Jökull


Jökull - 01.12.1975, Side 27

Jökull - 01.12.1975, Side 27
rock as granite is about 1% at a shallow depth. On the whole one must, therefore, consider interstitial groundwater to have been present in rocks. Its role in connection with radiometric dating is a very complicated question, as we have pointed out in sections 1 and 3. But generally, one should think that it is of import- ance, whether or not the groundwater has flow- ed through the rock, and even through the minerals. The flow would be largely determin- ed by groundwater pressure gradients, which in turn depend on the landscape at each time. A dip of a flat land, corresponding to a surface gradient of 200 m per 100 km, would suffice to cause a groundwater flow through granite, which could be serious from the point of view of radioactive dating. But also the geology of the area as a whole, of which the rock under consideration is a part, influences groundwater flow through the rock. One cannot possibly reconstruct the history of groundwater flow for rocks of some consicler- able age. One could try to test the influence of the geological factors in the following way: 1) The age for each productive mineral type is measured, using a variety of sizes and shapes, as said before, to test losses of a daughter ele- ment. 2) Whether loss is indicated or not in this way, the ages of the receptive minerals are also tested in the same way. This treatment might enable the analyst to make necessary cor- rections to lead him at least closer to the true age of the rock. In practice the grouping of Precambrian ages, for instance, is interpreted as metamorphic phases which are dated in this way, and beyond which a correction is not thought to be possible. The possibility of reconstructing the history of groundwater flow is, however, considerably greater in connection with the dating of such young rocks as Neogene or even Paleogene ones. In this case the tectonic history, in parti- cular uplifts and the following dissection of such areas may be known in the main outlines, and these factors have increased the flow of cold groundwater down to some depth and lowered the temperature within rocks which are now exposed. The “age” of such rocks may date this cooling phase. We shall come back to this topic in the next section. As to still younger rocks, such as those of 1—4 My age (cf. the geomagnetic time scale), we must keep in mind that they have been soaked with water for much of their life time, and that even cold groundwater dissolves sub- stances frorn such igneous rocks as fresh basalts. One way of testing loss of 40Ar into ground- water is to test 40Ar/38Ar in water emerging from different depths, as indicated by the temp- erature. Another way is to grind a sample down to the coarseness of its grains, and test experi- mentally the rate of loss of 40Ar into water at various temperatures ánd pressures. If such tests indicate the possibility of significant loss of 40Ar in 1 to 4 My old rocks, a correction of the measured ages must take into consideration the history of the grounclwater flow, as far as it could be reconstructed from the geomorpho- logic and climatic history. The present author has not seen a single report on radiometric dating, or text books, in which the physical principles statecl in section 4, i. e. the importance of grain size, have been mentioned, although the tests of reliability following from these principles seem most es- sential. Discordant ages for any rock mass are very common, sometimes to such a degree that the analyst has the only choice to postulate some unknown geological events, to explain the gross discordance in the ages given by various dating methods. In such cases radiometric dating is just no independent method of dating. The omission of some of the physico-geological prin- ciples in radiometric dating which we have discussed leaves one in doubt about the extent of the necessary revision of some of the radio- metric material sofar collected. 5. NON-RADIOMETRIC, SEMI-ABSOLUTE TIME SCALES FOR TERTIARY ROCKS Distinction between the radiometric age of rocks and of geomorphological phases, by consideration of groundwater changes. When it is realized how often it is difficult to measure geological time with certainty by radio- metric methods, it seems worthwhile to recon- sider those older methods which were abandon- ed when radiometry in its early clays seemed to be the only sensible geological clock. We shall not enter upon the possibility, JÖKULL 25. ÁR 25

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