Geochemistry Geophysics Geosystems最新文献

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.

Understanding reactive melt flow is crucial for advancing our knowledge of crustal differentiation; however, the mechanisms governing melt migration remain debated, particularly in deep magmatic arc environments. A composite sample from the Central Qilian continental arc, NE Tibet, preserves the transition from hornblende gabbronorite to garnet granulite, offering a rare opportunity to study reactive melt flow in the arc root. Thermodynamic modeling showed that the hornblende gabbronorite was metastable under lower-crustal conditions (6.2–8.2 kbar, 900–931°C). To equilibrate with the normal thermal regime of the middle to lower crust, it underwent near-isobaric cooling to 816 ± 16°C, whereas its transformation into garnet granulite occurred under higher pressure and temperature conditions (10.2–12.2 kbar, 833–865°C). The sample records melt-rock interactions during the transition from the magmatic stage to garnet granulite facies metamorphism. Reactive melts infiltrated grain boundaries, inducing mineral replacement via dissolution-precipitation and metasomatism. Enriched rare earth elements (REEs) in blue-green pargasite, reaction microstructures and hydrous products attest to melt-rock interactions involving Mg-Sr-REE-enriched silicate melts. Trace element mapping reveals a correlation between reaction microstructures and high-Sr plagioclase bands, highlighting grain boundary pathways for melt migration. Replacement microstructures illustrate permeable reactive melt flow pathways within the lower arc crust. Reactive melt flow enhanced chemical disequilibrium and mineralogical reorganization, driving textural maturation through coupled dissolution-reprecipitation. This pervasive melt-rock interaction mechanism likely governs both crustal differentiation and the development of high Sr arc magmatic signatures.

The interaction between aging oceanic plates and their underlying mantle is a crucial component of the plate tectonic cycle. Sub-lithospheric small-scale convection (SSC) explains why plates appear not to thicken after a certain age. Here, we link grain-scale processes, dynamic models of asthenospheric flow, and seismic observations to gain new insights into the mechanisms of SSC. We present high-resolution 3D geodynamic models of oceanic plate evolution with an Earth-like rheology including coupled diffusion/dislocation creep and their interplay with evolving olivine grain size. Our models quantify how rheology affects the morphology and temporal stability of SSC, and we directly relate these quantities to geophysical observations from the Pacific OBS Research into Convecting Asthenosphere (ORCA) experiment. We convert variations in temperature, pressure, grain size, water content and stable melt fraction to seismic velocity and attenuation, seeking to match the wavelength and pattern of observed longitudinal convective rolls, the young SSC onset age, the large seismic velocity heterogeneity, low absolute seismic velocities, and high seismic attenuation. This requires low ( Pa s) asthenospheric viscosity, the contribution of both diffusion and dislocation creep to deformation, and the presence of volatiles and melt. Although SSC occurs at plate ages $��\ll �$60 Ma in our best-fit model, the plate thermal structure approximately matches global observations of heat flux and bathymetry, indicating an important role of vigorous SSC in Earth's plate dynamics. However, reconciling all seismological observations is challenging, and additional mechanisms are required to explain the strong velocity heterogeneities suggested by body wave tomography.

The Marion Rise (MR) at the central Southwest Indian Ridge (SWIR) is an ultra-slow spreading ridge with thin crust, shallow ridge depth, sparse basaltic coverage, and exposed peridotite. Clinopyroxenes from the MR peridotites have highly variable Hf-Nd isotopic composition extending to extreme ε_Nd of 94 and ε_Hf of 417, which requires extensive melting and evolution with high Lu/Hf for more than 1 Ga. The Yb content of clinopyroxenes is negatively correlated with the Cr# (molar Cr/Cr + Al) of spinel, but not with ε_Hf, indicating a multi-stage evolution of depletion and melt-rock reaction. The highly variable Hf-Nd isotopic compositions of the MR basalts are not systematically correlated and range from ε_Nd −8 to 9.1 and ε_Hf −10 to 32. Therefore, the basalts are probably a mixture of melts from several lithologies, for example, a recycled crustal component with exceptionally low Hf-Nd isotope ratios, in addition to melts from the volumetrically predominant, isotopically highly variable peridotites. The ancient melt-depletion of the MR peridotites with high Hf isotope ratios also reduced their density. A peridotitic mantle melted to <10% can support the Marion Rise without the need of increased mantle temperature. Ultra-depleted peridotites like those from the MR ones have been documented at multiple localities, indicating that they are ubiquitous in the sub-ridge mantle. Hence, melts from such ultra depleted peridotite influences mid-ocean ridge basalt (MORB) compositions and variably melt depleted sub-ridge peridotites should be considered when evaluating ridge depth variations.

Coral-based temperature reconstructions and gridded sea-surface-temperature (gSST) data sets both provide valuable insights into tropical climate variability. However, coral records often exhibit greater interannual to decadal variability than is observed in gSST products or Earth System Models (ESMs). This discrepancy is often attributed to large differences in spatial scale: coral records reflect conditions over areas of only a few square centimeters, while gSST and ESM grid cells span 1 to 10,000 km². In situ temperature loggers on coral reefs allow us to isolate the effects of spatial scale from other non-climatic influences on coral temperature records. Many logger studies focus on hourly to monthly timescales, temperature biases, and whether gSST can capture temperature extremes associated with coral bleaching and mortality; however, paleoclimate reconstructions provide an understanding of variability on longer timescales. Here, we compare the power spectral density and coherence of logger temperature and gSST on daily to decadal timescales using logger data from 42 sites on the Great Barrier Reef. We find that temperature variations recorded by loggers on reefs are well correlated with and have the same amplitude as gSST variations at decadal to annual timescales. Therefore, the excess decadal variability commonly seen in coral-based temperature reconstructions cannot be attributed to a general effect of spatial scale.

Volcanic material preserved in marine and lacustrine sediments is a key high-resolution archive for studying the past eruptive history of volcanic regions. In this work, we use the geochemical and isotopic compositions of marine volcanic glass shards, the thicknesses, and age models of tephra layers preserved in the deep sediments of the eastern equatorial Pacific, to study their volcanic source, the long-term evolution of volcanism, and its relationship with the regional geodynamics. We highlight that explosive eruptions associated with the Galápagos hotspot occurred in the Late Miocene and Early Pleistocene, which may reflect plume-ridge interplays. We also show that the oldest products of the Northern Andean arc were deposited at ∼4.8 Ma, shortly before the extinction of volcanic activity in northern Peru-southern Ecuador, due to the gradual flattening of the slab. The eruptive activity, apparently restricted to the Eastern Cordillera of Ecuador during the Pliocene, intensified and expanded from 2 Ma, with products of more varied compositions reflecting the construction of stratovolcanoes. This increase in volcanic activity, coeval with episodes of uplift of the Coastal Cordillera and with the development of the regional fault system that accommodates crustal deformations, may reflect the presence under the Ecuadorian Andes of the young Nazca oceanic crust, which carries the Carnegie Ridge. Finally, our results suggest that tephra of the Northern Andean arc recorded in sediments of the Panamá Basin were essentially emplaced by Plinian eruptions of a VEI-5-6 (Volcanic Explosivity Index), except one VEI-7 caldera-forming eruption, which occurred at 216 ± 5 ka.

Hydrous minerals transported by cold subducting slabs to the lowermost mantle are believed to significantly influence mantle properties and the heterogeneities in the core-mantle boundary (CMB) region. FeOOH is one of the essential iron-bearing hydrous minerals whose high-pressure phases, ε-FeOOH and Pyrite-type FeOOH (Py-FeOOH), can remain stable at the pressure-temperature conditions pertaining to the deep lower mantle. Using the first-principles density functional theory (DFT) based methods, we compute the thermoelastic properties of ε-FeOOH and Py-FeOOH and investigate the role of these two minerals in the deep Earth. Our calculations suggest that the phase transition of ε-FeOOH to Py-FeOOH will result in an increase of V_P and V_S by roughly 9% and 10%, respectively, which can contribute to the positive velocity anomalies in the high-velocity zones (HVZs) found in the lower mantle regions below the eastern and western pacific. Our anisotropy studies for ε-FeOOH depict a decreasing trend of anisotropy with temperature, which might indicate the presence of ε-FeOOH in the upper regions of the LLSVPs. Due to its high density and thermoelastic properties, Py-FeOOH might contribute to ULVZs, ORPs and other features at the core-mantle boundary.

Intraplate magmatism is widespread in continental and oceanic domains. Some occurrences, such as Hawai'i, fit the predictions of the mantle-plume hypothesis well. However, many occurrences require alternative explanations. The Cameroon volcanic line (CVL) in West Africa is one of the most voluminous and long-lived non-plume intraplate magmatic provinces, providing an ideal location for testing alternative intraplate magma-generation hypotheses. Two CVL volcanoes, Etinde and Mount Cameroon, located on the African continental margin are composed mostly of nephelinite and basanite, respectively. We present 12 new Ar-Ar dates and show that the Etinde mafic nephelinites (0.572 ± 0.032 and 0.5152 ± 0.0073 Ma) predate the Mount Cameroon basanites (0.442 ± 0.014 Ma to present). Basanite samples from Etinde had much younger ages (0.113 ± 0.019 and 0.073 ± 0.011 Ma) and likely originated in the Mount Cameroon magmatic system. Indistinguishable radiogenic isotope ratios and similar primitive-mantle-normalized incompatible-element patterns indicate that the magmas feeding the two volcanoes share the same mantle source. The temporal progression from nephelinite (Etinde) to basanite (Mount Cameroon and minor, late eruptions on Etinde) is marked by a reduction in La/Yb and an increase in SiO₂ content in the most mafic magmas. These features are consistent with the progressive melting of a common carbonate-enriched mantle source in which the proportion of carbonate in the melt declined with increasing melt fraction. We propose that the carbonate-enriched mantle flowed outwards from beneath Africa and decompressed as it encountered a thinner lithosphere at the continent-ocean boundary, leading to magmatism at Etinde and Mount Cameroon.