HR: 08:35h
AN: V51F-03    [Abstracts]
TI: Global Cycling of Carbon Constrained by Partial Melting Experiments of Carbonated Mantle Peridotite and Eclogite
AU: * Dasgupta, R
EM: dasg0007@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Hirschmann, M M
EM: hirsc022@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Withers, A C
EM: withe012@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AB: The mass of carbon stored in the mantle exceeds that in all other Earth's reservoirs combined¹ and large fluxes of carbon are cycled into and out of the mantle via subduction and volcanic emission. Outgassing of CO₂ from the mantle has a critical influence on Earth's climate for time scales of 10⁸-10⁹ yr¹. The residence time for carbon in the mantle is thought to exceed the age of the Earth^1,2, but it could be significantly less owing to pervasive deep melting beneath oceanic ridges. The chief flux of subducted carbon is via carbonate in altered ocean-floor basalts, which survives dehydration during subduction. Because solidi of carbonated eclogite remain hotter than average subduction geotherms at least up to transition zone³, significant subducted C is delivered to the deep Earth. In upwelling mantle, however, partial melting of carbonated eclogite releases calcio-dolomitic carbonatite melt at depths near ~400 km and metasomatically implants carbonate to surrounding peridotite. Thus, volcanic release of CO₂ to basalt source regions is controlled by the solidus of carbonated peridotite. We conducted experiments with nominally anhydrous, carbonated garnet lherzolite (PERC: MixKLB-1+2.5 wt.% CO₂) using Pt/C capsules in piston cylinder (3 GPa) and Walker-style multi-anvil presses (4 to 10 GPa) and between 1075-1500 °C. The stable near-solidus crystalline carbonate is dolomite_ss at 3 GPa and magnesite_ss from 4 to 10 GPa. Carbonate melt is stabilized at the solidus and crystalline carbonate disappears within 20-60°. The solidus increases from ≥1075 °C at 3 GPa to 1110-1140 °C at 4.1 GPa as the stable carbonate transforms from dolomite_ss to magnesite_ss. From 4.1 GPa, the solidus of PERC magnesite lherzolite increases to ~1500 °C at 10 GPa. In upwelling mantle the solidus of carbonated lherzolite is ~100-200 km shallower than that of eclogite+CO₂, but beneath oceanic ridges, initial melting occurs as deep as 300-330 km. For peridotite with ~120-1200 ppm CO₂, this initial melting yields 0.03-0.3% carbonatite melt. Extraction of such melts from the mantle above 300 km implies residence times of 1 to 4 Gyr for carbon and other highly incompatible elements in the convecting mantle. Such short residence times suggest that large fractions of mantle carbon must be recycled rather than primordial. Implied CO₂ fluxes are 0.12-3.4 × 10¹⁵ g/yr, which matches or exceeds direct estimates for CO₂ fluxes at ridges (0.04-0.66 × 10¹⁵ g/yr)^1,4. However, not all of this deep extracted CO₂ may reach ridges; some may instead be implanted into oceanic lithosphere, providing a widespread source for metasomatic fluids highly enriched in incompatible elements. ¹Sleep, N. H. and Zahnle, K. 2001, JGR 106, 1373-1399. ²Zhang, Y. and Zindler, A. 1993, EPSL 117, 331-345. ³Dasgupta et al. 2004, EPSL 227, 73-85. ⁴Javoy, M. and Pineau, F. 1991, EPSL 107, 598-611.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005