Early Cambrian black shales of the Yangtze Block, South China, show anomalous heavy rare earth element (HREE) enrichment, but the mineral hosts and enrichment processes of rare earth elements and yttrium (REY) in organic-rich, redox-stratified black-shale systems remain poorly constrained. We combine TIMA mineral mapping, SEM–EDS observations, and quantitative EPMA analyses with whole-rock major- and trace-element concentrations, total organic carbon (TOC) contents, and Nd-isotope compositions from the Gongxi (GX) and Ganshanpo (GSP) sections of the basal Niutitang Formation. Mineralogical data from three thin sections identify xenotime-(Y) and monazite-(La) as the principal REY-bearing phases observed. Apatite accounts for <0.01 mass% of the mapped mineral assemblages, although a minor fine-grained or unresolved contribution cannot be excluded. Contrasting xenotime textures in GX and GSP suggest different local precipitation and post-depositional modification conditions. HREE-enriched patterns, genuine negative Ce anomalies, and elevated Y/Ho ratios indicate that the precursor REY inventory underwent marine cycling and acquired seawater-like fractionation characteristics, whereas the Ba-corrected GX dataset does not support a dominant high-temperature hydrothermal contribution. Whole-rock εNd(t) compositions provide qualitative constraints on provenance and source mixing, whereas positive ΣREY–lithogenic-element covariation suggests a possible terrigenous influence on the precursor REY inventory but cannot quantify its magnitude or establish detrital hosting of enriched REY. Mo–U systematics and TOC relationships indicate heterogeneous redox-sensitive element sequestration, whereas hydrographic restriction is supported mainly by regional and stratigraphic evidence; V-based ratios provide supplementary constraints. These results support a marine authigenic–diagenetic model in which marine cycling, basin-regulated redistribution, and early-diagenetic phosphate fixation may have contributed to HREE enrichment.
The classical single-site lattice strain model fails systematically for light and middle rare earth elements (LREE and MREE) in zircons from hydrous, phosphorus-undersaturated magmatic systems, predicting concentrations one to three orders of magnitude below observed values. We develop a two-population model in which the bulk partition coefficient is the mole-fraction-weighted sum of contributions from the ideal ZrO8 site and a second, enlarged site generated by REE3+–H+ coupled substitution. Both sites are parameterised by independent Brice parabolas; the fractional occupancy of the defect site, fd, follows a Langmuir saturation function of water content with half-saturation constant H0 = 422 ppm. Initial calibration of defect-site parameters against the 32 published SIMS spot analyses of Vema Fracture Zone oceanic gabbro zircons reported by De Hoog et al. (2014) gives an unconstrained Nd/Lu optimum of r0REE−H = 0.9916 Å, EREE–H = 592.1 GPa, D0REE−H= 344.5 (RMSE = 0.154 log10 units), whereas physically constrained weighted global optimisation, within-grain REE-pattern inversion, and DFT-consistent modest site expansion together indicate a preferred effective defect radius near 0.984 Å with an admissible envelope of ~0.975–0.993 Å. Forward modelling defines a detection threshold in temperature–[H2O] space below which the defect-site signal is unresolvable from geological noise, requiring [H2O]zrc > ~30–80 ppm depending on temperature. The model predicts that in hydrous, P-undersaturated systems the REE3+–H+ mechanism alone can produce Lazrc exceeding 0.1 ppm through genuine lattice incorporation, demonstrating that a fixed La threshold is not a universal inclusion-free criterion but depends on temperature, water content, and melt La concentration. The charge-balance ratio fdCB= (REY–P)/REY, recoverable from routine LA-ICP-MS data without water measurement, provides an independent estimate of defect-site occupancy and correlates with water-based estimates where the REE3+–H+ mechanism is active, thereby providing a quantitative test of mechanism activation across natural systems. The model is applicable specifically to P-undersaturated, hydrous, low-temperature magmatic systems where the REE3+–H+ mechanism has been independently confirmed by FTIR spectroscopy and charge-balance analysis.
The geochemical heterogeneity of the mantle beneath mid-ocean ridges is documented by the variable radiogenic isotope ratios of abyssal peridotites and basalts. Incompatible element-depleted peridotites also occur in ophiolites of variable ages world-wide. The time when peridotites in ophiolite massifs last melted constrains the evolution of the paleo-ocean basin they originate from. Yet, estimating the ages of peridotites in ophiolite massifs becomes significantly more complicated if the strong geochemical heterogeneity documented in present-day abyssal peridotites also characterizes the ancient mantle of ophiolite massifs. Here, we apply a challenging approach, analysing the NdHf isotopic composition of coupled clinopyroxene and orthopyroxene within single samples to calculate dates for the last melting and melt-rock interaction events in peridotites from the Internal Liguride and Monte Maggiore ophiolites of the Alpine Tethys Ocean. This approach avoids the use of a theoretical average ‘Depleted Mantle’. The time of magmatic processes inferred from the new pyroxenes NdHf isotope data ranges from Late Triassic to Middle Jurassic, contemporaneous to the opening and maturing of the Alpine Tethys Oceanic basin. The NdHf isotope ratios of pyroxene couples in peridotites and pyroxenites indicate that MORB-like (on average εNdi 7.5 ± 2.5 and εHfi from 13 ± 4) to slightly more melt-depleted peridotites (on average εNdi 14 ± 3 and εHfi from 19.5 ± 3.5) coexisted in the subridge mantle of the Alpine Tethys Ocean. Finding remnants of peridotites which were variably melt-depleted prior to the embryonic opening of the Alpine Tethys Ocean (Internal Liguride) agrees well with the ubiquitous isotopic heterogeneity of peridotites from modern mid-ocean ridges, which indicates they have melted once, or several times, before recent processing under modern mid-ocean ridges.
This study presents trace-element enrichments, aerosol contents, and emission rates at two Chilean volcanoes with fumarolic activity: Guallatiri and Callaqui. Sampling techniques included filter packs, ground-based remote sensing, and direct sampling, while analytical techniques involved ICP-MS and SEM. Solid particles in fumarolic plumes suggest erosive processes at the vent, whereas rounded sulfur particles agree with temperatures above the melting point of sulfur. However, unique dendritic salammoniac minerals are found, suggesting evaporative concentration of condensed steam droplets during storage. High chalcophile contents, especially for arsenic, indicate high mobility of these elements in fumarolic emissions, in contrast to the poorly to moderately enriched lithophile and siderophile elements. Arsenic emission rates in Guallatiri reach up to 55 kg/yr (increasing up to 270 kg/yr during periods of high SO2 degassing), while Callaqui's rates are <5.9 kg/yr. These values are modest compared to global emission rates from arc volcanoes like Etna, Stromboli, Masaya, and Ambrym. Nevertheless, Guallatiri and Lascar, the latter with ~110 kg/yr, can be regarded as significant sources of pollution in the Central Andes. Current and past degassing of Andean volcanoes, along with the circulation of hydrothermal solutions through the upper crust, are hypothesized to drive As contamination in this area. Magmatic (magma composition, sulfide saturation) and hydrothermal (variable water/rock ratio in the hydrothermal reservoir) processes are postulated to control trace-element distributions in fumarolic volcanoes. Future research should quantify the total budget of trace elements released by Central Andean volcanoes and deepen understanding of the physicochemical processes governing their transport and dispersal in the surrounding environment.
The environmental fate of heavy metals is strongly influenced by ferrihydrite (Fh) transformations. However, the influence of coexisting organic ligands on heavy metal mobility remains poorly understood, due to their complex and multifunctional effects on heavy metal speciation and transformation pathways. Here, we investigated how oxalic acid (Ox) affects Zn2+, Cu2+, and Pb2+ fate during Fh transformation. In binary Fh-metal systems, metal mobility was primarily governed by ionic compatibility and mineral crystallography: Zn2+ (0.74 Å) and Cu2+ (0.73 Å) were less mobile and showed greater retention in hematite (Hm), whereas Pb2+ (1.19 Å) exhibited greater release, mainly retained in goethite (Gt). In ternary Fh-Ox-metal systems, Ox reversed these trends through two-stage control over metal speciation and mineral binding. At the Fh stage, Ox decreased metal-mineral affinity for Zn2+/Cu2+ through electrostatic repulsion of bioxalate complexes while enhancing Pb2+ binding via ternary surface complexation. During transformation, Ox reduced Hm's surface energy from 350 to 220 mJ·m−2, increasing nucleation rates ~85-fold over Gt. This kinetic advantage amplified affinity differences, leading to enhanced Zn2+/Cu2+ mobility but strong Pb2+ immobilization through coupled surface complexation and structural retention. These findings establish a mechanistic link between molecular-scale interactions, mineral crystallization kinetics, and heavy metal dynamics, providing insight into ligand-driven processes that govern heavy metal fate in natural and engineered environments.
