^1,2W. Akram, ^1,2M. Schönbächler, ³S. Bisterzo, ³R. Gallino
¹School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
²Institute for Geochemistry and Petrology, ETH, Clausiusstrasse 25, 8092 Zürich, Switzerland
³Dipartimento di Fisica, Università di Torino, Via P. Giura 1, I–10125 Torino, Italy

A growing number of elements show well–resolved nucleosynthetic isotope anomalies in bulk–rock samples of solar system materials. In order to establish the occurrence and extent of such isotopic heterogeneities in Zr, and to investigate the origin of the widespread heterogeneities in our solar system, new high–precision Zr isotope data are reported for a range of primitive and differentiated meteorites. The majority of the carbonaceous chondrites (CV, CM, CO, CK) display variable ε96Zr values (⩽ 1) relative to the Earth. The data indicate the heterogeneous distribution of 96Zr–rich CAIs in these meteorites, which sampled supernova (SN) material that was potentially synthesized by charged–particle reactions or neutron-captures. Other carbonaceous chondrites (CI, CB, CR), ordinary chondrites and eucrites display variable excesses (ε96Zr ⩽ 1) correlated with small depletions in 91Zr (ε91Zr ⩽ 0.2) relative to the Earth and enstatite chondrites. In contrast to the CAI–related heterogeneity, this correlation provides evidence for variable contributions of average solar system s–process material to different regions of the solar system, with the Earth representing the most s–process enriched material. New s–process model calculations indicate that this s–process component was produced in both low and intermediate mass asymptotic giant branch (AGB) stars. The bulk rock heterogeneity is different to the s–process signature resolved in a previous Zr leaching experiment, which was attributed to low mass AGB stars. The bulk rock heterogeneity requires several nucleosynthetic sources, and therefore opposes the theory of the injection of material from a single source (e.g., supernova, AGB star) and argues for a selective dust–sorting mechanism within the solar nebula. Thermal processing of labile carrier phases is considered and, if correct, necessitates the destruction and removal of non–s–process material from the innermost solar system. New Zr isotope data on mineral separates and a fusion crust sample from chondrites indicate that this non–s–process material could be silicates.

Reference
Akram W, Schönbächler M, Bisterzo S, Gallino R (2015) Zirconium isotope evidence for the heterogeneous distribution of s–process materials in the solar System. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.013]

Copyright Elsevier

¹C.Pan,¹A. D. Rogers,^2,3J. R. Michalski
¹Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
²Planetary Science Institute, Tucson, Arizona, USA
³Department of Earth Sciences, Natural History Museum, London, UK

Central peaks of impact craters contain materials exhumed from depth and therefore, investigation of these materials provide clues to subsurface geology and mineralogy. A global spectral survey of central peaks of Martian impact craters between 10–200 km diameter was completed using Mars Odyssey Thermal Emission Imaging System (THEMIS) data. Twenty-six central peaks with distinctive spectral signatures from surrounding plains were identified and characterized with thermal infrared and visible/near-infrared data. The distribution of spectrally distinct central peaks (SDCPs) shows some degree of regional clustering, with most craters found in western Noachis Terra, Tyrrhena Terra, within the northern rim of Hellas Basin, and fewer in the northern lowlands. With the exception of four craters in western Noachis Terra, SDCPs contain only one spectrally distinct unit at THEMIS resolution (100 m/pixel). The maximum number of spectrally distinct units observed was three, in Jones and Ostrov craters. The western Noachis Terra SDCPs may expose crustal stratigraphies of multiple igneous compositions or impact materials from Argyre. In the highlands, most SDCP units are consistent with enrichments in olivine or pyroxene relative to surrounding plains, suggesting olivine- and pyroxene-basaltic lithologies; few are olivine- and pyroxene-poor. No spatial trend in spectrally-derived compositions of SDCPs was observed. Three SDCPs contain THEMIS signatures consistent with high abundances of phyllosilicates, which may contain the most phyllosilicate-rich lithologies found in central peak-associated materials globally.

Reference
Pan C, Rogers AD, Michalski JR (2015) Thermal and Near-Infrared Analyses of Central Peaks of Martian Impact Craters: Evidence for a Heterogeneous Martian Crust. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004676]

Published by arrangement with John Wiley&Sons