Earth and Planetary Science Letters最新文献

The latest stages of the lunar magma ocean (LMO) crystallization led to the formation of ilmenite-bearing cumulates and urKREEP, residual melts enriched in K, rare earth elements (REEs), P, and other incompatible elements. Those highly evolved lithologies had major impacts on the petrogenesis of lunar volcanic rocks and the compositional diversity of post-LMO magmatism resulting from mantle remelting. Here, we present new experimental results constraining the composition of the very last liquids produced during LMO crystallization. To test the potential role of silicate liquid immiscibility in the formation of urKREEP, synthetic samples representative of residual melts of bulk Moon compositions were placed in double platinum-graphite capsules at 1020–980 °C and 0.08–0.10 GPa in an internally-heated pressure vessel. The produced silicate liquids are multiply saturated with plagioclase, augite, silica phases, and ilmenite (± fayalitic olivine ± pigeonite). Our experiments show that the liquid line of descent reaches a two-liquid field at 1000 °C and >97% crystallization for a range of whole-Moon compositions. Under these conditions, a small proportion of silica-rich melt (70.0–71.4 wt.% SiO₂, 6.4–7.3 wt.% FeO, 5.4–6.1 wt.% K₂O, 0.2–0.3 wt.% P₂O₅) coexists within an abundant Fe-rich melt (42.6–44.1 wt.% SiO₂, 27.6–28.8 wt.% FeO, 0.9–1.0 wt.% K₂O, 2.8–3.2 wt.% P₂O₅) with sharp two-liquid interfaces. Our experimental results also constrain the relative onset of ilmenite crystallization compared to the development of immiscibility and indicate that an ilmenite-bearing layer formed in the lunar interior before immiscibility was attained. Using a self-consistent physicochemical LMO model, we constrain the thickness and depth of the ilmenite-bearing layer during LMO differentiation. The immiscible K-Si-rich and P-Fe-rich melts together also produced an immiscible urKREEP layer ∼2–6 km thick and ∼30–50 km deep depending on the trapped liquid fraction in the cumulate column (≤10%) and the thickness of the buoyant anorthosite crust (30–50 km). We provide constraints on the relationship between the compositions of immiscible urKREEP melts and those of KREEPy rocks. By modeling the mixing of KREEP-poor basalt and the immiscible melt pairs, we reproduce the K and P enrichments and apparent decoupling of K from P in KREEPy rocks. Our results highlight that processes such as the assimilation of evolved heterogeneous mantle lithologies may be involved in hybridization during post-LMO magmatism. The immiscible K-Si-rich lithology may also have contributed to lunar silicic magmatism.

Recent observational models of the paleomagnetic field have revealed new details about geomagnetic field variability, which have yet to be adequately explored in numerical dynamo simulations. Here we present results from a new suite of dynamo simulations with computationally accessible rotating rates and diffusivities, an Earth-like magnetic Reynolds number, and a force balance that is consistent with the expected regime of the geodynamo, allowing comparison of simulated data and observational models. We find that such simulations are able to simultaneously reproduce the observed extreme rates of change in intensity and direction as well as the general amplitude of field variability over the last 100 ky, if the mean dipolarity is in the range 0.4-0.5. We use the paleosecular variation (PSV) index to identify a broad spectrum of polarity excursions and show that the PSV index is closely linked to the dipolarity of the simulation. Simulated excursional events are mostly associated with a decrease in the axial dipole moment with generally modest changes in dipole tilt. The excursions range from global events characterised by a reduction in the field contribution from solely the axial dipole component and a decrease in mean VDM in the manner of the Laschamp excursion, to localised events with anomalous activity in small regions reminiscent of the Mono Lake/Auckland excursion. Global events are generally longer than regional excursions, and reflect a drop in the total magnetic energy of the dynamo.

The Archean mantle redox state played an important role in degassing of the Earth's interior and thus influenced atmospheric oxygen levels of the early Earth. But it is unclear if any parts of the uppermost mantle were significantly oxidized by a certain point in the Archean. Here, we investigate oxygen fugacity (fO₂) of Archean (> 2535–2517 Ma) peridotites in the North China Craton. Petrology and geochemistry reveal that they experienced strong Neoarchean subduction-related metasomatism. These Neoarchean subduction-metasomatized peridotites record fO₂ of ΔFMQ +1.3 ± 0.4 (SD) [relative to the fayalite-magnetite-quartz (FMQ) buffer], which are more oxidized than the Archean ambient mantle, but similar to the modern sub-arc mantle. We propose that this Neoarchean rise of mantle oxidation in the North China Craton was induced by plate subduction, during which the Neoarchean sub-arc mantle in the North China Craton could have been metasomatized and oxidized, and its oxygen fugacity was increased. This process may have had connections with the Great Oxidation Event in the Early Proterozoic.

Using petrography, in situ garnet Lu–Hf geochronology, garnet rare-earth element (REE) analysis, zircon U–Pb geochronology and phase equilibrium modelling, we provide unambiguous evidence for Eoarchean granulite-facies metamorphism in the northern Itsaq Gneiss Complex (IGC), southwest Greenland. In situ garnet Lu–Hf geochronology from two samples of variably migmatitic metabasic rocks least affected by subsequent (Neoarchean) reworking yield Lu–Hf isochron ages of 3641 ± 62 Ma (MSWD = 1.7, n = 45/67; all age uncertainties at 2σ level) and 3652 ± 69 Ma (MSWD = 1.8, n = 83/84) from garnet with REE patterns typical of single-stage prograde growth. From the same two samples, zircon grains with textures consistent with metamorphic growth give weighted-mean ²⁰⁷Pb/²⁰⁶Pb ages of 3620 ± 8 Ma (MSWD = 1.2, n = 45) and 3630 ± 8 Ma (MSWD = 0.6, n = 44), respectively. Phase equilibrium modelling constrains peak P–T conditions of Eoarchean (3640–3630 Ma) metamorphism to 8.3–9.0 kbar and 730–820 °C. The thermobaric ratios (T/P) of 800–1000 °C/GPa recorded by the investigated samples are considerably higher (warmer) than previously proposed for granulite-facies metamorphism in the northern IGC, and broadly similar to Archean metamorphic P–T data globally, with no evidence for the bimodality in T/P that characterizes younger metamorphism. Either subduction-driven metamorphism (and plate tectonics) did not operate in the Eoarchean, or the Eoarchean lithosphere had a rheology that prohibited exhumation of subducted rocks.

The ongoing collision between the Indian and Eurasian plates propels the eastward movement of the Tibetan plateau (TP), leading to substantial crustal deformation around the southern Sichuan-Yunnan block (SYB). Using ambient noise data from multiple temporary seismic arrays and permanent stations, we construct a high-resolution regional crustal azimuthally anisotropic Vs model in the SYB. Our new model reveals two significant low-velocity anomalies with strong azimuthal anisotropy near the block boundary faults in the middle-and-lower crust. The extensive low-velocity anomalies around the middle-south segment of the Xiaojiang Fault (XJF) possibly result from partial melting due to spontaneous deformation caused by crustal thickening and increased felsic components, as well as the superimposition of shear heating faults and local upwelling asthenosphere. The N‒S trending low-velocity anomaly at the northwest end of the Red River Fault (RRF) may be associated with weak material migration from the TP, potentially serving as a conduit for mantle upwelling. The azimuthal anisotropy along the block boundary faults exhibits spatial variations linked to segmented distortion resulting from southeastward crustal movement and various geological activities. A key finding is that the crustal channelized low-velocity along the XJF is clearly blocked by the RRF, instead of going through. Notably, the azimuthal anisotropy in the E‒W direction, observed above the Moho and at depths deeper than 30 km in the intersection end, implies the potential intrusion of localized mantle materials into the lower crust. Therefore, lithospheric deformation is significantly affected by block boundary faults and the properties of the crust and mantle.

The movement of carbon in subduction zones plays a crucial role in regulating the global carbon cycle, controlling Earth's climate, and maintaining its habitability. Recent work suggests that only a fraction of the carbon released from subducting slabs at sub-arc depths is ultimately released from volcanic arcs, necessitating the existence of hidden carbon reservoirs within the slab-to-arc pathways. However, the precise location of these reservoirs remains enigmatic. Slab fluid serves as the primary medium for carbon transport in subduction zones; thus, a comprehensive understanding of fluid-rock interaction during slab fluid migration is essential for reconciling the carbon flux imbalance between the slab and the arc. In this study, we explore rock carbonation along a fluid conduit in the Southwestern Tianshan HP metamorphic belt in northwest China. Field evidence and petrologic observation reveal significant carbonation of a siliciclastic metasediment at its contact with a high-pressure garnet-bearing calcite (formerly aragonite) vein. We find that rock carbonation (by progressive Fe-bearing magnesite, dolomite, then aragonite precipitation) occurred when slab-derived carbonic fluids migrated through the metasedimentary sequence at approximately 80 km depth. Furthermore, modeling demonstrates that the metasedimentary layer atop the slab has the capacity to sequester 20%–50% of the fluid carbon from the ascending slab devolatilization flux. We propose that the metasedimentary veneer at the plate interface functions as a “carbon filter”, hindering the transfer of carbon from the slab to the arc and helping to reconcile the carbon flux imbalance between the amount released by the slab and that emitted by the arc. This study also provides insights into decarbonation efficiency and mechanisms, carbon-transfer pathways, and temporal aspects of the subduction zone carbon cycle.

We propose a new weak fault model in which isolated viscoelastic regions are distributed along the fault. Numerical simulations using the finite element method show that the viscoelastic regions relax and the shear stress supported by them is applied to surrounding elastic regions after a time sufficiently longer than their relaxation time, while the normal stress continues to be supported by the viscoelastic regions, and then the normal stress in the elastic regions remain unchanged. Since the shear stress is amplified but the normal stress remains unchanged in the elastic regions, a macroscopic weakening of the fault occurs even under a constant coefficient of friction. The fault can be weakened without assuming high pore pressure. As a result of examining the effect of the geometry of the viscoelastic regions on the fault strength by changing their shape and spatial distribution in various ways, we found that the fault strength decreases as the ratio of the area of the elastic regions remaining unrelaxed to the total area of the fault decreases. It is known that faults can be weakened by fault rocks such as clay minerals, but the frictional properties of these fault rocks are basically velocity strengthening, making it difficult to weaken seismic faults. The fault model in this study is a model for deformation characteristics of the host rock around a fault, which does not place any constraints on the frictional properties of the fault, and thus can weaken a seismic fault.

The Coso geothermal field is a major geothermal power production site in the western United States. It has been observed that low-magnitude seismic events (M < 3.71) are unevenly distributed in three distinct zones, namely, nearfield (<3 km), midfield (3–6 km), and farfield (> 6 km) from the Coso geothermal plant. These zones exhibit distinct changes in earthquake location before and during geothermal production episodes that began in 1986. After 1986, the midfield region of the main flank experiences a significantly lower seismicity rate than the surrounding areas before production episodes. During 2014–2019, the farfield earthquakes cluster in the eastern and western parts of the greater Coso area, which is discernably different from how those pre-production earthquake events were distributed along the conjugate NW-SE and SW-NW trending structures across the main flank. Here, we analyze the stage of stress with finite-element-based poroelastic simulations to illustrate how the spatiotemporal evolution of the seismicity is associated with the pattern of stress perturbations caused by fluid migration amid the operations of geothermal power plants. Generally, ∼70% of co-production seismicity is found in zones of increased Coulomb stress between 2014 and 2019 at >99% confidence. Meanwhile, the midfield zone of seismic paucity overlaps with the zone of decreasing pore-fluid pressure. Overall, the results provide a physical explanation of how decadal geothermal operations at Coso have perturbed stress-field changes and contributed to the evolving characteristic seismic pattern, shedding insights into assessing the seismic hazard in other geothermal settings.

We assess the growth of anomalously high relief on Denali, located in the Alaska Range, Alaska, and the tallest mountain in North America (6190 masl). Denali is 3000 m taller than most surrounding peaks. It lies inside a 19° restraining bend in the active Denali fault system that is moving at about 7 mm/yr, providing a tectonic and structural driver for ongoing rock uplift. High relief around Denali is also due, in part, to its granitic rock type and low fracture density relative to adjacent metasediments. Here we show that unique climatic conditions at high elevations also contribute to the growth of relief. We examine ¹⁰Be concentrations in 1) three new gravel samples between 3500 and 5200 m elevation from sites unaffected by recent glacial erosion, 2) previously published samples from a sidewall of the Kahiltna Glacier from 2400 to 2800 masl, 3) previously published data for samples collected from medial moraines along the length of the Kahiltna Glacier from ∼500 to 1400 masl, and 4) previously published data for alluvial samples collected along the Kahiltna River at an elevation of ∼200 masl. These samples constitute a transect extending >5000 vertical meters, and the data establish that erosion rates decrease with elevation and contribute to the growth of relief. Erosion rates for the three new high-elevation samples are calculated to 4.6 ± 0.6 mm/ka at 5200 masl, 28.6 ± 3.7 mm/ka at 4000 masl, and 38±5 mm/ka at 3500 masl. Erosion rates at intermediate elevations, on the sidewall of the Kahiltna Glacier, range between 160 and 327 mm/ka. Along the medial moraines inferred erosion rates range between 140 and 537 mm/ka, and basin-wide erosion rates calculated from sediments in the river below the glacier range between 450 and 896 mm/ka. These differences in erosion rates can create relief of 3 km within 1–10 Ma, well within the estimated period of increase in rock uplift and exhumation on Denali over the last ∼6 Ma. Meteorological data from 2130 to 5550 masl at 5 sites show temperatures rarely exceed freezing above 4000 masl elevation, indicating that frost weathering currently plays a diminished role in erosion at high elevations. The immediate implication of this temperature and erosional correlation is an increase in relief. This is the first study to directly measure a significant decrease in erosion rates at high elevations in the relative absence of frost weathering. The results highlight the combined influence of rock type, glacial erosion, and permanent sub-zero temperatures on erosion rates. In combination with active faulting, the data explain the resultant increase in relief along the southern side of the Alaska Range over the past 100 ka.