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The Multiple-Diffusion Domain (MDD) model empirically describes the diffusive behavior of noble gases in some terrestrial materials and has been commonly used to interpret 40Ar/39Ar stepwise degassing observations in K-feldspar. When applied in this manner, the MDD model can be used to test crustal exhumation scenarios by identifying the permissible thermal paths a rock sample could have undergone over geological time, assuming the diffusive properties of Ar within the mineral are accurately understood. More generally, the MDD model provides a framework for quantifying the temperature-dependent diffusivity of noble gasses in minerals. However, constraining MDD parameters that successfully predict the results of step-heating diffusion experiments is a complex task and the assumptions made by existing numerical methods used to quantify model parameters can bias the absolute temperatures permitted by thermal modeling. For example, the most commonly used method (Lovera et al., 1997) assumes that no domains lose more than 60 % of their gas during early heating steps. This assumption is unverifiable, and we show that Lovera et al.'s (1997) procedure may bias predicted temperatures towards lower values when it is violated. To address this potential bias and to provide greater accessibility to the MDD model, we present a new, open-source method for constraining MDD parameters from stepwise degassing experimental results, called the "MDD Tool Kit." This software optimizes all MDD parameters simultaneously and removes any need for user-defined Ea or regression-fitting choices used by other tools. In doing so, this new method eliminates assumptions about the domain size distribution. To test the validity of our thermal predictions, we then use the MDD Tool Kit to interpret the 40Ar/39Ar results of Wong et al. (2023) from the Grayback Fault, AZ, USA. Although the resulting thermal histories are consistently ~ 60–75 °C higher than those found by Wong et al. 2023), they agree with independent observations from apatite fission track, zircon fission track, and (U-Th)/He (Howard and Foster, 1996). [ABSTRACT FROM AUTHOR]

While thermochronological studies have constrained the landscape evolution of several of the crustal blocks of West and East Antarctica, the tectono-thermal evolution of the Ellsworth Mountains remains relatively poorly constrained. These mountains are among the crustal blocks that comprise West Antarctica and exhibit an exceptionally well-preserved Palaeozoic sedimentary sequence. Despite the seminal contribution of Fitzgerald and Stump (1991), who suggested an Early Cretaceous uplift event for the Ellsworth Mountains, further thermochronological studies are required to improve the current understanding of the landscape evolution of this mountain chain. We present new zircon (U–Th) / He (ZHe) ages, which provide insights into the landscape evolution of the Ellsworth Mountains. The ZHe ages collected from near the base and the top of the sequence suggest that these rocks underwent burial reheating after deposition. A cooling event is recorded during the Jurassic–Early Cretaceous, which we interpret as representing exhumation in response to rock uplift of the Ellsworth Mountains. Moreover, our results show that while ZHe ages at the base of the sequence are fully reset, towards the top ZHe ages are partially reset. Uplift and exhumation of the Ellsworth Mountains during the Jurassic–Early Cretaceous was contemporaneous with the rotation and translation of this crustal block with respect to East Antarctica and possibly the Antarctic Peninsula. Furthermore, this period is characterized by widespread extension associated with the disassembly and breakup of Gondwana, with the Ellsworth Mountains playing a key role in the opening of the far southern Atlantic. Based on these results, we suggest that uplift of the Ellsworth Mountains during the disassembly of Gondwana provides additional evidence for major rearrangement of the crustal blocks between the South American, African, Australian and Antarctic plates. Finally, uplift of the Ellsworth Mountains commenced during the Jurassic, which predates the Early Cretaceous uplift of the Transantarctic Mountains. We suggest that the rift-related exhumation of the Ellsworth Mountains occurred throughout two events: (i) a Jurassic uplift associated with the disassembly of southwestern Gondwana and (ii) an Early Cretaceous uplift related with the separation between Antarctica and Australia, which is also recorded in the Transantarctic Mountains. [ABSTRACT FROM AUTHOR]

A promising primary reference material for U-Pb LA-ICP-MS carbonate dating is analysed and reported here. The new RM is a botryoidal cement (C1) from sample RA138. The sample was collected in outcrop strata of mid-Carboniferous (Uppermost Mississippian, upper Serpukhovian) in northern Spain near La Robla, and multiple aliquots have been meticulously prepared for distribution. The RA138 is characterised by variable U/Pb ratios (from ~1 to ~19) and a relatively high and homogeneous U content (~4 ppm). This material exhibits a low age uncertainty (0.2 %, 2s; unanchored, ID-TIMS), allowing for the establishment of a well-defined isochron, particularly when anchored to the initial Pb ratio using LA-ICP-MS. ID-TIMS analyses of micro drilled C1 cement (17 sub-samples) produce a lower intercept age of 321.99 ± 0.65 Ma, an initial 207Pb/206Pb ratio of 0.8495 ± 0.0065, and a Mean Square of Weighted Deviations (MSWD) of 5.1. The systematic uncertainty of 1.5 % observed in repeated LA-ICP-MS analyses challenges previous estimations of 2–2.5 % based on repeated analyses of ASH-15D and JT using WC-1 as primary reference material, underscoring the precision and reliability of RA138 for U-Pb dating applications. [ABSTRACT FROM AUTHOR]

Central Dronning Maud Land (cDML) is part of the late Mesoproterozoic Maud Belt in East Antarctica, which was metamorphosed and deformed during the Ediacaran–Cambrian Gondwana assembly. Here we study high-pressure (HP) mafic rocks in Gjelsvikfjella, cDML, which occur as lenses and pods transposed in highly strained, upper amphibolite-facies gneisses. We present a P – T – t history for the HP rocks based on mineral assemblages, reaction textures and new U–Pb zircon data. Relics that indicate an early HP granulite-facies stage have been identified in anhydrous garnet–clinopyroxene rocks. The peak-pressure assemblage was plagioclase-free and contained garnet, titanite, clinopyroxene and quartz. The HP assemblage has been extensively overprinted by lower-pressure phases and exhibits a variety of symplectite and corona textures that record the post-peak-pressure evolution of the rocks. Decompression and heating in the granulite-facies field resulted in the replacement of titanite by ilmenite–clinopyroxene symplectite, formation of clinopyroxene–plagioclase intergrowths and resorption of garnet by plagioclase–clinopyroxene kelyphite. Formation of late orthopyroxene in symplectites and kelyphites demonstrates that the P – T evolution entered the medium-pressure granulite-facies field. The peak metamorphic stage was followed by retrograde cooling into the amphibolite facies. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicate Mesoproterozoic protolith ages (1150–1000 Ma) and Ediacaran–Cambrian metamorphic reworking at ca. 568 and ca. 514 Ma. The HP granulites were formed and exhumed during a clockwise P – T evolution related to continental collision during Gondwana amalgamation, followed by post-collisional extension and orogenic collapse. [ABSTRACT FROM AUTHOR]

The European Beyond EPICA project aims to extract a continuous ice core of up to 1.5 Ma, with a maximum age density of 20 kyr m -1 at Little Dome C (LDC). We present a 1D numerical model which calculates the age of the ice around Dome C. The model inverts for basal conditions and accounts either for melting or for a layer of stagnant ice above the bedrock. It is constrained by internal reflecting horizons traced in radargrams and dated using the EPICA Dome C (EDC) ice core age profile. We used three different radar datasets ranging from a 10 000 km 2 airborne survey down to 5 km long ground-based radar transects over LDC. We find that stagnant ice exists in many places, including above the LDC relief where the new Beyond EPICA drill site (BELDC) is located. The modelled thickness of this layer of stagnant ice roughly corresponds to the thickness of the basal unit observed in one of the radar surveys and in the autonomous phase-sensitive radio-echo sounder (ApRES) dataset. At BELDC, the modelled stagnant ice thickness is 198±44 m and the modelled oldest age of ice is 1.45±0.16 Ma at a depth of 2494±30 m. This is very similar to all sites situated on the LDC relief, including that of the Million Year Ice Core project being conducted by the Australian Antarctic Division. The model was also applied to radar data in the area 10–15 km north of EDC (North Patch), where we find either a thin layer of stagnant ice (generally <60 m) or a negligible melt rate (<0.1 mm yr -1). The modelled maximum age at North Patch is over 2 Ma in most places, with ice at 1.5 Ma having a resolution of 9–12 kyr m -1 , making it an exciting prospect for a future Oldest Ice drill site. [ABSTRACT FROM AUTHOR]

Chemical abrasion in hydrofluoric acid (HF) is routinely applied to zircon grains prior to U-Pb dating by isotope dilution thermal ionization mass spectrometry (ID-TIMS) to remove radiation-damaged portions of grains affected by Pb loss. Still, many chemically abraded datasets exhibit evidence of residual Pb loss. Here we test how the temperature and duration of chemical abrasion affects zircon U-Pb and trace element systematics in a series of 4-hour, single-crystal stepwise dissolution experiments at 180 °C and 210 °C. Microtextural data for the zircon samples studied is presented in a complementary paper by McKanna et al. (2023). We find that stepwise dissolution at 210 °C is more effective at eliminating U, common Pb (Pbc), and light rare earth element (LREE) enriched material affected by open system behavior; reduces the presence of leaching-induced artefacts that manifest as reverse discordance; and produces more consistent and concordant results in zircon from the three rocks studied. We estimate that stepwise dissolution in three 4 h steps is roughly equivalent to a single ~8 h leaching step due to the insulating properties of the PTFE sleeve in the Parr pressure dissolution vessel, whereas traditionally labs utilize a single 12-hour leaching step. To better understand the causes of Pb-loss in zircon, we calculate time-integrated alpha dose estimates for leachates and residues from measured radionuclide concentrations to determine: 1) the alpha dose of the material dissolved at the two leaching conditions, and 2) the apparent minimum alpha dose required for Pb loss to occur: ≥ 6 × 1017 α/g. We conclude that a single 8 h leaching step at 210 °C should yield crystallization ages in the majority of zircon and that this can be used as an effective approach for routine analysis. However, Ultimately, the effectiveness of any chemical abrasion protocol will be sample-dependent. By framing Pb loss and zircon solubility in terms of alpha dose, however, workers can better tailor the chemical abrasion process to specific zircon samples to improve the accuracy and precision of U-Pb results. [ABSTRACT FROM AUTHOR]

The Martabe gold deposits in Sumatra formed in a shallow crustal epithermal environment associated with intermediate mafic intrusions adjacent to an active right-lateral wrench system. Gas/fluid temperatures reached 200–350 ∘ C. The structural geology suggests episodic switches in stress orientations during a Plio-Pleistocene seismotectonic evolution. Different mineralisation events may have been associated with oscillations in this earthquake cycle, so samples containing alunite were collected for 40 Ar / 39 Ar geochronology to constrain the timing. 39 Ar diffusion experiments were performed to constrain variation in argon retentivity. The age spectra were produced by incremental step-heating with heating times chosen so similar percentages of 39 Ar gas release occurred during as many steps as possible. This ensured the detail necessary for analysis of the complex morphology of these spectra by applying the method of asymptotes and limits, which enabled recognition of different growth events of alunite in overprinting fluid systems. It was possible to provide estimates as to the frequency of individual events and their duration. The heating schedule also ensured that Arrhenius data populated the inverse temperature axis with sufficient detail to allow modelling. Activation energies were between 370–660 kJ mol -1. Application of Dodson's recursion determined closure temperatures that range from 400–560 ∘ C for a cooling rate of 100 ∘ C Ma -1. Such estimates are higher than any temperature to be expected in the natural system, giving confidence that the ages represent the timing of growth during periods of active fluid movement and alteration: a hypothesis confirmed by modelling age spectra using the MacArgon program. We conclude that gold in the Purnama pit resulted from overprinting fluid rock interactions during very short mineralisation episodes at ∼2.25 and ∼2.00 Ma. [ABSTRACT FROM AUTHOR]

Chemical abrasion is a technique that combines thermal annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U–Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µ CT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µ CT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. Acid frequently reaches crystal interiors via fractures spatially associated with radiation damage zoning and inclusions to dissolve soluble high-U zones, some inclusions, and material around fractures, leaving behind a more crystalline zircon residue. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically controlled with dissolution proceeding almost exclusively along the c axis. Increasing the leaching temperature from 180 to 210 ∘ C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. How a grain dissolves strongly depends on its initial radiation damage content and defect distribution as well as the size and position of inclusions. As such, the effectiveness of any chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely sample-dependent. We also briefly discuss the implications of our findings for deep-time (U-Th)/He thermochronology. [ABSTRACT FROM AUTHOR]

The Oman ophiolite (Samail massif, Sultanate of Oman) is the largest sub-aerial exposure of oceanic lithosphere on Earth and provides the opportunity to study the accretion and alteration of oceanic lithosphere formed under fast-spreading conditions. Drill hole GT3A (23 ∘ 06 ′ 50.7 ′′ N, 58 ∘ 12 ′ 42.2 ′′ E) of the ICDP (International Continental Scientific Drilling Program) Oman Drilling Project with a length of 400 m aimed at penetrating the dike–gabbro transition of the Samail ophiolite paleocrust in order to shed light on the role of the axial melt lens (AML) during accretion of the lower plutonic crust. AMLs beneath fast-spreading mid-ocean ridges are sandwiched between the sheeted dike complex and the uppermost gabbros and are believed to feed the upper crust and, at least partially, the underlying crystal mush. Typical gabbroic rocks from dike–gabbro transitions of fast-spreading systems are the so-called "varitextured gabbros", often showing considerable variations in mineral mode, texture and grain size, which are regarded as the frozen fillings of axial melt lenses. Here, we present a detailed petrographic, microanalytical and bulk-chemical investigation of 36 mafic rocks from the drill hole GT3A, which represent mostly varitextured gabbros, revealing a complex formation with several evolution stages. Poikilitic domains formed first, corresponding to an early crystallization stage, where only plagioclase and clinopyroxene of more primitive composition crystallized. Later, domains of granular textures containing also interstitial amphibole and Fe–Ti oxide were formed. This stage is characterized by a magma evolution that underwent crystal fractionation established by lower temperatures due to more efficient hydrothermal cooling at the margin of the AML. A last stage is characterized by pervasive hydrothermal alteration, where all primary minerals have been altered under temperature conditions, varying from the magmatic regime down to greenschist facies. A highlight of this stage is amphiboles showing noticeable compositional zoning. The observation of peculiar microgranular domains, representing relics of stoped exogenic material from the sheeted dike complex, documents the upward migration of an AML in a replenishment event, forcing the AML to burn through previously altered sheeted dikes. This process is responsible for significant assimilation of hydrothermally altered components, indicated by a marked Cl enrichment in the outer zones of magmatic amphiboles. Petrological modeling involving gabbros and basalts revealed that the GT3A rock suite followed a fractional crystallization evolution trend, with a primitive MORB as parental melt with an estimated water content of 0.2 wt % to 0.8 wt %. The modeled liquid lines of descent suggest a magmatic evolution via fractional crystallization, where the basalts correspond to frozen liquids, while the gabbros, especially the more primitive ones, show a significant cumulate component. [ABSTRACT FROM AUTHOR]

In situ carbonate U-Pb dating studies have proliferated dramatically in recent years. Almost all these studies have targeted relatively young calcite up to Carboniferous in age. To assess the robustness of the carbonate U-Pb chronometer in deep-time, we carried out in situ U-Pb analyses in magnesite-ankerite-calcite carbonates in the martian meteorite Allan Hills (ALH) 84001. Carbonates in ALH 84001 formed at ca. 3.94 Ga, and there is little evidence that much happened to this rock since then, making it an ideal sample to test the robustness of the U-Pb system in old carbonates. We obtained a concordant date of 3941 ± 49/110 Ma (n = 14, MSWD = 2.0), which is identical to the step-leaching Rb/Sr date determined previously. These results thus confirm that old carbonates are amenable to U-Pb dating in samples that have had a relatively simple history post-carbonate formation. [ABSTRACT FROM AUTHOR]

Chemical abrasion is a technique that combines laboratory annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U-Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. We find that most grains do not dissolve predominantly from rim-to-core. Acid frequently reaches crystal interiors via fracture networks spatially associated with radiation damage zoning and inclusions to dissolve higher U zones, material in the vicinity of fractures, and some inclusions. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically-controlled with dissolution proceeding almost exclusively along the c-axis. Increasing the leaching temperature from 180 °C to 210 °C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. We discuss the implications of our findings for zircon ID-TIMS U-Pb geochronology and paired trace element analyses, radiation damage annealing models, and for using µCT for imaging radiation damage zoning for (U-Th)/He thermochronology. [ABSTRACT FROM AUTHOR]

We collected a debris-rich ice core from a buried ice mass in Ong Valley, located in the Transantarctic Mountains in Antarctica. We measured cosmogenic nuclide concentrations in quartz obtained from the ice core to determine the age of the buried ice mass and infer the processes responsible for the emplacement of the debris currently overlaying the ice. Such ice masses are valuable archives of paleoclimate proxies; however, the preservation of ice beyond 800 kyr is rare, and therefore much effort has been recently focused on finding ice that is older than 1 Myr. In Ong Valley, the large, buried ice mass has been previously dated at > 1.1 Ma. Here we provide a forward model that predicts the accumulation of the cosmic-ray-produced nuclides 10 Be, 21 Ne, and 26 Al in quartz in the englacial and supraglacial debris and compare the model predictions to measured nuclide concentrations in order to further constrain the age. Large downcore variation in measured cosmogenic nuclide concentrations suggests that the englacial debris is sourced both from subglacially derived material and recycled paleo-surface debris that has experienced surface exposure prior to entrainment. We find that the upper section of the ice core is 2.95 + 0.18 / - 0.22 Myr old. The average ice sublimation rate during this time period is 22.86 + 0.10 / - 0.09 m Myr -1 , and the surface erosion rate of the debris is 0.206 + 0.013 / - 0.017 m Myr -1. Burial dating of the recycled paleo-surface debris suggests that the lower section of the ice core belongs to a separate, older ice mass which we estimate to be 4.3–5.1 Myr old. The ages of these two stacked, separate ice masses can be directly related to glacial advances of the Antarctic ice sheet and potentially coincide with two major global glaciations during the early and late Pliocene epoch when global temperatures and CO 2 were higher than present. These ancient ice masses represent new opportunities for gathering ancient climate information. [ABSTRACT FROM AUTHOR]

Recent developments in analytical capabilities in the field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded the applications of U–Pb geochronometers in low-U minerals such as carbonates or garnets. The rapid evolution of the technique relies on well-characterized matrix-matched reference materials. In this article, we explore the suitability of using carbonate as an "almost-matrix-matched reference material" for in situ U–Pb dating of sulfates. For such purpose, we have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97–5.33 Ma) and compared these dates with the U–Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common Pb content and low 238U/204Pb ratios, five of the samples showed a higher dispersion on U/Pb ratios. The obtained dates in four of these samples are comparable with the expected ages, while another gave an unexpected younger age, each of them with 6 %–11 % of uncertainty. The pit depth of the spots showed that the sulfates ablate similar to carbonates, so the offset due to the crater geometry mismatch or downhole fractionation can be assumed to be negligible. To sum up, the bias between the U–Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty, and thus the results obtained here suggest that carbonate reference material is currently the best option for standardization of in situ U–Pb sulfate analyses. [ABSTRACT FROM AUTHOR]

In situ carbonate U–Pb dating studies have proliferated dramatically in recent years. Almost all these studies have targeted relatively young terrestrial calcite up to Carboniferous in age. To assess the robustness of the carbonate U–Pb chronometer in deep time, we carried out in situ U–Pb analyses in magnesite–ankerite–calcite carbonates in the martian meteorite Allan Hills (ALH) 84001. Carbonates in ALH 84001 formed at ca. 3.94 Ga , and there is little evidence that much happened to this rock since then, making it an ideal sample to test the robustness of the U–Pb system in old carbonates. We obtained a concordant date of 3941 ± 49/110 Ma (n=14 , MSWD = 2.0), which is identical to the step-leaching Rb/Sr date determined previously. These results thus confirm that old carbonates are amenable to U–Pb dating in samples that have had a relatively simple history post-carbonate formation. [ABSTRACT FROM AUTHOR]

We present a simple inexpensive design for a filter wheel to modify a broadband cathodoluminescence CL) detector in a scanning electron microscope (SEM) to provide spectroscopic information along with image processing and visualization tools that enhances the quality and utility of that imagery. This system was initially tested using a ZnS standard known to produce a strong CL signal in the visible spectrum. Enhanced images and spectroscopic data could also be used for grain and spot targeting of natural zircons that commonly precedes isotopic analysis by laser ablation or ion microprobe. The filter-wheel system was made from inexpensive or 3D printed materials, allows for interchangeable filters for a variety of applications, and the switching between those filters without the user breaking SEM vacuum. The color filtered images were used to create color composite images similar to those generated by higher-end spectrometers with built-in color filtering capability. The embedded data from the RGB pixels of the color filtered images were then used to generate 2D and 3D graphs providing another visual path for comparing relative intensities. A color composite image and associated 3D graphs for a prototypical zircon sample was generated to demonstrate the usefulness of the device and utility of plots. Design documents and specifications for the color filter wheel, and codes to generate images and plots are available on https://weber.edu/cos/instrument%5Finformation.html. [ABSTRACT FROM AUTHOR]

Experimental homogenization of olivine-hosted melt inclusions representative of near-primary basic and ultrabasic magmas is a powerful approach to investigate the nature of their source regions and the melting conditions in Earth's mantle. There is growing evidence that the total CO 2 contents of olivine-hosted melt inclusions may reach values of the order of a single to several weight percent, especially in intraplate continental basalts. To be able to homogenize melt inclusions with such high CO 2 contents, we developed a technique allowing for heat treating of the melt inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder, using thick-walled Au 80 –Pd 20 containers and molten NaCl as the surrounding medium for the inclusion-bearing olivines. We applied this technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais volcanic province, French Massif Central. Thueyts melt inclusions were chosen because of their high CO 2 contents, as indicated by up to 1.19 wt % dissolved CO 2 in the glasses and by the presence of shrinkage bubbles containing abundant carbonate microcrystals in addition to a CO 2 fluid phase. The homogenization experiments were conducted at pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 ∘ C, and run durations of 30 min. In all the melt inclusions treated at 2.5 GPa–1300 ∘ C and half of those treated at 2 GPa–1300 ∘ C, we were able to completely homogenize the inclusions, as indicated by the disappearance of the starting bubbles, and we obtained total CO 2 contents ranging from 3.2 wt % to 4.3 wt % (3.7 wt % on average). In all the other melt inclusions (equilibrated at 1.5 or 2 GPa and 1300 ∘ C or at 2.5 GPa–1275 ∘ C), we obtained lower and more variable total CO 2 contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest total CO 2 contents, the size of the shrinkage bubble was in most cases small (<5 vol %) to medium (<10 vol %): this is a strong argument in favor of an origin of these melt inclusions by homogeneous entrapment of very CO 2 -rich basanitic liquids (∼ 4 wt %) at pressures of 2 to 2.5 GPa. The lower total CO 2 contents measured in some inclusions could reflect a natural variability in the initial CO 2 contents, due for instance to melt entrapment at different pressures, or CO 2 loss by decrepitation. An alternative scenario is heterogeneous entrapment of basanitic liquid plus dense CO 2 fluid at lower pressures but still at least on the order of 1 GPa as indicated by dissolved CO 2 contents up to 1.19 wt % in the glasses of unheated melt inclusions. Whatever the scenario, the basanites from the Bas-Vivarais volcanic province were generated in a mantle environment extremely rich in carbon dioxide. [ABSTRACT FROM AUTHOR]

We describe a new method for the measurement of U/Pb ratios by isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the dating of geologically young clean carbonates, particularly speleothems. The method is intended for materials containing little or no initial 232 Th. We illustrate and validate the method with four examples ranging from 0.57 to 20 Ma. The new method is capable of applying the 235 U– 207 Pb and 238 U– 234 U– 206 Pb chronometers, common Pb and quantifiable residual 234U/238U disequilibrium permitting. These provide an alternative to the more widely used 238 U– 206 Pb chronometer, which can be highly inaccurate for samples that are < ca. 20 million years old, owing to uncertainties in the excess initial 234 U (hence, excess radiogenic 206 Pb) commonly observed in speleothems. [ABSTRACT FROM AUTHOR]

The North Pamir, part of the western syntax of the India–Asia collision zone, preserves remnants of a poorly understood Paleozoic intra-oceanic subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with U – Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Dating of radial fibrous to equant spary calcite yielded three meaningful Visean–Serpukhovian ages. Rare-earth elements and yttrium (REE + Y) data reveal that the basaltic host rock of the calcite and oxidizing seawater are major sources of trace elements during calcite precipitation. U – Pb ages seem to be independent of REE + Y concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors. We challenge the hypothesis that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir. [ABSTRACT FROM AUTHOR]

Amphibole represents an important repository of water (among other volatiles, e.g., chlorine and fluorine) in the lithosphere in all those environments characterized by the circulation of fluids and hydrous melts, such as subduction zones and subcontinental lithospheric mantle. Therefore, detailed knowledge of the mechanisms ruling water incorporation in amphibole is essential to assess the amount of water that can be fixed in the lithosphere by this mineral and, ultimately, gain a better insight into the deep water cycle. Water is incorporated into the structure of amphibole as hydroxyl (OH -), which is hosted in the anion site O(3) , and the incorporation is mainly controlled by the oxo-substitution mechanism M(1) Ti 14++O(3) O 22- M(1) (Mg 2+ , Fe 2+)-1+ O(3) (OH -)-2-. However, the fluids and melts circulating in the lithospheric mantle can be variably enriched in halogens (Cl - and F -) that can substitute OH - in the anion site O(3) of amphibole, thus potentially affecting its water budget. The aim of this study is to evaluate the effect of Cl on the oxo-substitution and the incorporation of water in amphibole. End-loaded piston cylinder experiments were conducted at pressure and temperature conditions compatible with the upper-mantle depth (1.4 GPa and 1015–1050 ∘ C) in order to favor the crystallization of amphibole at equilibrium with the coexisting melt. Alkali basalt powder was used as starting material, and water doped with different contents of Cl was added to each experiment. Two ranges of oxygen fugacity (f O 2) were investigated at Δ FMQ = -2.6 (log f O 2 [experiment] - log f O 2 [FMQ buffer]) and Δ FMQ = +1.7 , where FMQ is fayalite–magnetite–quartz, in order to preliminarily identify the potential influence of the f O 2 on the water budget in amphibole. In this contribution, we propose a new method to quantify water in amphiboles using confocal micro-Raman spectroscopy. The H 2 O contents range from 2.20 ± 0.10 wt % to 5.03 ± 0.47 wt % in glasses and from 0.93 ± 0.08 wt % to 1.50 ± 0.12 wt % in amphiboles, resulting in a partition coefficient of water between amphibole and glass (Amph/L D H2O) ranging from 0.29 ± 0.06 to 0.52 ± 0.08. Our results show a positive correlation between the Cl content of amphibole (from 0.18 wt % to 0.88 wt %) and the Amph/L D H2O. This effect is ascribed to the incorporation of Cl at the anion site O(3) that influences the oxo-substitution mechanism by impeding the entrance of Ti 4+ at the M(1) sites and thus preventing the amphibole dehydrogenation. The effect of Cl reported in this study, which is related to a change in the amphibole crystal structure, highlights that high Cl concentrations in magmatic systems favor the incorporation of water in amphibole rather than in the coexisting melt, although the exchange coefficient between Amph/L D H2O and Amph/L D Cl supports a preferential incorporation of water over Cl in amphibole. Therefore, the presence of abundant Cl influences the hydration state of magmas evolving from upper-mantle conditions towards crustal roots with the crystallization of amphibole. [ABSTRACT FROM AUTHOR]

Foreland fold-and-thrust belts (FTBs) record long-lived tectono-sedimentary activity, from passive margin sedimentation, flexuring, and further evolution into wedge accretion ahead of an advancing orogen. Therefore, dating fault activity is fundamental for plate movement reconstruction, resource exploration, and earthquake hazard assessment. Here, we report U–Pb ages of syn-tectonic calcite mineralizations from four thrusts and three tear faults sampled at the regional scale across the Jura fold-and-thrust belt in the northwestern Alpine foreland (eastern France). Three regional tectonic phases are recognized in the middle Eocene–Pliocene interval: (1) pre-orogenic faulting at 48.4±1.5 and 44.7±2.6 Ma associated with the far-field effect of the Alpine or Pyrenean compression, (2) syn-orogenic thrusting at 11.4±1.1 , 10.6±0.5 , 9.7±1.4 , 9.6±0.3 , and 7.5±1.1 Ma associated with the formation of the Jura fold-and-thrust belt with possible in-sequence thrust propagation, and (3) syn-orogenic tear faulting at 10.5±0.4 , 9.1±6.5 , 5.7±4.7 , and at 4.8±1.7 Ma including the reactivation of a pre-orogenic fault at 3.9±2.9 Ma. Previously unknown faulting events at 48.4±1.5 and 44.7±2.6 Ma predate the reported late Eocene age for tectonic activity onset in the Alpine foreland by ∼10 Myr. In addition, we date the previously inferred reactivation of pre-orogenic strike-slip faults as tear faults during Jura imbrication. The U–Pb ages document a minimal time frame for the evolution of the Jura FTB wedge by possible in-sequence thrust imbrication above the low-friction basal decollement consisting of evaporites. [ABSTRACT FROM AUTHOR]