From dry to damp and stiff mantle lithosphere by reactive melt percolation atop the Hawaiian plume

Abstract

Predicting the incorporation of hydrogen (H) in the atomic structure of upper mantle minerals is a major target in geodynamics since hydration impacts essential physico-chemical properties such as the melting temperature or viscosity. Here we quantify the hydrogen concentration in olivine and pyroxenes in nine mantle peridotites from Pali (Oahu island, Hawaii), which experienced variable degrees of reactive melt percolation leading to refertilization and olivine recrystallisation. Hydrogen concentration is quantified using Fourier transform infrared spectroscopy with unpolarized and polarized light. Despite important contamination by H2O-bearing and CO2-bearing melt/fluid inclusions, quantitative analysis of H in olivine was successful in all samples. Quantification of H concentration in orthopyroxene was successful in most samples, excepted the two most reacted peridotites. Olivines are very H-poor (0-5 ppm H2O wt, on average 0.8 ppm H2O wt) and orthopyroxenes contain very small amounts of hydrogen (4 to 44 ppm H2O wt, on average 19.5 ppm H2O wt). The hydrogen concentration in clinopyroxene could not be quantified due to ubiquitous water-derived species trapped at the interfaces with spinel exsolutions and in hydrous melt inclusions. Despite low concentrations, H and Al in orthopyroxene are positively correlated, but comparison with a database encompassing pyroxene from peridotites from different geological settings shows that variations in degree of melting/type of metasomatism may totally overcome the crystallographic-control of Al on H incorporation. The whole-rock hydrogen concentrations estimated based on the FTIR data (2-26 ppm H2O wt) are among the lowest in the current database for the mantle lithosphere (average at 150 ppm H2O wt). The hydrogen concentration in orthopyroxene is positively correlated with both the recrystallized olivine fraction and the pyroxene mode, indicating that the reactive melt percolation is responsible for the limited hydration of the Pali peridotites. However, as this metasomatism did not hydrate the olivine, it did not induce mechanical weakening of the oceanic mantle lithosphere.