The Role of Water in Basaltic Magma I.

Abstract

The crystallization of a hydrous melt in the system K₂SiO₃—SiO₂—H₂O is studied in detail to determine what are the external factors exerting the greatest influence over the character of the final crystallization product. These are found to be (a) the nature of the stresses present in the crustal horizon at which crystallization takes place and, (b) the effective pressure of the water in the pores of the surrounding rock, a pressure which is determined by the potential rate of escape of vapour from the magma as well as by the gas pressure of the water itself. Various types of environment are considered and the products of consolidation of an ideal magma of this system compared for the various cases. The probable mode of assimilation of a xenolith composed of one of the solid phases of the system in such an ideal magma is also investigated.

With this preliminary study as a guide an attempt is made to determine the probable influence of dissolved water on the course of crystallization of basaltic magma, the possibility of an analogy between the theoretical and the natural systems being first investigated. The conclusion is arrived at that for plutonic basaltic magmas the concentration of water is from 4 to 5 per cent and that for less deep-seated horizons the amount in solution is less. The effect of this dissolved water on the mineralogy of the rocks is discussed, principally with reference to the plagioclases, pyroxenes, hornblende, biotite and the reaction relation of olivine and pyroxene. Its control over assimilation processes is also considered, the examples being taken from theKeweenawan diabases of Ontario. Finally an attempt is made to evaluate the importance of the water content in modifying the products of crystallization differentiation from basaltic magma, first through its displacement of the equilibrium relations in the anhydrous part of the melt and secondly, through tis control over the nature of the minerals separating.