Dacite Melt at the Puna Geothermal Venture Wellfield, Big Island of Hawaii

摘要

During the drilling of injection well KS-13 in 2005 at the Puna Geothermal Venture (PGV) wellfield, on the island of Hawaii, a 75-meter interval of diorite containing brown glass inclusions was penetrated at a depth of 2415 m. At a depth of 2488 m a melt of dacitic composition was encountered. The melt flowed up the wellbore and was repeatedly redrilled over a depth interval of ～8 m, producing several kilograms of clear, colorless vitric cuttings at the surface. The dacitic glass cuttings have a perlitic texture, a silica content of 67 wgt.%, are enriched in alkalis and nearly devoid of mafic minerals with the exception of rare pyroxene phenocrysts and minor euhedral to amorphous magnetite. The melt zone is overlain by an interval of strong greenschist facies metamorphism in basaltic and di-oritic dike rock. The occurrence of an anhydrous dacite melt indicates a rock temperature of approximately 1050℃ (1922°F) and sufficient residence time of underlying basaltic magma to generate a significant volume of highly differentiated material. Heat flux from the magma into the overlying geothermal reservoir is ～3830 mW/m~2, an order of magnitude greater than for mid-ocean ridges. The geologic conditions at PGV combine tensional tectonics with magmatic temperatures at readily drill-able depths (＜2500 m).