GEOCHRONOLOGY BY THE ~(40)AR/~(39)AR METHOD AT THE SERNAGEOMIN LABORATORY, SANTIAGO, CHILE

摘要

An ~(40)Ar/~(39)Ar geochronology laboratory has been in operation at the Servicio Nacional de Geologia y Mineria (SERNAGEOMIN) in Santiago, Chile, since 1998. This laboratory has the capacity to date potassium-bearing geological materials with ages ranging from the age of the solar system (4600 million years), into the historical record (2000 years). The youngest sample analysed to date in this laboratory is a sanidine crystal with an age of 12 000 years. Samples are irradiated at the Comision Chilena de Energia Nuclear (CCHEN) nuclear reactor, using the Fish Canyon sanidine (FCs) standard (28.03 million years) as a neutron flux monitor. The most important factor in obtaining precise and accurate ages is the measurement of neutron flux (J factor), which depends on the use of high quality geological standards and a relatively homogeneous distribution of J factors across the sample holder discs. A SERNAGEOMIN biotite standard (METRI), with an age of 12.88 ± 0.12 million years, is currently being developed as a complement to the FCs standard. The large (20%) variation of J factors across the sample discs, due to a steep gradient in neutron flux in the irradiation site near the margin of the reactor, will be reduced by the implementation of a motorized sample rotation system, currently under testing. Changes in isotope production ratios, due to the gradual replacement of the enriched fuel elements by less enriched fuel, are monitored using artificial standards such as kalsilite glass and calcium fluoride. Problems of ~(39)Ar loss from fine grained samples by recoil effects are being addressed by the implementation of a sample encapsulation technique. The successful development of the SERNAGEOMIN ~(40)Ar/~(39)Ar laboratory has been possible only with the close working relationship which exists with the CCHEN .