Subduction zones play a major role in governing chemical cycling between the Earth's crust, hydrosphere and atmosphere and the deeper mantle. In these environments volatile elements (H, C, F, Cl, S and others) mediate mass transfer and elemental cycling, and also strongly influence the processes that lead to the generation, transport and eruption of magmas within subduction zone environments. To investigate the distribution and role of volatile elements and their effect on mass transfer and melting in single subduction system, this study will involve a systematic study of major, trace and volatile elements trapped in melt inclusions from primitive magmas in the Cascade arc in California, Oregon and Washington. The aims are to determine pre-eruptive volatile concentrations and oxygen fugacity conditions for the different primitive magma types found in the Cascade arc, and to use this data to examine the composition of slab- and mantle-wedge derived components along and across the arc, and to test current models for melting and magma genesis in subduction zones. Although the Cascade subduction zone is recognized as an endmember "hot and dry" subduction zone, the role of volatiles in element cycling and magma genesis remains unclear. Existing studies and our preliminary data show that water contents vary and that some primitive Cascade magmas are quite "wet" - with H2O up to 4 to 7 wt.%. Two sampling strategies will be pursued, firstly involving study of lavas from along an east-west cross-arc traverse in the Central Oregon Cascades (Lat. ~43-45 degrees). The Oregon High Cascades region is characterized by the highest recent erupted volumes of mafic magma and contains all major primary magma types found in the arc (HAOT, OIB-like, CAB, and shoshonite). This work will be combined with work on other lavas from selected well-studied locations along the arc. Overall this approach will allow use to identify the volatile composition of the range of different magma types produced within the Cascade arc, and provide insight on the variations in volatile abundances across and along the arc.
