Geochemistry Geophysics Geosystems最新文献

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.

We present ²³⁰Th-²³⁸U crystallization ages and trace element compositions for zircons spanning the late Pleistocene to Holocene rhyolite eruptive record at South Sister volcano in the central Oregon Cascade Range. Most zircon ages are between 100 and 20 ka, with very few in secular equilibrium (>350 ka). The weighted mean of zircon ages for the two oldest South Sister rhyolites, 31.5 ± 2.1 and 39.1 ± 2.4 ka, are significantly younger than the associated ⁴⁰Ar/³⁹Ar ages, 47.4 ± 9.7 and 51.4 ± 9.7 ka. We propose that these ⁴⁰Ar/³⁹Ar dates, performed on plagioclase separates, are compromised by a subtle amount of excess Ar and therefore the younger weighted mean zircon ages yield more reliable eruption ages. These results imply that the interval of rhyolite eruption at South Sister during the late Pleistocene was both shorter and more productive than previously thought and that eruption at South Sister initiated after Middle Sister. Compositionally, zircons from the Pleistocene rhyolites are broadly similar and show down-temperature zircon and plagioclase crystallization trends. However, we argue that destabilized amphibole and titanite in a common mush also exert leverage on the Pleistocene zircon trace element compositions. Divergence in the Eu/Eu* ratio between the Pleistocene and Holocene lavas implies chemically distinct magma reservoirs originating from the Pleistocene rhyolite eruptive sequence and the Holocene eruptive sequence. This work suggests a higher flux of rhyolite volcanism than previously thought and characterizes magmatic storage distinctions between the Pleistocene and Holocene rhyolites, aiding in the assessment of future eruptive hazards at South Sister volcano.

The investigation of tectonic controls on CH₄ and CO₂ emissions was conducted by measuring the fluxes of the gases in the different tectonic units along the northwestern margin of the Ordos Block in China, a region renowned for its intricate tectonic configuration. The mean fluxes of CH₄ ranged from −1.5 to 1.1 mg m⁻² d⁻¹, while CO₂ fluxes spanned from 2.0 to 29.2 g m⁻² d⁻¹. Notably, the Minqin, Ordos, and Haiyuan blocks primarily exhibited absorption characteristics for CH₄. In contrast, within the Hetao and Yinchuan grabens, both degassing and absorption processes coexist. A striking observation was that blocks with high internal deformation exhibited significantly higher CH₄ and CO₂ fluxes compared to those in the stable blocks. Additionally, regions experiencing extensional deformation demonstrated greater gas emission than those undergoing compressional deformation. The spatial distribution of CH₄ and CO₂ fluxes at the study points exhibited a similar trend to faults in the Yinchuan Graben. Our findings revealed that CH₄ and CO₂ are mainly of biogenic origin, accompanied by abiotic emissions from underground. And the gas source, migration pathway, and tectonic stress were the primary factors influencing gas emission, with tectonic stress playing a pivotal role. This stress controlled the formation of tectonic structures, changed the degassing pathway, and served as the driving force for gas migration. The results of this study offer valuable insights into the mechanisms governing CH₄ and CO₂ emission in faulted regions. Furthermore, our results may contribute to future assessments aimed at quantifying the contribution of geological sources to greenhouse gas emissions.

The Molucca Sea subduction zone is unique for its active divergent double-subducted slab located in the north of the Java and Banda subduction zones. The spatial proximity of these subduction zones would cause a complex mantle flow field. To clarify the mantle dynamics, here we present P-wave tilting-axis anisotropic tomography obtained by inverting a large number of local and teleseismic travel-time data recorded at 254 seismic stations in eastern Southeast Asia. Our results suggest that the mantle structure and dynamics of the western Molucca Sea subduction zone may be remotely controlled by the Java and Banda subduction zones. Mantle convection in the big mantle wedge west of the Molucca Sea subduction zone is possibly influenced by east-west mantle flow associated with compression of the Indo-Australian slab as well as north-south mantle flow related to rollback of the Indo-Australian slab. In contrast, the eastern Molucca Sea subduction zone is virtually unaffected by other subduction zones, probably due to the ongoing slab subduction there.

Euxinia, a crucial geological condition, usually signifies more severe extinction events attributed to deoxygenation in Earth's history. Despite extensive exploration of various proxies in paleoredox studies, most are primarily utilized to reconstruct atmospheric pO₂, the proportion of anoxic water relative to the entire basin, and broader trends in redox states. Few, however, hold the capacity to precisely delineate local euxinia within confined areas. To address this gap and gain insights into the temporal and spatial extent of benthic euxinia, we propose leveraging the synergistic analysis of total nitrogen isotopes (δ¹⁵N_TN) and pyrite sulfur isotopes (δ³⁴S_py). Our study focuses on the Triassic Chang 7 Member from the Yanchang Formation, Ordos Basin, North China. Through coupling the δ¹⁵N_TN and δ³⁴S_py systematics on 11 drill cores within the Ordos Basin, we reconstruct the temporal and spatial distribution of the benthic euxinia zone during the Chang-7 period. Our results suggest strong spatial heterogeneity of benthic redox conditions, with the euxinia boundary shifting from the central lake to the southwestern sections. Moreover, we identify redox-controlling factors, including organic carbon loading, water depth, and potential water circulation, and evaluate their interplay with benthic euxinia. Furthermore, the discernment of water circulation patterns may provide an innovative approach to restore the paleowind direction. These findings highlight the effectiveness of coupling δ¹⁵N_TN and δ³⁴S_py in reconstructing the local benthic redox landscape of benthic environments, and enrich our understanding of biogeochemical processes.

In a deforming partially molten rock, melt concentrates into a grain-scale melt pocket aligned at a preferred orientation (melt-preferred orientation, or MPO). However, observing this texture alone provides limited information on the 3D orientation and geometry of these melt pockets, which are critical parameters for estimating permeability. Here, we modeled the MPO of experimentally deformed peridotites by simulating melt streaks arising from melt pockets of various shapes and 3D orientations. The model aims to identify 3D distribution and characteristics of melt pockets that could account for the observed length, thickness, and the probability of melt streaks. Results show that melt pockets at preferred orientation exhibit greater length, thickness, and number density compared to those perpendicular. These results can be incorporated into the simulation of melt flow through individual melt pockets, which allows us to estimate the permeability corresponding to the observed MPO. We found that the permeability of vertically compressed peridotites increases with increasing compressive strain and a more elongated and thickened shape for melt pocket aligned at preferred orientation. The vertical permeability in the sample with 30% compressive strain is at least 40 times larger than that of an undeformed sample. For peridotites deformed under simple shear, the permeability exhibits an anisotropy of at least three. Such anisotropic permeability, coupled with the formation of melt-rich bands and other melt channels, is believed to cause lateral melt focusing beneath mid-ocean ridges.

Boron to calcium (B/Ca) records in benthic foraminifera, used for reconstructing the carbonate ion saturation state (ΔCO₃) of the deep ocean, suggest that carbon sequestration in the Southern Pacific contributed to lowering atmospheric CO₂ during the last glacial interval. However, the spatial and temporal extent of this storage is debated due to limited ΔCO₃ records. To increase available ΔCO₃ records, we explored using strontium and sulfur to calcium (Sr/Ca, S/Ca) in Planulina wuellerstorfi as additional proxies for ΔCO₃ based on comparison with paired B/Ca down-core records from Pacific Sites U1486 (1,332 m depth) and U1487 (874 m depth) cored during the International Ocean Discovery Program Expedition 363. The Sr/Ca and S/Ca records from P. wuellerstorfi closely covary with the B/Ca-derived ΔCO₃ records. Temperature, reconstructed using Uvigerina peregrina magnesium to calcium (Mg/Ca), has no discernible effect on Sr/Ca, whereas S/Ca also varies with Mg/Ca in both U. peregrina and P. wuellerstorfi, suggesting an additional temperature effect. Mg/Ca records from P. wuellerstorfi are affected by both temperature and ΔCO₃. We assess calibrations of Sr/Ca to ΔCO₃ for the Atlantic, Pacific, and Indian Oceans and recommend using the down-core rather than core-top calibrations as they yield consistent sensitivity, though with offsets, in all ocean basins. Reconstructing Pacific ΔCO₃ records from sites U1486, U1487, and DSDP 593, we demonstrate the benefit of using Sr/Ca as an additional ΔCO₃ proxy to assess the contribution of the Southern Pacific to the increase of atmospheric CO₂ at glacial terminations.

The ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain-states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so-called pseudo-single-domain particles with little understanding of the implications for domain-states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated-octahedral geometries containing single domain (SD), single-vortex and occasionally multi-vortex states. We show that these domain states exhibit a well-defined trend in the Day plot that extends from the SD region well into the multi-domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability.