Earth and Planetary Science Letters最新文献

The extensional detachment systems of metamorphic core complexes (MCC) have the potential to facilitate significant fluid movement between Earth's surface and the warm, ductile middle crust. One long-standing and influential detachment fault-fluid interaction model called the “footwall refrigeration hypothesis” (Morrison and Anderson, 1998) postulates that cool meteoric fluids circulating downward along a detachment can facilitate large-scale heat loss at depth before rocks are exhumed by faulting. Stable isotopes of oxygen and hydrogen have long been recognized as potentially useful tracers of surface-to-depth fluid flow in core complexes; however, typical sampling methods and the competing effects between temperature and fluid composition have made oxygen isotope signatures of meteoric fluid infiltration difficult to detect and/or interpret in the rock record.

Here, we use in situ δ¹⁸O measurements by secondary ion mass spectrometry (SIMS) to address the rock record of meteoric fluid interactions and temperature variation within the Whipple Mountains MCC footwall beneath the Whipple detachment fault (WDF). The micro-scale sampling of SIMS (10 µm diameter x 1–2 µm deep pits) allows investigation of isotope variability as a function of mineral geochemistry and microstructure and enables more accurate interpretation of the causes of isotope variability. In the Whipple footwall mylonites, quartz and epidote show grain-to-grain oxygen-isotope variability within samples and as a function of distance from the main detachment. Approaching the WDF, both quartz and epidote show increasing spread toward low δ¹⁸O values. The lowest SIMS-measured δ¹⁸O epidote values require exchange with a low δ¹⁸O, meteoric fluid at high temperature. However, SIMS data also reveal that meteoric fluid-rock interactions were spatially heterogeneous at the scale of individual grains, resulting in local preservation of metamorphic quartz-epidote oxygen isotope equilibrium unaffected by infiltrating meteoric fluid. Within preserved metamorphic domains at all investigated structural levels of the WDF footwall, quartz and epidote δ¹⁸O values are tightly clustered and yield similar fractionations (∆¹⁸O_qz-ep). For samples from the longest footwall traverse, these ∆¹⁸O_qz-ep values are 4.0 ± 0.4‰, consistent with δ¹⁸O equilibrium at 521+43/-37 °C. We conclude that there is no evidence of a large paleo-thermal gradient associated with the Whipple Detachment Fault (i.e., no evidence for footwall refrigeration).

Two contrasting age models for initial mountain building in the northeastern (NE) Tibetan Plateau (Paleocene-early Eocene versus late Oligocene-early Miocene) have led to the debate on how the deformed continental lithosphere absorbs plate convergence in general. The initial compressional deformation in the West Qinling (WQL) of the NE Tibetan Plateau figures prominently in this ongoing debate. Here, apatite (U-Th)/He (AHe) thermochronology combined with geomorphological analysis are used to refine the onset of compressional deformation in the WQL. New AHe ages from two vertical transects and an updated reconstruction of an obliquely-tilted erosion surface document the accelerated exhumation in the northern WQL at 23-22 Ma, interpreted as the onset of north-vergent thrusting. The AHe results, together with sedimentary records in the intermontane and foreland basins, suggest that the entire WQL began experiencing compressional deformation in the late Oligocene-early Miocene. When integrated with previous studies, our findings show that the northern plateau boundary has not remained stationary since the collision, but has instead experienced ∼750 km of outward expansion during the late Oligocene to middle Miocene. This phase of rapid plateau growth is coeval with the ∼30–50 % reduction of the India-Eurasia convergence rate, which suggests that the increased gravitational potential energy of orogenic belts played a key role in plate motion changes.

Load on a planet's lithosphere can often form a well-defined flexural bulge, including a permanent (or long-lasting) forebulge, which preserves important information on the force of the load and properties of the lithosphere itself. On Pluto, aspects of the outer ice shell (i.e. the lithosphere) have become increasingly ascertainable, as recent work using data from the New Horizons space probe has revealed evidence of ongoing surface cryovolcanism and a subsurface water ocean. However, the precise thickness and elasticity of the ice shell has yet to be fully established. Sputnik Planitia, one of the largest surface features on Pluto, is an elliptical depression that may have formed during an impact event and subsequently infilled with nitrogen ice. It is characterized by a smooth, radially asymmetrical, forebulge which has been retained in places along the border of the depression. However, the proportion of influence on the formation of the forebulge between the impact load and the load induced by the infill remains unknown. Here, we report results from the analysis of the forebulge of Sputnik Planitia to explore the characteristics of the ice shell and the nitrogen infill. By utilizing multiple Converging Monte Carlo (CMC) simulations within the material and environmental parameters of Pluto, the best fit flexure surface was able to replicate the topography of the flexure (including the forebulge) from ten profiles. Results show an ice shell thickness ranging from 65 to 90 km, with an average of 78 km. The density of the ice shell is 50 kg/m³ less than the density of the subsurface water ocean. We demonstrate that if the formation of the forebulge occurs solely from the nitrogen ice infill load, the infill must reach >18 km of thickness. Furthermore, a southeast-northwest central load symmetry may have been produced by an impacting object with a southeast-northwest trajectory.

The Ediacaran Period was punctuated by major environmental and evolutionary milestones, including large-scale phosphogenesis during intervals of elevated phosphorous (P) flux. Although increased P flux could be an important driver of animal evolution and associated ocean oxygenation, the processes responsible for triggering the remarkable Ediacaran phosphogenesis episodes remain unclear. In this study, we present new δ¹³⁸Ba, ⁸⁷Sr/⁸⁶Sr, and major and trace element data from Ediacaran phosphorites of the Doushantou Formation (Nanhua Basin, China). Low-to-negative δ¹³⁸Ba (−0.31 to 0.06 ‰, analogous to δ¹³⁸Ba of modern and ancient restricted depositional settings) and elevated ⁸⁷Sr/⁸⁶Sr values (0.7100 ± 0.0002, greater than the global average) from lower Doushantuo phosphorites indicate intense local continental weathering enhanced P delivery to the restricted Nanhua Basin at ∼630–600 Ma. During this early phosphogenesis stage, macroeukaryote blooms stimulated atmospheric oxygenation and eutrophication of deep seawater in the Nanhua Basin. In contrast to the lower phosphorites, the elevated and open-marine-like δ¹³⁸Ba of 0.17 to 0.35 ‰, as well as lower ⁸⁷Sr/⁸⁶Sr values (mostly lower than 0.7096, comparable to the global average), from overlying ∼587–574 Ma phosphorites indicate that this later phosphogenesis interval was characterized by upwelling of pelagic deep ocean waters rich in re-mineralized P. Global-scale P burial (as phosphorite) could not lead to decreased eutrophic conditions in the global ocean, but, together with increasing oxygen levels in deep seawater and/or the atmosphere, this process may have finally facilitated the evolution of Ediacaran fauna.

Subduction processes usually involve extensive seismicity and create voluminous magmatic arcs by mantle wedge melting caused by dehydration of the subducting slab, but the Makran subduction zone has anomalously low seismicity and magmatism. Here we explain these anomalous features by 60–65% serpentinization in the peridotitic shallow mantle wedge based on our new integrated seismic, magnetic, gravity and isostatic model across the Makran subduction zone. The low-angle, slow Makran subduction provides ample time for the slab to release sufficient amounts of fluids for creating a large volume of rheologically weak serpentinite. This reduces seismicity by lowering the friction between the slab and surrounding rocks. Further, very little fluid is left in the slab when it reaches the melting depth, which explains the limited arc magmatism. Around 100 km depth, the subduction switches from low-angle to almost vertical. Our model demonstrates the combined effects of subduction rate and dip on mantle serpentinization with implication for assessment of seismic and volcanic hazards in subduction systems.

Magmatic processing in the lower crust of thick-crusted (>40 km) arcs generates garnet-rich clinopyroxenite residues (arclogites) that are denser than the underlying mantle and therefore susceptible to foundering. Arclogites, which can contain substantial amphibole, may undergo partial melting as they sink into the subarc mantle. To constrain the geochemical composition of arclogite-derived melts and the role of arclogite in magmagenesis, we conducted partial melting experiments on average amphibole-bearing arclogite ARC15 at 2 GPa from 1040 to 1300 °C. At subsolidus conditions, garnet and clinopyroxene coexist with lesser amounts of amphibole, ilmenite, and biotite. Hydrous ARC15 begins melting between 1040 and 1080 °C. Once amphibole and biotite are exhausted by 1140 and 1190 °C, respectively, garnet becomes the primary melting phase according to the reaction 0.8 Garnet + 0.2 Clinopyroxene = 1 Melt. At 1300 °C, ARC15 yields 18 wt.% melt, indicating an anomalously low melt productivity of 0.12%/°C and rendering it the least melt-productive of all experimental pyroxenites investigated at 2 GPa. Geochemically, melts sourced from ARC15 are predominantly basanitic with low SiO₂ (43–52 wt.%) and high FeO (10–17 wt.%), TiO₂ (2.5–3.7 wt.%), and alkali (Na₂O + K₂O = 3–6.5 wt.%) contents. Such compositions do not resemble magmas erupted at thick-crusted arcs, indicating that arclogites are not a dominant source of magmatism in these settings. Rather, the compositions of ARC15 melts more closely match those of magmas emplaced at post-collisional alkaline-silicate complexes, which form as a result of orogenic collapse and subsequent continental rifting and are common hosts of economic rare-earth element (REE) deposits. Trace element modeling further indicates derivation of alkaline-silicate magmas from variably metasomatized, light REE-enriched arclogite. Our results suggest a geodynamic model in which subduction beneath thickened crust leads to arclogite formation, foundering, and metasomatism as the material descends through the subduction-influenced mantle. During this stage, partial melts sourced from arclogite are likely modified by subduction-related secondary processes which dilute their diagnostic geochemical signatures. Subsequent collision and orogenic collapse induce extensional stresses that facilitate asthenospheric upwelling, leading to partial melting of the arclogite-bearing mantle and efficient extraction of primitive arclogite melts to the surface to form alkaline-silicate complexes and associated REE deposits.

High-pressure experiments conducted at upper mantle conditions reveal increased Fe³⁺ contents in majoritic garnet with increasing pressure. Consequently, at ∼8 GPa, Fe²⁺ disproportionation is expected to generate Fe⁰, leading to a higher metal/sulfur ratio of the initially present monosulfidic melt. This study experimentally investigates equilibrium systematics between the reduced Fe-Ni-S-C(-O) melt and mantle silicates at 5–16 GPa and near-adiabatic temperatures (1420–1590 ^∘C), aiming to constrain the alloy melt composition in equilibrium with bulk silicate Earth-like (BSE) mantle silicates. As analysis of unquenchable liquid alloys requires large melt pools, experimental charges were designed towards a 50:50 ratio of alloy:silicates, with Fe/Ni ratios, sulfur and carbon scaled to correspond to a BSE with 200–250 ppm S and 100–150 ppm C.

Observed phase assemblages consist of Fe-Ni-S-C(-O) melt (metal/(S+O): 0.9–3.5, Ni/(Fe+Ni): ∼0.3–0.6 and 0.2–1.6 wt.% oxygen) saturated with graphite/diamond and coexisting with olivine/wadsleyite ($X_{\mathrm{Mg}}$: 0.76–0.91), garnet and cpx ± low-Ca pyroxene. Oxygen fugacities ($f_{O_2}$) range from -0.5 to +4.0 relative to the iron-wüstite (IW) buffer. In most runs, Fe⁰–Fe²⁺ redox equilibration affected silicate $X_{\mathrm{Mg}}$'s, resulting in values <0.90. Therefore, an empirical model was developed to predict $K_{D\mathrm{Ni-Fe}}^{\mathrm{alloy-olivine}}$, allowing to recalculate the experimental results for a primitive BSE mantle composition with $X_{\mathrm{Mg}}$ of 0.90.

Modeling BSE, the alloy's metal/(S+O) increases from 1.1 to ∼7 while Ni/(Fe+Ni) decreases from 0.35 to 0.19 at 5–16 GPa. With BSE-like S concentrations, the abundance of the Fe-Ni-S-C(-O) melt increases from 600–750 ppm at 5 GPa to ∼3800 ppm at 16 GPa, 100–150 ppm bulk carbon lead to graphite/diamond saturation to ∼14 GPa. Finally, a method is presented to derive $f_{O_2}$ from modeled liquid alloy compositions, which act as a “redox-sensor”. For a BSE-like mantle with 250 ppm S, $f_{O_2}$ decreases from IW+2.6 at 5 GPa to IW at 8 GPa, further decreasing to IW-1.0 at 12–16 GPa.

Petrographic, geochemical, C-, O-, and clumped isotope measurements of drill core samples from the Paleoproterozoic (1.88 Ga) Gunflint Iron Formation containing siderite, ankerite, Fe-silicate clays (dominantly greenalite) and silica, reveal variable but light carbonate δ¹³C values (-21.3 to -7.87‰, VPDB), with heavy and less variable carbonate δ¹⁸O values (20.6 to 24.9‰, VSMOW). These measurements are consistent with a model in which siderite is formed during early diagenesis as a by-product of the metabolism of dissimilatory iron reducing (DIR) microbes that consume Fe(III)-oxide and organic carbon delivered from the water column to the sediment-water interface. This model indicates that the oxygen isotope compositions of reactant Fe-oxide, organic matter and dissolved inorganic carbon were set in equilibrium with seawater at T = 5–35 °C and δ¹⁸O = -1.0 to -6.5‰, prior to being metabolized to form siderite. Clumped isotope measurements on Fe-bearing carbonates yield temperatures (T(Δ₄₇)) between 40 °C-132 °C, which constrain δ¹⁸O_fluid to values between -6.22 to 7.32‰. These measurements record the onset temperature of DIR followed by low water-to-rock ratio diagenetic recrystallization at elevated temperature. Combined petrographic, chemical, and isotopic measurements reveal that the major phases delivered to the shallow seafloor were a disequilibrium assemblage of Fe-oxide, greenalite, silica, and organic matter that underwent microbially mediated modification to form an assemblage of siderite, greenalite, silica, and later diagenetic ankerite. We contend that observed differences between carbonate derived from Gunflint core and outcrop may be reconciled by removal of siderite during exposure and weathering, leaving outcrop enriched in late diagenetic ankerite.

Understanding changes in bacterial-algal communities is vital for unraveling climato-environmental factors linking ultimate triggers (e.g., volcanic, bioevolutionary, and tectonic events) and proximate killers (e.g., temperature and redox) in biocrises. However, the dynamics of marine community evolution during the two main extinction pulses of the Late Ordovician Mass Extinction (LOME), the first at the Katian-Hirnantian boundary and the second in the late Hirnantian, remain poorly investigated. Here, using multiple biomarkers (i.e., steranes, hopanes, moretanes, gammacerane, and n-alkanes), we document changes in red-algal/green-algal and bacteria/eukaryote ratios, along with the response of certain microbes sensitive to oceanic stratification and soil erosion through the Ordovician-Silurian transition. The distributions of these biomarkers reveal increased primary productivity and decreased continental weathering and oceanic stratification during the LOME-1, with the opposite trend during the LOME-2. Correlating these finding with global climato-environmental records, we infer that elevated productivity stimulated local expansion of oceanic anoxia and contributed to the culmination of a long-term cooling trend during the LOME-1, whereas rising temperatures and sea levels promoted intensified oceanic stratification and regionally anoxic conditions during the LOME-2. The results of the present study provide significant new biological constraints on the causation of the LOME.

We study the postseismic deformation of the 2005 Mw7.6 Kashmir earthquake in Pakistan, with a combination of the near- and far-field geodetic observations from the Kashmir Himalaya and the Tibetan Plateau, respectively. We construct a 3D finite-element model that integrates topographic relief, curved fault plane, and layered lithospheric structures to analyze stress-driven afterslip and estimate the rheological parameters of the Tibetan lithosphere. Our findings reveal that the ten-year afterslips of up to 30 cm were primarily localized at the downdip edges of co-seismic slip patches, causing near-field surface deformation of up to ∼5 cm within the first year. In contrast, viscoelastic relaxation in the lower crust of western Tibet and Pamir was required to reproduce far-field displacements of up to 1–2 cm recorded by GPS during the first 7 years. The model that best fits to the GPS displacements suggests a Maxwell viscosity exceeding 10²⁰ Pa s for the Indian lower crust, while a much lower viscosity of ∼4–10×10¹⁸ Pa s is inferred for the lower crust beneath Pamir and western Tibet, indicating a softer Tibet and Pamir in response to the continental collision between India and Eurasia.