The in vacuo release of Ar from minerals: 2. The role of structural modifications of K-feldspar during heating revealed by Raman microprobe analyses

Abstract

The release of Ar during stepwise heating 39Ar40Ar dating experiments may be controlled by Fick's Law diffusion in an inert matrix, and/or by structural modifications of the host mineral. A ca. 1 mm3 irradiated cleavage fragment of a low sanidine crystal from Itrongay, Madagascar was degassed isothermally at 888 ± 2 °C for 264 h, acquiring 67 stepwise 39Ar release data. The 39Ar release was observed to follow a smooth sigmoid curve and not a line as would be predicted by Fick's Law. If the 39Ar release is controlled by crystallographic changes, the implication is that these changes undergo time-dependent variations. The long-term bulk degassing, being the sum of various structural modifications, approximates a Fickian behavior that is not verified in detail in short-term experiments, as it averages over different 39Ar release regimes. This would make the downslope extrapolation of laboratory data to geological conditions highly underconstrained.In order to constrain the behavior of the crystal structure of K-feldspar during laboratory heating, we measured the Raman spectra of a different, ca. 1 mm3 cleavage fragment of the same irradiated sanidine crystal. The sample was heated in air in a Linkam heating stage, and Raman spectra were acquired at temperatures increasing from 300 to 1000 °C, including a 6-h isothermal run at 900 ± 1 °C.Raman modes between 50 and 1200 cm−1 were observed to record two kinds of change, defining robust trends. The positions of most peaks were shifted towards lower wavenumbers (lower energies) and broadened; in addition, the relative heights of different peaks showed robust variation trends. The larger changes coincide with discrete temperature increases, but all changes also progressed over time at constant temperature. These peaks mirror the excitation of phonon modes, which are associated with interatomic bond stretching and deformation, with Si,Al ordering and with deformation and rotation of SiO2 tetrahedra. Most, but by far not all, of the change is reversible (such as e.g. the differential activation of phonon modes), but irreversible structure modifications (such as e.g. Al,Si disordering) are also recorded. We conclude that the K-feldspar structure violates mathematical requirements of matrix inertness during laboratory heating.