Serpentine, serpentinite

In addition to anthropogenic pollution, some serpentine soils derived from Fe and Mg-rich ultramafic rocks are enriched in Ni, Cr and Co. In North America, ultramafic rocks form two discontinuous bands along the east and west side of the continent. The largest area of ultramafic terrain is in the Klamath Mountains province of northern California and southern Oregon (Lee et al., 2001). Serpentinite is a metamorphic rock formed from low... 

Serpentinite Igneous or metamorphic rock chiefly composed of serpentine minerals such as chrysotile or lizardite (Jackson 1997). Chrysotile, when found, can occur in localities with serpentinite rock (Churchill et al. 2001). 

Hurley P. M., Fairburn H. W., and Pinson W. H. (1964) Rb-Sr relationships in serpentine from Mayaguez, Puerto Rico and dunite from St. Paul s rocks a progress report. In A Study of Serpentinite The AMSOC Core Hole near Mayaguez, Puerto Rico. (ed. C. A. Burke) Natl. Tes. Counc. vol. 1118, pp. 149-151. 

Figure 23. Incremental reaction model of seawater-harzburgite interaction at the MARK serpentinite drilled in ODP hole 920. This model inclndes high temperatnre serpentinization at 400°C, followed by additional harzbnrgite reaction as the fluids cool to 300°C. The model closely duplicates sulfur isotope values and sulfide sulfur contents of MARK serpentinites.

A higher content of magnesium is characteristic of basic and ultrabasic magmatic and metamorphic rocks (e.g., basalt and serpen-tinite) and their weathering products. Mg-rich rock-forming minerals are biotite and other dark-colored silicate minerals, as well as serpentine. In the case of the serpentinite soils, a Mg-adapted natural vegetation has developed. In the case of sediments, dolomite is a Mg-rich limestone young sediments deposited from seawater are also Mg-rich, as can be seen in marsh soils. 

The key reaction catalyzed by the rock is called serpentinization, because it makes serpentinite. It s rare but can be found in the shadow of the Golden Gate Bridge and on a particular Washington beach (if this amateur geologist guessed right). Serpentinite is made when the mass of the deep ocean presses hot water into olivine. 

The results of our computations are, of coinse, dependent on the assumed compositions of Hadean hydrothermal fluid and seawater, and will vary somewhat if the compositions of these fluids on the early Earth differ from those used here. However, because hydrothermal fluid compositions in serpentinizing systems are predominantly controlled by reactions with minerals in the subsurface and there is no reason to believe that the composition of ultramafic rocks on the early Earth were substantially different from those today (73), the composition of fluids discharged from serpentinites has probably changed very little over the past 4 billion years. One aspect of the assumed hydrothermal fluid composition, however, that may warrant further exploration is the level of dissolved inorganic carbon (DIC). 

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