Low Concentrations Of Trace Elements Research Articles

Dissolution–reprecipitation of igneous zircon in mid-ocean ridge gabbro, Atlantis Bank, Southwest Indian Ridge

Zircons recovered from oceanic gabbro exposed on Atlantis Bank, Southwest Indian Ridge, typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas. In one rock (of twenty investigated), weak-oscillatory-zonation patterns are overprinted by secondary textural features characterized by mottled, convoluted and wavy internal zonation patterns that are frequently associated with secondary micron- to submicron-scale micro-porosity. These zircons are hosted in a felsic vein that intruded an oxide gabbro, both of which are cross-cut by monomineralic amphibole- and quartz-rich veinlets. Zircons with weak-oscillatory-zonation patterns record a weighted-average 206Pb/ 238U age of 12.76 ± 0.20 Ma (mswd = 1.5), and have high trace element concentrations [e.g., ΣREEs (∼ 0.4–2.2 wt.%), Y (∼ 0.6–2.8 wt.%), P (∼ 0.4–0.9 wt.%)], and Th/U (0.1–0.5). These zircons are anomalously old (≥1 Myr) relative to the magnetic age for this portion of oceanic crust (11.75 Ma). In contrast, zircons with non-igneous, secondary textures have a younger weighted-average 206Pb/ 238U age of 12.00 ± 0.16 Ma (mswd = 1.7), and have lower trace element concentrations [e.g., ΣREEs (∼ 0.2–0.8 wt.%), Y (∼ 0.3–1.0 wt.%), P (∼ 0.1–0.3 wt.%)], and slightly lower Th/U (0.1–0.3). The weighted-average age of these zircons is similar to the magnetic anomaly age, and other 206Pb/ 238U ages of nearby rocks. We do not observe a correlation between crystallographic misorientation, internal texture, or trace element chemistry. We suggest that the decrease in trace element concentrations associated with the development of non-igneous alteration textures is attributed to the purging of non-essential structural constituent cations from the zircon crystal lattice at amphibolite-facies conditions. The mechanism of alteration/re-equilibration was likely an interface-coupled dissolution–reprecipitation processes that affected pre-existing, anomalously old zircons during shallow-level magmatic construction of Atlantis Bank at ∼ 12.0 Ma.

Geochemical Signatures of Early Paleogene Source Rocks in the Sanshui Basin, South China

Abstract:The Honggang member of the early Paleogene Buxin Formation is the main source rock in the Sanshui Basin, characterized by organic‐rich black shales with the cyclic recurrence of organic‐poor sediments. The geochemical characteristics of the Honggang member have been documented to determine the organic matter types and depositional environments in this paper. The organic matter of the black shales mainly consists of a mixture of land plant‐derived and phytoplankton‐derived organic matter. Total organic carbon content (TOC)‐sulfur‐iron (Fe) relationships suggest that the organic‐rich black shales were deposited under dysoxic‐to‐euxinic water conditions. The time that iron minerals remained in contact with H2S in anoxic waters possibly influenced the formation of syngenetic pyrite, and organic carbon controlled the formation of diagenetic pyrite. Organic‐poor intervals usually show pyrite sulfur enrichment and higher degree of pyritization values relative to low organic carbon contents. This resulted from HS− diffusing downward from overlying organic‐rich sediments and formed Fe sulfides through reactions with sufficient Fe. Trace elements generally exhibit low concentrations and little TOC dependence, suggesting some degree of depletion in these elements in the early Paleogene sediments of the Sanshui Basin. This probably resulted from cyclic recurrences of oxic benthic conditions, which promoted the remobilization of trace elements and caused the low concentration of trace elements.

Experimental study of the incorporation of Li, Sc, Al and other trace elements into olivine

We performed a series of synthesis experiments at 1 atm pressure to investigate the substitution mechanisms of 1+ and 3+ ions into olivine. Forsterite crystals were grown from bulk compositions that contained the element of interest (e.g. Li) and different amounts of additional single trace elements. By working at constant (major element) liquid composition and temperature we eliminated all compositional effects other than those due to the trace elements. Mineral–melt pairs were then analysed to determine the compositional-dependence of the partition coefficient ( D), which corresponds to [ element ] mineral [ element ] glass , and where [element] refers to weight concentration of the element in the respective phase. We find that Li forms a stable coupled substitution with Sc and, at above ∼500 ppm Sc in the crystal, Li + and Sc 3+ ions form an ordered neutral complex ([LiSc]). This complex dissociates at lower trace element concentrations and a second, concentration-independent, mechanism begins to dominate. This second solution mechanism is most likely 2Li + ⇔ Mg 2+ where one of the Li atoms is in an interstitial position in the crystal lattice. Natural olivines show Li contents slightly greater than Sc (on an atomic basis), indicating that both substitution mechanisms are significant. Unlike Sc, Al does not appear to form a stable complex with Li in the olivine structure. Sodium is highly incompatible in olivine with D Na Fo – melt of ∼0.00015–0.03. Olivine–liquid partitioning of Na + is independent of Sc 3+ or Al 3+ concentration. This indicates that the coupled substitution of Na + with any 3+ ions is unlikely. Instead, the relevant substitution mechanism appears to be 2Na + ⇔ Mg 2+. Although independent of 3+ ion concentration, D Na Fo – melt is inversely correlated with the Li concentration of both melts and crystals, implying that Na competes (unsuccessfully) with Li to replace Mg in the olivine structure. Aluminium is highly incompatible in forsterite D Al Fo – melt = 0.006 ± 0.0005 n = 7 . Values D Al Fo – melt of are similar for all phase pairs synthesised from starting materials containing between 10 and 100,000 ppm Al. This suggests that Al is principally incorporated in forsterite by replacing one Mg and one Si atom Mg Mg x + Si Si x ↔ Al Mg • + Al Si ′ , where the Al atoms on octahedral (Mg) and tetrahedral (Si) sites are dissociated from one another. The incorporation of gallium into forsterite is influenced by the presence of Li. Where Li concentration in the crystal is much greater than that of Ga (on an atomic basis) we find an excellent correlation between D Ga Fo – melt and melt Li content. This relationship indicates that Ga 3+ and Li + replace 2Mg 2+ on octahedral sites and that the Ga and Li atoms are, like Sc and Li, strongly associated in the crystal structure. The mechanism by which scandium is incorporated into forsterite is strongly governed by the presence Li. As discussed above, ordered Li Mg ′ Sc Mg • complexes form readily in forsterite in Li-rich experiments. Under Li-absent but Sc-rich conditions (Sc in the crystal >∼500 ppm), D Sc Fo – melt is proportional to the concentration of Sc in the melt. This indicates that Sc incorporation is charge-balanced by the formation of magnesium vacancies 3 Mg Mg x ↔ 2 Sc Mg • + V Mg ″ , and that both species are associated Sc Mg • V Mg ″ . At lower Sc concentrations (<500 ppm in the crystal), the concentration-dependence of partitioning indicates that the Sc Mg • V Mg ″ complexes dissociate. Our results demonstrate that partitioning of 1+ and 3+ ions into olivine is complex and involves a range of point defects which yield strongly composition-dependent crystal–melt partition coefficients. Since physical and chemical properties of natural olivine, such as diffusion of 6Li and 7Li and H 2O solubility, depend on the concentrations of the defects identified in this study, our results provide an important insight into how determining substitution mechanisms can improve our understanding of large-scale mantle processes and properties.

Geochemistry and mineralogy of coal in the recently explored Zhundong large coal field in the Junggar basin, Xinjiang province, China

Abstract The Zhundong coalfield is a very large coal deposit, currently under exploration, and promises to be an important coal mining resource in Western China. The coal reserves amount to 164 Gt. At Xiheishan and Wucaiwan, the coal-bearing units are the Badaowan (Lower Jurassic) and, especially, the Xishanyao formations (Middle Jurassic). These contain from one to 31 coal seams (one to 10 workable coals reaching the rank of high volatile A bituminous) with an accumulated workable coal thickness of 20–87 m. These recently explored coal reserves are of a high quality and are characterized by low ash, aluminum, iron, sulfur, and trace element contents. The minerals present in this coal are mainly quartz, kaolinite, siderite, and pyrite. The trace element contents in both Xiheishan and Wucaiwan coals are much lower when compared with the usual concentration ranges present in Chinese coals, with the exception of Ba and Sr. Thus, Ba contents in Xiheishan coal are higher than Swaine's worldwide concentration range. A large number of elements have mainly aluminosilicate affinity, and occur in clay and other detrital coal minerals. Ca and Mn have mainly carbonate affinity; Fe and S have mainly sulphide affinity in Wuchaiwan coal. In Xiheishan, Fe, Mn and Mg have mainly carbonate affinity (siderite), and probably, B, Co, Ni and S have an organic affinity. Thus, the Zhundong coalfield contains coal with a very high quality and very low levels of impurities. The very low ash yields, S, Fe, and trace element contents may be attributed to the sedimentological setting, with intensive peat bog aggradation in a very shallow lake environment with a low detrital supply. The very low sulfur content, the relatively high Ba-sulfate (barite) content and the high siderite occurrence in Xiheishan coal (in contrast to the pyrite occurrence in Wucaiwan coal) may be attributed to the rapid aggradation of peat with the consequent oxidation and leaching of minerals, elements, and degradation of organic components. Under this scenario, sulfate is trapped by the continuous precipitation and accumulation of barium sulfate (very low soluble).

Trace elements and antioxidant enzymes in extremely low birthweight infants

Oxygen radicals are believed to contribute to typical diseases of prematurity, such as bronchopulmonary dysplasia (BPD), intraventricular haemorrhage (IVH), retinopathy of prematurity (ROP) and necrotising enterocolitis (NEC). Our aim was to investigate whether these disorders are associated with disturbances in antioxidant enzyme activities and with low trace elements, which are co-factors of antioxidant enzymes. 209 infants with birthweight less than 1000 g were enrolled into a European multicentre randomised erythropoietin (rhEPO) trial; 155 developed one or more of the above mentioned diseases. We analysed Zn, Cu, Fe, Se in plasma and red blood cells (RBCs), superoxide dismutase (CuZn-SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in RBCs on the 3rd and 68th day of life. Zn, Fe, Se in plasma, and Se in RBCs decreased ( p<0.01), and Zn in RBC ( p<0.001), CuZn-SOD ( p<0.01) and CAT increased ( p<0.05), whereas GSH-Px remained unchanged. No differences were observed between the rhEPO and control groups. Antioxidant enzyme activities did not correlate with gestational age. In infants with BPD, IVH, ROP, or NEC, CuZn-SOD and CAT ( p<0.05) were higher at day 68 than in infants without these diseases. CuZn-SOD and GSH-Px at 3 days and CuZn-SOD at 68 days correlated positively ( p<0.05) with the duration of oxygen treatment. In conclusion, in ELBW infants, trace element concentrations decreased over the first 10 weeks of life. Lower trace element concentrations, did not affect the activities of CuZn-SOD, GSH-Px, and CAT. Typical diseases of prematurity were not associated with decreased antioxidant enzyme activities.

Analysis of major and trace elements by INAA to predict the thousands year old sediment deposition environment in the Meghna river delta

Major, trace and rare earth elements have been measured in sediments of different layers to determine the controls of constituent minerals on the distribution of elements and sediment deposition environment in the Meghna river delta. The geochemical composition of sediments was enriched in SiO2, Ca, Mg and Ba and depleted in Al, Fe, Ti, Mn, and Sr relative to PAAS (Post-Archaen Average Shale) value. The X-ray diffraction and elemental analyses demonstrate the dominant presence of quartz, micas, feldspar, chlorite, amphibole and clay minerals in sediments. The very low contents of trace elements suggest that the oxic condition was more prevalent during sediment deposition of Pleistocene-Holocene period and reflect the massive chemical weathering by biogeochemical reactions. The enrichment of light rare earth elements and La/Yb ratio reflect the intense silicate weathering of crustal materials and the high sediment depositional rate in the Meghna river delta.

Towards a black-box for biological EXAFS data analysis – III. A universal post-processor for fluorescence XAS: KEMP2

Because of the typically rather low trace element concentrations in biological samples XAS is collected in fluorescence mode with the help of a multi-element detector. In addition, several scans have to be accumulated and thus post-processing operations have to be carried out after the measurement, e.g. selecting good detector channels, checking for radiation damage, fitting of background and normalization. KEMP2 is a freely available post-processor implementing all these steps, while maintaining universal applicability by separating the instrument- and beamline-dependent steps like energy-calibration etc. from XAS-generic tasks. Thus, KEMP2 has the potential to serve as a highly automated data pipeline assisting both senior and novice users.

Determination of Trace Elements in Calcite Using Solution and Laser Ablation ICP‐MS: Calibration to NIST SRM Glass and USGS MACS Carbonate, and Application to Real Landfill Calcite

We report new data on the trace element concentrations of Mg, Cr, Mn, Co, Ni, Cu, Zn, Sr, Cd, Ba, La, Ce, Nd, Pb and U in USGS carbonate reference materials (MACS‐1 and MACS‐2) and compare solution ICP‐MS and LA‐ICP‐MS trace element determinations on landfill calcites using calibration to different reference materials (MACS‐1 and MACS‐2 carbonate and NIST SRM 612 glass). Very good agreement (differences below 10% relative) was found between laser ablation and solution ICP‐MS data for MACS‐1 with higher concentrations of trace elements (values between 100 and 150 μg g−1), with the exception of Cu and Zn. Similarly good agreement was found for MACS‐2 with lower trace element concentrations (units to tens of μg g−1), with the exception of Cr, Co and Zn. The MACS‐1 reference material for calibration of LA‐ICP‐MS was found to be extremely useful for in situ determination of trace elements in real‐world carbonate samples (landfill calcites), especially those present in calcite in higher concentrations (Mn, Sr, Ba; &lt; 5% RSD). Less accurate determinations were generally obtained for trace elements present at low concentrations (∼ units of μg g−1). In addition, good agreement was observed between the instrument calibration to MACS and NIST SRM 612 glass for in situ measurements of trace elements in landfill calcites K‐2, K‐3 and K‐4 (differences below 15% relative for most elements). Thus, the application of MACS carbonate reference materials is promising and points to the need for the development of new carbonate reference materials for laser ablation ICP‐MS.

Metamorphic growth and recrystallization of zircon: Distinction by simultaneous in-situ analyses of trace elements, U–Th–Pb and Lu–Hf isotopes in zircons from eclogite-facies rocks in the Sulu orogen

Simultaneous in-situ analyses of trace elements, U–Th–Pb and Lu–Hf isotopes were carried out on distinct domains of zircons in ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks from the main hole of the Chinese Continental Scientific Drilling (CCSD) in the Sulu orogen. For the first time, trace elements are directly linked to Lu–Hf isotopes in metamorphic zircons with reference to their U–Pb dates. This enables methodological integration to distinguish four types of metamorphic zircon: solid-state, replacement and dissolution recrystallizations of protolith zircons, and new growth from the aqueous fluid. Metamorphically grown zircons are characterized by concordant U–Pb ages for the metamorphism, flat HREE patterns typical of the garnet effect, low contents of REE (especially HREE), Y, Nb + Ta and Th + U, high contents of Hf, low (Lu/Gd) N, Lu/Hf and Th/U (< 0.1) ratios, and elevated 176Hf/ 177Hf ratios relative to solid-state recrystallized zircons. This suggests the effects of both garnet and fluid on the growth of metamorphic zircons. In contrast, metamorphic recrystallization has reset the U–Th–Pb isotope system of protolith zircons to different extents, depending on the extents of fluid action during metamorphism. Solid-state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons, which are characterized by discordant U–Pb ages close to or below the protolith age, steep MREE–HREE patterns typical of magmatic origin, high contents of trace elements and their ratios, and low 176Hf/ 177Hf ratios. On the other hand, dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant U–Pb ages for the metamorphism, steep MREE–HREE patterns, lowered contents of trace elements such as REE, Th, U, Y, Nb, Ta and Ti relative to the protolith zircons, and almost unchanged Hf isotope ratios. Replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between. While the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of HREE with flat pattern and the low contents of trace elements, the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of MREE to HREE without the flat HREE pattern. Therefore, the simultaneous in-situ analyses of metamorphic zircons have the advantage over single-term analyses in making distinction between the new growth and the different types of recrystallization.

The Soils of Kiritimati (Christmas) Island, Kiribati, Central Pacific: New Information and Comparison with Previous Studies

Kiritimati, the largest land area atoll in the world, is undergoing rapid population increase, and, given the isolation of the island, local food production will have to be expanded to support the residents. Two soils investigations were completed in the 1960s, but no additional information on the soil resources of the island has been produced since that time. In this study, 15 soil profiles were described and analyzed. Where possible, comparison has been made with previous work, and discussion of the soil-forming factors is presented. Results confirm that soils are weakly developed (Entisols) with relatively low organic matter contents and low water-retention capacity. These properties are expected from the age of the parent materials and the relatively dry climate of the island. Total elemental analyses show that the soils contain very low concentrations of potassium and important trace elements (iron, manganese, copper, and zinc), which will limit any plant production. Classification of the soils identified eight soil families, mainly separated on the basis of content of larger coarse fragments and soil moisture regime, including the influence of groundwater. Comparison with previous studies showed that although different nomenclature and classification systems were used, similar soil patterns were observed, and the soils of Kiritimati are relatively unique in the Pacific islands.

Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high‐grade metamorphism

AbstractU–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and 176Lu/177Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE‐enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean 207Pb/206Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ LuN≥2213; 14.7 ≤ LuN/SmN ≤ 354) and 176Lu/177Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean 207Pb/206Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two‐stage Hf model (TDM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.

Petrography and geochemistry of the Dales Gorge banded iron formation: Paragenetic sequence, source and implications for palaeo-ocean chemistry

Banded iron formations (BIFs) have long been considered marine chemical precipitates or, as more recently proposed, the result of episodic density flows. In this study, we examined the mineralogy, petrography and chemistry of the Dales Gorge BIF to evaluate the validity of these models. Microbands reflect a compositionally variable primary precipitation/sedimentation pattern with diagenetic modifications. “Iron-rich” bands are characterized by massive anhedral aggregates and xenomorphic hematite, commonly showing overgrowths of subhedral magnetite with minor apatite and late diagenetic ankerite-Fe dolomite. “Iron-poor” bands consist of fine-grained quartz, siderite and Fe-talc in variable amounts. Amorphous silica, ferrihydrite (precursor of hematite), greenalite and possibly some siderite constitute the primary precipitates, while ankerite-Fe dolomite, Fe-talc and magnetite and the bulk of siderite are secondary mineral phases. Ankerite and Fe-dolomite most likely represent by-products of siderite dissolution and the subsequent reaction of dissolved bicarbonate with Ca, Mg and Fe. Ferroan-talc is thought to be formed from the following reactions: (i) greenalite + chert, and (ii) siderite + chert. Magnetite formed from the conversion of hematite, likely through bacterial Fe(III) reduction. Geochemical mineral analyses show that all the phases have very low concentrations of trace elements with the exception of Ba, As, Cr, Zn and Sr. This partitioning was presumably controlled by both sorptive reactions occurring during primary precipitation in the water column and secondary remobilization during diagenesis. Whole-rock analyses indicate two decoupled sources for BIF and S macrobands. While BIF macrobands have a major hydrothermal influence, data from the S macrobands supports a dominantly mafic provenance. Nonetheless, when all the lithologies (i.e., source rocks, S and BIF macrobands) are evaluated together, continuous geochemical trends can be observed. This suggests that at least part of the precursor material of BIF macrobands was sourced from the same material that gave origin to the S macrobands. Similar relationships are seen in other BIF successions, where the evolution from an ultamafic- to a mafic-dominated upper continental crust is distinctly reflected in BIF compositions through time. Interpretation of this data implies that any model developed to explain BIF deposition must consider: (i) processes involving low-temperature weathering of the continental and ocean basement rocks, mainly (ultra)mafic lithologies; and (ii) high temperature water–rock reactions associated with hydrothermal activity at spreading ridge centers or seamounts. In either case, the influence of the compositional change of the upper continental crust played a major role in the chemical compositions of BIFs through time.

Mineralogical and geochemical characterization of the Jurassic coal from Egypt

The Jurassic coal deposit in the Maghara area, Sinai, Egypt contains at least 11 coal seams of lenticular shape. The thickness of the main coal seams ranges from 130 cm to 2 m and are underlain and overlain by thin black shale beds. Mineralogical analysis indicated that this coal is characterized by low mineral matter with traces of quartz in some samples. However, coal ash is made up of quartz with traces of calcite, anhydrite, and hematite. Analysis of coal rank parameters indicated that the Maghara coal can be classified as medium volatile bituminous coal. The high sulfur contents and the relatively high proportion of pyritic sulfur suggest a possible marine transgression after the deposition of precursor peat. This interpretation is supported by the relatively high B contents. The relatively high Ge in the Maghara coal could be attributed to an infiltration of Ge enriched water from the surrounding siliceous sediments probably during diagenesis. The high Au contents were contributed to an Au-rich provenance of the ash contents of this coal. Rare earth elements geochemistry indicated low concentrations of these elements with slight enrichment of light rare earth elements (LREEs), slight negative Eu anomaly, and relatively flat heavy rare earth elements (HREEs) patterns. The low contents of trace and rare earth elements, particularly those with environmental relevance, compared to the usual concentration ranges in worldwide coal gives an advantage for this coal.

Trace element concentration in wheat grain: results from the Swedish long-term soil fertility experiments and national monitoring program

Concentrations of trace elements in wheat grain sampled between 1967 and 2003 from the Swedish long-term soil fertility experiments were analyzed using ICP-MS. The long-term effect of inorganic and organic fertilization on trace metal concentrations was investigated including the impact of atmospheric deposition and myccorhiza, whereas other factors such as soil conditions, crop cultivar, etc. are not discussed in this paper. Mean values derived from 10 experimental sites were reported. Significantly declining Pb and Cd concentrations in wheat grain could be explained by lower atmospheric deposition. Mean Se contents in all samples were 0.031 mg kg(-1) grain dry weight. No samples had sufficiently high Se concentrations for human (0.05 mg Se kg(-1)) or animal demand (0.1 mg Se kg(-1)). Concentrations of Co in wheat grain were extremely low, 0.002-0.005 mg Co kg(-1) grain dry weight, and far below the minimum levels required by animals, which applied to all fertilizer treatments. A doubling of Mo concentrations in grain since 1975 resulted in Cu/Mo ratios often below one, which may cause molybdenosis in ruminants. The increase in Mo concentrations in crops correlated with the decline in sulfur deposition. Concentrations of Cu and Fe declined in NPK-fertilized wheat as compared to unfertilized or manure-treated wheat. Very low concentrations of Se and Co and low concentrations of Fe and Cu require attention to counteract risks for deficiencies. The main characteristic of the study is that there are few significant changes over time between different fertilizer treatments, but throughout there are low concentrations of most trace elements in all treatments. In general, good agreement between concentrations in wheat from the long-term fertility experiments and the national monitoring program indicate that values are representative.

Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy)

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO2-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO2-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge.

Geochemistry of a long in-situ section of intrusive slow-spread oceanic lithosphere: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)

IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30°N on the Mid-Atlantic Ridge (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on 228 samples representative of the different lithologies sampled at this location. Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the section; they have depleted MORB compositions similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt–rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (> 70% modal olivine) and troctolites having high Mg# (82–89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb 0.06–0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60–86 and low trace element contents (Yb 0.125–2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (< 50), low Ni (∼ 65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere. Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30°N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere.

An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.

Cosmic-Ray Exposure Ages of Large Presolar SiC Grains

AbstractAmong presolar SiC grains found in the Murchison carbonaceous meteorites (average size less than 0.5 μm) are very large grains, ranging in size up to 50 μm. We interpret 6Li excesses measured in eight of these grains as being the result of spallation reactions by Galactic cosmic rays during the time the grains spent in the interstellar medium before their incorporation into the meteorite. Derived interstellar exposure ages range from 40 My to 1 Gy, the highest values being consistent with theoretical expectations of interstellar grain lifetimes. Although six grains have almost identical C and Si isotopic compositions, their exposure ages are very different. This observation, combined with low trace element contents, and unusual grain sizes, raises fundamental questions about their stellar sources.

Geochemistry and genesis of Fe-Mn mineralization in island arcs in the west Pacific Ocean

This paper presents materials on the chemical and mineralogical composition of Fe-Mn mineralization in island arcs (Kurile, Nampo, Mariana, New Britain, New Hebrides, and Kermadec) in the western part of the Pacific Ocean. The mineralization was proved to be of hydrothermal and/or hydrogenic genesis. The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments. Some Fe oxi-hydroxides can be derived via the halmyrolysis of volcaniclastic material. Crusts of this stage are characterized by fairly low concentrations of trace and rare elements, and their REE composition is inherited from the volcanic-terrigenous material. The minerals of the Mn oxi-hydroxides are todorokite and “Ca-birnessite.” The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions. The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group (Co, Ni, Tl, and Mo) and the Fe group (REE, Y, and Th). The hydrogenic crusts consist of Fe-vernadite and Mn-feroxyhyte. Some of the hydrothermal crusts originally had a hydrothermal genesis. The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc, which contained anomalously high concentrations of Co (2587 ppm) and other Mn-related trace elements in the absence of hydrogeneous Fe oxi-hydroxides.

The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling

Trachyte and peralkaline rhyolite (pantellerite and comendite) frequently comprise the felsic end-member in bimodal suites in continental rift and oceanic island settings. In these settings, the relationship between the mafic (mildly alkaline, or transitional, basalt) and felsic lavas is ambiguous; major- and trace-element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization. In this paper, we present representative mineral analyses and whole-rock analyses from forty samples of a basalt–trachyte–pantellerite suite collected at Pantelleria, Italy, in the Strait of Sicily Rift Zone, and compare the results of major- and trace-element modelling with the results of thermodynamic (MELTS) modelling. From these results we conclude that metaluminous trachyte formed as a result of 70 to 75% low-pressure (0.1 GPa) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, magnetite, and apatite from a hydrous (1.0–1.5 wt.% H 2O) transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite–magnetite–quartz buffer (FMQ−1). The “Daly gap”–a lack of intermediate (~ 49–62 wt.% SiO 2) volcanic rocks–at Pantelleria is concluded to be primarily the result of rapid differentiation through that interval. Relatively rapid crystallization at low pressure may have effected the partial degassing of water-saturated (~ 4 wt.% H 2O) metaluminous trachyte magma. Some metaluminous trachyte lavas have positive Eu anomalies, high K/Rb ratios, high concentrations of Ba, and low concentrations of incompatible trace elements; these are interpreted to be the result of up to 40% accumulation of alkali feldspar. Comenditic trachyte, pantelleritic trachyte, and pantellerite formed after an additional 20 to 80% fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma. The most evolved pantellerite lavas and tuffs are the result of a total of ~ 95% fractional crystallization of transitional basalt, with phenocrysts that equilibrated at low temperatures (< 700 °C), low oxygen fugacities, and high (> 4 wt.%) water contents.