  The most abundant terrestrial lavas, mid-ocean ridge and ocean island basalt
(MORB and OIB), are commonly considered to be derived from a depleted
MORB-mantle component and more specific, variably enriched mantle plume
sources. However, findings of oceanic lavas and mafic cumulates issued from
melts, enriched in chlorine and having a radiogenic Sr ratio, can be attributed
to an interaction between the asthenosphere-derived melts and lithospheric
peridotite variably hydrated due to penetration of hydrothermal water down to
and below Moho level. To constrain mechanisms and rates responsible for the
interaction, we report results of experiments of reaction between serpentinite
and tholeiitic basaltic melt. Results show that the reaction proceeds via a
multi-stage mechanism: (i) transformation of serpentinite into Cr-rich
spinel-bearing harzburgite containing pore fluid, (ii) partial melting and
dissolution of the harzburgite assemblage with formation of interstitial
hydrous melts, and (iii) final assimilation of the Cr-rich spinel-bearing
harzburgite/dunite and formation of hybrid basaltic melts with high MgO and
elevated Cr and Ni contents. Assimilation of serpentinite by basaltic melt may
occur under elevated melt/rock ratios and may lead to chromitite formation. Our
experiments provide evidence that MORB and high-Mg-Cr orthopyroxene-rich
cumulates depleted in incompatible elements can be produced from common
mid-ocean ridge basaltic melts modified by reaction with hydrated lithospheric
peridotite. We established that the rate of assimilation of serpentinized
peridotite is controlled by silica diffusion in the reacting hydrous basaltic
melt.
