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

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 _OH 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 _OH-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 _OH-bands that were found in the whole set, 17 are caused by NSI, 53 by [ILD/CED]. Total mean integrated _OH-band intensities, (̄_int)_total with ̄_int=(_||a+_||b+_||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<wt. ppm (H₂O)_total<419. Olivines included in diamonds were found to be free of hydroxyl (detection limit of the single crystal IR-spectrometry, 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 _OH in the range 3,704–3,657 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) Mg-edenite or Mg-pargasite occur rarely and with _OH at 3,709–3,711 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 _OH-band, polarised c>a>b, at 3,327–3,328 cm^–1, an energy typical of _OH 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 _OH-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 _OH 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.Editorial responsibility: J. Hoefs

---

Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes

A suite of metasomatised xenoliths from the Letlhakane kimberlite (Botswana) forms a metasomatic sequence from garnet peridotite to garnet phlogopite peridotite to phlogopite peridotite. Before the modal metasomatism, most of the Letlhakane xenoliths were depleted harzburgites that had been subjected to an earlier cryptic metasomatic event. Modal phlogopite and clinopyroxene - Cr-spinel increase at the expense of garnet and orthopyroxene with increasing degrees of metasomatism. The most metasomatised xenolith is a wehrlite. With progressive modal metasomatism, the clinopyroxene becomes enriched in Sr, Sc and the LREE, orthopyroxene becomes depleted in Ca and Ni, but enriched in Al and Mn, and olivine becomes depleted in Al and V. Garnet chemical composition largely remains unchanged. The garnet replacement reaction seen in most xenoliths allows the measurement of the flux of trace elements through detailed modal analysis of the pseudomorphs. Mass balance calculations show that the modally metasomatised rocks became enriched in incompatible elements such as Sr, Na, K, the LREE and the HFSE (Ti, Zr and Nb). Major elements (Al, Cr and Fe) and garnet-compatible trace elements (V, Y, Sc, and the HREE) were removed during this metasomatic process. The modal metasomatism caused a strong depletion in Al, and the results challenge previous suggestions that this metasomatic process merely occurred within an Al-poor environment. The data suggest that the xenoliths represent the mantle wallrock adjacent to a major conduit for an alkaline basic silicate melt (with high contents of volatile and incompatible elements). The volatile and incompatible element-enriched component of this melt percolated into the wallrock along a strong temperature gradient and caused the observed range of metasomatism.     

Composition of the Siberian cratonic mantle: evidence from Udachnaya peridotite xenoliths

Bulk compositions and mineral analyses for forty-one, large, garnet- and spinel-facies peridotite xenoliths from the Udachnaya kimberlite in the central Siberian platform have many similarities to those of well-studied peridotites from the Kaapvaal craton in southern Africa. Coarse Mg-rich lherzolites and harzburgites with equilibration temperatures below 1000 °C are abundant and are believed to form the principal rock type in the Siberian lithosphere. The low-temperature Udachnaya peridotites have an average mg number [Mg/(Mg+Fe)] of 92.6 with a wide dispersion in modal enstatite, ranging to over 40 wt%. High-temperature peridotites are relatively richer in Fe and Ti and are commonly deformed, with porphyroclastic or mosaic-porphyroclastic textures, some of the latter having fluidized enstatite. The Udachnaya peridotites have experienced late-stage metasomatism before, during and after eruption. Garnets and pyroxenes in many of the high-temperature rocks are zoned, probably by reaction with melt prior to eruption. Virtually all the peridotites contain secondary diopside, inhomogeneous on a micron scale, that mantles primary orthopyroxene. It is believed to have crystallized along with lesser amounts of intergranular calcite and monticellite during eruption. Bulk analyses for total Fe in many specimens are higher than whole-rock Fe calculated from the electron probe analyses and the modes. The magnitude of the difference between the two measurements of total Fe correlates with loss-on-ignition, suggesting that Fe has been introduced during serpentinization following eruption. These late metasomatic processes have thus affected some major as well minor and trace element compositions. The similarities in bulk composition of peridotites from Udachnaya and the Kaapvaal are evidence of a common origin. Low-temperature cratonic peridotites differ from oceanic peridotites in having higher mg numbers (>92) and in having relatively high but wide-ranging modal enstatite (Mg/Si = 1.06–1.49 weight fraction). The Udachnaya low-temperature peridotites have an inverse correlation between FeO (calculated from the probe analyses and modes) and SiO₂. This correlation is also present in the Kaapvaal data but is complicated by a greater range in fertility that produces a positive variation of Fe with Si. A negative trend for Fe/Si can be seen within a portion of the Kaapvaal data, that for low-Ca harzburgites, in which the variation in fertility is restricted. The negative trends for Fe/Si can be interpreted as a consequence of either segregation of olivine and orthopyroxene by metamorphic differentiation or partial sorting during cumulate formation. Received: 18 June 1996  / Accepted: 11 February 1997     

Water content in minerals of mantle xenoliths from the Udachnaya pipe kimberlites (Yakutia)

Distribution of water among the main rock-forming nominally anhydrous minerals of mantle xenoliths of peridotitic and eclogitic parageneses from the Udachnaya kimberlite pipe, Yakutia, has been studied by IR spectroscopy. The spectra of all minerals exhibit vibrations attributed to hydroxyl structural defects. The content of H₂O (ppm) in minerals of peridotites is as follows: 23–75 in olivine, 52–317 in orthopyroxene, 29–126 in clinopyroxene, and 0–95 in garnet. In eclogites, garnet contains up to 833 ppm H₂O, and clinopyroxene, up to 1898 ppm (~ 0.19 wt.%). The obtained data and the results of previous studies of minerals of mantle xenoliths show wide variations in H₂O contents both within different kimberlite provinces and within the Udachnaya kimberlite pipe. Judging from the volume ratios of mineral phases in the studied xenoliths, the water content varies over narrow ranges of values, 38–126 ppm. At the same time, the water content in the studied eclogite xenoliths is much higher and varies widely, 391–1112 ppm.     

Tychite in mantle xenoliths from kimberlites: The first find and a new genetic type

Tychite Na₆Mg₂(CO₃)₄(SO₃) is a rare natural Na and Mg sulfatocarbonate. It is found only as minor mineral in deposits of saline lakes in the United States, Canada, Uganda, and China. In these continental evaporites tychite has sedimentary genesis. In this study, we report the first occurrence of tychite as a crystal phase in the melt inclusions in olivine from mantle xenoliths of the Udachnaya-East kimberlite pipe. This find provides an evidence for the probability of tychite crystallization from melts; i.e., this rare sulfatocarbonate may have a magmatic origin as well.     

Petrologic origin of exsolution textures in mantle minerals: evidence in pyroxenitic xenoliths from Yakutia kimberlites

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite, rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains (i.e. > 2 cm; megacrystalline) to a significantly finer-grained texture (i.e. < 1 cm).

These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystallization. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions (690–910°C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not contain primary garnet may have been derived from shallower depths.

Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith studies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution towards a greater understanding of the Siberian lithospheric mantle.     

胶辽半岛金伯利岩中地幔捕虏体岩石学特征：古生代岩石圈地幔及其不均一性

华北东部古生代岩石圈地幔经历过复杂的多旋回多阶段熔融岩浆、重结晶、变质和剪切变形历史,因而岩石组成、结构复杂.地幔橄榄岩的主元素、地幔矿物特别是金刚石中的包裹体的Mg#值及Cr#值等显示古生代岩石圈地幔高度难熔亏损玄武质的成分特征,而地幔橄榄岩的不相容元素、同位素显示高度富集的特点.这种高难熔和强富集两重性是克拉通岩石圈地幔的特征.Pb同位素反映华北东部古生代岩石圈地幔具冈瓦纳大陆地幔的特点.复县和蒙阴古生代岩石圈地幔尽管都表现为太古代克拉通地幔的特征,但存在着不均一性,前者受克拉通化后的深部作用影响强于后者.与新生代地幔Sr、Nd同位素对比表明富集的古生代岩石圈地幔向亏损新生代的地幔转变和减薄伴有新生地幔物质对古老岩石圈的置换和混合作用.     

Chemistry of micas from kimberlites and xenoliths—I. Micaceous kimberlites

Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with $\text{mg [ = Mg/(Mg + Fe)] 0.45–0.65}$, TiO₂ 3–6 wt%, Al₂O₃ 14–16wt%, no Fe³⁺ required in tetrahedral sites, low NiO (~0.02 wt%), and relatively high na [Na₂O/(Na₂O + K₂O)] 0.02–0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium (mg 0.80-0.93), lower in TiO₂ (0.7–4.0 wt%) and Al₂O₃ (6.8–14.2 wt%), have substantial Fe³⁺ in tetrahedral sites, and have relatively low na. Both types may have rims with compositions indicative of mica-‘serpentine’ mixtures resulting from reaction with a highly aqueous fluid. The petrographically-determined ‘serpentine’ is chemically of two types: Fe-rich serpentine and Fe-rich talc. Associated phases in the ground-mass vary from one kimberlite to another: calcite, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, millerite?, heazlewoodite?, quartz.Inter-grain variations in composition of Type II micas may result from establishment of local reservoirs on a mm scale, consequent upon mechanical mixing and competition of other phases for minor elements (e.g. chromite for Cr, serpentine for Ni).Type I micas may result from an intrusive precursor (carbonatitic?) to kimberlite, perhaps genetically related, which was incorporated into a later pulse of kimberlite from which the Type II micas crystallized.     

Chrome-rich garnets from the kimberlites of yakutia and their parageneses

The chrome-rich magnesian garnets (6.6–18.9% Cr₂O₃) of kimberlitic concentrates and some peridotite xenoliths contain variable admixtures of CaO: from 0.69 to 26.0% (1.7–72% Ca-component). All the garnets both in respect of Ca and Cr-contents make up a continuous series.The variability in the Ca-content is caused by differences in paragenesis. Most of the Ca-poor pyropes are related to a paragenesis without clinopyroxene (mostly dunitic). Garnets rich in calcium are related to a paragenesis without entstatite. All the parageneses listed are of an ultramafic type, i.e. contain magnesian olivine. The solubility of knorringite—Mg₃Cr₂(Si₃O₁₂)—in kimberlitic garnets is possibly limited by pressure and does not exceed 50–60% mol.     

Nature and origin of eclogite xenoliths from kimberlites

Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.     

Compositional heterogeneity of the continental lithospheric mantle beneath the Early Precambrian and Phanerozoic structures: Evidence from mantle xenoliths in kimberlites and alkaline basalts

Data on the petro- and geochemical characteristics of mantle xenoliths in kimberlites, which sampled the mantle beneath Early Precambrian tectonic structures (Archean cratons: the basement of the Eastern Siberian Platform, Karelian, Kaapvaal, Wyoming, Western Dharvar; Early and Middle Proterozoic foldbelts: Western Olenek, Natal, and Halls Creek), and xenoliths in alkaline basalts, which sampled the mantle benath Late Proterozoic-Phanerozoic structures (foldbelts: Central Asian, Mozambique, southern tip of South America, and Central German) indicate the following: (1) The major and trace element and REE composition of the mantle is different beneath Early Precambrian structures and Late Proterozoic-Phanerozoic foldbelts and reflects the degree of partial melting of the primitive mantle and its depletion in magmaphile components beneath ancient structures compared to young ones. (2) The original composition of the mantle was different beneath the Early Precambrian and Late Proterozoic structures in terms of both major oxides and incompatible trace elements and REE and their ratios; the composition of the mantle beneath the Eastern Siberian Platform, Wyoming, and Karelian cratons is different in terms of Zr/Y, La/Sm, Ce/Sm, Gd/Yb, and Lu/Hf. (3) The degree of melting of the primitive mantle decreases with depth, as follows from the negative correlation between the MgO/SiO₂ ratio and pressure (i.e., depth) and the positive correlation between the Al₂O₃/MgO ratio and pressure in the xenoliths. (4) The Y, Zr, Ti, Sm, Gd, and Yb conncentrations and the sum of HREE in the mantle decrease with increasing degree of melting; correspondingly, the material most strongly depleted in these incompatible trace elements and REE composes the upper levels of the lithospheric continental mantle.     

Eclogite xenoliths from Wajrakarur kimberlites, southern India

Summary Mineralogical characteristics of eclogite xenoliths from three kimberlite pipes (KL2, P2 and P10) of the Proterozoic Wajrakarur kimberlite field of southern India have been studied. In a rare sample of enstatite eclogite from the KL2 pipe garnet contains microscopic triangular arrays of needles or blebs of omphacite, enstatite and rutile consistent with an origin by exsolution parallel to the isometric form {111}. Discrete omphacite grains in the sample contain exsolved needles or blebs of enstatite and garnet. Kyanite eclogites are abundant in the KL2 pipe which occasionally show a secondary ring of pure celsian around kyanite grains. Omphacite Na₂O contents in the eclogites of the KL2 and P2 pipes are typically between 3 and 6 wt%, and garnet has widely variable composition with end member ranges of Prp22-81Grs0-47Alm10-30Sps0-1Adr0-5Uv0-3. Eclogites of the P10 pipe comprise chromian omphacite and garnet. Phase relations in the ACF projection exhibit systematic increase of the Ca-Tschermak’s component in omphacite from enstatite eclogite through biminerallic eclogite to kyanite eclogite. Garnet-clinopyroxene Fe–Mg geothermometry yields temperatures mostly in the range of 900–1100 °C. A formerly supersilicic nature of garnet in enstatite eclogite as inferred from exsolution mineralogy indicates minimum peak pressure of 5 GPa.     

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al₂O₃ and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na₂O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.Electronic Supplementary Material Supplementary material is available for this article at     

Petrology of highly aluminous xenoliths from kimberlites of Yakutia

Highly aluminous xenoliths include kyanite-, corundum- and coesite-bearing eclogites, grospydites and alkremites. These xenoliths are present in different kimberlites of Yakutia but have most often been found in Udachnaya and other pipes of the central Daldyn–Alakitsky region. Kimberlites of this field also contain eclogite-like xenoliths with kyanite and corundum that originate in the lower crust or the lower crust–upper mantle transition zone. Petrographic study shows that two rock groups of different structure and chemistry can be distinguished among kyanite eclogites: fine- to medium-grained with mosaic structure and coarse-grained with cataclastic structure. Eclogites with mosaic structure are characterized by the occurrences of symplectite intergrowths of garnet with kyanite, clinopyroxene and coesite; only in this group do grospydites occur. In cataclastic eclogites, coarse-grained coesite occurs, corresponding in size to other rock-forming minerals. Highly aluminous xenoliths differ from bimineralic eclogites in their high content of Al₂O₃ and total alkali content. Coesite-bearing varieties are characterized by low MgO content and higher Na/K and Fe²⁺/Fe³⁺ ratios, as well as high contents of Na₂O. Geochemical peculiarities of kyanite eclogites and other rocks are exhibited by a sloping chondrite-normalized distribution of rare earth elements (REE) in garnets and low Y/Zr ratio, in contrast to bimineralic rocks. Coesite is found in more than 20 kyanite eclogites and grospydites from Udachnaya. Grospydites with coesite from Zagadochnaya pipe are described. Three varieties of coesite in these rocks are distinguished: (a) subhedral grains with size of 1.0–3.0 mm; (b) inclusions in the rock-forming minerals; (c) sub-graphic intergrowths with garnet. The presence and preservation of coesite in eclogites indicate both high pressure of formation (more than 30 kbar) and set a number of constraints on the timing of xenolith cooling during entrainment and transport to the surface. Different ways of formation of the highly aluminous eclogites are discussed. Petrographic observations and geochemistry suggest that some highly aluminous rocks have formed as a result of crystallization of anorthosite rocks in abyssal conditions. δ¹⁸O-estimations and other petrologic evidence point out the possible origin of some of these xenoliths as the result of subduction of oceanic crust. Diamondiferous samples have been found in all varieties except alkremites. Usually these eclogites contain cubic or coated diamonds. However, two sample corundum-bearing eclogites with diamonds from the Udachnaya pipe contain octahedra that show evidence of resorption.     

Major element composition of concentrate garnets in Proterozoic kimberlites from the Eastern Dharwar Craton,India: Implications on sub-continental lithospheric mantle

This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr₂O₃ contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF.     

Garnet lherzolite xenoliths in the kimberlites of northern Lesotho: revised P-T equilibration conditions and upper mantle Palaeogeotherm

Evidence is presented that the inflected palaeogeotherm for northern Lesotho, previously highlighted by Boyd (1973), Boyd and Nixon (1973, 1975), Finnerty and Boyd (1984, 1987), is essentially an artifact of the unsatisfactory, over-simplified barometer formulation (based on MacGregor 1974) employed. The absence of an inflection in the palaeogeotherm for Udachnaya, Siberia based on P-T estimates for garnet lherzolite xenoliths calculated with the same barometer, does not prove the reality of an inflected palaeogeotherm for northern Lesotho. Rather, it reflects, at least in part, chemical differences between the equivalent deformed, high-T xenoliths in these two areas — most importantly expressed in the respective contents of Jadeite relative to ureyite in the constituent orthopyroxenes. Accurate estimation of P-T equilibration conditions for garnet lherzolite xenoliths requires both complete and precise mineral analyses and adequate consideration of the influence of minor elements, such as Cr and Na, on the element exchange reaction thermometers and barometers employed. The barometer formulation of Nickel and Green (1985) is judged to be the best currently available. As no single thermometer is entirely satisfactory and dependable throughout the P-T range of interest, equilibration temperatures are currently best assessed as a mean value obtained from application of the most accurate formulations for both the two-pyroxene solvus thermometer (Bertrand and Mercier 1985) and Fe²⁺-Mg²⁺ exchange reactions between garnet-clinopyroxene (Powell 1985), garnet-orthopyroxene (Harley 1984a) and garnet-olivine (O'Neill and Wood 1979) mineral pairs. Such best P-T estimates for xenoliths in the kimberlites of northern Lesotho indicate a somewhat elevated, non-inflected, upper mantle palaeogeotherm, compatible with a 120–145 km thick thermally conductive lithosphere above a convecting asthenosphere. The common coarse textured, chemically depleted, garnet lherzolite xenoliths appear mostly to have originated from close to the base of the lithosphere whilst the contrasting deformed, higher T, more chemically fertile xenoliths have come from the underlying asthenosphere. There is evidence for slight variations in the heat flux within the mantle beneath northern Lesotho at the time of emplacement of the Thaba Putsoa and Mothae kimberlites, only some 16 km apart, and also possibly for a regional variation in the thickness of the lithosphere.     

Compositional zonation in garnets in peridotite xenoliths

Garnets in 42 peridotite xenoliths, most from southern Africa, have been analyzed by electron probe to seek correlations between compositional zonation and rock history. Xenoliths have been placed into the following 6 groups, based primarily upon zonation in garnet: I (12 rocks)-zonation dominated by enrichment of Ti and other incompatible elements in garnet rims; II (10 rocks)-garnet nearly homogeneous; III (8 rocks)-rims depleted in Cr, with little or no related zonation of Ti; IV (3 rocks)-slight Ti zonation sympathetic to that of Cr; V (3 rocks)-garnet rims depleted or enriched in Cr, and chromite included in garnet; VI (6 rocks)-garnets with other characteristics. Element partitioning between olivine, pyroxene, and garnet rims generally is consistent with the assumption of equilibrium before eruption. Although one analyzed rock contains olivine and pyroxene that may have non-equilibrated oxygen isotopes, no corresponding departures from chemical equilibrium were noted. Causes of zoning include melt infiltration and changes in temperature and pressure. Zonation was caused or heavily influenced by melt infiltration in garnets of Group I. In Groups III, IV, and V, most compositional gradients in garnets are attributed to changes in temperature, pressure, or both, and gradients of Cr are characteristic. There are no simple relationships among wt% Cr₂O₃ in garnet, calculated temperature, and the presence of compositional gradients. Rather, garnets nearly homogeneous in Cr are present in rocks with calculated equilibration temperatures that span the range 800–1500 °C. Although the most prominent Cr gradients are found in relatively Cr-rich garnets of rocks for which calculated temperatures are below 1050 °C, gradients are well-defined in a Group IV rock with T1300 °C. The variety of Cr gradients in garnets erupted from a range of temperatures indicates that the zonations record diverse histories. Petrologic histories have been investigated by simulated cooling of model rock compositions in the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃. Proportions and compositions of pyroxene and garnet were calculated as functions of P and T. The most common pattern of zonation in Groups III and IV, a decrease of less than 1 wt% Cr₂O₃ core-to-rim, can be simulated by cooling of less than 200 °C or pressure decreases of less than 1 GPa. The preservation of growth zonation in garnets with calculated temperatures near 1300 °C implies that these garnets grew within a geologically short time before eruption, probably in response to fast cooling after crystallization of a small intrusion nearby. Progress in interpreting garnet zonations in part will depend upon determinations of diffusion rates for Cr. Zonation formed by diffusion within garnet cannot always be distinguished from that formed by growth, but Ca–Cr correlations unlike those typical of peridotite suite garnets may document diffusion.     

Majoritic garnets in diamonds from placers of the Northeastern Siberian Platform

Doklady Earth Sciences -     

Dislocation deformation mechanisms in peridotite xenoliths in kimberlites

The dislocation substructures in olivine from coarse-grained peridotite xenoliths in kimberlites from the Lesotho region have been determined. The [100] dislocations may be located in simple (100) tilt boundaries while the density of free or individual [100] dislocations is 10⁶/cm² or less. The [001] dislocations form (010) twist arrays or more complex (100) subboundaries with the [100] dislocations; the density of free [001] dislocations increases to 8 × 10⁸/cm² in those grains in which tangles are observed. The simple (100) subboundaries are considered to result from a high temperature, slow strain-rate deformation (creep-like process) while the more complex subboundaries composed of [100] and [001] dislocations, as well as the high density of [001] dislocations, indicate faster strain rates and/or lower deformation temperatures than the creep deformation. These two broad phases of deformation have been interpreted as an early stage of mantle-type flow followed by deformation during or subsequent to the emplacement of the kimberlite.