¹Allan H. Treiman, ²Justin Filiberto
¹Lunar and Planetary Institute, Houston, Texas, USA
²Department of Geology, Southern Illinois University, Carbondale, Illinois, USA

The chemical compositions of shergottite meteorites, basaltic rocks from Mars, provide a broad view of the origins and differentiation of these Martian magmas. The shergottite basalts are subdivided based on their Al contents: high-Al basalts (Al > 5% wt) are distinct from low-Al basalts and olivine-phyric basalts (both with Al < 4.5% wt). Abundance ratios of highly incompatible elements (e.g., Th, La) are comparable in all the shergottites. Abundances of less incompatible elements (e.g., Ti, Lu, Hf) in olivine-phyric and low-Al basalts correlate well with each other, but the element abundance ratios are not constant; this suggests mixing between components, both depleted and enriched. High-Al shergottites deviate from these trends consistent with silicate mineral fractionation. The “depleted” component is similar to the Yamato-980459 magma; approximately, 67% crystal fractionation of this magma would yield a melt with trace element abundances like QUE 94201. The “enriched” component is like the parent magma for NWA 1068; approximately, 30% crystal fractionation from it would yield a melt with trace element abundances like the Los Angeles shergottite. This component mixing is consistent with radiogenic isotope and oxygen fugacity data. These mixing relations are consistent with the compositions of many of the Gusev crater basalts analyzed on Mars by the Spirit rover (although with only a few elements to compare). Other Mars basalts fall off the mixing relations (e.g., Wishstone at Gusev, Gale crater rocks). Their compositions imply that basalt source areas in Mars include significant complexities that are not present in the source areas for the shergottite basalts.

Reference
Treiman AH, Filiberto J (2014) Geochemical diversity of shergottite basalts: Mixing and fractionation, and their relation to Mars surface basalts. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12363]

Published by arrangement with John Wiley and Sons

¹Xie, X., ^“Chen, M., ³Zhai, S., ¹Wang, F.
¹Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
²State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
³Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing, China

An assemblage with FeNi metal, troilite, Fe-Mn-Na phosphate, and Al-free chromite was identified in the metal-troilite eutectic nodules in the shock-produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na-bearing graftonite (Fe,Mn,Na)3(PO4)2, whereas most others correspond to Mn-bearing galileiite Na(Fe,Mn)4(PO4)3 and a possible new phosphate phase of Na2(Fe,Mn)17(PO4)12 composition. The Yanzhuang meteorite was shocked to a peak pressure of 50 GPa and a peak temperature of approximately 2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock-produced Fe-Ni-S-O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, metal-troilite eutectic intergrowths, and silicate melt glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe-Ni-S-O melt combined with Fe and crystallized as Fe-Mn-Na phosphates within troilite, while Cr combined with Fe and crystallized as Al-free chromite also within troilite.

Reference
Xie X, Chen M, Zhai S, Wang, F (2014) Eutectic metal + troilite + Fe-Mn-Na phosphate + Al-free chromite assemblage in shock-produced chondritic melt of the Yanzhuang chondrite. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12379]

Published by arrangement with John Wiley & Sons

^1,2M.M.M. Meier, ³A. Reufer, ²R. Wieler
¹Centre de Recherches Pétrographiques et Géochimiques, CNRS Nancy, France
²Department of Earth Sciences, ETH Zurich, Switzerland
³School of Earth & Space Exploration, Arizona State University, AZ 85287-6004, USA

Knowing the isotopic composition of Theia, the proto-planet which collided with the Earth in the Giant Impact that formed the Moon, could provide interesting insights on the state of homogenization of the inner Solar System at the late stages of terrestrial planet formation. We use the known isotopic and modeled chemical compositions of the bulk silicate mantles of Earth and Moon and combine them with different Giant Impact models, to calculate the possible ranges of isotopic composition of Theia in O, Si, Ti, Cr, Zr and W in each model. We compare these ranges to the isotopic composition of carbonaceous chondrites, Mars, and other Solar System materials. In the absence of post-impact isotopic re-equilibration, the recently proposed high angular momentum models of the Giant Impact (“impact-fission”, Cúk, M., Stewart, S.T. [2012]. Science 338, 1047; and “merger”, Canup, R.M. [2012]. Science 338, 1052) allow – by a narrow margin – for a Theia similar to CI-chondrites, and Mars. The “hit-and-run” model (Reufer, A., Meier, M.M.M., Benz, W., Wieler, R. [2012]. Icarus 221, 296–299) allows for a Theia similar to enstatite-chondrites and other Earth-like materials. If the Earth and Moon inherited their different mantle FeO contents from the bulk mantles of the proto-Earth and Theia, the high angular momentum models cannot explain the observed difference. However, both the hit-and-run as well as the classical or “canonical” Giant Impact model naturally explain this difference as the consequence of a simple mixture of two mantles with different FeO. Therefore, the simplest way to reconcile the isotopic similarity, and FeO dissimilarity, of Earth and Moon is a Theia with an Earth-like isotopic composition and a higher (∼20%) mantle FeO content.

Reference
Meier MMM, Reufer A, Wieler R (2014) On the origin and composition of Theia: Constraints from new models of the Giant Impact. Icarus 242, 316–328 Link to Article [DOI: 10.1016/j.icarus.2014.08.003]

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