Geochemistry Geophysics Geosystems最新文献

Basaltic fissure eruptions emit volatile and environmentally reactive gases and particulate matter (PM) into the lower troposphere (e.g., SO₂, HCl, and HF in the gas phase; Se, As, Pb as complexes in the PM phase). Lava flows from fissure eruptions can be spatially extensive, but the composition and fluxes of their emissions are poorly characterized compared to those from main vent(s). Using uncrewed aircraft systems-mounted (drone) samplers and ground-based remote Fourier Transform Infrared Spectroscopy, we investigated the down-flow compositional evolution of emissions from active lava flows during the Fagradalsfjall 2021–2023 eruptions. The calculated fluxes of volatile trace metals from lava flows are considerable relative to both main vent degassing and anthropogenic fluxes in Iceland. We demonstrate a fractionation in major gas emissions with decreasing S/halogen ratio down-flow. This S-Cl fractionation is reflected in the trace element degassing profile, where the abundance of predominantly sulfur-complexing elements (e.g., Se, Te, As, Pb) decreases more rapidly in down-flow emissions relative to elements complexing as chlorides (e.g., Cu, Rb, Cs), oxides (e.g., La, Ce) and hydroxides (e.g., Fe, Mg, Al, Ti). Using thermochemical modeling, we explain this relationship through temperature and composition dependent element speciation as the lava flow ages and cools. As a result, some chloride-complexing elements (such as Cu) become relatively more abundant in emissions further down-flow, compared to emissions from the main vent or more proximal lava flows. This variability in down-flow element fluxes suggests that the output of metals to the environment may change depending on lava flow age and thermal evolution.

Interpreting the paleomagnetic records of altered rocks, especially those from Earth's earliest history, is complicated by metamorphic overprints and recrystallization of ferromagnetic minerals. However, these records may be as valuable as a primary signal if the timing and mechanism of alteration-related remagnetizations can be ascertained. We illustrate the success of this approach in the case of seafloor hydrothermal alteration by integrating simple rock magnetic and magnetic microscopy data with petrography, hyperspectral imagery, aeromagnetic surveys, field mapping, and geochronology of Paleoarchean basalts from North Pole Dome located in the East Pilbara Craton, Western Australia. We identify 12 hydrothermal episodes during the deposition of the stratigraphy between ∼3490 and 3350 Ma. These episodes produced stratabound zones of hydrothermal alteration with predictable facies successions of mineral assemblages reflecting sub-seafloor gradients in fluid temperature, pH, composition, and water/rock ratios. Rock magnetic data and magnetic microscopy pinpoint the secondary ferromagnetic minerals within each alteration assemblage, revealing a specific single-domain magnetite population within leucoxenes (titanite and anatase after primary titanomagnetites) that always accompanies low-water/rock alteration in fluids buffered to pH equilibrium with the host basalts. Highly uniform magnetic properties indicate that once formed, these magnetites remain unchanged upon further exposure to rock buffered fluids, stabilizing them against later alteration events and making them durable paleofield recorders. The altered basalts hosting this magnetite have unique and consistent appearances, mineralogy, IR absorption features, aeromagnetic signatures, and magnetic properties across all hydrothermal systems studied here, highlighting how integrating these data sets can identify and interpret this alteration style in future paleomagnetic investigations.

The first borehole 5071_1_B of the ICDP-Drilling the Ivrea-Verbano zonE (DIVE) project in Italy, which intersects the Massone antiform, provides a unique opportunity to integrate downhole geophysical measurements with observations from 100% recovered drill core in rarely drilled lithologies. The objective of this study is to petrophysically and structurally characterize the rock mass and constrain factors influencing the seismic velocity in the lower continental crust. A comprehensive data set, comprising core, well log and vertical seismic profiling data, was collected. The structural analysis indicates that the axial plane of the intersected tightly folded antiform is slightly tilted at the borehole location and thus the borehole intersects the hinge zone at the top and its limb in the lower part of 5071_1_B. Numerous open natural fractures with variable dips and two dominant dip azimuthal orientations are identified along the borehole, which affect the electrical and acoustic properties. The velocities at the core, well log and seismic scale are consistent but lower than intrinsic seismic velocities of the lower continental crust, since they are not only affected by fractures but also by micro cracks at the 5071_1_B in situ conditions. A systematic lithology correlation is not evident for these properties. However, a cluster analysis of gamma ray and magnetic susceptibility logs shows an excellent agreement with the logged core lithologies in the presence of remarkable spatial variability. Furthermore, the main lithologies are grouped into three distinct clusters, suggesting two types of kinzigites with distinct magnetic and radiogenic properties.

Continuous reconstructions of past variations of the Earth's magnetic field are based mainly on paleomagnetic and cosmogenic ¹⁰Be records in marine sediments. In both cases, the recording mechanisms can be affected by environmental processes. Climatic overprints are only partially removed by normalization procedures, so that stacking is used to further remove site-specific effects. Regionally or globally correlated artifacts, however, cannot be removed by stacking. Here we present a modified approach where geomagnetic records are complemented by environmental proxies representing processes that might affect the field recording mechanism. Geomagnetic and environmental records are jointly processed with principal component analysis to obtain a set of components supposed to represent true variations of the geomagnetic field and climatic overprints, respectively. After discussing the theoretical background of this new approach and its underlying assumptions, a practical example is presented, using a worst-case scenario based on a single ¹⁰Be record from the North Atlantic with strong climatic overprints, covering the last 600 ka. The first two principal components, which represent the modulation of ¹⁰Be by global climatic variations and by the geomagnetic field, respectively, explain 66.3% of the signal variance. Comparison of the geomagnetic principal component with global relative paleointensity stacks shows that the original climatic overprint can be reduced by a factor of 2, outperforming a ¹⁰Be/⁹Be stack obtained from two sites with little glacial-interglacial variability. The proposed method for removing climatic overprints can be applied to multiple sites more efficiently than conventional stacking.

The crystallographic preferred orientation (CPO) of polycrystalline olivine affects both the viscous and seismic anisotropy of Earth's upper mantle with wide geodynamical implications. In this methods paper, we present a continuous field formulation of the popular directors method for modeling the strain-induced evolution of olivine CPOs, assuming the activation of a single preferred crystal slip system. The formulation reduces the problem of CPO evolution to a linear matrix problem that can easily be integrated alongside large-scale geodynamical flow models, and conveniently minimizes the degrees of freedom necessary to represent CPO fields. We validate the CPO model against existing deformation experiments and naturally deformed samples, as well as the popular discrete grain model D-Rex. A numerical model of viscoplastic thermal convection is built to illustrate how flow and CPO evolution may be two-way coupled, suggesting that CPO-induced viscous anisotropy does not necessarily strongly affect convection time scales, boundary (lid) stresses, and seismic anisotropy, compared to isotropic viscoplastic rheologies. As a consequence, geodynamical modeling that relies on an isotropic rheology (one-way coupling) might suffice for predicting seismic anisotropy under some circumstances. Finally, we discuss limitations and shortcomings of our method, such as representing D- and E-type fabrics or modeling flows with mixed fabric types, and potential improvements such as accounting for the effect of dynamic recrystallization.

Subduction zones exhibit heterogeneities in composition due to different mineral assemblages transported into the mantle by the descending slabs, thus affecting the seismic properties of the region. These minerals are typically rich in alumina and silica and often contain hydrous phases. Nacrite, Al₂Si₂O₅(OH)₄, a mineral consisting of these components, forms in basaltic crust through hydrothermal alteration and is frequently overlooked due to its structural alikeness with its polytypes, making it hard to distinguish by traditional methods. Its occurrence in oceanic sediments and altered basaltic crust significantly impacts the subduction process by facilitating the transport of water into deeper mantle regions. In this study, we investigate the equation of state and elasticity of nacrite using first-principles calculations based on density functional theory corrected for dispersive forces over its pressure stability range. Anomalous behavior of elastic coefficients are suggestive of a polytypic transformation, evidenced by anomalous softening in the shear modulus and a decrease of approximately 3% in shear wave velocity observed at low pressures ($��\sim �$ 2 GPa). Our studies indicate that nacrite exhibits a significantly lower shear wave velocity compared to the surrounding mantle, resulting in very high V_P/V_S ratios. These findings emphasize the role of nacrite in the subduction zones of Japan and Alaska, particularly in the formation of low-velocity layers. We propose that nacrite's presence is a significant factor explaining these observations, alongside other hydrous minerals like lawsonite, glaucophane, etc., contributing to the low-velocity layers in these regions.

Upwelling and decompression of mantle plumes is the primary mechanism for large volumes of intraplate volcanism; however, many seamounts do not correlate spatially, temporally, or geochemically with plumes. One region of enigmatic volcanism in the ocean basins that is not clearly attributable to plume-derived magmatism is the Geologist Seamounts and the wider South Hawaiian Seamount Province (∼19°N, 157°W). Here we present new bathymetric maps as well as ⁴⁰Ar/³⁹Ar age determinations and major and trace element geochemistry for six remote-operated vehicle recovered igneous rock samples (NOAA-OER EX1504L3) and two dredged samples (KK840824-02) from the Geologist Seamounts. The new ages indicate that volcanism was active from 90 to 87 Ma and 74 to 73 Ma, inferring that in conjunction with previous ages of ∼84 Ma, seamount emplacement initiated near the paleo Pacific-Farallon spreading ridge and volcanism spanned at least ∼17 m.y. Geochemical analyses indicate that Geologist Seamount lava flows are highly alkalic and represent low-degree partial mantle melts primarily formed from a mixture of melting within the garnet and spinel stability field. The ages and morphology inferred that the seamounts were likely not related to an extinct plume. Instead, we build upon previous models that local microblock formation corresponded to regional lithospheric extension. We propose that the microblock was bounded by the Molokai and short-lived Kana Keoki fracture zones. Regional deformation and corresponding volcanism among the Geologist Seamounts associated with the microblock potentially occurred in pulses contemporaneous to independently constrained changes in Pacific Plate motion—indicating that major changes in plate vectors can generate intraplate volcanism.

Over the vast area of present-day Europe, the Tithonian–Berriasian transition was a time of climate aridization, which was supposedly related to the more general trend of the latest Jurassic–earliest Cretaceous cooling and restrictions in atmospheric circulation. Recent studies suggest that such conditions affected also some other paleoenvironmental processes such as monsoonal upwellings, seafloor ventilation and circulation of nutrients within the water column. In order to test this model, the uppermost Jurassic–lowermost Cretaceous sedimentary succession of the Slovenian Basin was correlated with a reference data from the Bakony Basin (Transdanubian Range, Hungary). Stratigraphic calibration was ensured by integrated stratigraphy, utilizing bio- (calpionellids, calcareous dinocysts) and chemostratigraphic tools (δ¹³C stratigraphy) as well as regional correlations of magnetic susceptibility and terrigenous input. Paleoclimate, paleoredox and paleoproductivity conditions were evaluated based on various geochemical proxies. Both the Slovenian and the Bakony basin sections were found to document late Tithonian–early Berriasian climate aridization as well as related signals of seafloor hypoxia and elevated accumulations of micronutrients. Significant geochemical contrast between the basal (lower Tithonian) radiolarites and overlying upper Tithonian–Berriasian carbonates evidences the inverse relation between the surface productivity and the amount of nutrient-type trace metals buried in sediments. The rhythm of paleoclimatically controlled environmental changes, with relatively humid early Tithonian, arid late Tithonian–early Berriasian, and again humid late Berriasian, correlates with those estimated for Vocontian Basin (SE France) and the Sub-Boreal domain of Western and Central Europe. This indicates that climatic stratigraphy is a useful tool for global correlation of the Jurassic/Cretaceous boundary interval.

CO₂ is the first volatile to exsolve in magmatic systems and plays a crucial role in driving magma ascent and volcanic eruptions. Carbon stable isotopes serve as valuable tracers for understanding the transfer of CO₂ from the melt to the gas phase during passive degassing or active eruptions. In this study, we present δ¹³C measurements from the 2021 Fagradalsfjall eruption, obtained from (a) volcanic gases emitted during the eruption and collected via unmanned aerial systems (UAS), and (b) a series of mineral-hosted melt inclusions from the corresponding tephra deposits. These data sets jointly track the carbon isotopic evolution of the melt and gas phases during the last 10 km of magma ascent. The isotopic evolution of both phases indicates that kinetic degassing, a process previously only identified in mid-ocean ridge basalts, took place in the 10 to 1 km depth range, followed by equilibrium degassing at near-surface conditions in the last kilometer. Postulating that the melt was first saturated with CO₂ at 27 km depth and that degassing from then to 10 km depth took place via equilibrium isotopic fractionation, the melt inclusion data constrain the initial δ¹³C signature of the Icelandic mantle to −6.5 ± 2.5‰ but also show indications of possible isotopic heterogeneity in the mantle source.