Pyroxenes water content

The Cl chondrites represent one of the most curious paradoxes of cosmochemistry. Despite their unfractionated compositions, the Cl chondrites are the most altered of all chondrites, with water contents of —19.5 wt.% (Nagy et al., 1963). Anhydrous phases (olivines and pyroxenes) represent less than 1 vol.% of these meteorites (Leshin et al., 1997). Cl chondrites are complex meteorites that consist of a dark, fine-grained matrix comprised of phyllosilicates with magnetite, sulfides, carbonates, and sulfates embedded within it (e.g., DuFresne and Anders, 1962 Nagy, 1966). They have experienced extensive breccia-tion on their asteroidal parent bodies that caused... 

Dobson et al. (1995) measured water contents in melt inclusions in pyroxenes in boninites from the Bonin Islands. These melt inclusions contained 2.8-3.2 wt.% H2O, whereas quenched glass from pillow lava rims had 2.2-2.4wt.% H2O, which Dobson et al. interpreted as a result of degassing between the time of entrapment of the melt inclusions and the eruption on the seafloor. They also measured the water content of the orthopyroxene, which contained 80-120 ppm H2O. 

Peslier A. H., Luhr J. F., and Post J. (2002) Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge. Earth Planet Sci. Lett. 201, 69-86. 

The amount of water that can dissolve in orthopyroxene depends on pressure, temperamre, and composition. In a study at 1,100 °C, Rauch and Keppler (2002) found that water content in pure enstatite increased from 55 ppm H2O at 0.2 GPa to 867 ppm at 7.5 GPa, then decreased slightly to 714 ppm at 10 GPa. They also looked at the effect of changing pyroxene composition the most impressive change occurred with increasing aluminum content of the pyroxene. The addition of 1 wt.% AI2O3 increased water solubility from 199 ppm to 1,102 ppm H2O at 1.5 GPa and... 

C. They suggest this implicates a substi-mtion of Al -f H for Si", and argue that examining the aluminum distribution between octahedral and tetrahedral sites in mantle-derived pyroxenes would allow estimation of original water contents even for samples that have lost hydrogen during ascent. 

Clnysotile belongs to the serpentine group of minerals, v arieties of which are found in most of the important mountain ranges and precambrian shields (8). Only a small part of these serpentine occurrences are in the asbestiform clnysotile variety. Chrysotile fibers are found as veins in serpentines or related minerals in serpentinized ultramafic rocks and in serpentinized dolomitic marbles (9). It has been suggested that the ultrabasic rocks (forsterite, Mg-rich pyroxenes, and ampliiboles) are first attacked in an hydrothermal process and transformed in serpentines in a later hydrothermal event, the serpentines are partially redissolved and crystallized as chrysotile fibers (9). (Heady, the genesis of each chrysotile deposit must have involved specific features related to the composition of the precursor minerals, the stress and defomiations in the host matrix, the water content, the temperature cycles, etc. Nonetheless, it is generally observed that the chemical composition of the fibrous phase is closely related to that of the surrounding rock matrix (9). 

The association pyroxene + olivine + cummingtonite + quartz, often encountered in highly metamorphosed BIF, is also very important for judging the thermodynamic parameters of mineralization. This association fixes the temperature at 700-720°C, which depends little on pressure and iron content of the silicates, while pressure can be estimated fairly precisely from the iron content of orthopyroxene. A decrease in partial pressure of water in conjunction with mechanical equilibrium P =Ff) due to dilution of the fluid by other volatiles, for instance carbon dioxide, can lead to some shift of the P-T curve into the lower-temperature region. However, the amount of such a shift, especially at high pressures (8-10 kbar) cannot be significant because Fe-Mg carbonates which are stable in carbon dioxide fluids already appear at 610-650°C (Fig. 93b), and their paragenesis with anhydrous silicates is not typical of iron formations. 

When olivine and pyroxene are oxidized the same regularities are observed as in the oxidation of carbonates. In particular, it was established in a study of the rich and unique olivine-magnetite iron ores of the Volodarsk deposit (Ukrainian shield) that the magnesium content of the silicates increases as the magnetite content of the ore increases (Mel nik and Yaroshchuk, 1966). In this case the most likely oxidants may be water and carbon dioxide, the main components of the fluids causing metasomatic reworking of the olivine-and pyroxene-bearing iron formations. 

There have been tremendous strides made recently towards understanding how volatiles in general and water in particular is transported and stored in the mantle. This progress is based on research on a number of fronts studies of mantle-derived samples have provided insight into the nature and occurrence of hydrous phases such as amphibole, mica, and chlorite, and have provided constraints on the capacity of nominally anhydrous minerals (NAMs) such as olivine, pyroxenes, and garnet to contain water by a variety of substimtion mechanisms. Experimental studies on mantle-derived magmas have provided constraints on volatile contents in their source regions. Other studies have constrained the pressure, temperature, and composition conditions over which hydrous phases are stable in the mantle. 

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