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

• Published in: Geochimica et cosmochimica acta 1999-12, Vol.63 (23), p.4119-4138
• Main Authors: Sims, K.W.W., DePaolo, D.J., Murrell, M.T., Baldridge, W.S., Goldstein, S., Clague, D., Jull, M.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(99)00313-0

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.