Geochemistry Geophysics Geosystems最新文献

The spatiotemporal interaction of large- and regional-scale upper mantle forces can prevail in collisional settings. To better understand the role of these forces on post-subduction tectonics, we focus on mantle dynamics in the East Anatolian Plateau, a well-documented segment of the Arabian-Eurasian continental collision zone. Specifically, we analyze multiple forces in the upper mantle, which have not been considered in previous studies in this region. To this end, we use a state-of-the-art 3D instantaneous geodynamic model to quantify the dynamics of thermally defined upper mantle structures derived from seismic tomography data. Results reveal a prominent SW-NE-oriented mantle flow from the Arabian foreland to the Greater Caucasus–a plumelet–through a lithospheric channel under the East Anatolian Plateau. This plumelet induces localized dynamic topography (∼500 m) around the extensional Lake Van province, favoring NE-directed compression and westward escape of the Anatolian plate. We suggest that the Lake Van region is an active magma-rich intraplate rift in the Africa-Arabia-Anatolian plume-rift system. The rift zone was probably initiated by Neotethyan subduction-related forces and has been reactivated and/or sustained by the plumelet-induced convective support. Our findings are consistent with numerous observations, including the recent low-ultralow seismic velocities with a SW-NE splitting anisotropy pattern, geochemical and petrological studies, and local kinematics showing upper mantle-induced extensional tectonics in the collisional region.

The compositional diversity of primitive arc basalts has long inspired questions regarding the drivers of magmatism in subduction zones, including the roles of decompression melting, mantle heterogeneity, and the amount and composition of slab-derived materials. This contribution presents the volatile (H₂O, Cl, and S), major, and trace element compositions of melt inclusions from basaltic magmas erupted at three volcanic centers in the Washington Cascades: Mount St. Helens (two basaltic tephras, 2.0–1.7 ka), Indian Heaven Volcanic Field (two <600 ka basaltic hyaloclastite tuffs), and Glacier Peak (late Pleistocene to Holocene basaltic tephra from Whitechuck and Indian Pass cones). Compositions corrected to be in equilibrium with mantle olivine display variability in Nb and trace element ratios indicative of mantle source variability that impressively spans nearly the entire range of arc magmas globally. All volcanic centers have magmas with H₂O and Cl contributions from the downgoing plate that overlap with other Cascade Arc segments. Volatile abundances and trace element ratios support a model of melting of a highly variably mantle wedge driven by a subduction component of variably saline fluid and/or slab partial melt. Magmas from Glacier Peak in northern Washington have unusually high Th/Yb ratios that are similar to Lassen region basalts, indicating possible contributions of “subcreted” metasediments that geophysical data suggest are not present beneath central Oregon and southern Washington. This data set adds to the growing inventory of primitive magma volatile concentrations and provides insight into spatial distributions of mantle heterogeneity and the role of slab components in the petrogenesis of arc magmas.

Serpentinites are major carriers of volatiles in deep subduction zones, releasing most fluids in the 500–650°C range. Despite fundamental implications for mass transfer and intermediate-depth seismicity, the mechanical role of these fluids is unclear. To characterize the mechanical role of fluids at (ultra)high-pressure conditions, we perform a petro-structural analysis on olivine-rich veins from the Western Alps meta-ophiolite. Some veins formed through dilational and mixed dilational-shear fracturing without significant shear-related deformation. However, field and microstructural observations indicate transient shearing and dilational fracturing at high pore fluid pressures. These include: (a) foliated sheared veins; (b) newly formed olivine and Ti-clinohumite within mineral lineations coating sheared veins and shear bands; (c) Olivine and Ti-clinohumite mineral fibers sealing porphyroclasts; (d) mutual crosscutting relationships among dilational and shear features. Dilational veins prevail in low-strain areas, while sheared veins and shear bands dominate within high-strain zones toward the ultramafic sliver boundaries. These strain variations underscore the role of local stress regimes during serpentinite dehydration. Consequently, areas experiencing stronger shear stresses around large-scale blocks or mechanical weakening during fluid circulation are prone to draining overpressurized fluids. These interface-parallel and fracture-controlled pathways thus facilitate fluid escape from the dehydrating downgoing slab. Transient events of dilational fracturing and brittle-ductile shearing, along with strain localization in highly comminuted olivine-bearing sheared veins, may have resulted from strain rate bursts potentially related to (sub)seismic deformation. These observations are in line with geophysical data indicating high pore fluid pressures within the intermediate-depth seismicity region.

Magnetic hysteresis measurements are routinely made in the Earth and planetary sciences to identify geologically meaningful magnetic recorders, and to study variations in present and past environments. Interpreting magnetic hysteresis data in terms of domain state and paleomagnetic stability are major motivations behind undertaking these measurements, but the interpretations remain fraught with challenges and ambiguities. To shed new light on these ambiguities, we have undertaken a systematic micromagnetic study to quantify the magnetic hysteresis behavior of room-temperature magnetite as a function of particle size (45–195 nm; equivalent spherical volume diameter) and shape (oblate, prolate and equant); our models span uniformly magnetized single domain (SD) to non-uniformly magnetized single vortex (SV) states. Within our models the reduced magnetization associated with SV particles marks a clear boundary between SD (≥0.5) and SV (<0.5) magnetite. We further identify particle sizes and shapes with unexpectedly low coercivity and coercivity of remanence. These low coercivity regions correspond to magnetite particles that typically have multiple possible magnetic domain state configurations, which have been previously linked to a zone of unstable magnetic recorders. Of all the hysteresis parameters investigated, transient hysteresis is most sensitive to particles that exhibit such domain state multiplicity. When experimental transient hysteresis is compared to paleointensity behavior, we show that increasing transience corresponds to more curved Arai plots and less accurate paleointensity results. We therefore strongly suggest that transient behavior should be more routinely measured during rock magnetic investigations.

The tectonic configuration of the Caribbean plate is defined by inward-dipping double subduction at its boundaries with the North American and Cocos plates. This geometry resulted from a Paleogene plate reorganization, which involved the abandonment of an older subduction system, the Great Arc of the Caribbean (GAC), and conversion into a transform margin during Lesser Antilles (LA) arc formation. Previous models suggest that a collision between the GAC and the Bahamas platform along the North American passive margin caused this event. However, geological and geophysical constraints from the Greater Antilles do not show a large-scale compressional episode that should correspond to such a collision. We propose an alternative model for the evolution of the region where lower mantle penetration of the Farallon slab promotes the onset of subduction at the LA. We integrate tectonic constraints with seismic tomography to analyze the timing and dynamics of the reorganization, showing that the onset of LA subduction corresponds to the timing of Farallon/Cocos slab penetration. With numerical subduction models, we explore whether slab penetration constitutes a dynamically feasible set of mechanisms to initiate subduction in the overriding plate. In our models, when the first slab (Farallon/Cocos) enters the lower mantle, compressive stresses increase at the eastern margin of the upper plate, and a second subduction zone (LA) is initiated. The resulting first-order slab geometries, timings, and kinematics compare well with plate reconstructions. More generally, similar slab dynamics may provide a mechanism not only for the Caribbean reorganization but also for other tectonic episodes throughout the Americas.

Coral-based stable oxygen isotopes (δ¹⁸O) have been used as a proxy for sea surface temperature (SST) since the 1970s, and δ¹⁸O–SST calibration studies have been fundamental to assure robust and faithful SST reconstructions. Paleoclimatic studies based on corals from the tropical western South Atlantic (TWSA) are scarce, and the available coral species need to be calibrated to improve climate and environmental reconstructions. Siderastrea stellata, a slow-growing coral, is a potential species to be explored as a coral archive in the TWSA. We provide the first multi-site δ¹⁸O–SST calibration for the coral S. stellata from three locations at the TWSA: Todos os Santos Bay, Tamandaré and the Rocas Atoll. Pseudo-coral δ¹⁸O calculations derived from gridded SSS and SST show that the contributions of SSS and SST to coral δ¹⁸O are expected to be different at each site. Weighted least squares linear regressions performed between the δ¹⁸O and SST generated the following calibrations equations: δ¹⁸O = −0.18 (±0.02) × SST (°C) + 1.90 (±0.47) for Todos os Santos Bay; δ¹⁸O = −0.18 (±0.02) × SST (°C) + 1.54 (±0.67) for Tamandaré; and δ¹⁸O = −0.16 (±0.03) × SST (°C) + 1.24 (±0.71) for the Rocas Atoll. The δ¹⁸O-SST sensitivity of S. stellata from the TWSA is similar to that of other slow-growing species of the genus and consistent with the expected δ¹⁸O-SST sensitivity of other species reported in the literature. These calibrations will allow future SST reconstructions based on δ¹⁸O records from sub-fossil and fossil S. stellata, an abundant species in the TWSA.

The powerful eruption of Hunga volcano (15-January-2022) excavated ∼6.3 km³ of pre-existing material, leaving behind an 855 m deep crater. The scientific and humanitarian response to this event was challenging due to the remote location, safety concerns, and COVID-19 pandemic restrictions. To investigate the status of ongoing eruptive/hydrothermal activity, this study used, for the first time, an un-crewed surface vessel operated remotely from >16,000 km away to make direct water column measurements within the crater and map its structure in detail. Intense turbidity and oxidation-reduction potential (ORP) anomalies located ongoing activity at sites on the steep inside crater slopes near both remaining islands. Mid-water acoustic reflectors indicated ongoing degassing, and positive ORP anomalies suggested gas composition was dominated by CO₂. At least 75% of the crater rim is shallower than 100 m, so any exchange with the surrounding ocean is limited by the depths of breaches in the rim (185 m between the islands and 290 m on the ENE side). This post-eruption bathymetry results in accumulation of emission products within the deep crater. There were no indications of the ongoing activity visible at the ocean surface, which highlights the limitations and inherent biases associated with relying on discolored surface water and/or atmospheric disturbances to determine eruption start/end dates at submarine volcanoes. This study demonstrates the value and need to add repeat hydrothermal plume and bathymetric surveys to our toolbox for monitoring submarine volcanoes, and the potential for un-crewed, remotely operated vessels to contribute significantly to these efforts.

The structure of the lithosphere and the associated magmatic systems found in different locations along slow-spreading ridges can vary dramatically, from melt-starved to magmatically robust segments. A growing number of studies suggest that the evolution of the magmatic crust being governed solely by fractional crystallization is too simplistic. Reactions between migrating melts and their surroundings play a key role during accretion, yet the full extent of their impact is still to be resolved. We present here the results of a petrological, microstructural, and in situ geochemical study of two drilled sequences from the Kane Megamullion and Atlantis Massif oceanic core complexes. We show that melt-mush reactions generate locally strong textural and/or geochemical heterogeneity at the cm-scale, but their impact can also be identified at the 100 m-scale. We found evidence for assimilation at various degrees of primitive lithologies of potential mantle origin within the gabbroic sequence at both locations, in addition to typical melt-mush reactions previously described in other slow-spread magmatic systems. Observations and numerical modeling confirm the similarity of the reactions impacting both sequences. However, the regime of the reactions (ranges of assimilation to crystallization ratios) seems to vary between Kane Megamullion and Atlantis Massif, variations which likely result from differences in melt fractions present during melt-mush reactions. We infer relying on our observations and previous studies that the regime of the reactions is most likely controlled by the melt flux during the formation of the two sections.

The brittle-plastic transition (BPT), the strongest part of the crust, is critical to continental geodynamics but is poorly understood relative to simpler crust above and below. It is typically represented as a depth transition from brittle/frictional to plastic/viscous deformation controlled by temperature and pressure. Footwalls of low-angle normal faults (LANFs) exhumed through the BPT provide rock records that challenge this view. Three well-studied LANF footwalls are reviewed. All record geochemical, mineralogical and fluid-related controls on embrittlement, not just monotonic P-T decrease. Two quartz-rich examples record embrittlement at unexpectedly high T (≥450–500°C) that was modulated by wetting characteristics of fluids. One had an inverted BPT: brittle fracture beneath contemporaneous mylonites. In another study, a brittle LANF grew from plastic mylonites due to mineralogic changes that strengthened parts, causing initial frictional slip and cataclasis on weak planes that ultimately linked. In all, geologically abrupt small-scale processes controlled behavior at kilometer scales. Similar processes likely affect other tectonic settings and seismic cycles. Such processes offer fertile research opportunities in continental geodynamics; they will be increasingly tractable as computational abilities improve. Adaptive, multi-scale approaches including the effects of fluid-rock geochemistry and mineralogical changes on rock strength and deformation are needed. Thoughtful modeling approaches may yield key insights into the positive and negative feedbacks that are likely. Discontinuous deformation is probably needed explicitly along with exploration of initial and boundary conditions.

The upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H₂O, S, and CO₂, and Fe³⁺/ΣFe and S⁶⁺/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine-, plagioclase-, and pyroxene-hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H₂O, and fO₂ of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H₂O, not elevated in fO₂ compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar in fO₂ and H₂O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H₂O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H₂O contents, are the cause for the prominent geophysical anomaly observed in this region.