H2O CONCENTRATIONS IN PRIMARY MELTS FROM SUPRA-SUBDUCTION ZONES AND MIDOCEAN RIDGES - IMPLICATIONS FOR H2O STORAGE AND RECYCLING IN THE MANTLE

A total of 145 inclusions, trapped and isolated in Mg-rich olivine phe nocrysts (Fo(0.87-0.94)) from basalts and ultramafic lavas, and repres enting the most primitive mantle melts known, have been analysed by io n microprobe for their H2O contents. This approach allows us to conduc t a general Survey of the distribution of water in primary melts deriv ed from the mantle beneath mid-ocean ridges and above subduction zones . The primitive melts included in MORE olivines have low H2O contents (mean at 0.12 wt% for N-MORB (14 samples), 0.17 wt% for T-MORB (9 samp les) and 0.51 wt% for E-MORB (14 samples)). A strong decoupling betwee n H2O and K2O has been found in some MORE primary melts which might we ll be explained by the presence of a H2O-bearing CO2-rich fluid. In co ntrast with mid-ocean ridges, primitive melts of subduction zones basa lts are very rich in H2O (between 1.0 and 2.9 wt% (mean at 1.7 wt%, 84 samples) for boninites and between 1.2 and 2.5 wt% (mean at 1.6 wt%, 24 samples) for island are tholeiites). In addition, most of these mel ts have high H2O/K2O ratios which are consistent with a transfer of H2 O as a fluid phase from the subducted slab to the mantle wedge. For bo ninites and island are primary melts, the present H2O contents are app roximate to 2.5 x higher than commonly assumed, which suggests that th e amount of H2O released to the surface in are magmatism has been prev iously underestimated.