Geochemistry Geophysics Geosystems最新文献

Subduction zones are important regions for understanding plate tectonic processes. New Zealand experiences slow slip, volcanism, and back-arc rifting, and has evidence of large megathrust events and tsunamis. We use S-to-P receiver functions to image lithospheric discontinuities beneath the North Island of New Zealand. A positive discontinuity interpreted as the Moho is imaged at 15–30 ± 3 km depth beneath the overriding Australian Plate. In some locations, near the interface of the Pacific and Australian Plates, we don't image the Pacific Plate Moho, and the Australian Plate Moho is faint or absent. The former could be related to the increasing dip or eclogitization of the Pacific Plate crust, and the latter is likely related to mantle wedge serpentinization. A negative velocity discontinuity associated with the lithosphere-asthenosphere boundary (LAB) of the Australian Plate is imaged at 63–80 ± 8 km depth across the northwestern side of the island. Negative discontinuities are imaged beneath the southern Pacific Plate at 85–105 ± 10 km and 130 ± 13 km depth, representing either a mid-lithospheric discontinuity (MLD) and a deeper LAB, or more likely a shallow LAB and a deeper artifact, given that the latter is better aligned with previous work. Beneath the Australian Plate, asthenospheric melt is inferred in the northwest beneath several regions of active volcanism. Beneath the Pacific Plate, asthenospheric melt is inferred near the trench, also corresponding to the transition to where the plates become locked; therefore, plate locking could be related to the buoyancy of the melt.

Using high-resolution remote sensing images and detailed field investigations, this study provides the spatiotemporal distribution and geometry of the Neoproterozoic giant mafic dyke swarm in the southeastern Tarim Craton, which is crucial for reconstructing the paleogeographic position of the Tarim Craton within the Rodinia supercontinent. This dyke swarm extends over 600 km in length and 20 km in width, spanning more than 12,000 km². Individual dykes ranged from 1.9 to 168 m in thickness and from 27 to 2,778 m in length. The mafic dykes were emplaced at 933–914 Ma, and exhibit trace element and isotope compositions similar to those of enriched mid-ocean ridge basalts (E-MORBs). The mantle potential temperature and pressure of the primary magma were estimated at ∼1,570°C and ∼4.0 GPa, respectively. The large-scale dyke swarm, together with its geochemical signatures and estimated P-T conditions, proposed that these dykes were generated by interaction between a Neoproterozoic mantle plume and the asthenospheric mantle. In addition, the Neoproterozoic giant mafic dyke swarm shares a close spatial and temporal relationship with the ∼890 Ma Sailajiazitage large igneous province in the southwestern Tarim Craton. Similar 930–890 Ma mafic dyke swarms have also been identified in North China Craton, São Francisco Craton, and Congo Craton, indicating the presence of a single giant mantle plume beneath the Rodinia supercontinent. Together with the sedimentary records and paleomagnetic data, it is proposed that the Southern Tarim Terrane was located adjacent to the São Francisco Craton and Congo Craton in the Northern Hemisphere.

Precise dating of Quaternary volcanism is vital for risk mitigation, understanding volcano-climate interactions, and deciphering the evolution of large silicic magmatic systems. The Atitlán caldera in Guatemala has experienced major eruptions that challenge radiometric dating techniques and complicate eruption chronology in this densely populated area. This study refines the eruptive history of Atitlán caldera using zircon double-dating (ZDD: combined [U-Th]/He and ²³⁸U-²³⁰Th disequilibrium dating). We present new ZDD eruption ages for previously undated events, including the I-tephra and the newly discovered Atitlán Early Tephra (AET). Additionally, we provide crystallization dates for the Los Chocoyos (LCY) supereruption, utilizing ultra-distal samples from the Pacific Ocean, Lake Petén Itzá, and Mexico. ZDD was also applied to the ⁴⁰Ar/³⁹Ar sanidine-dated W-tephra confirming its reliability. Our findings yield an internally consistent chronology, with the first radiometric ages of 64 ± 8 ka for the I-tephra and 497 ± 12 ka for AET. The ZDD eruption age of 160 ± 9 ka for W-tephra corroborates the existing ⁴⁰Ar/³⁹Ar sanidine age. Bayesian eruption age modeling (BEAM) of new LCY ²³⁸U-²³⁰Th disequilibrium dates consistently yields ages younger than previous estimates based on overdispersed zircon and plagioclase dates. Regardless of the prescribed zircon age distribution, BEAM results indicate the youngest zircon crystallization at ca. 88–76 ka, supporting the established ZDD eruption age of 75 ± 2 ka for LCY. This refined chronology provides insights into the Atitlán caldera volcanic activity, enhances hazard assessment and understanding of regional geological evolution, and highlights the pitfalls of Bayesian age modeling when integrating different chronometers.

Great volumes of water are carried downward into the mantle transition zone (MTZ, 410–670 km depth) by subducting slabs. If this water is later drawn upward, the resulting mantle melting may generate continental intraplate volcanism (IPV). Despite water's importance, its amount and spatial distribution within the MTZ, and its impact on IPV, are poorly constrained. Here we use plate tectonic reconstructions to estimate the rates and positions of water injection into the MTZ by subducted slabs during the past 400 Myr. This allows us to construct global maps of heterogeneous MTZ hydration, which we then compare to IPV eruption locations from the past 200 Myr. We detect a statistically significant correlation between wet MTZ regions and IPV locations at the surface, but only if slabs sink faster than 1 cm/yr, water remains stored in the MTZ for periods of 30–100 Myr, and IPV eruptions occur 10–30 Myr later. We find that 42%–68% of continental IPV is underlain by wet MTZ, with greater fractions associated with longer MTZ residence time. Hydrous underpinning of continental IPV was highest during the Jurassic, when more extensive slab interaction with the MTZ hydrated a wider area of the MTZ. Since the Cretaceous, continents have been moving over the wet MTZ, increasing IPV possibilities. MTZ regions near the northern Pacific, southern Africa, and western Europe have remained dry by avoiding wet slabs. We suggest that subducted water shapes global patterns of intraplate volcanism, with hydrous upwellings rising from the MTZ to generate continental IPV above wet MTZ regions.

Stable and inert under most conditions, zircon can be dissolved and precipitated by aqueous fluids in the upper crust. Geochemical models using currently available thermodynamic properties for Zr aqueous species at 0.2 GPa predict that zircon solubility increases with temperature from 400 to 900°C in fluids saturated with quartz or baddeleyite. Zircon solubility is low in near-neutral pH fluids and enhanced in acidic and alkaline fluids. Adding NaOH and to a lesser extent NaF to the solution significantly increases the solution pH values and Zr concentrations at zircon saturation. Modeled Zr concentrations are often orders of magnitude different from zircon solubilities measured experimentally under similar conditions. Metamict (amorphous) ZrSiO₄ is more soluble than crystalline zircon and is replaced through a coupled dissolution-precipitation process. Reaction path kinetics models were constructed to simulate experiments described in the literature and extract rate constants for replacement of metamict ZrSiO₄. Replacement is rate limited by zircon precipitation and is nearly complete after 1 week when fluid is present at 600°C, with the rate of replacement increasing with temperature. In a closed system, hydrothermal zircon may form by replacement of radiation-damaged zircon but not fully crystalline zircon. Replacement of metamict ZrSiO₄ forms characteristic porosity. Geochemical models identify the conditions that promote zircon solubility, metamict ZrSiO₄ replacement, and the formation of hydrothermal zircon, and provide constraints on the interpretation of zircon U-Pb dates of hydrothermal events.

To understand the exhumation history of orogens and their fold-thrust belts, it is important to accurately reconstruct their time-temperature evolution. This is often done by employing thermokinematic models. One problem of current approaches is that they are limited in prescribing geometric constraints only as far as they affect transient thermal conditions. This often results in 2-D plane strain assumptions, and a simple treatment of structural and kinematic uncertainties. In this work, we combine 3-D kinematic forward modeling with a random sampling approach to automatically generate an ensemble of kinematic models in the range of assigned geometric uncertainties. Using Markov Chain Monte Carlo, each randomly generated model is assessed regarding how well it fits the available paleo-depth data taken from low-temperature thermochronology. The resulting, more robust model can then be used to re-interpret the thermal resetting data. We first apply this method to synthetic experiments with variable structural complexity and sample uncertainties, and later to the Alpine fold-thrust belt, the Subalpine Molasse. Results show that it is possible to use thermochronological data to make predictions about exhumation, which can be translated into likelihood functions to obtain the range of 3-D kinematic forward models explaining the data. Though the method performs well for the synthetic models, additional thermochronological parameters may need to be considered to improve the inversion results for structurally complex settings. The method is useful, however, to study alternative mechanisms of exhumation for the thermochronological samples that are not respected by the modeling, even when uncertainty is considered.

Estimating the equilibration temperatures of mantle-derived garnets is crucial for assessing the diamond potential of kimberlites. Traditional garnet geothermometers require co-existing mineral data or costly trace element analysis, limiting their practical use. As an alternative approach, based on the major and minor element composition of garnet alone, we first re-calibrated an Mn-in-garnet thermometer using a newly compiled data set of garnets from well-equilibrated peridotitic xenoliths with well-constrained pressure-temperature (P-T) conditions. The re-calibrated Mn-in-garnet thermometer, however, is only of intermediate accuracy, with a relatively large discrepancy relative to the most reliable multi-phase thermometry, indicated by a high root mean square error value (RMSE = 79°C) across a temperature range from 900 to 1,400°C. In a second improve approach, we developed a new machine learning (ML)-based garnet thermometer that demonstrated superior performance, achieving significantly better accuracy and reduced discrepancies (average RMSE = 61°C). The ML-based garnet thermometer outperforms the Mn-in-garnet thermometer because it considers not only MnO but also other major and minor elements, particularly TiO₂, revealed by the ML model to be critical for accurate prediction of garnet temperatures. Applying the ML-based thermometer to garnet xenocrysts from kimberlites on the Slave and Kaapvaal cratons reveals that high numbers of sublithospheric (superdeep) diamonds are associated with significantly higher proportions of high-T (>1,200°C) high-Ti garnets, compared to kimberlites in which superdeep diamonds are either few or absent. This finding indicates that a number of kimberlites, not currently identified as containing superdeep diamond populations, are promising hosts of such diamonds.

The motion of the Adriatic microplate is thought to be highly sensitive to the surrounding subduction zones and the convergence of Africa and Eurasia. However, our understanding of the mantle dynamics in the Mediterranean region and its effect on plate motion remains incomplete. Here, we present a large set of 3D thermomechanical models of the entire Mediterranean region over the last 35 Myr to understand what controls the motion of the Adriatic microplate. The simulations take the convergence of the African and Arabian plates with the Eurasian plate into account, along with the dynamics of the subduction systems in the western (Apennines-Calabria), central (Dinarides-Hellenides) Mediterranean and in the Alpine-Carpathian region. Our results demonstrate that the subduction systems around Adria are highly coupled, which gives rise to complex asthenospheric flow in the central Mediterranean. We find that the plate motion of the Adriatic microplate over the last 35 Myr is controlled by the interplay of three main factors: (a) the convergence between the African and Eurasian plates, (b) the retreat of the Alpine subduction zone to the north of Adria, and (c) the distance between the Calabrian and Hellenic subduction zones around Adria. Furthermore, in a system characterized by active convergence between Africa and Eurasia, the slab pull exerted by nearby subduction zones can only notably influence the motion of the Adriatic microplate if these subduction zones are located within a few hundred kilometers of Adria.

Gypsum is one of the major evaporite minerals that can be used as a sensitive archive for past climate change, revealing variations in the hydrological cycle and dry-wet climatic alternations. Measuring variations in gypsum content, rapidly and with high accuracy, will contribute to more extensive paleoclimate reconstructions. However, a fast, convenient, and non-destructive method for quantifying gypsum in sedimentary rocks is still lacking. Diffuse reflectance spectroscopy (DRS) is a commonly employed technique for rapidly identifying and quantifying minerals; however, its application to semi-quantifying gypsum in paleoclimate studies has received comparatively less attention. Here, we synthesized artificial and natural samples with precisely controlled gypsum content and analyzed the visible and near-infrared DRS characteristics of the gypsum crystals. We report a linear correlation between gypsum content and the intensity of negative peaks in the DRS spectra at 1,540 nm (I₁₅₄₀) and 1,942 nm (I₁₉₄₂), converted by the second derivative of the Kubelka-Munk (K-M) remission functions. The robustness of this I₁₅₄₀/I₁₉₄₂-based DRS proxy (I_gyp) for semi-quantifying gypsum in sediments was further validated against powder X-ray diffraction results. Applying the new I_gyp proxy to the Eocene gypsum sequence from the Xining Basin (China) shows a strong correlation with calibrated gypsum contents, demonstrating its potential for the identification and quantification of gypsum in sedimentary rocks. This method offers a promising new approach for gypsum analysis in paleoclimate studies.

Volcanic basalt samples originating from two historic eruptions, that is, the 1991 C.E. Hekla, Iceland, and 1944 C.E. Vesuvius, Italy, have been studied to determine the 3D tomographic and spatial distributions of their constituent (titano)magnetite minerals using SEM-FIB slice-and-view. Determining the morphology is key to quantifying the magnetic recording fidelity of a rock, as grain morphology is a primary control of the magnetic (domain) state of a grain, which in turn determines magnetic recording fidelity. Smaller grains are magnetically uniform and are termed single domain (SD). A surface morphology resolution of $��\sim �$2 nm was achieved and the smallest grains that were resolved with $��\sim �$21 nm in diameter; a total of 971 particles were analyzed. We determined a median equivalent-volume spherical diameter of 70 nm for the Hekla sample, and 135 nm for the Vesuvius sample. The particles had nearest-neighbor distances of 184 and 355 nm, indicate the majority of grains were free from magnetostatic interactions. In both samples there was a roughly even split between oblate and prolate grains. This number of oblate grains is much higher than traditionally assumed, and will have implications for many paleomagnetic methods which assume prolate grains, for example, anisotropy of magnetic susceptibility analysis. Numerical micromagnetic analysis of the grain-morphologies, predict that $��\sim �$64% of the Hekla grains have SD ground-states ($��\sim �$6% by volume), but only $��\sim �$26% of the Vesuvius grains have SD ground-states ($��\sim �$1% by volume). Both samples are predicted to be excellent paleomagnetic recorders, with median relaxation times far larger than the length of the Universe.