Osmium isotope systematics of ureilites

Abstract

The 187 Os/ 188 Os for twenty-two ureilite whole rock samples, including monomict, augite-bearing, and polymict lithologies, were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body (or bodies). The Re/Os ratios of most ureilites show evidence for a recent disturbance, probably related to Re mobility during weathering, and no meaningful chronological information can be extracted from the present data set. The ureilite 187 Os/ 188 Os ratios span a range from 0.11739 to 0.13018, with an average of 0.1258 ± 0.0023 (1σ), similar to typical carbonaceous chondrites, and distinct from ordinary or enstatite chondrites. The similar mean of 187 Os/ 188 Os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites. From the narrow range of the 187 Os/ 188 Os distribution in ureilite meteorites it is further concluded that Re was not significantly fractionated from Os during planetary differentiation and was not lost along with the missing ureilitic melt component. The lack of large Re/Os fractionations requires that Re/Os partitioning was controlled by a metal phase, and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body.3

Highly siderophile elements in ureilites

“…We further consider the effect of differentiation on the heat-producing elements (e.g., 60 Fe) in ureilites, and evaluate the prospect of using the short-lived 60 Fe-60 Ni chronometer for dating metallic partial melt extraction in the UPB. The Re-Os abundances have been reported by Rankenburg et al (2007). The HSE data were presented in preliminary form by Rankenburg et al (2005), and some of the modeling aspects of this study were presented by Humayun et al (2005).…”

“…The suite of samples and analytical techniques employed are identical to those of Rankenburg et al (2005Rankenburg et al ( , 2007. In this contribution, isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) data for Ru, Pd, Ir, and Pt, measured at the National High Magnetic Field Laboratory, are presented in conjunction with Re-Os abundances measured previously on the same sample aliquots (Rankenburg et al, 2007).…”

“…In this contribution, isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) data for Ru, Pd, Ir, and Pt, measured at the National High Magnetic Field Laboratory, are presented in conjunction with Re-Os abundances measured previously on the same sample aliquots (Rankenburg et al, 2007). Five of our 23 samples were also analyzed in a recent elemental study of 25 ureilites (Warren et al, 2006) that also compiles earlier neutron activation analyses of another 12 of these ureilites.…”

“…It is the exception among known achondrites that a compositional or oxygen isotopic link exists between the members of the achondrite class and its parent chondrite. However, there is increasing consensus from experimental data (Walker and Agee, 1988), oxygen and osmium isotopes (Clayton and Mayeda, 1988;Rankenburg et al, 2007), and high carbon abundances (Grady et al, 1985;Hudon et al, 2004;Mittlefehldt et al, 2005) that ureilites began their existence as materials broadly similar to carbonaceous chondrites. Although the specific chemical composition, oxidation state, and size of the parent body, all influence its exact differentiation path, the first partial melts in a body undergoing progressive heating should be generated along the (Fe, Ni)-FeS cotectic at $950 to 988°C (Villars et al, 1995).…”

Siderophile Elements in Tracing Planetary Formation and Evolution

“…As with two of the diogenites, four of the angrites evolved to sub-chondritic present day 187 Os/ 188 Os, indicating that Re behaved less compatibly than Os as the rocks formed either as residues of partial melting or olivine-rich cumulates. This is unusual for achondrites, although some ureilites are also characterised by present day sub-chondritic 187 Os/ 188 Os (Rankenburg et al, 2007). CI chondrite normalised HSE abundances for angrite meteorites.…”