Jf. Molina et S. Poli, Carbonate stability and fluid composition in subducted oceanic crust: an experimental study on H2O-CO2-bearing basalts, EARTH PLAN, 176(3-4), 2000, pp. 295-310
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.