T294 Subduction Top to Bottom 2 (ST2B-2), Cascadia and Beyond (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

EVALUATING SUBDUCTION COMPONENT CONTRIBUTIONS TO THE SOUTHERN CASCADE ARC: INSIGHTS FROM GEOCHEMICAL ANALYSES OF MONOGENETIC VOLCANO MAFIC LAVAS AND GORDA PLATE SEDIMENTS

Paper number: T294-5
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Subducted sediment at convergent margins plays a crucial role in worldwide arc magmatism by fertilizing the mantle and introducing volatiles and other elements to the mantle wedge (Plank and Langmuir, 1998). In the Cascadia Subduction Zone, sediment subduction may increase from the north to the south; this hypothesis is based on more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd ratios in basalts erupted in the southern arc (Green and Harry, 1999; Schmidt et al., 2008; Jicha et al., 2009). Furthermore, subduction signatures in the northern Cascades (Garibaldi Belt) have been shown to be negligible (Mullen and Weis, 2013). To constrain subducted sediment contributions in the southern Cascades, this research uses whole-rock major and trace elements, whole-rock radiogenic isotopes, and olivine δ18O from basaltic monogenetic volcanoes from the southern Cascade arc and ten drill core sediment samples from the Gorda Plate. Concentrations of fluid-mobile trace elements in primary melt compositions (re-equilibrated with Fo90) indicate a subduction component contribution similar to the northern Cascade arc (Ba 175-360 ppm in the south, 150-675 ppm in the north (Mullen and Weis, 2013); Pb 3-9 ppm in the south, 1-6 ppm in the north (Mullen and Weis, 2013)). However, 87Sr/86Sr and 143Nd/144Nd signatures are similar to other southern Cascade arc lavas (87Sr/86Sr 0.70354± 0.014 - 0.70375±0.014, 143Nd/144Nd 0.51285±0.01 - 0.51291±0.01); these lavas have lower 143Nd/144Nd and higher 87Sr/86Sr ratios than northern Cascades basalts. Pb isotope signatures (208Pb/204Pb and 206Pb/204Pb) of lavas and Gorda Plate sediments from this study overlap, which may indicate a contribution from Gorda Plate sediments to the arc lavas. Preliminary δ18O olivines are similar to mantle δ18O, which may indicate that at least some magmas are derived primarily from decompression melting of a minimally hydrated mantle. Additional 18O of olivine phenocrysts, as well as further trace element and radiogenic isotopes of basaltic lavas and Gorda Plate sediments, will be used to model sediment input into the sub-arc mantle, and to investigate along-arc trends in sediment subduction.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306813

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