Melting Processes and Fluid and Sediment Transport Rates Along the Alaska-Aleutian Arc from an Integrated U-Th-Ra-Be Isotope Study

Authors

Rhiannon George, University of Bristol
Simon Turner, University of Bristol
Chris Hawkesworth, University of Bristol
Julie Morris, Washington University
Chris Nye, Alaska Division of Geological and Geophysical Surveys
Jeffrey G. Ryan, University of South FloridaFollow
Shu-Hui Zheng, University of California

Document Type

Article

Publication Date

5-15-2003

Keywords

uranium series, beryllium isotopes, Aleutians, Alaska; arc petrogenesis

Digital Object Identifier (DOI)

https://doi.org/10.1029/2002JB001916

Abstract

A comprehensive data set for young lavas erupted along the Alaska-Aleutian arc is used to examine how fluid and sediment transport rates and melting processes vary in response to systematic changes in subduction rate and dip along the arc and across the ocean-continent boundary. Positive correlations between convergence rate, volcano volume, and ²³⁸U excesses suggest that magmatic output is closely linked to the size of the fluid flux which occurred <10 kyr prior to eruption. Sediment-sensitive tracers like Th/Nb, Ce/Ce*, and ¹⁰Be/⁹Be also increase with convergence rate. However, the inferred ¹⁰Be/⁹Be ratio of this component is low relative to that in the incoming sediments, and this could reflect either sediment removal by accretion or storage in the mantle wedge for ∼0.5–1 Myr. Th/Nb and Th/Nd ratios exceed those expected from bulk sediment addition in the center of the arc suggesting that the enhanced sediment signal reflects transfer in a partial melt phase, whereas partial melts from the subducted oceanic crust appear to be restricted to the western tear in the Pacific plate. Therefore the thermal structure at the slab-wedge interface in the center of the arc must lie close to, and possibly between, the sediment and basalt solidii and is thus constrained to be in the region of 670–740°C at 3 GPa. Elevated (²³⁰Th/²³²Th) ratios, the occurrence of ²³⁰Th excesses and only modest ²²⁶Ra excesses in many of the lavas are modeled as the products of dynamic melting effects superimposed upon a fluid-fluxed mantle wedge. The transition from oceanic to continental lithosphere seems to play an important role in determining the relative effects of fluid addition and partial melting upon U-Th disequilibria. We infer that the change from fast and steep plate subduction in the Aleutians to shallow and slow subduction, coupled with increasing lithospheric lid thickness, in Alaska together control slab and wedge temperatures and the rate of matrix flow through the melting region.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Journal of Geophysical Research, v. 108, issue B5, art. 2252

© Copyright 2003, American Geophysical Union.

Scholar Commons Citation

George, Rhiannon; Turner, Simon; Hawkesworth, Chris; Morris, Julie; Nye, Chris; Ryan, Jeffrey G.; and Zheng, Shu-Hui, "Melting Processes and Fluid and Sediment Transport Rates Along the Alaska-Aleutian Arc from an Integrated U-Th-Ra-Be Isotope Study" (2003). Geology Faculty Publications. 16.
https://digitalcommons.usf.edu/gly_facpub/16

Full Text URL

https://doi.org/10.1029/2002JB001916