BIOTITE-SANIDINE 40Ar/39Ar AGE DISCORDANCES REFLECT Ar PARTITIONING AND PRE-ERUPTION CLOSURE IN BIOTITE

Biotite and sanidine are two frequently used K-rich phases in K-Ar and ⁴⁰Ar/³⁹Ar dating of volcanic rocks. In many cases, the ages obtained from these minerals are not concordant, and this can be problematic where uncertainties of a few hundred thousand years or less are essential, such as in stratigraphic correlation. Understanding the origin of ⁴⁰Ar/³⁹Ar age discrepancies between biotite and sanidine requires an independent geochronometer, and in this study we document how U-Th disequilibria may be used to interpret the ⁴⁰Ar/³⁹Ar ages. Here we show that extraneous ⁴⁰Ar in biotite, and its absence in sanidine may be a natural consequence of small violations to assumptions implicit in ⁴⁰Ar/³⁹Ar geochronology on volcanic samples, namely that (1) prior to eruption, minerals are devoid of ⁴⁰Ar due to rapid loss to an ‘infinite reservoir’ such as the atmosphere, and (2) that closure to volume diffusion is geologically instantaneous and coincident with eruption.

At Parinacota Volcano and numerous silicic ignimbrites from the Altiplano-Puna Volcanic Complex in the Andes, biotite ⁴⁰Ar/³⁹Ar apparent ages are discordant with one another and up to 600 ka older than coeval sanidine. In 47 – 40 ka rhyodacite domes of Parinacota Volcano, U-Th disequilibria indicate that both sanidine and biotite crystallized ~120 ka prior to eruption, implying that in addition to ⁴⁰Ar produced in situ, there must be a partitioned excess ⁴⁰Ar component, which is retained in biotite but not sanidine.

Because closure temperature (T_c) varies with pre-eruption cooling rate, use of plutonic cooling rates underestimates T_c for most volcanic systems. Recalculated T_c for biotite and hornblende are sub-equal to eruption temperatures for silicic lavas, thereby allowing preservation of non-atmospheric Ar partitioned into biotite prior to volcanic degassing and eruption. We propose a mechanism whereby presence of extraneous Ar in minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) closure with respect to volume diffusion of Ar in the minerals, (3) equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies in the geosciences that depend on biotite geochronological data.