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The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa

Bell, David R. and Rossman, George R. (1992) The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa. Contributions to Mineralogy and Petrology, 111 (2). pp. 161-178. ISSN 0010-7999. doi:10.1007/BF00348949.

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166 garnets of dominantly mantle origin were analyzed for OH content by infrared (IR) spectroscopy. IR spectra in the 3400–3700 cm^(-1) region display consistent absorption patterns attributable to OH structurally bound within the garnet crystal, occasionally contaminated by low intensity OH absorptions from microscopic inclusions. The principal structural OH absorption occurs near 3570 cm^(-1), with the appearance of additional absorptions near 3512 cm^(-1) and 3650 cm^(-1) dependent on garnet composition or paragenesis. Samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern Africa. OH abundances, using the best currently available calibration, range from less than 1 up to 135 ppm H_2O, and increase in the general order as follows: on-craton eclogites<coarse-granular peridotites<Ti-rich deformed peridotites<some off-craton eclogites<Cr-poor megacrysts. OH abundances in garnet are closely linked to host rock paragenesis and cannot be explained purely by any crystal chemical factors which we have investigated. Cr-poor garnet megacryst nodules display striking inverse correlations between OH contents and Mg/(Mg+Fe) ratios, which we interpret to reflect the progressively increasing water content of the differentiating parental megacryst magmas. OH abundances in garnet megacrysts decrease in the host rock order Group 2 (micaceous) kimberlite>Group 1 (basaltic) kimberlite>alnöite>alkali basalt. The OH contents of common lithospheric granets from coarse peridotites, including several phlogopite-bearing samples are typically less than 20 ppm H_2O, for tectonic settings of kimberlites both on and off the Archaean Kaapvaal craton. Ti-rich garnets from deformed peridotites are richer in OH, supporting previous suggestions of association of these xenoliths with putative megacryst magmas. Subcalcic Cr-rich xenocrysts, diamond inclusion garnets and garnets from diamondiferous eclogites have very low OH contents, similar to eclogites and depleted peridotites without macroscopic diamonds. The OH content of southern African peridotite and eclogite garnets are significantly lower on average than those previously examined from the Colorado Plateau diatremes. While details of emplacement-related H mobility in garnets remain to be established, our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle. Although garnets do not appear to constitute a large reservoir of mantle hydrogen, the large stability range of OH-bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 1992 Springer-Verlag. Received November 10, 1990. Accepted December 15, 1991. Revised manuscript submitted to Contributions to Mineralogy and Petrology November 13, 1991. We would like to thank all the sample donors listed in Table 1 for their willingness to contribute samples to this study. In particular, J.J. Gurney and De Beers Consolidated Mines Ltd. are gratefully thanked for access to their impressive xenolith collections. G.H. Read and R.O. Moore were also especially helpful with sample provision and assistance in the field. J.T. Armstrong provided instruction and general wisdom in electron microprobe analysis and P. Carpenter gave analytical assistance. The manuscript benefitted from thoughtful reviews by D. Smith and S.E. Haggerty. DRB gratefully acknowledges financial support from the Harry Crossley Foundation and the American Federation of Mineralogical Societies. This work was funded in part by NSF grants EAR-86-18200 and EAR-88-16006. Editorial responsibility: I.S.E. Carmichael.
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Harry Crossley FoundationUNSPECIFIED
American Federation of Mineralogical SocietiesUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences4938
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Official Citation: The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa David R. Bell, George R. Rossman pp. 161-178
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39690
Deposited By: Ruth Sustaita
Deposited On:31 Jul 2013 22:56
Last Modified:09 Nov 2021 23:46

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