The Influence of Magma Ocean Processes on the Present-day Inventory of Deep Earth Carbon

Rajdeep Dasgupta

CIDER post-AGU workshop December 10, 2011

Depleted mantle

50-200 ppm CO2

Enriched mantle

up to 1000 ppm CO2

What is the influence of the modern plate tectonic cycles on the deep Earth carbon inventory and distribution?

Is the Earth’s present-day mantle carbon budget shaped by early Earth processes?

Behavior of C during early Earth differentiation (metal-silicate equilibration and reduced magma ocean)?

In what form and capacity the magma ocean retained carbon?

Experimental simulation of metal-silicate equilibration in a magma ocean environment

Experiments: 1 – 7 GPa; 1500 – 2000 °CDevice: Piston cylinder (Rice) and Multi-anvil (Lamont-Doherty Earth Observatory)

Starting Mix: Basalt/komatiite/peridotite + Fe-Ni + C ± S

Han Chi

FeO (silicate melt) = Fe (alloy melt) + 1/2O2

2log(aFeO/aFe) = IW

2 GPa, 1625 °C

fO2 = IW-1 to IW-2

Carbon Solubility at Graphite Saturation, CCGC (fO2 ~ IW-1.6)

Chi et al. (in progress)

Silicate Melt

Fe-5%Ni Melt

CCGC in basaltic melt is small and increases with increasing T and decreases with increasing P

CCGC in Fe-rich metallic melt is significant

Chi et al. (in progress)

Carbon partitioning during metal-silicate equilibration

Depleted (MORB) mantle – 50-200 ppm CO2; Enriched (OIB) mantle – up to 1000 ppm CO2

Carbon partitioning into segregating metallic liquid

1. Trapped metallic liquid, metal carbide, and metal alloy in crystallizing magma ocean

2. C-ingassing from early atmosphere

C in atmosphere

1

2

Closing Remarks

• Carbon dissolution in reduced magma ocean was primarily in the form of neutral or hydrogenated species AND solubility was likely low (≤100 ppm C)

• Most of early Earth carbon was partitioned into metallic Fe-Ni liquid and subsequently sequestered in the metallic core OR trapped as interstitial alloy or carbide phase

• Convective overturn of lower mantle materials (with interstitial carbide or alloy) may have supplied a sizeable portion of the initial carbon budget of the mantle

• Depth dependence of carbon concentration in reduced basaltic magma at graphite/diamond saturation may also have facilitated ingassing of carbon from the early atmosphere

How much carbon was dissolved in the silicate magma ocean during core segregation?

C measurement in silicate glasses using 12C/30Si versus basaltic melt CO2 calibration using ion probe

Chi et al. (in progress)

Cameca IMS 1280 ion probe at WHOI

Graphite

Fe-Ni metallic melt

Silicate melt

SIMS pits

1 mm

2 GPa, 1625 °C, Fe-Ni melt saturated basaltic melt in a graphite capsule

At fO2 of IW-1 to IW-2, CO2 solubility (in graphite and metal saturated conditions) in basaltic silicate melt is minimal

Chi et al. (in progress)

Typical carbon bearing basalt FTIR spectra at oxidized (~IW+5) conditions