A gas chromatography system for measuring carbonaceous gases released at high-pressure and high-temperature conditions.

Carbonates or carbon-bearing materials may release gases under high pressure and high temperature (HP-HT) conditions. Characterizing the species and quantifying the volumes of these carbonaceous gases are critical for understanding carbon chemistry. However, the volatile nature of carbonaceous gas poses technical challenges in their collection, speciation, and quantification during HP-HT experiments. To address these challenges, we have developed a system that integrates sample collection, gas transportation, chemical conversion, and measurement of carbonaceous gases trapped within the large volume press capsules. The system comprises a capsule-crushing device for thorough sample pulverization, a mechanizer coupled with a flame ionization detector, a gas-sealing and transport interface, and gas chromatography for detection. To evaluate the system's capabilities, we quantified the gas volumes released from encapsulated kerogen quenched from 1.9 GPa to 873, 973, and 1073 K. The collected gas chromatography signals were compared to those obtained from standard mixed-gases. The volumes of CO2, CH4, and C2H6 in the samples were successfully derived from the signal peak area through calibration. The relative standard deviation value of two runs at 3 GPa and 1073 K is 1.956%, suggesting good reproducibility. Our system thus provides a robust solution for investigating carbon chemistry under HP-HT conditions.

Massive abiotic methane production in eclogite during cold subduction.

Methane (CH4) is a critical but overlooked component in the study of the deep carbon cycle. Abiotic CH4 produced by serpentinization of ultramafic rocks has received extensive attention, but its formation and flux in mafic rocks during subduction remain poorly understood. Here, we report massive CH4-rich fluid inclusions in well-zoned garnet from eclogites in Western Tianshan, China. Petrological characteristics and carbon-hydrogen isotopic compositions confirm the abiotic origin of this CH4. Reconstructed P-T-fO2-fluid trajectories and Deep Earth Water modeling imply that massive abiotic CH4 was generated during cold subduction at depths of 50-120 km, whereas CO2 was produced during exhumation. The massive production of abiotic CH4 in eclogites may result from multiple mechanisms during prograde high pressure-ultrahigh pressure metamorphism. Our flux calculation proposes that abiotic CH4 that has been formed in HP-UHP eclogites in cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH4 on Earth.

Mid-mantle water transportation implied by the electrical and seismic properties of ε-FeOOH.

Water in the mantle transition zone and the core-mantle boundary plays a key role in Earth's stratification, volatile cycling, and core formation. If water transportation is actively running between the aforementioned layers, the lower mantle should contain water channels with distinctive seismic and/or electromagnetic signatures. Here, we investigated the electrical conductivity and sound velocity of ε-FeOOH up to 71 GPa and 1800 K and compared them with global tomography data. An abrupt three-order jump of electrical conductivity was observed above 50 GPa, reaching 1.24(12) × 103 S/m at 61 GPa. Meanwhile, the longitudinal sound velocity dropped by 16.8% in response to the high-to-low spin transition of Fe3+. The high-conductivity and low-sound velocity of ε-FeOOH match the features of heterogenous scatterers in the mid-lower mantle. Such unique properties of hydrous ε-FeOOH, or possibly other Fe-enriched phases can be detected as evidence of active water transportation in the mid-lower mantle.

Realization of parallel experiments in a diamond anvil cell and their application to water-mineral interactions at high-pressure and high-temperature conditions.

Parallel experiments are normally used to compare different chemical systems and conditions simultaneously. In the field of high-pressure experimental science, parallel experiments are hard to realize due to very limited reaction chamber size for the generation of high-pressure conditions, especially in diamond anvil cells (DACs). Multiple holes, instead of a single hole, can be drilled into a gasket (i.e., multihole gasket technique) to realize parallel experiments in a DAC. In this study, we conducted a series of systematic calibration experiments on multihole gasket techniques using statistical methods. Multiple (two or three or four) holes 100 µm in diameter were symmetrically drilled into a gasket by a laser drilling instrument with the help of a coded Python program. The pressure deviations among different holes in a gasket at average pressures below 10 GPa are constrained to less than 0.2 GPa in all calibration experiments at room temperature. We further checked the influences of the gasket material, hole number, pre-indented gasket thickness, and temperature on the pressure deviations among different holes in a gasket. Finally, we applied the multihole gasket technique in a DAC experiment and compared the solubility of calcite in different chemical environments at the same pressure and temperature conditions. The experimental results showed that the multihole gasket technique could be widely applied to study water-mineral interactions at high-P (<10 GPa) and high-T (<700 °C) conditions because multiple parallel experiments can be efficiently realized simultaneously.

Temperature-induced amorphization in CaCO3 at high pressure and implications for recycled CaCO3 in subduction zones.

Calcium carbonate (CaCO3) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling. However, its phase relations above 3 GPa and 1000 K are controversial. Here we report a reversible temperature-induced aragonite-amorphization transition in CaCO3 at 3.9-7.5 GPa and temperature above 1000 K. Amorphous CaCO3 shares a similar structure as liquid CaCO3 but with much larger C-O and Ca-Ca bond lengths, indicating a lower density and a mechanism of lattice collapse for the temperature-induced amorphous phase. The less dense amorphous phase compared with the liquid provides an explanation for the observed CaCO3 melting curve overturn at about 6 GPa. Amorphous CaCO3 is stable at subduction zone conditions and could aid the recycling of carbon to the surface.

Cold deep subduction recorded by remnants of a Paleoproterozoic carbonated slab.

The absence of low-thermal gradients in old metamorphic rocks (<350 °C GPa-1) has been used to argue for a fundamental change in the style of plate tectonics during the Neoproterozoic Era. Here, we report data from an eclogite xenolith in Paleoproterozoic carbonatite in the North China craton that argues for cold subduction as early as 1.8 Ga. The carbonatite has a sediment-derived C isotope signature and enriched initial Sr-Nd isotope composition, indicative of ocean-crust components in the source. The eclogite records peak metamorphic pressures of 2.5-2.8 GPa at 650-670 °C, indicating a cold thermal gradient, 250(±15) °C GPa-1. Our data, combined with old low-temperature events in the West African and North American cratons, reveal a global pattern that modern-style subduction may have been established during the Paleoproterozoic Era. Paleoproterozoic carbonatites are closely associated with granulites and eclogites in orogens worldwide, playing a critical role in the Columbia supercontinent amalgamation and deep carbon cycle through time.

Recovery of an oxidized majorite inclusion from Earth's deep asthenosphere.

Minerals recovered from the deep mantle provide a rare glimpse into deep Earth processes. We report the first discovery of ferric iron-rich majoritic garnet found as inclusions in a host garnet within an eclogite xenolith originating in the deep mantle. The composition of the host garnet indicates an ultrahigh-pressure metamorphic origin, probably at a depth of ~200 km. More importantly, the ferric iron-rich majoritic garnet inclusions show a much deeper origin, at least at a depth of 380 km. The majoritic nature of the inclusions is confirmed by mineral chemistry, x-ray diffraction, and Raman spectroscopy, and their depth of origin is constrained by a new experimental calibration. The unique relationship between the majoritic inclusions and their host garnet has important implications for mantle dynamics within the deep asthenosphere. The high ferric iron content of the inclusions provides insights into the oxidation state of the deep upper mantle.