Other
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..872g&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Other
Scientific paper
Ureilites are olivine-pigeonite bearing achondrites with interstitial carbonaceous material and metal. The latter is present as <1 μm metal inclusions in reduced rims of silicate grains, 5-20 μm spherical inclusions in clear silicate grains, and in association with interstitial carbonaceous material. We have studied the composition of metal and silicates in 9 ureilites with a fayalite content of Fa2 to Fa21. Vein metal contains 3.7 to 5.4 wt% Ni and 0.35 to 0.54 wt% Co. Cobalt and Ni contents in vein metal and fayalite in olivine do not correlate. Mass balance calculations assuming a chondritic parent body yield a metal core with 7 to 11 wt% Ni and 0.3 to 0.55 wt% Co. Thermodynamic calculations of Fe-Ni and Fe-Co exchange between olivine and vein metal show that vein metal cannot be in equilibrium with the olivine at any temperature. We conclude that the vein metal is genetically not linked to the ureilite olivine and may have been injected into the parent body by an impactor. Recently published data show a deficit in 60Ni of - 0.24+/-0.02 ɛ-units for various achondrites including ureilites [1]. This has been interpreted as evidence for a late injection of 60Fe after formation of these achondrites. However, our chemical data for vein metal, which is the dominant Ni host in ureilites, demonstrate that bulk Ni isotope data have little meaning with respect to the formation of ureilite silicates. In this work we present Ni isotope data for bulk samples but also vein material and the silicate phase of 4 ureilites (ALHA77257, EET87157, EET96041, Kenna). Bulk ureilites have a ɛ60 between -0.05+/-0.12 and 0.08+/-0.12; the vein metal gives ɛ60 = -0.05+/-0.13 to 0.11+/-0.16. No resolvable deficit in ɛ60 was found, in disagreement with results reported in Bizzarro et al. (2007). The vein material and the bulk samples have, within uncertainty, the same isotopic composition, confirming that the global Ni budget is controlled by the vein material. In ureilite silicates ɛ60 varies from -0.77+/-0.31 to -0.12+/-0.21. Due to the high Fe/Ni ratio of silicates, clear excesses of 60Ni (at least several ɛ-units) are expected if they formed early in the solar system. This is not observed. There is thus no evidence for life 60Fe in ureilites, which may be interpreted in different ways: either 60Fe was injected at a later time into the protoplanetary disk as suggested by [1] (but this is difficult to reconcile with data obtained in other meteorites [2]), or several isotopically distinct reservoirs co-existed at the beginning of the solar system. The isotopic difference between vein material and silicates rather supports the second hypothesis, even if further studies are required to confirm it. It may also be that the Fe-Ni system has been disturbed at a later stage after formation of ureilites. References [1] Bizzarro M. et al. 2007. Science 316 (5828), 1178- 1181. [2] Quitté G. et al. 2007. LPSC 38, abstract #1900.
Gabriel A. D.
Pack Andreas
Quitté Ghylaine
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