isotope data of CLAYTONet ul. (1976) suggest that the ureilitic ultramafic silicates are somewhat more closely related to carbonaceous chondrites than to ordinary or enstatite chondrites.
As discussed in the previous section, the intensity of shock effects in ureilites decreases through the suite extending from Goalpara to Kenna. SI~EROPHILE F~~ONATIO~ UREILITES
AMONG
WKNKE et al. (1972) showed that the C-rich veins were enriched not only in the rare gases but in siderophiles such as Ga, As and Ir as well. WKNKE et al. (1972) and WLOTZKA(1972) noted that Ni-poor metal is found near the margins of olivine grains and Nirich metal is associated with the veins, and inferred that most of the siderophiles found in bulk Haverii were associated with the C-rich veins. Figure 2 shows that, with the exception of Ge and Ir, siderophile/Ni ratios are constant to within a factor of 1.5. The variation in the ratio of Ir to other siderophiles among the ureilites appears to be inconsistent with the hypothesis that the bulk of the siderophiles were introduced with one component of constant composition. Since ureilitic Ni is found in the olivine. we will henceforth use Au as our normalizing element. Figure 3 is a diagram of Ni/Au vs Ir/Au, both relative to the ratios in Cl meteorites. Our data span a range of 4.2 in h/Au ratio and are significantly correlated at the 98% confidence level. The linearity of the distribution indicates that the data can be adequately explained by two compoilents, and that the amount of oxidized Ni in the silicate portion is negligible~ as expected from the low (40pgjg) concentration in HaverG olivine reported by NEUVONENet al. (1972); even the somewhat higher levels (-230 pg/g) in
Fig. 3. A strong correlation si~ific~t at 98% level is observed between Ni/Au and k/Au ratios in the five ureilites. These indicate the presence of two dominant components. The low-k component is similar to the E4 chondrites; it is attributed to metal associated with the C-rich veins. The high-k component appears to be similar to that in interiors of coarse-grained Ailende Ca-Al-rich clasts; it is attributed to refractory siderophiles retained during the formation of the ultramafic silicates by partial melting.
1453
Classification of and elemental fractionation among ureilites Kenna (BERKLEY et al., 1976) account for only N 107: of the total Ni. The large amount of metal in the veins indicates that at least one of these components originated together with the vein material; the other component may be associated with the mafic silicates, or may also originate in the vein material. The three possible two~om~nent hypotheses are: (1) vein metal has a low Ir/Au ratio, mafic silicate metal a high Ir/Au ratio; (2) vein metal has a high Ir/Au ratio, mafic silicate metal a low Ir/Au ratio; or (3) both components originated in the vein material. We also show the locations of certain types of chondritic materials in Fig. 3. Bulk concentrations in E!4 chondrites (BAEDECKERand WASSON,1975) fall near an extension of the regression line to low Ir/Au ratios, whereas the Ni/Au ratio in CI chondrites (CHOU et al., 1976) lies about 20% higher than the line. The interiors of Ca-Al-rich clasts (CARCs, WARK and LOVERING,1976) in Allende inclusions have very high Ir/Au ratios and very low Ni/Au ratios; the interiors of two coarse-grained CARCs plot near the opposite extrapolation of the mixing line (W~~NKEet al., 1974; CHOU et al., 1976). The first hypothesis seems the most attractive. The O-isotope data of CLAYTONet al. (1976) suggest that the nebular formation location of the ureilites was near that of the carbonaceous chondrites, and it is possibfe that the chondritic parental material of the ureilites intone Ca-Al-rich clasts. If the ureilitic mafic silicates were produced by partial melting, it is reasonable that some siderophiles could have heen trapped in refractory minerals. BOYNTONet al. (1976) show that rare earth data indicate that the ureilitic mafics were formed by this mechanism rather than by fractional crystallization. In this scenario the loci of the ureilite whole-rock data in Fig. 3 can be understood if the siderophiles in Goalpara largely originated in the C-rich vein material, whereas the Kenna siderophiles reftect substantial ~ntributions from both components. By assuming reasonable concentrations regarding elemental concentrations in the end components we can estimate the amounts of these components in each ureilite. The least ad hoc assumptions are that ratios in the low Ir component should lie intermediate between the E4 and CI chondrites, and that those in the high-Ir component should be similar to the plotted CARC concentrations. The values chosen are marked with asterisks. As Au con~ntrations in the low& com~nents we took 350 rig/g,, identical to that in F!4 chondrites, and in the high-Ir component we took 180 p&/g, a value typical of Allende inclusions. Combining these with the ratios estimated from Fig. 3 yields Ni, 0.96 mg/g and Ir, 7.3 pg/g in the high-Ir component, and Ni, 19.7 mg/g and Ir, 0.70 pg/g in the low Ir component. Abundances of the two hypothetical components were estimated using only Au and Ir data. Two caveats are in order before proceeding. First, the listed mounts of the components are subject to sys-
tematic errors because of uncertainties in the elemental concentrations in the end components. If we are correct in our assignment of the elemental ratios, these uncertainties should be about +30x in the high-Ir component but as much as f a factor of 2 in the low-Ir component. Second, although we believe that our choice of the positions of the end component on the mixing line is the most reasonable, they are not in fact constrained by the available ureilitic data. Moving an end component nearer the observed ureilite range (Fig. 3) increases the observed range of that component. The best way to confirm the correctness of our component assignments would be to show that they could be isolated as separate fractions of ureilites. Separated fractions of Haverij analyzed by WKNKE et af. (1972) are shown in Fig. 3. Their whole rock data differ from ours chiefly in terms of Ni content, which is 15% lower than ours. It appears that the same difference is found in their separated phase data, which plot well below our mixing line. Their hand separated samples of vein material (13 and 18-l) lie down and to right of their whole rock, in seeming contradiction of our assignment of the high-Ir component to the mafic silicates. However, metal separated magnetically from the bulk meteorite plots still lower and farther to the right, despite the fact that WLOTZKA(1972) infers that the fine portion of this metal was formed by reduction of olivine. Fraction 19-1, a residue after treatment with aqua regia and HF, is clearly anomalous. Fractions with high Ni/Au and low Ir/Au ratios needed to account for the whole rock composition were not found. We can find no simple interpretation of these data. Similar experiments in which additional fractions are analyzed are needed. Bearing in mind the uncertainties and returning to Table 3, we see that there is an apparent variation of a factor of 24 in the high-h component, but only slightly more than a factor of two in the low-Ir component. Concentration of C (WIIK, 1969,1972; GIBBON and MOORE,1976) tends to correlate with the amount of the low-Ir component listed in Table 3. This observation and the fact that Ni-rich metal tends to be found in veins (e.g. WLOTZKA,1972) tend to support the first hypothesis. According to the second hypothesis, vein metal has a high-Ir content and residual metal a low Ir content. We find this less plausible for two reasons: (1) the low-Ir metal is com~sitionally highly reduced E4 chondrites,
similar to that in the suggesting a closer rela-
Table 3. Relative amounts of hypothetical siderophile components in ureilites based on Au and Ir data. Assumed concentrations in carriers: high-h, llOng/g Au, 7300 rig/g Ir; low-k, 350 rig/g Au, 700 rig/g Ir component
Dyalpur
Goalpara
HavcrU Kenna Nova-Urei
High-Ir (%)
2.5
0.45
2.2
9.4
4.9
Low-xr
6.4
3.8
5.4
8.0
7.5
(%f
J. T. WASSONet al.
1454
tionship to the highly reduced vein material than to the oxidized matic silicates. (2) The high-Ir metal resembles the Ca-Al-rich clasts of CV chondrites, but differs greatly in composition from bulk chondrites; the high planetary rare gas contents of ureilite vein material indicate a closer relationship to the latter than to refractory materials such as the CARCs. According to the third hypothesis, both types of metal originated in the veins. In support of this can be cited the fact that the amounts of the two components (Table 3) seem to be correlated. This correlation would disappear, however, if the compositions assumed for the components were somewhat nearer the observed ureilite range. Further, the BERKLEYet al. (1976) observation of indigenous, sausage-shaped metal inclusions inside olivine and pyroxene grains tends to rule out this model. In fact, if these inclusions are really trapped, they offer an excellent means for choosing between the first two hypotheses. However, it is necessary that they be completely encased in a single crystal of olivine or pyroxene, else metamorphic reheating may have resulted in equilibration with vein metal. This may be the explanation for the relatively constant Ni content found in the metal of Kenna by BERKLEYer al. (1976). On balance, we feel the evidence favors the interpretation that two components of constant composition are responsible for the siderophile distributions observed in ureilites, and that the low-Ir component is associated with the C-rich veins whereas the high-Ir component is indigenous metal trapped in the mafic silicates.
similar to those in CV chondrites (CHOU et al., 1976). Zinc is a useful classificational indicator, and if it is associated with the ultramafic portion of the ureilites, the high ureilite Zn levels indicate a closer relationship to carbonaceous than to ordinary chondrite groups. The low Na, Ga, and Ge abundance ratios are probably associated with loss of low-melting silicate and metal fractions. These latter elements show a systematic increase in abundance from Goalpara to Kenna, suggesting a systematic decrease in the efficiency of extraction of the low-melting liquids. The distributions of highly volatile elements (Fig. 4) show several striking features: (1) the range of abundances is small (factor of N 30 for the rare gases, 334 for Cd and In) compared to the range observed among the ordinary chondrites. (2) Rare gas abundances tend to increase (though not monotonically) through the suite from Goalpara to Kenna. Since the amount of vein material in the ureilites seems to be the same within a factor of 2, this suggests greater diffusive loss of rare gas at the Goalpara-Havero location than at the Kenna-Novo-Urei location, and that following the introduction of the C-rich material, the temperature was higher at the former than at the latter location. (3) The abundance ratios of Cd and In are much lower than those of the rare gases. (4) The more volatile the rare gas, the higher the abundance ratio. The third and fourth points may be related. Recalling that the abundances of heavy rare gases in CI chondrites are lower than the cosmic abundances by factors ranging from 2 x 10’ for Ar to 7 x lo3 for
VOLATILE FRACTIONATION AMONG UREILITES Figure 4 shows ureilite volatile-element abundances divided by the abundances in Cl chondrites; Si concentrations of 19.0 and 10.3% were assumed for ureilites and CI chondrites, respectively; CI concentrations of elements other than rare gases are from CHOU et al. (1976) and MACON (1971). Data on Na and Mn in ureilites are from SCHMITT et al. (1972), WXNKE et al. (1972) and Table 2. Rare-gas data for Dyalpur, Goalpara and Novo-Urei and CI chondrites are from MAZOR et al. (1970) those for Havero are from WEBEX et al. (1971), and those for Kenna from WILKENING and MARTI (1976). In the ordinary chondrites the first 5 elements in Fig. 4 are moderately volatile, the second 5 highly volatile (WASSON and CHOU, 1974). The elements increase in volatility (under ordinary chondrite formation conditions) from left to right. A horizontal dashed line indicates the abundances of the 5 moderately volatile elements in the L group, the ordinary chondrite group most similar to ureilites in degree of oxidation. The abundance of Mn in ureilites is similar to that in CI and ordinary chondrites. The ureilite Zn abundance is 3 x higher than in ordinary chondrites, and
i
Cd
Fig. 4. Ureilite abundances normalized to CI abundances for 5 moderately volatile and 5 highly volatile elements: left-to-right ordering is roughly in terms of increasing volatility. The highest abundances are generally in Kenna, Novo-Urei or Dyalpur, and the lowest abundances in Goalpara. Abundances of highly volatile elements Cd and In are surprisingly low considering the high rare-gas abundances. The high Zn and Mn abundances are attributed to the ultramafic component of the ureilites. There is no evidence that volatiles other than the rare-gases and C have high abundances in the C-rich component.
Classification of and elemental fractionation among ureilites Xe (SUESS,1949; see WASSON,1974, p. 78), the inverse correlation of rare gas abundance with volatility may indicate that the CI chondrite material lost more of its original gas than did the C-rich veins. In this case one would expect that the lighter gases would be lost to a greater degree. It is of interest that when nonfissiogenic and nonradiogenic average carbonaceous chondrite (AVCC) Kr and Xe abundances are divided by those in Kenna (WILRENINGand MARrI, 1976), a small mass-dependent fractionation of light from heavy isotopes is observed in AVCC Kr, with the hint of a similar fractionation in Xe. This appears to be in keeping with our ideas regarding gas loss in carbonaceous chondrites as well as the proposal of WILKENINGand MARTI (1976) that the isotopic composition of Kenna Xe approaches that of a primitive end component. It is not certain that Cd and In originated in the vein material, but if they did, their concentration in that material were similar to those in CI chondrites. LOCATIONS OF INDIVIDUAL UREILITF.23 WITHIN THE PARENT BODY The parallel systematic differences in the abundance of elements associated with the low-melting silicates (Na) and with the indigenous metal (Ir) indicate decreasing efficiency in separation of these phases through the suite from Goalpara to Kenna. The simplest explanation for this difference is that magmatic tem~ratures were higher in the region where Goalpara formed. That these regions were an appreciable distance from each other can also be inferred from the relative differences in shock intensity. Going one step further in our speculation, since the source of the shock energy must have been a collision on the exterior of the parent body, we can infer that Goalpara formed nearer to the surface than Kenna, and that the higher maximum temperatures inferred at the Goalpara region implies an external heat source. Although this is our preferred scenario, an internal heat source cannot be ruled out at this time. It would be useful to know the approximate size of the ureilitic region. The similarity in distribution and amount of C-rich vein material suggest that its thickness could not have been much greater than 1 km, and was probably less; the horizontal dimensions could have been substantially greater. OLIVINE COMPOSITION AND THE TEMPERATURE HISTORY OF THE UREILITE PARENT BODY
The fayalite content of olivine, the dominant mineral, is substantially lower at the borders than in the centers of grains. This is the opposite of the zoning expected during nebula condensation or igneous crystallization. ANDERS’(1964) suggestion that this resulted from reaction with the C-rich vein material is surely correct. However, this reaction can only
1455
occur at low CO pressures; for example, at 1400 and 1OOOK and an FeSi,,50z activity of 0.1 the equilibrium CO fugacity is only 13 and 0.05 atm, respectively (we are indebted to E. M. Stolper for pointing this out to us). Thus, the presence of the reaction rims demands both high tem~ratur~ and low confining pressures during the period that reaction was occurring. The Ca contents (- 0.4 wt %) of the centers of large olivine grains (WLOTZKA,1972; BERKLRYet al., 1976) are much higher than those found in equilibrated ordinary chondrites (KELLand FREDRMSSON,1964) or in slowIy cooled plutonic rocks (SIMKM and SMITH, 1970). High Ca contents are found in unequilibrated chondrites, in the highly metamorphosed Shaw ordinary chondrite (DODD, 1972), and in rapidly cooled basalts (SIMKWand SMITH,1970). It seems quite likely that high Ca contents reflect a fre~ng-in of a high temperature equilibrium state of the olivine, implying that the olivine was hot at the time the C-rich material and Fe2Si04 began to react, since the observed zoning of the olivine clearly indicates that re-equilibration of the centers of olivine grains did not occur after this event. During slow cooling the Ca would have formed diopside, or if sufficient Al were present, anorthite. The final equilibration temperature of the olivine was probably somewhere in the range of 135&1550K estimated by FREDRIKS~~Nand MA~CIN (1967) and ONUMAet al. (1972) for Shaw. The olivine data allow several conclusions: (1) the high temperatures needed to bring about the reaction between the C-rich material and the ohvine were not entirely supplied by the shock event, but may have mainly been residual magmatic temperatures. (2) If the heat source which produced the ultramafic silicates by partial melting was a Hayashi phase of the sun or solo-wind induced electrical currents (SoNETTer al., 1970) that was available only during the first 103-lo6 yr of solar system history, injection of the C-rich material occurred very early in the history of the solar system. (3) If the ultramafic materials of ureilites were initially formed under a thick insulating layer, much of this i~ulation was no longer present following the shock event, otherwise the observed compositional gradients in the olivine would not have formed, and extensive loss of rare gas from the C-rich material would probably have occurred. The voids found in ureilites other than Kenna also suggest that, following the shock event, the overburden resulted in relatively low pressures. The most serious problem in this scheme is that it appears necessary to store the C-rich material at appreciable distances from the ultramafic silicates prior to the shock event, and then, within an exceedingly brief interval, to transport the material to the silicates and divide it evenly throughout the moderately large region needed to account for the compositional suite extending from Goalpara to Kenna. This event seems required by the high rare-gas content of the C-rich material and from the fact that loss of
1456
J. T. WASSONet al.
a metal phase should extract most of the elemental c.
that it originated together with the enstatite chondrites in the innermost portion of the solar nebula.
ON THE ORIGIN OF THE CARBON-RICH MATERIAL
RELATIONSHIPS OF UREILITES TO OTHER GROUPS OF METEORITES
In the previous section we showed that the evidence indicated that the C-rich material was injected into the silicates while they were hot, and that cooling occurred soon thereafter. However, it is possible that the injection of the C-rich material and the shock event that produced the diamonds and other shock effects were separated in time (BERKLEYet al., 1976). It seems quite likely that a massive deposit of C-rich material was present before the mixing occurred. BERKLEYet al. (1976) estimate the volume abundance of the C-rich material in Kenna to be z 1l:/,. Some portion of this material must be indigeous ultramafic silicates incorporated during the injection event, but C alone accounts for 20-25x of the material. It seems unavoidable that the linear dimensions of the deposit were at least 10% the thickness (5 1 km) of the ultramafic silicates. Shock waves move faster than material can be transported. If the C-rich material was introduced at the time of the shock event, it follows that most diamonds were formed in or near the original C-rich deposit, not in situ. Since this implies formation of the diamonds at higher pressures than those experienced by the silicate phases, it helps explain the remarkably high efficiency of diamond formation: the abundance of diamond-lonsdalitegraphite aggregates (0.36%) reported by VDOVYKIN(1972) in Havero accounts for _ 16% of the C in this meteorite. The preferred orientation observed in the diamonds by LIPSCHUTZ (1964) and NEUVONEN et ul. (1972) is an inherent property of the aggregates and would still be observed even if these were transported following their production. Graphite is rare in primitive meteorites. It is an accessory in enstatite chondrites (KEIL, 1968). Figure 3 shows that the siderophiles in Goalpara are closely related to those in E4 chondrites: we believe that most of the siderophiles other than Ir in Goalpara is from vein metal. The moderately reduced, primitive group IAB irons contain graphite as large nodules; roughly equal amounts of C are found in this form and as cohenite, Fe& The cohenite is formed by low-temperature exsolution from Fe--Ni, and it is likely that the graphite formed by exsolution at higher temperatures, and is not a primitive material. What, then, is the source of the ureilitic C-rich material? The high abundance of rare gases indicates that it probably existed as small particles in the solar nebula or in some other environment conducive to the incorporation of these gases, and was never strongly heated thereafter. This and the inference that in at least one location it was present as a deposit with dimensions of _ l&100 m suggests that it was a distinct and significant type of primitive material. Our best guess is
The O-isotope data of CLAYTON et a!. (1976) the Zn abundance and Fe/(Fe + Mg) ratios suggest a relationship between the ureilites and the carbonaceous chondrites. As discussed above, there is evidence for appreciable amounts of reduced metal in the parental material from which the ultramafic silicates formed. Since the CI and CM chondrites are essentially metal free, the parent material must have been more similar to the metal-bearing CV or CO groups of carbonaceous chondrites, or perhaps intermediate between these and the ordinary chondrites. BILD and WASSON (1976) report that the Lodran meteorite shares several properties with ureilites (e.g. zoned olivine decreasing in Fe content toward its periphery, abundant planetary-type rare gases). However, olivine cores in Lodran are Fal5, substantially lower than the value of Fa21 typical of the ureilites and the two groups must be assigned to separate parent bodies. BINZ et cd. (1975) suggest that Chassigny might be related to the ultramafic component of the ureilites. However, since the olivine in Chassigny is Fa32 (Pamz et al., 1974), 1.5 x greater than that in the ureilites, separate parent bodies are indicated. If the proto-ureilitic parent body had a chondritic bulk composition, upon differentiation it should have had a core of Fe-Ni and FeS and a feldspar-rich crust. The O-isotope data indicate that the achondrites analyzed by CLAYTON et ~2. (1976) are not samples of its crust. It is not now possible to infer the trace-element composition of the core in order to relate it to a known group of iron meteorites.
A SCENARIO The above discussion leads to the following model of ureilite formation. Some aspects are more speculative than others; in the preceding discussion we have attempted to provide the reader with qualitative indications of the strength of our conclusions, and also reference to the sources of ideas not our own. In this
section we will limit ourselves to a list of the sequence of processes needed to form the ureilites, and of some properties produced by these processes. (1) A parent body formed consisting of material similar to the metal-bearing carbonaceous chondrites. (2) A heat source produced partial melting, and led to the sinking of a metal-troilite melt and the ascension of a feldspar-rich melt. As a result of higher temperatures, the extraction of melts was more efficient in the Goalpara region than in the Kenna region. (3) An impact event injected C-rich material into the ultrarnafic silicates residual from the partial melting process. Diamond formation occurred during this
Ciassification of and elemental fraetianation among ureilites
event or resulted from a later irhpact. Prior to the injection event the C-rich material was present as a massive deposit on the proto-ureilitic body or, more likely, on the projectile responsible for the injection. (4) High” &vine Ca contents show that the ultram&c silicates were hot before the injection event. Shock heating resulted in still higher temperatures. As a result of these high temperatures and a low confining pressure following the shock, C and the oxidied Fe in the silicates began to react to give CO and metallic Fe. (5) The impact event had removed most of the strata overlying the ureilitic region, and within a short period the material’ cooled to a temperature (- 1000 K) too low to permit further reaction between C and oxidized Fe. (6) Further low-velocity collisions broke up the ureilitic material into small bodies that were eventually captured by the Earth as meteorit& Acknowledgements-We are grateful to R. Si CLARKG,G: I. Huss, R. HUTCHISON, V. D. KOLOMENSKII and K. J. NEU-
VONENfor the provision of samples, and to W. V. BOYNN. CLAYTON,K. KEIL,M.E. LEPSCHUTZ,K.MART& R. A. Smrrr, E. M. STOLI’?~,L. L. W~~K~~~ and their
TON, R
coworkers for providing copies of their manuscripts prior to publication. We are indebted to J. WOOD and F. BEGEMANNfor critical reviews and to W. V. BOYNTONfor constructive arguments regarding sideophiles in ureilites. Laboratory and technical assistance was provided by W. V. BOYNTON, H. J. CHLJN,S. KASS,G. PUSAVAT,K. ROBINSOS, L. L. SIlNDBER&,and P. H. WARREN.This research was primarily supported by NSF grants GA-32084 and
DES74-22495.
1457
Chemical evidence for the genesis of the ureilites, the achondrite Chassigny and the nakhlites. Geochim. Cosmochim. Acta 40, 1439-1447. CHOU C.-L.,BAEDECKER P. A.
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