¹Takashi Yoshizaki,^1,2,3William F.McDonough
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2021.125746]
¹Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
²Department of Geology, University of Maryland, College Park, MD 20742, USA
³Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
Copyright Elsevier

Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk’s lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth’s mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (∼0.5% of the mass of the Earth’s mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.

^1,2,3Michael K.Weisberg,⁴Noriko T.Kita,⁴Kohei Fukuda,⁴Guillaume Siron,^2,3Denton S.Ebel
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.02.027]
¹Dept. Physical Sci, Kingsborough College CUNY, Brooklyn, NY 11235
²Dept. Earth and Environmental Sci, CUNY Graduate Center, New York, NY 10016
³Dept. Earth and Planetary Sci, American Museum of Natural History, New York, NY 10024
⁴WiscSIMS, Dept. of Geoscience, University of Wisconsin-Madison, Madison, WI 53706
Copyright Elsevier

We report petrology and high precision, in situ oxygen isotope analyses of silicates in chondrules, fragments, metal-rich nodules, refractory inclusions from the ALH 81189 (EH3), ALH 85159 (paired with ALH 81189) and from the MAC 88136 (EL3) chondrite. This is the first report of oxygen isotope ratios for individual objects in an EL3 and for the silicates associated with the metal-rich nodules that are characteristic of unequilibrated enstatite (E3) chondrites. The oxygen isotopic data from the chondrules and other objects form a trend, on a 3-isotope plot, that coincides with the slope∼1 primitive chondrule mineral (PCM) line (initially defined by chondrules from the Acfer 094 primitive carbonaceous chondrite), with most objects clustering at the intersection of the PCM line with the terrestrial fractionation (TF) line, near whole rock E3. The data from EH3 and EL3 overlap and show a similar distribution, suggesting they formed from a similar pool of precursors or in similar gaseous environments, but their mineral compositions suggest differences in their nebular environments and/or parent bodies. Silicates in the metal-rich nodules we analyzed (in both EH3 and EL3) have oxygen isotope ratios (as well as mineral compositions) similar to the silicate (metal-free) chondrules. This is consistent with formation of the metal-rich nodules prior to chondrite accretion, in an environment and from a process similar to that which formed the coexisting chondrules, but from more metal-rich mixtures of precursors. Olivine in an AOA from ALH 81189 is 16O-rich with δ18O = –46.5‰, δ17O = –48.0‰, similar to the AOAs and refractory inclusions previously reported in E3 and in all other chondrite groups. There is a clear distinction in oxygen isotopic compositions between the chondrules in the E3 chondrites and those in the LL and R as well as those in CV and CM chondrite groups. Chondrules from CR and E chondrites plot closer to the PCM line than all other chondrite groups with E3 chondrules having a different distribution toward more 16O-poor compositions. Chondrules in other chondrite groups form trends above and below the PCM. From the distribution of EC chondrules along the PCM line, we propose that similar pools of chondrule precursors were present in the different (carbonaceous, CR and Acfer 094 and non-carbonaceous, E) chondrule forming regions in the protoplanetary disk but with different amounts of 16O-rich refractory materials, prior to development of the postulated Jupiter divide that potentially separated inner (non-carbonaceous) from outer (carbonaceous chondrite) Solar System materials or the Jupiter barrier was inefficient in completely separating these materials.