Hydroxyl in olivines from mantle xenoliths in kimberlites of the Siberian platform

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 microscope-absorption-spectrometry in the νOH vibrational range, 3,000–3,800 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 νOH-spectra allowed distinction between hydroxyl in non-intrinsic 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]⇆[CED]n, we propose to symbolise them as [ILD/CED]. Of the total of 70 polarised νOH-bands that were found in the whole set, 17 are caused by NSI, 53 by [ILD/CED]. Total mean integrated νOH-band intensities, ∑(ᾱint)total with ᾱint=(α||a+α||b+α||c)int/3, were determined from the spectra. They yielded the contents of structurally unallocated “water”, using the recent calibration of the IR-method (Bell et al. 2003). The range is 0 a>b, at 3,327–3,328 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 3,175 and 3,260 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, i.e. liquid-like water with its characteristic broad band centred at ca. 3,400 cm−1. We did also not find any humite minerals as an NSI. Of the 53 νOH-bands 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 3,672 and 3,535 cm−1 are assigned to boron-related defects, [ILD/CED]B. Five bands at 3,573, 3,563, 3,541, 3,524 and 3,512 cm−1 are intensity-correlated and are assigned to Si-depleted “titan-clinohumite-like” defects, [ILD/CED](thl). The other, so far unidentified νOH of [ILD/CED] are suggested to originate from OH− in different types of (Mg, Fe)-depleted defects recently predicted and discovered by TEM. These are called [ILD/CED](KWK). Eight mostly strong bands of them occur at energies higher than 3,573 cm−1, [ILD/CED](KWK)-H, 13 strong to medium strong bands occur below 3,500 cm−1, [ILD/CED](KWK)-L. Such intrinsic defects may occur alone, [ILD/CED](thl) and [ILD/CED](KWK)-H, or in different combinations with each other, [ILD/CED](KWK)-H+[ILD/CED](thl), [ILD/CED](KWK)-H+[ILD/CED](KWK)-L and [ILD/CED](KWK)-H+[ILD/CED](thl)+[ILD/CED](KWK)-L. 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 discussed.