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

Marc M. Hirschmann, Rajdeep Dasgupta

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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
Issue number1-2
StatePublished - May 2009

Bibliographical note

Funding Information:
We thank Cyril Aubaud, Tony Withers, Lindy Elkins-Tanton, and Pierre Cartigny for stimulating discussions that have lead to some of the ideas in this paper. We are especially grateful for the detailed editorial comments of Jackie Dixon and two anonymous referees that prompted many key improvements to the original manuscript and that saved us from the embarrassment of some errors. This work supported by NSF Grant OCE-0623550 and NASA grant NNX08AN07G. RD acknowledges support of Lamont-Doherty Earth Observatory post-doctoral fellowship and a Rice University start-up grant.


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


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