Trace Element Mapping Research Articles

Deep subduction, melting, and fast cooling of metapelites from the Cima Lunga Unit, Central Alps

AbstractThe Cima Lunga unit in the Central Alps is dominated by quartzofeldspathic gneisses with subordinate mafic, ultramafic, and metacarbonate rocks. Only mafic and ultramafic lithologies were thought to preserve clear evidence of Alpine high‐P metamorphism. This led to the questions of whether the different rock types were subducted and exhumed as a coherent unit or underwent different pressure–temperature (P–T) histories. New petrological and geochemical data from a metapelite associated with garnet peridotite from Cima di Gagnone (Cima Lunga unit, Switzerland) were obtained using major and trace element mapping. Complex zoning patterns in garnet and white mica are observed. In particular, high Ti content in phengite and increasing P, Zr, and HREE contents in pyrope‐rich garnet indicate that this metapelite underwent high‐P and high‐T (HP–HT) metamorphism involving fluid‐fluxed partial melting. A P–T path is reconstructed by combining textural analysis with petrological–geochemical data and thermodynamic simulations. We show that the mineral record preserves an evolution from prograde to HP–HT peak conditions (2.7 ± 0.1 GPa and 800℃) followed by near‐isobaric cooling (~2.5 GPa and 700–750℃) prior to decompression (1.0 GPa and ~620℃). The reconstructed P–T path suggests that the studied metapelites were subducted to depths where the slab gets heated by proximity to asthenospheric mantle related to slab break‐off. This heating resulted in the dehydration of chlorite‐ to garnet peridotite and the liberated fluids triggered partial melting in the associated metapelites, which might have favoured the fast exhumation of the entire Cima Lunga unit. Metapelites and garnet peridotite from Cima di Gagnone underwent a common prograde to peak and retrograde P–T path without significant tectonic pressure difference between the different lithologies, and deviation from lithostatic pressure is excluded. Lastly, the peak metamorphic conditions of metapelite from Cima di Gagnone are comparable with P–T estimates of ultramafic lithologies from the southern Adula nappe and the Dascio Bellinzona zone, thus opening new scenarios for the geodynamic interpretation of the Central Alps.

Combined fluorescence imaging and LA-ICP-MS trace element mapping of stalagmites: Microfabric identification and interpretation

Here, we map a series of stalagmites from Asturias, Spain, by laser ablation inductively-coupled-plasma mass spectrometry and confocal laser scanning (fluorescence) microscopy and discuss the origins of trace element and fluorescence variations. Seasonal banding is evident with both methods and may be attributed to lignins/humic acids based on fluorescence absorption and emission characteristics. Some lateral variations in fluorescence present as saw-tooth fluorescent “spires” and demonstrate disruptions of seasonal banding, corroborated by trace element variations (most prominently Mg and Na). Such features likely reflect the differential partitioning of trace elements by sectoral zoning as a result of low supersaturation and/or high organic matter load, combined with the effects of dissolved organic matter on the calcite growth surface and the association of each element with colloidal organic matter. The lateral variability of trace elements demonstrates the pitfalls of obtaining trace element information from one-dimensional transects without prior reconnaissance mapping. It is, however, possible that traditional drilling with ~1 mm holes homogenizes these features and provides reliable trace element estimates.

Texture, trace elements, sulfur and He-Ar isotopes in pyrite: Implication for ore-forming processes and fluid source of the Guoluolongwa gold deposit, East Kunlun metallogenic belt

The Gouli goldfield is the most important gold producers of the East Kunlun metallogenic belt. Gold mineralization in this region is enigmatic, primarily because of uncertainties as to the mineralization processes, and the nature and sources of ore-forming fluids. Here, we investigate the ore-forming fluids and processes through a comprehensive study of the texture and geochemistry (LA-(MC)-ICPMS trace element analysis, mapping, S isotopes, and He-Ar isotopes) of pyrite from the Guoluolongwa gold deposit, the largest deposit in the Gouli goldfield. Four types of pyrite with distinct textures, trace element contents and sulfur isotopes were identified, and provide evidence for a multi-stage hydrothermal fluid evolution. The sedimentary-diagenetic Py0, representing the pre-ore stage, is characterized by high contents of Co and Ni, and negative δ34S values (−11.6‰ to −4.0‰), which are notably different from the subsequent hydrothermal Py1 to Py3. Hydrothermal Py1, comprising subtypes Py1a and Py1b, was deposited in pyrite-quartz veins during stage I. The Py1a subtype shows characteristic Co-Ni-As oscillatory zoning that replaced Py0 via dissolution-reprecipitation, and hence inherited high concentrations of Co and Ni from Py0. In contrast, the Py1b subtype was devoid of Py0 influence and represented the initial ore-forming fluid with low concentrations of Co and Ni, moderate Cu, Pb, Zn, Sb, and Bi, but high As, Au, and Ag. The next stage, Py2, formed in the quartz-pyrite veins during stage II and showed clear textural and compositional contrasts to Py1. Specifically, Py2 has elevated Cu, Pb, Zn, Ag, and Bi contents and somewhat lower δ34S values (+1.5‰ to +3.2‰) compared to Py1 (+1.6‰ to +5.6‰). In addition, Py2 shows irregular compositional zones, which we attribute to fluid phase separation caused by pressure fluctuation. Abundant Py2 precipitation triggered native gold deposition due to desulfidation of the ore-forming fluid. The next stage, Py3 in polymetallic sulfides-calcite-quartz veins at stage III, implies a new pulse of Cu-Pb-Zn-As-Au-Ag-rich and Bi-poor ore-forming fluid. This latter pyrite is characterized by inclusion-rich cores (Py3c), which contain abundant inclusions of chalcopyrite, galena, sphalerite, and native gold, and inclusion-free rims (Py3r). The cores, Py3c, likely represent abrupt destabilization of metal complexes and rapid precipitation during vigorous phase separation, whereas the euhedral Py3r rims formed under steadier physico-chemical conditions, contributing to lower δ34S values of Py3c (+0.5‰ to +3.0‰) than Py3r (+1.9‰ to +3.6‰). Noble gas isotopes of Py1-Py3 have crust-dominated 3He/4He (0.04–0.4 Ra) and 40Ar/36Ar (623–2081), mixed with minor mantle components. Sulfur isotopes and low Co and Ni contents of hydrothermal pyrite, and He-Ar isotopes of fluid inclusions in pyrite, are consistent with a model in which multiple pulses of ore-forming fluids were exsolved during sequential episodes of primarily crustal-derived felsic magmatism with lesser mantle influences. This model is also supported by previous H-O isotope analyses, as well as widespread granitoid magmatism in the eastern Central East Kunlun Belt.

Tracking geochemical signatures of rare earth and trace elements in spring waters and outcropping rocks from the hidden geothermal system of Acoculco, Puebla (Mexico)

A geochemical mapping of rare earth (REE) and other trace elements in water and rock samples of the hidden geothermal system of the Acoculco caldera was carried out. REE and other trace elements were measured at low concentrations in waters (ΣREE <50 μg/L) and rocks (ΣREE <773 μg/g), and used, for the first time, to understand the water-rock interaction processes of this geothermal system. A bimodal frequency distribution of pH was found among acidic waters (pH <3.5) that emerge from cold springs located inside the caldera, and slightly-acid to hot alkaline spring waters (pH 5 to 9.2) from outside the caldera. The geochemical modelling of REE showed that minerals containing Al3+, Fe, and F− were the main phases that control their concentrations in acidic waters. Different degrees of REE complexation in waters located inside the Acoculco caldera (Alcaparrosa and Los Azufres) clearly showed new signatures of geothermal activity. LnSO4+ complexes and free ions of Ln3+ were identified as major species of the acidic waters of Alcaparrosa, whereas in less acidic waters of Los Azufres, LnF2+ show an increase in their complexation percentages. The REE chondrite-normalized patterns observed in acid waters display a depletion from light-REE (LREE) to heavy-REE (HREE), and an inverse correlation between high REE concentrations and acid pH's. The REE concentration in felsic and intermediate rocks shows “gull-wing” chondrite-normalized patterns characterised by an enrichment of LREE, a negative anomaly of Eu, and depletion of HREE with a flat tendency. The present study of water-rock interaction contributed to the identification of new geochemical signatures of major and trace elements that provide new evidences of deep formation temperatures between 200 °C and 300 °C, becoming in an effective tool for finding out convincing anomalies in hidden geothermal systems.

Element mobility and oxygen isotope systematics during submarine alteration of basaltic glass

Abstract Studies of the submarine alteration of basaltic glass may improve the understanding of crust-seawater interactions and the oceanic elemental cycle. Natural alteration processes such as the palagonitization of basaltic glass are complex, and their elucidation is crucial to the understanding of fluid-rock interactions. This study involves major and trace element and isotope mapping of a submarine-altered basaltic glass grain and reveals sequential development of alteration textures toward the grain core through the transition between unaltered glass and palagonite. The conversion from basaltic glass to palagonite in a low-temperature submarine environment results in enrichment in B, Rb, K, Li, H2O, U, Nb, Th, Ti, Cu, Ta, Zr, Hf, Ni, Sc, Fe, Cr, Pb, and Zn; and depletion in Si, Mg, Al, Sr, Na, Co, rare-earth elements, Ca, P, and V (in order of decreasing distribution coefficient). The glass-palagonite interface region has the highest high field strength element (HFSE) and Fe-Ti contents but the lowest Mg content, indicating that the Fe-Ti phases that host HFSE were precipitated first during initial palagonitization. At the same time, an inferred exchange of oxygen occurred, based on variation in δ18O, with values increasing from basaltic glass to palagonite. However, initial palagonite compositions were affected by subsequent precipitation and/or incorporation/adsorption of additional compounds such as Mg(OH)2 scavenged from pore water.

A magmatic‐hydrothermal origin of the Xinfang gold deposit, Liaodong Peninsula, China, revealed by in‐situ S–Pb isotopes and trace element analyses of pyrite

AbstractThe Xinfang deposit is a large gold deposit (&gt;20 t of Au) on the southern edge of the Liaodong Peninsula, China. A total of 37 gold‐bearing orebodies are identified along NS‐ and NW‐striking faults and are mainly hosted by the Archean and Neoproterozoic metamorphic rocks. Three stages of hydrothermal mineralization are distinguished, including Stage I of dominant quartz ± pyrite (Py1), Stage II of quartz‐polymetallic sulfide (Py2), and Stage III of quartz‐calcite ± pyrite (Py3). Gold primarily occurs at the Stage II in association with intensive silicification, pyritization, and sericitization. Here, we report on high‐precision in‐situ LA‐MC‐ICP‐MS analyses of S and Pb isotopic compositions of the sulfide minerals and LA‐ICP‐MS analyses of trace element signature of the pyrite from the Xinfang gold deposit. The δ34S values of the sulfides range from 1.3 to 5.3‰, which is consistent with a magmatic origin of S. The sulfides yield 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of 37.497–37.773, 15.323–15.410, and 17.004–17.229, respectively, which deviate from the values of the wall rocks, suggesting an external Pb source. The in‐situ LA‐ICP‐MS spot analyses of the pyrite reveals that gold occurs in two generations of pyrite (Py2 and Py3) but at low concentrations (≤0.06 ppm for Py2, ≤0.24 ppm for Py3). Visible gold, however, only occurs in the Stage II assemblage as inclusions in pyrite and quartz, and is distributed along the margins or within the fractures of the sulfides. The Py2 shows high Co, Ni, and As concentrations and is depleted in other trace elements typical of a magmatic‐hydrothermal derivation. In contrast, Py3 exhibits low concentrations in most trace elements. Trace element mapping of Py2 revealed correlations between Fe and Co, as well as Ni and As, indicating their couped substitution in the pyrite structure. The available geological, petrographic and geochemical data consistently suggest that the Xinfang gold deposit formed during the Early Cretaceous by hydrothermal fluids of magmatic contribution.

New constraints on the formation of main group pallasites derived from in situ trace element analysis and 2D mapping of olivine and phosphate

Pallasites are stony‑iron meteorites consisting mainly of olivine and Fe-Ni metal with a formation history that remains widely debated. Despite their simple mineralogy, relatively limited data are available on their geochemistry and the lateral elemental distribution in individual pallasite olivine crystals. In this work, laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) was used for the elemental analysis of both pallasite olivine and phosphate phases, including 2D trace element mapping of olivine crystals. While the results obtained are in good agreement with literature values, important differences are observed for Al and Ni concentrations compared with bulk analytical methods. The element distribution maps reveal complex zoning in pallasite olivine, which can be explained based on i) diffusion gradients formed during olivine cooling, ii) the crystal chemistry of element substitution due to charge-balancing, and iii) inherited features of olivine before the metal-olivine mixing. Oscillatory zoning in an olivine crystal from Imilac provides strong evidence for the olivine not being a restite of partial melting, but rather having crystallized from a melt. Concentrations of Cr and Al are correlated both within single olivine crystals and between olivine crystals of different pallasites, forming a 1:1 linear trend. This likely results from a spinel-type charge-balancing substitution mechanism in pallasite olivine. Additionally, principle component analysis of laterally resolved multi-element concentration data of pallasite olivine provides an independent measure of the genetic relationships between the different main group pallasites within their parent body (−ies). Rare earth element (REE) contents of pallasite phosphate grains range from concentrations typically measured for chondritic or primitive achondrite-like phosphates to more (light) REE-depleted signatures, revealing the overall primitive nature of the melts from which the phosphate minerals crystallized. Phosphate in pallasites may thus have formed through the consumption of apatites during pro-grade metamorphism, melting, and melt extraction. Combined, the trace element signatures of olivine and phosphate in pallasites of the main group (PMG) suggest that these meteorites are common products of planetary processes on chondritic or primitive achondrite-like precursor bodies, with the different subgroups highlighting distinct formation and evolution histories.

In situ S isotope analysis and source tracing of pyrite from lacustrine hydrothermal sedimentary rocks: the Chang 7-3 sub-member, Triassic Yanchang Formation, Ordos Basin

The Ordos Basin developed a large depression-type lake basin in the Late Triassic. Here, we report on pyrites from lacustrine hydrothermal sedimentary rocks that were found at the bottom of a black shale layer from the Chang 7-3 sub-member of the Triassic Yanchang Formation in Tongchuan and Yaoqu and Well Z9 sections, on the southern margin of the Ordos Basin. Observations of field outcrops and underground cores were used to identify vein and brecciated pyrites that are distributed within water-explosive dolomite breccias or sandwiched in a textured layer of hydrothermal sedimentary rock. Based on previous work, scanning electron microscopy observations, electronic probe microanalysis, nano-SIMS trace-element mapping and in situ sulfur isotope analyses were completed. The crystal form of pyrites is mainly cubic with surfaces mostly smooth and clean. Pyrites contain trace elements, such as As, Au, Co, Cu, Cr and Zn. The pyrite sulfur isotope values are enriched in 34S (δ34SV-CDT = 7.31–10.05‰; average 8.49‰) and show that the pyrite is related to hydrothermal deposition. Late Triassic hydrothermal activity at the bottom of the lake provided sulfate for pyrite formation, and hydrothermal sulfide may have also been an important sulfur source. KEY POINTS Pyrites from lacustrine hydrothermal sediments were formed at the bottom of a black shale layer from the Chang 7-3 sub-member of the Triassic Yanchang Formation on the southern margin of the Ordos Basin. The pyrite contains trace elements such as As, Au, Co and Ni. In situ pyrite sulfur isotope values are enriched in 34S and show that the pyrite is related to hydrothermal deposition with sulfur derived from sulfate and hydrothermal sulfide

Monazite geochronology in melt-percolated UHP meta-granitoids: An example from the Erzgebirge continental subduction wedge, Bohemian Massif

Coupled age and trace element mapping in monazite grains using the Laser-Ablation Split-Stream ICP-MS technique was carried out for a sequence of samples from (U)HP crustal section in the Eger Crystalline Complex interpreted as a Variscan continental subduction wedge. The sampled section comprises banded orthogneiss with monazite protolith ages of 486 ± 3[10] Ma. The orthogneiss was heterogeneously transformed to migmatite, HP felsic granofels and U(HP) fine-grained Ky-Kfs granulite during coupled deformation and percolation of silicate melts in a subduction channel. Age of the (U)HP metamorphism and related melt percolation in the granulite is dated by Th-rich and HREE-poor monazite cores at 353.6 ± 2.7[8] Ma, that are in equilibrium with HREE-rich garnets. Th-rich cores of monazites in the migmatite, granofels and granulite that lack the (U)HP signatures show younger ages of 347.8 ± 3.4[8] Ma, 345.8 ± 2.4[8] Ma and 339.3 ± 1.3[7] Ma, respectively. The monazite rims with lower Th and higher HREE contents than the monazite cores are likely a result of (re)crystallization in equilibrium with melt that partly dissolved the HREE-rich prograde garnet during decompression. This (re)crystallization is constrained by ages of 344.2 ± 1.6[8] Ma in (U)HP granulites and 338–340 ± [8] Ma in migmatites, granofelses and granulites without the (U)HP signature. The groups of ages from 353 Ma to 338 Ma, are significantly different when only their in-run uncertainties (7–10 Ma) are compared and indicate a 15 ± 3 Ma period of monazite (re)crystallization from burial/peak to exhumation.

Tracing mineralization history from the compositional textures of sulfide association: A case study of the Zhenzigou stratiform Zn-Pb deposit, NE China

The Qingchengzi Orefield located in northeastern China hosts more than ten Pb-Zn deposits and several Au-Ag deposits. Among those, the Zhenzigou Zn-Pb deposit displays a typical stratiform morphology, comprising ore bodies in layers and lenses. The ore minerals are simply composed of sphalerite, galena, pyrite, and pyrrhotite, together with minor arsenopyrite, marcasite, and argentite. A sulfide association of sphalerite-galena-pyrite-arsenopyrite was selected for this study; the interior compositional texture of the sulfide association was investigated by means of LA-ICPMS spot and mapping analysis, with the aim of constraining the ore genesis, fluid evolution history and element partitioning behavior. The results show that three distinct fluids were involved in the mineralization, namely an episode of sedimentary fluids at ∼2052 Ma which is responsible for the precipitation of pyrite core (Py0), an episode of metamorphic fluids at ∼1800 Ma which formed an overgrowth of pyrite (Py 1) and new crystallization of sphalerite (Sph1) and arsenopyrite (Aspy1), and a final episode of hydrothermal fluids related to the Triassic magmatism, forming another overgrowth of pyrite (Py2), sphalerite (Sph2) and arsenopyrite (Aspy2), together with minor precipitation of galena (Ga). The sedimentary fluids are relatively low in temperature (<150 °C), enriched in Cu, Pb, Zn, Ag, and depleted in Co, Ni, and As. Pyrite is the main precipitated sulfide mineral, scavenging most of the available trace elements. The late metamorphic and magmatic fluids are high in temperature (>300 °C) and have remobilized As, Co and Ni but only at mineral grain scale. The late metamorphic and magmatic fluids are enriched in In, Cd, Au, As, Ag, Cu, Pb, Sb, Sn, Co and Ni, and depleted in Bi, W, Mo, Te, and Se. The partitioning of most of the metals from the fluids into the sulfide minerals was controlled by the metals’ availability in the fluids and by the compatibility with crystal structure through direct or coupled substitutions. Pyrite acts as the main host of Co, Ni, Pb, and As; arsenopyrite hosts a major amount of Sb and Pb, and a minor amount of Au; sphalerite is the primary host of Cd, In, Cu, and the secondary host of Ag and Sn; galena is the primary host of Ag, Sb, Sn, and subordinately hosts Cd, In and As. In particular, a remarkable amount of In and Cd was carried by the hydrothermal fluids associated with the Triassic magmatism, indicating that the contribution of magmatic hydrothermal fluids to the mineralization is significant. In conclusion, our in-situ geochemical evidence of sulfide minerals reinforces a sedimentation-metamorphism-magmatic-hydrothermal reworking model for the Zhenzigou stratiform Zn-Pb mineralization. This case study underlines the significance of compositional textures of coexisting sulfide minerals when assessing the ore genesis. Overall, our study demonstrates that, by integrating trace element mapping and spot analysis on coexisting sulfide minerals, it is possible to re-construct the fluid evolution history, resolve the metal behaviors associated with fluids evolution, and review the contribution of individual fluids to mineralization.

Major and trace element mapping of garnet: Unravelling the conditions, timing and rates of metamorphism of the Snowcap assemblage, west‐central Yukon

AbstractGarnet crystallization has been simulated in the MnNCKFMASHT model system using a simple nucleation and growth scenario, calibrated with three‐dimensional garnet crystal size distribution data as well as garnet compositional data obtained by electron probe micro‐analysis and laser ablation inductively coupled plasma mass spectrometry. Results indicate wide‐spread Barrovian‐type metamorphism for garnet‐zone rocks from the Snowcap assemblage along a hairpin‐shaped pressure–temperature loop with garnet growth from ~515°C and 4 kbar to metamorphic peak conditions of ~600°C and 6 kbar. Lu–Hf garnet‐whole geochronology points to initial garnet growth at c. 192.2 ± 4.7 Ma. Sm–Nd garnet–whole‐rock geochronology applied to a sample with garnet rims enriched in Sm indicates that the metamorphic peak conditions have been attained at c. 172.9 ± 2.4 Ma. Older garnet growth at c. 245.3 ± 0.8 Ma during a low‐P–high‐T event has been preserved as garnet cores separated from the Jurassic garnet rims by a sharp microstructural and compositional discontinuity. These polyphase garnets are restricted to Mn‐rich metapelitic lithologies. Trace element zoning in the outermost ~50 μm thin segments of the Early Triassic garnet cores reflects a short garnet growth episode in the presence of melt at peak conditions of ~710°C and 2.5 kbar, supported by phase equilibrium and diffusion geospeedometry calculations. Diffusion simulations across the interface between the Early Triassic garnet core and the Jurassic garnet rim indicate that the Barrovian‐type metamorphism during the Jurassic lasted for 20–25 Myr, in line with the radiometric data.

Late orogenic gold mineralization in the western domain of the Karagwe-Ankole Belt (Central Africa): Auriferous quartz veins from the Byumba deposit (Rwanda)

Gold mineralization in the KAB (Karagwe-Ankole Belt) of Central Africa dominantly occurs as quartz lode and ferruginous breccia systems. Primary gold mineralization in the KAB is possibly linked to a deformational event in the early-Neoproterozoic which is accompanied by shear movements and post compressional magmatism. The Byumba deposit is chosen due to its position in the KAB Mesoproterozoic metasedimentary stratigraphy (Akanyaru Supergroup), interesting structural trend (NW trending shear zone) and its closeness to magmatic intrusions. Explorative drillings from the Byumba deposit in Rwanda were logged and sampled for further petrographic investigation, in situ Rb-Sr geochronology (Laser Ablation Inductively-Coupled-Plasma Triple Quadrupole Mass Spectrometry) and Micro X-ray fluorescence (µXRF) major and trace element mapping. Distinct phases of folding and shear deformation are present at Byumba and three main phases of quartz veining are identified (pre-, syn-, post-folding). The first quartz vein generation is layer-parallel and folded along the bedding, indicative of a pre-folding origin (V1 generation). A second generation of quartz veins (V2a) is concentrated in fold hinges and related to the primary folding of the layers. Cleavage parallel and often boudinaged V2b quartz veins are emplaced during late-stage folding. Post-folding (V3) massive or sigmoidal quartz veins, crosscut the folds and cleavage. Element mapping of the gold by using µXRF shows that Chlorite-rich quartz veins (V3) host the primary gold mineralization in the form of sub-micron gold and of small disseminated blebs. Cores additionally recorded a supergene enrichment phase of gold at reduction zone boundaries. In situ Rb-Sr geochronology on white mica of the crosscutting post-folding quartz veins (V3) point towards an Early-Neoproterozoic age of ∼900 Ma. The gold mineralization at Byumba is interpreted to be related to the regional early-Neoproterozoic compressional deformation event based on 1) the post-folding timing of the auriferous veins, 2) the observation of shear-related textures both in the host-rock and the veins, and 3) the location of the Byumba deposit within a regional shear structure. This association with compressional shearing points towards an orogenic origin for gold mineralization. However, a magmatic-hydrothermal influence from contemporaneous granitic magmatism cannot be excluded.

New horizons in quantitative compositional mapping – Analytical conditions and data reduction using XMapTools

Quantitative compositional mapping is a key technique for imaging solids and, therefore, for trying to understand compositional zoning systematics in a broad range of geological materials. Compositional mapping requires an adapted analytical method for efficient data collection, a post-processing calibration stage to obtain elemental concentration values, and specific computer tools for exploring and visualising the data. In this contribution, we describe optimal analytical conditions, possible physical limitations and data reduction strategies for compositional maps of major, minor and trace elements obtained by electron probe microanalysis (EPMA) and laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Data reduction is performed using the computer software XMapTools, which incorporates a large variety of options and tools for data processing, data calibration and data visualisation based on single and multi-channel maps or via binary, ternary and spider diagrams.

Sub-millimeter distribution of labile trace element fluxes in the rhizosphere explains differential effects of soil liming on cadmium and zinc uptake in maize

Trace element concentrations in the rhizosphere were quantified to better understand why soil liming often fails to reduce cadmium (Cd) uptake by plants. Maize seedlings were grown on a soil with natural background levels of Cd and zinc (Zn). Soil liming increased soil pH from 4.9 to 6.5 and lowered the soil solution free ion activities by factor 7 (Cd) and 9 (Zn). In contrast, shoot Cd concentrations were unaffected by liming while shoot Zn concentrations were lowered by factor 1.9. Mapping of labile soil trace elements using diffusive gradients in thin films (DGT) in combination with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) revealed an almost complete depletion of Cd in the rhizosphere in all soil treatments, showing that Cd uptake is controlled by diffusion. The flux of Cd from soil to the DGT, with direct contact between the soil and the binding gel, was unaffected by liming whereas it decreased by factor 3 for Zn, closely mimicking the contrasting effects of liming on Cd and Zn bioavailability. This evidence, combined with additional flux data of freshly spiked Cd and Zn isotopes in soil and with modelling, suggests that the diffusive transport of Cd in unsaturated soil is more strongly controlled by the labile adsorbed metal concentration than by its concentration in solution. This is less the case for Zn because of its inherently slower desorption compared to Cd.

Identification of growth mechanisms in metamorphic garnet by high-resolution trace element mapping with LA-ICP-TOFMS

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800–850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core–mantle boundary occurred over a length scale of ~ 200 µm. Garnet in an eclogite from the Sesia Zone, Western Alps (P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P–T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications.

Large S isotope and trace element fractionations in pyrite of uranium roll front systems result from internally-driven biogeochemical cycle

Complex pyrite textures associated with large changes in isotopic and trace element compositions are routinely assumed to be indicative of multi-faceted processes involving multiple fluid and sulfur sources. We propose that the features of ore-stage pyrite from roll front deposits across the world, revealed in exquisite detail via high-resolution trace element mapping by LA-ICP-MS, reflect the dynamic internal evolution of the biogeochemical processes responsible for sulfate reduction, rather than externally driven changes in fluid or sulfur sources through time. Upon percolation of oxidizing fluids into the reduced host-sandstones, roll front systems become self-organized, with a systematic reset of their activity cycle after each translation stage of the redox interface down dip of the aquifer. Dominantly reducing conditions at the redox interface favor the formation of biogenic framboidal pyrite (δ34S from −30.5 to −12.5‰) by bacterial sulfate reduction and the genesis of the U mineralization. As the oxidation front advances, oxidation of reduced sulfur minerals induces an increased supply of sulfate and metals in solution to the bacterial sulfate reduction zone that has similarly advanced down the flow gradient. Hence, this stage is marked by increased rates of the bacterial sulfate reduction associated with the crystallization of variably As-Co-Ni-Mo-enriched concentric pyrite (up to 10,000′s of ppm total trace contents) with moderately negative δ34S values (from −13.7 to −7.5‰). A final stage of pyrite cement with low trace element contents and heavier δ34S signature (from −6.9 to +18.8‰) marks the end of the roll front activity cycle and the transition from an open to a predominantly closed system behavior (negligible advection of fresh sulfate). Blocky pyrite cement is formed using the remaining sulfate, which now becomes quickly heavy according to a Rayleigh isotope fractionation process. This ends the cycle by depleting the nutrient supplies for the sulfate-reducing bacteria and cementing pore spaces within the host sandstone, effectively restricting fluid infiltration. This internally-driven roll front activity cycle results in systematic, large S isotope and trace element fractionation. Ultimately, the long-time evolution of the basin and fluid sources control the metal endowment and evolution of the system; these events, however, are unlikely to be preserved by the roll front, as a direct result of its hydrodynamic nature.

Pyrite chemistry: A new window into Au-Te ore-forming processes in alkaline epithermal districts, Cripple Creek, Colorado

Tellurium has a wide variety of applications, most importantly in the solar energy industry and is eco-toxicologically significant; however, the magmatic-hydrothermal processes causing the pronounced Te enrichment together with Au in some epithermal districts are still poorly constrained. Hydrothermal and alkaline magmatic activity in post-subduction environments are suggested to be a critical component in the evolution of this Te-rich sub-class of low-sulfidation epithermal deposits. Cripple Creek represents an example for a world-class low-sulfidation epithermal Au-Te anomaly in the continental crust. This area represents a natural laboratory to investigate the processes of ore-formation and Au-Te enrichment in alkaline igneous rock-hosted epithermal systems.Here, we present the first micro-analytical approach that combines petrographic observations with in situ LA-ICP-MS analyses and trace element mapping to define the key ore-forming processes of Au and Te in the Cripple Creek epithermal complex. Two main styles of mineralization can be distinguished: (1) low-grade Au disseminated pyrite-rich ores in the permeable brecciated host rocks and (2) high-grade quartz-fluorite veins rich in calaverite (AuTe2), coloradoite (HgTe), petzite (Ag3AuTe2), altaite (PbTe) and native Au.Pyrite trace element mapping revealed distinct variations and decoupling of elements (Au-Te) and element pairs (Au-As vs. Co-Ni) within and between different sites in the epithermal district. This is reflected by the concentric/growth zoning of these elements in pyrite that were interpreted to be caused by fluid boiling associated with the deposition of the low-grade disseminated host rock ores at temperatures between 220 and 350 °C leading to the precipitation of Au, As, Co and Ni from the liquid phase and the preferential partitioning of Te into the vapor phase. The subsequent condensation of the Te-rich vapors in metal-bearing meteoric waters led to the Te precipitation being decoupled from Au in the low-grade ores.The high-grade Au-Te mineralization was likely initiated by the influx of magmatically derived oxidized fluids. Subsequently, fluid temperatures dropped due to mixing with meteoric waters resulting in fluid boiling under low pressure conditions at shallower crustal levels (<1000 m) compared to the low-grade ores. Elevated fO2 minimized the loss of Te to the vapor phase and the strong boiling conditions at lower fluid temperatures (105–200 °C) caused the contemporaneous precipitation of Au and Te in the high-grade veins.

Trace element mapping of high-pressure, high-temperature experimental samples with laser ablation ICP time-of-flight mass spectrometry – Illuminating melt-rock reactions in the lithospheric mantle

Laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) is a fast, relatively high-spatial resolution method which allows to measure and visualize major, minor and trace elements in fine-grained samples. Thus, it is a promising tool for the investigation of high-pressure, high-temperature (HP-HT) reaction experiments that typically produce small grain sizes. Here, we apply LA-ICP-TOFMS mapping to HP-HT reaction experiments by Gervasoni et al. 2017 between a) hydrous eclogite and fertile peridotite, which simulates metasomatism in subduction zones, and b) ultramafic silicate-carbonate melt and peridotite, which simulates metasomatism at the base of cratonic lithospheric mantle. A spatial resolution (i.e., laser spot size) of 5 μm and an acquisition rate of 20 multi-element pixels per second (i.e., laser repetition rate of 20 Hz) were used for mapping of the experimental samples. We demonstrate the applicability of LA-ICP-TOFMS by comparison of our results to maps created with electron probe micro-analysis (EPMA) and elemental concentrations measured by conventional (i.e., crater-drilling) LA-ICP-MS. Our new data suggest that metasomatism in subduction zones is expected to result in the formation of Al–Ti-rich amphiboles that strongly fractionate the rare earth elements (REE) and high-field strength elements (HFSE) from other trace elements. In contrast, metasomatism of the cratonic lithosphere involves fractionation in the light and middle REE, large ion lithophile elements (LILE) and HFSE, leaving Ni and heavy REE behind in olivine and garnet, respectively. The trace element enrichment patterns of the resulting melt bear a general resemblance to those of natural Group I kimberlites.

International GEOTRACES Program: A Japanese Research Vessel Contributes to 3-D Mapping of Trace Elements in the Ocean

International GEOTRACES Program: A Japanese Research Vessel Contributes to 3-D Mapping of Trace Elements in the Ocean

Trace elemental modification in magnetite from high-grade metamorphosed BIFs in the southern North China Craton

Magnetite, as a common oxide mineral in banded iron formations (BIFs) and ore deposits, is an ideal provenance indicator for mineral exploration, and its composition has long been used for genetic studies of ore deposits. However, many ore deposits, particularly the BIFs worldwide, have undergone various grades of secondary metamorphism or hydrothermal alteration. It is still unclear whether the original magnetite composition was modified during the secondary processes and, if so, to what extent the compositions were modified. In this study, we conduct mineralogical and LA-ICP-MS trace elemental investigations on magnetite from the amphibolite- to granulite-facies metamorphosed BIFs in the southern North China Craton. The new results were compared with those of unmetamorphosed and greenschist-facies metamorphosed BIFs worldwide to understand how the original composition of magnetite was modified during different grades of metamorphism. Magnetite grains from the amphibolite- to granulite-facies BIFs have low Cr, Co, Ni and Ga (less than 10 ppm) and slightly variable V and Zn. These elements do not show remarkable changes during high-grade metamorphism when compared with the unmetamorphosed and greenschist-facies metamorphosed magnetite, indicating that these elements in magnetite are immobile during metamorphism. A very narrow range of Fe2+/Fe3+ mole ratios of the high-grade metamorphosed magnetite roughly suggest limited changes of oxygen fugacity during metamorphism, which is also supported by the limited change of Cr and V contents. High Mn contents in these magnetite grains are associated with low Mg contents possibly due to the fact that these elements occupy the same site in magnetite structure. Compared with unmetamorphosed magnetite, the high-grade metamorphic magnetite in the BIFs of the southern North China Craton has elevated Al, Ti and Mn. Such a change of magnetite compositions is mainly controlled by coexisting Fe-Mg silicates that formed during high-grade metamorphism. For example, our new results of trace elemental mapping on magnetite show that the edge of magnetite grains that are in contact with Fe-Mg silicates (e.g., hornblende, grunerite and pyroxene) are remarkably enriched in Mg, Mn, Al, Si, and Na compared to the cores (some elements up to 10 times more). All these features indicate that elemental diffusion and exchange between magnetite and coexisting silicate minerals during high-grade metamorphism have extensively modified original compositions of magnetite from BIFs.