铀系组分法测定年青火山岩年龄的研究

铀系组分法是测定火山岩不同组分中的２３０Ｔｈ／２３２Ｔｈ值和２３８Ｕ／２３２Ｔｈ值、计算年龄的方法。岩石中的各组分是用磁选或浮选分离的。经过论证，组分法与等时线方法是等价的，但不需要从全岩中精选纯的单矿物，不会发生铀和钍同位素的分馏。根据组分法模式测定了云南腾冲盆地北来凤山、老龟坡山和马鞍山火山岩样品的年龄，分别为０．１１ＭａＢ．Ｐ．、５１０００。Ｂ．Ｐ．和２５０００ａＢ．Ｐ，与地层顺序相吻合。组分法对于年青火山岩定年有着极大的潜力。

STUDIES ON THE URANIUM SERIES COMPONENT DATING METHOD FOR YOUNG VOLCANIC ROCKS

The dating of young volcanic rocks is a geological problem that has not been satisfatorily resolved. Study of Uranium series disequilibrium in Late Quaternary volcanic rocks offers a unique opportunity to obtain information on the timing of the geological processes involved in their formation, which is difficult to obtain by other dating techniques. The application of Uranium series techniques to young volcanic rocks is feasible because disequilibrium commonly occur between Uranium parents and some daughter nuclides. Observed disequilibrium can thus be used to determine the age of crystallization provided the magma reached the surface in a time which is short compared to the half-lives of the nuclides involved.The dating of young volcanic rocks is a relatively recent application of Uranium series methods. Kigoshi and Taddeucci et al. reported isochron techniques for 230Th-dating of young volcanic rocks. In spite of potential problems with incomplete leaching or isotope during leaching, the results appear satisfactory.Two problems are involved in the isochron techniques: (1) the physical means is heavy and difficult to completely separate the pure minerals as zircon, magnetite,quartz-plagioclase, biotite-hornblende from volcanic rock matrix; (2) a critical requirement of the methodology of Kigoshi is that no fractional extraction of uranium and thoriurn isotopes takes place during the dilute acid leaching of volcanic rock samples.The uranium series component-dating method is besed on the measurements of 230Th/232Th and 238U/232Th in the differential fractions designed to isolate by flotation or magnetic seperation procedure according to specific gravity and magnetic property. These fractions tottally are dissolved. In their complete dissolution, the differential extration of uranium retative to thorium is not allowed during the dilute acid leaching of volcanic rock samples. There is no need to completely separate the pure minerals from volcanic rock matrix. Much improvement has been made in the 230Thmineral isochron technique and the metodology of Kigoshi.In general, the heavy fraction or strong magnetic fraction of the analysed rock contains ferrugionous minerals and pyroxene to 50% by mineralogical distribution.The points of the measured data for these fractions were located in the down lefthand side of the isochron line on the 230Th/232Th-238U/232Th diagram. The lightly weight fraction or>5A magnetic fraction contains essentially major minerals posed of alkai feldspar and sodic plagioclase as much as 80 per cent. The data of these fractions were displased in the upper right hand on the 230Th/232Th-238U/232Th diagram.For the system initially at isotopic disequilibrium, the 230Th/232Th-238U/232Th activity ratios for the differential fractions including heavy, middle and light magnetic fractions as well as whole rock will define a isochron and thus the age of the rock will be determinable. Simple multiple-fraction mixing of a differentiating magma would theoretically generate a linear correlation between 230Th/232Th and 238U/232Th.In order to obtain the accurate ages by this component-model, four criteria must be met: (1) the differential fractions and whole rock must have identical 230Th/232Th ratios but different 238U/232Th ratios at the time of crystallization; (2) the transit time of the magma to the surface must be short with respect to the half-life of 230Th; (3) the mineral phases of the differential fractions must have formed at essentially the same time; (4) the system must have remained closed with respect to post-crystallization isotope migration. Once the initial 230Th/232Th and 238U/232Th isotope ratios are set at the time of crystallization, a volcanic rock must remain closed to either inward or outward isotope migration.Adopting the mentioned-above component-isochron model, the authors dated three Quaternary volcanic rock samples from the Tengchong Basin west of Yunnan province: Beilaifengshan, Laoguipo and Maanshan. The age data of these samples were 110×103 yr, 51×103 yr, 25×103 yr, respectivery. For