Geochemistry Geophysics Geosystems最新文献

We present the first documented occurrence of the Laacher See tephra (LST) in the Eastern Alps, identified in lake Plansee, Austria. The LST is a key chronostratigraphic marker for correlating and dating Late Glacial sedimentary archives. This discovery was made by progressively narrowing down observational limits from rough estimates of potential cryptotephra presence and position to pinpointing the volcanic ash layer with great precision, using a rapid, non-destructive scanning workflow that integrates core-scanning techniques (CT, MS, XRF) with targeted high-resolution methods (μ-XRF, EPMA). This core-scanning workflow bridges the gap between rapid scanning and detailed analysis of discrete sediment samples. Notably, the detection relies not only on glass shards but also on associated tephra mineral phases, demonstrating the method's effectiveness in chemically altered, glass-poor sediments and more unconventional settings. The presence of LST in alkaline sediments of Plansee indicates a more extensive and spatially variable tephra dispersal than previously mapped, highlighting the need for a re-evaluation of established fallout models for the 13 cal ka BP eruption. This finding emphasizes the potential of sediment archives in more challenging sedimentary records (e.g., alpine lakes, paleo-lakes, alkaline lake sediments) to host cryptotephra layers and underscores the value of incorporating mineral-based detection strategies into tephrochronological workflows. By refining regional chronologies and expanding the spatial scope of eruption impact assessments, this work contributes to a deeper understanding of Late Glacial environmental changes within a precise temporal framework and paves the way for advancements in future volcanic risk assessments.

Stromatolites archive critical information on Precambrian marine environments, but their geochemical signals are often obscured using complex diagenetic processes. Tonian stromatolites from the Weiji Formation, North China, show selective dolomitization in dark stromatolitic laminae, forming zoned dolomites. The zoned dolomite crystals with well-preserved growth zoning record the evolution of diagenetic fluid, yet their genesis remains controversial. To constrain selective dolomitization processes and zoned dolomite formation in stromatolites, and further assess their reliability as paleoceanographic proxies, we use LA-ICP-TOF-MS elemental mapping to reveal crystallization mechanisms and early diagenetic controls in stromatolitic carbonates. Results show distinct geochemical distribution patterns between dark and light stromatolitic laminae, reflecting complex microbial-induced diagenetic reactions during early diagenesis. Fe is preferentially concentrated in dark stromatolitic layers, while Mn anomalously accumulates in light layers (with Mn/Fe ratio up to ∼2). Meanwhile, high terrigenous-indicative elemental contents (Al-Si-K) are observed in dark laminae and the adjacent clay-rich matrix. The anomalous Fe-Mn distribution is attributed to the redox oscillations and terrigenous pulses (ROTP) model. Selective dolomitization proceeds through an Ion-Exchange Motor mechanism during penecontemporaneous to early diagenesis, where Mg²⁺ derived from seawater undergoes dehydration via clay mineral adsorption and/or microbial mediation. Within ferroan zoned dolomite, elemental zoning exhibits alternating Mn-enriched bands and Fe-rich zones, indicating redox-controlled diagenesis with preferential Mn(IV) reduction prior to Fe(II) mobilization. While Precambrian stromatolites remain valuable proxies for paleo-ocean chemistry, our results emphasize the critical need for in situ analytical approaches to distinguish primary signals from diagenetic overprints in Precambrian carbonate systems.

The formation, storage, and evolution of granitic magmas are fundamental processes driving the growth of continental crust. While traditionally attributed to crystal fractionation in high-melt fraction magma chambers, the model invoking low-melt fraction crystal mushes has gained wide acceptance. However, the chemical and textural impacts of crystal mush rejuvenation remain elusive and the precise petrological record is relatively poorly studied. The rapakivi K-feldspar identified in the early Eocene monzogranitic porphyry of the Caina intrusive complex, Gangdese batholith, is an ideal candidate for investigating these issues, as feldspar can record clues to magmatic processes. Field survey, optical and mineral flake scanning observations, X-ray fluorescence analysis, in situ Sr and mineral Sm-Nd isotopic analyses, TESCAN integrated mineral analysis, electron probe microanalysis, and three-dimensional crystal shape modeling were performed on the collected samples. K-feldspars can be divided into three types based on chemical zonation: normal, reverse, and oscillatory zoning crystals. Varying isotopic signatures between the K-feldspar and associated mantle suggest that the rapakivi texture originated in heterogeneous magmatic pulse recharge. Crystal shape modeling of the plagioclase chadacryst, mantle, and matrix plagioclase, combined with compositions, indicates that mantle plagioclase originated from the quenching of recharge magmas. We propose a model for the formation of rapakivi K-feldspar and the rejuvenation of crystal mush. Repeated hot magma pulses recharged the mush, triggering magma convection and thermal perturbations. This process enabled the prolonged growth of K-feldspar megacrysts, which were subsequently capped by plagioclase, resulting in the formation of the rapakivi texture.

Understanding the transition from oceanic to continental subduction is critical for reconstructing the geodynamic evolution of orogens and constraining ancient plate boundaries. The Sulu orogenic belt in eastern China was formed by Triassic deep subduction of the South China Block (SCB) beneath the North China Block (NCB). Its architecture was reformed by multi-phase exhumation of high-pressure (HP) to ultra-high-pressure (UHP) metamorphic rocks, obscuring the early syn-convergence process. The Stenian to Tonian Wulian group—a non-(U)HP tectonic unit—likely records the geodynamic process preceding deep continental subduction and is a key to understanding the transition from oceanic to continental subduction. Its debated tectonic affinity (SCB vs. NCB) further constrains the location of the plate boundary. We integrate structural, EBSD, and geochronological and geochemical investigations on the Wulian meta-sediments. This group comprises a lower amphibolite-facies unit and an upper greenschist-facies unit. Detrital zircon U–Pb ages and Lu–Hf isotopic data indicate its affinity for SCBs. Our structural analysis reveals a Late Permian–Early Triassic (ca. 260–250 Ma) norma-sense shearing, with top-to-the-NNE kinematics accommodating the southward extrusion of the Wulian group. Deformation temperatures were 400–500°C at the lower unit and 280–400°C at the upper unit. By comparison with tectonic events of HP–UHP units, we suggest that the Wulian group was decoupled from subducting SCB during oceanic slab break-off (ca. 260–250 Ma), while the trailing continental crust continued to subduct and experienced HP–UHP metamorphism. This model implies that the NCB–SCB plate boundary lies north of the Wulian group.

Understanding the pressure conditions of the formation of tonalite-trondhjemite-granodiorite (TTG) series rocks is important, as these rocks are key constituents of the Archean continental crust. However, the detailed pressure history of Archean TTG gneisses remains ambiguous due to uncertainties surrounding their sources and the effects of fractional crystallization. To address this problem, we developed a machine learning-based tool to predict the melting pressures of Archean TTG gneisses from their major element compositions. This method provides high accuracy, supports batch processing, and incorporates the effects of fractional crystallization, mineral segregation, and other factors, making it suitable for application to global data sets. Our results indicate that the Earth's oldest rocks record two key transitions from low to medium pressure conditions, suggesting that the formation of the earliest continental crust may not have required deep subduction-related geodynamic processes. High-pressure rocks and melting conditions first emerged around 3.8 Ga (up to 1.5 GPa), becoming more extensive globally after 3.5 Ga (up to 1.9–2.0 GPa). Fractional crystallization, source, and other petrogenetic factors significantly influence the identification of high-pressure TTGs, suggesting that true global high-pressure TTGs may make up only ∼8% of the total rock record, much lower than earlier estimates. Our findings suggest that after 3.5 Ga, increased crustal rigidity likely facilitated widespread high-pressure melting and plate tectonic activity across numerous terranes. This research provides new insights into Archean geodynamic processes and demonstrates the power of machine learning in advancing our understanding of ancient Earth systems.

During subduction, the downgoing oceanic crust is exposed to high temperatures in the mantle wedge, causing volatile-bearing minerals to break down and release hydrous fluids into the forearc. These fluids percolate upwards, reacting with the mantle wedge to form hydrated ultramafic lithologies, including serpentinite. To accurately track the fate and impact of water on the forearc, we develop time-dependent models that self-consistently capture both serpentinite ingrowth and the associated rheological weakening of the plate interface. Unlike many subduction models that investigate forearc serpentinization and prescribe plate velocities, geometries, or steady-state conditions, our approach allows plates to evolve dynamically without predefined velocities or geometries. During subduction infancy, serpentinite accumulates rapidly. As subduction matures, serpentinite ingrowth decreases, and more serpentinite is also dragged downward by the slab. To elucidate the links between subduction dynamics and serpentinization, we consider variations in serpentinite strength and hydration state of the incoming plate. Subducting fully water-saturated sediments yield ∼3× greater forearc serpentinite than within the moderately hydrated reference case. The water-saturated case produces a weaker interface and, in turn, subduction zone convergence rates ∼40% higher than in an endmember case with anhydrous sediment. A lower serpentinite strength also produces higher convergence rates despite more downdragging of serpentinite from the forearc. We find that hydrous sediments not only lubricate the interface directly by weakening it, as previously suggested, but also by dehydrating and releasing water that produces weak serpentinite in the mantle wedge, with such feedback only able to be captured within fully coupled dynamic models.

The northern Sierra Nevada batholith was emplaced into and across a series of accreted crustal belts that vary considerably in their ages and lithologies. Unlike batholithic segments to the south, the northern Sierra comprises smaller, spatially distinct plutons where geologic relations with the host basement can be observed. Intermediate to felsic plutons were sampled as arc-perpendicular transects at the latitude of Lake Tahoe and zircon Lu-Hf and trace element analysis was performed in order to assess the relative impacts of temporal and spatial variability of arc magmatism on zircon geochemistry. Trends through time in the Hf data are complex, whereas there is an abrupt step from juvenile values in plutons intruding western belts (+12.3 to +14.4) to more evolved values in those intruding the Northern Sierra terrane to the east (−0.6 to +5.2). A similar pattern is observed in several zircon trace element signatures, including pronounced steps toward higher U/Yb, Dy/Yb and Ce/Y from the western belts into the Northern Sierra terrane to the east. The step is approximately coincident with the Feather River terrane, which is interpreted to mark the suture between the oceanic lithosphere to the west and the North American continental lithosphere to the east. The observed links between variation in zircon Lu-Hf and trace element concentration and basement domain indicate that northern Sierran zircons incorporate, and are sensitive to, the crustal tracts into which they are emplaced. Preliminary application of our results to provenance analysis of Great Valley strata indicates changing provenance through time in the adjacent forearc.

Hydrothermal systems at mid-ocean ridges (MORs) mediate the transfer of heat and geochemical fluxes from the mantle to the hydrosphere, facilitating fluid-rock interaction and metal cycling. While short-term hydrothermal dynamics are well studied, the long-term response of these systems at slow-spreading ridges to glacial-interglacial sea-level change remains poorly constrained. Here, we measured Pb isotope ratios and trace metal concentrations (Co, Ni, Cr) in authigenic Fe–Mn oxyhydroxide coatings from sediment core SSD77-GC03, recovered from the Carlsberg Ridge, to reconstruct hydrothermal activity over the past 49 kyr. Pb isotope compositions of basalt fragments were also analyzed to constrain the local hydrothermal end-members. The Fe–Mn coating data delineate six discrete hydrothermal events (HY1–HY6), revealing two distinct modes of sea-level forcing. Events HY4–HY6 (∼46–36 ka) follow the Marine Isotope Stage (MIS) 4 sea-level lowstand and reflect a delayed, melt-driven hydrothermal response, consistent with decompression-induced mantle melting and subsequent magma ascent. In contrast, events HY1–HY3 coincided with rapid sea-level falls during the MIS 3–2 transition and Last Glacial Maximum (LGM ∼ 24–22 ka), and reflect a hybrid mechanism in which fault-enhanced permeability and transient magmatic heat sustained hydrothermal discharges. These dual mechanisms highlight the sensitivity of hydrothermal systems to both mantle melting and upper crustal stress reconfiguration driven by sea-level change. During these events, trace metal enrichments in authigenic coatings indicate enhanced trace metal fluxes to the deep Indian Ocean. Our findings demonstrate that hydrothermal systems at slow-spreading ridges respond dynamically to sea-level forcing and modulate deep-ocean chemistry over glacial-interglacial cycles.

Electrical conductivity measurements on well-characterized materials in the laboratory allow accurate interpretations of high-conductivity anomalies within the lithosphere and asthenosphere, both affected by substantial deformation over geological times. However, only a few experiments so far have measured rock conductivity during controlled deformation at high pressures (≥1 GPa) and temperatures (500–1,000°C). Here, we report the first successful deformation experiments performed in a new-generation Griggs-type apparatus adapted for electrical measurements. As a proof of concept, one successful experiment was conducted on Carrara marble at a confining pressure of 1 GPa and temperatures of 500, 650, and 800°C. Three other experiments were then performed at the same pressure to explore the electrical conductivity of Åheim dunites at 500, 650, and 800°C, respectively. Our results show very different electrical responses in the elastic and plastic regimes. Stress and strain can significantly impact the electrical conductivity of peridotites by changing the thickness, number, or geometry of grain boundaries. At fixed P-T conditions, the electrical conductivity varies within an order of magnitude during elastic loading and unloading, which motivates reappraisal of interpretations of electrical anomalies in mantle rocks, at least in tectonically active regions. Upon additional development to achieve deformation up to 4 GPa (≃120 km depth), the design presented here opens a fully new research field, which will help to more deeply understand electrically conductive anomalies in rocks under stress at depth, notably within the lower crust, upper mantle and subducting slabs.

The thermal evolution of the crust during continental collision evolves from cold to hot with time, which impacts crustal reworking and differentiation. However, it remains elusive as to the mechanism driving the crust to be hot during the protracted collision. Here, we describe crust thermal evolution via detailed petrographic and geochronological analyses, and P−T calculations on different metamorphic rocks from east-central Himalaya, which record a wide range of P−T conditions and ages from the early to the late collision stage. The Eocene (ca. 44 Ma) metamorphism, represented by the Kangmar garnet amphibolite, exhibits P = ∼12 kbar, T = 670°−690°C, and a geothermal gradient of 17.0°–17.4°C/km. Rocks in the Tsona area yield metamorphic ages of 39–36 Ma and peak P−T conditions of 13.0–14.5 kbar and 760°−770°C (16.0°−17.9°C/km). Mafic granulites recorded variable peak conditions of 18–25 kbar and 720°−870°C (8.72°−14.6°C/km) and were overprinted by granulite-facies metamorphism of ∼8 kbar, 916°−932°C (∼33.3°C/km) at ∼15 Ma. These results indicate that the Himalayas exhibited elevated thermal gradients during protracted collisions. Given the thick felsic crust and high rate of heat production, thermal modeling results indicate that radiogenic heating during prolonged collision caused the Himalayan crust to be hot, even to ultra-high temperature conditions, and led to the elevated geothermal gradients. As a premier example of continental orogenesis, the Himalaya is distinctly hotter than the cold Alpine-type orogens. This thermal difference could stem from a reduced convergence rate, low-angle underthrusting, vigorous felsic magmatism, and persistent shear heating.