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What is the role of Subduction in Deep Earth Volatile Cycles?PowerPoint Presentation

What is the role of Subduction in Deep Earth Volatile Cycles?

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### Influence of Modern Tectonics on Deep Earth Volatile Cycles

### Why is the H/C Ratio of the Exosphere Much Greater than the Mantle H/C Ratio?

What is the role of Subduction

in Deep Earth Volatile Cycles?

Marc Hirschmann

University of Minnesota

Possibilities:

1. Net Outgassing

2. Net Ingassing

3. Quasi-steady state

H/C Ratios of Chondritic Meteorites

Kerridge, 1985; Robert+Merlivat, 1979;

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

Evidence from undegassed basalts

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.

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

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

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

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

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

Carbon may not be subductable Mantle H/C Ratio?

in early, hot subduction zones

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