Mineral chemistry and geochronology of the Rajasthan emerald deposits, NW India

ABSTRACT The emerald deposits in Rajasthan, northwest India, are situated in a narrow NE–SW belt in the Aravalli Mountains. The studied deposits were formed by the metasomatic reaction between muscovite (± garnet ± tourmaline) pegmatites and lenticular bodies of altered ultramafic rocks that are hosted by the Delhi Group gneisses. This reaction produced phlogopite schists containing the exometasomatic emeralds, as in all other granite-related emerald deposits. Endometasomatic changes of the mineralogy of the pegmatites is indicated by the geochemistry of the muscovite (phengitic substitution) and the feldspars (disappearance of the potassic feldspar and calcification of the plagioclase). The K-Ar analyses of syngenetic phlogopite (from the phlogopite schist) and muscovite (from the pegmatites) give an age of ca. 790 Ma, close to that of the last major orogeny affecting the region. This is in accordance with the ages of other granite-related deposits, which all formed in conditions of active orogeny. The ages of the biotite are lower than those of the muscovite, indicating limited radiogenic argon loss as a result of deformation.

Progress and Challenges in K-Ar and40Ar/39Ar Geochronology

K-Ar and more recently the40Ar/39Ar variant are well established dating methods. The40Ar/39Ar method requires irradiation with neutrons, posing some complications that are greatly outweighed by the benefits. The40Ar/39Ar method is particularly powerful due to the availability of internal reliability criteria, the ability to analyze single crystals, and the amenability of the analyses to automation.40Ar/39Ar dating has the capability for unsurpassed precision and is applicable to the broadest range of geologic environments and time scales of any radioisotope dating technique. For chronostratigraphic applications,40Ar/39Ar is most important in the Cenozoic, becoming progressively less useful into the early Phanerozoic due to alteration and loss of radiogenic argon. Precision and accuracy of40Ar/39Ar dating have been improved considerably in recent years, but an uncertainty of about 1% in the decay constant for40K, probably mainly in the electron capture decay branch, still limits accuracy at about this level. Inconsistent use of standards (neutron fluence monitors) and attribution of variable ages to standards is still a source of confusion, but straightforward recalculation procedures can overcome the underlying problems provided that appropriate standards are used.