Cohen, Anthony and O'Nions, R. Keith
Melting rates beneath Hawaii: Evidence from uranium series isotopes in recent lavas.
Earth and Planetary Science Letters, 120(3-4),
U-238, Th-232, Th-230 and Ra-226 abundances have been measured in six samples of recent Hawaiian basalt by high precision mass spectrometry, in an attempt to compare the melting process in plumes and at spreading ridges. The data reveal a very small range in (Th-230/U-238) activity ratio up to a maximum value of 1.02+/-0.01, and (Ra-226/Th-230) activity ratios which lie between 1.10+/-0.015 and 1.19+/-0.02. U-Th and Ra-Th abundances are linearly correlated demonstrating that the disequilibria predate crystallisation and differentiation. Using recently published estimates for the bulk partition coefficients of U and Th, the results are consistent with melting rates >10(-3) kg m(-3) a(-1) at porosities <10(-3) for dynamic fractional melting in the garnet stability field.
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