Parental Silicate Research Articles

Zircon and Whole-Rock Trace Element Indicators of Magmatic Hydration State and Oxidation State Discriminate Copper Ore-Forming from Barren Arc Magmas

Abstract To meet surging requirements of copper for the green energy revolution, minable resources subequal to all copper production in history must be found in the next two decades. We show that trace elements in zircon and whole-rock samples that are diagnostic of unusually high-pressure magmatic differentiation and high hydration state and oxidation state of their parent silicate melt are effective for discriminating copper sulfide-ore-productive arc magmas from infertile arc magmas. Tests on our database of 5,777 zircons from 80 igneous complexes, including 2,220 zircons from ore-generative intrusions in 37 major porphyry and high-sulfidation epithermal Cu(-Au-Mo) deposits worldwide, demonstrate that our magmatic copper fertility discriminants apparently perform equally well in intraoceanic arcs, continental margin arcs, and continental collision orogens of Ordovician to Quaternary age. That performance consistency means that the tectono-magmatic controls on development of magmatic-hydrothermal copper ore-forming fertility are essentially the same in all those plate-convergence settings. The ratio Ce/√(U × Ti) in zircon is a quantitative indicator of the relative oxygen fugacity of the silicate melt and its sulfur-carrying capacity. The ratio of the europium anomaly to ytterbium in granitoid melts and zircon is an uncalibrated but empirically useful indicator of the melt’s hydration state and ability to provide chloride-complexed metals to exsolving hydrothermal fluids. Plots of (EuN/Eu*)/YbN vs. Ce/√(U × Ti) in zircon are remarkably effective for discriminating igneous complexes, arc segments, and time intervals within them that can generate and are likely to host magmatic-hydrothermal Cu(-Au-Mo) ore deposits. Arrays of cognate zircons on such plots have slopes that vary with pressure-dependent chlorinity of exsolving fluid and its efficacy in scavenging CuCl from the melt. Our zircon indicators of Cu metallogenic fertility are applicable to detrital as well as in situ zircons and can assist with ore discovery in watersheds upstream from a sediment sampling site. We formulated a composite zircon copper fertility index (ZCFI) that can be applied to each microbeam spot analysis—ZCFI = 104 (EuN/EuN*)/YbN + 5 Ce/√(Ui × Ti)—and substantially decreases the number of zircon analyses needed for reliable prospectivity assessment in a detrital grain population, thereby making this watershed-scale exploration tool cost-competitive with other methods of geochemical exploration.

GEMAS: Chemical weathering of silicate parent materials revealed by agricultural soil of Europe

The chemical composition of soil represents to a large extent the primary mineralogy and geochemistry of the source bedrock, the effects of pre- and post-depositional weathering and element mobility, either by leaching or mineral sorting with the addition of secondary products such as clays. Agricultural soil in Europe (0–20 cm, 33 countries, 5.6 million km2) was sampled during the continental-scale soil mapping project (GEochemical Mapping of Agricultural and grazing land Soil - GEMAS). Total element concentrations were determined by wavelength dispersive X-ray fluorescence spectrometry. Bulk content of major elements (SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, P2O5) in soil has been used to calculate weathering indices such as chemical index of alteration (CIA), reductive and oxidative mafic indices of alteration (MIA), the change in mass balance (τ calculation relative to immobile Nb) for soil derived from silicate bedrocks (granite, gneiss and schist) at the European continental-scale, which are useful tools for evaluating chemical weathering trends.The greatest variation of CIA values is exhibited by soil derived from granite, followed by soil derived from schist. The CIA results show clearly that Ca and Na are removed from agricultural soil (0–20 cm) during soil development. Geographical distribution of weathering indices shows that weak chemical weathering of agricultural soil samples, sourced from gneissic and granitic bedrock, occurs in the Fennoscandian Shield; intermediate chemical weathering in granite, schist and gneiss occurring in the Iberian Peninsula and France, and intense chemical weathering in schist sourced agricultural soil occurring in areas with intense rainfall and moderate temperature (Wales, England, Ireland and western Iberia). In addition, values of CIA and MIA, higher than those for standard rock compositions, suggest more active enrichment/depletion processes during soil formation and subsequent soil weathering/maturation.The range of the elemental mass transfer coefficient τ is wide for agricultural soil derived from gneiss and granite, and narrow for agricultural soil sourced from schist parent rocks. The weathering behaviour of chemical elements, assessed by using the τ mass transport model, suggests an order of susceptibility of K ≈ Na > Al > Fe > Ca > Mg > Ti.Weathering indices and gain-loss mass transfer coefficients, tested on agricultural soil samples, provide an insight to the weathering processes affecting the silicate parent rocks and their impact on soil development at the European scale. This can have further implications for evaluation of soil nutritional conditions and mitigating of soil erosion and deprivation.

The Origin of Ultramafic Complexes with Melilitolites and Carbonatites: A Petrological Comparison of the Gardiner (E Greenland) and Kovdor (Russia) Intrusions

Abstract In many alkaline complexes, large amounts of ultramafic rocks occur together with carbonatites, melilitolites and other alkaline silicate rocks. There is an ongoing debate if and how these contrasting lithologies were formed by differentiation of a common, mantle-derived silicate magma or rather by metasomatic processes between carbonatite and country rocks. In order to find petrological evidence for one or the other, two key examples, the Gardiner (E Greenland) and Kovdor (Russia) complexes are compared in this study. Despite their similar tectonic setting and succession of rock types, they show significant differences in the texture and mineral composition of ultramafic rocks. Ultramafic rocks from Kovdor include calcite- and biotite-rich dunites and pyroxenites without typical cumulate textures. They consist of Ni-poor forsterite, Cr-poor diopside and Ni-Cr-poor spinel and are possibly metasomatic reaction products between mantle-derived carbonatite melts and silicic host rocks. Similar ultramafic rocks are associated with carbonatites e. g. at Palabora (South Africa), Afrikanda (Russia), and Salitre (Brazil). In contrast, the ultramafic rocks from Gardiner show well-preserved cumulate textures and consist of Ni-rich forsterite, Cr-rich diopside as well as Cr-Ni-Ti-rich spinel and also contain F-Cl-rich apatite. They record an increase in aSiO2 from dunite to pyroxenite at similar fO2 (ΔFMQ ~ +1.2, with FMQ = fayalite-magnetite-quartz buffer), indicating that these rocks represent cumulates of an evolving, moderately oxidized mafic melt derived from a Ti-rich mantle source, similar to other rocks of the North Atlantic igneous province. In contrast to systems like Kovdor where carbonatite metasomatism is likely dominant, Ti-rich parental silicate magmas can abundantly crystallize Ti phases, as recorded by massive perovskite cumulates in Gardiner melilitolites. This can effectively scavenge HFSE from the magmatic system early in its evolution and likely explains HFSE-barren carbonatites at Gardiner, while those from Kovdor are highly HFSE-enriched. In summary, the results of our study provide strong textural and mineral chemical evidence that ultramafic rocks in alkaline complexes can be of both cumulate and metasomatic origin; the specific type has an important bearing on their HFSE enrichment and on the types of ores present in such complexes.

Genesis of the Kiruna-type Nixintage iron deposit, Chinese Western Tianshan, NW China: Constrains of ore geology, geochemistry and geochronology

The Nixintage iron deposit is located in the central segment of the Awulale metallogenic belt, Chinese Western Tianshan. Iron ores are mainly hosted by andesitic tuff and andesite of the Carboniferous Dahalajunshan Formation. The paragenetic sequence includes a magmatic magnetite-apatite stage (I); a magmatic–hydrothermal stage (II), and three substages can be divided, including magnetite-albite substage (II-1), magnetite-chlorite-pyrite substage (II-2), magnetite-quartz-calcite substage (II-3); and a late hydrothermal quartz-calcite-sulfides stage (III). Anhedral, fine-grained magnetite associated with apatite in brecciated and matrix ores formed in stage I with relatively variable compositions, most probably indicate the fast crystallization under a disequilibrium condition. The high-grade magnetite orebodies mainly formed in the stage II, are featured by widespread hydrothermal mineral assemblages associated with the ores. Magnetite formed in this stage has medium-to-high concentrations of V, low-to-medium concentrations of Ti, Cr, relative depletion of Mg, Ni, K, and these data mainly plot in the Kiruna and porphyry fields of discriminant diagrams that comparable to magnetite from typical Kiruna-type iron deposits. Both stages I and II magnetites show similar wide δ18O ranges and with peak values at 6.0–10.0 ‰, heavier than typical Kiruna-type deposits, while could be attributed to the relatively low crystallization temperature inferred by microthermometric data of a previous study. The δ34S values of pyrite formed in different stages range between 0.1 and 2.4 ‰, comparable to the sulfur isotopic composition of the mantle-derived magma. These features indicate that stage I iron mineralization was generated by the extrusion of iron phosphorous oxide melts separated from the parental silicate magma. Subsequent deuteric fluids exsolved from basaltic to andesitic melts during the late stage of volcanism, generated magmatic–hydrothermal iron mineralization in stage II, and the continuous evolution of the deuteric fluids formed stage III hydrothermal veins. In summary, integrated evidence suggests the Nixintage is a Kiruna-type iron deposit.Zircon U-Pb ages of ore-hosting andesite and andesitic tuff are 317.6–314.4 Ma, and timing for iron mineralization was inferred at the same time, i.e., ca. 315 Ma. Previously published petro-geochemical data combined with our geochronological data of these rocks, suggest the ore-related volcanism in Nixintage probably originated from the partial melting of the mantle wedge modified by subducted fluids under a Late Carboniferous continental arc setting. Consequently, a genetic model for the studied deposit could be created. Furthermore, the southeast of the Nixintage nearer to the vent might have potential for prospecting large Kiruna-type iron deposits.

The Role of Soil Type in Triggering Shallow Landslides in the Alps (Lombardy, Northern Italy)

Shallow landslides due to the soil saturation induced by intense rainfall events are very common in northern Italy, particularly in the Alps and Prealps. They are usually triggered during heavy rainstorms, causing severe damage to property, and sometimes causing casualties. A historical study and analysis of shallow landslides and mud-debris flows triggered by rainfall events in Lombardy was carried out for the period of 1911–2010, over an area of 14,019 km2. In this study, intensity–duration rainfall thresholds have been defined using the frequentist approach, considering some pedological characteristics available in regional soil-related databases, such as the soil region, the textural class, and the dominant soil typological units (STU). The soil-based empirical rainfall thresholds obtained considering the soil regions of the study area were significantly different, with a lower threshold for landslide occurrence in the soil region M1 (Alps), where soils developed over siliceous parent material, with respect to the whole study area and the soil region M2 (Prealps), where soils developed over calcareous bedrocks. Furthermore, by considering textural classes, the curves were differentiated, with coarse-textured soils found more likely to triggerlandslides than fine soils. Finally, considering both texture and main soil groups, given the same rainfall duration, the rainfall amount and intensity needed to initiate a landslide increased in the following order: “coarse-skeletal” Cambisols < Umbrisols < Podzols < “fine” Cambisols. The results of this study highlighted the relevant role of pedological conditioning factors in differentiating the activation of rainfall-induced shallow landslides in a definite region. The information on soils can be used to define more precise rainfall–pedological thresholds than empirical thresholds based solely on meteorological conditions, even when they are locally defined. This knowledge is crucial for forecasting and preventing geo-hydrological processes and in developing better warning strategies to mitigate risks and to reduce socio-economic damage.

Crystal-Liquid Segregation in Silicocarbonatite Magma Leads to the Formation of Calcite Carbonatite

Abstract A suite of silicocarbonatite and lamprophyre rocks from SW Ireland, with mantle affinity and primitive composition, are used as a proxy for parental carbonated silicate magmas to model early magmatic evolution. Reconstruction of volatile ratios is validated using global occurrences. At 1200°C, the point at which melts transition from ionic liquids with exceptionally low viscosity (0.06 PaS) to covalently polymerised liquid (viscosity up to 1.3 PaS) is 33 mol% SiO2. Incremental and significant increase in magma density accompanies magma ponding, due to dehydration of magmas from model molar CO2/(CO2 + H2O) of 0.60 in plutonic settings to 0.75 for initial subvolcanic magmas. Magma-crystal density differences dictate that repeated influxes of magmas into an inflating magma chamber sustain a mechanical boundary layer between dense (silicate and oxide) mineral layers and a calcite ± phlogopite flotation assemblage. The range of critical CO2 concentration at which calcite floats (10–13 wt% CO2) may be extended by the presence of additional volatiles and fluid bubbles. The model accommodates a range of phenomena observed or inferred for alkaline/carbonatite complexes, including the following: 1, a growing calcite-dominated flotation assemblage with an apparently early magmatic mineralisation; 2, a residual liquid with high concentrations of incompatible metals; 3, variable carbonatite–pyroxenite–phoscorite rock relations; and 4, multiple phases of overprinting metasomatism.

The Calatrava paradox to decipher the origin of carbonatites: A petrological insight on Finca La Nava, Calatrava Province (central Spain)

The paper presents a detailed petrographic and geochemical study of the pyroclastites outcropping at La Nava maar (Calatrava Volcanic Province-CVP). Here, extrusive carbonatites mix with melilite nephelinite lapilli and bombs, crustal and mantle debris. Nephelinite and carbonatite are not characterised by a HFSE distribution typical of immiscible pairs. ΣREE and La/Lu ratios reflect primitive mantle carbonates and kimberlites. Igneous carbonates have δ13C and δ18O isotopic ratios in the range of extrusive carbonatites and differ from local sedimentary carbonates. Carbonatites have 87Sr/86Sr and 144Nd/143Nd isotopic ratios similar to Calatrava leucitites but different to melilite nephelinite. The difference indicates that melilite nephelinites and carbonatites are co-eruptive but not comagmatic. The mingled magmatic convoy, underwent decarbonation processes, releasing Ca and Mg that reacted with parental silicate melt. Consequently, the nephelinitic melt evolved towards a melilititic composition producing Ca-rich overgrowths on mafic phenocrysts and reacted to produce high Mg#, low Cr + Ni forsterite. Mass balance calculations indicats that the nephelinite magma assimilated up to 30% of the carbonatite magma. Mixing modelling based on the Sr and Nd isotopic composition of mantle xenoliths and carbonatites indicate that La Nava carbonatite may represent the mantle metasomatic agent.

Relationships between vegetation of Macedonian pine (Pinus peuce Griseb.) and different types of soils on which it develops

Abstract This paper deals with relationships between vegetation of Macedonian pine (Pinus peuce) and soils developed on different parent materials on the territory of North Macedonia. We analysed the floristic composition at localities on limestone, on scree of dolomite marble and on scree of silicate. On limestone and scree of dolomite marble, rendzinas on hard limestone and dolomite have developed, and on silicate parent material brown forest soils. The vegetation was sampled according to the Braun-Blanquet approach. DCA and indicator values were used for ecological interpretation of the vegetation patterns. The mechanical and chemical properties of soil and textural classes were also processed. An evident increased presence of carbonates in the soil of scree of dolomite marble on Nidže Mountain was observed, unlike that on Shar Mountain which has formed on typical limestone. Although it is a forest community dominated by the same species, differences between the massifs, the precipitation regime, geology, differences in soil properties in relation to the appearance of carbonates and pH values, and other factors, result in differences in their floristic composition and are the reason for the distinction between the two groups. On silicate on Nidže Mountain, Macedonian pine forests have also developed on brown forest soils, with a different floristic composition to that of the other group on carbonate (dolomite and limestone).

Soil phosphorus status and P nutrition strategies of European beech forests on carbonate compared to silicate parent material

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry,https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENIP) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition.

Rubidium measured by XRF as a predictor of soil particle size in limestone and siliceous parent materials

PurposeInformation about particle size distribution (PSD) and soil texture is essential for understanding soil drainage, porosity, nutrient availability, and trafficability. The sieve-pipette/gravimetric method traditionally used for particle size analysis is labour-intensive and resource-intensive. X-ray fluorescence (XRF) spectrometry may provide a rapid alternative. The study’s aim was to examine the use of XRF for rapid determination of PSD in Irish soils.MethodsSoils (n = 355) from existing archives in Ireland were analysed with a benchtop energy-dispersive XRF (EDXRF). Correlation and regression analyses were determined to compare Rb, Fe, Al, and Si concentrations to % clay, % silt, and % sand. Also, linear regression models were developed to compare % clay, % sand, and % silt measured by the gravimetric method to values predicted by EDXRF.ResultsThe relationship between element concentration and PSD was dependent on parent material. Rb, Al, and Fe showed a significant (p < 0.05) correlation (r > 0.50) with % clay and % sand in soils derived from limestone and siliceous stone parent materials. Rb was the best predictor for % clay (R2 = 0.49, RMSE = 10.20) in soils derived from limestone and siliceous stone-derived soils.ConclusionGeochemistry and clay mineralogy of the soils’ parent material strongly influenced the EDXRF’s ability to predict particle size. The EDXRF could predict % clay in soils from parent materials which weather easily, but the opposite was true for soils with parent material recalcitrant to weathering. In conclusion, this study has shown that the EDXRF can screen % clay in soils derived from limestone and siliceous stone parent materials.

Lunar feldspathic meteorite Dhofar 081: Petrochemical constraints on petrogenesis

Two alternative petrogenetic models, plagioclase flotation and serial magmatism have been proposed to explain the origin of the lunar anorthositic crust, covering ~80% of the lunar highland. In this study, we re-examine the possible relict igneous texture present in an inferred lunar highland breccia clast (area ~1 mm2) in the Dhofar 081 meteorite. Our new petrographic and in-situ mineral microprobe chemical data on this clast show this coarse grained (average grain size ~0.5 mm) clast preserves relict igneous texture where subhedral, prismatic low-Ca pyroxene has intergrown with anhedral anorthitic plagioclase, suggesting its eutectic crystallization from its parent silicate magma. Absence of maskelynite and similarity of Na, K contents of plagioclase with the FAN assemblages negate the possibility of crystallization of the studied relict clast from an impact melt. The mineral-chemical data of Dhofar 081 suggest it is FAN (Ferroan anorthosite) in composition (after Warren in Annu. Rev. Earth Planet. Sci. 13:201–240, 1985). Hence, intergrown crystallization of minerals in the present relict igneous clasts and other reported FAN samples argues against a cumulate origin of the lunar anorthosite. The orthopyroxenes present in the unbrecciated portion of this meteoritic clast include bimodal low- and high-iron geochemical sub-groups. The application of orthopyroxene and plagioclase thermobarometry (after Gasparik in Contrib. Mineral. Petrol. 96:357–370, 1987) on our new microprobe data, and also two-pyroxene thermometry (after Lindsley in Am. Mineral. 68:477–493, 1983; Putirka in Rev. Mineral. Geochem. 69(1):61–120, 2008) on our new microprobe data and synthesis of literature data constrain the pressure and temperature of crystallization of lunar anorthosite parent magma close to 8 kbar and 1050°C, respectively. Application of Fo–An–Q experimental phase diagram at high pressure (up to 20 kbar) negates the possibility of generation of lunar anorthosite from a lherzolite source, the parent magma of these anorthosites probably lie on or close to Fo–An join of this phase diagram close to the spinel field.

Barium stable isotopes as a fingerprint of biological cycling in the Amazon River basin

Abstract. The biological cycle of rock-derived nutrients on the continents is a major component of element transfer between the Earth's surface compartments, but its magnitude currently remains elusive. The use of the stable isotope composition of rock-derived nutrients, which can be fractionated during biological uptake, provides a promising path forward with respect to quantifying biological cycling and its overall contribution to global element cycling. In this paper, we rely on the nutrient-like behaviour of the trace element barium (Ba) and use its elemental and stable isotope compositions in dissolved and sediment load river samples to investigate biological cycling in the Amazon Basin. From these measurements, we show that dissolved Ba mainly derives from silicate rocks, and a correlation between dissolved Ba and K abundances suggests that biological cycling plays a role in the Ba river budget. Furthermore, the isotope composition of Ba (δ138Ba) in the dissolved load was found to be significantly different from that of the parent silicate rocks, implying that dissolved Ba isotopic signatures are affected by (i) the precipitation of soil-forming secondary phases as well as (ii) biological uptake and release from dead organic matter. Results from an isotope mass balance method applied to the river dissolved load data indicate that, after its release to solution by rock weathering, Ba is partitioned between the river dissolved load, secondary weathering products (such as those found in soils and river sediments), and the biota. In most sub-catchments of the Amazon, river Ba abundances and isotope compositions are significantly affected by biological cycling. Relationships between estimates of Ba cycled through biota and independent metrics of ecosystem dynamics (such as gross primary production and terrestrial ecosystem respiration) allow us to discuss the role of environmental parameters such as climate or erosion rates on the biological cycling of Ba and, by extension, the role of major rock-derived nutrients. In addition, catchment-scale mass and isotope budgets of Ba show that the measured riverine export of Ba is lower than the estimated delivery of Ba to the Earth surface through rock alteration. This indicates the existence of a missing Ba component, which we attribute to the formation of Ba-bearing particulate organics (possibly accumulating as soil organic matter or currently growing biomass within the catchments) and to organic-bound Ba exported as “unsampled” river particulate organic matter. Given our findings on the trace element Ba, we explore whether the river fluxes of most major rock-derived nutrients (K, Mg, Ca) might also be significantly affected by biological uptake or release. A first-order correction of river-derived silicate weathering fluxes from biological cycling shows that the carbon dioxide (CO2) consumption by silicate weathering at the mouth of the Amazon could be several times higher than the previously reported value of 13 × 109 mol CO2 yr−1 (Gaillardet et al., 1997). Overall, our study clearly shows that the chemical and isotope compositions of rivers in the Amazon – and most likely in other large river basins – bear a biological imprint, thereby challenging common assumptions made in weathering studies.

The Nature and Composition of the J-M Reef, Stillwater Complex, Montana, USA

Abstract In this contribution, we analyze 30 years of mine development data and quantitatively identify the processes that control the grade and tenor of the mineralized rock. An assay database of more than 60,000 samples was used to examine variations in ore grade and tenor of the sulfide mineralization in the J-M reef horizon of the Stillwater Complex along the strike and down the dip of the deposit in the area of the Stillwater mine. We compare these results with data from the East Boulder mine and whole-rock lithogeochemistry of samples collected along the entire strike length of the complex. We find significant variation in the composition of the reef sulfides in different spatial domains of the Stillwater mine area and between the Stillwater and East Boulder mines. Most of the variation in the grade and tenor can be explained by a variation in the mass of silicate magma with which the sulfide liquid equilibrated (i.e., R factor); however, geochemical and textural evidence suggests that parts of the reef may have experienced significant S loss following initial sulfide melt segregation. Some variability in the reef tenor and grade can be attributed to variable amounts of sulfur loss due to low-temperature hydrothermal fluids and the overestimation or underestimation of metal concentrations in reef assays due to the nugget effect. Furthermore, we address the Pd/Pt ratio of the reef samples and suggest that the lower solubility of Pt in the parental silicate melt may have caused the crystallization and removal of Pt alloys at some point before the melt reached sulfide saturation and Pt could partition into the sulfide liquid. This disparity between the prior evolution of Pt and Pd in the silicate melt resulted in the observed Pd/Pt ratio of ~3.65 across all areas of the reef—a value significantly larger than anticipated for primitive mantle-derived magmas.

Magnetite Chemistry by LA-ICP-MS Records Sulfide Fractional Crystallization in Massive Nickel-Copper-Platinum Group Element Ores from the Norilsk-Talnakh Mining District (Siberia, Russia): Implications for Trace Element Partitioning into Magnetite

Abstract Mineralogical and chemical zonations observed in massive sulfide ores from Ni-Cu-platinum group element (PGE) deposits are commonly ascribed to the fractional crystallization of monosulfide solid solution (MSS) and intermediate solid solution (ISS) from sulfide liquid. Recent studies of classic examples of zoned orebodies at Sudbury and Voisey’s Bay (Canada) demonstrated that the chemistry of magnetite crystallized from sulfide liquid was varying in response to sulfide fractional crystallization. Other classic examples of zoned Ni-Cu-PGE sulfide deposits occur in the Norilsk-Talnakh mining district (Russia), yet magnetite in these orebodies has received little attention. In this contribution, we document the chemistry of magnetite in samples from Norilsk-Talnakh, spanning the classic range of sulfide composition, from Cu poor (MSS) to Cu rich (ISS). Based on textural features and mineral associations, four types of magnetite with distinct chemical composition are identified: (1) MSS magnetite, (2) ISS magnetite, (3) reactional magnetite (at the sulfide-silicate interface), and (4) hydrothermal magnetite (resulting from sulfide-fluid interaction). Compositional variability in lithophile and chalcophile elements records sulfide fractional crystallization across MSS and ISS magnetites and sulfide interaction with silicate minerals (reactional magnetite) and fluids (hydrothermal magnetite). Estimated partition coefficients for magnetite in sulfide systems are unlike those in silicate systems. In sulfide systems, all lithophile elements are compatible and chalcophile elements tend to be incompatible with magnetite, but in silicate systems some lithophile elements are incompatible and chalcophile elements are compatible with magnetite. Finally, comparison with magnetite data from other Ni-Cu-PGE sulfide deposits pinpoints that the nature of parental silicate magma, degree of sulfide evolution, cocrystallizing phases, and alteration conditions influence magnetite composition.

The types, ore genesis and resource perspective of endogenic REE deposits in China

China hosts a large proportion of global REE resources. This paper presents a preliminary review of the types and ore genesis of endogenic REE deposits in China. Three major types of REE deposits are identified, namely carbonatite-type, alkaline and alkali granite-type and hydrothermal-type deposits. The carbonatite-type deposits mainly include Bayan Obo in northern China, Maoniuping in southwestern China, Weishan in eastern China, and Miaoya in Central China, all of which are genetically related to carbonatite/carbonatite-alkaline complex. This type of deposit mainly contains economic light REE (LREE), which are mainly hosted in fluoro-carbonate and/or phosphate. Apart from LREE, some deposits may also show obvious enrichments in heavy REE (HREE), Nb, Th, and Sc. The above-mentioned carbonatite-type REE deposits were formed in different periods, including Meso-Proterozoic (Bayan Obo), Paleozoic (Miaoya), Mesozoic (Weishan) and Cenozoic (Maoniuping). They were generated in intra-continental extension (Bayan Obo, Miaoya, and Weishan) or post-collisional (Maoniuping) setting. The ore-related intrusions were mainly derived from very low degrees partial melting of the sub-continental lithosphere mantle (SCLM). Before the formation of carbonatite, the SCLM had been metasomatized by recycled crustal components, which elevated the contents of REE and other components (e.g., Nb, Ba, F, and P). The parental carbonatitic or CO2-rich silicate magmas have experienced intensive fractional crystallization or immiscibility of carbonate- and silicate-rich melts, which further improve the contents of REE in the later-stage magmas. A subset of REE minerals was crystallized in the magmatic evolution process, while a large proportion of REE were retained in the residual melts. Furthermore, the REE were transported out of the magma chamber by volatile-rich fluids (e.g., F−, SO42− and Cl−), and were deposited adjacent to the carbonatitic intrusions. The alkaline and alkali granite-type deposits mainly include Baerzhe and Saima in northeastern China, which have genetic associations with SiO2-saturated or -unsaturated alkaline intrusions, respectively. Compared with the carbonatite-type deposit, this type of deposits contains relatively high contents of HREE, which are mainly hosted in phosphate or silicate. Notably, this type of deposits may also contain economic Zr, Nb, and Ta. The above-mentioned deposits were formed in the Mesozoic under an intra-continental extensional setting. Similar to the carbonatite, the ore-related alkaline intrusions were derived from the sub-continental lithosphere mantle (SCLM), which had been metasomatized by REE- and volatile-bearing components. The parental silicate magmas have experienced intensive fractional crystallization during the evolution process. Many REE-, Nb-, and Zr-bearing minerals were crystallized in the later magmatic process. These minerals were later modified by magmatic-hydrothermal fluids, which lead to remobilization and further enrichment of REE, Zr and Nb. A few IOCG deposits also show obvious enrichment in LREE, which are represented by the Paleoproterozoic Yinachang and Lala deposits in southwestern China. Diverse minerals occur as REE hosts in this type of deposits, mainly including fluoro-carbonate, phosphate, and silicate. No spatially associated magmatic intrusions can be identified in the IOCG-type deposits, but available data indicate that REE enrichment was genetically related to some hidden magmatic intrusions. The intrusions were generated in a rift-setting and vary from mafic to felsic in composition. In this type of deposit, REEs were initially deposited in apatite and allanite. In the later stages, apatite and allanite were modified by multiples phases of fluids. During the fluid/mineral interaction processes, REEs were mobilized and reprecipitated in other REE minerals, including monazite, bastnaesite, xenotime, synchysite. Multiple stages of fluid/mineral interaction are important for the final enrichment of REE in this type of deposit. In the future, more studies are needed to accurately evaluate the resources of REE in China, to target more deposits that are relatively enriched in medium-heavy REE, and to provide guidance for the effective utilization of accompanied metals in the REE deposits.

Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russia)

Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide immiscibility in the crust without requiring magma contamination and/or extensive fractionation. However, the records of incipient sulfide melts in equilibrium with primitive high-Mg olivine and Cr-spinel are scarce. Sulfide globules in olivine phenocrysts in picritic rocks of MORB-affinity at Kamchatsky Mys (Eastern Kamchatka, Russia) represent a well-documented example of natural immiscibility in primitive oceanic magmas. Our study examines the conditions of silicate-sulfide immiscibility in these magmas by reporting high precision data on the compositions of Cr-spinel and silicate melt inclusions, hosted in Mg-rich olivine (86.9–90 mol% Fo), which also contain globules of magmatic sulfide melt. Major and trace element contents of reconstructed parental silicate melts, redox conditions (ΔQFM = +0.1 ± 0.16 (1σ) log. units) and crystallization temperature (1200–1285 °C), as well as mantle potential temperatures (~1350 °C), correspond to typical MORB values. We show that nearly 50% of sulfur could be captured in daughter sulfide globules even in reheated melt inclusions, which could lead to a significant underestimation of sulfur content in reconstructed silicate melts. The saturation of these melts in sulfur appears to be unrelated to the effects of melt crystallization and crustal assimilation, so we discuss the reasons for the S variations in reconstructed melts and the influence of pressure and other parameters on the SCSS (Sulfur Content at Sulfide Saturation).

New Magmatic Oxybarometer Using Trace Elements in Zircon

Abstract We derive a novel method for determining the oxidation state of a magma as zircon crystallized, with a standard error of ±0·6 log unit ƒO2, using ratios of Ce, U, and Ti in zircon, without explicit determination of the ionic charge of any of them, and without independent determination of crystallization temperature or pressure or parental melt composition. It yields results in good agreement with oxybarometry on Fe–Ti oxide phenocrysts and hornblende phenocrysts quenched in eruptive I- and A-type dacites and rhyolites, but our zircon oxybarometer is also applicable to slowly cooled plutonic rocks and applicable to detrital and xenocrystic zircons. Zircon/melt partition coefficients of Ce and U vary oppositely with ƒO2 variation in the silicate melt. The Ce/U ratio in zircon also varies with the Ce/U element ratio in the silicate melt. During mafic-to-felsic magmatic differentiation, Ce and U are incorporated mainly in calcium-dominated lattice sites of clinopyroxene, hornblende, apatite, and occasionally titanite and/or allanite, all of which have a similar degree of preference for Ce over U. We employ the U/Ti ratio in zircon and in silicate melt as a magmatic differentiation index. Convergent- and divergent-plate-margin differentiation series consistently follow the relation log (Ce/U) ≈ –0·5 log (U/Ti) + C' in silicate melts of basaltic to rhyolitic composition. That correlation permits thermodynamic derivation of the oxybarometry relation among those elements in zircon: log fO2(sample)−log fO2(FMQ)≈42n+1log[Ce/(Ui×Ti)z]+C, wherein Ui denotes age-corrected initial U content, FMQ represents the reference buffer fayalite + magnetite + quartz, superscript z denotes zircon, and n varies with the average valence of uranium in the zircon’s parental silicate melt. We empirically calibrate this relation, using 1042 analysed zircons in 85 natural populations having independently constrained log ƒO2 in the range FMQ – 4·9 to FMQ + 2·9, to obtain the equation log fO2(sample)−log fO2(FMQ)=3·998(±0·124) log[Ce/(Ui×Ti)z]+2·284(±0·101) with a correlation coefficient R = 0·963 and standard error of 0·6 log unit ƒO2 in calc-alkalic, tholeiitic, adakitic, and shoshonitic, metaluminous to mildly peraluminous and mildly peralkaline melts in the composition range from kimberlite to rhyolite. Thermodynamic assessment and empirical tests indicate that our formulation is insensitive to varying crystallization temperature and pressure at lithospheric conditions. We present a revised equation for Ti-in-zircon thermometry that accounts appropriately for pressure as well as reduced activity of TiO2 and SiO2 in rutile- and quartz-undersaturated melts. It can be used to retrieve absolute values of ƒO2 from values of ΔFMQ obtained from a zircon analysis.

Iron isotope fractionation during sulfide liquid segregation and crystallization at the Lengshuiqing Ni-Cu magmatic sulfide deposit, SW China

Abstract We report the first measurements of Fe isotope fractionation between coexisting pyrrhotite and chalcopyrite from the Lengshuiqing Ni-Cu magmatic sulfide deposit, Southwest China. The δ56Fe values of chalcopyrite (Cp) range from 0.24‰ to 1.25‰, much higher than those in the coexisting pyrrhotite (Po, −1.25‰ to −0.13‰). The δ56Fe values of pyroxene separates from the Lengshuiqing intrusions are in a range from −0.03‰ to 0.18‰. Pyrrhotite in sulfide-mineralized rocks with greater than ∼2 wt.% S has δ56Fe values of −0.3 ± 0.2‰, indicating Δ56FePo-sil (pyrrhotite – silicate melt) in the range of −0.4 ± 0.2‰ with parental silicate magma δ56Fe of 0.1‰ as calculated from the least evolved pyroxene δ56Fe value. This fractionation is similar to that experimentally determined by Schuessler et al. (2007) for pyrrhotite and a rhyolitic melt, suggesting that Δ56FePo-sil of approximately −0.4‰ represents a near-equilibrium value for sulfide-rich mineralization in the Lengshuiqing intrusions. Pyrrhotite-dominated sulfide separates in samples with less than ∼2 wt.% S show δ56Fe values as low as −1.25‰. The low δ56Fe values can be modeled by a non-equilibrium process, where the fractionation between immiscible sulfide liquid and silicate magma decreases from −0.4 to −1.4‰ as the R-factor (mass ratio of silicate melt to coexisting sulfide liquid) increases from 1 to 1000. We suggest that the relative proportion of externally introduced Fe from sulfidic sedimentary country rocks may be higher in low-S samples, controlling the low δ56Fe values of the sulfide assemblage in the mineralized rocks. The trend of Fe isotope enrichment at Lengshuiqing is consistent with Fe-S bond strengths, but the difference between coexisting pyrrhotite and chalcopyrite is not consistent with equilibration at high temperatures. The Δ56FeCp-Po values at Lengshuiqing (1.06–1.77‰) are similar to those measured for samples from the Voisey’s Bay and Eagle Ni-Cu deposits, and suggest that in a cooling sulfide liquid Fe isotope equilibration may continue to temperatures less than 200 °C.

Evolution of soil pores and their characteristics in a siliceous and calcareous proglacial area

Soil chronosequences in Alpine areas have often been applied to trace physical, mineralogical and chemical soil properties over time. How the soil pore system of undisturbed soils evolves, however, has not yet been clarified. We therefore investigated two soil chronosequences in the Swiss Alps spanning a time period from a few decades up to the Early Holocene/Late Pleistocene. One soil sequence developed on siliceous parent material in the Sustenpass (proglacial area of the Stein glacier), with soils aged from 30a to 10 ka while the second developed on calcaric parent material in the Klausenpass, with soils aged from 110a to 14 ka (proglacial area of the Griess glacier). We hypothesised that the overall pore size of the topsoil and consequently the relative proportion of macropores decrease with increasing soil age. The pore size characteristics were determined by using X-ray computed tomography. Independent of the parent material (calcareous vs siliceous), macroporosity decreased with time. The greatest changes seemed to occur between the Little Ice Age (LIA) and about 3–4 kyr of soil evolution. Vegetation composition and the functional richness of the plant community exert an influence on soils that developed on siliceous parent material by giving rise to more small pores. Besides pore sizes, pore characteristics also changed with time. In general, the tortuous structure of the soils became more complex with time. The evolution of the pore characteristics such as tortuosity, number and length of pore networks seem also to be related to biologic factors such as root properties, plant species and soil organic matter composition. It remains, however unanswered how these time-dependent biologic factors interact with and influence in detail the pore network. This aspect certainly merits investigation in greater detail in the future.

The Soil Microbiome of GLORIA Mountain Summits in the Swiss Alps.

While vegetation has intensively been surveyed on mountain summits, limited knowledge exists about the diversity and community structure of soil biota. Here, we study how climatic variables, vegetation, parent material, soil properties, and slope aspect affect the soil microbiome on 10 GLORIA (Global Observation Research Initiative in Alpine environments) mountain summits ranging from the lower alpine to the nival zone in Switzerland. At these summits we sampled soils from all four aspects and examined how the bacterial and fungal communities vary by using Illumina MiSeq sequencing. We found that mountain summit soils contain highly diverse microbial communities with a total of 10,406 bacterial and 6,291 fungal taxa. Bacterial α-diversity increased with increasing soil pH and decreased with increasing elevation, whereas fungal α-diversity did not change significantly. Soil pH was the strongest predictor for microbial β-diversity. Bacterial and fungal community structures exhibited a significant positive relationship with plant communities, indicating that summits with a more distinct plant composition also revealed more distinct microbial communities. The influence of elevation was stronger than aspect on the soil microbiome. Several microbial taxa responded to elevation and soil pH. Chloroflexi and Mucoromycota were significantly more abundant on summits at higher elevations, whereas the relative abundance of Basidiomycota and Agaricomycetes decreased with elevation. Most bacterial OTUs belonging to the phylum Acidobacteria were indicators for siliceous parent material and several OTUs belonging to the phylum Planctomycetes were associated with calcareous soils. The trends for fungi were less clear. Indicator OTUs belonging to the genera Mortierella and Naganishia showed a mixed response to parent material, demonstrating their ubiquitous and opportunistic behaviour in soils. Overall, fungal communities responded weakly to abiotic and biotic factors. In contrast, bacterial communities were strongly influenced by environmental changes suggesting they will be strongly affected by future climate change and associated temperature increase and an upward migration of vegetation. Our results provide the first insights into the soil microbiome of mountain summits in the European Alps that are shaped as a result of highly variable local environmental conditions and may help to predict responses of the soil biota to global climate change.