Geochemistry Geophysics Geosystems最新文献

We present a mid-to-late Holocene record of relative paleosecular variation from the Ross Sea region of Antarctica. The 6,700-year-long record of inclination, declination, and relative paleointensity from a marine sediment core collected near Cape Adare is independently dated using a combination of ramped pyrolysis oxidation and carbonate radiocarbon dates. Agreement between the large-scale features of the relative paleointensity record and the virtual axial geomagnetic dipole moment suggests that changes in the record are dominated by the dipole component of the Earth's geomagnetic field. Correspondence between the record and a non-independently dated reconstruction from the Antarctic Peninsula indicates regionally coherent changes in the geomagnetic field intensity in the southern high latitudes during the mid-to-late Holocene. The prominent features of the record serve as stratigraphic markers for hard-to-date Antarctic sedimentary records and a constraint on Holocene geomagnetic field behavior when incorporated into the next generation of geomagnetic field models.

Melting for the generation of intracontinental diffuse basaltic fields is generally ascribed to the effects from lithospheric dynamic processes, but how the asthenospheric dynamics influence the overlying lithosphere remains poorly known. Here, this is explored via a combined geochemical and numerical modeling study on the long-lived Middle Yangtze basaltic field adjacent to the Dabie orogenic belt, central China. This basaltic field with a southwestard younging trend consists of 110–40 Ma basaltic rocks that show alkaline to sub-alkaline major-oxide compositions, oceanic island basalt-like trace-element patterns and MORB-like Sr-Nd-Pb isotopic ratios. The elemental and isotopic signatures show that the basaltic episodes generally underwent fractionation in the crust-mantle transition zone from sub-alkaline parental melts, which were produced by decompression melting of asthenosphere under a thinned (<2 GPa) lithosphere. Thermodynamic-based Bayesian probabilistic inversions on rare-earth element data further show that the melting for different basaltic episodes occurred at consistently shallow depths (40–60 km) and ambient mantle-like potential temperatures (1350–1400°C). Together with regional lithospheric seismic imaging and the orogenic evolution nearby, the southwestard younging trend likely records progressive erosion of the basal lithosphere from the northern margin of the Yangtze block to its interior; the lithospheric thinning was likely triggered by an inland propagating chain of mantle instabilities at lithosphere-asthenosphere boundary initiated by the delamination of the Dabie orogenic root. We suggest that the proposed model should have general relevance to many other intraplate diffuse basaltic fields with similar spatiotemporal features.

High-pressure and high-temperature experiments were conducted to investigate the partitioning behaviors of Si, O, and Mg between molten Fe-alloys and silicate melts in the Fe-Si-O-Mg system under conditions of 2–72 GPa and 2000–5500 K, using both laser-heated diamond anvil cells and a multi-anvil press. Combing our new experimental results with previously published data, we evaluated the effects of pressure, temperature, and metallic compositions on the partitioning behaviors of Si, O, and Mg. A set of internally consistent interaction parameters between Si, O, and Mg were obtained by the simultaneous fitting of distribution coefficients for all three elements in the Fe-Si-O-Mg system. The composition-dependent distribution coefficients were applied in calculating the compositional evolution of metallic melts during multi-stage core formation. Our results suggest that the core-forming metallic melts would contain more Si and O than previously estimated due to the attractive interactions of light elements in the metal. Compared to the geophysically constrained core composition, these findings imply the exsolution of light elements, likely in the form of SiO₂, from the outer core upon cooling.

Tracking the supply and demand of nitrate in the past ocean can help to constrain the role of biology and circulation in regulating climate. The nitrogen isotopic composition of marine phytoplankton (δ¹⁵N_biomass) tracks nitrate supply and demand but may not be well preserved in the sediments, while the isotopic composition of nitrogen contained within the frustules of diatoms (δ¹⁵N_DB) is thought to be protected from alteration. Here we document the relationship between δ¹⁵N_biomass and δ¹⁵N_DB in cultures of seven Southern Ocean diatom species. Average δ¹⁵N_biomass values are 1.8 ± 0.8‰ lower than δ¹⁵N_DB values for 6 of the 7 species grown. The exception, Fragilariopsis rhombica, records δ¹⁵N_biomass that are greater than δ¹⁵N_DB values, opposite in sign from the other studied species and statistically significantly different from each other. Because most of the diatom species measured indistinguishable isotopic offsets between biomass and diatom-bound N, we assert that sedimentary δ¹⁵N_DB values largely reflect surface ocean nitrate δ¹⁵N and therefore nitrate supply and demand. F. rhombica was not only different in terms of its relationship between δ¹⁵N_biomass and δ¹⁵N_DB but also in mean N:Si uptake and nitrogen content of cleaned frustules, suggesting a role for N allocation relative to Si in setting δ¹⁵N_DB. While this result is largely based on only one species of diatom in culture, it is possible that significant contribution of groups with a similarly elevated ε_DB could decrease the overall value of the sedimentary δ¹⁵N_DB signal.

Authigenic carbonates in the exhumed Miocene Shimanto accretionary complex (the Nabae carbonates) were studied in the Cape Muroto area to better constrain the formation mechanisms and tectonic setting during carbonate precipitation. Three main types of carbonates were recognized on the basis of morphology: nodular, bedded, and tubular. The δ¹³C values of the carbonates define two distinct groups with relatively low (−31.9‰ to −20.3‰) and relatively high values (−14.7‰ to −4.7‰). The former suggests carbonate precipitation from methane-enriched fluids, while the latter indicates mixing of fluid sources in a range of depositional settings. Most carbonates have REE + Y patterns characterized by well-defined positive Eu and Y anomalies, which we interpret to have formed by mixing between defused hydrothermal fluids and seawater. Other samples are enriched in MREE, a signature that is found in methane-derived cold seep carbonates. Additionally, the carbonates exhibit positive correlations between δ¹³C and various proxies for detrital input (e.g., REE + Y, Zr content, Y/Ho ratios). Based on comparing our observations with drill core and geophysical measurements from the active Nankai accretionary complex, we conclude that the Nabae carbonates were precipitated from relatively low-temperature hydrothermal fluids or as cold seeps near the deformation front at the toe of the Shimanto accretionary complex in a high heat flow environment. As the first onshore example of ancient authigenic carbonates from an accretionary complex, the Nabae carbonates have the potential to reveal important information regarding the characteristics and distribution of hydrothermal areas in active accretionary wedges.

The sedimentary successions of several basins in Europe show evidence of widespread Late Jurassic aridification that is considered a long-standing conundrum in paleoclimate modeling. The distinctive feature of this event is that it appears concentrated in a discrete time interval between the Kimmeridgian (Late Jurassic) and the Berriasian (Early Cretaceous), and that it extended to eastern and southern-central Asia for a total of ∼10 Mkm² in roughly the same time interval. Climate modeling has not provided a convincing explanation for this event. We compiled and reviewed paleomagnetic data from several continents including Adria, the African promontory, showing that this large-scale aridification was produced by an abrupt and transient southward migration of Eurasia toward arid tropical latitudes, while its demise coincided with a “retromotion” to more humid northern latitudes in the Early Cretaceous. This movement is part of a global plate motion event, most likely due to True Polar Wander, that profoundly affected the depositional environments, the ecosystems, and the architecture of sedimentary basins worldwide.

Garnet has been widely used to decipher the pressure-temperature-time history of rocks, but its physical properties such as elasticity and diffusion are strongly affected by trace amounts of hydrogen. Experimental measurements of H diffusion in garnet are limited to room pressure. We use atomistic simulations to study H diffusion in perfect and defective garnet lattices, focusing on protonation defects at the Si and Mg sites, which are shown to be energetically favored. Transient trapping of H renders ab-initio simulations of H diffusion computationally challenging, which is overcome with machine learning techniques by training a deep neural network that encodes the interatomic potential. Our results from such deep potential molecular dynamics (DeePMD) simulations show high mobility of hydrogen in defect-free garnet lattices, whereas H diffusivity is significantly diminished in defective lattices. Tracer simulations focusing on H alone highlight the vital role of atomic vibrations of heavier atoms like Mg on the release of H atoms. Two regimes of H diffusion are identified: a diffuser-dominated regime at high hydrogen content with low activation energies due to saturation of vacancies by hydrogen, and a vacancy-dominated regime at low hydrogen content with high activation energies due to trapping of H atoms at vacancy sites. These regimes account for experimental observations, such as a H-concentration dependent diffusivity and the discrepancy in activation energy between deprotonation and D-H exchange experiments. This study underpins the crucial role of vacancies in H diffusion and demonstrates the utility of machine-learned interatomic potentials in studying kinetic processes in the Earth's interior.

Strain localization is crucial in developing crustal-scale shear zones; however, a systematic analysis of deformation characteristics and their impact on seismic properties is still lacking. This study provides a comprehensive analysis of migmatites and granitic mylonites in the Ailaoshan-Red River shear zone (ASRR-SZ), incorporating detailed field observations, microstructure analysis, mineral crystallographic preferred orientations, rheological parameters, and seismic properties. Pre-existing compositional and mechanical anisotropies significantly influence strain localization in the ASRR-SZ. The southern part of the ASRR-SZ is primarily characterized by crustal anatexis, suggesting these regions as potential initiation sites for strain localization. Strain characteristics in the ASRR-SZ manifest in deformation temperatures (three ranges, ∼400–440°C, ∼470–500°C, and ∼730°C), differential stress (concentrated at 15.9–65.1 MPa), and strain rate (concentrated at 10⁻¹³–10⁻¹¹ s⁻¹). Notably, strain localization significantly alters the rock fabric and further affects the seismic properties of rocks. Significant differences in Vp values and orientations are noted between melanosomes (anisotropy of P-waves: AVp = 6.8%–17.9%, Max. Vp along the X-axis) and leucosomes (AVp = 3.4%–3.7%, Max. Vp along the Y-axis). The seismic velocities and AVp in granitic mylonites exhibit a linear correlation with quartz content, and deformation conditions strongly influence their orientation. For a middle to lower crust thickness of ∼25 km, the delay times between fast and slow polarized shear waves are 0.3–0.66 s for granitic mylonites, 0.37–0.7 s for melanosomes, 0.22–0.31 s for leucosomes, 0.27–0.58 s for amphibolites, and 0.57–2.7 s for schists. The average delay time (dt) of these rocks along the ASRR-SZ accounts for the observed delay time (dt = 0.58 s).

Serpentinization, a consequence of water-rock interaction in mafic and ultramafic rocks, refers to the hydrothermal alteration of olivine and pyroxene with serpentine as the typical product. Magnetite is produced in variable amounts during this serpentinization process. Here, we conducted a systematic rock magnetic study on the serpentinite of Ocean Drilling Program (ODP) Hole 1070A from the Iberia Abyssal Plain. From bottom to top, three units are distinguished rock-magnetically: (a) serpentinized peridotite dominated by single-domain (SD) ± multidomain (MD) magnetite particles; (b) gabbro with SD ± vortex state magnetite; (c) breccia dominated by MD ± vortex and SD ± vortex state maghemite/hematite and magnetite. Samples of the three units are highly serpentinized with serpentinization degrees >60%. The magnetite content increases exponentially with the degree of serpentinization. Two phases of serpentinization are proposed: (a) massive serpentinization that occurred before mantle exhumation and (b) maghemitization-serpentinization that occurred near or at the seafloor after the final exhumation of mantle peridotites. The latter reduces the magnetization of the breccia unit significantly. Serpentinized peridotite associated with strong magnetization is the dominant contributor to the marine magnetic anomalies in ocean to continent transition areas.

This study provides insights into the tectonic evolution of the normal Mt Morrone Fault System (MMFS) in Central Italy and highlights the utility of multidisciplinary approaches in reconstructing the seismic history of dormant fault systems. The MMFS comprises two parallel normal faults that traverse the western slope of Mt. Morrone, and although the system can produce M > 6 earthquakes, it has been aseismic in post Roman times. Here, we combine geochemical analysis of carbonate fault-scarp samples with new structural fault data and Lidar-based topographic analysis to provide new constraints on fault geometries and kinematics, the paleo-earthquake history of MMFS since the Last Glacial Maximum and its slip rates. Structural analysis reveals kinematic similarities between the two parallel strands, reflecting their response to the same stress regime. Rare Earth Elements analyses on 53 limestone samples reveal a minimum of eight concentration fluctuations upscarp, here interpreted as tectonic exhumation of the fault scarp due to post LGM earthquakes. Slip per event ranges from 30 to 110 cm typical of earthquakes with 6.3 ≤ M ≤ 6.8. Lidar analysis reveals triangular slip profiles on both fault strands. We estimate that an earthquake with an average M = 6.5–6.6 would have a recurrence interval of ∼2,125 ± 125 years. Slip rates were calculated to be 0.5–0.65 mm/yr on the lower and 0.65–0.7 mm/yr on the upper fault strand, with the combined system having slip rates of 0.62–0.69 mm/yr. Our findings indicate that both strands of the MMFS are active and accumulate slip interdependently, a finding that is critical for seismic hazard assessment.