The Fingerprint of Water in Mantle Melting

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Intellectual Merits. This project seeks to study the role of water and hydrous melt in melting of the Josephine Peridotite (SW Oregon). This body comprises lherzolite (5% clinopyroxene) and harzburgite (5% clinopyroxene) rocks interpreted to the residue of mantle melting: first at an ocean ridge under dry conditions that produced local websterite layering, and then in a wet hydrous melting environment that drove the residue to extreme depletions not seen in abyssal peridotite suites. This is expressed by a wide range in mineral composition spanning that for both abyssal peridotites from mid-ocean ridges, and for ophiolite peridotites believed formed in supra-subduction zone environments. A second stage of melting is attributed to introduction of water and/or hydrous melt from a down-going ocean lithosphere slab into the mantle wedge - lowering the mantle melting point and allowing it to proceed well beyond the Cpx-out phase boundary that limits melting beneath ocean ridges. Whereas ridge magmatism can be attributed to adiabatic upwelling and melting of nearly dry mantle, arc-related volcanism involves introduction of volatiles and hydrous melts into the mantle that induce hydrous (or 'flux') remelting. The relative contribution of the slab-derived and mantle wedge components in the production of arc-related magmas has been long debated. Most studies constrain magmatic processes by inverting isotopic and trace element compositions of arc-related lavas. This approach is hindered by the complexity of the melt generation process, that may involve as yet poorly understood interactions between migrating melts and the overlying mantle, as well as transport through and interaction with arc crust. The Josephine Peridotite offers the opportunity to directly study and constrain how water and hydrous melts are introduced into, interact with and are transported through the subarc mantle - and how this effects melt chemistry. A combined major and trace element whole rock and mineral study is proposed to document how the composition of the Josephine Peridotite massif was influenced by the introduction of volatiles and hydrous melt, and whether this is a local or pervasive influence.Broader Impacts: This project relates directly to the goal of understanding the role of volatiles in arc-related magmatism - a subject of broad interest and significant societal impact due to the explosive nature of arc volcanism. In addition, the project will support a Ph.D. student.