APPLICATION OF FINE MAGNETIC-SEPARATION T ECHNOLOGY IN K-AR, AR-39-AR-40, AND RB-SR METHODS OF ROCK AND MINERAL DATING

The technique and examples of using magnetic-separation technology in isotope-geochronological investigations are considered. Crushed sample s, in their passage through a specially constructed magnetic separator , are separated according to their magnetic susceptibilities as follow s: mineral mixtures - minerals - mineral fractions. The efficiency of this technique is demonstrated by three examples. 1. Isotope-geochrono logical investigations by minerals magnetically separated from cryptoc rystalline rocks. Using Ar-40-Ar-39 and K-Ar methods, by the example o f andesite-basalt from the dike crossing the Kuznetsov Formation (East Sikhote-Alin) it is shown that the age of basaltoid magmatism is 72.7 -74.6 Ma. Determined from matrix fractions of this rock, its isochrono us Rb-Sr age is 72+/-8.7 Ma at r.m.s. = 0.164. The age of xenogene-mat erial protolith is no less than 91.5+/-1.7 Ma. The primary isotopic co mposition of Sr in basaltoid magma and in xenogene material is 0.70401 +/-0.00007 and no higher than 0.70286, respectively. 2. Isotope-geochr onological investigations of minerals intensely affected by superimpos ed processes. Using a magnetic separation method, primary kersutite an d chlorite are isolated from the subalkaline volcanite found in Gornay a Shoriya. Analysis of kersutite and chlorite by a Ar-40-Ar-39 method shows that amphibole formed 485+/-5 Ma ago, and the superimposed proce sses took place 254 Ma ago. 3. Monomineral isochronous Rb-Sr dating by one of mineral varieties. Fractions of feldspar and phlogopite are is olated from alkaline feldspathic rock by magnetic separation. Determin ed by the phlogopite fractions, the isochronous Rb-Sr age is 40.9-42 M a. Applying a fine magnetic separation method for sample preparation p ermits one to reveal the irregularity of characteristics of Ar-40-Ar-3 9 (K-Ar) and Rb-Sr isotope systems within rocks and individual mineral s and to detalize the obtained data on age and origin of objects.