In situ Serpentinization and Hydrous Fluid Metasomatism in Spinel Dunite Xenoliths from the Bearpaw Mountains, Montana, USA

The Bearpaw Mountains, Montana, USA, form part of an Eocene high-K magmatic province on the northern margin of the Wyoming craton. Spinel dunite xenoliths from the Bearpaw Mountains contain evidence of interaction with a hydrous fluid with a subduction-related signature. These xenoliths fall into three categories. In Category 1, the rock has been subjected to moderate in situ serpentinization with antigorite appearing along olivine grain boundaries and forming reaction pockets through the breakdown of pyroxene, amphibole and spinel. In Category 2, newly formed clinopyroxene, amphibole, mica and calcite overprint and partially replace the antigorite in reaction pockets. In Category 3, metasomatic fibrous orthopyroxene appears at the expense of olivine in veins and in reaction pockets and sometimes forms spherulitic or radiating aggregates. The xenoliths have high B, Cl and H2O contents, and also whole-rock and mineral trace element patterns characteristic of subduction zones. It is suggested that the source of the hydrous fluid may have been part of the subducting slab of the Farallon plate, which collided with the craton keel similar to 55 Myr ago. Spinel dunite xenoliths may have originated from the topmost part of the mantle lithosphere below the abnormally thick crust of the Wyoming craton. Hydrous fluids rising through the mantle lithosphere, but unable to penetrate the thick mafic-underplated crust, may have accumulated beneath the Moho, causing this topmost part of the mantle to become hydrated and initiating serpentinization. Subsequent heating halted serpentinization and was accompanied by the growth of new minerals, which may have been partially due to deserpentinization but was also the result of metasomatism by a silica-rich hydrous fluid.