Hydroxyl in olivines from mantle xenoliths in kimb

Authors: S S Matsyuk K Langer

Publish Date: 2004/02/26

Volume: 147, Issue: 4, Pages: 413-437

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Abstract

From a total of 335 olivine crystal grains crystallographically orientated platelets and where possible parallelepipeds were prepared chemically analysed by electron microprobe examined under the polarisation microscope and studied by polarised FTIR microscopeabsorptionspectrometry in the νOH vibrational range 3000–3800 cm−1 The 335 crystal grains were extracted from 174 different specimens of Yakutian upper mantle material including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the Siberian shield The other specimens were mega and macrocrysts or inclusions in diamonds and garnets Analysis of the polarised νOHspectra allowed distinction between hydroxyl in nonintrinsic separate inclusions NSI and in isolated local or condensed extended defects intrinsic to the olivines ILD or CED respectively As the two latter types cannot be distinguished by vibrational spectroscopy and as they are presumably interconnected by intracrystalline condensation reactions of the type n ILD⇆CEDn we propose to symbolise them as ILD/CED Of the total of 70 polarised νOHbands that were found in the whole set 17 are caused by NSI 53 by ILD/CED Total mean integrated νOHband intensities ∑ᾱinttotal with ᾱint=αa+αb+αcint/3 were determined from the spectra They yielded the contents of structurally unallocated “water” using the recent calibration of the IRmethod Bell et al 2003 The range is 0wt ppm H2Ototal419 Olivines included in diamonds were found to be free of hydroxyl detection limit of the single crystal IRspectrometry ca 1 wt ppm “water” The total “water” contents of the different types of olivines increase in the sequence groundmass crystals megacrysts macrocrysts NSI are 1 Serpentine plus talc with νOH in the range 3704–3657 cm−1 either polarised along a of the olivine matrix Pbnm setting or unpolarised Approximately 232 olivines out of the 335 contain such NSI Serpentine and talc occur mostly together in rare cases one of them alone and if so mostly talc 2 Mgedenite or Mgpargasite occur rarely and with νOH at 3709–3711 cm−1 NIS types 1 and 2 are presumably formed by metasomatic alterations of the host olivines the orientated ones probably in the mantle the unorientated ones during later stages 3 The spectra of 23 olivine crystals displayed specifically a νOHband polarised cab at 3327–3328 cm−1 an energy typical of νOH in hydrous wadsleyite We assume this phase to be present as NIS in the respective olivines possibly as relic phase 4 Weak bands between 3175 and 3260 cm−1 polarised along c are tentatively assigned to molecular water NSI with relatively strong hydrogen bonds to the matrix We did not find larger clusters of molecular water ie liquidlike water with its characteristic broad band centred at ca 3400 cm−1 We did also not find any humite minerals as an NSI Of the 53 νOHbands intrinsic to olivine the 29 most abundant and strong ones were subject to further analysis in terms of OH−bearing structural defects ILD/CED Nearly all these bands are strongly polarised along a Two bands at 3672 and 3535 cm−1 are assigned to boronrelated defects ILD/CEDB Five bands at 3573 3563 3541 3524 and 3512 cm−1 are intensitycorrelated and are assigned to Sidepleted “titanclinohumitelike” defects ILD/CEDthl The other so far unidentified νOH of ILD/CED are suggested to originate from OH− in different types of Mg Fedepleted defects recently predicted and discovered by TEM These are called ILD/CEDKWK Eight mostly strong bands of them occur at energies higher than 3573 cm−1 ILD/CEDKWKH 13 strong to medium strong bands occur below 3500 cm−1 ILD/CEDKWKL Such intrinsic defects may occur alone ILD/CEDthl and ILD/CEDKWKH or in different combinations with each other ILD/CEDKWKH+ILD/CEDthl ILD/CEDKWKH+ILD/CEDKWKL and ILD/CEDKWKH+ILD/CEDthl+ILD/CEDKWKL Though there are indications that the occurrences of such types and combinations of the intrinsic OH−bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks straightforward and simple correlations do not exist The reasons for this and also for the great number of varieties of intrinsic ILD/CED are discussedThe Yakutian diamond company ALROSA provided the senior author with the authorisation to collect xenolith specimens from the Yakutian kimberlite pipes N V Sobolev Russian Academy of Sciences kindly provided olivine grains from Uv406 I Krüger R T Schmitt and G Wappler Naturkundemuseum Berlin provided us with chondrodite and humite samples S HertingAgthe D Alscher and Ch Kotré Institute of Applied Geosciences Technical University Berlin made available samples of norbergite and humite helped in problems of data processing and manuscript preparation and in XRDorientation of some of the crystals respectively M Wildner Geocenter Universiy of Vienna identified chondrodite 1223 by XRD F Galbert ZELMI TU Berlin helped with some electron microprobe analyses The Deutsche Forschungsgemeinschaft DFG BonnBad Godesberg generously provided research stipends every year since 1996 to the senior author The DFG financed also the FTIR microspectrometer used under grant no La 324/32 as well as other costs of the project To all these persons and institutions our sincere thanks are due

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