查看更多>>摘要：Remobilization of upper crustal crystal-rich magma reservoirs ("crystal mush") following melt extraction has been widely invoked in the formation of crystal-rich erupted products. However, the melt storage duration and conditions, and the pre-eruptive magma chamber processes remain poorly understood. Here, we present new whole-rock elemental and Sr-Nd-Pb isotope data, and zircon U-Pb-Hf isotope and trace element data of the Nageng (sub-)volcanic complex in the East Kunlun Orogen of NW China. This complex consists of the highlyevolved (high SiO2, Rb/Sr) crystal-poor rhyolite and less-evolved crystal-rich rhyodacite and dacite porphyry. These rock types are genetically linked, based on their similar chondrite-normalized REE patterns and Nd-Hf isotopes, and their linear correlation trends of (208)pb/(204)pb and (207)pb/(204)pb vs. (208)pb/(204)pb. Their depleted whole-rock epsilon(Nd) (t) (-9.74 to -7.37) and zircon epsilon(Hf)(t) (-7.16 to -3.48) values, together with their Proterozoic two-stage Hf model ages (1707-1474 Ma), are indicative of an ancient lower crustal magma source. The overlapping zircon ages but distinct composition (74-80 vs. 60-66 wt% SiO2) and crystallinity (similar to 5 vs. 30-40 vol%) between the crystal-poor rhyolite and dacite porphyry can be explained by melt extraction from the crystal mush zone. The lack of resorption texture and large age interval (similar to 8 Myr) between the zircon core (ca. 228 Ma) and rim (ca. 220 Ma) indicate that the crystal mush remained partially molten for protracted duration. The melt storage condition was constrained by Ti-in-zircon thermometry to be low-temperature, around the granitic solidus (650-700 degrees C). The crystal-rich nature and slightly negative Eu anomalies suggest that the dacite porphyry was formed in the roof zone (solidification front) of the crystal mush. The occurrence of resorbed zircon core (ca. 220 Ma) overgrown by much younger rim (ca. 213 Ma) for the crystal-rich rhyodacite demonstrates long-term rejuvenation of a semi-solidified crystal mush. The similar zircon core-rim Ti contents and Th/U, Yb/Gd, Zr/ Hf ratios preclude that the crystal mush was rejuvenated by hotter, more mafic magmas. However, the widespread fluorite-rich magmatic-hydrothermal veining, combined with the reduced nature (indicated by the zircon Ce4+/Ce3+ drop from core to rim) of the crystal-rich rhyodacite, imply that the volatile-rich fluid influx may have been important in reactivating the crystal-mush for later crystal-rich rhyodacite eruption.
查看更多>>摘要：Radon-222 (222Rn) is routinely used as a tracer of surface water (SW) - groundwater (GW) interactions. However, methods that rely on this tracer to quantify SW-GW exchange fluxes, GW residence time, SW-GW mixing ratios, or GW flow velocities usually depend on the assumption of spatially constant 222Rn production within aquifers and/or hyporheic sediments. In the present study, we measure the natural variability of 222Rn emanation rates in a highly instrumented alluvial aquifer and underline some important issues in assuming homogenous 222Rn production in alluvial systems. A robust experimental setup was used to determine 222Rn emanation rates from sediments. High-resolution measurements reveal that 222Rn emanation rates are highest within the first few meters below the surface. These results have significant implications for the interpretation of 222Rn activities in GW, notably in the vicinity of infiltration zones, as changes in hydraulic conditions can lead to variations in 222Rn signatures that can only be resolved with proper knowledge of the spatial distribution of 222Rn production rates. In such contexts, the assumption of constant 222Rn production leads to an inadequate understanding of SW infiltration patterns, and biased 222Rn-based estimates of SW-GW exchange.
查看更多>>摘要：Dehydration of the subducted slab is a crucial process in the generation of arc magmas. However, the mineralogical controls of slab dehydration remain uncertain. Thallium (Tl) isotopes have emerged as tracers of sub ducted slab components, but the Tl isotope characteristics of slab fluids are not known. High-pressure metamorphic veins and host eclogites in the Dabie terrane (China) provide important information on the composition and evolution of subduction zone fluids. In this study, we present the first Tl concentration and Tl isotope data for two high-pressure metamorphic epidote-rich veins formed through breakdown of lawsonite, as well as host eclogites and phengite separates, from the Ganghe and Hualiangting areas. The high Tl concentrations in phengite separates and identical Tl isotope compositions between phengites and their host bulk eclogites demonstrate that phengite controls the overall inventory of Tl in the host eclogites. Generally, the vein samples from both areas have much lower Tl concentrations than their host eclogites, suggesting that the fluids from which the veins precipitated were derived from phengite-bearing eclogites that retained Tl in the residue. The Ganghe eclogites display similar Tl isotope compositions compared with Ganghe omphacite-epidote vein. This similarity indicates that the vein-forming fluid was likely derived from the host eclogites, and that Tl isotopes did not fractionate during fluid-release. Three generations of veins in the Hualiangting area record variable Tl isotope values and were likely derived from multi-stage dehydration of heterogeneous host eclogites, some of which may have contained a sediment component. We argue that fluids derived from eclogite dehydration are likely characterized by higher Cs/Tl ratios compared to their host eclogites. Arc magmas that display high Cs/Tl (i.e. >15) that cannot be caused by variations in slab source composition, coupled with relatively low Ba/Th ratios, are likely to directly record the presence of residual phengite at subarc depths.
查看更多>>摘要：To study the incorporation of magnesium (Mg) and strontium (Sr) in calcite precipitated over synthetic calcite seeds (overgrowths) as a function of the precipitation rate (R*, mu mol/m(2) h), we performed precipitation experiments wherein temperature and precipitation rates were decoupled at intervals of approximately 3.63-5.22 mu mol/m(2) h. In most sample reactions, high-magnesium calcite (HMC) overgrowths co-precipitated with aragonite from the stirred solutions exposed to an atmosphere of NH3 and CO2 gases throughout the spontaneous decomposition of (NH4)(2)CO3. The percentage of aragonite in the solid CaCO3 increased with both temperature and dissolved inorganic carbon (DIC). The order of reaction with respect to the [DIC] is temperature dependent and is 1.9, 2.4, and 2.9 at temperatures of 12.5, 25.0, and 37.5 degrees C, respectively. The magnesium distribution coefficient (D-Mg) increases significantly with increasing R*, temperature, and Mg/Ca ratio in the fluid. The strontium distribution coefficient (D-Sr) increases with R* and with increasing MgCO3 concentrations in the calcite overgrowths. However, it is independent of temperature.
查看更多>>摘要：Bacterially-induced sequestration of atmospheric CO2 is at the forefront of geomicrobiological research due to high potential of this process in the mitigation of climate warming. Cyanobacteria have been known to form stromatolites since the Precambrian and could be used to enhance this process by sequestering carbon via the biomineralization of Mg and Ca carbonates. Currently, olivine (MgSiO4) is considered as one of the most efficient silicate minerals suitable for CO2 capture in the form of secondary Mg carbonates. However, the role of dissolved Si on the efficiency of biomineralization is not sufficiently well understood. The present study intended to reproduce in the laboratory the processes of biomineralization by Synechococcus sp. cyanobacteria extracted from modern stromatolites in a carbonate- and Mg-bearing medium containing various Si concentrations, in order to characterize the rates and stoichiometry of reactions as well as mineralogical nature of precipitates. The results demonstrated the dominant role of cyanobacterial metabolism in the precipitation of carbonate minerals by increasing the pH of the medium via photosynthesis and providing a template in the form of cell walls and their EPS for mineral nucleation. Transmission electron microscopy and other microscopic and spectroscopic observations and analyses identified magnesium carbonates and silicates, such as nesquehonite (MgCO3 center dot 3H(2)O) and/or hydromagnesite (Mg-5(CO3)(4)(OH)(2)center dot 4(H2O) together with amorphous analogue of sepiolite (Mg4Si6O15(OH)(2)center dot 6H(2)O) as dominant precipitated minerals. Apparent inorganic C precipitation rates were not affected by the concentration of Mg and Si in the initial solution. However, the carbon sequestration potential was 20-40% higher in the presence of Si. Overall, the experimental approach developed in this study allows efficient reproduction of combined Mg hydroxy-carbonate and hydrous silicate precipitation under cyanobacterial activity and helps to constrain optimal conditions of cyanobacteria-induced CO2 sequestration.
Bian, LeiboChappaz, AnthonySchovsbo, Niels HemmingsenNielsen, Arne Thorshoj...
9页
查看更多>>摘要：The late Cambrian witnessed significant biological and environmental changes. However, the main triggers responsible for that formidable turnover remain largely unknown. This study investigates mercury (Hg) enrichments from the late Miaolingian to Furongian section in a unique Alum Shale drilled core obtained from southeast Sweden. Our results show that the background level of Hg is relatively high because of the presence of reducing conditions and continuous Hg supply from water mass. However, our most spectacular findings are the presence of four Hg anomalies. The most pronounced Hg anomalies are characterized by the high values in total Hg (similar to 900 ppb) and Hg/TOC ratio (similar to 80), as well as specific signatures of mass independent fractionation of Hg (MIF5; Delta Hg-199, Delta Hg-201, and Delta Hg-200) that we attribute to volcanic-derived Hg. This Hg loading event occurred immediately after the globally recognized Steptoean Positive Carbon Isotope Excursion (SPICE) event and could be attributed to volcanic activities during that period.
NSTL
Elsevier