T75 Magmas and Their Sources II: A Tribute to the Distinguished Career of Fred Frey

Sunday, 19 October 2014
1:00 PM - 5:00 PM
205/206

WANING DEHYDRATION OF MANTLE SERPENTINITE IN THE SUBDUCTING PLATE DRIVES SLAB MELTING BENEATH THE CASCADE ARC

Paper number: T75-9
Program: Technical Sessions
Time: 3:05 PM – 3:20 PM

Authors

Abstract

In subduction zones, the mantle wedge is fluxed by fluids and/or melts derived from the downgoing plate, lowering the mantle solidus to produce melts that rise through the overlying crust and form arc volcanoes [1]. However, the thermo-petrologic model for the Cascade subduction zone predicts that the young subducting crust reaches high temperatures at relatively shallow depths and that little H2O may reach sub-arc depths [2] to initiate melt production. We investigated this problem using analyses of δD in olivine-hosted melt inclusions, because slab dehydration reactions in subduction zones strongly fractionate H isotopes. Here, we present volatile, trace element, and δD analyses of melt inclusions from 6 cinder cones (calc-alkaline and calc-alkaline transitional basalts) in the Lassen region of the southern Cascades.

Initial δD values, corrected for post-entrapment H loss, range from -95 to -70‰. These values are isotopically lighter than those of melt inclusions from the Marianas (-55 to -12‰)[3], suggesting a more dehydrated subducted plate source. Using temperatures predicted for the subducted plate from geodynamic models, we modeled the δD values of fluids released from the plate as a function of depth. The model predicts that waning dehydration of the last 5-15% of bound H2O within the deep slab interior (serpentinized mantle) occurs beneath the arc, and the predicted δD values closely match those of the melt inclusions. Because the upper, completely dehydrated portions of the plate are likely at or above the MORB + H2O and sediment + H2O solidi (based on H2O/Ce and geodynamic models), dehydration of the deeper parts of the slab should cause partial melting of the slab top. Our results are consistent with published evidence [4] for a slab component with MORB-like isotopic composition and demonstrate the likelihood of wet partial melting of the upper part of the subducted plate beneath the arc.

[1] Grove et al. (2002). [2] van Keken et al. (2011). [3] Shaw (2008) EPSL 275, 138-145.[4] Borg (1997) Can. Min. 35, 425-452.