Mantle hydrous

Paul UH (2001) Melt retention and segregation beneath mid-ocean ridges. Nature 410 920-923 Feineman MD, DePaolo DJ, Ryerson FJ (2002) Steady-state Ra/ °Th disequilibrium in hydrous mantle minerals. Geochim Cosmochim Acta 66 A345 (abstr)... 

In oceanic fracture zones as well as in actively-upwelling forearc mud volcanoes, seawater or other marine fluids interact directly with mantle rock (Bonatti 1976 Fryer 1985). Serpentinization of mantle rocks at temperatures broadly <350°C involves influx of water, as well as general increase in Li concentration in the newly-formed hydrous assemblage. Considering the low temperatures involved, Li isotope exchange during seawater-mantle... 

Studies of 2.65 Ga gold systems in the Yilgarn Craton, Western Australia, have emphasised the interaction of intrinsically reduced (CH4 H2 N2 HCI) and oxidized (CO2 SO2) anhydrous fluids as well as hydrous fluids in deposit formation. Carbonate veins have between -4 and -6 %o, typical of mantle values and primary sulfate have of 8 to 12 %o. 

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 ). 

Ion microprobe analyses of hydrous minerals in Martian meteorites reveal two different sources of hydrogen. One is interpreted as magmatic water, with 5D = 900 permil, and thought to reflect the mantle composition the other is thought to reflect the atmospheric composition, with 5D =4000 permil (Leshin, 2000). The incorporation of atmospheric water into these meteorites suggests some kind of cycling of water between the atmosphere and lithosphere on Mars. 

Li, X., Jeanloz, R. (1991b) Phases and electrical conductivity of a hydrous silicate assemblage at lower-mantle conditions. Nature, 350, 332-4. 

Peridotite fertilization may also result from the fractional solidihcation of exotic (deep-seated) melts inhltrated in wall rocks of translithospheric magma conduits. This process was hrst described in composite mantle xenoliths (Wilshire and Shervais, 1975 Gurney and Harte, 1980 Irving, 1980 Wilshire et al., 1980 Boivin, 1982 Harte, 1983 Harte et al, 1993 Menzies et ah, 1987), where it is referred to as modal metasomatism when new (generally hydrous) minerals are precipitated (Dawson, 1984 Kempton, 1987), or Fe-Ti metasomatism (Menzies et al., 1987) when the attention is focused on chemical enrichment. In contrast with ultramafic xenoliths, the tectonically emplaced and oceanic peridotites contain only sparse rock types attributable to mantle metasomatism by deep-seated melts. Examples of wall-rock, modal, and Fe-Ti metasomatism were nevertheless described in IP orogenic Iherzolites, notably in the Pyrenees (Fabrics et al., 1989 Bodinier et al., 1988, 1990, 2003 McPherson et al., 1996 Woodland et al., 1996). 

In an extensive review of the geochemistry of volatile-bearing minerals in mantle xenoliths, Ionov et al (1997) have pointed out that although minerals such as mica, amphibole, and apatite are often referred to as hydrous, in many cases they have very low H2O contents (Boettcher and O Neill, 1980). In such cases, these minerals may have significant amounts of fluorine, chlorine and CO2. Mica, amphibole, and apatite, together with the oxide phases, are important hosts for titanium, potassium, rubidium, strontium, barium, and niobium (Table 9). 

Hydrogen isotope data for mantle xenoliths is usually acquired on hydrous minerals such as amphibole and mica. One problem is that early studies did not texturally characterize mica occurrences and so the information is of limited value. Perhaps the best-documented study is that... 

Chazot G., Lowry D., Menzies M. A., and Mattey D. (1997) Oxygen isotopic composition of hydrous and anhydrous mantle peridotites. Geochim. Cosmochim. Acta 61, 161-169. 

Johnson K. E., Davis A. M., and Bryndzia L. T. (1996) Contrasting styles of hydrous metasomatism in the upper mantle an ion microprobe investigation. Geochim. Cosmochim. Acta 60, 1367—1385. 

Menzies M. and Murthy V. R. (1980b) Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts imphcations for local heterogeneities in metasomatically veined mantle. Earth Planet. Sci. Lett. 46,... 

Schrauder M. and Navon O. (1994) Hydrous and carbonatitic mantle fluids in fibrous diamonds from Jwaneng, Botswana. Geochim. Cosmochim. Acta 58, 761—771. 

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. 

At some point, however, because of the limited thermal or pressure stability of the hydrous phases, water will be liberated from the slab into the surrounding mantle. At this point, the water will either exist as a fluid, a melt—or something intermediate if we are above the second critical end point in the relevant system (Wyllie and... 

These samples may provide a biased perspective of the Earth s mantle because of the association of the host magmas with certain tectonic environments. For example, xenolith-bearing kimberlites are usually found on cratons, which have been shown to be underlain by mantle with anomalous geophysical, and presumably geochemical, nature (see Chapter 2.05). Another caveat is that at many localities, samples containing hydrous minerals may be uncommon, even if they are disproportionately represented in the literature published on a given site—Dish Hill, California, is a good example of this phenomenon. 

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