GeoScienceWorld
GeoRef Record 2008-040005

Water partitioning between mantle minerals from peridotite xenoliths

Water partitioning between mantle minerals from peridotite xenoliths

Latitude & Longitude N44°00′00″ – N47°00′00″ and E45′00″ – E00′00″N31°30′00″ – N37°00′00″ and W109°05′00″ – W103°00′00″S35°00′00″ – S22°00′00″ and E16°00′00″ – E33°00′00″

Abstract

The speciation and amount of water dissolved in the nominally anhydrous silicates of eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H (sub 2) O. Orthopyroxene, olivine and garnet contain 169-201, 3-54 and 0 to <3 ppm H (sub 2) O, respectively. Pyroxenes water contents and the distribution of water between orthopyroxene and clinopyroxene is identical regardless of sample mineralogy. The total water contents of each xenolith are similar at (113 + or - 14 ppm H (sub 2) O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region. Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy.