Incremental heating of hornblende in vacuo: implications for ⁴⁰Ar/ ³⁹Ar geochronology and the interpretation of thermal histories

Increasing evidence of the incompatibility between the geologic thermal histories deduced from the ⁴⁰Ar/³⁹Ar incremental heating technique and those derived from other methods has necessitated a careful evaluation of the assumptions inherent in the former approach. Aliquots of Mmhb-1 were heated in vacuo at temperatures of 500, 700, 930 and 1050°C for 30 min. Up to 700°C, no physical changes in the hornblende were observed. The onset of the structural decomposition of the oxyhornblende occurs at ~930°C and is directly correlated with the initial release of 6% of its radiogenic Ar (⁴⁰Ar*). The breakdown of the oxyhornblende by the internal nucleation of incongruent melt is almost (93%) complete by 1050°C. Because Ar is released simultaneously from the reaction products and is subsequently homogenized in vacuo, this breakdown can yield plateau age spectra, regardless of any original Ar spatial gradients in the mineral. -from Authors