Abstract:
Minerals in the upper mantle are nominally anhydrous and contain water in the form of OH. Although the water content is extremely low, the physical and chemical properties of minerals can be strongly influenced by these hydrous components. This study focuses on two clinopyroxenite veins within mantle peridotite from the Trinity ophiolite in Eunice Bluff, Northern California (USA). It is hypothesized that these veins are the pathways for water migration from melts into host mantle though crystallization and infiltration. In order to examine the spatial variations associated with major elements, water content, and hydrogen isotope in pyroxene, samples were collected perpendicular to the direction of the pyroxenite vein in transects. Water content was measured as hydrogen (H/O and D/O), with a Cs+ ion source on a Cameca 6f secondary ion mass spectrometer at Arizona State University. Water contents in clinopyroxene range from 548 to 1577 ppm with an average 903 ppm and a median 876 ppm, while the range in orthopyroxene is from 204 to 573 ppm with an average 394 and a median 391 ppm. δD in clinopyroxenes varies from -114 to +72 ‰, while the range in orthopyroxene is -189 to -42 ‰. Both water content and δD in the surrounding peridotite adjacent to the vein have a higher value than the peridotite further away, which indicates there is a hydrogen gradient between veins and host mantle. A model in this study reveals the mechanism of water migration from vein to peridotite, which indicates the intrusion of the melts into the mantle cause not only recrystallization but also hydrogen diffusion within a distance of at least ~20 cm from the contact.
