Nominally anhydrous minerals

Water in the mantle is fonnd in different states as a fluid especially near sub-duction zones, as a hydrous phase and as a hydroxyl point defect in nominally anhydrous minerals. 8D-values between -90 and -110%c have been obtained by Bell and Ihinger (2000) analyzing nominally anhydrous mantle minerals (garnet, pyroxene) containing trace quantities of OH. Nominally anhydrous minerals from mantle xenoliths are the most D-depleted of all mantle materials with 5D-values 50%c lower than MORE (O Leary et al. 2005). This difference may either imply that these minerals represent an isotopically distinct mantle reservoir or that the samples analyzed have exchanged hydrogen dnring or after their ascent from the mantle (meteoric/water interaction ). 

Halides (fluorine, chlorine, bromine, and iodine). Fluorine as F substitutes readily for OH in hydroxy minerals, implying that it probably occurs in all nominally anhydrous minerals in the same way as OH. Fluorine is not significantly soluble in seawater. Both these properties make its geochemical behavior quite different from the other halides. F/K and F/P ratios in basalts are reasonably constant (Smith et al, 1981 Sigvaldason and Oskarsson, 1986) with ratios of 0.09 0.04 and 0.29 0.1, respectively. Both ratios yield the same value of 25 ppm for the PM abundance which is listed in Table 4. However, the F/K ratio in the continental crust appears distinctly higher and the F/P ratio lower (Gao et al, 1998), indicating that the incompatibility of these elements increases in the order P < F < K. 

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. 

Bell D. R. and Rossman G. R. (1992a) Water in Earth s mantle the role of nominally anhydrous minerals. Science 255, 1391-1397. 

Bolfan-Casanova N., Keppler H., and Rubie D. C. (2000) Water partitioning between nominally anhydrous minerals in the Mg0-Si02-H20 system up to 24 GPa imphcations for the distribution of water in the Earth s mantle. Earth Planet. Sci. Lett. 182, 209-221. 

Keppler H. and Rauch M. (2000) Water solubility in nominally anhydrous minerals measured by FTIR and H MAS NMR the effect of sample preparation. Phys. Chem. Mineral. 21, 371-376. 

Figure 9.3. H MAS NMR spectra of the nominally anhydrous minerals enstatite, forsterite and three synthetic clinopyroxenes. Note the sharp resonance at 4.8 ppm from fluid-like water inclusions. The peak at about 1.5 ppm is attributed to organic contamination. From Kohn (1996), by permission of the Mineralogical Society of America.

Recent work on the effects of mantle metasomatism on water solubihty in nominally anhydrous minerals (Peslier et al. 2000) has shown that the (OH) contents of these minerals decrease during the increase in oxidation state accompanying metasomatism. This has the possible implication that the rheological strength of cratons actually increases as a result of metasomatism. The ubiquitous nature of metasomatism in cratonic peridotites (Menzies Hawkesworth 1987 Pearson 1999h) may have the result of providing additional strength to an already robust lithospheric keel. There are specific instances... 

Peslier, a. H., Luhr, j. Post, J. 2000, Water in mantle xenolith pyroxenes from Mexico and Simcoe (WA USA) the role of water in nominally anhydrous minerals from the mantle wedge. Geological Society of America Abstracts with Programs, 32, A387. 

A key point here is that the correlations based on either ionic or anion porosity, however well they work, are strictly empirical and cannot have much of a microscopic basis. This is because of the fact that the porosities of nominally anhydrous minerals do not change at all due to the presence of trace amounts of water, but oxygen diffusion rates do, typically by orders of magnitude. One can speculate that this behavior is linked. 

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