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

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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 titanomagnetite, 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 composition 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. Chondrite-normalized REE concentrations show strong light REE enrichment and relative flat patterns with negative Eu anomaly. Strontium isotope values (0.70328) are consistent with mantle-derived melt, but oxygen and hydrogen isotope values are depleted (3.1 and −118 ‰, respectively) relative to mantle values. The hydrogen isotope values overlap those of hydrothermal epidote from rocks altered by the meteoric-water-recharged Krafla geothermal system. The rhyolite melt was emplaced into and has reacted with a felsic intrusive suite that has nearly identical composition. The felsite is composed of quartz, alkali feldspar, plagioclase, titanomagnetite, and augite. Emplacement of the rhyolite magma has resulted in partial melting of the felsite, accompanied locally by partial assimilation. The interstitial melt in the felsite has similar normalized SiO2 content as the rhyolite melt but is distinguished by higher K2O and lower CaO and plots near the minimum melt composition in the granite system. Augite in the partially melted felsite has re-equilibrated to more calcic metamorphic compositions. Rare quenched glass fragments containing glomeroporphyritic crystals derived from the felsite show textural evidence for resorption of alkali feldspar and quartz. The glass in these fragments is enriched in SiO2 relative to the rhyolite melt or the interstitial felsite melt, consistent with the textural evidence for quartz dissolution. The quenching of these melts by drilling fluids at in situ conditions preserves details of the melt–wall rock interaction that would not be readily observed in rocks that had completely crystallized. However, these processes may be recognizable by a combination of textural analysis and in situ analytical techniques that document compositional heterogeneity due to partial melting and local assimilation.

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Acknowledgments

This research would not have been possible without the generous cooperation of Landsvirkjun and the IDDP consortium. The work was supported by NSF grants EAR 0507518 and EAR 0506882. Reviews by Bruce Marsh, Christy Till and anonymous reviewers improved the manuscript.

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Author notes

  1. N. E. Marks

    Present address: Lawrence Livermore National Labs, 7000 East Avenue, Livermore, CA, 94550, USA

  2. E. C. Pope

    Present address: Natural History Museum of Denmark, Copenhagen University, Øster Voldgrad 5-7, 1350, København K, Denmark

Authors and Affiliations

  1. Department of Geology, University of California-Davis, Davis, CA, 95616, USA

    R. A. Zierenberg, P. Schiffman, G. H. Barfod, C. E. Lesher & N. E. Marks

  2. U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025, USA

    J. B. Lowenstern

  3. Iceland GeoSurvey (ISOR), Grensásvegur 9, IS 108, Rekjavik, Iceland

    A. K. Mortensen

  4. Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, 94305, USA

    E. C. Pope & D. K. Bird

  5. Department of Geological Sciences, 1272 University of Oregon, Eugene, OR, 97403, USA

    M. H. Reed

  6. HS Orka hf, Brekkustigur 36, IS 260, Reykjanesbær, Iceland

    G. Ó. Friðleifsson

  7. Department of Earth Sciences, University of California-Riverside, Riverside, CA, 92521, USA

    W. A. Elders

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  1. R. A. Zierenberg
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Correspondence to R. A. Zierenberg.

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Communicated by T. L. Grove.

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Zierenberg, R.A., Schiffman, P., Barfod, G.H. et al. Composition and origin of rhyolite melt intersected by drilling in the Krafla geothermal field, Iceland. Contrib Mineral Petrol 165, 327–347 (2013). https://doi.org/10.1007/s00410-012-0811-z

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