Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.593w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 593
Other
2
Elements, Siderophile, Eucrites, Fugacity, Oxygen, Mars, Meteorites, Snc, Osmium, Rhenium
Scientific paper
We have employed neutron activation, including radiochemical NAA, to investigate SNC/martian meteorites ALH 77005, ALH 84001 and LEW 88516, along with 15 eucrites. Our data for 10 manifestly monomict eucrites confirm previous indications [e.g., 1] that compositionally pristine eucrites are generally extremely siderophile-poor, although for several of the most extremely siderophile-depleted eucrites we find slight enhancements in Re/Os (Figure). Our RNAA data are the first for highly siderophile elements in polymict eucrites, and show a broad similarity with lunar polymict breccias. In general, our data (e.g., Ga/Al = 4.3x10^-4) confirm SNC affinity [2] for ALH84001. However, siderophile concentrations are, by SNC standards, extraordinarily low: Ni = 5.8 micrograms/g and (in pg/g) Au = 9.4, Ir = 80, Os = 10.2, and Re = 1.66+/-0.25(1-s); Ge (1080 ng/g) is typical for SNCs. Like terrestrial basalts [1], other SNCs have relatively constant Re, ranging from 28 (Lafayette [3]) to 102 pg/g (ALH 77005) among seven analyzed meteorites of various types, in which Os ranges from <2.3 to 4400 pg/g. A plot of Os vs. Re/Os (Figure) shows that ALH 84001 has 23x lower Re than expected for a young SNC of similar Os content. On Earth, Re generally behaves as a mildly incompatible element, whereas Os behaves as a strongly compatible element. A plausible explanation for this divergence [1] is that Re is more prone to enter higher oxidation states, such as Re^4+, which would tend to behave like W^4+. This model is consistent with the Os-like behavior of Re in the highly reduced lunar and eucritic environments, and Birck and Allegre [1] interpret the typically intermediate Re contents of SNCs as suggestive of origin from a mantle source region at intermediate fO(sub)2 (they also considered, but rejected, an implausible "contamination" model). Extended to ALH 84001, this model implies that the mantle source was at a substantially (roughly 1.7 log(sub)10 units) lower fO2 than the analogous sources of the younger SNCs. Conceivably ALH84001 siderophiles were altered by metasomatic processes [cf. 2]. However, near-surface processing on a heavily cratered body would generally tend to add siderophile material, rather than remove it. Also, Treiman [4] argues that alteration took place strictly at low temperatures. Another possible objection is that unlike eucrites and lunar basalts, ALH84001 is rich in Fe^3+ [2]. However, if the parent magma encountered even a small proportion of water in the upper crust of Mars, the final fO(sub)2 would be substantially raised. Many authors have proposed that the terrestrial planets in general, and Mars in particular, were originally very dry, implying low fO(sub)2, and that only late in accretion history did substantial proportions of oxidizing volatiles accrete as a "veneer". Possibly when ALH84001 formed, 4.50+/-0.13 Ga [5], oxidation had not yet altered the primordial, low fO(sub)2 nature of its mantle source region. It is also conceivable that large portions of the martian mantle never reached fO(sub)2 as high as inferred for the younger SNCs (possibly derived from a single crater). In any case, the siderophile-depletion pattern of ALH 84001 is unique among SNCs. References: [1] Birck J. L. and All gre C. J. (1994) EPSL, 124, 139-148. [2] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [3] Treiman A. H. et al. (1986) GCA, 50, 1071-1091. [4] Treiman A. H. (1995) Meteoritics, 30, 294-302. [5] Nyquist L. E. et al. (1995) LPS, XXVI, 1065-1066.
Kallemeyn Gregory W.
Warren Harry P.
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