Early release of H2O during subduction of carbonated ultramafic lithologies

To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO3. For a bulk CaCO3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO3-bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1–2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H2-rich fluids

The efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at earth surface conditions

San Carlos forsterite was dissolved in initially pure H2O in a batch reactor in contact with the atmosphere for five years. The reactive fluid aqueous pH remained relatively stable at pH 6.7 throughout the experiment. Aqueous Mg concentration maximized after approximately two years time at 3x10-5 mol/kg, whereas aqueous Si concentrations increased continuously with time, reaching 2x10-5 mol/kg after 5 years. Element release rates closely matched those determined on this same forsterite sample during short-term abiotic open system experiments for the first 10 days, then slowed substantially such that the Mg and Si release rates are approximately an order of magnitude slower than that calculated from the short-term abiotic experiments. Post-experiment analysis reveals that secondary hematite, a substantial biotic community, and minor amorphous silica formed on the dissolving forsterite during the experiment. The biotic community included bacteria, dominated by Rhizobiales (Alphaproteobacteria), and fungi, dominated by Trichocomaceae, that grew in a carbon and nutrient-limited media on the dissolving forsterite. The Mg isotope composition of the reactive fluid was near constant after 2 years but 0.25‰ heavier in δ26Mg than the dissolving forsterite. Together these results suggest long-term forsterite dissolution in natural Earth surface systems maybe substantially slower that estimated from short-term abiotic experiments due to the growth of biotic communities on their surfaces

Influence of inorganic solution components on lithium carbonate crystal growth

Lithium-bearing brines are an increasingly attractive source of Li for extraction. One extraction mechanism is the removal of Li from the fluid phase through the precipitation of zabuyelite (Li2CO3). The chemistry of the brine plays an important role in this process because ions in solution can compete for the components of the Li-carbonate phase. Here we explore the effect of different brine components on the precipitation of zabuyelite using experiments and computational simulations. Crystals formed in all solutions showed morphological evidence for potential transformation from a precursor phase. Our study indicates that Ca2+ and SO42– are incorporated into the precipitated zabuyelite crystals. Sulfate also interacts directly with specific surfaces on the growing crystal and is expected to form ion pairs with Li+ in solution. Similarly, Na+ appears to form ion pairs in solution with the carbonate ion, slowing nucleation of zabuyelite in the experiments. K+ and Cl– may interact with the growing zabuyelite crystals but do not appear to affect zabuyelite nucleation and growth times. These experiments highlight the importance of understanding the solution chemistry on zabuyelite formation in order to predict the efficiency of extraction processes and the purity of the solids

Geological records of transient fluid drainage into the shallow mantle wedge

Pore fluid pressure on subduction zone megathrusts is lowered by fluid drainage into the overlying plate, affecting subduction zone seismicity. However, the spatial and temporal scales of fluid flow through suprasubduction zones are poorly understood. We constrain the duration and velocity of fluid flow through a shallow mantle wedge based on the analyses of vein networks consisting of high-temperature serpentine in hydrated ultramafic rocks from the Oman ophiolite. On the basis of a diffusion model and the time-integrated fluid flux, we show that the channelized fluid flow was short-lived (2.1 × 10−1 to 1.1 × 101 years) and had a high fluid velocity (2.7 × 10−3 to 4.9 × 10−2 meters second−1), which is close to the propagation velocities of seismic events in present-day subduction zones. Our results suggest that the drainage of fluid into the overlying plate occurs as episodic pulses, which may influence the recurrence of megathrust earthquakes

Heitt Mjölnir: a heated miniature triaxial apparatus for 4D synchrotron microtomography

Third- and fourth-generation synchrotron light sources with high fluxes and beam energies enable the use of innovative X-ray translucent experimental apparatus. These experimental devices access geologically relevant conditions whilst enabling in situ characterization using the spatial and temporal resolutions accessible at imaging beamlines. Here, Heitt Mjölnir is introduced, a heated miniature triaxial rig based on the design of Mjölnir, but covering a wider temperature range and larger sample volume at similar pressure capacities. This device is designed to investigate coupled thermal, chemical, hydraulic and mechanical processes from grain to centimetre scales using cylindrical samples of 10 mm × 20 mm (diameter × length). Heitt Mjölnir can simultaneously reach confining (hydraulic) pressures of 30 MPa and 500 MPa of axial stress with independently controlled sample pore fluid pressure < 30 MPa. This internally heated apparatus operates to temperatures up to 573 K with a minimal vertical thermal gradient in the sample of <0.3 K mm−1. This new apparatus has been deployed in operando studies at the TOMCAT (Swiss Light Source), I12 JEEP (Diamond Light Source) and PSICHÉ (Synchrotron SOLEIL) beamlines for 4D X-ray microtomography with scan intervals of a few minutes. Heitt Mjölnir is portable and modular, allowing a wide range of 4D characterizations of low-grade metamorphism and deformational processes. It enables spatially and temporally resolved fluid–rock interaction studies at conditions of crustal reservoirs and is suitable for characterization of material properties in geothermal, carbonation or subsurface gas storage applications. Technical drawings and an operation guide are included in this publication

Dynamic Evolution of Porosity in Lower-Crustal Faults During the Earthquake Cycle

Earthquake-induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake-derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post- and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid-assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain-size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake-induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep

High-magnitude stresses induced by mineral-hydration reactions

Fluid-rock interactions play a critical role in Earth’s lithosphere and environmental subsurface systems. In the absence of chemical mass transport, mineral-hydration reactions would be accompanied by a solid-volume increase that may induce differential stresses and associated reaction-induced deformation processes, such as dilatant fracturing to increase fluid permeability. However, the magnitudes of stresses that manifest in natural systems remain poorly constrained. We used optical and electron microscopy to show that one of the simplest hydration reactions in nature [MgO + H2O = Mg(OH)2] can induce stresses of several hundred megapascals, with local stresses of as much as ∼1.5 GPa. We demonstrate that these stresses not only cause fracturing but also induce plastic deformation with dislocation densities (1015 m−2) exceeding those typical of tectonically deformed rocks. If these reaction-induced stresses can be transmitted across larger length scales, they may influence the bulk stress state of reacting regions. Moreover, the structural damage induced may be the first step toward catastrophic rock failure, triggering crustal seismicity

Перебіг анемії вагітних у хворих на туберкульоз легень

В статье представлены данные течения анемии беременных и нарушения фетоплацентарного комплекса у 86 женщин, больных туберкулезом легких. Выявлено, что анемия на фоне туберкулезного процесса приводит к существенным функциональным и морфологическим нарушениям в плаценте. показано диагностическое значение оценки плацентарной дисфункции для профилактики и лечения анемии у данного контингента женщин.The article presents the data of course of pregnancy anemia and failures of fetoplacental complex in 86 women with pulmonary tuberculosis. It is found that anemia under tuberculous process leads to significant functional and morphological disturbances in placenta. Diagnostic significance of assessment of placental dysfunction for the prevention and treatment of anemia in this contingent of women is shown