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What is the role of Subduction in Deep Earth Volatile Cycles?. Marc Hirschmann University of Minnesota. Earth’s Deep Volatile Cycles. Hydrogen. Carbon. (diagram courtesy of J Phipps-Morgan). Influence of Modern Tectonics on Deep Earth Volatile Cycles. Possibilities: 1. Net Outgassing

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What is the role of Subduction

in Deep Earth Volatile Cycles?

Marc Hirschmann

University of Minnesota


Earth’s Deep

Volatile Cycles

Hydrogen

Carbon

(diagram courtesy of J Phipps-Morgan)


Influence of modern tectonics on deep earth volatile cycles

Influence of Modern Tectonics on Deep Earth Volatile Cycles

Possibilities:

1. Net Outgassing

2. Net Ingassing

3. Quasi-steady state


Phanerozoic sea level variation

H2O must be near

steady state

Miller et al., Science 2005


H/C Ratios of Chondritic Meteorites

Kerridge, 1985; Robert+Merlivat, 1979;

McNaughton et al. 1981; Grady et al., 1989; Javoy, 1998


H/C ratio of the mantle:

Evidence from undegassed basalts


Simple model for h and c in the bulk silicate earth bse
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.




H/C ratios of

principal Earth

and cosmochemical

materials


Why is the h c ratio of the exosphere much greater than the mantle h c ratio

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


Dasgupta et al. (2004) Mantle H/C Ratio?


Evolution of exosphere by preferential Mantle H/C Ratio?

subduction of C

(Total H subducted, C/H ratio of subducted mass)


Evolution of exosphere by preferential Mantle H/C Ratio?

Subduction of C


carbonates Mantle H/C Ratio?

mantle

organic carbon

Hayes and Waldbauer, 2006


The exosphere carbon reservoir is dominated by Mantle H/C Ratio?

continental sediments……

Age distribution of

present-day continental

sediments

…and so the surface carbon reservoir

grows with the continents.

Veizer and Mackenzie, 2004



Zahnle et al. 2007 Mantle H/C Ratio?


Impact Ejecta Mantle H/C Ratio?

(lunar glass beads)

(highly susceptible to

reaction with

atmospheric CO2)

Sleep and Zahnle, 2001



Carbon may not be subductable Mantle H/C Ratio?

in early, hot subduction zones


H/C ratios of Mantle H/C Ratio?

principal Earth

and cosmochemical

materials


H/C evolution of the Exosphere Mantle H/C Ratio?

Net C ingassing

Net C outgassing


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