Earth and Planetary Science Letters最新文献

Calcium-aluminum-rich inclusions (CAIs) commonly observed in chondritic meteorites are the oldest dated solids formed in the Solar System. Short-lived isotope chronologies (²⁶Al-²⁶Mg, ¹⁸²Hf-¹⁸²W) suggest a ∼2 Ma gap between the formation of CAIs and the accretion of the final chondrite parent bodies. One thin section, 3.27 cm² in size, of an ordinary chondrite NWA 3358 (H3.1) studied contains 52 refractory inclusions (CAIs and amoeboid olivine aggregates (AOAs)) comprising 0.14 % of its area, which is the highest abundance of refractory inclusions among non-carbonaceous chondrites containing on average ∼0.009 area % of CAIs and AOAs. In combination with a low chondrule/matrix ratio of ∼1.5, this makes NWA 3358 a unique ordinary chondrite. The aqueously-formed fayalites (Fa_>99) in NWA 3358 have the inferred initial ⁵³Mn/⁵⁵Mn ratio of (5.56 ± 0.44) × 10⁻⁶ which is the highest measured value for secondary minerals in chondrites and corresponds to the formation time of ∼1.0–1.5 Ma after CAIs. Based on the ⁵³Mn-⁵³Cr chronology of fayalite formation and the thermal modeling, we infer that the first-generation of an H chondrite parent body, ∼6–12 km in diameter, accreted within 1.0 Ma after formation of CAIs, filling the gap of ∼2 Ma between CAIs and the earliest chondrite parent bodies. This early accretion provides a possible mechanism of CAIs/AOAs storage in the inner solar nebula and could explain the high amount of refractory inclusions in NWA 3358. A later destruction of these first-generation bodies may also explain the presence of CAIs and chondrules of different ages within later formed chondrite parent bodies.

Climate's role in governing landscape evolution has been intensely studied for several decades, but few studies clearly document climate-landscape interactions in natural landscapes. This study aims to improve understanding of climate-landscape linkages using hotspot volcanic islands in the tropics as natural laboratories. Relatively uniform lithology, strong precipitation and climate gradients, and known initial topographic conditions on Réunion and Mauritius islands (Réunion hotspot) and Kaua'i (Hawaii hotspot) enable us to explore the impact of climate on erosion rates and geomorphic process. We reconstruct paleo-topography of drainage basins based on preserved remnants of relict topography from past volcanic events that repaved the landscapes that are differenced from the modern-day topography to determine eroded volumes. Existing geochronology of the volcanic flows allows us to constrain the timing of repaving (a proxy for the initiation of erosion) and basin average erosion rates. The initial and final conditions and the duration of erosion are used to calibrate a simple stream power model for bedrock river incision for each basin using a Bayesian inversion. We compare the erosion rate and calibrated stream power parameters to precipitation and climate data for each drainage basin on each island to explore potential relationships. Results show that basin average erosion rates for basins eroding < ∼1 mm/yr show a positive relationship with mean annual precipitation (MAP) and a negative relationship with the duration of erosion. Importantly, MAP and erosion duration are correlated, so we infer that the negative correlation between erosion rate and duration is coincidental. The stream power slope exponent and erodibility coefficient only exhibit significant correlations with climate parameters for Réunion Island, particularly mean annual cyclonic precipitation. Our results demonstrate that both mean annual precipitation and extreme events control long-term landscape evolution on volcanic islands.

The region between central Europe and the centre of the Mediterranean is characterised by complex tectonics and kinematics. Here, the interaction between thickened crust, subducting lithosphere and surrounding asthenosphere produces strong and pervasive anisotropy in the upper mantle. Shear wave splitting measurements, the most adopted method to image seismic anisotropy so far, when interpreted in a ray-based framework result in little or no depth resolution, hampering a correct image of the anisotropy distribution with depth. In this study, we aim to better constrain the depth-dependent seismic anisotropy beneath Italy and surrounding regions, by isolating for the first time the source region of anisotropy at different depths. To do that, we perform an anisotropy tomography, adopting the splitting intensity inversion method. It is entirely based on the finite-frequency effect in the splitting of SKS waves. We first computed the splitting intensity using SKS waves recorded at all available permanent and temporary stations over the region, obtaining a huge dataset of measurements used as an input for the tomographic inversion. The large-scale 3D model of seismic anisotropy obtained with the inversion shows a clear change of anisotropy properties in terms of fast polarisation direction and intensity for different depths, thus improving the characterization of the main sources of anisotropy in the mantle as a function of depth. Shallower layers (70–100 km depth) are characterised by a complex and variable oriented pattern of anisotropy fast direction and intensity, which becomes progressively more organised with depth (100–300 km). This pattern suggests a strong control exerted by the geometry and motion of the different slab segments and the large-scale asthenospheric flow generated by subduction and roll-back processes. The strength of anisotropy increases with depth, with high values affecting the bulge of the Alps and Apennines chains and the southern Tyrrhenian subduction system. On the contrary, weaker anisotropy characterises the transition zone from the Apennines to Alpine domains beneath the Po plain, and both the Adriatic and European domains.

The immense outburst floods that have occurred on the Tibetan Plateau during the Late Quaternary are closely linked to tectonic and climatic factors. These floods likely induced very rapid, short-term geomorphic impacts on the evolution of mountain drainage systems and patterns of sedimentary movement. In this study, four glacially-dammed lake outburst flood events that occurred along the middle reaches of the Yarlung Tsangpo River since the Middle Pleistocene were reconstructed by combining comprehensive geomorphic, stratigraphic and geochronologic investigations. The most recent outburst flood sequence occurred at ∼10.5 ka, with a peak discharge of ∼5 × 10⁵ m³/s. The tilted uplift of the Cona Normal Fault has resulted in localized topographic lift and the formation of river knickpoints, contributing to the development and stabilization of glacial dams. River damming and outburst events have also been influenced by glacial-interglacial climate fluctuations since the Middle Pleistocene. The focused erosion and extensive mobilization of sediment by these low-frequency, high-energy floods have resulted in a repeated pattern of material transport and deposition from the Tibetan Plateau interior to its exterior. Coupled with glacial activity, the primary factor impacting the sustained stability of knickpoints on the Yarlung Tsangpo River along the Tibetan plateau's southern margin has been differential rock uplift, which results in a distinct geomorphic pattern characterized by knickpoints, glacial dams and alternating wide valleys and deep gorges.

Stable lead (Pb) isotopes have been regarded as tracers of ocean circulation, both in the present time and geological past. Here we present a new dataset of seawater Pb concentrations and isotope compositions for ten depth profiles from the South Atlantic Ocean (GEOTRACES cruises GA02 and GA10). By comparing Pb isotope data collected on the two cruises, and by modelling the distribution of Pb with an extended optimum multiparameter analysis, we find evidence of vertical transport of anthropogenic Pb pollution due to reversible scavenging. Surface to depth transfer of polluted Pb is aided by high suspended particulate matter loads at the Brazil – Malvinas Confluence and along ∼40°S in the South Atlantic. Overall, our findings caution the use of Pb isotope ratios as ventilation tracers in the South Atlantic and emphasize the importance of particle-seawater interaction for biogeochemical cycles.

Marine carbonate rocks are the major reservoir of carbon through CaCO₃ and CaMg(CO₃)₂ precipitation extracting CO₂/HCO₃^- from the atmosphere/oceans; hence dolomite is one of the major sinks in the global carbon cycle. Most ancient dolomite has been considered as mainly precipitated under Earth surface conditions. Previous studies have demonstrated that microbes can mediate dolomite formation. However, the “microbial dolomite” model is not sufficient to explain the dolomite that shows no or little evidence of a microbial origin. Although attempted for decades, the synthesis of inorganically-produced dolomite at normal temperatures (<50 °C) has been relatively unsuccessful. Hence, this "dolomite enigma" has been one principal research focus this century. Here we demonstrate that NH₃ catalyzes proto-dolomite precipitation inorganically with higher Mg/Ca molar ratios and CO₃^2- activity at normal temperatures of 30 °C and 40 °C. NH₃ dissolution increases the alkalinity of the solution and transformes into NH₄⁺ ions, which prefer to bond with H₂O on Mg[(H₂O)₆]²⁺ rather than free H₂O, thus releasing Mg²⁺ to facilitate proto-dolomite nucleation. Furthermore, the low dielectric constant and low dipole moment allow NH₄⁺ absorbed on crystal surfaces to lower the energy barrier of Mg[(H₂O)₆]²⁺ dehydration, promoting proto-dolomite nucleis growth. The system for proto-dolomite precipitation in our experiments closely simulates the natural aqueous environment. This study brings new insights to understanding the mechanisms of dolomite precipitation in natural waters.

The canonical Moon-forming giant-impact models allow for substantial chemical differences between the bulk silicate Moon and Earth due to incomplete mixing of the impactor and the proto-Earth. In comparison, the emerging high-energy giant-impact (Synestia) model requires the refractory element compositions of the Earth and Moon to be nearly identical, owing to extensive chemical homogenization of the Moon-forming disk in a vigorously mixed silicate fluid. These distinct chemical predictions make the lunar refractory element composition crucial for testing Moon-formation hypotheses, yet it remains highly controversial and necessitates new approaches to resolve. In this study, we develop a novel method using the composition of pristine lunar anorthosite samples to constrain the Moon's refractory lithophile element compositions. We obtained a very close match of refractory major and trace element compositions for the lunar magma ocean model, suggesting indistinguishable refractory element abundances between the bulk silicate Moon and Earth. This striking refractory element similarity is difficult to reconcile with the relatively poor mixing conditions of the canonical giant-impact models. The compatibility of this result with disk equilibration models other than the Synestia has yet to be quantitatively verified. Our results further constrain that the formation of the Earth-Moon system requires a thoroughly-mixed protolunar disk of chemical and isotopic homogenization with an initially fully-molten Moon, as enabled by emerging models like the Synestia.

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.