Geochemistry Geophysics Geosystems最新文献

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.

Sedimentary uranium (U) and thorium (Th) isotopes are invaluable proxies to assess bottom water redox conditions, site-specific sediment focusing and vertical rain rates. We investigate if authigenic uranium (aU) can serve as a proxy for bottom water ventilation at International Ocean Discovery Program Site U1537 in the Scotia Sea and we provide Th-normalized vertical rain rates and focusing factors. The presented data set is complemented by bulk sediment δ²³⁴U, porewater U concentrations and biogenic barium. Furthermore, we introduce a method to check temporal variations in the detrital factor for the calculation of aU by comparing measured and modeled δ²³⁴U. We observed partial uranium remobilization in the core for sections older than 70 ka, identified by δ²³⁴U anomalies and porewater U concentrations. During interglacials, the accumulation of aU in the sediment is regulated by the decomposition of substantial quantities of organic matter, ultimately controlled by high export productivity and associated high particulate organic carbon fluxes. Conversely, during glacial times, low export productivity coincides with low aU concentrations, suggesting well-oxygenated bottom waters. However, during the Last Glacial Maximum, a rise in aU likely indicates reduced ventilation, suggesting an absence of Weddell Sea Deep Water and/or enhanced water column stratification between 23 and 17.5 ka.

In Earth history, our understanding of how large-bodied herbivores shape a variety of ecosystem processes is limited by the quality of paleoecological proxies for herbivore composition and abundance. Fecal stanols are lipids that can be produced by microbes within animal digestive systems and that could remedy this dearth of proxies. We used two multi-decadal herbivore exclosures in Kruger National Park, South Africa, to constrain whether and how biomarker signatures preserve signals of herbivore abundance. Soil samples and dung counts were collected along transects across crests, mid-slopes, and sodic sites inside and outside exclosures. Soils were analyzed for steroid (sterols and stanols) concentrations and distributions. We found that stanol concentrations were significantly greater in sodic soils outside exclosures, where herbivore dung densities were greatest. In contrast, sterol concentrations did not differ between treatments. Ratios of stanol isomers to sterols, which account for both compound degradation and source, increased strongly with herbivore dung counts. Finally, while herbivore species compositions influenced steroid distributions, total herbivore abundance was their strongest predictor. Further calibration is needed, but this work provides strong preliminary evidence that wild herbivore populations are quantitatively recorded by fecal biomarker distributions.

International Ocean Discovery Program Expedition 398 recovered more than 2,200 m of volcaniclastic deposits from 12 sites and 28 holes from Santorini Caldera, Greece, and the surrounding rift basins in the South Aegean Volcanic Arc. We compare and contrast discrete shipboard measurements of physical properties (density, P-wave velocity) of these volcaniclastic sediments with other uncemented marine sediments in the cores. The grain density (mass of solids divided by their volume, including any isolated vesicles) of volcaniclastic deposits is typically lower than that of volcanic glass and crystals and is sometimes less than 2 g/$��{�c�m�}^3�$, indicating the preservation of isolated gas-filled vesicles in erupted materials. Volcaniclastic deposits typically have higher P-wave velocities but lower bulk densities than oozes and other marine sediments. In volcaniclastic deposits, lapilli have higher P-wave velocities and lower bulk density than ash, the opposite trend of most sediment in which higher density is correlated with higher seismic velocity. We use granular physics models to show that the higher volcaniclastic P-wave velocity originates from two effects: (a) lower pore volume outside clasts that increases elastic moduli and (b) isolated gas vesicles in volcanic clasts that lower bulk density. In volcaniclastic sediments there is relatively little change in physical properties to depths of several hundred meters below the seafloor, which we attribute to rough grain surfaces and lower intergranular (external) porosities that hinder compaction and the decrease of intergranular pore space. These trends lead to distinctive signatures of volcaniclastic sediments in reflection seismic images.

Several studies have shown that H₂ and CH₄ emissions are recorded within Precambrian areas, where the origin of gases has yet to be determined. The Mid-Continent Rift (MCR) is an aborted rift aged from 1.1 Ga and composed of volcanic and (ultra)-mafic rocks. Wells presenting free or dissolved gas were sampled in NE-Minnesota along the MCR. High pH water (up to 11.4) and low Eh (down to −300 mV) with elevated concentrations (up to 756 µmolar) of formate and acetate are associated with gases consisting of He, CH₄, CO_2, heavier hydrocarbons, and up to 500 ppmv of H₂. The CH₄ presents different ¹³C and D isotopic values, suggesting that several abiotic and biotic pathways might be active in these shallow systems. The alkaline and reducing waters associated with the igneous rocks of the MCR suggest that H₂ could have been produced through water-rock interactions at deeper levels not reached by the shallow sampled wells. The associated high concentrations of gaseous and dissolved carbon compounds (VOA and methane) suggest that subsequent redox reactions have occurred in most of the rocks crossed by the wells, consuming a part of the H₂ as it was migrating toward the surface. Those results highlight potentially active H₂ production and consumption processes, providing keys for targeting source rocks in Precambrian environments. Those results suggest that direct H₂ detection in soil gas may not be the most effective exploration strategy. Searching for biogenic methane associated with deep He and N₂ may prove to be more effective.

Percolation through magma mush is a key transport mechanism for melts in the crust and is influenced by the permeability of the crystal framework. Existing models for mush permeability do not account for the range of microstructures that can evolve as mushes crystallize or compact to low melt fractions. Here, we use numerically generated domains of cuboids at the random maximum packing as a starting geometry for a loose magma mush. We then expand the cuboid edges into the pore spaces sequentially, representing a geometrical simulation of crystal overgrowth and crystallization. At each iterative step, we measure the melt fraction, specific surface area, and melt permeability via 3D fluid flow simulations. We find that (a) the permeability drops proportional to the drop in surface area as the melt fraction reduces, (b) the permeability falls to zero at a percolation threshold $��{\varphi }_c�=�0.0187�\pm �0.0010�$ that is independent of scale and insensitive to the starting cuboid geometry, and (c) once the percolation threshold is determined, our data match a universal percolation model without requiring any free fitting parameters. We show how this percolation model accounts for any 3D shape of the crystals that comprise the evolving mush. Importantly, this approach demonstrates that mush permeability can remain non-zero in texturally unequilibrated mushes, down to very low melt fractions. Our model outperforms previous models, which overestimate mush permeability by up to three orders of magnitude, and our model can be used to accurately predict how mush permeability changes as mushes mature and crystallize, with implications for quantifying melt extraction, percolation rates, and melt reservoir assembly.