Hg Isotopes and Enhanced Hg Concentration in the Meishan and Guryul Ravine Successions: Proxies for Volcanism Across the Permian-Triassic Boundary

High-resolution organic carbon isotope (δ13C), Hg concentration and Hg isotopes curves are presented for the Permian-Triassic boundary (PTB) sections at Guryul Ravine (India) and Meishan D (China). The total organic carbon (TOC)-normalized Hg concentrations reveal more intense environmental changes at the Latest Permian Mass Extinction (LPME) and the earliest Triassic Mass Extinction (ETME) horizons coinciding with major δ13C shifts. To highlight palaeoredox conditions we used redox-sensitive elements and Rare Earth Element distribution. At Meishan, three Hg/TOC spikes (I, II, and III) are observed. Spike I remains after normalization by total aluminum (Al), but disappears when normalized by total sulfur (TS). Spike III, at the base of Bed 26, corresponds with excursions in the Hg/TS and Hg/Al curves, indicating a change in paleoredox conditions from anoxic/euxinic in the framboidal pyrite-bearing sediments (Bed 26) to oxygenated sediments (Bed 27). At Guryul Ravine, four Hg/TOC spikes were observed: a clear spike I in Bed 46, spike II at the base of the framboidal pyrite-rich Bed 49, spike III at the PTB, and spike IV at the LPME horizon. Some of these Hg/TOC spikes disappear when TS or Al normalization is applied. The spike I remains in the Hg/TS and Hg/Al curves (oxic conditions), spike II only in the Hg/TS curve (anoxic/euxinic), and spikes III and IV only in Hg/Al curves (oxic). In both sections, Hg deposition was organic-matter bound, the role of sulfides being minor and locally restricted to framboidal pyrite-bearing horizons. Positive mass-independent fractionation (MIF) for Hg odd isotopes (odd-MIF) was observed in pre-LPME samples, negative values in the LPME–PTB interval, and positive values above the ETME horizon. Most Hg-isotope patterns are probably controlled by the bathymetry of atmospheric Hg-bearing deposits. The source of Hg can be attributed to the Siberian Traps Large Igneous Province (STLIP). In the LPME-PTB interval, a complex of STLIP sills (Stage 2) intruded coal-bearing sediments. The negative δ202Hg, the mercury odd-MIF Δ201Hg patterns, and the Δ199Hg–Hg plot in both sections are compatible with volcanic mercury deposition. Our study shows the strength of Hg/TOC ratios as paleoenvironmental proxy and as a tool for stratigraphic correlation.

Rare-earth element distribution features in natural-technogenic complexes of some polymetallic deposits of Eastern Transbaikalia

The purpose of the study is to investigate rare-earth elements distribution features in all components of the natural-technogenic complexes of Akatuevsky, Blagodatsky and Novo-Shirokinsky polymetallic deposits of Eastern Transbaikalia. Due to increasing demand for rare-earth elements (REE) in various fields of industry, identification of features of REE distribution in natural-technogenic complexes of polymetallic deposits of Eastern Transbaikalia is relevant. The chemical elements of the REE group include 15 elements, yttrium (Y) and the lanthanoid group consisting of 14 elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). Determination of the elemental composition of samples was carried out by X-ray fluorescence method in the Geological Institute of the Geological Institute of the Siberian Branch of the Russian Academy of Sciences (Ulan-Ude) and by ICP- MS method in the laboratory of SGS Vostok Limited (Chita). In the process of investigations REE concentrations in all components of natural-technogenic complexes of Akatuevsky, Blagodatsky and Novo-Shirokinsky polymetallic deposits (lead-zinc ores, technozems, soils) were determined. Their regular decrease of REE concentrations in the order: soils→technozems→lead-zinc ores was established. In the most of studied samples there is a decrease of heavy lanthanides content relative to light ones, as well as negative europium anomaly, in some samples of lead-zinc ores positive europium anomaly is observed. Europium anomaly magnitude (Eu/Eu*) is an indicator of the degree of differentiation of magmatic melts, determined by plagioclase fractionation processes. It is known that feldspars serve as the main controller of the Europium anomaly. The phenomenon of a negative europium anomaly is observed if plagioclase remains in the source after fractional crystallization or partial melting. Positive europium anomaly in sulphide ores is caused by the presence of barite, as well as by the accumulation of plagioclase in the liquid phase due to the fractionation process. REE is known to have adverse effects on the environment due to its high biological and biochemical activity. Modern methods of remediation of the potential damage to the environment have been proposed

Petrological, geochronological, and geochemical potential accounting for continental subduction and exhumation: A case study of felsic granulites from South Altyn Tagh, northwestern China

Deciphering the formation and geodynamic evolution of high-pressure (HP) granulites in a collisional orogeny can provide crucial constraints on the geodynamic evolution of subduction-exhumation. To fully exploit the geodynamic potential of metamorphic rocks, it is necessary to constrain the metamorphic ages, although it is difficult to link zircon and monazite ages to metamorphic evolution. A good case study for understanding these geodynamic processes is felsic granulites in the Bashiwake area, South Altyn Tagh. Petrographic observations suggest that the studied felsic granulites have suffered multi-stage metamorphism, and the distinct metamorphic events were documented by compositional zoning and high Y + heavy rare earth element (HREE) concentrations in the large garnet porphyroblast. Zircon U-Pb dating yielded two major age clusters: one age cluster at ca. 900 Ma represents the age of the protolith for the felsic granulite, and another age cluster at ca. 500 Ma represents the post-UHT (ultrahigh temperature) stage based on the rare earth element distribution coefficients between zircon and garnet. Meanwhile, in situ monazites U-Pb dating yielded a weighted mean 206Pb/238U age of 482 ± 3.5 Ma, and the monazite U-Pb age was interpreted to be in agreement with the metamorphic zircon rims data, which together with zircon recorded the cooling time after the UHT stage. Whole-rock major and trace elements as well as Sr-Nd isotopes suggest that the protolith of the felsic granulite derived from partial melting of ancient crustal materials with the addition of mantle materials. Integrating these results along with previous studies, we propose that the felsic granulites metamorphosed from the Neoproterozoic granitic rocks, and the granitic rocks with associated mafic-ultramafic rocks suffered a common high-pressure–ultrahigh temperature (HP-UHT) metamorphism and subsequent granulite-facies metamorphism. A tentative model of subduction-relamination was proposed for the geodynamic evolution of the Bashiwake unit, South Altyn Tagh.

Mantle xenoliths from the Komsomolskaya-magnitnaya kimberlite pipe (Upper Muna kimberlite field, Siberian Craton) Evidences of the composition of the SCLM

<p>Peridotite xenoliths from kimberlites provide important information about the composition, structure and thermal regime of the lithospheric mantle of ancient cratons. In this paper, we present the results of mineralogical studies of peridotite xenoliths from kimberlites of the Upper Muna field. The Middle Paleozoic (D3-C1) high diamondiferous kimberlite pipe Komsomolskaya-Magnitnaya was chosen as the object of research.</p><p>We studied a collection of 180 peridotite xenoliths of the Komsomolskaya-Magnitnaya pipe, of which 104 belong to dunite-harzburgite paragenesis, 74 to lherzolite and 4 websterites.</p><p>The chemical composition of basic minerals from xenoliths was determined using JEOL JXA-8100 electron microprobe. Chemical analysis of xenolith garnet compositions was also performed using the Agilent 7700cs LAM-ICPMS method.</p><p>Based on a study of the collection of deep xenoliths, we found that the lithospheric mantle under the Upper Muna kimberlite field is composed mainly of garnet-bearing and chromite-bearing dunites and harzburgites, as well as coarse grained garnet lherzolites.</p><p>The olivine Mg# varies from 88.4 to 94.12%, while the magnitude of the majority (60%) of the studied olivines does not exceed 92% and 30% of olivines have Mg#> 93%. We identified 2 groups according Mg # olivine from xenoliths. Group 1 with “typical” mantle values Mg # 88.39-90.70mol%, it is characteristic for fertile peridotites. And group 2 with highly depleted compositions Mg # 91.20-94.12mol%. A high proportion (~ 30%) of peridotites with high magnesian olivines (Mg #> 93 mol%) indicates the presence of a block of highly depleted rocks in the lithospheric mantle beneath the Upper Muna kimberilte field.</p><p>According to the distribution of calcium and chromium in garnets, 10 out of 35 studied garnets from xenoliths belong to diamondiferous harzburgite-dunite paragenesis. According to the distribution of rare-earth elements, we distinguish two groups of garnets. Group 1 includes garnets with typical rare earth element distribution spectra typical for fertile garnets, and group 2 garnets with S-shaped spectra that are characteristic of garnet mineral inclusions in diamonds. We noted a high proportion of garnets with S-shaped REE distribution spectra (~ 66%), as well as garnets belonging to the harzburgite-dunite paragenesis, it indicate a moderate role of metasomatic changes associated with silicate melts, as well as interaction with carbonatite melts enriched in LREE.</p><p>Using clinopyroxene monomineral thermobarometry, we found that the “diamond” window in the lithosphere mantle beneath the Upper Muna field, at the time of kimberlite magmatism (~ 360 Ma) was significant (about 95 km) and was located at a depth of 125 to 220 km.</p><p>The study was supported by the Russian Science Foundation (grant No. 18-17-00249).</p>

Hydrochemical Characteristics and Evolution Mode of Cold Seeps in the Qiongdongnan Basin, South China Sea

Submarine cold seeps have recently attracted significant attention and are among the most effective indicators of gas hydrate in the oceans. In this study, remotely operated vehicle (ROV) observations, seismic profiles, core sediments, bottom seawater, and fluid vented from cold seeps in the deep-water Qiongdongnan Basin were used to investigate the origin and evolution of cold seeps and their relationships with gas hydrate. At stations A, B, and C, inactive cold seeps with dead clams, cold seep leakage with live clams, and active cold seeps with a rich mussel presence, respectively, were observed. The salinity and Na+ and Cl- concentrations of the cold seeps were different from those of typical seawater owing to gas hydrate formation and decomposition and fluid originating from various depths. The main ion concentrations of the bottom seawater at stations B and C were higher than those at station A, indicating the substantial effects of low-salinity cold seep fluids from gas hydrate decomposition. The Na+-Cl-, K+-Cl-, Mg2+-Cl-, and Ca2+-Cl- diagrams and rare earth element distribution curves of the water samples were strongly affected by seawater. The concentrations of trace elements and their ratios to Cl- in the bottom seawater were high at the stations with cold seeps, suggesting the mixing of other fluids rich in those elements. Biochemical reactions may also have caused the chemical anomalies. Samples of HM-ROV-1 indicated a greater effect of upward cold seep fluids with higher B/Cl-, Sr/Cl-, and Ba/Cl- values. Moreover, the Re/Cl- value varied between fluid vents, possibly due to differences in Re precipitation strength. Differences in cold seep intensity are also believed to occur between areas. The cold seep fluxes changed from large to small before finally disappearing, showing a close connection with gas hydrate formation and decomposition, and influenced the local topography and ecosystems.