SERPENTINE 



55 



tirely explained and are well worth further study. 

 The chemical reactions involved in the formation of 

 serpentine are relatively simple. They can most easily 

 be shown by equations based on reactions involving the 

 pure magnesium member of the olivine group, for- 

 sterite; and although the natural olivine occurring in 

 dunite or peridotite does contain some iron, its pres- 



ence will not alter the conclusions presented below. 

 The pressure-temperature fields of stability of the 

 minerals in the MgO-SiO 2 -H 2 O system have been in- 

 vestigated by Bowen and Tuttle (1949, p. 439-460), 

 and their results provide important data regarding 

 the reactions of forsterite and water. The two perti- 

 nent equations are as follows : 



Forsterite Water Serpentine Brucite 



-450 



2(2MgO-SiO 2 )+3H 2 O ; - 3MgO-2SiO 2 -2H 2 O+MgO-H 2 O 



+450 



Forsterite Silica Water Serpentine 



-500 



3(2MgO-SiO 2 )+SiO 2 +4H 2 O . 2(3MgO-2SiO 2 -2H 2 O) 



+ 500 



Bowen and Tuttle's experiments were performed at 

 pressures as much as 40,000 pounds per square inch, 

 equivalent to the lithostatic pressure at a depth of 

 about 6 miles. In this range, each of the reactions 

 takes place near the indicated temperature, with pres- 

 sure having very little effect, suggesting the tempera- 

 ture of the reaction would be little changed even at 

 greater pressure or depth. Two important conclusions 

 can be drawn from these equations, namely : 



1. Serpentine cannot exist at temperatures appreci- 



ably above 500C, or in the presence of excess 

 magnesia above 450 C. 



2. Forsterite by hydration alone forms serpentine and 



brucite, but with the addition of silica (or loss 

 of magnesia) forms serpentine. If volume rela- 

 tions remain equal, as in the direct replacement 

 of forsterite by serpentine, both magnesia and 

 silica must be removed. 



Three different theoretical conditions for the state 

 of the serpentine bodies when intruded would seem 

 to be possible, and each of these has been proposed for 

 serpentines in other areas (Benson, 1918). First, the 

 material may have been a liquid or partly crystallized 

 ultramafic magma, which, after solidification to perido- 

 tite, became hydrated in place by solutions either 

 having their origin in deeper parts of the magma 

 chamber, or in the adjacent wall rocks, or in a com- 

 pletely unrelated younger magma. Second, the mate- 

 rial may have been intruded at a low temperature as 

 an extremely hydrous magma which crystallized either 

 directly or indirectly through an olivine stage to ser- 

 pentine, without the addition of water (Hess, 1938, 

 p. 321-344 and Sosman, 1938, p. 353-359). And third, 

 the serpentine bodies may have been injected plasti- 

 cally as serpentine forming the so-called "cold intru- 

 sions" (Clark, B. L., 1935, p. 1060, 1074; Bailey, 1942, 



p. 150-151; Bailey, 1946, p. 211; and Eckel and Myers, 

 1946, p. 94). 



The first of these possibilities, involving the emplace- 

 ment of a peridotitic magma, its solidification in place, 

 and its subsequent hydration, has been accepted by 

 many for the serpentine bodies in the California Coast 

 Ranges (Kramm, 1910, p. 315-349) and seems to be 

 required to explain the few masses which show differ- 

 entiation banding. It also has been believed by some 

 to explain satisfactorily the prevalent blocky variety 

 of serpentine if one invokes expansion during the hy- 

 dration process to account for the internal shearing 

 in the masses (Taliaferro, 1943b; p. 154-155). The 

 source for the water, if considered at all, is generally 

 regarded to be the same magma, for in most parts of 

 the Coast Ranges there are no other intrusive rocks 

 that can supply the large quantity of water required. 



Many objections to this theory of origin have been 

 raised. One of them is that the melting point of pe- 

 ridotite is so high that a peridotite magma would be 

 expected to produce widespread metamorphic effects 

 in the surrounding rocks, whereas these -effects are 

 generally lacking. The temperature of such a magma 

 would be about 1,400C (Daly, 1933, p. 67), although 

 it has been pointed out that the presence of abundant 

 water or other mineralizers would lower this perhaps 

 a few hundred degrees. Such mineralizers, however, 

 if present, would presumably migrate at least to a 

 small extent into the wallrocks, and therefore would 

 promote the development of metamorphic aureoles 

 around the serpentine bodies. 



Any proposed source for the water can likewise be 

 countered by objections in the light of experimental 

 work by Bowen and Tuttle (1949, p. 439-460). If 

 the water were included in a peridotite magma, on 

 cooling to a temperature of 900C the rock would be 



