Physicochemical Controls on the Compositions of the Earth and Planets

Abstract

Despite the fact that the terrestrial planets formed from the protoplanetary disk, their compositions show marked departures from that of solar nebula condensates. Metallic cores fix oxygen fugacities ($f$O${}_2$s) of the planets to 5 (Mercury) to 1 log units below the iron-wüstite (IW) buffer, orders of magnitude higher than the nebular gas. Their oxidised character is coupled with a lack of volatile elements with respect to the solar nebula. Condensates from a solar gas at different temperatures ($T_0$) have Fe/O (by mass) of 0.93 ($T_0$ = 1250 K) to 0.81 ($T_0$ = 400 K), far lower than that of Earth (1.06). Because the reaction Fe(s) + H2O(g) = FeO(s) + H2(g) proceeds <600 K, temperatures at which most moderately volatile elements (MVEs) have condensed, oxidised planets should be volatile-rich, and vice-versa. That this is not observed suggests that planets did not accrete from equilibrium nebular condensates and/or underwent additional volatile depletion/$f$O${}_2$ changes. Indeed, MVEs in small telluric bodies (Moon, Vesta) indicate near equilibrium evaporation/condensation at IW-1 and 1400-1800 K. Volatile-depleted elemental yet near-chondritic isotopes of larger telluric bodies (Earth, Mars) reflect mixing of bodies of variable volatile depletion, overprinted by volatile-undepleted material. From the Cr- and Ti isotopes in the BSE, such undepleted matter has been proposed to be CI chondrites. 6% CI added late to an enstatite chondrite-like proto-Earth would match the Earth. However, because Earth is an end-member in isotopic anomalies of heavier elements, no combination of existing meteorites alone can account for its chemical- and isotopic composition. Instead, the Earth is made partially or essentially entirely from an NC-like missing component. If so, the oxidised-, yet volatile-poor nature of inner solar system bodies, including Earth and Mars, is intrinsic to the NC reservoir.

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