Laser microprobe measurement of chlorine and argon zonation in biotite

Neutron activation of chlorine yields ³⁸Ar which can be liberated by the laser microprobe (LMP) in sufficient quantities to be measured by gas mass spectrometry. When used in conjunction with electron microprobe (EMP) analyses, this technique can precisely measure low concentrations of Cl in minerals. The accuracy is limited by lack of an adequate standard. LMP analyses of standard Mmhb-1 indicate its Cl concentration is quite heterogeneous and is not a useful standard for LMP Cl analyses. In natural phlogopite and hydrothermally treated (HT) biotite Cl concentration gradients have been measured with the LMP. The zonation probably reflects partial re-equilibration by diffusive anion exchange with Cl transport occurring more rapidly parallel to the silicate sheets than perpendicular to the sheets. The LMP also detected ⁴⁰Ar gradients in the phlogopite and HT biotite, suggesting that the rate of Cl transport is comparable to that for Ar diffusion. LMP analyses also indicate that ⁴⁰Ar migration is more rapid parallel to the silicate sheets than perpendicular to the sheets and that the radius of diffusion is on the order of the grain size. The latter observation apparently contradicts the relatively much smaller estimates of the radius of diffusion for Ar in biotite (∼ 150 μm) inferred from ⁴⁰Ar analyses of bulk separates of hydrothermally treated biotite. The discrepancy may be related to the numerous pores occurring at layer terminations and pinch outs and at the edges of Guinier-Preston zones detected by high-resolution transmission electron microscopy (HRTEM) and which appear to be more abundant in the HT biotite than in the original untreated sample biotite. If these pores are sufficiently large to permit a conduit for rapid Cl and Ar transport via pipe diffusion, then they may be the cause of the apparently enhanced diffusivity of ⁴⁰Ar observed in large-grain-size fractions of hydrothermally treated biotite.