¹M.E. Varela, ²S-L. Hwang, ³P. Shen, ⁴H-T. Chu, ⁵T-F. Yui, ⁵Y. Iizuka, ⁶F. Brandstätter, ⁷Y.A. Abdu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.027]
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE), Avenida España 1512 sur, J5402DSP, San Juan, Argentina
²Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC
³Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan, ROC
⁴Central Geological Survey, PO Box 968, Taipei, Taiwan, ROC
⁵Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
⁶Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, 1010 Wien, Austria
⁷Department of Applied Physics and Astronomy, University of Sharjah, P.O.Box 27272, Sharjah, United Arab Emirate
Copyright Elsevier

Olivinites, together with olivine megacrysts, are the most magnesian phases found in angrites. Their chemical composition (mg# 90) is out of equilibrium with the groundmass and far away from that of possible precipitates from angrite parent melts. Therefore olivinites, as well as olivine megacrysts, were considered as xenoliths and xenocrysts. We report here a detailed study of five olivinites from the angrite D’Orbigny. Our results indicate that D’Orbigny experienced metasomatic alteration processes, which led to enrichments in FeO and MnO (relative to the original composition), changing the initial Mg-rich composition of the olivines to the one seen now. As this process took place in equilibrium with a chondritic reservoir (e.g., Fe/Mn ratios spreading around primitive values), the primitive (Mg-rich) olivine chemical composition was changed towards a more fayalitic one while preserving a chondritic signature. This chondritic signature was preserved in the Fe/Mn ratio of the olivinites, olivine megacrysts, augite grains in olivinites and groundmass olivine of D’ Orbigny. Therefore the fayalite content of about 35 mol.% that characterizes the groundmass olivine of this rock – as well as other angrites- does not correspond to its original composition but may be the result of a late metasomatic process that affected these rocks. If so, olivinites and Mg-rich olivines might not be compositionally exotic phases but are an early constituent phase that retained the pristine more reducing conditions that have been preserved in some angrites, where they form either a small part of the rock (e.g., Asuka 881371 and D’Orbigny) or the majority of it (NWA 8535).

^1,2Stepan M. Chernonozhkin, ³Mona Weyrauch, ^1,2Steven Goderis, ³Martin Oeser, ²Seann J. McKibbin, ³Ingo Horn, ⁴Lutz Hecht, ³Stefan Weyer, ²Philippe Claeys, ¹Frank Vanhaecke
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.022]
¹Ghent University, Department of Analytical Chemistry, Campus Sterre, Krijgslaan, 281 – S12, 9000 Ghent, Belgium
²Vrije Universiteit Brussel, Analytical, Environmental, and Geo- Chemistry, Pleinlaan 2, 1050 Brussels, Belgium
³Leibniz Universität Hannover, Institute of Mineralogy, Callinstrasse 3, 30167 Hannover, Germany
⁴Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany
Copyright Elsevier

In this work, a femtosecond laser ablation (LA) system coupled to a multi-collector inductively coupled plasma-mass spectrometer (fs-LA-MC-ICP-MS) was used to obtain laterally resolved (30-80 μm), high-precision combined Ni and Fe stable isotope ratio data for a variety of mineral phases (olivine, kamacite, taenite, schreibersite and troilite) composing main group pallasites (PMG) and iron meteorites. The stable isotopic signatures of Fe and Ni at the mineral scale, in combination with the factors governing the kinetic or equilibrium isotope fractionation processes, are used to interpret the thermal histories of small differentiated asteroidal bodies. As Fe isotopic zoning is only barely resolvable within the internal precision level of the isotope ratio measurements within a single olivine in Esquel PMG, the isotopically lighter olivine core relative to the rim (Δ56/54Ferim-core = 0.059 ‰) suggests that the olivines were largely thermally equilibrated. The observed hint of an isotopic and concentration gradient for Fe of crudely similar width is interpreted here to reflect Fe loss from olivine in the process of partial reduction of the olivine rim. The ranges of the determined Fe and Ni isotopic signatures of troilite (δ56/54Fe of -0.66 to -0.09 ‰) and schreibersite (δ56/54Fe of -0.48 to -0.09 ‰, and δ62/60Ni of -0.64 to +0.29 ‰) may result from thermal equilibration. Schreibersite and troilite likely remained in equilibrium with their enclosing metal to temperatures significantly below their point of crystallization. The Ni isotopic signatures of bulk metal and schreibersite correlate negatively, with isotopically lighter Ni in the metal of PMGs and isotopically heavier Ni in the metal of the iron meteorites analyzed. As such, the light Ni isotopic signatures previously observed in PMG metal relative to chondrites may not result from heterogeneity in the Solar Nebula, but rather reflect fractionation in the metal-schreibersite system. Comparison between the isotope ratio profiles of Fe and Ni determined across kamacite-taenite interfaces (Δ56/54Fekam-tae = -0.51 to -0.69 ‰ and Δ62/60Nikam-tae = +1.59 to +2.50 ‰) and theoretical taenite sub-solidus diffusive isotopic zoning broadly constrain the cooling rates of Esquel, CMS 04071 PMGs and Udei Station IAB to between ∼25 and 500 °C/Myr.

¹A.A. Nemchin, ²H. Jeon, ³J.J. Bellucci, ¹N.E. Timms, ³J.F. Snape, ²M.R. Kilburn, ³M.J. Whitehouse
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.024]
¹Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
²Centre for Microscopy Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
³Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
Copyright Elsevier

Pb-Pb isochron ages of ca. 3.92 Ga for three K-feldspar-rich clasts from Apollo 14 breccias 14303 and 14083 were determined using Secondary Ion Mass Spectrometry (SIMS). These ages are interpreted to represent the resetting of the U-Pb system in the clasts as a result of brecciation during the Imbrium impact. One of the clasts contains zircon grains that record a significantly older crystallization age (ca. 4.33-4.35 Ga) for the rock represented by that clast. Initial Pb compositions determined for the clasts, combined with the previously measured Pb isotopic compositions of K-feldspar grains from several Apollo 14 breccia samples, constrain a range of initial Pb compositions in the ca. 3.9 Ga Fra Mauro formation at the Apollo 14 landing site. This range in initial Pb compositions indicates that the rocks represented by these clasts, or the sources of those rocks, evolved with a high 238U/204Pb (μ-value) for substantial periods of time, although the precise crystallization ages of the rocks represented by at least two of the clasts investigated here are unknown.