Geochemistry Geophysics Geosystems最新文献

Analysis of magnetotelluric (MT) data across the northern Appalachian region reveals significant mantle heterogeneity. By inverting a subset of long-period EarthScope USArray MT data, we constructed a three-dimensional electrical resistivity model that provides new insights into the seismic low-velocity Northern Appalachian Anomaly (NAA). Comparison with empirical conductivity models indicates that the low-resistivity anomalies along the northern and western edges of the NAA cannot be explained by temperature alone and likely require the presence of volatiles, such as $��{\text{CO}}_2�$-rich or hydrous melts, or other volatile-bearing phases, to reduce mantle resistivity to the observed levels. In addition, our modeling suggests that certain alternative lithologies, particularly hydrous clinopyroxenites, may also contribute to the observed conductivity, implying that compositional heterogeneity plays a role alongside fluids or melt. These conductive features may reflect partial melting or metasomatic enrichment of carbonated and hydrated mantle domains introduced during past subduction or plume interactions, potentially mobilized by edge-driven convection at lithospheric boundaries. We also resolve a deep resistive feature in western New England, interpreted as a dry and depleted lithospheric block, though its nature remains uncertain due to limited seismic expression and the relatively low sensitivity of MT to resistive structures. Our results suggest that the upper mantle beneath New England is both compositionally and thermally heterogeneous, shaped by a complex tectonic history involving subduction, metasomatism, lithospheric thinning, and ongoing asthenospheric processes.

Mn/Ca of foraminiferal calcite has been proposed as a tool to reconstruct past oxygen conditions, but the impact of the concentration of Mn ([Mn]) in seawater on partitioning of Mn in foraminiferal calcite remains unclear. Here, we explore Mn incorporation of different species of foraminifera across a gradient of seawater [Mn] by culturing small and large benthic foraminifera in hypoxic conditions using a controlled laboratory set-up. Our observations confirm previous results that Mn incorporation varies greatly between species and underline the need for species-specific calibrations and mono-species application of Mn-based proxies. We explore whether the observed species-specific incorporation of Mn could be due to an interaction between Mn incorporation and Mg content, or other processes related to the co-uptake, -transport and -precipitation of Mn and Mg to the calcification site and ultimately foraminiferal calcite. Furthermore, results show that for several species (Ammonia confertitesta, Bulimina marginata, Cassidulina laevigata and Amphistegina lessonii) partitioning of Mn increases below a species-specific threshold of seawater [Mn]. For application of the Mn-proxy, this slightly higher Mn partitioning can lead to a slight overestimation (up to 5-fold) of reconstructed seawater Mn/Ca at very low concentrations close to 0 (e.g., well oxygenated conditions), when assuming a constant distribution coefficient for this proxy. However, this only occurs at extremely low seawater Mn/Ca, below 1.06 mmol/mol. This trend is less clear for the larger benthic foraminifera (Amphistegina lessonii and Operculina ammonoides), where it is potentially masked by the larger range of Mn/Ca data and additional impacts on Mn incorporation, such as ontogeny.

Obtaining accurate estimates of the absolute intensity of the past geomagnetic field (paleointensity) is one of the major challenges in paleomagnetic research. Paleointensity data are typically determined by replacing the ancient thermoremanent magnetization (TRM) acquired in an unknown field with a laboratory TRM acquired under controlled conditions. A major source of uncertainty in paleointensity experiments arises from cooling rate effects, as the two TRMs are acquired under significantly different cooling conditions. Néel theory for single-domain (SD) particles predicts that ancient (slow-cooled) TRM is larger than laboratory (fast-cooled) TRM, and that the ratio between them is linearly proportional to the logarithm of the cooling rate ratios. Here, this relationship is tested for non-ideal SD materials, providing an empirical basis for the validity of cooling-rate correction experiments. Eighty-two archeological baked-clay artifacts and basalt samples were given eight TRMs under an exponential cooling process, using seven different cooling-rate constants spanning 2.5 orders of magnitude, resulting in cooling times ranging from 30 min to 1 week. These samples exhibited a range of domain state properties, including SD, vortex, strongly interacting particles, and mixtures of different populations. The results show that the ratio of slow-to fast-cooled TRMs is a linear function of the logarithm of the exponential cooling rate constants, regardless of the domain state. Cooling rate corrections, calculated for more than 2,100 archeological specimens using three different exponential cooling constants, are analyzed and provide practical guidelines for TRM effects in typical archeological materials. The results highlight that cooling rate correction should always be measured.

We assemble the highest-resolution data set of Love wave phase velocities measured across the continental U.S. to date, combining observations from ambient noise and teleseismic earthquakes to span a period range between 10 and 120 s. We jointly invert this data set and Rayleigh wave phase velocity maps for 3-D models of isotropic shear velocity $��V_S�$ and radial anisotropy $��\xi �$. To rigorously investigate the necessity of radial anisotropy, we use as our starting model the $��V_S�$ results from Shen and Ritzwoller (2016, https://doi.org/10.1002/2016jb012887), which were derived from Rayleigh wave data. We find that the starting model consistently underpredicts the observed Love wave phase velocities and fits well the Rayleigh wave phase velocities, highlighting the need for radial anisotropy to explain both data sets simultaneously. Our inverted model fits the Love and Rayleigh wave data sets well. We show that the crust in the western and easternmost U.S. is characterized by strong anisotropy, with a clear connection between positive anisotropy and regions of Cenozoic and Mesozoic extension, and between negative anisotropy and orogenesis. The central U.S. crust is mostly isotropic. We find positive upper-mantle anisotropy across the U.S., with the exception of the Colorado Plateau, and show that it may be linked to diverse geodynamic processes including horizontal mantle flow in the asthenosphere and melt sills in the lithosphere.

Continental intraplate magmatism remains a fundamental challenge in Plate Tectonics. Triassic magmatism represents a critical phase in the evolution of the Central Asian Orogenic Belt and continues to provoke debate about its tectonic setting. This study presents an integrated analysis of field, petrographic, geochronologic, geochemical, and isotopic data from two newly identified Triassic granitoids in the Bogda Range: an adakitic porphyry and a low-Sr/Y granite porphyry. Zircon U-Pb dating yields nearly coeval crystallization ages of 221–220 Ma and 219–217 Ma, respectively. Both granitoids are characterized by high SiO₂ and K₂O content, low MgO (Mg^#) and Ni content, and elevated K₂O/Na₂O and Th/Nb ratios. They have also depleted whole-rock Sr-Nd and zircon Hf isotopic compositions, indicative of juvenile crustal sources. The adakitic porphyry exhibits high Sr/Y and (La/Yb)_N ratios, low Rb/Sr ratios, and weak Eu anomalies, suggesting derivation from garnet-stable mafic lower crust. In contrast, the granite porphyry displays low Sr/Y and (La/Yb)_N ratios, consistent with a plagioclase-stable source. By integrating these results with regional data, we propose that the diversity of Triassic magmatism in the Eastern Tianshan is due to the reworking of juvenile and ancient crust at different depths and temperatures, resulting in arc-like geochemical signatures. This process was associated with transcurrent tectonics and variable mantle contributions in a continental intraplate setting.

Subduction zones are integral to Earth's deep water cycle, influencing magmatism, lithosphere dynamics and seismicity. They often exhibit development of extensional processes in the upper plate, which may lead to volcanic arc splitting and backarc basin opening. Here, we investigate numerically the factors controlling arc rifting and back-arc opening and basin evolution, emphasizing the roles of slab dehydration, and melting processes linked to upper plate thermal structures and variable sediment fluxes. Using an extensive suite of 2D and 3D thermo-mechanical models, we assess intra-arc rift initiation timing in the upper plate, lithospheric thinning, and arc rifting patterns. Model results reveal that higher crustal temperatures and sedimentation rates enhance melting and crustal weakening along the arc, facilitating strain localization and faster roll-back, but may simultaneously diminish stress transfer efficiency. Additionally, 3D modeling captures along-strike variations, including slab tearing and toroidal mantle flows, offering insights into the complex interplay of subduction dynamics and their effect for arc rifting. These findings are compared with natural laboratories in the Mediterranean and in retreating subduction systems of SE Asia.

The study of the serpentinization process in ultramafic rocks has been revived in recent years in response to the problems of natural hydrogen generation. Serpentinization results in the formation of magnetite, which is often observed in the most serpentinized peridotites, at concentrations of up to a few percentage. In this study, we propose a novel experimental approach to test the formation of magnetite at temperatures of 100°C, 200°C and 300°C. The aim is to test whether magnetite can be produced at low temperatures in variably serpentinized peridotites, and how the degree of serpentinization can influence this production. This experimental approach exploits the properties of newly formed magnetites to record the magnetic field applied during the experiment. To achieve this, a study was conducted on Pyrenean peridotites, which encompassed variable degrees of serpentinization. The findings suggest that magnetite formation commences at 100°C, irrespective of the initial rock's degree of serpentinization. At 200°C, the production of magnetite remains comparable to that at 100°C. However, at 300°C, the production of magnetite is an order of magnitude higher. The most noteworthy outcome of our investigation is the demonstration that the production of magnetite is directly proportional to the initial nanomagnetite content of the peridotite. This finding suggests that magnetite may act as a catalyst, with strongly serpentinized peridotite exhibiting a greater propensity for magnetite production than weakly serpentinized peridotite under natural conditions.

Sediment thermal history controls the progress of diagenetic reactions that can alter the mechanical behavior of material entering a subduction zone that then: accretes to the margin, hosts the plate boundary interface, or is carried deeper within the Earth. On the Cascadia margin offshore Oregon (USA), hydrothermal circulation in the oceanic crust affects thermally controlled processes, enhancing sediment alteration above the MARGIN seamount, which is buried by the Astoria Fan. Hydrothermal circulation increases temperatures at the summit of the seamount and in the overlying sediment by up to ∼100°C. We use sediment thermal history constrained by heat flux observations to model the expected progress of the smectite-to-illite reaction around the MARGIN seamount. Above the seamount, the smectite-to-illite reaction is expected to progress to completion ∼250 m below the seafloor; away from the seamount, smectite is likely unaltered to a burial depth of ∼800 m. The altered sediment above the seamount has higher rigidity and p-wave velocity than the surrounding sediment. Spatial variability in sediment alteration may be present around other buried seamounts. We use vertical gravity gradient anomalies to estimate the locations and heights of additional seamounts. Each of these seamounts may have altered sediment around it, which could affect deformation and seismicity in the margin wedge. Because cemented sediment with greater elastic strength is better able to store elastic strain energy, enhanced sediment alteration and cementation above seamounts entering the subduction zone could facilitate earthquake nucleation for material in the margin wedge that was above a seamount prior to subduction.

Understanding surface heat flux is essential for evaluating geothermal system dynamics and resource potential, particularly in tectonically active but non-volcanic regions. This study presents the first integrated assessment of surface heat flux in three representative geothermal areas in Taxkorgan County, western China, including Taheman (THM), Liaoyangyuan (LYY) and Dabudar (DBD). Field measurements were conducted using a combination of soil temperature gradient analysis, water vapor flux estimation, soil CO₂ flux measurement and the desiccant-based CO₂:H₂O ratio determination method. Sequential Gaussian simulation of the geostatistical method was applied to mapping spatial distributions of heat flux and CO₂ flux. The results show that the total surface heat fluxes of THM, DBD, and LYY are 68.6, 19.2, and 57.7 W·m⁻², respectively, with CO₂-derived heat flux accounting for 51.8%–54.6% of the total, highlighting the dominance of gas-phase convective heat release. THM exhibits the highest thermal output (1.3 MW), driven by deep volatile exsolution and enhanced fault permeability, characteristic of a fracture-controlled geothermal reservoir. In contrast, LYY shows a high thermal gradient and dominant conductive heat flux, reflecting a shallow heat source. DBD displays the lowest heat flux, possibly representing a peripheral or capped thermal zone. These findings provide critical measured data and theoretical basis for the identified hybrid heat transport mechanisms and also offer broader implications for understanding tectonically controlled geothermal systems in continental collision orogenic belts.

Supercritical fluids are ideal media for mass transfer from the subducting slab into the mantle wedge. However, little is known about the Mg-Fe isotope compositions of supercritical fluids in subduction zones. Here, we present the Mg-Fe isotope data for a coesite-bearing eclogite-vein system, which is closely associated with supercritical fluids, from the Dabie Orogen in China. The results reveal the geochemical effects of supercritical fluids under subarc conditions. The eclogites close to the eclogitic vein formed by supercritical fluids have not only lighter Mg isotope compositions for whole-rock but also higher δ²⁶Mg and δ⁵⁶Fe values in separate minerals than those distant from the vein. The ultrahigh-pressure eclogitic vein has δ²⁶Mg values of +0.17 to +0.23‰ and δ⁵⁶Fe values of +0.26 to +0.35‰. These observations indicate that vein-forming supercritical fluids have heavy Mg-Fe isotope compositions and are produced by the contributions of omphacite from eclogite during the dissolution-precipitation process. The supercritical fluids released from eclogite at subarc depths are recovered to have high δ²⁶Mg values of +0.30 to +0.37‰ and δ⁵⁶Fe values of +0.34 to +0.49‰ and thus can contribute to arc lavas with heavy Mg-Fe isotopic compositions. On the basis of the mixing modeling between subduction zone fluids and mantle wedge peridotites, we propose that the supercritical fluids have an effect on the mantle wedge to drive it to incorporate slightly heavier Fe isotopes.