Introduction to Hydrous Minerals

What is/are Hydrous Minerals?

Hydrous Minerals - The hydrous minerals and carbonates contained in fluid inclusions of generations (1) and (2) indicate that original constituents of the early homogenous magmatic fluid must have been H2O and CO2. ^[1] Over the last decade, however, evidence has been found in apatites, lunar volcanic glass beads, nominally anhydrous minerals and olivine-hosted melt inclusions, to support a relatively “wet” Moon. ^[2] The past decade has seen a great number of studies dealing with magmatic water contents and how these could be retrieved by the nominally anhydrous minerals’ (NAMs) trace structural hydroxyl (water. ^[3] Hydrous minerals are found on the surfaces of asteroids, but their origin is not clear. ^[4] Active and inactive magmatic belts are associated with a range of relatively low temperatures (~640°C – ~810°C in the depth range of 25 km – 100 km), which may be a consequence of the fluid content in hydrous minerals. ^[5] Clinopyroxene (Cpx) is commonly believed to be the best structural water (hydrogen) carrier among all major upper mantle nominally anhydrous minerals (NAMs). ^[6] Water can be incorporated into lattice defects of normally anhydrous minerals of the upper mantle as hydroxyl. ^[7] Knowledge of water content and distribution in the Earth's mantle is critical to understanding the geochemical evolution and geodynamic processes of the Earth, since water can incorporate into nominally anhydrous minerals at high pressure and dramatically affect the chemical and physical properties of mantle minerals. ^[8] Hydroxylation is a method that allows “decoration” of the pre-existing point defect structure of nominally anhydrous minerals, such as olivine. ^[9] In this paper, we present how even a small amount of ’water’ (more precisely structural hydroxyl) in nominally anhydrous minerals (NAMs) can affect the physical properties of the lithospheric mantle and how to quantify the amount of the structural hydroxyl. ^[10] The fluid inclusions have salinities of ~30–35 wt% NaCl equivalent, likely resulting from a multi-stage process that involves an increase in salinity during formation of hydrous minerals during subduction, leaving high salinity fluids and/or halide salts sequestered in pores. ^[11] Determining the H2O contents of nominally anhydrous minerals (NAMs) is one relatively common technique that has been applied to estimate mantle H2O contents. ^[12] Water may be transferred from the subducted oceanic slab to the mantle wedge in the form of aqueous fluids, released by the dehydration of hydrous minerals, or by sediment melts. ^[13] The simulator also includes mass porosity and permeability evolution in response to dehydration reactions of hydrous minerals. ^[14] The Dengdeer and Maodun samples are high-K calc-alkaline, rich in hydrous minerals, Rb, Th, and LREEs, depleted in Nb, Ta, and Ti, and show decoupling Hf-Nd isotopic signatures. ^[15] Earth's mantle is known to harbor water in the form of hydrous and nominally anhydrous minerals. ^[16] The nominally anhydrous minerals are indeed water-rich. ^[17] Oxidation of arc magmas may be a secondary feature, acquired as hydrogen from magmatic water is incorporated into anhydrous minerals in the mantle wedge, according to analyses of orthopyroxenes in mantle xenoliths from an arc setting. ^[18] Several recent studies have shown a link between intermediate‐depth earthquakes and the thermal‐petrological path of subducting slabs in relation to the stability field of hydrous minerals. ^[19] As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. ^[20] The structural hydroxyl content of the nominally anhydrous minerals (olivine and pyroxenes) in the upper mantle is among the important attributes that influence the physical and chemical features of the upper mantle. ^[21] Such melts resemble MORB-like melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. ^[22] We employ 3-D time-dependent thermomechanical numerical models that incorporate up-to-date slab geometry and phase diagrams of hydrous minerals in the subducting plate to study the north-central Chilean (19 °S–33 °S) coupled megathrust, including the region where the slab is subhorizontal (28 °S–32 °S). ^[23] Unlike most other CM chondrites, the bulk oxygen isotopes are extremely 16O rich, apparently dominated by the signature of anhydrous minerals, distributed on a steep slope pointing to the domain of intrinsic CM water. ^[24] The determinations of water status incorporated in hydrous minerals are of considerable significances in geoscience fields. ^[25] Interestingly, the changes in porosity caused by the formation of carbonated and hydrous minerals are small while a strong decrease in permeability is measured during the experiments. ^[26] During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. ^[27] Dehydration of hydrous minerals is the key for understanding the partial melting and earthquakes taking place at subduction zones. ^[28] Instead, a metasomatic origin is suggested, which is strongly supported for the clinopyroxenites by the occurrence of hydrous minerals and high contents of large-ion-lithophile elements (K, Rb, Ba, Th and Sr) and light rare earth elements, as well as elevated δ 18 O (9. ^[29] Therefore, water might have been transported to the deep Martian interior by hydrous minerals, such as phase E and phase D, in cold subduction plates. ^[30] Afterwards, the anhydrous minerals, which developed during the M0-ii stages, were variably replaced by amphibole-bearing assemblages (Miii stage). ^[31] Nominally anhydrous minerals were used to evaluate the detection limits and indicate a surface contamination level of about 200 ppm equivalent H2O in these conditions. ^[32]