Chemical Geology最新文献

The adsorption of soil dissolved organic matter (DOM) by minerals is crucial for long-term carbon storage and has been extensively studied. However, the variations in soil DOM composition resulting from adsorption by different minerals have rarely been investigated from a spectral perspective. This study examined the impact of non‑iron (kaolinite) and iron-bearing (ferrihydrite) minerals on the concentration and characteristics of DOM from dark brown soil during a 60-h-long incubation experiment. Soil dissolved organic carbon was removed more efficiently by ferrihydrite (66.2 %) than by kaolinite (14.8 %). UV–Vis spectroscopic analysis indicated that DOM with high aromaticity and molecular weight exhibited a strong affinity for both ferrihydrite and kaolinite. The fluorescence of soil DOM decreased in the ferrihydrite treatment, with a notable reduction in humic-like fluorescent components identified through fluorescent excitation-emission matrix coupled parallel factor analysis. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that carboxyl groups were critical in DOM adsorption on ferrihydrite, participating in ligand exchange, hydrogen bonding, and electrostatic attraction. This study highlights the important role of iron-bearing minerals in soil carbon sequestration and demonstrates how different mineral alter soil DOM composition, potentially influencing the cycling of organic carbon in terrestrial systems.

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ^98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ^98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ^98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ^98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ^98/95Mo (< −0.32 ‰). The basalts display low δ^98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high ⁸⁷Sr/⁸⁶Sr_i (0.7046 to 0.7059), TiO₂, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (⁸⁷Sr/⁸⁶Sr_i = 0.7035 to 0.7043, chondritic-like δ^98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO₂, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ^98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.

In this study, we make a quantitative assessment on the volatile flux of mantle-derived fluids and source of volatiles to explain the geologic controls on the transport of volatiles within thermal fluids of northeastern Anatolia. In line with this objective, we collected 22 samples (gas and water phase) from 16 geothermal fields in NE Turkey covering an area of nearly 100,000 km² that extends from the eastern termination of North Anatolian Fault towards the Georgian and Armenian borders. The ³He/⁴He ratios of the samples (R) normalized to the atmospheric ³He/⁴He ratio (Ra = 1.4 × 10⁻⁶) vary from 0.31 to 7.15, and are considerably higher than the crustal value of 0.02Ra. Regarding spatial distribution of helium isotope composition, samples collected from areas of tectonic unrest around the Erzincan and Erzurum pull-apart basins in the southern part are represented by a higher range of ³He/⁴He ratios (>5 Ra) than those in volcanic areas to the east and northeast of the region. δ¹³C_CO2 values of the gas samples varying from −20.76 to 5.43 ‰ are about 4–5 ‰ lower than δ¹³C_DIC values of the water samples that range from −16.90 to 8.85 ‰, indicating CO₂ removal from waters by degassing. CO₂/³He ratios of gas samples falling in the range of 3.81 × 10⁹ to 2.83 × 10¹² imply that carbon is derived from the mixing between the crustal lithologies and mantle, the latter having a contribution up to 40 %. The maximum flux of mantle-derived fluids is found 88 mm/a at Erzincan (eastern part of the North Anatolian Fault Zone). This value is higher by a factor of about 10 than the western part of the fault zone. Calculations show that degassing from magmatic bodies is the sole mechanism to explain the high helium isotope compositions in the region. Therefore, we suggest that mantle-derived He contents might be due to extensive melting associated with active tectonism. The ³He/Heat ratios of thermal waters in northeastern Turkey were calculated in range of 0.25 to 29.7 × 10⁻¹⁵ cm³(STP)/J, consistent with the data reported for waters along the North Anatolian Fault, indicating a crustal heat source for the geothermal systems in northeastern Turkey.

Evaporites have recently been suggested as a potential archive for recording the Mg isotope compositions (δ²⁶Mg) of coeval seawater. However, episodic dolomitization during the deposition of massive evaporites could cause considerable Mg removal and isotopic fractionation. To constrain the hydrological changes and influence of dolomitization on ambient brine δ²⁶Mg, we present petrographic and mineralogical features, as well as Mg-C-O-Sr isotope data extracted from carbonate phases of a middle Triassic (ca. 247 Myr) marine anhydrite-dolostone sequence from a drill core in eastern China. The drilled lithologies are characterized by massive dolostone layers in the lower part, followed by an upward decline in dolomite contents accompanied by a rise in anhydrite. Multiple lines of evidence consistently point to a syn-depositional origin for the dolostones in a marginal basin of the Tethys Ocean and a lack of diagenetic alteration since deposition. We reconstructed the dynamic changes of δ²⁶Mg values of basin waters based on Mg isotope compositions in dolomite leachates. According to our findings, the δ²⁶Mg values of the basin waters were remarkably high (about 0.38 ± 0.05‰) at the onset of evaporation, indicating a significant Mg sink of the massive dolomitization. The δ²⁶Mg of brine in the basin then changed towards the value of coeval seawater (about −0.32 ± 0.05‰) starting with the deposition of evaporites. Concurrently, ⁸⁷Sr/⁸⁶Sr ratios of dolomites shift from radiogenic values towards contemporaneous seawater composition. Our results demonstrate that the evaporite basin was not strictly restricted, and water exchange with the open ocean never ceased. Modeling calculation reveals that, even when the seawater exchange rate is far below the average ocean circulation, δ²⁶Mg of brine in the basin will reach the value of open ocean within 1 Myr, completely removing the influence of early dolomitization. We suggested that massive marine evaporite sequences have the potential to record the long-term evolution of seawater Mg isotopes.

Nickel and cobalt are potentially critical metals in many countries because of their great significance for national security and economic development, and they are mainly derived from magmatic Ni-Cu‑platinum-group element (PGE) sulfide ore deposits hosted in mafic-ultramafic intrusions. Although Cu isotopes have been used to trace the metallogenic processes in Ni-Cu-(PGE) sulfide deposits, the Cu isotopic fractionation mechanism during magma generation and evolution in convergent tectonic settings is still debated. The Xiarihamu magmatic NiCu sulfide deposit, located in the East Kunlun orogenic belt, northern Tibetan Plateau, is the world's largest known magmatic NiCu sulfide deposit in an orogenic setting. Here we report the whole-rock element and sulfide CuS isotope compositions of samples from the Xiarihamu deposit. The δ⁶⁵Cu and δ³⁴S values of the sulfide grains from the massive, heavily disseminated, and disseminated sulfide ores range from 0.19 ‰ to 0.79 ‰, and from 4.2 ‰ to 9.4 ‰, respectively. Most of the samples have δ⁶⁵Cu values higher than normal mantle values, except for two samples with δ⁶⁵Cu values within the mantle composition range. The whole rock S/Se ratios range from 3200 to 22,500, and the Cu/Pd ratios range from 330 to 820,000, which are also mainly higher than the corresponding mantle values. Modeling calculations of the δ⁶⁵Cu, S/Se, and Cu/Pd values reveal that the sulfide liquid to silicate melt mass ratio (R factor) is not the main reason for the observed δ⁶⁵Cu variations in the Xiarihamu deposit. The variations in the Cu/Pd ratios of the whole-rock samples, and the sulfide δ⁶⁵Cu and δ³⁴S values with depth indicate that sulfide segregation and crustal contamination can reasonably jointly produce the Cu isotope variations in the Xiarihamu deposit, and the variations of samples from different parts of the deposit are caused by variations in these controlling factors. Therefore, Cu isotope fractionation in convergent tectonic settings is mainly caused by magma evolution. The complicated controlling factors of the Cu and S isotopes indicate that the correlation between δ⁶⁵Cu and δ³⁴S values may not be helpful in evaluating the metallogenic potential of Ni-Cu-(PGE) sulfide deposits. Cu isotopes can be used to ascertain the migration path of magmas.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is the key biogeochemical process at marine seeps, seafloor environments sustaining lush chemosynthesis-based life. While an extensive molecular record of SD-AOM has been established for Cenozoic and Mesozoic seeps, to date only one reported case of SD-AOM exists for the Paleozoic. To get new insight into the dominant biogeochemical processes at Paleozoic seeps, a detailed lipid biomarker study was conducted on post-glacial early Permian seep carbonates from Western Australia. The encountered biomarker inventory comprises two diagnostic isoprenoid hydrocarbons with low δ¹³C values: mixed phytane and crocetane (−124 to −110‰) and 2,6,10,15,19-pentamethylicosane (PMI; −128 to −102‰), compounds known to be produced by anaerobic methane-oxidizing archaea (ANME). Other known biomarkers of ANME like glycerol dibiphytanyl glycerol tetraethers (GDGTs) and sn2-hydroxyarchaeol are not preserved in the Permian seep deposits despite the low to moderate thermal maturity of the Paleozoic limestones. Still, degradation products of these compounds including biphytanes and phytane, respectively, yield δ¹³C values (biphytanes: −117 to −111‰) typical of ANME lipids. The combined phytane/crocetane peaks show similar ¹³C depletion as other ANME lipids, suggesting a derivation of the precursor lipids of phytane from ANME. Among the detected lipids, biomarkers of sulfate-reducing bacteria, the syntrophic partners of ANME in SD-AOM, include the ¹³C-depleted terminally branched fatty acids iso- and anteiso-C_15:0 and -C_17:0 as well as iso- and anteiso-alkanes with 15 and 17 carbons (δ¹³C values: −97 to −63‰), the latter representing probable degradation products of fatty acid and bacterial mono- and diether precursors. ANME-derived lipids (phytane and PMI) are recognized as organic sulfur compounds (OSCs) in the free hydrocarbon fraction, comprising thiolanes, thianes, and thiophenes. The ANME-derived OSCs are accompanied by sulfurized alkanes with 16 and 18 carbons (δ¹³C values: −83 to −79‰), tentatively interpreted to derive from unsaturated glycerol ester or ether lipids synthesized by seep-dwelling sulfate-reducing bacteria, while a derivation of these compounds from sulfide-oxidizing bacteria can neither be substantiated nor excluded. We suggest that OSCs formed in the shallow sedimentary subsurface during early diagenesis, reflecting fast entombment and preservation in authigenic carbonates. Rapid OSC formation was probably caused by (1) the presence of excess hydrogen sulfide, which derived from SD-AOM and (2) the scarcity of reactive iron. The studied Permian seep limestones of Western Australia expand our knowledge of the biogeochemical processes at Paleozoic seeps and provide a unique example of how early sulfurization of organic compounds may aid the preservation of bi