Plagioclase archives of depleted melts in the oceanic crust

Mid-ocean ridge and ocean island basalts provide vital but incomplete insights into mantle chemistry. For example, high-anorthite plagioclase is generally too refractory and incompatible- element depleted to have crystallized from the melts that carry it to the surface. Moreover, erupted basalts rarely preserve the extreme isotopic and incompatible-element depletions found in some primitive melt inclusions and residual abyssal peridotites. By integrating experimental observations with analyses of natural crystals and glasses, we infer that high-anorthite plagioclase grows from high-degree melts of refractory but otherwise unexotic mantle sources with depleted incompatible element compositions. The widespread occurrence of high-anorthite plagioclase in oceanic basalts and the oceanic crust hence indicates that refractory melts pervade the uppermost mantle and lower crust. We thus suggest that refractory melts play much a greater role in crustal accretion than typically recognized, and that refractory and feasibly depleted peridotite is more prevalent in the upper mantle than previously thought.