Maria C. Valdes^a, Manuel Moreira^b, Julien Foriel^a and Frédéric Moynier^a,b

^aDepartment of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University in St. Louis, United States
^bInstitut de Physique du Globe de Paris, Université Paris Diderot, 1 rue Jussieu, 75005 Paris, France

Calcium is the fifth most abundant element in the Earth and in chondrites and is a pure lithophile element which does not partition into planetary cores. Therefore, the calcium isotopic composition of the mantle represents the bulk Earth and calcium isotopes have the potential to reveal genetic links between Earth and meteorites. However, whether calcium exhibits significant mass-dependent variations among Earth and the various chondrite groups, and the magnitude of these variations, is still contentious. Here we have developed a new method to analyze calcium isotope ratios with high precision using multiple-collector inductively-coupled-plasma mass-spectrometry. The method has been applied to a range of terrestrial and meteoritic samples. We find that the Earth, the Moon, and the aubrite parent body are indistinguishable from enstatite, ordinary, and CO chondritic meteorites. Therefore, enstatite chondrites cannot be excluded as components of Earth’s building blocks based on calcium isotopes, as has been proposed previously. In contrast, CI, CV, CM and CR carbonaceous chondrites are largely enriched in lighter calcium isotopes compared to Earth, and, overall, exhibit a wide range in calcium isotopic composition. Calcium is the only major element, along with oxygen, for which isotopic variations are observed among carbonaceous chondrite groups. These calcium isotope variations cannot be attributed to volatility effects, and it is difficult to ascribe them to the abundance of isotopically light refractory inclusions. The calcium isotope data presented in this study suggest that both ordinary and enstatite chondrites are representative of the bulk of the refractory materials that formed Earth. On the basis of calcium isotopes, carbonaceous chondrites (with the exception of CO) are not representative of the fraction of condensable material that accreted to form the terrestrial planets and can be excluded as unique contenders for the building blocks of Earth; however, on the basis of other isotopic systems, CO chondrites can be excluded as well.

Reference
Valdes MC, Moreira M, Foriel J and Moynier F (2014) The nature of Earth’s building blocks as revealed by calcium isotopes. Earth and Planetary Science Letters 394:135.
[doi:10.1016/j.epsl.2014.02.052]
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Indhu Varatharajan, Neeraj Srivastava and Sripada V.S. Murty

PLANEX, Physical Research Laboratory, Ahmedabad 380009, India

A comparative assessment of the mineralogy of young basalts (∼1.2 Ga to ∼2.8 Ga) from the western nearside, Moscoviense basin, and the Orientale basin of the Moon has been made using Level 2 Moon Mineralogy Mapper (M³) data from the Chandrayaan-1 mission. Spectral data characteristics of the individual units have been generated from fresh small craters to minimize the complications due to space weathering. Representative spectra for individual units and the derived spectral parameters (Band centers and Integrated Band Depth Ratio) have been used to study composition of these young basalts. A modified approach ofGaffey et al. (2002) (for olivine-pyroxene mixtures) and the methodology of Adams (1974) (for interpreting pyroxene type) have been used to improve our understanding of the spectral behavior of these basalts. Most of the young basalts of Oceanus Procellarum are characterized by abundant olivines and they show complex volcanic history. Vast exposures of olivine concentrated units having higher abundance of olivine content than high-Ca pyroxenes are emplaced in the northern Oceanus Procellarum region. Mostly, they show distinct stratigraphic gradation with the immediately underlying units of relatively lower olivine content. The Moscoviense unit shows signatures of Fe-rich glasses along with clinopyroxenes. The basalts of Orientale basin are typically devoid of olivine and are rich in high-Ca pyroxene. Thus, mineralogy of these mare basalts which erupted during the late stage volcanism vary across the Moon’s surface; however, broader observations reveal apparently higher FeO content in the younger basalts of western nearside and Orientale region.

Reference
Varatharajan I, Srivastava N and Murty SVS (2014) Mineralogy of young lunar mare basalts: Assessment of temporal and spatial heterogeneity using M3 data from Chandrayaan-1. Icarus
[doi:10.1016/j.icarus.2014.03.045]
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