The accumulation of helium in geological time.—III

Abstract

The present instalment of my experiments on this subject refers to the minerals of igneous rocks. A word of preface is necessary as to the interpretation of results obtained from these. An igneous magma, at the time of consolidation, contains large quantities of gases: the liquid carbonic acid which may be seen in the cavities of granitic quartz is a proof of this, as is also the copious emission of water vapour from consolidating lava streams. We are for the most part ignorant of the history of rock magmas previous to their intrusion into their present position among the strata, and consolidation there; consequently, it is impossible to say whether any helium which may be found in the consolidated rock as a whole was generated since the consolidation, or whether it partly represents what was entangled or dissolved in the molten magma previous to consolidation; for there is no doubt that helium was being generated in the molten just as in the consolidated material. All the evidence goes to show that the rate of radio-active change is nearly if not quite independent of the physical conditions in which the radio-active matter may happen to be, and of its state of chemical combination. We may evade the difficulty by concentrating attention on those con­stituent minerals of the rock which are exceptionally radio-active. These, although they may in some sense have been present in the magma, in solution, must at that time have added the helium they were generating to the common stock of the magma as a whole. It is only since they separated in the solid form that they can have begun to accumulate within themselves a store of helium greatly more concentrated than that present in the rock mass generally. Minerals like zircon and sphene, containing hundreds of times as much helium as the average of the rock of which they are constituents, may accordingly be regarded as having generated the whole of their supply since consolidation.