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

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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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slide1
What is the role of Subduction

in Deep Earth Volatile Cycles?

Marc Hirschmann

University of Minnesota

slide2
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

slide5
Phanerozoic sea level variation

H2O must be near

steady state

Miller et al., Science 2005

slide8
H/C Ratios of Chondritic Meteorites

Kerridge, 1985; Robert+Merlivat, 1979;

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

slide9
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.
slide13
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

*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

slide17
Evolution of exosphere by preferential

subduction of C

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

slide19
carbonates

mantle

organic carbon

Hayes and Waldbauer, 2006

slide22
The exosphere carbon reservoir is dominated by

continental sediments……

Age distribution of

present-day continental

sediments

…and so the surface carbon reservoir

grows with the continents.

Veizer and Mackenzie, 2004

slide26
Impact Ejecta

(lunar glass beads)

(highly susceptible to

reaction with

atmospheric CO2)

Sleep and Zahnle, 2001

slide28
Carbon may not be subductable

in early, hot subduction zones

slide29
H/C ratios of

principal Earth

and cosmochemical

materials

slide30
H/C evolution of the Exosphere

Net C ingassing

Net C outgassing

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