Journal of Petrology Advance Access published online on July 1, 2004
Journal of Petrology, doi:10.1093/petrology/egh015
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1 DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY 170-25, PASADENA, CA 91125-2500, USA
* To whom correspondence should be addressed. E-mail: David.R.Bell{at}asu.edu.
Concentrations of OH, and major and trace elements were determined in a suite of mantle-derived megacrysts that represent the crystallization products of a kimberlite-like magma at
Accepted December 15, 2003
Article
Abundance and Partitioning of OH in a High-pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa
2 MINERAL SERVICES CANADA INC., 205-930 HARBOURSIDE DRIVE, NORTH VANCOUVER, B.C. V7P 3S7, CANADA
Abstract 
5 GPa and 1400-1100°C. OH concentrations, determined by single-crystal Fourier transform infrared spectroscopy, display the following ranges (ppmw H2O): olivine 54-262, orthopyroxene 215-263, garnet 15-74, clinopyroxene 195-620, and zircon 28-34. High OH concentrations in olivine imply mantle conditions of origin, with limited H loss during ascent. OH is consistently correlated with megacryst composition, exhibiting trends with Mg-number that are similar to those of other minor and trace elements and indicating a record of high-pressure magmatic evolution. H substitution is not coupled to minor elements in olivine, but may be in ortho- and clinopyroxene. The OH-Mg-number trends of garnet and clinopyroxene show inflections related to co-precipitation of ilmenite, suggesting minor element (Ti) influence on OH partitioning. During differentiation, relative OH enrichment in clinopyroxene and olivine is consistent with proportional dependence on water activity, whereas that in garnet suggests a higher power-law dependence and/or influence of temperature. Inter-mineral distribution coefficients for OH between cpx, opx, olivine and zircon are thus constant, whereas partitioning between these minerals and garnet shows a factor 4-10 variation, correlated regularly with composition (and temperature). Calculation of solid-melt partition coefficients for H at 5 GPa over a range of magmatic evolution from 1380 to 1250°C yields: ol 0·0053-0·0046, opx 0·0093-0·0059, cpx 0·016-0·013, gt 0·0014-0·0003, bulk (garnet lherzolite-melt) 0·0063-0·0051. These are consistent with experimental studies and similar to values inferred from mid-ocean ridge basalt geochemistry, confirming the moderate incompatibility of H in mantle melting.
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