Trace Element Variations Research Articles

Slab window–related magmatism as a probe for pyroxenite heterogeneities in the upper mantle

Abstract New high-precision trace-element analyses of magmatic olivines point to a pyroxenite-dominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<~5%) melts of pyroxenite and peridotite.

A new look at rubber recycling and recreational surfaces: The inorganic and OPE chemistry of vulcanised elastomers used in playgrounds and sports facilities

We revisit current understanding of the chemical complexity of different kinds of particulate vulcanised elastomers widely used in playgrounds and sports fields, adding new data on trace element and organophosphate ester contents of used tyre and EDPM crumb rubber. Enrichments in elements such as Zn, S, Co, Bi and Nd relate to the vulcanised mixtures created during manufacture. Zinc concentrations vary across an order of magnitude, being highest in our used tyre particulate samples (up to 2.4 %). In contrast, other trace element variations are due to coloured pigments causing increases in Cu and Zr (blue), Cr (green), and Fe and Sn (red and yellow) concentrations. The use of pale fillers such as CaCO3 and clay minerals (rather than carbon black) strongly influences Ca and Al concentrations, which are much higher in the coloured EPDM than in black used tyre crumb rubber. Representatives from all three of the main organophosphate ester (OPE) groups, namely chlorinated (e.g. TCEP, TCIPP, TDCPP), alkyl (e.g. TEHP, TEP, TNBP, TBOEP) and aryl (e.g. TPHP, EHDPP, TCP) were identified, confirming how these chemicals are commonly used in modern rubber compounding as flame retardants and plasticisers. Elevated concentrations of TEHP (up to 117 μg/g) in coloured-coated used tyre turf infill crumb rubbers were traced to the coating rather than the crumb rubber itself. The presence and weathering behaviour of OPEs in recreational crumb rubber materials deserve closer investigation. The ecological “onehealth” impact of potentially toxic substances present in recreational crumb rubbers will depend on if and how they are released during play and sports activities, maintenance procedures, and natural weathering. We argue that detailed chemical data on these materials should be available to buyers, as manufacturers strive to reduce ecotoxin content as part of the quest towards sustainable use and recycling of vulcanised elastomers.

Genesis of the Xianghualing tin-polymetallic deposit in southern Hunan, South China: Constraints from chemical and boron isotopic compositions of tourmaline

The Xianghualing skarn Sn polymetallic deposit (Nanling Range), with ca. 0.25 Mt Sn metal, 0.85 Mt Pb + Zn metal, and 22 Kt (Ta, Nb)2O5, is a Late Jurassic biotite granite-related Sn-Pb-Zn-Nb-Ta deposit. However, our understanding of how magmatic-hydrothermal processes contributed to its mineralization remains poor. Tourmaline has the capacity to retain primary elemental and isotopic information throughout the tin-related ore-formation process, allowing us to use the in-situ major, trace elemental, and boron isotopic compositions of tourmaline to research the origin of tin-bearing fluids and to trace the magmatic-hydrothermal evolution associated with tin mineralization. Five types of tourmaline are recognized based on petrography: i) late-magmatic tourmaline (Tur-I) disseminated in the biotite granites; ii) hydrothermal tourmaline (Tur-II) in massive schorl-fluorite association from the endoskarn zone; iii) hydrothermal tourmaline (Tur-III) with long columnar crystals in massive schorl-epidote association from the exoskarn zone, is related to Sn-Pb-Zn mineralization; iv) hydrothermal tourmaline (Tur-IV) from tourmaline-fluorite veins in exoskarn within the marble; and v) hydrothermal tourmaline (Tur-V) from cassiterite-sulfide-tourmaline-fluorite-topaz veins hosted by quartz sandstone, and is directly related to Sn-Pb-Zn mineralization.Tourmaline from the Xianghualing tin-polymetallic deposit dominates the alkali group and the schorl-dravite solid-solution series. Distinct Fe/(Fe + Mg) and Na/(Na + Ca) ratios of Tur-II to Tur-IV reveal different degrees of B-rich fluid-rock (granite or carbonate) interaction. Trace element variations in tourmaline are principally governed by local fluid-rock interactions and melt/fluid compositions in a low fO2 environment. Late-magmatic tourmaline (Tur-I) and hydrothermal tourmaline (Tur-V) related to vein-type Sn-Pb-Zn mineralization have similar Ca, Na, Al, Mg, Fe, and REE + Y contents, indicating that magmatic-hydrothermal exsolution controls Sn-Pb-Zn enrichment. This process occurs in a reducing environment without significant water–rock reactions, demonstrating that high fO2 or acid-consuming reactions are unnecessary for cassiterite precipitation. Variations in water–rock reactions between magmatic fluids and the carbonates cause differences in tourmaline elemental concentrations (e.g., Ca, Al, and Pb) between the magmatic and skarn phases. There are two main paths of tin mineralization in the Xianghualing deposit: (1) fluorine- and boron-rich fluid exsolution at the magmatic-hydrothermal transition stage is crucial for releasing tin and forming cassiterite during the direct cooling process; (2) fluid-rock interaction between tin-bearing magmatic fluids and carbonates facilitates the precipitation of cassiterite. The small variation in δ11B values for all types of tourmaline (-14.8 to −11.6 ‰) suggests that B-rich fluids are mainly derived from magmatic differentiation and fluid exsolution, with insignificant B isotopic fractionation. Magmatic-hydrothermal evolution at the Xianghualing Sn-polymetallic deposit can be unraveled by using the element and B-isotope fingerprints of tourmaline, which might be used as a prospecting aid for Sn-polymetallic deposits.

Geochemistry of Precambrian dyke swarms in the Singhbhum craton, India: Implications for recycled crustal components in the mantle source

The Singhbhum craton, eastern India records multiple stages of emplacement of Precambrian dyke swarms with contrasting petrogenetic models proposed for their formation. In this study, we document elemental and Sr-Nd isotopic data for three major dyke swarms in the southern part of the craton, including the ca. 2.7 Ga Ghatgaon dyke swarm, the Early Proterozoic Keonjhar dyke swarm and the ca. 1.76 Ga Pipilia dyke swarm. Dyke compositions are dominated by basalt and basaltic andesite with minor andesite, showing trace element signatures typical of continental crustal rocks. Age-corrected Nd isotopic data for Ghatgaon (εNdt = −4.8 to + 4.6), Keonjhar (εNdt = −11.9 to + 3.8), and Pipilia (a single sample with εNdt = −8.8) dyke swarms display substantial variations. The lack of magma compositions that could indicate the presence of elevated mantle potential temperature among the rocks suggests melting regime was likely similar to the ambient mantle. The Dy/Yb and Dy/Dy* systematics of the rocks indicates melting occurred between spinel-stable depths and the spinel-garnet transition zone. The dominantly mafic compositions of the rocks and ubiquitous continental crustal trace element signature are best explained by peridotite source with recycled crustal components, probably in the form of pyroxenites. Our new Nd isotopic data, which argue against any simple secular evolution trend invoked in previous studies, indicate that crustal recycling was likely an episodic phenomenon rather than a discrete, single-stage process since the Archean. Geochemical modelling indicates that a sublithospheric mantle source with (10% or less) recycled crustal components satisfactorily explains the trace element variations of the dyke swarms.

Spatio-temporal variation of trace elements distributed over surface water of Upper Ganga River Basin in Western Himalayan Region

Spatio-temporal variation of trace elements distributed over surface water of Upper Ganga River Basin in Western Himalayan Region

Sulfur from the subducted slab dominates the sulfur budget of the mantle wedge under volcanic arcs

Sulfur is of a crucial importance in the Earth system influencing biological, climate, ore-forming, and redox processes. Subduction zones play a key role in the global sulfur cycle. Arc magmas have higher sulfur contents and are more oxidised than mid-ocean ridge basalts (MORBs) due to either an oxidised mantle source or magma differentiation. Melt oxidation state and sulfur content may interrelate, as sulfur is a potential oxidising agent during slab-mantle interaction. Here, we use melt inclusions (MIs) to determine the sulfur isotopic composition (δ34S) of primary arc magmas from three volcanic centres along the Central American Volcanic Arc (CAVA): Fuego (Guatemala), Cerro Negro (Nicaragua), and Turrialba (Costa Rica). These three locations sample much of the global arc magma trace element variability: Ba/La ratios range from 22 (Turrialba) to 118 (Cerro Negro). Melt δ34S values are between -0.5‰ and +4.9‰. Sulfur contents and δ34S values of homogenised and naturally quenched MIs overlap, indicating post-entrapment processes do not affect sulfur contents and sulfur isotope ratios in the studied MIs. Degassing causes limited sulfur isotope fractionation; calculated gas-melt isotope fractionation factors are between 0.998-1.001. Our model calculations predict that most volcanic gases along the CAVA have δ34S between -1‰ and +6‰, becoming enriched in 34S as degassing progresses. We estimate initial melt δ34S values for Fuego, Cerro Negro, and Turrialba to be +0.7±1.4‰, +2.2±1.0‰, and +1.6±0.8‰ (two standard errors), respectively. All these values are elevated compared to MORBs (-0.9‰). Addition of oxidised slab material enriched in 34S to the mantle wedge can explain elevated arc primary melt δ34S and the oxidising conditions observed in arc magmas globally. Based on mass balance, a slab component with δ34S between +2‰ to +5‰ is present in the mantle wedge under the CAVA, elevating local arc mantle S contents to 360±30 ppm at Fuego, 462±11 ppm at Cerro Negro. Modelling suggests that 40-70% of sulfur in the mantle wedge originates from a slab-derived component. Slab subduction is expected to have major control on the evolution of Earth's sulfur cycle and mantle oxidation state over its geological history.

Trace element and Nd isotope analyses of apatite in granitoids and metamorphosed granitoids from the eastern Central Asian Orogenic Belt: Implications for petrogenesis and post-magmatic alteration

We present in situ trace element and Nd isotopic data of apatites from metamorphosed and metasomatized (i.e., altered) and unaltered granitoids in the Songnen and Jiamusi massifs in the eastern Central Asian Orogenic Belt, with the aim of fingerprinting granitoid petrogenesis, including both the magmatic and post-magmatic evolution processes. Apatites from altered granitoids (AG) and unaltered granitoids (UG) are characterized by distinct textures and geochemical compositions. Apatites from AG have irregular rim overgrowths and complex internal textures, along with low contents of rare earth elements (REEs), suggesting the re-precipitation of apatite during epidote crystallization and/or leaching of REEs from apatite by metasomatic fluids. εNd(t) values of the these apatites are decoupled from zircon εHf(t) values for most samples, which can be attributed to the higher mobility of Nd as compared to Sm in certain fluids. Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd-Hf isotopic compositions. Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt, fractional crystallization of other REE-bearing minerals, and changes in partition coefficients. Sr contents and Eu/Eu* values of apatites from UG correlate with whole-rock Sr and SiO2 contents, highlighting the effects of plagioclase fractionation during magma evolution. Apatites from UG can be subdivided into four groups based on REE contents. Group 1 apatites have REE patterns similar to the host granitoids, but are slightly enriched in middle REEs, reflecting the influence of the parental melt composition and REE partitioning. Group 2 apatites exhibit strong light REE depletions, whereas Group 3 apatites are depleted in middle and heavy REEs, indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization, respectively. Group 4 apatites are depleted in heavy REEs, but enriched in Sr, which are features of adakites. Some unusual geochemical features of the apatites, including the REE patterns, Sr contents, Eu anomalies, and Nd isotopic compositions, indicate that inherited apatites are likely to retain the geochemical features of their parental magmas, and thus provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry.

Spatiotemporal Variability of Trace Elements Fingerprints in Otoliths of Japanese Eel (Anguilla japonica) and Its Use in Tracing Geographic Origin.

To secure traceability along supply chains of foodstuffs, the spatiotemporal variability of trace elements' fingerprints (TEF) in fish otoliths provides a powerful tool to determine and discriminate the origin. Spatiotemporal variability of TEF was examined in a commercially important seafood, Japanese eel (Anguilla japonica), by means of laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Six elemental ratios (Na:Ca, Mg:Ca, P:Ca, K:Ca, Sr:Ca, and Ba:Ca) were determined in the otoliths of specimens originating from four aquaculture farms to examine the spatial variability and from one wild habitat over three years to examine the temporal variation. Significant temporal variation was found in Mg:Ca and Sr:Ca ratios; however, discriminant function analysis showed a lower temporal variation (50%) for the three years. Spatial variations were significant in Sr:Ca and Ba:Ca ratios, and discriminant function analysis showed high (80%) spatial variation among the four farms. Otolith TEF in the Japanese eel showed specific spatial variation among aquaculture farms but intangible temporal variation, suggesting the otolith TEF reflect each aquaculture environment. The present study shows that otolith TEF can be a reliable tool to discriminate the geographic origin of the Japanese eel.

Variable element enrichment sources and contributions to volcanic rocks along the Lesser Antilles Island Arc

Understanding relationships between different components involved in subduction-related recycling of sediment at arc volcanic systems has long been a focus of study. It is generally understood that volcanic arc lavas incorporate materials from the down-going slab including fluids, fluid mobile elements (FME), and fluid modified large ion lithophile elements (LILE), largely derived from subducted sediments and serpentinized ultramafic rocks. Past studies have presumed bulk sediment contributions rather than addressing localized variations in sediment composition. We examine trace element variability along the Lesser Antilles Island Arc (LAIA) and show that sediment inputs via fluid transfer to the mantle wedge are necessary to explain high B (up to 50 ppm), high B/La ratios, and higher As, Cs, Rb in the central LAIA. In contrast, the northern portion of LAIA is enriched in Ba, Sr and U/Th, with low FME. Grenada has low FME abundances, low Ba/La, but higher Sm/La. Our results suggest that modification of LAIA mantle source by fluid addition is important for the central islands but find that sediment enrichment is more likely in the northern section of the LAIA. Li/Yb ratios suggest the Grenada signature is chemically more comparable to sediment from DSDP Site 144 (Demerara Rise), while Martinique magmas in the central LAIA are affiliated to sediments from Site 543 (offshore Dominica). Unexpectedly, the northern portion of LAIA is not directly relatable to Site 543 sediments but instead trends back toward a sediment source similar in composition to Site 144 but with lower B, lower La/Sm and higher Ba. Binary mixing models suggest the northern islands are likely influenced by small amounts of sediment melt similar to average Site 144 sediments, but with Ba concentrations ranging from 1,250 to 1,500 ppm. Trace element ratios then likely result from different levels of contribution from sediment packages both spatially and temporally along the arc, with some variation evident among samples from the same island. We suggest that the process of recycling subducted sediment along the arc is variable along the length of the LAIA, wherein the trace element relationships are likely strongly influenced by the types of sediment being subducted along strike.

Distribution of Trace Elements in K-Feldspar with Implications for Tracing Ore-Forming Processes in Pegmatites: Examples from the World-Class Kolmozero Lithium Deposit, NW Russia

This study utilizes LA-ICP-MS-determined minor and trace element contents of megacrystic blocky K-feldspar to reveal the chemical variability and fractionation degree of albite-spodumene and barren feldspar pegmatites of the Kolmozero lithium deposit in the Kola region, Russia. K-feldspar from albite-spodumene pegmatite is represented by two generations: early microcline-I and late microcline-II. Rb, Cs, Li, and Tl are the most typical impurity elements in K-feldspar that replace K in its crystal lattice. Microcline-II differs from microcline-I: (i) relatively high contents of Rb (6520 and 4490 ppm, respectively), Cs (146 and 91 ppm), and Li (86 and 68 ppm), Tl (34 and 28 ppm); and (ii) low contents of Ba (13 and 29 ppm), Sr (8 and 24 ppm), and Pb (14 and 26 ppm). K-feldspar from feldspar pegmatites of the Kolmozero pegmatite field differs from those in the Kolmozero Li deposit in (i) low contents of Rb, Cs, Li, Tl, and an orthoclase component; and (ii) high contents of Sr, Ba, Pb, and an albite component. K/Sr, K/Ba, Rb/Ba, and Rb/Sr element ratios increase, while K/Rb, K/Cs, K/Tl, and K/Li element ratios decrease in K-feldspar, from feldspar pegmatites to albite-spodumene pegmatites. These trends reflect different fractionation degrees of pegmatite evolution. The implications of the detected trace element variations in K-feldspar are discussed in respect of tracing the rare element enrichments in pegmatite systems. A model is proposed for the formation of the Kolmozero pegmatites by differentiation from a hypothetical parental granite, rather than by anatexis of the host rock.

Sub‐Micrometre Resolution FIB‐SEM‐based ToF‐SIMS Used to Map Geochemical Zoning in Four Zircon Reference Materials

Zircon geochemistry can vary over micrometre scales; therefore, natural reference materials need to be well characterised before being used to calculate trace element mass fractions in unmeasured samples. Moreover, reference material homogeneity needs to be ensured with the accelerating rate of geoanalytical developments to map mineral chemistry at increasingly finer scales. Here, we investigate trace element zoning in four widely used zircon reference materials: 91500, Mud Tank, Temora and Plešovice, as well as zircon crystals from the Mount Dromedary/Gulaga Igneous Complex, Australia. Sub‐micrometre resolution focused ion beam scanning electron microscope (FIB‐SEM) based time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and 5 μm resolution LA‐ICP‐MS mapping show that trace elements are zoned in all reference materials, though 91500 exhibited the least zonation. We demonstrate that FIB‐SEM‐based ToF‐SIMS can rapidly resolve variations in trace elements (e.g., U, Th, Sc, Y, Gd, Dy, Yb and Li) at sensitivities down to the μg g‐1 level with a spatial resolution of 195 nm for areas 100 × 85 μm to 959 × 828 μm. Zircon 91500 is recommended for future quantitative analyses provided that (1) the spatial distribution of elements is imaged before analysis of unknown samples and (2) it is used in conjunction with a doped glass as the primary reference material.

Garnet geochemical compositions of the Bailongshan lithium polymetallic deposit in Xinjiang Province: Implications for magmatic-hydrothermal evolution

The Bailongshan deposit is a newly-discovered super-large lithium polymetallic pegmatite deposit. Garnets occur widely in muscovite granites and pegmatites at Bailongshan, but comprehensive study is yet to be done on the geochemical evolution of garnet from the magmatic to the hydrothermal stages. Here, we present a systematic study of in-situ major and trace elemental variations of garnet from Bailongshan, and discuss the geochemical behaviors during magmatic to hydrothermal stage and the mineral indicator of large-scale mineralization of rare metal. Three types of garnet have been identified at Bailongshan, including disseminated garnet in muscovite granite (Grt I), medium-coarse-grained garnet in barren pegmatites (Grt II), and medium-grained euhedral-subhedral garnet in ore-bearing pegmatites (Grt III). EPMA and LA-ICP-MS geochemical analyses reveal that the garnet belong to almandine-spessartine solid-solution series. The garnets have varied REE contents (∑REE = 3.64～382 ppm), and HREE-enrichment, negative Eu anomalies, and LREE below or close to their detection limits. Grt III has higher Mn and Zn, but lower Fe, Y and HREE than those of Grt I and Grt II. The contents of Li, Zr, Hf, Nb and Ta increase from Grt I to Grt II, but decrease abruptly to Grt III. During the evolution of granitic magma, the increasing magma fractionation and fluid-phase accumulation and the crystallization of zircon lead to the continuous decrease of Y and HREE contents from Grt I to Grt III, and the gradual decrease of Y and HREE contents from core to rim of garnet crystals. In the quartz-albite-spodumene pegmatites, the crystallization of spodumene and columbite-group minerals removes Li, Nb and Ta from the melt, which significantly reduces the contents of these elements in garnet. The crystallization of spessartine-rich garnet is a reflection of a high degree of melt evolution at low pressure which facilitates the formation of LCT type pegmatite deposit. The Zr/Hf and Y/Ho ratios also indicate that magma fractionation degree and fluid activity increase from muscovite granites to barren-pegmatites and ore-bearing pegmatites, The combination of some major (e.g., Fe and Mn) and trace elements (e.g., Li, Ta, Nb, Sc and HREE) signatures, as well as the Zr/Hf, Y/Ho ratios in garnets can indicate the magma fractionation degree and the crystallization of some ore minerals (e.g., spodumene and columbite-group minerals), and may be a potential exploration indicator for rare-metal mineralization.

Black shale–gray shale transitions in a Late Devonian shale succession, Central Appalachian Basin (Northern Ohio): Sedimentary and geochemical evidence for terrestrial organic matter input driving anoxia events

In a 140 m-thick sediment core through Upper Devonian shale units in the Central Appalachian Basin, gray shale units (TOC ≤1%) and black shale units (TOC ≤9%) are separated by “intercalated zones” (ICZs) of thinly interbedded gray and black shales. In accordance with Walther’s law of facies successions, ICZs imply environments producing black shales were coeval and adjacent to environments producing gray shales. To understand these facies transitions, we studied event layer sedimentology, organic geochemistry, and trace elements. This study found black shales to be shallow-water deposits (with tempestites and wave-modified turbidites), associated with significant terrestrial input (with abundant hyperpycnites, and terrestrial-derived humic and fulvic acids). The shales were deposited under varying redox conditions, with extensively bioturbated intervals alternating with intervals preserving intact micro-lamination, and demonstrating significant variations in trace element V/Cr, Ni/Co, and Th/U ratios. These observations are reconciled by attributing shale TOC values to sediment dynamics of deltas draining the Neo-Acadian Highlands. Progradation of individual delta distributary lobes resulted in localized offshore regions receiving increased terrestrial nutrient input and causing freshwater-induced water column stratification and oxygen stress (similar to the “dead zones” of many modern deltas), producing black shales. In contrast, gray shales formed between deltas or after delta distributary lobe abandonment. The intercalated black shale-gray shale transition intervals (ICZs) are interpreted to represent dynamic water and sediment conditions during the time interval of delta distributary lobe avulsion events. This model contrasts with previous models calling for basin-wide anoxia or assigning black shales to basinal deposits.

Geochemical and isotopic (Sr-Nd-Pb) signature of crustal contamination in Na-alkali basaltic magmas of South-East Turkey

Widespread sodic alkali basaltic activity developed from Early Miocene to Pleistocene, around the northern termination of the Dead Sea Fault Zone, and in the North-West foreland of the Arabian Plate, in South-East Turkey. Volcanic activity started within the Gaziantep Basin during the Early-Late Miocene (21.2-7.0 Ma), and further East, abundant magmatism brought to the formation of the Karacadağ Shield Volcano, starting from Middle Miocene throughout the Holocene (12.1-0.01 Ma). During the Pleistocene, volcanism developed also around the Anatolia-Africa-Arabia triple junction, along the Karataş-Osmaniye Fault, and along the northern segment of the Dead Sea Fault, in the Karasu Basin.Volcanic rocks emplaced in these areas mainly consist of abundant basalts and basanites, with subordinate hawaiites and basaltic andesites, mostly showing Na-alkaline affinity, whereas some of the older products show calc-alkaline signatures. As a whole, these volcanic rocks show LREE/HREE and HFSE/LILE ratios similar to those of OIB-like magmas. Some trace element and radiogenic isotope variations are observed amongst the less differentiated samples and they are explained by partial melting of a heterogeneous subcontinental mantle source at variable depths, mostly in the garnet stability field.The largest compositional variability observed, however, is attributed to differentiation processes, resulting in quite remarkable chemical and isotopic variations (e.g., 87Sr/86Sr = 0.70301-0.70529; 143Nd/144Nd = 0.51263- 0.51294; 206Pb/204Pb = 18.78-19.47; 207Pb/204Pb = 15.57-15.72; 208Pb/204Pb = 38.75-39.27). This implies the occurrence of open-system magma evolution, characterised by crystal fractionation at low pressure of mineral assemblage made up by olivine ± plagioclase ± clinopyroxene + apatite + Fe-Ti oxides, coupled with assimilation of crustal materials. The extent and the chemical modifications induced by this process are such that in some of the oldest and most evolved samples, the pristine Na-alkaline affinity is obliterated, and a secondary subalkaline signature is overprinted. This effect is less evident or negligible in more recent lavas, which preserve their primary features, being erupted in a well-developed fault system or in a long lasting, chemically homogeneous, magmatic feeding system.The magmatism studied here is quite unusual and not frequent in the frame of Na-alkaline intraplate volcanism, being characterised by abundant intermediate products and by evolutionary processes strongly affected by crustal contamination. In our view, this is due to the peculiar setting of the region, which is the locus of a significant mantle upwelling, accompanied by very limited or absent extensional rate.

Genesis of the Longtougou gold deposit, South Qinling Orogen, China: Constraints from ore geology, trace element, and S–Pb–H–O isotopes

Orogenic gold deposits are a significant source of gold production, although the geological processes and metallogenic material sources remain somewhat controversial. The Longtougou deposit is located in the South Qinling Orogen, China, and is a newly discovered moderate tonnage gold deposit. The deposit can be split into four stages of mineralization and contains three types of pyrite (Py0, Py1, and Py2). The Py0 is fine-grained, present as subhedral–anhedral aggregates, and formed during diagenetic stage of sandstones of the Devonian Dafenggou Formation. Coarse- to medium-grained euhedral Py1 is hosted by early-ore stage quartz–pyrite veins. The last pyrite to form was fine-grained anhedral Py2, which coexists with barite within main-ore stage quartz–barite–polymetallic sulfide veins. The final stage is associated with the formation of quartz–barite–carbonate veins and stockworks. Early-ore stage samples yield δ18OH2O values from 9.8‰ to 13.9‰ and δDV-SMOW values from −75.4‰ to −65.3‰, indicative of fluids may be derived from a mixing of metamorphic and magmatic fluids. Main-ore stage samples yield δ18OH2O and δDV-SMOW values of 5.4‰ and −70.6‰, respectively, whereas late-ore stage samples yield δ18OH2O values from −9.0‰ to 3.3‰ and δDV-SMOW values from −74.5‰ to −56.7‰. These data indicate that the main-ore stage involved meteoric and organic water, whereas the late-ore stage was dominated by meteoric water. The Py0 has significantly negative δ34S values (−15.1‰ to −14.5‰), suggesting that the sulfur in this pyrite was derived from bacterial sulfate reduction. Early-ore stage sulfides have restricted ranges of δ34S values (−5.7‰ to 5.6‰) that are indicative of a magmatic–hydrothermal fluid origin. The Py2 yields lower positive δ34S values (2.4‰ to 5.3‰) and coexisting barite has higher positive δ34S values (15.3‰ to 17.5‰), indicative of a transition into an oxidizing environment. The Δ34SA–B−δ34SB diagram yields a δ34SΣS value of 8.2‰, indicative of sulfur derived from a mix of sedimentary and magmatic sources. All of the sulfides have Pb isotopic ratios that plot within the field of underlying Paleozoic strata and are close to the Indosinian and Yanshanian granites, suggesting that the metals were derived from a combination of underlying Paleozoic sedimentary and granitic sources. Most of the hydrothermal pyrites have Co/Ni and S/Se ratios that are indicative of a magmatic-hydrothermal origin, suggesting that the Au was derived predominantly from magma but with possible contributions from sedimentary and metamorphic fluid sources. In addition, hydrothermal pyrites have trace-element variations that differ from Py0, suggesting that the gold was not derived from the local host rocks. An underlying Paleozoic sedimentary sequence enriched Au that underwent greenschist facies metamorphism may be another source of the gold. The majority of invisible Au is present as structurally bound, with visible gold present as inclusions or as fracture fills within Py2. The gold was precipitated as a result of fluid mixing, a process that caused an increase in fO2 values and a change from reduced to oxidized conditions during the main-ore stage. This study suggests that the Longtougou deposit is an orogenic gold deposit that formed during the transition between compression and extension in an intracontinental orogeny.

Whiskers provide time-series of toxic and essential trace elements, Se:Hg molar ratios, and stable isotope values of an apex Antarctic predator, the leopard seal

In an era of rapid environmental change and increasing human presence, researchers need efficient tools for tracking contaminants to monitor the health of Antarctic flora and fauna. Here, we examined the utility of leopard seal whiskers as a biomonitoring tool that reconstructs time-series of significant ecological and physiological biomarkers. Leopard seals (Hydrurga leptonyx) are a sentinel species in the Western Antarctic Peninsula due to their apex predator status and top-down effects on several Antarctic species. However, there are few data on their contaminant loads. We analyzed leopard seal whiskers (n = 18 individuals, n = 981 segments) collected during 2018–2019 field seasons to acquire longitudinal profiles of non-essential (Hg, Pb, and Cd) and essential (Se, Cu, and Zn) trace elements, stable isotope (ẟ15N and ẟ13C) values and to assess Hg risk with Se:Hg molar ratios. Whiskers provided between 46 and 286 cumulative days of growth with a mean ~ 125 days per whisker (n = 18). Adult whiskers showed variability in non-essential trace elements over time that could partly be explained by changes in diet. Whisker Hg levels were insufficient (<20 ppm) to consider most seals being at “high” risk for Hg toxicity. Nevertheless, maximum Hg concentrations observed in this study were greater than that of leopard seal hair measured two decades ago. However, variation in the Se:Hg molar ratios over time suggest that Se may detoxify Hg burden in leopard seals. Overall, we provide evidence that the analysis of leopard seal whiskers allows for the reconstruction of time-series ecological and physiological data and can be valuable for opportunistically monitoring the health of the leopard seal population and their Antarctic ecosystem during climate change.

TRACE ELEMENTS IN QUARTZ: INSIGHTS INTO SOURCE AND FLUID EVOLUTION IN MAGMATIC-HYDROTHERMAL SYSTEMS

AbstractQuartz trace elements record information about fluid evolution as well as metal migration and precipitation. Here, we summarize most of the reported (including this study) quartz trace element data (N = ~4,600) generated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on various textural types and paragenetic stages of quartz in I-type porphyry-epithermal (Cu-Mo-Au-Ag-Te) and S- and A-type granitegreisen (Sn-W and rare metal) systems in the world. The results show that Li versus Al diagrams, combined with Ti-Ge-As-Sb contents, can be used to decipher the source and evolution of fluids in magmatic-hydrothermal systems. In I-type porphyry-epithermal systems, magmatic quartz has low Li/Al ratios from 0.001 to 0.173 (N = 483) with a mean of 0.039 ± 0.032. Hydrothermal quartz has progressively higher Li and Al concentrations that are dominated by cooling along fluid pathways. Quartz evolves from Ti rich to Ge rich from early to late stages in porphyry hydrothermal veins and is As and Sb rich in epithermal veins. In S- and A-type granite-greisen systems, magmatic quartz has high Li/Al ratios from 0.007 to 0.502 (N = 604) with a mean of 0.130 ± 0.063 and from 0.009 to 0.327 (N = 325) with a mean of 0.126 ± 0.065, respectively. Hydrothermal quartz has progressively lower Li and Al concentrations that are dominated by fluid-rock reactions and cooling along fluid pathways. Quartz evolves with decreasing Ti concentrations from magmatic to hydrothermal stages. Ge is abundant in pegmatite quartz in S-type systems. Variations in pH or precipitation rate along fluid pathways have a small influence on Li/Al ratios. The variation of quartz trace elements with elevation in individual systems suggests that they can be used as a vector to guide exploration in magmatic-hydrothermal systems.

Mixing Loops, Mixing Envelopes, and Scattered Correlations among Trace Elements and Isotope Ratios Produced by Mixing of Melts Derived from a Spatially and Lithologically Heterogenous Mantle

Abstract Mixing has been widely used in the interpretation of radiogenic isotope ratios and highly incompatible trace element variations in basalts produced by melting of a heterogeneous mantle. The binary mixing model is constructed by considering mass balance of endmember components, which is independent of physical state and spatial distribution of the endmembers in the mantle source. Variations of radiogenic isotope ratios and highly incompatible trace elements in basalts also depend on the size and spatial distribution of chemical and lithological heterogeneities in the mantle source. Here we present a new mixing model and a mixing scheme that take into account of the size, spatial location, and melting history of enriched mantle (EM) and depleted mantle (DM) parcels in the melting column. We show how Sr, Nd, and Hf concentrations and isotope ratios in the aggregated or pooled melt collected at the top of the melting column vary as a function of location of the EM parcel in the melting column. With changing location of the EM parcel in the upwelling melting column, compositions of the pooled melt do not follow a single mixing curve expected by the binary mixing model. Instead, they define a mixing loop that has an enriched branch and a depleted branch joined by two extreme points in composition space. The origin of the mixing loop can be traced back to four types of EM distribution or configuration in the melting column. The shape of the mixing loop depends on the relative melting rate of the EM to that of the DM and the number and spacing of EM parcels in the melting column. Probabilities of sampling the enriched and depleted branches in the pooled melt are proportional to volume fractions of the enriched and depleted materials in the mantle source. Mixing of pooled melts from a bundle of melting columns results in mixing envelopes in the isotope ratio correlation diagrams. The mixing envelope is a useful tool for studying chemical variations in mantle-derived melts. As an application, we consider scattered correlations in 87Sr/86Sr vs. 143Nd/144Nd and 143Nd/144Nd vs. 176Hf/177Hf in mid-ocean ridge basalts. We show that such correlations arise naturally from melting of a spatially heterogeneous mantle.

High-latitude climatic response across the Triassic-Jurassic boundary recorded by Mg-Cu-Zn isotopes

The Triassic-Jurassic boundary is one of the most important geological boundaries in the Earth's evolutionary history. Previous paleoclimate studies focused primarily on Tethys realm at low and middle latitudes, while the northern hemisphere has yet to be studied. Here, we report Mg-Zn-Cu isotopic data for a continuous terrestrial sedimentary section in the southern margin of the Junggar Basin to constrain chemical weathering intensity and climatic change during this critical interval at high latitude. This section is enriched in heavy Mg isotopes with δ26Mg ranging from 0.08‰ to 0.43‰. It displays a decreasing Zn isotopic composition with δ66Zn ranging from −0.05‰ to 0.24‰, as well as limited Cu isotope fractionation (δ65Cu = −0.06‰ to 0.09‰). These isotopic results, combined with major and trace elemental variations, suggest a reduced sedimentary environment and increasingly intensive chemical weathering across the boundary, responding to a warmer and more humid paleoclimate. Compared with other places located around Tethys, this regional climatic change in high latitude areas can be linked to known global climate change during this period. Our study provides new insights from high-latitude Asia to reconstruct the global pattern of climate change at the Triassic-Jurassic boundary.

Formation of Late Paleoproterozoic Gaositai Hornblendite in Northern North China Craton: Evidence from Zircon U-Pb Isotopes and Amphibole Trace Elements

Paleoproterozoic tectonic evolution of the northern North China Craton has been a hot research topic. We firstly identified a 1.85 Ga hornblendite from the Gaositai mafic–ultramafic complex, in northern Hebei. Systematic studies of petrology, zircon U-Pb geochronology, and in situ mineral major and trace elements of hornblendite are the key to revealing the petrogenesis of the Paleoproterozoic ultramafic rock and the tectonic evolution of northern North China Craton. LA-ICP-MS zircon U-Pb dating suggests the Gaositai hornblendite formed at 1851 ± 44 Ma. The late Paleoproterozoic ultramafic rocks, together with coeval post-collisional granites, formed a bimodal igneous assemblage. Both hornblende and its equilibrium melt compositions show strongly fractionated HREE patterns, relative enrichments in LREEs and LILEs, and depletions in HREEs and HFSEs. The phlogopite-bearing hornblendite magma could have originated from a hydrous garnet-facies mantle source metasomatized by slab-derived silicate melt. Furthermore, the variations of major and trace elements in hornblende from core to rim also reveal the mineral fractional crystallization and magma recharge. Zircon trace elements, melt composition equilibrium with hornblendes, and the bimodal igneous assemblage suggest that the generation of the Gaositai Paleoproterozoic hornblendite was likely the product of post-collisional extension related to the collision between eastern and western North China blocks.