Shaw, S.D., DeBari, S., Wallace, P. and Sisson, T., 2010, Volatile contents in olivine-hosted melt inclusions from primitive magmas in the Northern Cascade arc: 2010 AGU Fall Meeting, V53C-2260

Volatile contents in olivine-hosted melt inclusions from primitive magmas in the Northern Cascade arc

The Cascade volcanic arc is the hot endmember of arcs worldwide, and its unique thermal structure has prompted debate about the role of fluid-flux melting versus decompression melting in Cascade arc magma generation. While the slow subduction of the hot Juan de Fuca slab is expected to result in strong dehydration prior to reaching sub-arc depths, there is no consensus on whether the slab is entirely dehydrated at this point, and whether volcanism is the result of water-poor, decompression melting, or fluid-flux melting. An accurate measurement of primitive arc magma composition and volatile contents is critical to resolve this debate. In the northern region of the arc, at Mount Baker and Glacier Peak, we provide the first measurements of pre-eruptive volatile contents in olivine-hosted melt inclusions from primitive magmas. Tephra deposits were collected from two mafic, monogenetic satellite cinder cones at Glacier Peak and a mafic satellite cinder cone at Mount Baker. The Glacier Peak cinder cones have erupted borderline low-K olivine tholeiite at the Whitechuck cinder cone (50.1-51.6 wt % SiO2; 7.29-7.62 wt % MgO; Fo84-86) and calk-alkaline basalt at the Indian Pass cinder cone (50.7-52.47 wt % SiO2; 7.54-9.05 wt % MgO; Fo85-88), whereas the cinder cone at Mount Baker has erupted tephra of calcalkaline affinity (associated flow units: 51.2-54.6 wt % SiO2; 5.0-5.5 wt % MgO; Fo80-84). If decompression melting is solely responsible for magma generation, then we would expect that all magmas, regardless of geochemical affinity, would have low H2O contents (<1%). In contrast, if fluid-fluxed melting is responsible for the generation of calcalkaline magmas, we would expect much higher H2O contents similar to values found for arc magmas elsewhere. Results of FTIR spectroscopic analysis of 12 olivine-hosted melt inclusions from the low-K olivine tholeiitic cinder cone south of Glacier Peak (Whitechuck), show H2O contents of 0.5-2.1 wt% H2O. CO2 contents range from below detection (≤ 50 ppm) to 704 ppm. This range reflects variable degassing of melts before inclusion entrapment, and the highest value provides a minimum estimate for the initial H2O content of the magma. Analysis of 13 melt inclusions from the calc-alkaline Indian Pass cinder cone, also south of Glacier Peak, yields H2O contents of 0.3-2.9 wt% H2O. CO2 contents range from below detection to 1382 ppm. Analysis of 6 melt inclusions from the calc-alkaline Schriebers Meadow cinder cone at Mount Baker, yields H2O contents of 1.0-2.4 wt.% H2O. CO2 contents range from below detection to 1505 ppm. These preliminary volatile contents show that flux melting continues to play a significant role in magma generation in the northern arc.