H 2O concentrations in primary melts from supra-subduction zones and mid-ocean ridges: Implications for H 2O storage and recycling in the mantle

Abstract

A total of 145 inclusions, trapped and isolated in Mg-rich olivine phenocrysts (Fo 0.87–0.94) from basalts and ultramafic lavas, and representing the most primitive mantle melts known, have been analysed by ion microprobe for their H 2O contents. This approach allows us to conduct a general survey of the distribution of water in primary melts derived from the mantle beneath mid-ocean ridges and above subduction zones. The primitive melts included in MORB olivines have low H 2O contents (mean at 0.12 wt% for N-MORB (14 samples), 0.17 wt% for T-MORB (9 samples) and 0.51 wt% for E-MORB (14 samples)). A strong decoupling between H 2O and K 2O has been found in some MORB primary melts which might well be explained by the presence of a H 2O-bearing CO 2-rich fluid. In contrast with mid-ocean ridges, primitive melts of subduction zones basalts are very rich in H 2O (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 arc tholeiites). In addition, most of these melts have high H 2O/K 2O ratios which are consistent with a transfer of H 2O as a fluid phase from the subducted slab to the mantle wedge. For boninites and island arc primary melts, the present H 2O contents are ≈ 2.5 × higher than commonly assumed, which suggests that the amount of H 2O released to the surface in arc magmatism has been previously underestimated.