Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, ¹⁸⁷Os/¹⁸⁸Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.     

Petrology of peridotite xenoliths in lamprophyre from Shingu,Southwestern Japan: Implications for origin of Fe-rich mantle peridotites

Summary Xenoliths of harzburgite, lherzolite, dunite and wehrlite (= Group I rocks) in lamprophyre dikes from Shingu are accompanied by large amounts of ultramafic-mafic xeno liths with Al- and Ti-rich clinopyroxene and/or kaersuite (websterite, clinopyroxenite, kaersutite rock, gabbro and anorthosite) (= Group II rocks). The latter rocks often crosscut the Group I rocks as veinlets, indicating that Group II rocks are younger. Although harzburgites and lherzolite from Shingu have ordinary modal compositions, the constituent minerals have extraordinary chemical characteristics; low Mg and Cr and high Ti, Al and Fe³⁺. Fo values of olivine range from 91 to 77. Cr/(Cr + Al) atomic ratios of spinel are lower than 0.5 even in harzburgites. Fe³⁺/(Cr+Al+Fe³⁺) atomic ratios of spinel are sometimes over 0.1. TiO₂ contents of clinopyroxene often exceed 0.5 wt%. These characteristics are revealed when Group I rocks are veined or selvaged by Group 11 rocks; chemical compositions of minerals in peridotites systematically change forwards the latter. This strongly suggests that injections of melts with alkali basaltic affinity which had precipitated Group 11 rocks resulted in diffusion metasomatism on the Group I rocks.It is likely that the metasomatized peridotites are widespread underneath the areas where alkali basalt magmatism had fluorished, such as southwestern Japan. Some of Fe-rich lherzolite and harzburgite xenoliths reported in the literature are possibly metasomatites.

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Petrologie von Peridotit-Xenolithen in Lamprophyren von Shingu, Südwest-Japan: Hinweise auf die Herkunft Fe-reicher Mantel-Peridotite

Zusammenfassung In lamprophyrischen Gängen von Shingu kommen Xenolithe von Harzburgit, Lherzolith, Dunit and Wehrlit (= Gesteinsgruppe I) vor. Sie werden von einer Vielzahl von ultramaf-isch-mafischen Xenolithen mit Al- and Ti-reichem Klinopyroxen and/oder Kaersutit (Websterit, Klinopyroxenit, Kaersutit-Gestein, Gabbro and Anorthosit) (=Gesteinsgruppe II) begleitet, die die Xenolithe der Gruppe I häufig gangförmig durchkreuzen, was auf ein jü ngeres Alter der Gesteinsgruppe II hinweist. Obwohl die Harzburgite and Lherzolithe von Shingu übliche modale Mineralbestände aufweisen, sind die Mineralchemismen außergewöhnlich: Niedrige Mg- and Cr- and hohe Ti-, Al- and Fe³⁺-Gehalte. Die Fo-Gehalte von Olivin reichen von 91 bis 77. Die Cr/(Cr+Al)-Atom-Verhältnisse der Spinelle sind kleiner als 0,5, sogar in den Harzburgiten; die Fe³⁺/(Cr+Al+Fe³⁺)-Atom-Verhaltnisse teilweise größer als 0,1. Der TiO₂-Gehalt im Klinopyroxen ist meist über 0,5 Gew.%. Diese Charakteristika zeigen sich dort, wo die Gesteinsgruppe II die Gesteinsgruppe I durchschlägt oder kontaktiert. Der Mineralchemismus in den Peridotiten ändert sich dabei systematisch. Es wird vermutet, daß Schmelzinjektionen mit alkali-basaltischer Affinität, von denen die Gesteinsgruppe II herstammt, eine Diffusions-Metasomatose der Gesteinsgruppe I verursacht hat.Es wird angenommen, daß metasomatisierte Peridotite an der Basis von alkali-basaltischem Magmatismus weft verbreitet sind, wie zum Beispiel in Südwest-Japan. Einige in der Literatur aufscheinende Fe-reiche Lherzolith- and Harzburgit-Xenolithe sind möglicherweise metasomatisch entstanden.

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With 6 Figures     

新疆西克尔碧玄岩中的地幔橄榄岩包体

在新疆西克尔地区发现了尖晶石相橄榄岩包体.这些包体的寄主岩石为碧玄岩,其K-Ar同位素年龄为19.76～21.90 Ma.岩相学和矿物化学研究表明西克尔橄榄岩包体具有典型的岩石圈地幔橄榄岩包体的特征.利用矿物温压计对包体的平衡温压进行估算,发现西克尔地幔橄榄岩包体的平衡温压为736～1017℃和1.7～2.2GPa,与西南天山托云地区晚白垩纪火山岩中尖晶石二辉橄榄岩包体(平衡温压为818～1113℃和1.5～2.0GPa)相比,具有温度明显偏低,而压力明显偏高的特点.这说明西克尔地区的地幔橄榄岩包体没有受到地幔热异常事件的影响,因此可以代表塔里木板块岩石圈地幔的原始性质.这对于研究塔里木盆地岩石圈地幔的热结构和地球化学特征以及塔里木盆地内大量幔源岩浆的成因具有重要意义.     

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.     

Highly siderophile elements in ureilites

The abundances of the highly siderophile elements (HSE) Ru, Pd, Re, Os, Ir, and Pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples, including monomict, augite-bearing, and polymict lithologies. This report adds significantly to the quantity of available Pt and Pd abundances in ureilites, as these elements were rarely determined in previous neutron activation studies. The CI-normalized HSE abundance patterns of all ureilites analyzed here except ALHA 81101 show marked depletions in the more volatile Pd, with CI chondrite-normalized Pd/Os ratios (excluding ALHA 81101) averaging 0.19 ± 0.23 (2σ). This value is too low to be directly derived from any known chondrite group. Instead, the HSE bulk rock abundances and HSE interelement ratios in ureilites can be understood as physical mixtures of two end member compositions. One component, best represented by sample ALHA 78019, is characterized by superchondritic abundances of refractory HSE (RHSE—Ru, Re, Os, Ir, and Pt), but subchondritic Pd/RHSE, and is consistent with residual metal after extraction of a S-bearing metallic partial melt from carbonaceous chondrite-like precursor materials. The other component, best represented by sample ALHA 81101, is RHSE-poor and has HSE abundances in chondritic proportions. The genesis of the second component is unclear. It could represent regions within the ureilite parent body (UPB), in which metallic phases were completely molten and partially drained, or it might represent chondritic contamination that was added during disruption and brecciation of the UPB. Removal of carbon-rich melts does not seem to play an important role in ureilite petrogenesis. Removal of such melts would quickly deplete the ureilite precursors in Re/Os and As/Au, which is inconsistent with measured osmium isotope abundances, and also with literature As/Au data for the ureilites. Removal of ²⁶Al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions.     

Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths

Osmium, Ru, Ir, Pt, Pd and Re abundances and ¹⁸⁷Os/¹⁸⁸Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al₂O₃. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al₂O₃ contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the ¹⁸⁷Os/¹⁸⁸Os ratio (a proxy for Re/Os) show positive correlations with Al₂O₃, indicating incompatible behavior of Pt, Pd and Re during mantle melting. The empirical sequence of peridotite-melt partition coefficients of Re, Pd and Pt as derived from peridotites () is consistent with previous data on natural samples. Some harzburgites and depleted lherzolites have been affected by secondary igneous processes such as silicate melt percolation, as indicated by U-shaped patterns of incompatible HSE, high ¹⁸⁷Os/¹⁸⁸Os, and scatter off the correlations defined by incompatible HSE and Al₂O₃. The bulk rock HSE content, chondritic Os/Ir, and chondritic to subchondritic Pt/Ir, Re/Os, Pt/Re and Re/Pd of many lherzolites of the present study are consistent with depletion by melting, and possibly solid state mixing processes in the convecting mantle, involving recycled oceanic lithosphere. Based on fertile lherzolite compositions, we infer that PUM is characterized by a mean Ir abundance of 3.5 ± 0.4 ng/g (or 0.0080 ± 0.0009*CI chondrites), chondritic ratios involving Os, Ir, Pt and Re (Os/Ir_PUM of 1.12 ± 0.09, Pt/Ir_PUM = 2.21 ± 0.21, Re/Os_PUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/Ir_PUM = 2.03 ± 0.12, Pd/Ir_PUM = 2.06 ± 0.31, uncertainties 1σ). The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217-228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.     

Origin of planetary cores: evidence from highly siderophile elements in martian meteorites

We present new bulk compositional data for 6 martian meteorites, including highly siderophile elements Ni, Re, Os, Ir and Au. These and literature data are utilized for comparison versus the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. The siderophile composition of ALH84001 is clearly anomalous. Whether this reflects a more reducing environment on primordial Mars when this ancient rock first crystallized, or secondary alteration, is unclear. QUE94201 shows remarkable similarity with EET79001-B for siderophile as well as lithophile elements; both are extraordinarily depleted in the “noblest” siderophiles (Os and Ir), to roughly 0.00001 × CI chondrites. As in terrestrial igneous rocks, among martian rocks Ni, Os and Ir show strong correlations vs. MgO. In the case of MgO vs. Ni, the martian trend is displaced toward lower Ni by a large factor (5), but the Os and Ir trends are not significantly displaced from their terrestrial counterparts. For Mars, Re shows a rough correlation with MgO, indicating compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows a weak anticorrelation vs. MgO, resembling the terrestrial distribution except for a displacement toward 2–3 times lower Au. The same elements (Ni, Re, Os, Ir and Au) show similar correlations with Cr substituted for MgO. Data for lunar and HED rocks generally show less clear-cut trends (relatively few MgO-rich samples are available). These trends are exploited to infer the compositions of the primitive Earth, Mars, Moon and HED mantles, by assuming that the trend intercepts the bulk MgO or Cr content of the primitive mantle at the approximate primitive mantle concentration of the siderophile element. Results for Earth show good agreement with earlier estimates. For Mars, the implied primitive mantle composition is remarkably similar to the Earth’s, except for 5 times lower Ni. The best constrained of the extremely siderophile elements, Os and Ir, are present in the martian mantle at 0.005 times CI, in comparison to 0.007 times CI in Earth’s mantle. This similarity constitutes a key constraint on the style of core-mantle differentiation in both Mars and Earth. Successful models should predict similarly high concentrations of noble siderophile elements in both the martian and terrestrial mantles (“high” compared to the lunar and HED mantles, and to models of simple partitioning at typical low-pressure magmatic temperatures), but only predict high Ni for the Earth’s mantle. Models that engender the noble siderophile excess in Earth’s mantle through a uniquely terrestrial process, such as a Moon-forming giant impact, have difficulty explaining the similarity of outcome (except for Ni) on Mars. The high Ni content of the terrestrial mantle is probably an effect traceable to Earth’s size. For the more highly siderophile elements like Os and Ir, the simplest model consistent with available constraints is the veneer hypothesis. Core-mantle differentiation was notably inefficient on the largest terrestrial planets, because during the final ∼ 1% of accretion these bodies acquired sufficient H₂O to oxidize most of the later-accreting Fe-metal, thus eliminating the carrier phase for segregation of siderophile elements into the core.     

Clinopyroxene-orthopyroxene major and rare earth elements partitioning in spinel peridotite xenoliths from Assab (Ethiopia)

Major element and rare earth element (REE) partitioning among coexisting clinopyroxene-orthopyroxene pairs from mantle xenoliths of the Assab Range (Ethiopia) are discussed in terms of crystal-chemistry.Major element partitioning indicates relatively uniform conditions of subsolidus equilibration over a narrow range of temperatures (mean value about 1100 C) in the spinel peridotite stability field. Major element distributions and correlations, moreover, seem to indicate that the mantle material studied underwent slightly different depletions prior to the metamorphic equilibration.In spite of the rather homogeneous major element compositions for both cpx and opx, clinopyroxenes show chondrite-normalized REE patterns which are widely variable both in shape and absolute values, whereas orthopyroxenes exhibit more restricted ranges and concordant profiles.REE activity ratios have been investigated by applying Iiyama's (Bull. Soc. fr. Minéral. Christallogr.97, 143–151) thermodynamic model: the estimated activity patterns exhibit a good coherence for the different pyroxene pairs, in spite of the contrasting features of their REE concentration ratios. The wide ranges in the measured partition values for the same rare earth element in different pyroxene pairs have been related to coupled substitutions involving A1 in the Z site and REE in the M₂ site of clinopyroxene.     

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.     

内蒙古集宁新生代玄武岩中橄榄岩包体和巨晶的发现及意义

内蒙古集宁新生代玄武岩产于华北克拉通北缘，属于广义的“汉诺坝玄武岩”。首次在三义堂附近的碱性玄武岩中发现了大量的橄榄岩包体、辉石和长石以及少量的含钛磁铁矿巨晶。包体主要是尖晶石二辉橄榄岩，造岩矿物为橄榄石＋斜方辉石＋单斜辉石＋尖晶石，辉石巨晶主要为透辉石和普通辉石，长石巨晶主要是歪长石和少量斜长石。1．5GPa条件下，尖晶石二辉橄榄岩样品所记录的平衡温度与汉诺坝相近，集中于950℃左右，大体上反映了华北克拉通北缘大陆岩石圈地幔尖晶石相部分的温度状态。单斜辉石巨晶的结晶温压大于幔源包体的平衡温压，表明巨晶的来源深度可能大于包体。包体的结构及矿物成分分析表明，这些包体是玄武质岩浆上升过程中偶然捕获岩石圈地幔的岩石碎块。二辉橄榄岩包体中橄榄石高Mg#值（89．5～91．7）和较高的NiO含量（0．29％～0．55％），暗示集宁玄武岩中的橄榄岩包体来自较高熔融程度的岩石圈地幔。     

Electron microscope study of peridotite xenoliths in kimberlites

Dislocation structures in naturally deformed olivine from garnet peridotite xenoliths from South African kimberlites have been studied by electron microscopy. The substructure consists mainly of straight subboundaries of dislocations with Burgers vectors [001]. Most of the dislocations have both edge and screw components, and the slip planes are mainly (100). The dislocation density between the subboundaries is low.The slip planes in olivine are discussed in relation to the olivine structure. The observed dislocation structures seem to indicate that the large difference in strain rate between natural and experimental deformation will produce a difference in the slip mechanisms.The nature of the deformation lamellae visible in optical microscope is discussed.     

Naturally decorated dislocations in olivine from peridotite xenoliths

Dislocations decorated by hematite and magnetite have been observed optically in the olivine grains of undeformed or highly annealed peridotite xenoliths from Hawaii and Baja California ( 5 × 10⁵ cm^–2). The observed structures include loops, low-angle boundaries, and structures produced by multiple cross-glide of [100] screws. Loops are almost invariably parallel to (001). Simple arrays of parallel dislocations lie predominantly in (100), (010) and (001) with dislocation lines subparallel to low-index directions. [100] screws pinned to (100) boundaries are frequently seen to bow out on (001). Preliminary electron petrography has confirmed that all dislocations are decorated.     

与侵入岩有关的金矿成矿体系中Au与成矿伴生元素相关关系研究

与侵入岩有关的金矿成矿体系是近十几年来金矿研究的热点之一。以澳大利亚新英格兰造山带尤腊拉地区发现的某金矿脉为研究对象,基于土壤-围岩-低品位矿化体-矿体,较系统地研究了Au与其他成矿元素的相关关系,发现Au与As一直呈正相关,而Au与Na在原生环境中却表现出明显的负相关;另外,Ag、S、Sb 3个元素与Au在矿体中表现出弱负相关,而在其他3种介质中表现出正相关。综合以上研究结果,提出以As+Na作为IRGS型金矿的找矿指标,同时应重视综合利用Ag、S、Sb 3个元素。     

Petrology and geochemistry of peridotite xenoliths from the Letlhakane kimberlites, Botswana

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m² continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996     

Two diamond-bearing peridotite xenoliths from the finsch kimberlite,South Africa

Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at 1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the wet peridotite solidus.     

内蒙古阿巴嘎地幔岩捕掳体与岩石圈地幔性质探讨

阿巴嘎地区新生代地幔捕掳体岩性主要为尖晶石二辉橄榄岩,矿物组成有橄榄石(Fo89-91)、单斜辉石(Wo43-49En44-49Fs3-11)、斜方辉石(Wo0.7-2.2En88-91Fs8-11)和铬尖晶石(Cr6#-20)。橄榄岩结构以碎斑结构为主,除此还可见海绵边结构、重结晶结构、粗粒结构和变晶结构,部分橄榄石发育膝折带和伊丁石化现象,这些特征暗示该区地幔经历了较强烈的熔体-岩石圈相互作用和复杂的演化过程。大地构造位置上,阿巴嘎与大兴安岭哈拉哈河-绰尔河及五大连池-科洛同处在兴蒙造山带上,但阿巴嘎表现出与华北克拉通西部北缘相似的热状态及饱满型-过渡型的岩石圈地幔性质,区别于大兴安岭中部具有古老残留的岩石圈地幔,暗示着兴蒙造山带之下的岩石圈地幔存在时空上的不均一性。而这种大尺度岩石圈地幔的不均一性可能与软流圈-岩石圈相互作用程度、上地幔复杂的地质演化及兴蒙造山带内部不同块体的活动强度、时间等性质有关。     

Sc,Cr, Co and Ni partitioning between minerals from spinel peridotite xenoliths

The partitioning of divalent (Co, Ni) and trivalent (Sc, Cr) trace elements between olivine, ortho- and clinopyroxene and spinel from spinel peridotite xenoliths has been investigated. These peridotites cover a wide range in modal composition from dunite to primitive lherzolites and have equilibrated in the upper mantle between >900° C and <1,200° C.The distribution of Co and Ni shows only minor variation through the whole sequence. In contrast, Sc partitioning between ortho- and clinopyroxene and olivine and clinopyroxene as well as Cr partitioning between olivine and clinopyroxene or olivine and orthopyroxene display high but systematic variations which can be assigned to dependences upon equilibration temperatues. Empirical temperature calibrations are given for Sc-orthopyroxene/clinopyroxene, Sc-olivine/clinopyroxene and Cr-olivine/clinopyroxene which, in principle, may permit to estimate equilibration temperatures not only for lherzolites or harzburgites but for orthopyroxene-free peridotites, too.Sc and Ni partition coefficients between spinel and mantle silicate minerals are primarily dependent upon the major element composition of spinel (e.g. Cr and Al) although a temperature dependence can still be identified. Probably such compositional effects are not observed for trace element partitioning between pyroxenes and olivine or ortho- and clinopyroxene only for the reason that in normal spinel peridotites these minerals show much less variation in major element composition than their coexisting spinels.     

Distribution of siderophile and chalcophile elements in serpentinites of the oceanic lithosphere as an insight into the magmatic and crustal evolution of mantle peridotites

The paper reports result of comparative analysis of the distribution of siderophile (Au, Pt, Pd, Co, Ni, and Cr) and chalcophile (Ag, Cd, Sb, Pb, Zn, Cu, As, and S) elements in ultramafic rock samples of various types of abyssal peridotites. One of the objectives of this research was to obtain reference estimates for the concentrations of siderophile and chalcophile elements in the two end members defining the geochemical evolutionary trends of the material of the suboceanic mantle: a sample of insignificantly depleted mantle material (spinel lherzolite) and extensively carbonated serpentinite after harzburgite, which was formed at the “endpoint” of the ascent of mantle material to the seafloor surface. The distribution of siderophile and chalcophile elements is proved to record information on the whole compositional evolution of abyssal peridotites corresponding to the trajectory of their exhumation from mantle depth levels to seafloor outcrops. These data indicate that the bulk-rock compositional parameters of abyssal peridotites can be utilized to estimate the contribution of magmatic and hydrothermal process to the distribution of siderophile and chalcophile elements.     

Rare earth elements in alpine peridotites

The samples from alpine peridotite massifs (Beni Bouchera, Lherz and the Alps) have been analyzed for rare earth elements. The peridotites as a whole are characterized by various degrees of light REE depletion (Ce varies from 1.2 to 0.02 times chondrite) and a small variation in heavy REE (Yb varies about a factor of 2, from 1.3 to 2.2 times chondrite). They show a restricted and regular distribution in a Ce-Yb diagram, giving two types of linear trends for individual massifs (trend A for the Alps and Lizard; trend B for Beni Bouchera and Lherz, branching from trend A). The model calculations of partial melting based on the partition relations of REE among silicate minerals and melts suggest that trend A could represent a series of residua left after partial melting of garnet peridotite despite the fact that there is no garnet observed in the peridotites studied here. It is suggested that trend A would represent a melting event which predated the emplacement of the massifs and occurred at higher pressure (in the presence of garnet) than expected from the present mineralogy. The calculations also suggest that trend B could represent a partial melting event at lower pressures than trend A after the massifs uplifted into spinel peridotite field. It is also suggested that the REE concentrations of the mantle could be estimated as 2–2.5 times chondrite.