Barium silicates of the Berisal Complex, Switzerland: A study in geochronology and rare-gas release systematics

Barium silicate minerals such as celsian, ganterite, armenite, as well as Ba-bearing and Ba-free white mica from the Berisal Complex, Simplon Nappe, Swiss Alps, were dated by Ar-39-Ar-40. Ages of Ba-free micas are ca. 17 Ma, while Ba and parentless Ar-40 are correlated in Ba silicates, suggesting common inheritance from the Paleozoic orthogneissic protolith. The release pattern of reactor-produced Ar-39 (or Ar-37) from hydrated and anhydrous minerals is very similar, with apparent activation energies of ca. 180 kJ/mole and a conspicuous kink around 900 degrees C. White micas release Ar at higher temperature than the literature determination of their dehydroxylation. In addition to Ar, we studied the degassing of monoisotopic Xe-131 produced from Ba during neutron irradiation. Xe is degassed at higher temperature than Ar, and again all analyzed silicates have the same apparent activation energy of ca. 300 kJ/mole. The decoupling of Ar and Xe rules out delamination as the dominant degassing mechanism in mica and implies that recoiled rare gas atoms mostly reside inside the T-O-T layers of the mica structure. The near-identical apparent activation energies in such diverse silicates as tecto-, phyllo- and cyclosilicate requires instead that the in-vacuo gas release kinetics are the same in all three silicates. As the only structural element common to these three silicate families are silica tetrahedra, it is possible that their well-known rotation plays a decisive role for in-vacuo Ar degassing; additional high-temperature in situ structural investigations on feldspars and micas would be needed to help constrain the mechanisms of laboratory Ar release. (c) 2007 Elsevier Ltd. All rights reserved.