Carbonate stability and fluid composition in subducted oceanic crust: an experimental study on H2O-CO2-bearing basalts

Carbonates and hydrates are common products of the alteration of the upper basaltic crust in modern oceans. However, phase relationships and devolatil ization reactions in altered CO2-bearing metabasalts during the subduction process are still poorly known. A series of fO(2)-buffered piston cylinder experiments were performed on three basaltic model compositions in the pres ence of a H2O-CO2 mixed fluid, at pressures from 1.0 to 2.0 GPa and tempera tures from 665 to 730 degrees C. Experimental results on a tholeiite compos ition demonstrate that amphibole coexists with calcite at P less than or eq ual to 1.4 GPa, with dolomite at 1.4 less than or equal to P less than or e qual to 1.8 GPa, and with dolomite+magnesite at pressures higher than 1.8 G Pa. The stability of calcite increases with pressure with increasing Fe/(Fe +Mg) of the bulk composition. Omphacite was found in tholeiite only at 2.0 GPa, 730 degrees C. Garnet, plagioclase, paragonite, epidote and kyanite fu rther complicate phase relationships in the pressure range investigated. Es timates of the coexisting fluid compositions, on the basis of mass-balance and thermodynamic calculations, demonstrate the continuous H2O enrichment w ith increasing pressure and decreasing temperature. An almost purely aqueou s fluid (X-CO2 < 0.05) is obtained at 2.0 GPa, 665 degrees C. Hydrous fluid s and relatively high modal proportions of carbonates at high pressure and low temperature conditions are responsible for the displacement of the appe arance of omphacite at higher pressures than in H2O-saturated, CO2-free sys tems. Modeling of devolatilization reactions along subduction zone geotherm s reveals that significant decarbonation is feasible only at low pressures (in the forearc region) and at relatively high temperatures, once young oce anic crust is subducted at slow convergent rates. When the subduction proce ss approaches steady-state conditions, CO2 is fractionated in the solid and deep recycling of CO2 is expected to account for the global-scale imbalanc e at convergent margins. (C) 2000 Published by Elsevier Science B.V. All ri ghts reserved.