Trace-element partitioning in perovskite: Implications for the geochemistry of kimberlites and other mantle-derived undersaturated rocks
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Two types of olivine occur in kimberlites from Greenland, Canada and southern Africa. The first, xenocrystic olivine, displays several different forms. Most distinctive are 'nodules', a term we use to describe the large (1–5 mm), rounded, single crystals or polycrystalline aggregates that are a common constituent of many kimberlites. Olivine compositions are uniform within single nodules but vary widely from nodule to nodule, from Fo81 to 93. Within many nodules, sub- to euhedral asymmetric tablets have grown within larger anhedral olivine grains. Dislocation structures, particularly in the anhedral grains, demonstrate that this olivine was deformed before being incorporated into the kimberlite magma. Olivine grains in the kimberlite matrix between the nodules have morphologies similar to those of the tablets, suggesting that most matrix olivine grains are parts of disaggregated nodules. We propose that a sub- to euhedral form is not sufficient to identify phenocrysts in kimberlites and provide some criteria, based on morphology, internal deformation and composition, that distinguish phenocrysts from xenocrysts. The second type of olivine is restricted to rims surrounding xenocrystic grains. Only this olivine crystallized from the kimberlite magma. Major and trace element data for the rim olivine are used to calculate the composition of the parental kimberlite liquid, which is found to contain between about 20 and 30% MgO. The bulk compositions of many kimberlites contain higher MgO contents as a result of the presence of xenocrystic olivine. The monomineralic, dunitic, character of the nodules, and the wide range from Fo-rich to Fo-poor olivine compositions, provide major constraints on the origin of the nodules. Dunite is a relatively rare rock in the mantle and where present its olivine is persistently Fo-rich. The dunitic source of the nodules in kimberlites lacked minerals such as pyroxene and an aluminous phase, which make up about half of most mantle-derived rocks. It appears that these minerals were removed from the mantle peridotite that was to become the source of the nodules, and the Fo content of the retained olivine was modified during interaction with CO2-rich fluids whose arrival at the base of the lithosphere immediately preceded the passage of the kimberlite magmas. Fragments of the resultant dunite were entrained into the kimberlite, where they were retained both as intact nodules and as disaggregated grains in the matrix.
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Abstract Olivine is the most abundant phase in kimberlites and is stable throughout most of the crystallization sequence, thus providing an extensive record of kimberlite petrogenesis. To better constrain the composition, evolution, and source of kimberlites we present a detailed petrographic and geochemical study of olivine from multiple dyke, sill, and root zone kimberlites in the Kimberley cluster (South Africa). Olivine grains in these kimberlites are zoned, with a central core, a rim overgrowth, and occasionally an external rind. Additional ‘internal’ and ‘transitional’ zones may occur between the core and rim, and some samples of root zone kimberlites contain a late generation of high-Mg olivine in cross-cutting veins. Olivine records widespread pre-ascent (proto-)kimberlite metasomatism in the mantle including the following features: (1) relatively Fe-rich (Mg# <89) olivine cores interpreted to derive from the disaggregation of kimberlite-related megacrysts (20 % of cores); (2) Mg–Ca-rich olivine cores (Mg# >89; >0·05 wt% CaO) suggested to be sourced from neoblasts in sheared peridotites (25 % of cores); (3) transitional zones between cores and rims probably formed by partial re-equilibration of xenocrysts (now cores) with a previous pulse of kimberlite melt (i.e. compositionally heterogeneous xenocrysts); (4) olivine from the Wesselton water tunnel sills, internal zones (I), and low-Mg# rims, which crystallized from a kimberlite melt that underwent olivine fractionation and stalled within the shallow lithospheric mantle. Magmatic crystallization begins with internal olivine zones (II), which are common but not ubiquitous in the Kimberley olivine. These zones are euhedral, contain rare inclusions of chromite, and have a higher Mg# (90·0 ± 0·5), NiO, and Cr2O3 contents, but are depleted in CaO compared with the rims. Internal olivine zones (II) are interpreted to crystallize from a primitive kimberlite melt during its ascent and transport of olivine toward the surface. Their compositions suggest assimilation of peridotitic material (particularly orthopyroxene) and potentially sulfides prior to or during crystallization. Comparison of internal zones (II) with liquidus olivine from other mantle-derived carbonate-bearing magmas (i.e. orangeites, ultramafic lamprophyres, melilitites) shows that low (100×) Mn/Fe (∼1·2), very low Ca/Fe (∼0·6), and moderate Ni/Mg ratios (∼1·1) appear to be the hallmarks of olivine in melts derived from carbonate-bearing garnet-peridotite sources. Olivine rims display features indicative of magmatic crystallization, which are typical of olivine rims in kimberlites worldwide; that is, primary inclusions of chromite, Mg-ilmenite and rutile, homogeneous Mg# (88·8 ± 0·3), decreasing Ni and Cr, and increasing Ca and Mn. Rinds and high-Mg olivine are characterized by extreme Mg–Ca–Mn enrichment and Ni depletion, and textural relationships indicate that these zones represent replacement of pre-existing olivine, with some new crystallization of rinds. These zones probably precipitated from evolved, oxidized, and relatively low-temperature kimberlite fluids after crustal emplacement. In summary, this study demonstrates the utility of combined petrography and olivine geochemistry to trace the evolution of kimberlite magmatic systems from early metasomatism of the lithospheric mantle by (proto-)kimberlite melts, to crystallization at different depths en route to surface, and finally late-stage deuteric or hydrothermal fluid alteration after crustal emplacement.
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Olivine is the dominant component in coherent kimberlite rocks and related pyroclastic rocks. Quantitative characterization of olivine crystals in kimberlite rocks may be used to better understand kimberlite emplacement and eruption. Here, we construct the first complete olivine crystal size distribution (CSD) for magmatic or coherent kimberlite using two-dimensional image analysis techniques with a new method for scale-integration. Crystal size and frequency data are collected from polished slabs and thin sections, normalized to the largest scale of observation, and stereologically corrected to create a complete CSD covering the full range of observed olivine crystal sizes.
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<p>Compared to xenoliths, kimberlite xenocrysts provide, although less accurate, more complete information about the deep structure and processes in the subcratonic lithospheric mantle (SCLM). This work is devoted to the study the composition of xenogenic olivine from kimberlites as the main mineral constituting SCLM. Olivine in kimberlites has a different origin, including those not related to depleted rocks of the lithosphere. It can crystallize directly from kimberlite or belong to the so-called Cr-poor megacryst association. In this regard, for the correct interpretation of data on its composition, it is necessary to have sufficiently clear criteria for the separation of olivine xenocrysts from kimberlites into various genetic types. In order to remove olivines crystallizing directly from kimberlite from consideration, in our study we used only central homogeneous parts of crystals larger than 0.5 mm in size [Giuliani, 2018].</p><p>Based on unique and literature data on the composition of olivines from 230 xenoliths of peridotites from 12 kimberlites of the North American, South African and Siberian cratons we proposed a general division into 4 genetic types: olivines of ultrahigh-temperature (HTP-1), high-temperature (HTP-2), low-temperature (LTP) peridotites, olivines of low-chromium megacrystal association (MCA). The separation scheme uses the CaO content as an indicator of the temperature of formation and the ratio Mg/Mg+Fe as an indicator of the degree of enrichment.</p><p>A study of more than 1,500 olivines from a number of kimberlite bodies of the Siberian platform according to this scheme revealed three characteristic distributions of olivine types in kimberlite bodies: 1) without high-temperature differences (Obnazhennaya pipe), 2) with significant development of HTP-2 (Olivinovaya and Vtorogodnitsa pipes) and 3) with significant development of HTP-1 (Dianga pipe). Only the latter type is characterized by the presence of a noticeable amount of olivines of the megaryst association.</p><p>In general, variations in the composition of LTP olivines correspond to granular ones, while HTP-1 and HTP-2 correspond to deformed (shared) peridotites. Interestingly, the enrichment of olivines with incompatible components in these three types does not correlate directly with the formation temperature. Olivines of ultrahigh-temperature peridotites (HTP-1) have unexpectedly small compositional variations and occupy an intermediate position between low-temperature and high-temperature in content of incompatible elements.</p><p>A study of the content of impurity elements (TiO2, NiO) in olivines made it possible to propose the way of formation of two different types of high-temperature olivines. It belongs to the model [Harte et al., 1993; Burgess and Harte, 1999; Burgess and Harte, 2004] where megacrystal melt of various stages of fractionation [Moore et al., 1992] effects depleted rocks of lithospheric mantle. According to it, HTP-2 olivines arose upon exposure to a fractionated melt characteristic of late crystallization stages, and HTP-1 olivines due to unfractionated (less enriched with incompatible components) megacrystal melt at higher temperatures characteristic of the initial crystallization stage.</p><p>Funded by RFBR grant 18-05-01143, T.V.A. was supported by RSF grant 16-17-10067.</p>
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