Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: inferences from 238U- 230Th- 226Ra and 235U- 231Pa disequilibria
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.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- December 1999
- DOI:
- 10.1016/S0016-7037(99)00313-0
- Bibcode:
- 1999GeCoA..63.4119S