A New Approach to 40Ar/39Ar Dating of Combustion Events: A Case Study from the Late Pleistocene Coal Fires in Goose Lake Depression (Transbaikalia)

Resumo

Based on the study results of combustion metamorphic (CM) rocks from the young (0.01–0.06 kyr) CM complex at the Goose Lake brown coal deposit, a new approach to ⁴⁰Ar/³⁹Ar dating of coal paleofires was substantiated. The results of ⁴⁰Ar/³⁹Ar dating of K–Na feldspars (0.10 ± 0.16 Ma) from fragments of quartz syenites (initial age 285–305 Ma), enclosed in paralavas and heated by fire (T ≥ 950°С) show that magmatic feldspars have lost all radiogenic argon during combustion event. As follows from the numerical modeling results, feldspars (diffusion domains of 100 μm in size) can lose all radiogenic argon under heating (T = 950°C) in five hours. The estimated data have proved that grains of detrital feldspars enclosed in sediments that were subject to heating must be completely degassed over the time of existence of large fire focus. It means that glassy (T ≥ 900°C) CM rocks with relics of K–Na feldspars are applicable for correct ⁴⁰Ar/³⁹Ar dating of combustion metamorphic paleoevents. The abundance of such rocks in the CM sections promotes opportunities for geochronological reconstructions of the Late Cenozoic history of sedimentary basins.