RETENTION OF INHERITED ARGON IN ALKALI FELDSPAR XENOCRYSTS: CONSTRAINTS FROM BA DIFFUSION PROFILES

Alkali feldspars (AF) are invaluable sources of high-precision ⁴⁰Ar/³⁹Ar data for volcanic materials due to their high K-contents. The apparently low solubility and relatively rapid diffusivity of argon in AF’s at magmatic temperatures generally mitigate against inheritance of radiogenic argon. However, entrainment and eruption of xenocrysts can occur sufficiently rapidly that they are incompletely degassed. A case in point is a Miocene trachytic lava from near Mbeya, Tanzania, whose sanidine phenocrysts yield irregularly climbing age spectra with integrated ages between 41 and 327 Ma. The phenocrysts contain cores of partially resorbed AF xenocrysts with successive, oscillatorily zoned overgrowths having large episodic enrichments in BaO (up to 5 wt-%) determined by EPMA. The eruption age of the lava is well constrained by analysis of the densest (Ba-rich) overgrowths, separated by heavy liquids, which yield an isochron age of 17.55 ± 0.13 Ma (σ) with an atmospheric ⁴⁰Ar/³⁶Ar intercept of 280 ± 70, indistinguishable from results for amphibole phenocrysts, which yield an isochron age of 17.9 ± 0.3 Ma and an atmospheric ⁴⁰Ar/³⁶Ar intercept of 281 ± 9. Thus excess Ar leading to erroneously old integrated ages appears to be hosted in the inherited xenocrystic cores. Modelling Ba concentration profiles at the interfaces between xenocrystic cores and overgrowths as diffusion couples enables estimation of the quantity D*t (the product of diffusivity and time), the maximum value for which is ~2.0 x 10^-8 cm², for diffusive relaxation of Ba gradients. Where the profiles yield robust fits to diffusion couples, the contacts between successive Ba-enriched overgrowths yield progressively lower values for D*t from core to rim, as expected. Using results of Cherniak (2002) the D*t data yield possible T-t histories such that e.g. for a magma temperature of 1000 °C, the maximum D*t value corresponds to a magma residence time of ~400 days. Diffusive lengthscales comparable to the phenocryst dimensions (ca. 0.1-0.5 cm) are required for 5-95% retention of inherited Ar during such magma residence, using published diffusion parameters (E_a and D_o) for Ar diffusion in AF (Lovera et al., 1997). These results illustrate that sanidine is not completely immune to inherited ⁴⁰Ar in cases of rapid xenocryst entrainment, ascent and extrusion.