Geochemistry Geophysics Geosystems最新文献

Rocks at various lithospheric depths commonly display a fabric, resulting in mechanical anisotropy. The mechanical response of such anisotropic rocks depends on both the intensity of the anisotropy and the orientation of the fabric relative to the applied stress. Despite its potential significance, the role of mechanical anisotropy in governing lithospheric strength and deformation style during extension remains poorly constrained. Here, we investigate how mechanical anisotropy influences the deformation of the lithosphere under tectonic extension. We use two-dimensional numerical models of lithospheric deformation that incorporate a non-linear, transversely isotropic model. Both viscous and plastic rheologies are direction-dependent, and fabric orientations evolve using the director-vector approach. We perform simulations of continental extension and show that mechanical anisotropy is a major factor in the development of continental rifts. It influences the architecture of rift basins and reduces the driving force required for rifting. We explore the role of extensional velocity and find that it has only a second-order influence on the evolution of rift systems. Furthermore, we investigate the relative contributions of crustal and mantle anisotropy, and highlight that mantle anisotropy plays a more significant role. The driving forces required for continental rifting are quantified and systematically analyzed. Compared to isotropic models, the required driving force is reduced by up to a factor of three when mechanical anisotropy is included. As a result, forces below 10 TN/m can be achieved, which is consistent with estimates from the geological record.

Submarine hydrothermal systems are critical for global geochemical cycles. However, hydrothermal fluid chemistry is influenced by multiple overlapping processes, making it difficult to isolate the effects of individual factors. In this study, we applied independent component analysis (ICA) to a global database of hydrothermal fluids to extract the key factors controlling fluid chemistry. The ICA results identified magma differentiation and phase separation as the key controls for major elements, gases, and rare earth elements (REEs). With increasing magmatic differentiation, the CO₂ and F concentrations increase, whereas the La/Yb values and Eu anomalies decrease. Associated mineral compositional changes reduce Ca and H₂ while increasing Mn/Fe and the K, Li, Pb, Sb, Au, and Ag concentrations. During phase separation, volatiles partition into the vapor phase, whereas metals exhibit element-specific partitioning. This leads to the vapor-rich fluids being enriched in trivalent middle REEs and reduced Eu anomalies. pH exerts a strong control on Fe, Mn, Co, Cu, Zn, and REE mobility but has a limited influence on REE patterns. Sediment-hosted systems show elevated CH₄ and NH₃ levels although the sediment interaction appears to minimally affect the major elements and REEs. Acid sulfate fluids, formed by reactions between mixed magmatic fluids and seawater with highly altered rocks at high water–rock ratios, exhibit distinct chemical compositions, such as flat REE patterns. These findings demonstrate the utility of ICA for resolving overlapping geochemical processes in hydrothermal systems. Expanding the hydrothermal fluid database will enhance future efforts to model the hydrothermal contributions to oceanic geochemical budgets.

Seismic anisotropy is observed in the lowermost few hundred kilometers of the mantle. This anisotropy likely signifies strong deformation, possibly caused by mantle flow interacting with the edges of Large Low-Shear-Velocity Provinces (LLSVPs) or by the ascent of mantle plumes originating near these regions. In this study, we explore generation of seismic anisotropy as a result of deformation at LLSVPs and the flow behavior of the lowermost mantle using 3-D global models of compressible mantle convection in the geodynamic modeling software ASPECT, coupled with the mantle fabric simulations code ECOMAN. In our simulations, we initiate the LLSVPs as a 100-km thick chemically flat layer in the lowermost mantle. Our models include a plate reconstruction spanning the Pangea breakup over the past 250 million years. We tested several geodynamic models with varying compositional densities and viscosity ranges for the LLSVPs and thermal conductivities of the ambient mantle and computed the mantle fabrics for each model. Our findings align with previous shear wave radial anisotropy seismic tomography models at the lowermost mantle, where fast vertically polarized shear waves are observed near the LLSVPs. Our modeled anisotropy is mostly accumulated at the edges of the LLSVPs, which is consistent with previous regional seismic anisotropy observations. Our preferred seismic anisotropy results are for LLSVPs compositionally 100 times more viscous and 2% denser than the surrounding mantle. An increased compositional viscosity within the LLSVP strengthens its margins, leading to vertical deflection of mantle flow along its boundaries.

Igneous sill intrusions are common features in volcanic basins worldwide, constituting important components of basin-scale magma plumbing systems. While sills exhibiting simple geometries, such as saucer-shaped sills, are commonly linked to distinct mechanical processes, emplacement mechanisms for sills exhibiting more complex geometries are debated. To better understand the emplacement of complex sills, this study aims to constrain the formation mechanisms associated with the Infinity Sill, a 237 km² elongated sill located in the SW Vøring Basin. Detailed seismic interpretation and attribute analysis of an industry-standard 15,000 km² 3D seismic data set reveal that the 36 km long and 4–8 km wide Infinity Sill intruded mud-dominated Late Cretaceous strata. The thickness of the sill has been estimated to be dominantly 50–150 m, with a volume of ∼20 km³ (ranging between 7.9 and 25.5 km³). The shape of the sill and geometry of the sill elements suggest that the sill propagated as a large, magma-filled fracture exploiting a pre-existing polygonal fault network during propagation. The sill originated at a source in the SW, and the asymmetric propagation away from the source facilitated a 36 km lateral elongation of the sill, contrasting along-axis dike models for elongated intrusion geometries. Local deformation around the sill initially facilitated continuous transgression, but with increasing sill length, forced folding of the overburden triggered abrupt transgression of the sill margins, resulting in a lateral change in geometry. The Infinity Sill, with its distinct geometrical features and changing cross-sectional geometry, demonstrates that complex sill geometries are formed by a combination of emplacement mechanisms.

Rift zones, both on land and submarine, are key pathways for mantle-derived CO₂ degassing to the Earth's surface. Fixation of CO₂ as carbonates plays a critical role in regulating these emissions. This study examines carbon abundance and isotope compositions in hydrothermal fluids, altered rocks, and carbonates from geothermal systems in both Iceland rift and off-rift. Hydrothermal fluids exhibit wide CO₂ concentrations (6.1–70.4 mmol kg⁻¹) but narrow δ¹³C values (−4.8 to −2.5‰) assuming limited to no fractionation between vapor and liquid phases, in contrast to the broader δ¹³C range of carbonates (−14.5 to +0.5‰). Altered rocks contain up to 4.33 wt.% carbon, with enrichment in the upper 1,000 m of geothermal systems. Isotopic and geochemical modeling indicate that carbon is sourced from the mantle. Decompression boiling and water-rock interaction drive carbonate formation, but only 1%–10% of mantle-derived CO₂ is sequestered in on-land rift carbonates, with the majority emitted via hydrothermal fluids. In contrast, Icelandic off-rift low-temperature systems represent significant CO₂ sinks due to limited boiling processes. This study suggests that submarine rift zones, lacking prominent boiling processes, may sequester mantle-derived CO₂ more effectively into oceanic crust, facilitating long-term recycling into the mantle. These findings highlight the limited capacity of on-land rift systems to retain mantle-derived CO₂, underscoring the contrasting roles of terrestrial and submarine environments in global carbon cycling.

A novel high-resolution, non-destructive method for diagenetic screening of fossil corals for geochronologic and paleoclimatic studies using neutron computed tomography (NCT) is proposed. NCT circumvents limitations of traditional techniques, such as destructive sampling and 2-D imaging by providing detailed 3-D visualizations of coral structure and carbonate mineral phases. This method differentiates aragonite and calcite phases in fossil coral, crucial for identifying well-preserved sections suitable for dating and paleoclimatic reconstructions. A key advantage of NCT is its ability to map hydrogen content, providing a reliable indicator for identifying regions of well-preserved skeletal aragonite, since aragonite typically retains more water organic-matter than calcite. NCT scans conducted on a Holocene Porites coral (ca. 1.36–1.87 ka BP) from Muschu Island, Papua New Guinea, successfully distinguished between secondary low-magnesium calcite and aragonite skeletal material. This technique was also applied to an Isopora palifera fossil coral (ca. 39.4 to 44.8 ka BP) from Ashmore Reef, Northwest Shelf, Australia, which presented a more complex diagenetic history. Comparisons were made with results from hyperspectral imaging, X-Ray CT, scanning electron microscopy, and geochemical and petrological analyses, following calibration using a modern Porites coral from One Tree Reef, Southern Great Barrier Reef, Australia. Additionally, NCT was applied to an altered Acropora humilis coral (ca. 600 ± 280 ka BP) from Ribbon Reef 5, Great Barrier Reef, revealing small, hidden aragonite sections undetected by surficial hyperspectral imaging. This study demonstrates the advantages of combining NCT with traditional screening methods in identifying well-preserved aragonite for accurate geochronologic and paleoclimatic reconstructions. Recommendations for applying NCT in fossil coral screening are provided.

The Chicxulub impact on the Yucatán Peninsula triggered the end-Cretaceous mass extinction 66 million years ago, but physical models still struggle to accurately describe ejecta generation and transport from this and other large meteorite impacts. To better constrain these processes, Kaskes et al. (2025), https://doi.org/10.1029/2024gc012151 completed detailed micro-X-ray fluorescence (μ-XRF) mapping of a K-Pg boundary sequence preserved at Starkville South (Raton Basin, Colorado, USA). Their results directly challenge the previous “dual layer” model of ejecta sequences exemplified in the North American K-Pg outcrops, where one layer hosts the ballistically emplaced impact spherules and the overlying layer hosts the shocked minerals that were atmospherically transported. The new Kaskes et al. (2025), https://doi.org/10.1029/2024gc012151 model describes four distinct layers: (a) the ballistically emplaced spherules, (b) the ballistically emplaced shocked minerals, (c) an initial settling of atmospherically transported Ni- and Cr-rich dust, and (d) a gradual decrease of impact-generated dust back to background concentrations. Kaskes et al. (2025), https://doi.org/10.1029/2024gc012151 provide high-resolution geochemical analyses offering new insights into the timing and mechanisms of ejecta production, transport and deposition after a large meteorite impact event, which the community can apply to other K-Pg sites around the world.

The subduction interface hosts megathrust earthquakes and ductile creep, is fluid-rich and chemically dynamic, and produces metasomatic rocks that may host episodic tremor and slow slip (ETS). However, determining the depths at which these metasomatic rocks form and deform remains challenging. We reconstruct the pressure-temperature-time (P-T-t) evolution of epidote amphibolite-facies subduction interface metasomatic rocks suggested to host slow slip (Pimu'nga/Santa Catalina Island, California) using accessory phase petrochronology, thermometry, and thermodynamic modeling. Talc-, actinolite-, and chlorite-rich metasomatic rocks were produced from ultramafic, metasedimentary and metamafic protoliths by a combination of local chemical exchange, fluid infiltration and mechanical mixing. Rutile thermometry constrains the prograde initiation of local chemical exchange to near the mantle wedge corner (450–550°C) where the slab top and mantle were first juxtaposed. Metasomatism continued through peak metamorphic conditions at the depths of modern ETS (∼1 GPa, 550°C), constrained by carbonaceous material thermometry and the stability of albite and titanite in actinolite-rich rocks. Periodic influx of Ca-rich fluid released by dehydration of downgoing oceanic crust occurred near peak metamorphism and is recorded by the growth of titanite and development of actinolite-rich layers within talc-rich rocks. These results suggest that chemical exchange throughout the depths of modern ETS produced weak talc-rich rocks that may have hosted slow slip events under high fluid pressures produced by infiltrating Ca-rich fluids. Such complex chemo-mechanical interactions profoundly influence deformation and seismicity in subduction zones.

To better understand the lithosphere of Antarctica, we imaged its lithosphere-asthenosphere boundary (LAB) and crust-mantle transition using Sp receiver functions from teleseismic events analyzed at individual stations and with common conversion point stacking. Results reveal a prominent negative velocity gradient at depths of 70–100 km across much of West Antarctica, consistent with the seismically defined base of the lithosphere identified in prior tomography studies. Beneath the West Antarctic Rift System, lithospheric thicknesses are typically 70–85 km, with isolated zones up to 100 km. These thicknesses do not correlate with the time since significant extension. Rather, they are consistent with ablation of the cooling mantle at the base of the lithosphere caused by later processes, including ongoing asthenospheric flow. Mantle upwelling beneath Marie Byrd Land is one possible driver of asthenospheric flow and is consistent with this region's thin lithosphere, higher topography, and low upper mantle seismic velocities. Lithospheric thicknesses vary significantly along-strike beneath the Transantarctic Mountains, and these gradients in thermal structure indicate variable support for the mountains from a warm buoyant mantle. In the interior of East Antarctica, the absence of Sp phases from depths comparable to the base of the lithosphere seen in tomography suggests a more gradual LAB velocity gradient beneath the thick cratonic lithosphere. In contrast, beneath the margin of East Antarctica that rifted with Australia, clear LAB negative velocity gradients are present at depths of 90–120 km.

Using 2D numerical subduction models, we compare the morphology of deep slabs in the presence of an oceanic or continental overriding plate and viscosity jumps at either 660 km or 1,000 km depth as suggested by the latest geoid inversions. We demonstrate that a continental plate, combined with a 1,000 km depth viscosity increase, promotes slab penetration into the lower mantle. The same slab will deflect at 660 km depth if it subducts under an oceanic plate into a mantle where the viscosity increases at 660 km depth. To quantify these dynamics, we introduce a slab-bending ratio, dividing the angle of the deepest tip of the slab (slab tip angle) by its dip angle below the plate interface (shallow slab angle), reflecting the overall steepness, and sinking history of the slab. Ocean-ocean convergence models with a viscosity increase coincident with the phase transition at 660 km depth have low ratios and flattened slabs comparable to ocean-ocean cases in nature (e.g., Izu-Bonin). Coupling a continental overriding plate with a 1,000 km depth viscosity increase separate from the endothermic phase change results in slabs with high ratio values, and stepped morphologies similar to those observed for the Nazca plate beneath Southern Peru. Our results highlight that slab morphologies ultimately express the interaction between the type of overriding plate, slab-induced flow, and phase transitions, modulated by the viscosity structure of the top of the lower mantle and transition zone, complementing studies of slab folding, buckling, and other deformation in the upper mantle.