Geochemistry Geophysics Geosystems最新文献

At slow to ultraslow spreading ridges, the limited melt supply results in tectonic accretion and the exhumation of mantle rocks. Melt supply is focused toward volcanic centers where magmatic accretion dominates. In areas where the ridges reorientate, both types of accretion can occur across the ridge axis with detachment faults developing on the inside corners and hydrothermal vent fields located in close proximity. Microseismicity studies improve the understanding of the tectonic processes at detachment faults and their interplay with hydrothermal vent systems, but are mostly limited to mature detachment faults or short deployment times. This study presents results from a ∼11 months ocean bottom seismometer deployment around the Loki's Castle hydrothermal vent field at the intersection of the slow to ultraslow spreading Mohns and Knipovich Ridge. We observe seismicity to be highly asymmetric with the majority of the plate divergence being accommodated by an emerging detachment fault at the inside corner of the intersection west of Loki's Castle. Seismic activity related to the detachment fault displays a distinct contrast, with continuous low-magnitude events occurring at depth and episodic large-magnitude events concentrated in clusters within the footwall. The detachment fault shows no significant roll-over at shallow depths and the locus of spreading is located east of the detachment. These results suggest that the detachment fault west of Loki’s Castle is at an early development stage.

The metaturbidite-hosted, ∼380 Ma Dufferin gold deposit, Meguma terrane, northeastern Appalachian Orogen (Nova Scotia, Canada) is an orogenic gold deposit with mineralized saddle reef-type quartz veins hosted by metasandstones and black slates in a tightly folded anticline. Together with native gold inclusions, genetically related hydrothermal carbonaceous material (CM) in veins occurs as pyrobitumen in cavities and along fractures/grain boundaries proximal to vein contacts and wallrock fragments. Integrating several microanalytical methods we document the precipitation of gold via coupled fluid-fO₂ reduction (via interaction with CM) and pH increase. These changes in fluid chemistry destabilized gold bisulfide complexes, leading to efficient Au precipitation from a gold-undersaturated (0.045 ± 0.024 ppm Au; 1σ; n = 58 fluid inclusions) aqueous-carbonic fluid (H₂O-NaCl-CO₂ ± N₂ ± CH₄). The proposed mineralization mechanism is supported by: (a) a complementary decrease in Au and redox-sensitive semimetals (As, Sb), and increase in wall rock-derived elements (i.e., Mg, K, Ca, Sr, Fe) concentrations in fluid inclusions with time; (b) a corresponding decrease in the X_CO2, consistent with CO₂ removal via reduction/respeciation and late carbonate precipitation; and (c) gold embedding in, or on, the surface of CM inside mineralized cavities and fractures. Despite mineralizing fluids transporting low concentrations of Au far from saturation, precipitation of gold was locally evidently high where such fluids interacted with CM, contributing to the overall gold endowment of Meguma deposits. This work re-emphasizes CM as a potential prerequisite for efficient gold precipitation within the overall genetic model for similar orogenic metasedimentary settings globally where the presence and/or role of CM has been documented.

Volcanic activity has been shown to affect Earth's climate in a myriad of ways. One such example is that eruptions proximate to surface ice will promote ice melting. In turn, the crustal unloading associated with melting an ice sheet affects the internal dynamics of the underlying magma plumbing system. Geochronologic data from the Andes over the last two glacial cycles suggest that glaciation and volcanism may interact via a positive feedback loop. At present, accurate sea-level predictions hinge on our ability to forecast the stability of the West Antarctic Ice Sheet, and thus require consideration of two-way subglacial volcano-deglaciation processes. The West Antarctic Ice Sheet is particularly vulnerable to collapse, yet its position atop an active volcanic rift is seldom considered. Ice unloading deepens the zone of melting and alters the crustal stress field, impacting conditions for dike initiation, propagation, and arrest. However, the consequences for internal magma chamber dynamics and long-term eruption behavior remain elusive. Given that unloading-triggered volcanism in West Antarctica may contribute to the uncertainty of ice loss projections, we adapt a previously published thermomechanical magma chamber model and simulate a shrinking ice load through a prescribed lithostatic pressure decrease. We investigate the impacts of varying unloading scenarios on magma volatile partitioning and eruptive trajectory. Considering the removal of km-thick ice sheets, we demonstrate that the rate of unloading influences the cumulative mass erupted and consequently the heat released into the ice. These findings provide fundamental insights into the complex volcano-ice interactions in West Antarctica and other subglacial volcanic settings.

The study of active fault zones is fundamental to understanding both long-term tectonics and short-term earthquake behavior. Here, we integrate lidar-enabled geomorphic-geologic mapping and petrochronological analysis to reveal the slip-history, tectonic evolution, and structure of the southern Alpine Fault in New Zealand. New petrographic, zircon U-Pb and zircon trace-element data from fault-displaced basement units provides constraint on ∼70–90 km of right-lateral displacement on the presently active strand of the southern Alpine Fault, which we infer is of Plio-Quaternary age. This incremental displacement has accumulated while the offshore part of the fault has evolved within a distributed zone of plate boundary deformation. We hypothesize that pre-existing faults in the continental crust of the Pacific Plate have been exploited as components of this distributed plate boundary system. Along the onshore southern Alpine Fault, detailed mapping of active fault traces reveals complexity in geomorphic fault expression. Our analysis suggests that the major geomorphic features of the southern Alpine Fault correspond to penetrative fault zone structures. We emphasize the region immediately south of the central-southern section boundary, where a major extensional stepover and restraining bend are located along-strike of each other. We infer that this geometry may reflect segmentation of the Alpine Fault between two distinct fault segments. The ends of these proposed segments meet near where several Holocene earthquake ruptures have terminated. Our new constraints on the evolution and structure of the southern Alpine Fault help contribute to improved characterization of the greatest onshore source of earthquake hazard in New Zealand.

Natural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (<50 m). Their formation has previously been linked to methane seepage from the seafloor. Here, we characterize over 50,000 pockmarks based on their morphology, geochemical signature, and the subsurface pre-conditions using a new integrated geoscientific data set, combining geophysical and sedimentological data with geochemical porewater and oceanographic analysis. We test whether the methane seepage is indeed responsible for pockmark formation. However, our data suggest that neither the seepage of light hydrocarbons nor groundwater is driving pockmark formation. Because of this lack of evidence for fluid seepage, we favor the previously suggested biotic formation but also discuss positive feedback mechanisms in ocean bottom currents as a formation process. Based on a comparison of pockmarks to the central and southeastern North Sea, we find that local lithology significantly affects pockmark morphology. Muddy lithologies favor the formation of larger, long-lived structures, while sandy lithologies lead to short-lived, small-scale structures that are large in area but with shallow incision depth. We conclude that pockmarks in sandy environments might have been overlooked globally due to their shallow incision depth and recommend reevaluating the role of hydrocarbon ebullition in pockmark formation.

Understanding the composition of lavas erupted at the surface of the Earth is key to reconstruct the long-term history of our planet. Recent geochemical analyses of ocean island basalt samples indicate the preservation of ancient mantle heterogeneities dating from the earliest stages of Earth's evolution (Péron & Moreira, 2018, https://doi.org/10.7185/geochemlet.1833), when a global magma ocean was present. Such observations contrast with fluid dynamics studies which demonstrated that in a magma ocean the convective motions, primarily driven by buoyancy, are extremely vigorous (Gastine et al., 2016, https://doi.org/10.1017/jfm.2016.659) and are therefore expected to mix heterogeneities within just a few minutes (Thomas et al., 2023, https://doi.org/10.1093/gji/ggad452). To elucidate this paradox we explored the effects of the Earth's rapid rotation on the stirring efficiency of a magma ocean, by performing state-of-the-art fluid dynamics simulations of low-viscosity, turbulent convective dynamics in a spherical shell. We found that rotational effects drastically affect the convective structure and the associated stirring efficiency. Rotation leads to the emergence of three domains with limited mass exchanges, and distinct stirring and cooling efficiencies. Still, efficient convective stirring within each region likely results in homogenization within each domain on timescales that are short compared with the solidification timescales of a magma ocean. However, the lack of mass exchange between these regions could lead to three or four large-scale domains with internally homogeneous, but distinct compositions. The existence of these separate regions in a terrestrial magma ocean suggests a new mechanism to preserve distinct geochemical signatures dating from the earliest stages of Earth's evolution.

The wide distribution of tuff layers, locally named the “green bean rocks” (GBRs) in the Yangtze Block straddling the Early Middle Triassic marine sequence indicates intense volcanic eruption(s). Sr, Nd, and S isotope compositions and trace elements of marine sediments were analyzed spanning the tuff layers to elucidate their responses to the volcanic eruptions and related environmental changes. The Sr isotope compositions of marine sediments are comparable to those of open seawater during the time interval of ca. 245–248 Ma. Sr and Nd isotope compositions of the samples show synchronous increases in the ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values during the deposition of GBRs. The elevated ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values are proposed to be caused by the input of volcanic tephra and increased influx of weathering product of mafic rocks (most likely the Emeishan flood basalts). The S isotope compositions of sulfates exhibit a negative shift in the GBRs, which could possibly be attributed to greater input of lighter ³²S from weathering products and volcanic eruptions. The variation of Th/U ratios indicate that the GBRs formed in an anoxic environment, resulting from high marine productivity as a consequence of more nutrients from weathering and volcanic materials. The responses of Sr, Nd, and S isotopes to volcanic eruptions during the Early Middle Triassic indicate this event resulted in adverse effects, namely enhanced eutrophication and low O₂ levels, acidic precipitation, toxic components, etc., that could cause ecological destruction both on land and in the sea.

Recent seismic tomography models suggest large-radius primary plumes originating from the core-mantle boundary, with grain size variations potentially explaining these observations. Additionally, grain size variations are thought to enhance the long-term stability of Large Low Shear Velocity Provinces (LLSVPs), identified as thermochemical piles near the core-mantle boundary. Nevertheless, geodynamic models investigating these hypotheses remain limited. To address this gap, we constructed a series of geodynamic numerical models incorporating grain size evolution, plate tectonics, and the spontaneous generation of deep mantle plumes above LLSVPs. Our results reveal that grain size evolution does not significantly affect the plume width, primarily because the increased strain rate in the mantle plume suppresses both its grain size and viscosity. The region adjacent to the plumes, characterized by the accumulation of mantle materials with larger grain size and low-temperature remnants of subducted slabs, displays a higher viscosity compared to the area near the subducted slabs. Furthermore, grain size evolution plays a crucial role in enhancing the stability of LLSVPs by increasing the viscosity ratio between LLSVPs and the ambient mantle. These findings underscore the need for incorporating grain size evolution in geodynamic models to gain a better understanding of the dynamics of plumes and lower mantle.

Underground storage in geologic formations will play a key role in the energy transition by providing low-cost storage of renewable fuels such as hydrogen. The sealing qualities of caverns leached in salt and availability of domal salt bodies make them ideal for energy storage. However, unstable boundary shear zones of anomalous friable salt can enhance internal shearing and pose a structural hazard to storage operations. Considering the indistinct nature of internal salt heterogeneities when imaged with conventional techniques such as reflection seismic surveys, we develop a method to map shear zones using seismicity patterns in the US Gulf Coast, the region with the world's largest underground crude oil emergency supply. We developed and finetuned a machine learning algorithm using tectonic and local microearthquakes. The finetuned model was applied to detect microearthquakes in a 12-month long nodal seismic dataset from the Sorrento salt dome. Clustered microearthquake locations reveal the three-dimensional geometry of two anomalous salt shear zones and their orientations were determined using probabilistic hypocenter imaging. The seismicity pattern, combined with borehole pressure measurements, and cavern sonar surveys, shows the spatiotemporal evolution of cavern shapes within the salt dome. We describe how shear zone seismicity contributed to a cavern well failure and gas release incident that occurred during monitoring. Our findings show that caverns placed close to shear zones are more susceptible to structural damage. We propose a non-invasive technique for mapping hazards related to internal salt dome deformation that can be employed in high-noise industrial settings to characterize storage facilities.