Geochemistry Geophysics Geosystems最新文献

Provenance of pre-obduction Eocene turbidites from New Caledonia is used to better constrain their geodynamic context and inform debate on subduction polarity. Chemical compositions of detrital clinopyroxenes in arenites are compared against potential sources. Basaltic source modeling using cpx/rock partition coefficients confirms E-MORB origins from the allochthonous Poya Terrane. Detrital zircon populations in the Bourail Flysch are similar to those of the autochthonous Upper Cretaceous sedimentary cover. In contrast, a predominance of early Eocene zircons in the Nepoui-Koumac Flysch suggests derivation from the supra-subduction dyke system of the Peridotite Nappe. Results were compared with other arenite components (bioclasts, oxides, sulfides, zircon) and whole-rock compositions of rock fragments and blocks of the breccia/olistostrome in the upper part of the turbidites. Together, the data set allows identification of multiple successive sources and processes. A syntectonic character for the flysch basins is inferred from the pre-turbidite unconformity and provenance evolution. An abundance of shallow-water bioclasts throughout the succession indicates the formation and continuous destruction of a rimming carbonate platform. The existence of two previously identified types of turbidite basins is confirmed by the characteristics of the Bourail (foreland) and the Nepoui-Koumac (wedge-top) flysch successions. These basins were located on the northern Norfolk Ridge (lower plate) and ultramafic allochthon (accretionary wedge), respectively, representing elements of a foreland basin. These observations are inconsistent with a hypothesized connection between New Caledonia and a continent-directed Pacific subduction zone. Together with all available geologic data, the results of this investigation confirm the model of northeast- or east-dipping pre-obduction subduction.

The presence of fluid plays a critical role in controlling the fault slips and the seismogenesis in subduction zones. However, the origins and pathways of the fluid remain poorly understood. In this study, we examined the Inuyama sequence in the Mino belt of Japan, which is an on-land analog of a cold subduction zone such as the Japan Trench. In this region, white chert layers are intercalated within red chert layers, and the white chert layers were originally fluid conduits within subducting sediments along the plate interface according to the mass balance of the precipitated silica that constitutes the white chert. Trace element concentrations and Sr and Nd isotopic compositions of the chert layers suggest that the seawater expelled from subducting sediments due to compaction was the main carrier of silica. The temperature for the silica precipitation was approximately 60°C based on the O isotopic composition of the white chert layers, equivalent to the depth condition of approximately 5 km below the seafloor based on the analogy with the modern Japan Trench. As the major element concentrations suggest that the compaction and dehydration of the surrounding sediments could not fulfill the fluid volume required for supplying silica to fluid conduits, there may be trench-parallel fluid migration to one location where silica was precipitated. Furthermore, considering the fluid volume that would lead to silica precipitation, the permeability of the fluid conduit could be higher than that of the typical subducting siliceous sediments, suggesting that fluid flow toward shallow depths was transient.

Long-term tephrostratigraphies of volcanic islands such as the Azores are often limited to young and incomplete subaerial records. Here, we present a Pleistocene-Holocene marine tephra archive around the eastern islands of the Azores based on 22 marine gravity cores. We provide a new extensive data set comprising major element glass compositions of 350 marine tephra layers and 26 tephra samples collected from subaerial outcrops on São Miguel Island. The marine tephra record is dominated by trachytic glass compositions with lesser amounts of trachy-basaltic/basaltic materials. We categorize the marine deposits into four facies: primary fall (F1), primary flow (F2), eruption-related secondary flow (F3), and non-eruption-related secondary flow deposits (F4). Cores collected close to the islands contain a high proportion (>50%) of volcaniclastic material deposited during secondary processes. Using machine-learning algorithms, we assign potential volcanic sources to individual tephra layers. Furthermore, our spatial characterization of primary/secondary deposits demonstrates the importance of carefully selecting core locations during sea-going campaigns, as the preservation and distribution of primary and reworked tephras considerably differ depending on several factors (e.g., on paleo-wind directions, island topography, seafloor bathymetry, and distance from source volcanoes). We also report the occurrence of mafic primary fall tephras tens to hundreds kilometers away from the closest island, indicating a potential hazard from mafic Plinian eruptions in the Azores. This study aims to improve the estimation of the eruptive frequency of the archipelago and opens new pathways for more detailed studies concerning long-term volcanic cyclicity in the region.

Sulfate salts are deposited most commonly as evaporites on Earth; however, this is not their only origin. Pyrite oxidation during subsurface weathering is another common process on Earth that also produces a suite of sulfate salts—including the acidic phases jarosite and alunite—in sedimentary deposits. On Mars, the occurrence of sedimentary (non-vein-forming) sulfate salts has typically been interpreted as the result of an environmental evaporitic process. While these processes certainly occurred on Mars, we observed several specific occurrences of crystal pseudomorphs in the sedimentary strata preserved in Gale Crater that are likely nonevaporitic. These occurrences are not associated with bedded precipitates that might indicate a primary origin, nor are they associated with diagnostic sedimentary structures, including desiccation cracks, breccias, and tepee structures, that might indicate an early diagenetic origin. The sporadicity of these crystals implies a localized post-depositional process, which we propose to be the oxidation of sulfide-bearing minerals such as pyrite and pyrrhotite. If post-depositional pyrite and pyrrhotite oxidation took place in Gale Crater strata as it does perennially in terrestrial examples, it would have generated a significant amount of acidity that could dissolve a large proportion of any carbonates in contact with the same fluids.

South Sister volcano, Oregon Cascade Range, USA, has repeatedly erupted rhyolite since ca. 40 ka. The youngest such eruptions are the ca. 2 ka Rock Mesa and Devils Chain rhyolites, erupted several hundred years apart from two multi-vent complexes separated by 3–6 km. Fe-Mg interdiffusion models of orthopyroxene rims from both rhyolites produce timescales up to several-thousand years, but dominantly decades-to-centuries. Notably, the timescales of step-normal zoned orthopyroxene rims (i.e., normally zoned with a steep chemical gradient) from the Rock Mesa rhyolite are longer than those of reversely zoned crystals, whereas the Devils Chain produced mostly decadal timescales for both zoning types. Despite the proximity and broadly similar products of these episodes, their respective timescales indicate distinct sequences of events leading up to each eruption. The Rock Mesa timescales record centuries of magma chamber growth followed by decades of predominantly magma rejuvenation, reorganization, and destabilization. In contrast, the Devils Chain episode was preceded by a single episode of coupled rhyolite extraction, rejuvenation, and hybridization. Rare, high-An plagioclase cores and evidence of reheating implicate cryptic emplacement of mafic magma at the base of the rhyolite reservoirs. However, the diffusion timescales do not unequivocally support a single magma recharge event that affected both. Fluid fluxing and the reorganization of melt into buoyant magma chambers likely provided the source of increasing pressurization that initiated each eruption after several decades. Geodetic models of ongoing deformation west of South Sister could consider these processes in addition to magma emplacement.

Fe-oxidizing microorganisms in deep-sea hydrothermal vent environments are often used as analogs for primordial life on Earth. In fact, Earth's oldest purported microfossils are preserved as hematite filaments in a jasper rock dated between 4,160 and 4,280 million years and are thought to have originated in a seafloor hydrothermal environment. However, the kinds of post-depositional processes that can alter their morphologies are not well-known, which has implications for recognizing morphologies of bona fide microbial origin in the deep-time rock record. Here, we show that more than 10 morphological types of filamentous Fe-oxyhydroxide microstructures occur in Fe-oxide specimens from deep-sea hydrothermal vents in the southwest Indian Ocean, including thick and thin filaments with the following morphologies: parallel-alignments, branching and pectinate-branching, curved and straight, hollow to tubular, twisted and coated with botryoidal silica. Botryoidal silica mineralization is documented on several filament morphotypes and exhibits pattern with spheroidal twins, circular concentricity, and cavities, whereas their elemental composition is dominated by Si, Fe, Mn, C, with minor S and halogens. Such patterns and substances point to an origin from chemically oscillating reactions, which provide a novel abiotic model based on C, Fe, Mn, S, and halogen redox reactions in colloidal silica, to explain occurrences of botryoidal minerals grown onto deep-sea filamentous Fe-oxyhydroxide microstructures. The documented filamentous morphologies and new model for silica botryoid formation help to understand abiotic carbon cycling in marine and lacustrine environments, ancient filaments preserved in the geological record, as well as a basis to seek similar structures in deep-space settings.

A detailed study of palagonitization in rocks from Deception Island—one of Antarctica's most active volcanoes—has been performed to advance our understanding of this alteration process. A detailed petrographic (optical and SEM), mineralogical (XRD), and mineral and glass spot geochemistry (EDS and EMP) characterization has been conducted on pyroclastic samples. Palagonitization occurred at 80–100°C and involved (a) initial glass to palagonite transformation by congruent glass dissolution and precipitation, followed by (b) palagonite maturation resulting in increasing crystallization into an assemblage of dominant smectite with minor illite, zeolites and Ti-bearing oxides. During the first stage, an optically amorphous phase is formed with an estimated average density of 1.7–1.8 g/cm³ and a very early mineralogical control on its composition indicating nucleation at the nm-scale. Major elements are typically leached except for Ti, which behaves as immobile throughout palagonitization. Palagonite maturation occurs in an open system (variable element depletion and supply) and is controlled by an interplay between crystal nucleation and growth, overall mass balance, and local equilibration between crystals and fluid. Mass balances control palagonite porosity and density. Highly local physicochemical conditions (e.g., fluid chemistry or water-rock ratio) play a major role in the chemical and mineralogical composition and evolution of palagonite. Variability of these controls at the microscale produces a large variability in palagonite characteristics even at the intraclast scale. Glass composition has not been observed to play a significant role. Textures observed in several samples indicate the contribution of microbial activity to glass alteration.

The southwest region of Western Australia is one of the oldest continental regions on Earth, hosting the Archean Yilgarn Craton, bounded by the Proterozoic Albany-Fraser and Pinjarra orogens. Here we calculate shear wave splitting of the PKS and SKS teleseismic phases using new broadband arrays with unprecedented station spacing across the region. We find evidence for coherent seismic anisotropy, with the regional average delay time ($��1.05�\pm �0.39�$ s) comparable to the global average, $��\delta �$t = 1 s. Although fast polarization orientations show variation, they are not aligned with current plate motion and the expected mantle flow direction. In the South West Terrane and Albany-Fraser Orogen, fast polarization orientations match the trend of ancient structural faults. In contrast, structural faults in the Youanmi Terrane are oriented at an angle compared to the E–W and NE–SW fast polarizations. Instead, seismic anisotropy patterns show an intriguing similarity to E–W trending Precambrian (2.42 Ga) dykes that extend uninterrupted across the Yilgarn Craton. We propose that lithospheric fabrics frozen-in at the time of craton formation (2.76–2.65 Ga) generated a mechanical weakness which subsequently influenced the orientation and emplacement of the dykes. Further evidence for a similar, ancient (2.73 Ga) architectural fabric comes from recent isotope geochemistry analysis of primary ENE-trends within the Yilgarn Craton. Overall, these results point toward large-scale, fossilized lithospheric fabric within the Yilgarn Craton, preserved for over two billion years, offering a unique window into the formation and early evolution of the continent.

The Goiás Magmatic Arc (GMA) is a Neoproterozoic juvenile terrain formed through oceanic-oceanic and oceanic-continental subduction. It comprises three main segments: Mara Rosa, Anicuns-Itaberaí (GMAAI), and Arenópolis (GMAA), whose tectonic evolution and connectivity remain uncertain. Debate persists on: (a) whether these segments evolved together from the start or were juxtaposed during collision; (b) metallogenic models lack information on the lower crust and upper mantle, essential for fertility analysis; (c) why the Cretaceous Goiás Alkaline Province (GAP) outcrops exclusively in the GMAA. To address these questions, we conducted a long-period magnetotelluric survey (34 stations, 10–13,000 s) across the GMAA/GMAAI. The resulting 3D resistivity model (nRMS = 1.21) reveals distinct lithospheric structures. GMAA exhibits significant crustal thickness and resistivity variations, while GMAAI shows higher resistivity and a thicker lithosphere, which likely controlled the confinement of the GAP within the GMAA. The absence of a continuous conductor along the Moiporá-Novo Brasil shear zone suggests that these segments may be splays of the Transbrasiliano Lineament. Thus, the GMAA and GMAAI likely evolved along the same subduction front, amalgamating in the final stages of the Brasiliano orogeny. Three major conductors were identified: C1, associated with the Bom Jardim deposit (Cu); C2, linked to the Americano do Brasil deposit (Ni–Cu sulfide); and C3, related to the Caldas Novas geothermal anomaly. These findings provide new insights into the GMA's tectonic evolution and metallogenic significance, revealing deep conductive zones linked to known mineral deposits and highlighting the control exerted by lithospheric architecture in the region.

This study explores the isotopologues m/z 47 (¹³C¹⁸O¹⁶O, denoted Δ₄₇) and m/z 48 (¹²C¹⁸O₂, denoted Δ₄₈) in CO₂ derived from carbonate minerals, focusing on their temperature dependency, kinetic isotope effects, and distinct reaction pathways. By conducting experiments at four temperatures (5°C, 10°C, 15°C, and 25°C) using the enzyme carbonic anhydrase (CA) at pH 8.3, we approached isotopic equilibrium in dissolved inorganic carbon and measured Δ₄₇, Δ₄₈, and δ¹⁸O. Our results were compared with data from Devils Hole cave calcite and existing temperature calibrations, yielding regression equations correlating Δ₄₇ and Δ₄₈ with temperature: Δ_{47 I-CDES} = (2.43 ± 0.289) Δ_{48 CDES 90} − (0.006 ± 0.074); r² = 0.96; Δ_{47 I-CDES} = (0.038 ± 0.003) × 10⁶T⁻² + (0.161 ± 0.032); r² = 0.99; Δ_{48 CDES 90} = (0.015 ± 0.002) × 10⁶T⁻² + (0.076 ± 0.025); r² = 0.94. Further, calcite precipitated at varying temperatures and pHs of 8.3–11 had kinetic enrichments of Δ₄₇ and depletions of Δ₄₈ and δ¹⁸O at pH ≥ 9.5, with CA presence leading to distinct kinetic slopes and more efficient Δ₄₈ catalysis. These findings are consistent with theoretical predictions for kinetic effects from CO₂ hydration/hydroxylation. Additionally, methodological details for Δ₄₈ measurements using the Nu Perspective mass spectrometer are provided, including shot noise calculations, baseline corrections, and how nonlinearity evolves over time. We show that these instruments do not have pressure baseline effects on m/z 48, which is a result of secondary electron suppression on the m/z 48 collector. Therefore, calibration data should be unbiased by these analytical effects.