U-Th Isotopes in Arc Magmas: Implications for Element Transfer from the Subducted Crust
Hawkesworth1, Turner, McDermott
1C. J. Hawkesworth, S. P. Turner, D. W. Peate, and P. van Calsteren are with the Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. F. McDermott is with the Department of Geology, University College, Dublin, Eire.
Science (New York, N.Y.)
|April 25, 1997
Summary
Uranium-thorium isotopes reveal fluid and sediment transfer rates from subducting tectonic plates. Island arc volcanic rocks indicate fluids mobilize uranium, while sediments contribute significantly to thorium in arc rocks.
Area of Science:
- Geochemistry
- Isotope Geochemistry
- Tectonics
Background:
- Subduction zones are critical for recycling Earth's crust.
- Understanding material transfer from subducting slabs to the mantle wedge is key to deciphering arc magmatism.
- Uranium (U) and Thorium (Th) isotopes offer powerful tracers for geological processes.
Purpose of the Study:
- To investigate the rates of fluid and sediment transfer from a downgoing slab to the mantle wedge using island arc volcanic rocks.
- To quantify the contribution of subducted sediments to arc rock composition.
- To determine the timescales of fluid and sediment melt transport.
Main Methods:
- Analysis of Uranium-Th-Nd isotope systematics in island arc volcanic rocks.
- Utilizing the differential mobility of Uranium and Thorium during fluid alteration.
- Interpreting isotopic covariations (e.g., Th/Ce vs. 143Nd/144Nd) to distinguish sources.
Main Results:
- Uranium is readily mobilized in fluids, while Thorium is less mobile.
- Negative correlations between Th/Ce and 143Nd/144Nd indicate significant sediment input from the downgoing slab.
- Estimated transfer times for fluids are 30,000-120,000 years, and for sediment melts, several million years.
- Only about 30% of subducted sediment Thorium is returned to the crust.
Conclusions:
- Island arc volcanism records the complex interplay of fluid and sediment recycling during subduction.
- The distinct isotopic signatures highlight the differential transport mechanisms of fluids versus sediment melts.
- Archaean crustal composition reflects distinct crustal generation processes, influencing average U/Th ratios.
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