A role for subducted albite in the water cycle and alkalinity of subduction fluids

A role for subducting clays in the water transportation into the Earth's lower mantle

Subducting sedimentary layer typically contains water and hydrated clay minerals. The stability of clay minerals under such hydrous subduction environment would therefore constraint the lithology and physical properties of the subducting slab interface. Here we show that pyrophyllite (Al2Si4O10(OH)2), one of the representative clay minerals in the alumina-silica-water (Al2O3-SiO2-H2O, ASH) system, breakdowns to contain further hydrated minerals, gibbsite (Al(OH)3) and diaspore (AlO(OH)), when subducts along a water-saturated cold subduction geotherm. Such a hydration breakdown occurs at a depth of ~135 km to uptake water by ~1.8 wt%. Subsequently, dehydration breakdown occurs at ~185 km depth to release back the same amount of water, after which the net crystalline water content is preserved down to ~660 km depth, delivering a net amount of ~5.0 wt% H2O in a phase assemblage containing δ-AlOOH and phase Egg (AlSiO3(OH)). Our results thus demonstrate the importance of subducting clays to account the delivery of ~22% of water down to the lower mantle.

Mechanochemistry and Eco-Bases for Sustainable Michael Addition Reactions

The Michael addition reaction was revisited with a full focus on sustainability combined with efficiency, using mechanochemistry in mild conditions. First, the synthesis of cyclopentenone derivatives was chosen as a model reaction to find optimal conditions in mechanochemistry while using classical but weak bases. The reaction was efficient (84–95% yields), fast (2–6 h), solvent free, and required 0.1 equivalent of base. Aiming to reach greener conditions, classical bases were then replaced using new bio-sourced bases, called Eco-bases, that were easily prepared from plants and led to heterogeneous catalysts. The composition and structure of Eco-bases were characterized by MP-AES, XRPD, EBSD/EDS, HRTEM/EDX and ion chromatography. Interestingly, a high ratio of potassium was observed with the presence of K₂Ca(CO₃)₂ for the most effective Eco-base. The new Eco-bases were used for the mechanical-assisted construction of functionalized alkenone derivatives. The versatility of the method has been successfully applied with good to excellent yields to different Michael donors and acceptors. Eco-bases were recycled and reused four times with the same performances. Combining Eco-bases and mechanochemistry in Michael addition reactions allowed reaching a maximum degree of sustainability (efficient, rapid, low catalyst loading, solvent-free reactions with bio-sourced catalysts) and participating in the development of mechanochemistry in sustainable chemistry.

Acidic fluids in the Earth's lower crust

Fluid flux through Earth’s surface and its interior causes geochemical cycling of elements in the Earth. Quantification of such process needs accurate knowledge about the composition and properties of the fluids. Knowledge about the fluids in Earth’s interior is scarce due to limitations in both experimental methods and thermodynamic modeling in high/ultrahigh pressure–temperature conditions. In this study, we present halogen (Cl, F) measurements in apatite grains from the mafic (metagabbro), and felsic (two-pyroxene granulite, charnockite, hornblende-biotite gneiss) rocks preserved in the Nilgiri Block, southern India. Previous experiments show that it is difficult to incorporate Cl in apatite compared to F at high pressure and temperature conditions. Based on regional trends in Cl and F content in apatite (with highest Cl content 2.95 wt%), we suggest the presence of acidic C–O–H fluids in the lower crust (~20–40 km deep) during the high-grade metamorphism of these rocks. These fluids are capable of causing extreme chemical alterations of minerals, especially refractory ones. They also have significant potential for mass transfer, causing extensive geochemical variations on a regional scale and altering the chemical and isotope records of rocks formed in the early Earth. Our findings have important relevance in understanding speciation triggered by acidic fluids in the lower crust, as well as the role of fluids in deep Earth processes.