The double-edged sword of ultra-high precision 40Ar/39Ar geochronology: Investigating previously unresolved complexities in sanidine age distributions

Abstract

Improvement in the precision of 40 Ar/ 39 Ar sanidine geochronology has demonstrated that single crystal sanidine dates from ignimbrites are dispersed, leading to ambiguous interpretations of eruption ages. This inhibits interpretation of temporally closely spaced geologic events such as nearly coeval caldera forming eruptions, paleomagnetic reversals, extinction events, etc. Possible age dispersion sources related to (1) neutron dose differences between individual sanidine grains, (2) mineral and melt inclusion variations between grains, and (3) mass spectrometry and data reduction details have been evaluated via detailed laboratory experiments on multiple sanidine bearing ignimbrites. The accuracy of derived eruption ages is cross validated through stratigraphically constrained 27 – 28 Ma ignimbrites from the San Juan Volcanic Field that may differ in age by less than 15 ka. The 40 Ar/ 39 Ar method is based on irradiating a sample to convert 39 K to 39 Ar with the 40 Ar/ 39 Ar value being proportional to age. However, multifarious neutron flux, spatially and temporally, leads to no two grains receiving the same neutron dose, thus, variation in grain-to-grain dosage is a possible source of age dispersion. Irradiation of Fish Canyon tuff sanidine (FC-2) grains in a tightly spaced geometry significantly reduced dispersion from the typical grain-to-grain date range of up to ca. 100 ka to as low as ca. 30 ka. Although better constraining the irradiation geometry demonstrated that neutron flux variation is a large source of age dispersion, in detail, populations still show excess dispersion that likely correlates to geologic complexities. Geologic dispersion is evaluated by handpicking inclusion-free and inclusion-rich sanidine grains. Inclusion-rich grains are characterized by having visible melt and mineral inclusions when viewed under a microscope. Detailed experiments of FC-2 revealed no significant age difference or degree of dispersion