HR: 0800h
AN: V41A-1418    [Abstracts]
TI: Effect of Variable Carbonate Concentration on the Solidus of Mantle Peridotite
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: * Dasgupta, R
EM: dasg0007@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AB: Experiments show that the carbonated lherzolite solidus intersects the oceanic geotherm at ~300 km beneath ridges¹, so CO₂ may incite the deepest melting in basalt source regions². However, experimental solidus detection requires an order of magnitude more CO₂ than that (< ~0.1-0.4 wt.%) present in mantle source regions, which may bias solidus determinations. To explore the effect of CO₂ concentration on the solidus of natural carbonated peridotite, we performed near solidus phase equilibria experiments in a multi-anvil for compositions with 1.0, 2.5 and 5.0 wt% CO₂ at 6.6 GPa, using Pt-graphite capsules. Solidi were bracketed using textural criteria and verified by tracking shifts in Na₂O in cpx with temperature. With increasing bulk CO₂, solidi are located in the intervals 1190-1220, 1250-1275, and 1300-1330 °C respectively and magnesite_ss is consumed within ≤25, ≤50, and ~60 °C of the solidus. Drops in Na₂O in cpx across the solidus indicate preferential partitioning of Na to carbonate melt, the effect of which diminishes with decreasing bulk CO₂, corresponding to decreasing near-solidus melt fraction. Reconstructed melt compositions also suggest that Na-enrichments in carbonate melts diminish with increasing bulk CO₂ (7.2 wt.% Na₂O for 1 wt.% CO₂; 2.7 wt.% Na₂O for 2.5 wt.% CO₂; 1.7 wt.% Na₂O for 5 wt.% CO₂) owing to dilution of Na in larger melt fractions, causing the solidus to rise. This suggests that the 6.6 GPa solidus of natural peridotite with 10s to 1000s of ppm CO₂ is ~1150-1190 °C, ~600 °C lower than that of volatile-free peridotite solidus. This solidus is applicable provided magnesite is present at the solidus. If concentrations are very low, up to ~5 ppm CO₂ may be incorporated as a trace point defect in nominally C-free silicates³. Cryoscopic approximation for decarbonation melting of nominally carbon-free silicate minerals with up to 5 ppm CO₂ indicates negligible (<5 °C) depression of the solidus. Thus, carbon dissolved in peridotite minerals has no appreciable effect on the solidus and the solidus has a discontinuous drop at ~5 ppm CO₂, the onset of magnesite stability. ¹Dasgupta et al. 2005, AGU Fall Meeting Abstract. ²Plank, T. and Langmuir, C. H. 1992, JGR 97, 19749-19770. ³Keppler, H. et al. 2003, Nature 424, 414-416.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005