398 Transactions. 



Art. LI I. — The Physical and Chemical State and Probable Role of Water 



in Rock-magmas. 



By P. G. Morgan, M.A., Director, New Zealand Geological Survey. 



[Read before the Wellington Philosophical Society, 25th September, 1912.] 



Introduction. 



During recent years there has accumulated much evidence showing that 

 an abundance of water is present in some or all rock-magmas, and may 

 play a prominent part in their formation and subsequent consolidation. 

 Thus a theory of so-called aqueo -igneous fusion has been evolved, more 

 especially with respect to plutonic and hypabyssal rocks of acidic com- 

 position. Though there is a considerable amount of vagueness in the acces- 

 sible literature dealing with the function of water in aqueo-igneous fusion, 

 apparently many geologists and others distinctly hold the belief that the 

 water present acts as a flux or solvent of silica, silicates, and other minerals 

 not only when it is in the liquid state, but also when it is in the gaseous 

 state. Moreover, some writers seem to assume that there is no practical 

 limit imposed by temperature conditions on the solvent action of water. 

 On the other hand, there are those who tacitly or explicitly make the 

 assumption that the presence or absence of water and other " mineralizers " 

 is a matter of secondary importance. For instance, Harker (6,* pp. 288 

 et seq.) certainly does not overemphasize the function of water as a solvent 

 and mineralizing agent. Such a standpoint seems to be supported by the 

 data obtained by Albert Brun, of Geneva, and published in his recent work 

 entitled " Eecherches sur I'Exhalaison Volcanique " (9). Brun maintains 

 (pp. 254-55) that water is not present in lavas during extrusion, and, in 

 fact, is wholly absent during ordinary volcanic activity. These conclusions 

 are of doubtful validity, but if admitted would appear to lead to the corol- 

 lary that water is not present in ordinary rock-magmas. It will, however, 

 generally be granted that, notwithstanding Brun's new data, water is by far 

 the most abundant of the volatile substances present in rock-magmas, and 

 therefore that its role is relatively much more important than that of other 

 " mineralizers," such as fluorine, chlorine, &c. Hence in this discussion 

 the latter are more or less neglected. They seem, indeed, to be of import- 

 ance only in connection with the presence of certain minerals in veins 

 genetically related to granitic intrusions. 



Physical State of Water. 



If water is to be regarded as a solvent for silicates and other constituents 

 of rock-magmas, the question of its physical state whilst so acting deserves 

 consideration. The critical temperature of pure water being 365° C, and 

 the critical pressure 200-5 atmospheres, some, if not most, writers assume 

 that at higher temperatures, whatever the pressure, the water in a magma 

 is necessarily in the gaseous state. As evidence of this the following quota- 

 tion may be made : " He [Arrhenius] considers a magma as a complex 

 solution containing various silicates, &c., and also gases, the latter, of course, 

 including in the first place water, which above its critical temperature of 

 about 365° must be ranked as a gaseous body " (6, p. 295). Again, with 



* This and other numbers enclosed in brackets refer to list of literature at end. 



