Geochemistry Geophysics Geosystems最新文献

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.

Mafic eclogites of the Tauern Window in the Eastern Alps preserve vein networks associated with eclogite-facies mineral assemblages. The structural and mineralogical diversity of these veins is encapsulated by Type I veins, which resemble deformed tension gashes, and Type II quartz segregates with non-planar morphologies. Within host eclogites, garnet growth occurred along a prograde P-T path between 2.05 ± 0.10 GPa, 580 ± 15°C and 2.50 ± 0.10 GPa, 630 ± 15°C, consistent with conditions on the slab-wedge interface of modern subduction zones. The dehydration of lawsonite and Na-amphibole released ∼5 wt.% H₂O over 20–35°C, creating ∼11% transient porosity. In situ oxygen isotope analysis of quartz-rutile pairs constrains formation temperatures to between 460°C and 610°C for Type I and II vein structures. Individual veins preserve records of protracted crystallization over ∼100°C, suggesting that fluids remained undrained in the oceanic crust for 10⁵–10⁶ years during subduction to ∼90 km. A simple petrological-mechanical model for the blueschist-to-eclogite transition shows that under extremely low permeability (10⁻²² to 10⁻³⁴ m²), Type I veins may form by tensile failure during periods of high pore fluid pressure, whereas Type II quartz segregates represent accumulations of derived fluids during periods of lower fluid pressure. These findings imply that domains of oceanic crust with extreme low permeability may retain fluids released during the blueschist-to-eclogite past the depths of arc magma genesis.

Stalagmites provide an invaluable archive at high-resolution for paleoclimate studies. However, it is challenging to extract independent hydroclimate signals from stalagmites due to the scarcity of reliable hydrological proxies. Although the magnetic parameters of stalagmites have shown great potential in recording regional hydrological signals, the mechanistic linkages between magnetic minerals and hydroclimate variability remain unresolved, limiting the broader application of stalagmite magnetism. This study addresses this knowledge gap through a 5-year monitoring campaign targeting Heshang (HSD), Haozhu (HZD), and Chang (CD) caves in central China. We systematically analyzed the magnetic minerals in coupled soil-bedrock-dripwater-stalagmite systems using integrated environmental magnetic techniques. The results demonstrate that magnetic minerals in the dripwater are dominated by magnetite/maghemite (detrital origin from overlying soils) and goethite (mixed sources including pedogenic, bedrock derived, and authigenic contributions, but not specifically). Seasonal analysis reveals that magnetite/maghemite flux (M_Mag/Mgh-flux) in the HSD dripwater exhibits pronounced wet-season (May to September) enhancement, which is closely correlated with the rainfall-driven soil flushing. This pattern attenuates in the HZD and CD systems due to their reduced soil-bedrock cover thickness. In contrast, the relative concentration of goethite (R_Gt) displays a consistent sensitivity to regional rainfall across all the monitored caves, especially HSD, suggesting its broader utility as a hydroclimate proxy. Our findings establish a mechanistic framework linking stalagmite magnetic mineralogy to rainfall dynamics, identifying M_Mag/Mgh-flux and R_Gt as robust dual proxies for reconstructing past hydrological variability in karst systems.

Eastern North America has hosted significant historical earthquakes, where seismicity clusters along tectonically inherited structures. Using the spherical finite-element code CitcomSVE and fully 3D viscosity structure, we model the intraplate stress response to glacial isostatic adjustment (GIA) using ICE-6G, both with and without low-viscosity intraplate weak zones. We find that present-day GIA-induced stresses are generally small ( MPa across most of eastern North America), both at present day and during deglaciation, and can locally reach 3–4 MPa where weak zones are present. Associated $��S_{\mathrm{Hmax}}�$ rotations are limited to $��\pm �$1°, which are insignificant relative to the spread of observed stress data and far smaller than the continental-wide clockwise rotations obtained from mantle-flow models. However, GIA can still locally modify fault stability. In the New Madrid Seismic Zone, GIA promotes stability on the Reelfoot thrust fault while making NE-SW strike-slip faults less stable, suggesting a role in modulating present-day seismicity patterns but not in triggering the 1811–1812 sequence. In the Western Quebec Seismic Zone, GIA increases Coulomb failure stress (CFS) on the Timiskaming Fault and nearby faults, but changes in CFS in the Charlevoix Seismic Zone are negligible at present day and only marginally higher during deglaciation. Overall, GIA perturbs CFS by only a few MPa, insufficient to independently drive fault failure under tectonic background stress (TBS) conditions derived from mantle flow models, which dominate regional-to-continental intraplate stress. However, alternate lithospheric viscosity structures and TBS states can greatly enhance GIA stresses and their impact on faulting in the crust.

The sources of granitoids are variably oxidized due to the diversity of environments in which they form. This environmental and consequent chemical variability leads to differences in mineral assemblages, proportions, and compositions of ferric and ferrous iron-bearing phases in these sources and in the resultant granitoids. Accessory minerals that grow or recrystallize during high-temperature metamorphism and/or melt crystallization can record such redox variations through the incorporation of redox-sensitive trace elements, notably europium. Here, we use petrological modeling to explore how variations in ferric/ferrous iron ratios of a metapelite and a hydrated (meta)basite influence the speciation of Europium (Eu³⁺ vs. Eu²⁺) in the melt and resultant Europium anomalies in zircon using phase equilibrium modeling and mass balance. Europium anomalies in zircon sourced from metapelites are generally insensitive to the proportions of ferric to ferrous iron, except at very reducing conditions. Europium anomalies in zircon sourced from metabasites are influenced by the proportion of ferric iron in the source, and more so in the absence of residual garnet. The amounts of plagioclase—which is commonly linked to pressure—play a relatively minor role in the Europium anomalies of zircon in the metabasite. Hence, Europium anomalies in zircon may not be an appropriate tool on their own to unravel past tectonic processes, including the thickness of Earth's continental crust.

In the southwest USA, the Colorado Plateau is encircled by Late Cenozoic volcanic fields, most of which have eruptive histories that are marginally constrained. Establishing the spatiotemporal evolution of these volcanic fields is key for quantifying volcanic hazards and understanding magma genesis. The Black Rock Desert (BRD) volcanic field covers ∼700 km² of west-central Utah. We present 46 new ⁴⁰Ar/³⁹Ar ages from the BRD ranging from 3.7 Ma to 8 ka, which includes ⁴⁰Ar/³⁹Ar plateau ages from olivine separates. These new ages are combined with 13 recently published ⁴⁰Ar/³⁹Ar ages from the Mineral Mountains to evaluate the spatiotemporal evolution of all five BRD subfields. The oldest lavas and domes are located to the southwest, whereas the youngest lavas, which are only a few hundred years old, are located ∼30 km to the NNE. However, BRD vent migration patterns over the last 2.5 Ma are non-uniform. They are also not consistent with North American Plate motion over a partial melt zone nor have they migrated toward the center of the Colorado Plateau. BRD eruptions are almost always coincident with mapped Quaternary faults. A shear-velocity (Vs) model beneath the BRD indicates that the lithosphere has been thinned and that asthenospheric melt has coalesced at the lithosphere-asthenosphere boundary, which is supported by the trace element compositions of BRD lavas that signify that they have incorporated continental lithospheric mantle. Our data and observations suggest that the asthenosphere-lithosphere-volcanic system in the BRD is inherently complex.