Volatile element abundance patterns and an early liquid water ocean on Earth

Abstract

The abundances of the elements H, C, N, Ne, primordial Ar, Cl, Br, Kr, I and Xe, in the atmo-hydrosphere, continental crust and MORB-source mantle (termed the outer Earth reservoirs (OER)) are examined and compared with solar matter and an average of carbonaceous chondrites (CC). The aim is to assess the likelihood of various possible sources for these volatile elements now on Earth, and discuss and possibly quantify processes whereby volatiles were lost, in order to narrow down scenarios for the earliest terrestrial environment. Al-normalized abundance patterns show a depletion relative to solar matter which varies from 2×10−11 for Ne to 4×10−3 for the halogens. The overall relative pattern is broadly similar to an average abundance pattern of CC, with two to three orders of magnitude depletion relative to this. For H, C, N and the halogens this similarity, against the background of the huge diversity compared to solar matter, makes CC-type material a much more likely source than a gravitationally captured solar atmosphere. A small admixture of solar matter (Alsolar/Altotal≈10−7) acquired in accretion can account for the solar Ne isotope signatures in the deep mantle. The abundance pattern of C, N, Ne, primordial Ar, Br and Kr relative to such a “solar spiked” CC-type source shows depletions dependent of the masses of the molecules (C as CO, N as N2, Br as Br2), which can be modeled assuming a hydrodynamic escape mechanism with H2 fluxes varying from 1 to 3×1013 molecules cm−2 s−1. Relative to this depletion pattern, H (as H2O) and Cl (as HCl) are overabundant by about an order of magnitude. Delivery by comets over Earth history is a highly unlikely explanation for the H overabundance, as measured cometary D/H ratios are ∼2×, and C/H ratios ∼4× higher than those of the OER. The overabundance of both H and Cl can be understood and quantitatively modeled if it is assumed that a liquid water ocean existed in which major portions (on average, between 50 and 70%) of H2O and HCl resided during hydrodynamic escape, so that their volume proportion in the atmosphere (and therefore their loss) was reduced.