- View All
Carbon in oceanic crust and upper mantle is subducted at convergent margins, but the phase transformations that carbon undergoes, and hence its mobility, during subduction and transport to the deep upper mantle are not well known. We are undertaking high-pressure experiments (multi-anvil) on a range of carbonate and natural mid-ocean ridge basalt compositions to determine the phase relations and melting temperatures of carbon-bearing lithologies in subducting slabs.
DCO Fellow Enggist was hired in March of 2012. Subduction of altered, calcite-bearing oceanic crust is the primary return cycle of carbon from the exosphere to the deep mantle. Some subducted carbonate survives subduction through the sub-arc environment and is transported to the deeper upper mantle, transition zone or lower mantle. At low pressures (<10 GPa) carbonate-silicate exchange equilibria convert calcite to calcite-magnesite solid solution, which melts at a low T eutectic. Na+K carbonate components flux melting and lower solidus temperatures. At higher pressures alkali-bearing, Ca-Mg-Fe carbonates become stable and melting is controlled largely by the melting relations of these phases.
Experimental work has focused on determining the phase relations of carbonate compositions on the calcite-magnesite join at pressures ≥6 GPa. Several multi-anvil experiments have been conducted by Drs Enggist and Rapp at higher pressures (8 and 10 GPa) in this system, also aimed at determining minimum melting temperatures and compositions. Further experiments with alkali carbonate components added to the calcite-magnesite composition systematically address their effects on minimum melting relations and partial melt compositions in this pressure range. These simple-system experiments were used to inform further experiments in complex natural systems (modeling subducted carbonate-bearing MORB) in which stable carbonate compositions are complex and difficult to interpret as either melt or stable crystalline carbonate in the absence of comprehensive knowledge of phase relations in (Na,K)2CO3-CaCO3-MgCO3 simple systems.
Unexpectedly low solidus temperature (1000°C at 6 GPa, 1250°C at 15 GPa) may relate to fluxing by TiO2, Na2O, SiO2 and other contaminants. Comparison of this solidus with estimated P-T paths of subducting oceanic crust suggest that carbonate melts would form at relatively low pressures during subduction of hot crust. In the case of cooler crust, melting will not occur and carbonate may therefore be subducted into the deeper mantle. We have also experimentally investigated the reaction carbonate + coesite = clinopyroxene + C + O2, which probably controls diamond versus carbonate stability in P-T-ƒO2-composition space in deeply subducted altered (i.e., carbonated) oceanic crust (eclogite).
- April 2011 -