Hydrous partial melting of lherzolite at 1 GPa: The effect of H2O on the genesis of basaltic magmas

Partial melting of a natural lherzolite (KLB-1) under H2O-undersaturated conditions was experimentally investigated at 1 GPa over the temperature range from 1100 to 1350°C. The effect of H2O added to the dry lherzolite ranged from 0.1 to 0.9 wt% of the melt composition and the extent of melting was quantitatively estimated by comparison with the results of anhydrous melting experiments on the same lherzolite [1]. With the new capsule configuration presented here, partial melt layers of more than 30 μm in width formed along the inner capsule wall. The composition of quenched glass could successfully be obtained by direct microprobe analyses of these melt layers unmodified during quenching. The degree of partial melting was estimated from mass balance calculations using sodium concentrations. Au75Pd25 and Ag70Pd30 tubes were used as sample containers to minimize iron loss. SIMS analyses of hydrogen in quenched glass showed that loss of hydrogen through the Au75Pd25 capsule was minimal during the experiment. A small amount of H2O in the lherzolite lowers the solidus temperature from slightly below 1250°C to less than 1100°C and significantly increases the extent of melting. The experimental results quantitatively show the increase in extent of melting as a linear function of H2O content in the partial melt above 1200°C. Partial melts with less than 2.5 wt% H2O formed at temperatures above 1200°C are close in major element composition to the anhydrous melts formed by the same degree of partial melting. On the other hand, melts with more than 3 wt% H2O formed at 1100°C are clearly enriched in SiO2 and depleted in MgO compared to the anhydrous melts. The larger amounts of H2O in the mantle source in some hotspot regions such as the Azores platform along the Mid-Atlantic Ridge contribute to a higher extent of melting, resulting in relatively thick oceanic crust.