^1,2G.K.Benedix et al. (>10)*
Geochmica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.030]
¹Curtin University, Applied Geology, Bentley, Western Australia, Australia
²Western Australia Museum, Department of Earth and Planetary Sciences, Welshpool, Western Australia,
*Find the extensive, full author and affiliation list on the publishers website
Copyright Elsevier

Bunburra Rockhole is the first recovered meteorite of the Desert Fireball Network. We expanded a bulk chemical study of the Bunburra Rockhole meteorite to include major, minor and trace element analyses, as well as oxygen and chromium isotopes, in several different pieces of the meteorite. This was to determine the extent of chemical heterogeneity and constrain the origin of the meteorite. Minor and trace element analyses in all pieces are exactly on the basaltic eucrite trend. Major element analyses show a slight deviation from basaltic eucrite compositions, but not in any systematic pattern. New oxygen isotope analyses on 23 pieces of Bunburra Rockhole shows large variation in both δ17O and δ18O, and both are well outside the HED parent body fractionation line. We present the first Cr isotope results of this rock, which are also distinct from a majority of HEDs. Detailed computed tomographic scanning and back-scattered electron mapping do not indicate the presence of any other meteoritic contaminant (contamination is also unlikely based on trace element chemistry). We therefore conclude that Bunburra Rockhole represents a sample of a new differentiated asteroid, one that may have more variable oxygen isotopic compositions than 4 Vesta. The fact that Bunburra Rockhole chemistry falls on the eucrite trend perhaps suggests that multiple objects with basaltic crusts accreted in a similar region of the Solar System.

¹Max Collinet, ²Bernard Charlier, ³Olivier Namur, ³Martin Oeser, ^4,5Etienne Médard, ³Stefan Weyer
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.029]
¹Department of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²Département de Géologie, Université de Liège, 4000 Sart Tilman, Belgium
³Institut für Mineralogie, Leibniz Universität Hannover, 30167 Hannover, Germany
⁴Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX 77058, USA
⁵Laboratoire Magmas et Volcans, Université Blaise Pascal-CNRS-IRD, 63178 Aubière, France
Copyright Elsevier

Martian meteorites are the only samples available from the surface of Mars. Among them, olivine-phyric shergottites are basalts containing large zoned olivine crystals with highly magnesian cores (Fo 70-85) and rims richer in Fe (Fo 45-60). The Northwest Africa 1068 meteorite is one of the most primitive “enriched” shergottites (high initial 87Sr/86Sr and low initial ε143Nd). It contains olivine crystals as magnesian as Fo 77 and is a major source of information to constrain the composition of the parental melt, the composition and depth of the mantle source, and the cooling and crystallization history of one of the younger magmatic events on Mars (∼180 Ma). In this study, Fe-Mg isotope profiles analyzed in situ by femtosecond-laser ablation MC-ICP-MS are combined with compositional profiles of major and trace elements in olivine megacrysts. The cores of olivine megacrysts are enriched in light Fe isotopes (δ56FeIRMM-14 = -0.6 to -0.9 ‰) and heavy Mg isotopes (δ26MgDSM-3 = 0 to 0.2 ‰) relative to megacryst rims and to the bulk martian isotopic composition (δ56Fe = 0±0.05 ‰, δ26Mg = -0.27±0.04 ‰). The flat forsterite profiles of megacryst cores associated with anti-correlated fractionation of Fe-Mg isotopes indicate that these elements have been rehomogenized by diffusion at high temperature. We present a 1-D model of simultaneous diffusion and crystal growth that reproduces the observed element and isotope profiles. The simulation results suggest that the cooling rate during megacryst core crystallization was slow (43±21 °C/year), and consistent with pooling in a deep crustal magma chamber. The megacryst rims then crystallized 1 to 2 orders of magnitude faster during magma transport towards the shallower site of final emplacement. Megacryst cores had a forsterite content 3.2±1.5 mol% higher than their current composition and some were in equilibrium with the whole-rock composition of NWA 1068 (Fo 80±1.5). NWA 1068 composition is thus close to a primary melt (i.e. in equilibrium with the mantle) from which other enriched shergottites derived.