Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: inferences from 238U-230Th-226Ra and 235U-231Pa disequilibria |
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| Institution: | 1. Siberian Research Institute of Geology, Geophysics, and Mineral Raw Materials, Krasnyi pr. 67, Novosibirsk, 630091, Russia;2. Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia |
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| Abstract: | Measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr?1 for tholeiites and from 1 to 3 cm yr?1 for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr?1 for tholeiites and from 0.1 to 1 cm yr?1 for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites. |
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