Previously melt-depleted mantle beneath the Cascades Range arc

Abstract

Abstract Mantle peridotite xenoliths from Lorena Butte, Simcoe Mountains volcanic field, Washington, USA, have been reported with radiogenic 187Os/188Os (≤0.149) and elevated oxidation states, which have been interpreted to reflect Farallon slab contributions to the mantle wedge. To examine subarc processes beneath the Cascades arc, mineral thermobarometry and bulk-rock compositions, including highly siderophile element (HSE) abundances and 187Os/188Os, were obtained for Lorena Butte harzburgite xenoliths. Harzburgites have 187Os/188Os ratios (0.1231 ± 20; 2 SD; n = 10) that only marginally overlap and are, on average, 7% less radiogenic than the previously reported xenoliths, with estimates of oxygen fugacity (ΔFMQ = +1.0 ± 0.5) within the range of both subduction-related and abyssal peridotites. Lorena Butte harzburgites experienced significant shallow-level melt depletion (>15%) prior to incorporation into the mantle wedge. After incorporation into the lithosphere, the peridotites were erupted within trachybasalt of intraplate origin. This heritage suggests that only low-Os harzburgites preserve radiogenic 187Os/188Os from subduction fluids and that previously melt-depleted mantle forms an important component of mantle wedges. Furthermore, the extent of melt depletion experienced by the peridotites is not necessarily coupled with mineral thermobarometry, which can be affected by melt infiltration and metasomatism.