Trace element abundances in garnets and clinopyroxenes from diamondites – a signature of carbonatitic fluids

Summary ?Silicates intergrown with diamonds from 10 diamondites (polycrystalline diamonds, framesites) have been analysed for trace element contents by laser ablation ICP-MS. The diamondites are fine- (< 100 μm) to coarse-grained (> 1 mm) rocks with abundant pores and cavities. The walls of the open cavities are covered by euhedral diamond crystals. Silicates (commonly garnets) are mostly interstitial or occupy the space in cavities and often contain inclusions of euhedral diamonds. Four diamondites contain lilac “peridotitic” garnets with low CaO contents (3.6–5.7 wt%), high Mg-numbers (0.83–0.84) and high Cr₂O₃ contents (3.9–6.4 wt%). Occasionally, they are accompanied by Cr-diopside. “Peridotitic” garnets have heavy rare earth element-enriched and light rare earth element-depleted chondrite-normalised patterns, occasionally with a small hump at Eu and Sm. The remaining six diamondites contain orange coloured “eclogitic” garnets with low Cr₂O₃ contents (< 1 wt%). “Eclogitic” garnets can be divided into two subgroups: E-I garnets have high Mg-numbers (0.84–0.85, as high as those of the “peridotitic” garnets) and higher Cr₂O₃ and TiO₂ and lower heavy rare earth element contents than the E-II garnets. The chondrite-normalised trace element patterns of the two subgroups of “eclogitic” garnets are similar to each other, all are depleted in light rare earth elements with respect to the heavy rare earth elements and show significant positive anomalies of Zr and Hf. “Eclogitic” garnets are more depleted in highly incompatible elements (light rare earth elements, Nb and Ta) than the “peridotitic” garnets. Diamondites and their silicates very likely crystallised from a fluid phase. The trace element contents of the hypothetical fluids in equilibrium with the “peridotitic” garnets are similar to the trace element contents of kimberlitic and carbonatitic liquids. Thus, crystallisation of these diamondites from a highly alkaline liquid in the presence of carbonates can be suggested. Hypothetical melts in equilibrium with “eclogitic” garnets are highly magnesian but depleted in light rare earth elements and other highly incompatible elements relative to the typical kimberlitic, lamproitic or carbonatitic liquids. This is an unexpected result because eclogites are richer in trace elements than peridotites and fluids in equilibrium with these rocks should reflect this. The different trace element contents of fluids which precipitated, beside diamonds, “peridotitic” and “eclogitic” garnets, respectively, therefore, must be the result of differences in the properties of these fluids rather then of different source rocks, as was already suspected by Kurat and Dobosi (2000). Received October 27, 2000; revised version accepted December 29, 2001     

Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis

In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation.     

Trace element abundances of high-MgO glasses from Kilauea, Mauna Loa and Haleakala volcanoes, Hawaii

We performed an ion-microprobe study of eleven high-MgO (6.7–14.8 wt%) tholeiite glasses from the Hawaiian volcanoes Kilauea, Mauna Loa and Haleakala. We determined the rare earth (RE), high field strength, and other selected trace element abundances of these glasses, and used the data to establish their relationship to typical Hawaiian shield tholeiite and to infer characteristics of their source. The glasses have trace element abundance characteristics generally similar to those of typical shield tholeiites, e.g. L(light)REE/H(heavy)REE_C1 < 1. The Kilauea and Mauna Loa glasses, however, display trace and major element characteristics that cross geochemical discriminants observed between Kilauea and Mauna Loa shield lavas. The glasses contain a blend of these discriminating chemical characteristics, and are not exactly like the typical shield lavas from either volcano. The production of these hybrid magmas likely requires a complexly zoned source, rather than two unique sources. When corrected for olivine fractionation, the glass data show correlations between CaO concentration and incompatible trace element abundances, indicating that CaO may behave incompatibly during melting of the tholeiite source. Furthermore, the tholeiite source must contain residual garnet and clinopyroxene to account for the variation in trace element abundances of the Kilauea glasses. Inversion modeling indicates that the Kilauea source is flat relative to C1 chondrites, and has a higher bulk distribution coefficient for the HREE than the LREE. Received: 4 February 1997 / Accepted: 27 August 1997     

Trace element abundances and mineral/melt distribution coefficients in phonolites from the Laacher See volcano (Germany)

Twenty six whole rocks, seven matrix and fifty three mineral separates from the compositionally zoned late Quaternary Laacher See tephra sequence (East Eifel, W Germany) were analyzed by instrumental neutron activation. These data document the chemical variation within the Laacher See magma chamber prior to eruption with a highly fractionated phonolite at the top and a more mafic phonolite at its base as derived from other data. Incompatible elements such as Zn, Zr, Nb, Hf, U, light and heavy rare earths are extremely enriched towards the top whereas compatible elements (e.g. Sr, Sc, Co, Eu) are strongly depleted. Semicompatible elements (Ta and some middle REE) are depleted at intermediate levels. This chemical variation is shown by whole rock and matrix data indicating the phonolite liquid was compositionally zoned regardless of phenocryst content. Hybrid rocks (phonolite-basanite) show the largest concentrations for compatible elements. All elements (except Rb) display continuous compositional variations with regard to the stratigraphic position of pumice samples. From these data we are able to distinguish three main units: An early erupted highly fractionated magma, the main volume of evolved phonolite and a mafic phonolite as the final products. The extreme variation of trace element distribution coef ficients (K) for 9 mineral phases with respect to stratigraphic position (resp. matrix composition) cannot be explained by conventional mechanisms. We postulate a significant modification of the trace element content of the phonolite melt by liquid-liquid controlled differentiation processes subsequent to and/or contemporaneous with (fractional) crystallization which caused disequilibrium between phenocrysts and host matrix. Therefore, our “distribution coefficients” deviate from equilibrium partition coefficients equivalent to the amount of this post crystallization modification of the matrix composition. The relationship between varying K and matrix composition is demonstrated by a ΔK-ΔM-diagram (variation of K versus variation of matrix, M). Different parts of this diagram relate to different parameters (T, P, polymerization, complex-building, equilibrium crystallization in a zoned magma column and post crystallization disequilibrium effects) which are responsible for the variation of distribution coefficients in general. The ΔK-ΔM-diagram may allow to distinguish between different processes affecting the distribution coefficients measured in natural volcanic rocks from a differentiating magma system.     

Oxygen and bulk element abundances in Luna 20 fines

Abundances of O, Si, Al and Mn have been determined in Luna 20 fines sample 22001,9 by instrumental neutron activation analysis. The abundances of O, Si and Al are among the highest we have observed in lunar samples and reflect a highlands origin for much of this regolith sample. The Luna 20 abundances reported here most closely resemble those we have determined in four samples of two Apollo 16 fines, rock 14310, and a clast from breccia 15459. The Luna 20 $\text{O}\text{Si}$ ratio of 1.96 ± 0.05 is similar to that in most other lunar samples, but the $\text{Al}\text{Si}$ ratio of 0.532 ± 0.024 is exceeded only by our data on the Apollo 16 fines. This $\text{Al}\text{Si}$ ratio is in agreement with the value of 0.55 ± 0.06 determined by the remote X-ray fluorescence experiment for the highlands between Mare Crisium and Mare Smythii which lie near the Luna 20 site (Adleret al., 1972).     

铂族元素在地壳、岩石和沉积物中的分布

随着20世纪90年代铂族元素分析测试技术的突破性进展以及大量可靠的岩石和沉积物铂族元素含量分析数据的积累,引用20世纪80年代前发表的有关铂族元素的地壳丰度和岩石、沉积物中的平均含量作为衡量标准已不适宜。根据近15年来获得的大量的有关铂族元素含量的可靠数据,综合给出了铂族元素在地壳、岩石、土壤和沉积物中的丰度。这种对地壳、岩石和沉积物中铂族元素丰度值的新认识,对理论地球化学、勘查地球化学甚至铂族元素矿产勘查工作都具有重要的意义。     

Strontium isotope and alkali element abundances in ultramafic rocks

The concentrations of the trace elements Na, K, Rb and Sr and the isotopic composition of Sr have been measured in a suite of ultramafic rocks, including alpine-type intrusions, inclusions in basalts and kimberlite pipes, zones from stratiform sheets, and a mica peridotite. From these data and those available in the literature the following conclusions can be drawn. Alpine-type ultramafic material appears to be residual in nature and can be neither the source material for the derivation of basalts nor the refractory residue of modern basalts. Alpine-type ultramafic intrusions appear to have no relationship with ultramafic zones in stratiform sheets and were probably derived from the upper mantle. A genetic relationship exists between basalts and their ultramafic inclusions, but it is extremely doubtful that this inclusion material could give rise to basalts by partial fusion. There is a possible genetic relationship between basalts and ultramafic inclusions in kimberlite pipes, and this ultramafic material is a potential source for the derivation of basalts. Ultramafic inclusions in basalts are probably not fragments of an alpine-type ultramafic zone in the mantle. An attempt has been made to synthesize the data and interpretations of this study by way of speculations on the role of ultramafic rocks in the differentiation history of the earth.     

Trace element distribution in Chilean ignimbrites

The geochemistry of Chilean ignimbrites is discussed in terms of major and trace elements. The variation in the major elements and in the distribution of Ba, Co, Cu, Ga, Pb, Sn, Sr, V, and Zr with the differentiation factor (1/3Si+K)-(Ca+Mg) has been studied. The general trend is that characteristic of calc-alkaline rhyolite-dacite-andesite associations. Exceptionally high values of copper reported and high values of Sn and Zr provide further evidence of the anatectic origin of these ignimbrites.     

Trace element partitioning in plagioclase feldspar

Compilation and interpretation of experimental and natural Nernst partition coefficient (^{plagioclase/melt}D) data show that, with a few exceptions, increases in ^{plagioclase/melt}D correlate with decreasing anorthite-content of plagioclase. In contrast, increases of ^{plagioclase/melt}D for Ga, Sc, Cu, Zn, Zr, Hf and Ti, are better correlated against decreasing melt MgO or increasing melt SiO₂ contents. ^{plagioclase/melt}D for Ti and the rare earth elements (REE) show little dependence on temperature, but increase as the melt water content increases. ^{plagioclase/melt}D for K and Sr are sensitive to pressure. Variations of D₀ (the strain compensated partition coefficient), r₀ (the size of the site into which REE substitute), and E (Young’s Modulus of this site) were parameterized against variations of melt SiO₂, the An-content of plagioclase, and other combinations of variables, allowing ^{plagioclase/melt}D_REE-Y to be calculated from a variety of input parameters. The interrelations of temperature, melt MgO and SiO₂ content, and plagioclase anorthite-content for wet and dry systems were also parameterized to facilitate interpolation where such data are lacking. When combined, these semi-empirical parameterizations yield ^{plagioclase/melt}D results comparable to available experimental and natural data.     

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.     

Trace element partitioning in the granulite facies

Analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust. Since granulites are often considered residues of partial melting processes, trace element characteristics of their mineral phases may record mineral/melt equilibria thus giving an opportunity to understand the nature and composition of melts in the lower continental crust. This study provides an extensive set of mineral trace element data obtained by LA-ICP-MS analyses of mafic and intermediate granulites from Central Finland. Mass balance calculations using the analytical data indicate a pronounced contribution of the accessory minerals apatite for the REE and ilmenite for the HFSE. Coherent mineral/mineral ratios between samples point to a close approach to equilibrium except for minerals intergrown with garnet porphyroblasts. Mineral trace element data were used for the formulation of a set of D ^mineral/melt partition coefficients that is applicable for trace element modelling under lower crustal conditions. D ^mineral/melt were derived by the application of predictive models and using observed constant mineral/mineral ratios. The comparison of the calculated D ^mineral/melt with experimental data as well as the relationship between mineral trace element contents and a leucosome with a composition close to an equilibrium melt provides additional constraints on mineral/melt partitioning. The D values derived in this study are broadly similar to magmatic partition coefficients for intermediate melt compositions. They provide a first coherent set of D values for Sc, V, Cr and Ni between clinopyroxene, amphibole, garnet, orthopyroxene, ilmenite and melt. In addition, they emphasize the strong impact that ilmenite exerts on the distribution of Nb and Ta.     

Trace element distribution coefficients in alkaline series

Mineral/groundmass partition coefficients for U, Th, Zr, Hf, Ta, Rb, REE, Co and Sc have been systematically measured in olivine, clinopyroxene, amphibole, biotite, Ti-magnetites, titanite, zircon and feldspars, in basaltic to trachytic lavas from alkaline series (Velay, Chaîne des Puys: Massif Central, France and Fayal: Azores). Average partition coefficients are denned within the experimental uncertainty for limited compositional ranges (basalt-hawaiite, mugearites, benmoreite-trachyte), and are useful for trace element modelling. The new results for U, Th, Ta, Zr and Hf partition coefficients show contrasting behaviour. They can thus be used as “key elements” for identifying fractionating mineral phases in differentiation processes (e.g. Ta and Th for amphibole and mica).Partition coefficient may be calculated using the two-lattice model suggested by Nielsen (1985). Such values show a considerably reduced chemical dependence in natural systems, relative to weight per cent D values. The residual variations may be accounted for by temperature or volatile influence. This calculation greatly enhances modelling possibilities using trace elements for comparing differentiation series as well as for predicting the behaviour of elements during magmatic differentiation.     

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D _REE ^Cpx/Grt decreasing with decreasing D _Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000     

Minor element abundances in olivines of the Sharps (H-3) chondrite

Olivine crystals in 21 chondrules from the Sharps (H-3) chondrite were analyzed for CaO, Al₂O₃, Cr₂O₃, MnO, TiO₂, NiO, and Na₂O. The chondrules studied include representatives of all major types found in Sharps, and the mean fayalite contents of their olivine range from 1 to 28 %. Those olivines which contain less than 18 mol.% fayalite typically contain or occur with metallic nickel-iron; the others are metal-free.Na₂O is below detectability (0.01 wt.%) in all cases, and the abundances of Al₂O₃, NiO and TiO₂ are also typically very low. MnO varies simply and directly with FeO.Cr₂O₃ varies widely (0.03–0.21%) and several lines of evidence suggest that Cr is dominantly trivalent. It is concluded that F_{O 2} was rarely less than 10^–11 atm. during the crystallization of the chondrules in Sharps.     

Trace element modelling of pelite-derived granites

The presence or absence of a vapour phase during incongruent-melt reactions of muscovite and biotite together with the composition of the protolith determines the trace-element characteristics of the resulting melt, provided that equilibrium melting occurs for those phases that host the tracc elements of interest. For granitic melts, Rb, Sr and Ba provide critical constraints on the conditions that prevailed during melting, whereas REE are primarily controlled by accessory phase behaviour. Mass-balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction, and deplete the restite in feldspar. Hence the melt will be characterized by low Rb/Sr and high Sr/Ba ratios. In contrast, vapour-absent melting will result in a smaller melt fraction, and an increase in the restitic feldspar. Consequently high Rb/Sr and low Sr/Ba ratios are predicted. Vapour-absent melting will also enhance the negative Eu anomaly in the melt. Granites that result from the incongruent melting of biotite in the source will be characterized by higher Rb concentrations than those that result from the incongruent melting of muscovite. The Himalayan leucogranites provide an example of unfractionated, crustally derived eutectic melts that are enriched in Rb but depleted in Sr and Ba relative to their metasedimentary protoliths. These compositions may be generated by the incongruent melting of muscovite as a low melt fraction (F0.1) from a pelitic source under vapour-absent conditions.     

Trace element geochemistry in topsoil from East China

The concentrations of 23 trace elements in 50 topsoil samples collected from sites ranging between 18°19′N and 49°13′N in East China were analyzed. Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, and Ta have mean contents more than two times higher than in the continental upper crust. Three elements, Rb, Sr, and Ba, are present at lower concentrations than in the continental upper crust. Finally, a group of elements consisting of Ge, Y, Zr, Nb, Sc, Hf, Pb, Th, and U are present at concentrations 1–2 times higher than in the continental upper crust. However, concentrations of trace elements are mainly affected by parent rock. The contents of Sc, Ti, V, Cr, Mn, Co, and Cu for 29 soils from basalt were found to increase from north to south, whereas Rb, Sr, and Ba contents were found to decrease. In addition, element concentration shows a close relationship with annual average temperature (AAT) as well as annual average precipitation (AAP). Since the 29 soils are all from basalt, the trends of the elemental contents should reflect the influence of climate, which determines the intensity of weathering. These elemental trends suggest that the content of certain elements may indicate the intensity of basalt weathering. Ba/Nb and Sr/Nb ratios were both found to have good correlations with AAT and AAP in this study, which means that these ratios can also indicate the intensity of chemical weathering of basalt.     

Rare earth element abundances and distribution patterns in hydrothermally altered basalts: Experimental results

Oceanic tholeiites that have been experimentally reacted with seawater at 500–600° C, 800–1000 bars, have rare earth element (REE) abundances and distribution patterns that are essentially identical to those of the unaltered basalts. Although the data indicate the possibility of some REE leaching and redistribution, the net effects are small and the REE patterns of basalts affected by submarine hydrothermal processes may still be valid petrogenetic indicators.     

Trace element and isotopic composition of baddeleyite

Summary Baddeleyite from Palabora Igneous Complex, South Africa, is among the purest natural ZrO₂ phases. This has been demonstrated by using various methods, i.e. microprobe, neutron activation, spark source and thermal ion mass spectrometry. HfO₂ with 1.87% is the only other major component. The concentrations of other HFSE are also relatively high, compared to most other elements, that reach only a few ppm.The REE display a U-shaped pattern that is interpreted to be superimposed by a strongly LREE enriched source component. The high⁸⁷Sr/⁸⁶Sr initial of 0.713085 and the negative Nd_t of –10.7 prove that this component was LILE enriched for a long time prior to the formation of the Palabora Igneous Complex. These data indicate that the baddeleyite crystallized from a magma which was derived from an enriched mantle reservoir, similar to that involved in the formation of group II kimberlites.

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Spurenelemente und Isotopenzusammensetzung von Baddeleyit

Zusammenfassung Baddeleyit vom Palabora Igneous Complex, Südafrika, gehört zu den reinsten natürlichen Vorkommen von ZrO₂. Dies wurde durch Analysen mit verschiedenen Methoden wie Mikrosonde, Neutronenaktivierung, Funken- und Thermionenmassenspektrometrie bestätigt. HfO₂ ist mit 1.87 die einzige andere Hauptkomponente, auch die Konzentrationen der anderen HFSE sind relativ hoch im Vergleich zu den anderen Elementen, die nur wenige ppm erreichen.Die REE bilden ein U-förmiges Muster, das als Ausdruck einer stark LREE angereicherten Komponente im Ausgangsgestein gedeutet wird. Das hohe⁸⁷Sr/⁸⁶Sr-Initialverhältnis von 0.713085 und das negative Nd_t von -10.7 belegen, daß diese Komponente über einen langen Zeitraum vor der Bildung des Palabora Igneous Complex angereichert gewesen war. Diese Daten deuten an, da der Baddeleyit aus einem Magma kristallisierte, das aus einem angereicherten Mantelreservoir stammte, ähnlich dem, das bei der Bildung der Gruppe 11 Kimberlite beteiligt war.

     

Trace element geochemistry of Archean volcanic rocks

K, Rb, Sr, Ba and rare earth elements of some Archean volcanic rocks from the Vermilion greenstone belt, northeast Minnesota, were determined by the isotopic dilution method. The characteristics of trace element abundances, supported by the field occurrences and major element chemistry, suggest that these volcanic rocks were formed in an ancient island arc system. A felsic rock is suggested to be derived by partial melting of a basaltic source, presumably in an ancient subduction zone.It is well known that the distribution coefficients (liquid/source) for the above trace elements are almost invariably greater than one. Continuous extraction of volcanic liquids from the upper mantle through geologic time would result in depletion of these elements in the upper mantle. However, all trace element abundances in many Archean volcanic rocks are almost identical to their modern equivalents. This gross constancy of trace element concentration in rocks of different geologic age raises some important questions as to the evolution of the upper mantle. It is proposed that the trace elements have been repeatedly and fully recycled in a restrictive and closed system of crust and upper mantle during the last three billion years (recycled mantle), or the trace elements have been replenished from the lower part of the mantle by some undefined process (replenished mantle). It is believed that interplay of both recycling and replenishment have been responsible for crust-mantle evolution in geological history.