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What is the role of Subduction in Deep Earth Volatile Cycles?
Marc HirschmannUniversity of Minnesota
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Earth’s Deep
Volatile CyclesHydrogen
Carbon (diagram courtesy of J Phipps-Morgan)
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Influence of Modern Tectonics on Deep Earth Volatile Cycles
Possibilities:
1. Net Outgassing
2. Net Ingassing
3. Quasi-steady state
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Hydrogen in the Exosphere (Exosphere=everything above the
Moho) (Lecuyer et al. 1998)
Grams H2O Grams H Oceans 1.4 X 1024 1.56 X 1023 Other 2 X 1023 2.22 X 1022 Total 1.6 X 1024 1.78 X 1023
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Miller et al., Science 2005
Phanerozoic sea level variation H2O must be nearsteady state
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Exosphere Carbon Moles CO2 Grams C Sleep&Zahnle ‘02 Sediments 5.88 X 1021 7.06 X 1022 Oceans 3.31 X 1018 3.97 X 1019 Oceanic Crust 1.20 X 1021 1.44 X 1022 Total 7.08 X 1021 8.50 X 1022 Hayes&Waldbauer ‘06 8.50 X 1021 1.02 X 1023 Holser ‘89 7.64 X 1021 9.17 X 1022 Average 9.29±0.86 X 1022
H in Exosphere (grams) 1.78 X 1023 C in Exosphere (grams) 9.29±0.86 X 1022 H/C ratio of Exosphere
1.95±0.15
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Kerridge, 1985; Robert+Merlivat, 1979; McNaughton et al. 1981; Grady et al., 1989; Javoy, 1998
H/C Ratios of Chondritic Meteorites
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H/C ratio of the mantle:Evidence from undegassed basalts
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Simple Model for H and Cin the Bulk Silicate Earth
(BSE)• H/C of exosphere=1.95±0.15• H/C of MORB source=0.75±0.25• H/C of OIB source=0.5±0.3• H2O in MORB source=100±50 ppm• H2O in OIB source=600±300 ppm• OIB source=10 to 70% of mantle• No hidden volatile-rich reservoirs• Sum these in a Monte Carlo simulation to give H/C of
the mantle and the BSE.
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H/C ratio of the mantle
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H/C ratio of the Bulk Silicate Earth
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H/C ratios of principal Earthand cosmochemicalmaterials
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Why is the H/C Ratio of the Exosphere Much Greater than the
Mantle H/C Ratio?Volcanic outgassing has the opposite effect
(Coutgassing>H outgassing)
Loss of volatiles to space has the opposite effect (Cescape<H escape)
*Preferential Subduction of Carbon
*Remnant of events in Deep Time- Loss of massive CO2-rich atmosphere
- Retention of C during magma ocean degassing
- Early massive return of C to mantle
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Dasgupta et al. (2004)
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Evolution of exosphere by preferentialsubduction of C
(Total H subducted, C/H ratio of subducted mass)
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Evolution of exosphere by preferentialSubduction of C
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carbonates
organic carbon
Hayes and Waldbauer, 2006
mantle
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The exosphere carbon reservoir is dominated bycontinental sediments……
…and so the surface carbon reservoirgrows with the continents.
Veizer and Mackenzie, 2004
Age distribution ofpresent-day continentalsediments
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H/C evolution of the Exosphere (?)
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Zahnle et al. 2007
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Sleep and Zahnle, 2001
Impact Ejecta(lunar glass beads)
(highly susceptible toreaction with atmospheric CO2)
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H/C evolution of the Exosphere
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Carbon may not be subductablein early, hot subduction zones
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H/C ratios of principal Earthand cosmochemicalmaterials
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H/C evolution of the Exosphere
Net C outgassingNet C ingassing