Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: Inferences from ~(238)U-~(230)Th-~(226)Ra and ~(235)U-~(231)Pa disequilibria

摘要

Measurements of ~(238)U-~(230)Th-~(226)Ra and ~(235)U-~(231)Pa 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 (~(230)Th/~(238)U) > 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.