Rare earth element partitioning between minerals from anhydrous spinel peridotite xenoliths

REE abundances in minerals from spinel peridotite xenoliths from West Germany, the south-western U.S. and Mongolia decrease in the order clinopyroxene > orthopyroxene > olivine > spinel. While clinopyroxenes are similar in absolute chondrite-normalized concentrations to those known from other studies, orthopyroxenes and olivines are significantly lower in LREE although comparable in HREE. Spinels are much lower in all REE than any previously reported values and are completely negligible for the REE budget of peridotites.Partition coefficients for most orthopyroxene/clinopyroxene pairs increase systematically from La to Lu. Olivine/clinopyroxene and spinel/clinopyroxene partition coefficients increase from the intermediate rare earth elements to Lu and normally are higher for La compared to Sm.The application of Nagasawa's (1966) elastic lattice model suggests that all heavy but only minor amounts of the light REE substitute into structural positions of orthopyroxene and olivine.Significant differences between orthopyroxene/clinopyroxene partition coefficients for various xenoliths may be assigned to dependences upon equilibration temperature and bulk chemistry.Apart from grain surface contaminations, fluid inclusions which are practically always present in mantle minerals, can highly concentrate light rare earth elements and thus may be responsible for unexpectedly high concentrations of incompatible elements frequently reported for mantle olivines or orthopyroxenes.     

Platinum-group minerals in chromitites from the Eastern Rhodope ultramafic complex,Bulgaria

Summary The podiform chromitites investigated in the course of this study occur in intensely serpentinized dunites and peridotites of unknown age (paleozoic or older) within a metamorphic complex consisting of gneisses, amphibolites and marbles. Concentrations of platinum group elements (PGE) and the distribution of platinum group minerals (PGM) have been investigated in the chromitite occurrences of Dobromirci and Pletene.PGE concentrations in chromitites vary from 787 to 891 ppb (Dobromirci). The highest value was recorded in chromite ore from Pletene (1274 ppb). The enrichment is due to high contents of Os, Ir and Ru, whereas the contents of Rh, Pt and Pd are relatively low. The Ru-contents (480-600 ppb) are remarkable and correspond to the average content in chondrite Cl. Chondrite-normalized PGE distribution patterns of chromitites of both localities reveal a distinctly negative trend from Ru to Pd, which is typical for chromites from ophiolites.Irrespective of their chemical composition, most chromites carry numerous PGM inclusions which have formed during the magmatic stage at high sulphur fugacity (fs₂). In addition to laurite, the main mineral, there are sulpharsenides of Ru-Ir-Os (ruarsite, irarsite, osarsite).Textural aspects and the results of chemical analyses show that the concentration of PGE is not caused by substitution in the lattice of chromite, but by magmatic formation of discrete PGM before or contemporaneously with chromite. All PGM apparently remained unaltered. No evidence for remobilization or redistribution of PGE by serpentinization has been found.

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Minerale der Platinggruppe in Chromititen des Ultramafit-Komplexes des Ost-Rhodopen Massivs, Bulgarien

Zusammenfassung Die untersuchten podiformen Chromite tretey in stark serpentinisierten Duniten und Peridotiten unbekannten Alters (paläozoisch oder älter) innerhalb eines hochmetamorphen Komplexes auf, der aus Gneisen, Amphiboliten und Marmoren besteht. In den Chromitit-Vorkommen von Dobromirci und Pletene wurden Konzentrationen der Elemente der Platingruppe (PGE) und die Verteilung der Minerale der Platingruppe (PGM) untersucht.Die PGE-Konzentration der Chromitite variiert zwischen 787 und 891 ppb (Dobromirci). Die höchste Konzentration wurde im Chromiterz aus Pletene (1274 ppb) gefunden. Die Anreicherung geht auf hohe Beteiligung von Os, Ir und Ru zurück, da die Gehalte an Rh, Pt und Pd relativ niedrig sind. Auffallend hoch sind die Ru-Gehalte (480-600 ppb), die dem mittleren Gehalt im Chondrit Cl entsprechen. Chondritnormalisierte PGE-Verteilungsmuster von Chromititen beider Lokalitäten zeigen einen stark negativen Trend von Ru zu Pd, der für Ophiolith-Chromite typisch ist.Unabhängig von ihrem Chemismus führen die meisten Chromite zahlreiche PGME-Einschlüsse, die sich magmatisch bei hoher Schwefelfugazität (fS₂) gebildet haben. Neben dem Hauptmineral Laurit, wurden Sulfarsenide von Ru-Ir-Os (Ruarsit, Irarsit, Osarsit) festgestellt.Texturelle Merkmale der PGM und Ergebnisse der chemischen Analysen führen zu der Schlußfolgerung, daß die Konzentration der PGE nicht auf eine Substitution in Chromit, sondern auf die Frühbildung der selbständigen PGM vor oder gleichzeitig mit den Chromiten zurückzuführen ist. Die PGM zeigen keine Alterationserscheinungen. Es wurden keine Hinweise für eine Remobilisation oder Umsetzung der PGE durch Serpentinisierung gefunden.

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With 7 Figures     

Lithium isotopic disequilibrium of minerals in the spinel lherzolite xenoliths from Boeun,Korea

Spinel lherzolite xenoliths found in Boeun, Korea, have protogranular to porphyroclastic textures and are enclosed in a Miocene alkali basalt. The lithium concentration and isotopic compositions of olivine, clinopyroxene, and orthopyroxene separates from the spinel lherzolite, and whole rocks of the spinel lherzolites and alkali basalt were determined by inductively coupled plasma mass spectrometry (ICP-MS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The lithium concentrations of the olivines and orthopyroxenes range from 2.2 to 5.0 ppm and from 2.1 to 6.4 ppm, respectively. In contrast, the clinopyroxenes have larger lithium concentrations, from 2.0 to 8.4 ppm, which reflect their preferential lithium enrichment. The lithium isotopic compositions (δ⁷Li) of olivines (-5.4 to + 3.5‰), orthopyroxenes (-11.4 to -0.1‰), and clinopyroxenes (-14.4 to -4.7‰) range far beyond the average mantle composition of + 4 ± 2‰. The lithium isotopic composition of the host rock, alkali basalt (3.4‰), is within the range of the intraplate and oceanic island basalts. The spinel lherzolites from Boeun exhibits strong elemental and isotopic disequilibria due to the different lithium and lithium isotope diffusion velocities in the olivine, orthopyroxene, and clinopyroxene minerals after eruption and magma cooling.     

Rare earth elements in a sampled coal from the Pirin deposit,Bulgaria

Rare earth elements (La, Ce, Sm, Eu, Tb, Yb, and Lu) in a columnar section of the coal of the Pirin deposit, Bulgaria, have been determined by neutron activation analyses.The REE content in the coals is lower than the average REE content of shales from North America, Europe, and the Soviet Union. The REE abundances increase with the increasing ash content of the coals. The bottom of the seam is slightly enriched in REE, the trend being more pronounced in HREE (Tb, Yb, Lu). The REE content depends on the thickness of the coal layers: the thin coal layers are enriched in REE as compared to the thick ones.The chondrite-normalized distribution patterns are very uniform. They are characterized by a negative Eu anomaly whose mean value is 0.30 (varying in the individual samples from 0.21 to 0.49) and a positive Lu anomaly. It is supposed that the Eu anomaly is inherited from the source rocks.The shale-normalized distribution patterns show a distinct relative enrichment in HREE and a negative Eu anomaly. The relative enrichment in HREE is a specific feature of the REE geochemistry in the Pirin deposit. The LREE/HREE ratio is lower than that of composite shale; it increases with the increasing ash content of the coals and from the bottom to the top of the coal bed.REE are bound predominantly to the aluminosilicates of the mineral matter in the coals. All REE are positively correlated to the ash, Si, Al, Fe, and Na.The source of REE in coals is mainly the suspended terrigenous material. The specific enrichment of REE in the ash of low-ash coals is a result of the interaction between the dissolved REE and the products of disintegration and decay of organic substances, mainly the humic acids.     

NAA analysis of gold in minerals from Upper Cretaceous massive copper deposits in Bulgaria

Minerals from deposits Chelopech, Elshitsa, Radka and Krasen have been studied and their gold contents determined by neutron-activation. In all deposits sulphide and sulphosalt minerals show a well-expressed ability to concentrate gold, although their role in individual deposits is not equal. The highest concentration of gold is in bornite, tennantite, enargite and the early generations of pyrite, chalcopyrite, galena and sphalerite. The low content of gold in the late generations of pyrite, chalcopyrite, galena and sphalerite is due to the process of recrystallization and redeposition. The data obtained show that gold has been extracted entirely from the solutions of the early quartz-pyrite stage mineralizations while during the later quartz-sulphosalt-sulphide and quartz-carbonite-sulphate stages it was only re-deposited and concentrated.     

Iron in the minerals of the sodalite group

The study of the problem of the black coloration of feldspathoids of the sodalitic group (nosean, haüyne, sodalite) reveals iron as a main factor.A statistical study of the global compositions of these minerals proves that their enriching inFe is accompanied by a decrease in Al, but also by an increase in Si.A substitution of Fe⁺⁺⁺ to Al in the structure in the feldspathoids is suggested.The analysis with an electron microprobe confirms this hypothesis and allows us to state a fondamental difference between the formation of miaskitic phonolites rich in Fe⁺⁺ and that of agpaitic phonolites rich in Fe⁺⁺⁺.     

Alpha-decay damage in minerals of the pyrochlore group

X-ray diffraction analysis and transmission electron microscopy have been used to study the effects of alpha-decay damage in pyrochlore group minerals, characterized by the general formula A _1?m B ₂O₆(O,OH,F)_1?n ·pH₂O. As defined by the XRD intensity ratio I/I ₀ , both the saturation dose (for which I/I ₀ =0.1?0.0) and the dose which signifies the initial loss of crystallinity (for which I/I ₀ =1.0?0.8) increase as a function of geologic age. The increase is attributed to annealing of isolated alpha-recoil tracks back to the original crystalline structure. The tracks have calculated mean lives, τ_a, on the order of 10⁸ years. In contrast, minerals which remain crystalline (e.g., uraninite, UO₂) despite doses of up to 10¹⁸ alpha-events/mg have mean alpha-recoil track lives ≈10⁴ years (Eyal and Fleischer 1985). After correcting the calculated dose for annealing of alpha-recoil damage, I/I ₀ is observed to decrease exponentially to zero over the dose range 0.02–1.0 × 10¹⁶ alpha-events/mg. The relationship between I/I ₀ and “corrected” dose was used to calculate an average alpha-recoil track diameter of 4.6 nm, in which < 2600 atoms are displaced. XRD line broadening due to strain dominates the first half of the crystalline-to-metamict transition, reaching a maximum of 0.003, then decreasing to < 0.001. Line broadening due to decreasing crystallite size dominates the latter half of the transition. Estimated crystallite dimensions decrease from 450 nm to < 15 nm prior to reaching the fully metamict state. With increasing dose HRTEM images of microlites from the Harding pegmatite sequentially exhibit: 1) mottled diffraction contrast, 2) isolated 1–5 nm aperiodic areas, 3) coexisting aperiodic and crystalline areas, 4) relict “islands” of crystalline material in an aperiodic matrix, and 5) complete loss of lattice fringe periodicity. With no consideration given to alpha-recoil track fading, the transition covers a dose range of 0.04–1.7 × 10¹⁷ alpha-events/mg. Using a value of τ_a=10⁸ years, this dose range is corrected down to 0.02–1.2 × 10¹⁶ alpha-events/mg. The metamict state is characterized by a range of M-M and M-O distances which give rise to bands of diffuse scattering centered at 0.30 nm and 0.18 nm, respectively, in x-ray and electron diffraction patterns. Random image contrast shown by HRTEM is consistent with a random network type structure, an interpretation supported by EXAFS/XANES studies (Greegor et al. 1985a, b, 1987). The structure of metamict pyrochlore consists of an aperiodic framework of corner-sharing B-O polyhedra. Compared to the crystalline precursor, the metamict state displays a reduced M-O coordination number and mean bond length, increased distortion of the B-site, and a slight increase in the average M-M distance.     

Boron in magnesium minerals of the humite group

Based on microprobe analysis and IR-spectroscopy of a representative collection of magnesium minerals of the humite group (282 samples from 80 various geological-genetic occurrences all over the world), it was shown for the first time that these minerals typically concentrate trace boron (from 0.5 to 4.9 wt % B₂O₃ in 63 samples). The diagnostic bands of B-O stretching vibrations in IF-spectra of magnesium minerals of the humite group occur within the 1170–1190, 1262–1289, and 1306–1331 cm^?1 ranges and are regularly shifted to the low frequencies in a norbergite-chondrodite-humite-clinohumite series. Boron isomorphically replaces silicon in tetrahedra, probably, by Pertsev’s scheme: Si⁴⁺ + O^2? → B³⁺ + (F,OH)^?.     

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.     

The sodalite group of minerals

The results of a study of the chemical composition, cell parameters, density and refractive index of two sodalites and twenty five members of the nosean-haüyne solid solution series are given. It is evident from this study that potassium is a significant component substituting in the nosean-haüyne series, and that the cell edge shows a linear variation with the K₂O content. The space groups of the minerals have been determined, and it is proposed that the space group be used as the criterion to distinguish nosean (space group P¯43m) from haüyne (space group P¯43n), owing to the lack of a suitable criterion in the composition, density, or refractive index of these minerals. Sharp compositional zoning in nosean and haüyne is reported. Data on the superstructure reflections of certain of the minerals are given and discussed. Finally means of identifying sodalite, nosean, and haüyne are suggested.     

Inclusions of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe,Yakutia

The results of study of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe are reported. Most of the studied samples are characterized by high concentrations of Sr, Ca, Na, and LREEs in comparison with minerals of the LIMA series from kimberlites of South Africa, whereas the average concentrations of Ba and K are significantly lower. Crichtonite group minerals in pyropes are characterized by predomination of Na over K in most samples and by a high concentration of Al₂O₃ (up to 4.5 wt %). Findings of inclusions of crichtonite group minerals with high concentrations of incompatible elements provide evidence for the metasomatic origin of host chromium-rich pyropes.     

Melting and metasomatism in the continental lithosphere: laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia

Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogranular lherzolite xenoliths from southeastern Australia have been analysed for their major and trace element compositions using electron microprobe and laser ablation ICPMS. Bulk compositions of the lherzolites range from fertile (12–13% modal diopside) to depleted (2–3% modal diopside), with equilibration temperatures of 850–900 °C indicating entrainment of these lherzolites from relatively shallow depths (probably ≤ 35 km) within the lithosphere. Mineral compositions and abundances indicate a primary control by partial melting, with decreasing abundance of modal diopside accompanied by increasing Mg# of olivine and pyroxene, decreasing Al and Ti contents of diopside, increasing Ni contents of olivine, and increasing Cr/Al of spinel. HREE, Y, and Ga in diopside also follow melting trends, decreasing in concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent behaviour that cannot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from the cpx-poor samples are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have strongly negative Nb anomalies relative to Th and the LREE. Trace element patterns of diopside in the fertile lherzolites can be reproduced by ≤ 5% batch melting of a primitive source. The negative Nb anomalies are a consequence of this melting, and do not require special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites cannot be produced by realistic degrees of batch melting, but can be accomplished by up to ∼20% fractional melting, suggesting multiple episodes of melt depletion. Os isotopic compositions of these lherzolites show that the melt depletion events occurred in the middle and late Proterozoic, demonstrating the long-term stability of lithospheric mantle beneath regions of eastern Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date the magmatism that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent. Received: 24 September 1996 / Accepted: 12 August 1997     

Geochemical diversity of minerals of the pyrochlore group

A systematic and rational nomenclature of minerals of the pyrochlore group are developed based on the results obtained by processing 671 chemical analyses of pyrochlore-group minerals from carbonatite complexes, alkali rocks, and their pegmatites, granite pegmatites, and alkali and albitized granites. The proportions of Nb, Ta, and Ti are typomorphic of pyrochlore from these four types of geological environments. The paper lists pervasive characteristics of the distribution of Na and Ca, REE, Th and U, Sr and Ba, K and Cs, Pb, Sn, Sb, and Bi in the minerals. Based on the occurrence of compositions with elevated concentrations of typomorphic elements at site B in the structure of the minerals, pyrochlore subspecies are recognized: pyrochlore, Ta-pyrochlore, Ta,Ti-pyrochlore, Zr-pyrochlore, Nb-betafite, Ta-betafite, Ti-betafite, Ti-microlite, Nb,Ti-microlite, Nb-microlite, and microlite, as well as 60 geochemically significant varieties with the predominance of certain cations at site A (REE-pyrochlore, U-pyrochlore, etc.). Aspects of a rational systematics of minerals of complicated isomorphic series are discussed.     

Rare earth abundances and distribution in some spinel peridotite xenoliths from Assab (Ethiopia)

REE partition data between solid phases in nine spinel peridotite xenoliths from Assab (Ethiopia) are presented together with bulk-rock REE composition. REE partitioning between clinopyroxene and other coexisting solid phases varies over a relatively wide range. The largest ranges are for LREE in clinopyroxene/orthopyroxene and clinopyroxene/olivine pairs, while clinopyroxene/spinel partitioning of REE is more restricted. The range of REE partition values between coexisting phases is due to compositional dependency effects and is correlated with systematic variations in major element composition of the bulk rocks. The measured REE concentration in the Assab mantle harzburgites do not match with the compositions calculated by mass balance from the modal proportions and REE analyses in individual phases. Inconsistencies for HREE may be due to variable HREE amounts in the clinopyroxene, orthopyroxene phases within a single specimen, while the high LREE contents in the whole rocks are due to contamination during transport to the surface. A geochemical model based on theoretical treatment of the REE partition data suggests that the Assab harzburgites acquired their residual character during a batch melting episode in the upper mantle under the Afar region.     

Rare minerals from Rajpura-Dariba,Rajasthan, India. VII: Renierite

Summary The first Indian occurrence of renierite, [Cu₁₀(Cu_0.09Zn_0.71Fe_0.15)_0.15Fe₄(Ge_1.68V_0.03-As_0.27)_1.98]_16.93S_16.08 is reported from the Cu-rich basal zone of the polymetallic deposit at Rajpura-Dariba. Optical, X-ray and electron microprobe data on the mineral are compared with those from other occurrences. A slight excess of the iron content compared with the formula Cu₁₀(Zn_1–x Cu _x )(Ge_2–x As _x )Fe₄S₁₆ ofBernstein (1986) and noticeable differences between Cu-coefficient (in brackets) and As-coefficient are obtained. The substitution scheme in renierite may be more complicated than that suggested byBernstein (1986), and apparently also includes the substitution Cu Zn Fe together with As Ge.

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Seltene Minerale von Rajpura-Dariba, India. VII: Renierit

Zusammenfassung Das erste indische Vorkommen von Renierit [Cu₁₀(Cu_0.09Zn_0.71Fe_0.15)_0.15Fe₄(Ge_1.68V_0.03As_0.27)_1.98]_16.93S_16.08 wird mitgeteilt; dieses liegt in der Kupfer-reichen liegenden Zone der polymetallischen Lagerstätte von Rajpura-Dariba. Optische, Röntgen- und Mikrosondendaten dieses Minerals werden mit solchen von anderen Vorkommen verglichen. Ein leichter Überschuß des Eisengehaltes verglichen mit der Formel Cu₁₀(Zn_1–x Cu _x )(Ge_2–x As _x )Fe₄S₁₆ vonBernstein (1986) und Unterschiede zwischen dem Cu-Koeffizient (in Klammer) und dem As-Koeffizient wurden festgestellt. Die Art der Substitution in Renierit dürfte komplizierter sein als vonBernstein (1986) vorgeschlagen. Die Substitution Cu Zn Fe zusammen mit As scheint hier eine Rolle zu spielen.

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With 4 Figures