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 ppm CO 2 Enriched mantle up to 1000 ppm CO 2

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 °C Device: 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) + 1/2O 2 2log(a FeO /a Fe ) =  IW 2 GPa, 1625 °C fO 2 = IW-1 to IW-2

Carbon Solubility at Graphite Saturation, CCGC (fO 2 ~ 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 – ppm CO 2 ; Enriched (OIB) mantle – up to 1000 ppm CO 2

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 12 C/ 30 Si versus basaltic melt CO 2 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 fO 2 of IW-1 to IW-2, CO 2 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