『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.
Keywords: Deep Earth water cycle; Deep Earth carbon cycle; Mantle
volatiles; Magmatic volatiles; Origin of the atmosphere; Late
veneer』
1. Introduction
2. Inventories of H and C in the exosphere and in the mantle
3. Discussion
3.1. The C contents of mantle domains
3.2. H/C ration of the bulk silicate Earth
3.2.1. Constraints on (H/C)BSE and comparison to
chondrites
3.2.2. Extraterrestrial origin of high (H/C)BSE?
3.2.3. Origin of high (H/C)BSE or high (H/C)Exosphere
by removal of a thick CO2 atmosphere?
3.2.4. Origin of high (H/C)BSE by C sequestration
in the core?
3.2.5. A role for a late veneer?
3.3. Mantle-exosphere fractionation of H/C
3.3.1. Retention of C in a crystallizing magma ocean
3.3.2. Preferential degassing of H during basalt extraction
3.3.3. Preferential subduction of carbon
3.3.4. Primitive mantle-normalized concentration of exosphere
volatiles compared to continental crust: the key role of C subduction
3.3.5. Secular variation in the exosphere carbon budget?
3.3.6. A hidden reservoir could change everything
4. Concluding remarks
Acknowledgments
References