Sodium peridotites

Secondary alteration processes are also responsible for element deviations, particularly the strong depletion of calcium and sodium relative to aluminum in some abyssal peridotites, as well as part of the scattered variations of silicon, magnesium, calcium, and sodium. Serpentinized, orogenic and ophiolitic peridotites may also be strongly depleted in calcium and sodium. However, these samples are rare in the selected database for peridotite massifs, which includes only moderately serpentinized samples. 

In a given facies, the sodium and titanium content of clinopyroxene shows a negative correlation with depletion (or chromium content) and can increase substantially in metasomatized xenoliths. The substimtion of sodium, chromium, and aluminum has a complex relationship with T, P and degree of depletion but in general, clinopyroxene from higher P samples contains more sodium. Representative analyses for different peridotite facies are shown in Table 4. 

The presence of coesite and diamond together with high sodium and potassium in garnet and chnopyroxene respectively, make a clear case for the high-pressure origin of cratonic eclogites (Gurney et al, 1991). In contrast to the various quantitative barometers available for peridotites. 

Table 3 lists examples of more than a dozen different chemical types of river water. Although Ca and HCO j" are generally dominant, Mg dominance over Ca + can be found in rivers draining various lithologies such as basalt, peridotite, serpentinite, dolomite, coal, or where hydro-thermal influence is important (Semliki). Sodium may dominate in sandstone basins, in black shales (Powder, Redwater in Montana), in evaporitic sedimentary basins (Salt), in evaporated basins (Saoura), and where hydrothermal and volcanic influence is important (Semliki, Tokaanu). rarely exceeds 4% of cations, except in some clayey sands, mica schists, and trachyandesite it exceeds 15% in extremely dilute waters of Central Amazonia and in highly mineralized waters of rift lake outlets (Semliki, Ruzizi). 

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