『Abstract
Carbon geochemistry of mantle-derived samples suggests that the
fluxes and reservoir sizes associated with deep cycle are in the
order of 12〜13 gC/yr and 1022〜23 gC, respectively.
This deep cycle is responsible for the billion year-scale evolution
of the terrestrial carbon reservoirs. The petrology of deep storage
modulates the long-term evolution and distribution of terrestrial
carbon. Unlike water, which in most of the Earth's mantle is held
in nominally anhydrous silicates, carbon is stored in accessory
phases. The accessory phase of interest, with increasing depth,
typically changes from fluids/melts→calcite/dolomite→magnesite→diamond/Fe-rich
alloy/Fe-metal carbide, assuming that the mass balance and oxidation
state are buffered solely by silicates. If, however, carbon is
sufficiently abundant, it may reside as carbonate even in the
deep mantle. If Earth's deep mantle is Fe-metal saturated, carbon
storage in metal alloy and as metal carbide cannot be avoided
for depleted and enriched domains, respectively. Carbon ingassing
to the interior is aided by modern subduction of the carbonated
oceanic lithosphere, whereas outgassing from the mantle is controlled
by decompression melting of carbonated mantle. Carbonated melting
at ＞ 300 km depth or redox melting of diamond-bearing or metal-bearing
mantle at somewhat shallow depth generates carbonatitic and carbonated
silicate melts and are the chief agents for liberating carbon
from the solid Earth to the exosphere. Petrology allows net ingassing
of carbon into the mantle in the modern Earth, but in the hotter
subduction zones that prevailed during the Hadean, Archean, and
Paleoproterozoic , carbonate likely was released at shallow depths
and may have returned to the exosphere. Inefficient ingassing,
along with efficient outgassing, may have kept the ancient mantle
carbon-poor. The influence of carbon on deep Earth dynamics is
through inducing melting and mobilization of structurally bound
mineral water. Extraction of carbonated melt on one hand can dehydrate
the mantle and enhance viscosity; the presence of trace carbonated
melt on other may generate seismic low-velocity zones and amplify
attenuation.
Keywords: deep carbon cycle; partial melting; carbonatite; carbonated
silicate melt; carbonate; iron carbide; CO2;
subduction; early Earth』
1. Introduction
2. Carbon budget in the Earth's interior - constraints from basalt
geochemistry
3. Carbon-bearing phases in the mantle
4. Modern fluxes of carbon in and out of the mantle
5. Ingassing of carbon - petrology of carbonate subduction
6. Petrology of carbon outgassing - partial melting of the carbon-bearing
mantle
6.1. First melting of carbonated mantle in oceanic provinces
6.2. Carbonated silicate melts in the mantle
7. The early Earth carbon cycle
7.1. Whole Earth carbon distribution in the Hadean
7.2. Ingassing and outgassing of carbon in the early Earth
8. The deep Earth C and H2O geodynamic cycles:
a brief comparison
Acknowledgements
References