Composition and origin of rhyolite melt intersected by drilling in the Krafla geothermal field, Iceland

Abstract

The Iceland Deep Drilling Project Well 1 was designed as a 4- to 5-km-deep exploration well with the goal of intercepting supercritical hydrothermal fluids in the Krafla geothermal field, Iceland. The well unexpectedly drilled into a high-silica (76.5 % SiO2) rhyolite melt at approximately 2.1 km. Some of the melt vesiculated while extruding into the drill hole, but most of the recovered cuttings are quenched sparsely phyric, vesicle-poor glass. The phenocryst assemblage is comprised of titanomagne- tite, plagioclase, augite, and pigeonite. Compositional zoning in plagioclase and exsolution lamellae in augite and pigeonite record changing crystallization conditions as the melt migrated to its present depth of emplacement. The in situ temperature of the melt is estimated to be between 850 and 920 C based on two-pyroxene geothermometry and modeling of the crystallization sequence. Volatile content of the glass indicated partial degassing at an in situ pressure that is above hydrostatic (*16 MPa) and below lithostatic (*55 MPa). The major element and minor element com- position of the melt are consistent with an origin by partial melting of hydrothermally altered basaltic crust at depth, similar to rhyolite erupted within the Krafla Caldera.