HR: 15:00h
AN: V42E-06    [PDF]
TI: Stability of Carbonated Eclogite in the Upper Mantle: Experimental Solidus from 2 to 9 GPa
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: 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
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
AB: Carbonates are pervasive alteration products of the oceanic crust and likely survive subduction-related dehydration and/or melting. Thus, significant quantities of carbonated refractory eclogite are probably delivered to the deeper mantle. The melting behavior of such recycled carbonate influences the fate of recycled carbon, determines the possible sources and depths of carbonated metasomatic melts in the mantle, and delimits the conditions under which carbonated eclogite may act as a source of carbonatite and other types of magmatic CO$_{2}$. We present partial melting experiments of carbonated eclogite that constrain the solidus and near solidus phase relations from 2 to 9 GPa. To simulate the near-isochemical nature of ocean floor carbonation, the starting material was prepared by adding 5 wt.$%$ CO$_{2}$ in the form of a mixture of Fe-Mg-Ca-Na-K carbonates to a bimineralic eclogite from Salt Lake crater, Oahu, Hawaii. The starting composition is a reasonable approximation of carbonated oceanic crust from which siliceous hydrous fluid has been extracted by subduction. We find that melt-present versus melt-absent conditions can be distinguished based on textural criteria. Garnet and cpx appear in all the experiments. Between 2 and 3 GPa, the subsolidus assemblage also includes calcite-dolomite$_{ss}$ + ilmenite, whereas above the solidus (950-975 $\deg$C at 2 GPa and 1050-1075 $\deg$C at 3 GPa) calcio-dolomitic liquid appears. From 3 to 4.5 GPa, dolomite$_{ss}$ becomes stable at the solidus and the near solidus melt becomes increasingly dolomitic. Appearance of dolomite above 3 GPa is accompanied by a negative Clapeyron slope of the solidus, with the cusp located between 995 and 1025 $\deg$C at ${\it ca.}$ 4 GPa. Above 4-4.5 GPa, the solidus again rises with increasing pressure to ${\it ca.}$ 1245 $\deg$C at 9 GPa and magnesite becomes the subsolidus carbonate. Dolomitic melt coexists with magnesite + garnet + cpx + rutile between 5 and 9 GPa. If extrapolated to higher pressures, the carbonated eclogite solidus intersects the oceanic geotherm deeper than 400 km. Thus, eclogite cannot host carbonates in the asthenosphere. Carbonated eclogite bodies entering the convecting upper mantle would release carbonate melt in the mantle transition zone. Upon release, this small volume, highly reactive melt could be an effective agent of deep mantle metasomatism. Comparison of our eclogite-CO$_{2}$ solidus with that of peridotite-CO$_{2}$ shows a shallower solidus-geotherm intersection for the latter. This implies that carbonated peridotite is a more likely proximal source of magmatic carbon in oceanic provinces. However, carbonated eclogite is a potential source of continental carbonatites, as its solidus crosses the continental shield geotherm at ${\it ca.}$ 4 GPa.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 4806 Carbon cycling
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting