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

Marc M. Hirschmann, Rajdeep Dasgupta

Research output: Contribution to journalArticlepeer-review

129 Scopus citations

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 CO2-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.

Original languageEnglish (US)
Pages (from-to)4-16
Number of pages13
JournalChemical Geology
Volume262
Issue number1-2
DOIs
StatePublished - May 2009

Keywords

  • Deep Earth carbon cycle
  • Deep Earth water cycle
  • Late veneer
  • Magmatic volatiles
  • Mantle volatiles
  • Origin of the atmosphere

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