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Journal of Petrology Advance Access originally published online on August 18, 2007
Journal of Petrology 2007 48(9):1681-1724; doi:10.1093/petrology/egm035
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Garnet-bearing Xenoliths from Salt Lake Crater, Oahu, Hawaii: High-Pressure Fractional Crystallization in the Oceanic Mantle

Shantanu Keshav1,2,*, Gautam Sen1 and Dean C. Presnall2,3

1Department of Earth Sciences, Florida International University, Miami, FL 33199, USA
2Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA
3Department of Geosciences, University of Texas at Dallas, Box 830688, Richardson, TX 75083, USA

RECEIVED NOVEMBER 1, 2004; ACCEPTED JUNE 14, 2007


  Abstract

The focus of this study is a suite of garnet-bearing mantle xenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnet constitute the bulk of the xenoliths, and orthopyroxene is present in small amounts. Clinopyroxene has exsolved orthopyroxene, spinel, and garnet. Many xenoliths also contain spinel-cored garnets. Olivine, clinopyroxene, and garnet are in major element chemical equilibrium with each other; large, discrete orthopyroxene does not appear to be in major-element chemical equilibrium with the other minerals. Multiple compositions of orthopyroxene occur in individual xenoliths. The new data do not support the existing hypothesis that all the xenoliths formed at 1c 6–2c2 GPa, and that the spinel-cored garnets formed as a consequence of almost isobaric subsolidus cooling of a spinel-bearing assemblage. The lack of olivine or pyroxenes in the spinel–garnet reaction zones and the embayed outline of spinel grains inside garnet suggest that the spinel-cored garnets grew in the presence of a melt. The origin of these xenoliths is interpreted on the basis of liquidus phase relations in the tholeiitic and slightly silica-poor portion of the CaO–MgO–Al2O3–SiO2 (CMAS) system at pressures from 3c0 to 5c0 GPa. The phase relations suggest crystallization from slightly silica-poor melts (or transitional basaltic melts) in the depth range ~110–150 km beneath Oahu. This depth estimate puts the formation of these xenoliths in the asthenosphere. On the basis of this study it is proposed that the pyroxenite xenoliths are high-pressure cumulates related to polybaric magma fractionation in the asthenosphere, thus making Oahu the only locality among the oceanic regions where such deep magmatic fractional crystallization processes have been recognized.

KEY WORDS: xenolith; asthenosphere; basalt; CMAS; cumulate; oceanic lithosphere; experimental petrology; mantle; geothermobarometry; magma chamber

*Corresponding author. Present address: Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany. Telephone: +49-921-55-3719. Fax: +49-921- 55-3769. E-mail: keshav{at}uni-bayreuth.de


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