Potential and opportunities for 40Ar-39Ar geochronology and thermochronology research in India. PD-3-4

Geochronology based on radiogenic isotopes has become an invaluable tool in earth sciences. Several radioactive parent-daughter systems of varying half-lives such as Rb-Sr, Sm-Nd, K-Ar have been traditionally used by researchers for determining the timing of geological and planetary processes. 40Ar-39Ar dating, a variant of the K-Ar system, is a well established and versatile method of determining the eruptive ages of volcanic rocks and the ∼150-500 deg C thermal histories of a variety of more slowly cooled igneous and metamorphic rocks. This technique has been the most popular tool for dating felsic and intermediate volcanic rocks. For slowly cooled rocks, the K-Ar system yields ages that generally reflect the time elapsed since cooling below the closure temperature for argon of the mineral being analyzed and therefore becomes an extremely powerful tool for thermochronologic studies of orogenic systems. In recent years application of laser microprobe heating has enhanced the capabilities of this technique. Conventional 40Ar-39Ar step-heating experiments are designed to determine the bulk closure ages of aggregates of minerals like hornblende, muscovite, and biotite. The use of multidomain diffusion (MDD) modeling of K-feldspar developed by Lovera et al.,1989, has today become a very powerful tool for thermochronologic studies which provide constraints on the unroofing history of orogenic belts. Recently several new areas of research have been explored, including total-fusion dating of mineral grains from volcanic and sedimentary samples, mapping of argon isotopic gradients in crystals, and selective dating of fabric-defining minerals in polydeformed specimens