Olivine 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... 

Sisson and Layne (1993) analyzed melt inclusions in olivines in samples from four arc volcanoes. Inclusions in olivines in basalts from the 1974 eruption of Fuego volcano, Guatemala, contain 2.1-4.6 wt.% H2O. Inclusions in olivines in basaltic andesites from the same eruption have a wider range, from 1.0 wt.% to 6.2 wt.% H2O. Inclusions hosted in olivines in basalts from three centers in the Southern Cascades have lower water contents, ranging from 0.2 wt.% to 1.4 wt.%H20. 

Jamtveit et al. (2001) measured water contents in olivines from basalts and picrites from the North Atlantic Volcanic Province using Fourier-transform infrared spectroscopy (FTIR). They found H2O contents from <0.5 ppm to —18 ppm, and suggested that oh vines with... 

Wilkins and Sabine (1973) used IR spectroscopy to determine water contents of kyanite, andalusite, sillimanite, grossular, andradite, pyrope, diopside, olivine, and feldspars. They found low water contents 0.008 wt.% in olivine, 0.02 wt.% in diopside, and 0.009 wt.% in pyrope. Zemann, Beran, and co-workers published a series of papers on IR spectroscopy of both hydrous minerals and NAMs (e.g., Tillmanns and Zemann, 1965 Beran and Zemann, 1971, 1986 Beran, 1971, 1986, 1987 Beran and Gotzinger, 1987 Beran et al., 1993). For the most part, these contributions were focused on the substimtional mechanisms by which hydrogen entered the crystal stmcture, rather than on the absolute amount of hydrogen in the crystal stmcture. 

From available experimental data, it appears that these values do not reflect saturation with a fluid phase. For example, Kohlstedt et al. (1996) synthesized olivines with water contents that increased with pressure from 135 ppm H2O at 2.5 GPa, 1,100 °C to 1,510 ppm at 12GPa, 1,100 °C and 1,090 ppm at 13 GPa, 1,100 °C. 

In the case of a high iron content in the carbonate, olivine is formed instead of orthopyroxene. A 10% increase in magnesium content leads to a 5-8° shift of the P-T curve into the higher temperature region. The shift is relatively small, but it plays a definite role in the metamorphic redistribution of iron among the minerals, particularly in reactions in which iron oxides are formed in the absence of water ... 

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. 

This can be illustrated by a natural example. In the coarse-grained Allanin magnesium-gabbro, infiltration of fluid caused the formation of reaction rims around olivine (Chinner and Dixon, 1973). The succession is olivine anthophyllite (2 wt.% H2O) — talc (4wt.% H2O + kyanite — chloritoid (8wt.% H20- -talc + kyanite. This reaction rim is H2O undersaturated, and the succession of mineral assemblages corresponds to an increase of H2O content towards the rim and can only be modeled by an increase in the availability of water towards the rim. The H20-undersamrated character of the inner rim zones does not necessitate (or justify) a CO2 component in the fluid, but rather reflects limited availability of an H2O fluid. 

First, volatiles exert an important control on the physical properties of the mantle. For example, the presence of water reduces the strength of olivine aggregates and seriously alters the viscosity of the mantle. Experimental studies show that at 300 MPa, in the presence of water, the viscosity of olivine aggregates deformed in the dislocation creep regime is reduced by up to a factor of 140. Thus a wet mantle is a low viscosity mantle. Conversely a mantle that is dried out by partial melting will be stiffer and more refractory, as is the case for the lithospheric "lid" to modern oceanic mantle. Thus, if it is possible to estimate the volatile content of the mantle both now and in the Archaean, it will be possible to set some physical constraints on models of mantle evolution over time. 

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