Subcalcic garnets of distinct purple colour are found as single grains in heavy mineral concentrates from kimberlites and lamproites, as inclusions in diamond and, more rarely, in harzburgitic or dunitic xenolith (Sobolev, 1977). Their worldwide distribution and strong association with the occurrence of diamond makes them extremely interesting as an indicator mineral in diamond prospection work, as well as objects of scientific study in the attempt to clarify the genesis of diamond. The garnets are geochemically characterised by their low CaO-content (typically less than 3 wt%) together with variably high Cr2O3-contents of 5 to 10 wt% (Sobolev, 1977). It has been shown that the unusually low Ca-concentrations in the garnets exclude a paragenesis with clinopyroxene (Boyd and Gurney, 1982; Sobolev et al., 1973; Sobolev et al., 1969) which makes them very unusual for the generally lherzolitic mantle.
Suggestions for the origin of subcalcic garnets range between two opposing views, one of them proposing an origin within the mantle (Boyd and Gurney, 1982;Richardson et al., 1984; Pearson et al., 1995; Boyd et al., 1986) as residues of komatiite formation in the Archean overprinted by several metasomatic events, while the other one favours an origin related to subduction (Schulze, 1986; Bulatov et al., 1991; Ringwood, 1977; Canil & Wei, 1992). In this study we have analysed 30 subcalcic garnet grains weighing each between about 2 and 14 mg extracted from heavy mineral concentrates from kimberlites of the Daldyn-Alakit (Udachnaya) and Malo-Botuobiya districts (23. Party Congress pipe) in Yakutia, Siberia. A small fraction from each single grain was analysed for major elements, while the remaining major part of the grain was processed for Sr, Nd and Pb isotopes. In this way we obtained combined major element and isotopic data on single minerals, enabling us to observe heterogeneities within the population of subcalcic garnets from one locality.
The garnet suite spans a range of Cr2O3 concentrations between 4.86 and 12.63 wt% with CaO contents of 0.82 to 5.65 wt%. Strontium-concentrations vary between 0.7 and 15.6 ppm, a rather restricted range compared to data of up to 37.97 ppm (Pearson et al., 1995) or 52.1 ppm (Jagoutz, unpubl. data) for subcalcic garnets from diamondiferous harzburgites from Siberia. C1-normalised Sm/Nd-ratios are between 0.154 and 2.585. Initial 143Nd/144Nd-ratios between 0.51012 and 0.51210 (-41 to -2 e) and initial 87Sr/86Sr-ratios of 0.7065 to 0.72501 cover a wide range and plot far off the mantle array. 206Pb/204Pb-ratios range between 18.21 and 23.57, thus cover a range to the right of the Geochron reaching further than the OIB-defined "HIMU"-field. Most interesting features of the garnet population are positive correlations between 87Sr/86Sr and Cr-, Mn- and Fe-concentration. These trends cannot be explained by any usual geochemical process such as fractionation or partial melting.
Subcalcic garnets from Siberia and South Africa show similar geochemical characteristics, implying a related petrogenetic history, although some specific differences are present. The existence of subcalcic garnets beneath cratons worldwide may be linked to a globally similar petrogenetic process. Our model envisages a suite of Archean spinel-harzburgites, depleted to variable degrees that are transported into the diamond stability field, where metasomatic reactions involving very low-degree melts produce subcalcic garnet and olivine from chromiumspinel and opx (± cpx). This model explains the observed correlations between radiogenic isotopes and major elements and is supported also by oxygen isotopic data. Although the reaction itself cannot be dated, the fact that no features of reequilibration to lherzolitic assemblages (e.g. correlations with Ca-contents) are observed, implies a relatively young age for the subcalcic garnets.
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