Behavior of argon gas release from manganese oxide minerals as revealed by 40Ar/39Ar laser incremental heating analysis

Manganese oxides in association with paleo-weathering may provide significant insights into the multiple factors affecting the formation and evolution of weathering profiles, such as temperature, precipitation, and biodiversity. Laser probe step-heating analysis of supergene hollandite and cryptomelane samples collected from central Queensland, Australia, yield well-defined plateaus and consistent isochron ages, confirming the feasibilityy dating very-fined supergene manganese oxides by 40Ar/39 Ar technique. Two distinct structural sites hosting Ar isotopes can be identified in light of their degassing behaviors obtained by incremental heating analyses. The first site, releasing its gas fraction at the laser power 0.2-0.4 W, yields primarily 40Aratm, 38Aratm, and 36Aratm(atmospheric Ar isotopes). The second sites yield predominantly 40Ar*(radiogenic 40Ar), 39Ark, and Ark (nucleogenic components), at -0.5-1.0 W. There is no significant Ar gas released at the laser power higher than 1.0 W, indicating the b