The H/C ratios of Earth's near-surface and deep reservoirs, and consequences for deep Earth volatile cycles

Abstract

The H/C mass ratio of the Earth's exosphere, which consists of the fluid envelopes plus the crust, is 1.95 ± 0.15. In contrast, the H/C ratios of undegassed oceanic basalts are significantly lower, ranging from 1.2 down to 0.05. Reconstruction of source H/C ratios by accounting for H/C fractionation during partial melting and addition of carbon-enriched low-degree partial melts suggests that the source regions of MORB have H/C ratios in the range of 0.75 ± 0.25 and those of OIB have ratios in the interval 0.5 ± 0.3. Combining these estimates with plausible limits on the relative proportions of the OIB and MORB sources indicates that the total H inventory of the mantle is equivalent to between 0.2 and 1.6 times the H in the exosphere, assuming that there are no significant hidden reservoirs unsampled by oceanic basalts. Combining the H contents and H/C ratios of the mantle and the exosphere suggests that the H/C ratio of the bulk silicate Earth, (H/C) BSE, is 0.99 ± 0.42, significantly greater than the H/C ratio of chondrites, which have H/C ratios no greater than 0.55. The superchondritic (H/C) BSE ratio likely results from preferential sequestration of C in the core, though it may also partly reflect a cometary origin for some portion of the BSE volatile inventory. The high (H/C) BSE ratio, combined with a D/H ratio similar to chondrites, argues strongly that the BSE volatile inventory is not chiefly derived from a late veneer. The large difference in H/C ratio between the exosphere and the mantle could reflect early Earth processes such as preferential retention of C in a crystallizing magma ocean in reduced phases such as diamond, or selective loss of a massive CO 2-rich atmosphere. Alternatively, it may have arisen by enhanced subduction of carbon relative to hydrogen. If the latter is the case, carbon in the mantle is likely dominantly recycled.