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Abundance and Partitioning of OH in a High-pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa

Bell, David R. and Rossman, George R. and Moore, Rory O. (2004) Abundance and Partitioning of OH in a High-pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa. Journal of Petrology, 45 (8). pp. 1539-1564. ISSN 0022-3530. doi:10.1093/petrology/egh015.

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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 ~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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Rossman, George R.0000-0002-4571-6884
Additional Information:© Oxford University Press 2004. Reprinted with permission. RECEIVED JULY 14, 2001; ACCEPTED DECEMBER 15, 2003; ADVANCE ACCESS PUBLICATION JULY 1, 2004 This paper has benefited from discussions with J. E. Dixon, P. D. Ihinger and E. M. Stolper, helpful reviews by J. Dixon and H. Keppler, and the perceptive comments of an anonymous reviewer. We thank Mr S. Gasson of Winburg, South Africa, for permission to visit and sample the Monastery kimberlite, and J. J. Gurney and A. P. le Roex (University of Cape Town) for supporting supplementary studies on the Monastery megacrysts. We acknowledge the assistance and expertise of J. T. Armstrong (Caltech) and C. Hadidiacos (Geophysical Laboratory) in electron probe trace microanalysis. This research was supported by NSF grants EAR 88-16006, EAR 91-040459 and EAR 92-18980 to G.R.R. D.R.B. acknowledges support from the DOE Office of Basic Energy Sciences (DE-FG02-93ER14400) and the Geophysical Laboratory, Carnegie Institution of Washington.
Subject Keywords:hydrogen; megacryst; mantle; trace element; water
Issue or Number:8
Record Number:CaltechAUTHORS:BELjpet04
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1460
Deposited By: Archive Administrator
Deposited On:19 Jan 2006
Last Modified:08 Nov 2021 19:10

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