Implications of sanidine K/Ca compositions for 40Ar/39Ar geochronology

Abstract

Relative concentrations of K and Ca in minerals are easily obtained as byproducts of the ⁴⁰Ar/³⁹Ar dating process, and are useful in revealing the ranges of K/Ca in feldspars of volcanic ejecta. Most important for geochronology is the alkali feldspar sanidine. The K/Ca of sanidines obtained from Ar isotope data and reported in the literature are often imprecise and/or inaccurate, hence recommendations are given for producing and reporting data to enhance their utility for clarifying the petrogenetic implications. Data of suitable quality from the literature reveal that large volume ash flow tuffs display the greatest range of K/Ca in sanidines that in many cases include anomalously old crystals. In some such cases there is a positive correlation between age and K/Ca, suggesting that the older grains are antecrysts or variably degassed xenocrysts. Modelling of five sanidine-bearing magmas with the rhyolite-MELTS code shows that high K/Ca sanidines observed in many cases are not in equilibrium with their host magmas at P-T-X_H2O conditions at which the magma is <60 % solid by volume, hence they would be uneruptible. The exceptions are a dome lava and a relatively small volume pyroclastic flow. Actual data from these latter extrusions show unusually restricted ranges of sanidine K/Ca, and in the case of the dome, statistically homogeneous ⁴⁰Ar/³⁹Ar ages. Sanidines with K/Ca > 200 in subalkaline magmas are most likely to occur in large volume explosive eruptions that disrupt and mobilize otherwise uneruptible crystal mushes, which in some cases resided in partial ⁴⁰Ar* retention conditions, i.e. cold storage, prior to eruption. Analysis of exsolution lamellae would be useful for clarifying the mechanism for retention of pre-eruptive ⁴⁰Ar* retention, e.g. in cases where cold storage is suspected. Sanidines with K/Ca > 200 from moderately to highly peralkaline magmas are likely to represent autocrysts that were in equilibrium with their host magma at eruptible degrees of crystallinity. Determining relative concentrations of Na in alkali feldspars in addition to K and Ca would extend the petrogenetic information attainable from modified ⁴⁰Ar/³⁹Ar experiments; a potentially viable method using nucleogenic Ne isotopes is outlined.