Origin of Halogens and Nitrogen in Enstatite Chondrites

Abstract

The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH 2 /pH 2 O ratios. Enstatite chondrites (particularly EH) have higher CI-and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas.

Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

“…(with the N 2 possibly being derived from nitride condensates; e.g., Rubin and Choi, 2009). Such high temperatures are far beyond those envisioned for thermal metamorphism in EL4 or even EL6 chondrites ($700-950°C) and would have caused wide-spread melting of metal and sulfide.…”

Impact melting in the Cumberland Falls and Mayo Belwa aubrites

“…The retention of F in impact melted enstatite meteorites is consistent with a high original abundance of F. Enstatite chondrites are particularly rich in F: e.g., F in EH chondrites has an average Mg-normalized abundance ratio of 3.40 · CI (Rubin and Choi 2009). The F in Abee, St. Sauveur and Y 82189 may have initially condensed as simple metal halides that were incorporated into enstatite-chondrite precursor materials (Rubin and Choi 2009).…”

Nitrogen Content in the Earth's Outer Core

“…Similarly, the delivery of nitrogen to the Earth might have postdated core formation as has been suggested for many other volatile elements (Albarede, 2009;Dasgupta et al, 2013;Wang & Becker, 2013). For example, nitrogen may be delivered during core formation in the form of relatively refractory nitrides rather than less stable organics (Rubin & Choi, 2009), minimizing the volatility-induced loss. Hence, arguments both against and in favor of nitrogen sequestration in the core exist from various geochemical considerations.…”

“…Despite the geochemical arguments against significant concentration of nitrogen in the core, nitrogen cannot be ruled out as an important light element in the Earth's core on the basis of cosmochemical and early accretion process alone. For example, nitrogen may be delivered during core formation in the form of relatively refractory nitrides rather than less stable organics (Rubin & Choi, 2009), minimizing the volatility-induced loss. Moreover, a fraction of terrestrial core growth might have happened through near-disequilibrium merger of a differentiated planetary embryo (e.g., Li, Marty, et al, 2016;Rudge et al, 2010), in which case merger of a nitride-rich metallic core of more exotic composition formed from a different bulk composition could take place.…”