Art. LII.—The Physical and Chemical State and Probable Role of Water in Rock-magmas.

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

reference to so-called pneumatolytic action, Vogt quotes Arrhenius as saying, “The solution in aqueous gas now gradually cools, and one substance after another gradually separates from it. By reason of the great mobility of the solution, and its consequent strong capability of diffusion, the minerals (provided the cooling be not too rapid) are segregated in large crystals, such as characterize a so-called pegmatitic structure. Gradually also the constituents which longest retain a gaseous form—such as water and carbonic acid—escape” (1, p. 644).

If we consider the water in a magma to be chemically combined, no difficulty arises as to its physical state until the temperature is sufficiently high to destroy the combination. The more general view is to assume that the water is in solution. Again, there need be no discussion as to its physical state so long as the water or aqueous gas is truly in solution; but the temperature may become so great that steam separates from the magma in the form of bubbles. In this latter condition it can no longer be regarded as acting the part of a solvent for the containing magma. There is certainly need both for a more exact terminology in order to prevent confusion of thought, and for experimental work in order to furnish a better foundation for magmatic hypotheses.

The difficulties involved in the assumptions made by some writers are thus stated from a chemist's point of view by Dr. James Moir: “All sorts of geological authorities accept the belief that water can be made red-hot and yet preserve its solvent properties. Now, every chemist and physicist knows that above the critical point water can only exist as steam, entirely devoid of solvent powers except for other vapours; and this however high the pressure…. As rock-magmas are certainly not vapours, there is no possibility of anything except an uncombined emulsion of rock and steam” (7, p. 4). Dr. Moir relies upon the experimental work of Andreas Smits and J. P. Wuite (5, abstracts, ii, p. 985), who found that the solubility of sodium sulphate in water became zero at 365° C. This result, however, cannot reasonably be extended to silicate solutions without experimental proof. We know that the boiling-point of water containing dissolved solids is above 100° C. at ordinary atmospheric pressure, and also that water at temperatures above 200° C. exerts a strong dissolving power on silicates. In the light of these facts it appears probable that an aqueous solution of silica or a silicate will not necessarily cease to exist as such at 365° C. Moreover, there is some doubt as to whether the so-called critical temperatures of weter and other volatile bodies are really constants irrespective of pressures exceeding the critical pressures (2, pp. 460–61), and therefore it is to be hoped that experimental data showing the solubility of silicates in water at temperatures above 300° C. will soon be forthcoming.

The whole matter may be regarded from another point of view. The most enthusiastic supporter of aqueo-igneous fusion would not ask for the presence of more than 10 per cent, of water in any magma. Is there any real objection to regarding this amount of water as chemically combined with the silica and silicates of the rock-magma? The immense pressures prevailing in magmas may bring about a real chemical combination such as would be impossible at atmospheric pressure.

Hypothetical Classes of Magma.

A clear definition of the difference between water acting as a solvent and water chemicaly combined in magmas must be left to the physical chemist. In the following statements a return is made to the nomenclature

If in rock-magma a distinction between solution and chemical combination cannot be made then classes I and II must be merged. In any case, there cannot well be any hard-and-fast lines between, the three classes. Transition temperatures will depend to some extent upon conditions of pressure and of chemical composition.

Hypothetical Distribution of Magma in the Earth's Crust.

If the influence of pressure, which is practically constant for a given depth and that of chemical composition, which is of minor importance in the present discussion, be discarded, the presence or absence of liquid rock at a stated depth in the earth's crust depends within certain limits both upon the temperature and upon the presence or absence of water. Provided sufficient water (or other “mineralizer” or flux) is present, we may have aqueo-igneous magma at quite moderate depths. Below this comes igneo-aqueous magma, followed by fusion magma. Assuming that temperature continues to increase towards the centre of the earth, and that ordinary physical laws are not essentially modified by immense pressure, we may suppose, with Arrhenius, that the fusion magma ultimately becomes gaseous.^* Into this zone, however, there is no present need to venture. With a patchy and limited distribution of water in the earth's crust we may have at relatively shallow depths disconnected reservoirs of aqueo-igneous or igneo-aqueous magma surrounded by solid rock, with fusion magma at some greater depth. Where water is absent we shall have fusion magma only, confined by a solid roof of considerable thickness. It is quite possible, and, indeed, probable, that the influence of increasing pressure is sufficient to prevent the formation of fusion magma, except in those parts of the earth's crust where the temperature gradient is somewhat above the average.

Hypothetical Formation of Magmas.

In order to narrow discussion, let us consider a segment of the earth's crust assumed originally to have a temperature gradient of 1° C. in 200 ft., and to contain appreciable amounts of water to a depth of ten or fifteen miles, but little or no water at greater depth. Such a segment will be solid to a depth of forty miles, below which may be ordinary fusion (universal) magma. If now, through earth-movements or other cause, the temperature of the segment rises, the fusion magma, owing to melting of the overlying rock, will extend upwards. Even though the arguments in favour of a solid earth be considered valid, yet it will doubtless be admitted that a sufficient rise of the temperature gradient will cause melting in places, and this is all that is here postulated. Sooner or later aqueo-igneous magma will begin to form at a depth of, say, fifteen miles, and will gradually work its way upward. Two cases, dependent on amount of heat supplied, are conceivable. In the less important of these, the temperature gradient failing to reach, say, 1° per 100 ft., the fusion magma does not eat its way upward into contact with the aqueo-igneous magma. The latter in this case will probably cease its upward movement six or seven miles below the surface. On the

other hand, if the temperature gradient becomes relatively steep the lower magma will ultimately junction with the upper. In view of the fact that the lower magma may reasonably be expected to contain at least a small amount of water and other mineralizers, either as gas or contained in an aqueo-igneous differentiate, such junction means transference of water and heat to the aqueo-igneous magma. Lateral extension of the fusion magma will enable it to maintain a supply of water in gaseous form to the aqueo-igneous magma. The latter, therefore, will work (or, in Daly's phrase, “stope”) its way upward at an accelerated pace. So the action will go on until either thermal equilibrium is established or some new condition connected with approach towards the earth's surface comes into play.

Most geologists will probably concede that in some such way as that indicated above solid rocks comparatively near the earth's surface may pass into the liquid condition. The prominent role here assigned to water will, however, be disputed, and doubtless strong arguments can be marshalled against statements resting on so hypothetical a basis.

Magmatic Differentiation.

Under the assumptions made in this paper a magma during formation will necessarily differentiate into two, or perhaps three, parts, distinguished by differences in water-content. The preference displayed by water for silica and alkaline silicates leads to the belief that the aqueo-igneous portion of the magma will be relatively light and acidic, whilst the fusion magma will be heavy, basic, and non-aqueous. The igneo-aqueous portion, intermediate in position, will probably be intermediate also in chemical composition, but may incline to acidity. Further differentiation, it is easy to imagine, will result through the cooling of portions of the magma to the point at which crystallization begins. The opening of a passage to the earth's surface, whereby part of the magma may be extruded, will also give rise to differentiation, which under some conditions may be of a varied nature. The absence of water probably limits differentiation very considerably; and if it be possible for a differentiated magma to rise as a whole to a temperature above that required for an ordinary fusion magma, then it may be assumed that convection currents will check differentiation, and tend to bring about an approximately uniform composition.

Formation of Granite.

An aqueo-igneous magma will in general have the composition of a granite or an acid diorite. Ultimate consolidation of a granitic magma is largely influenced not only by cooling, but also by conditions permitting the escape of water, such as obtain when a rising magma approaches the surface of the earth. The consolidation temperature of granite may be between 575° C. and 800° C.* (8, p. 342), but, according to some geologists of the French school, a lower temperature is more probable. It may be observed that under the assumptions made in this paper some granites probably represent more ancient re-fused granites, gneisses, and rocks of sedimentary origin, and thus their formation completes a cycle of change.

Pegmatites, etc.

Highly heated water escaping into fissures from a solidifying granite will naturally carry silica and silicates with it in solution. These will ultimately crystallize as pegmatite veins (6, pp. 294–96). The consolidation temperature may be below 365° C., and is with tolerable certainty below 575° C. (8, p. 342). No difficulty need be experienced in accounting for the various rare or peculiar minerals found in some pegmatite veins. These are generally due to the presence of volatile bodies rejected with water from the consolidating granite. Any special pneumatolytic hypothesis seems unnecessary. In New Zealand pegmatitic veins, as a whole, are remarkably poor in accessory minerals, tourmaline being the only one at all frequently observed. Hence arises one reason why the writer lays stress on water, and not on other volatile substances, as a “mineralizing agent.”

Syenites.

Syemtic rocks are quantitatively of no great importance, but present a great variety of types. Since they are usually found on the outskirts of granitic masses, it may be assumed that, as a rule, they represent differentiates from consolidating granite magmas. Their variety is perhaps due to varying conditions of temperature and of assimilation of solid rock, and more especially to variation in water and other mineralizers present. Some syenites may be derived from igneo-aqueous magmas.

Diorites.

Diorites, more especially the acid types, may in part be due to differentiation from a granitic magma. Some diorites may represent consolidated igneo-aqueous magmas, whilst others may be derived from fusion magmas.

Acid Hypabyssal Rocks.

The acid intrusives may be regarded as apophyses from granitic magmas. Water probably plays a prominent part in their formation, its presence reducing viscosity and preventing premature consolidation. Quartzporphyry is said to consolidate, like granite, at temperatures above 575° C. (8, p. 342).

Formation of Schists.

Where schistose rocks occur in highly folded mountain-chains, and, moreover, are associated with granitic masses, it is easy to account for their format on by invoking a theory of dynamo-thermo-metamorphism. Gently folded schists extending over a wide area, such as the quartz-mica-schists of Central Otago, present a more difficult problem. It has been suggested that the Otago schists are due to thermal metamorphism, induced by underlying granite. Such a theory implies the formation of igneous rock beneath a wide area. In Central Otago evidence in favour of such a view is afforded by the occurrence in the schists of quartz veins carrying the tungsten mineral scheelite.

Volcanic Action.

It is commonly held that steam-pressure has much to do with volcanic eruptions (4, pp. 42, 45). Other imprisoned gases assist, and Harker suggests that gravitational pressure alone may cause fissure eruptions. Arrhenius supposes that sea-water may penetrate by means of fissures to a

magma near the earth's surface. The magma eagerly absorbs the water, and in consequence swells and becomes more fluid. The resulting pressure, which is analogous to osmotic pressure, forces the magma outwards, and thus volcanic eruption begins (4, p. 18).

According to the assumptions made in this paper, a fusion magma has little or no inclination to absorb water, and, if so, the hypothesis advocated by Arrhenius fails to hold good. Moreover, it is not easily understandable how large quantities of sea water could find their way into a magma against enormous opposing pressure.

An aqueo-igneous magma stoping its way upward during folding movements of the earth's crust may find a plane of weakness leading to the surface. In this case the probability is that water and other volatile substances will escape, whilst the magma, or those portions losing water, will consolidate.

It may happen that heat has accumulated in part of an originally aqueoigneous magma to such an extent that the temperature of this portion is raised well above 1000° C., and all or nearly all its water is free from combination. The opening of communication with the surface will relieve the pressure that keeps the aqueous and other gases confined, or holds them in solution. The result will be a series of violent explosions in the volcanic vent, analogous to geyser eruptions, which will scatter volcanic ash, pumice, &c., far and wide. When the gases have nearly exhausted them-selves there may follow effusions of acidic lava. The cooling of the magma as it rises in the volcanic conduit, together with the loss of water and the extrusion of lava, promotes part crystallization and differentiation. If to these changes we add the influence of magma drawn laterally to the volcanic vent or vents from regions outside the original disturbance, we shall have hypothetical data quite adequate to account for all the phenomena of vulcanism.

Volcanic action may originate through a fusion magma, nearly devoid of water, finding its way to the surface. Such an occurrence is rare, or perhaps impossible.

Hot Springs and Geysers.

Hot springs and geysers are usually attributed to expiring volcanic action. Since the early stages of vulcanism may afford escape for all or most of the water in a magma, a magmatic source for the heated waters of the final stage can be postulated only with difficulty. If, however, an aqueo-igneous or other magma containing water approach the surface it may give rise to boiling springs, the water of which (in part at least) will be of magmatic origin. Thus hot springs may indicate rise of magma, and precede as well as follow a period of igneous activity.

Summary and Conclusions.

The physical or chemico-physical state of water in rock-magmas is not a matter of indifference. By making assumptions that are consistent with known facts, but have not as yet been experimentally proved or disproved, the following conclusions may be reached:—