Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform |
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| Authors: | Matsyuk S S Langer K Hösch A |
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| Institution: | (1) Institut für Angewandte Geowissenschaften 1, Technische Universit?t Berlin, Ernst Reuter-Platz 1, D-10527 Berlin, Germany, DE |
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| Abstract: | A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet
parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite
pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such
kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v
OH vibrational range in search of the occurrence, energy and intensity of the v
OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the
mantle beneath the Siberian platform. The v
OH single-crystal spectra show either one or a combination of two or more of the following major v
OH bands, I 3645–3662 cm−1, II 3561–3583 cm−1, III 3515–3527 cm−1, and minor bands, Ia 3623–3631 cm−1, IIa 3593–3607 cm−1. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the
host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective
garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO2 > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v
OH spectra do not correspond to those of OH− defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications
of v
OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v
OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems
I to III. Instead, they exhibit one weak, broad band (Δv
OH 200–460 cm−1) near 3570 cm−1, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative
evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of
all v
OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between
values below the detection limit of our single-crystal IR method, near 2 × 10−4 wt%, up to 163 × 10−4 wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large
number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization
of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role.
Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation
between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least
two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional
or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature
diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of
the graphite-pyrope facies. This latter result (2) may point to significantly lower f
H2O and f
O2 in the former as compared to the latter facies.
Received: 25 November 1997 / Accepted: 9 March 1998 |
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