GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 42-13
Presentation Time: 9:00 AM-5:30 PM


BENOWITZ, Jeffrey A.1, MIGGINS, Daniel P.2, KOPPERS, Anthony A.P.3 and LAYER, Paul W.1, (1)Geophysical Institute and Geochronology Laboratory, University of Alaska Fairbanks, Fairbanks, AK 99775, (2)College of Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Bldg, Corvallis, OR 97331-5503, (3)College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331

Defining the Pleistocene and historic eruptive history of volcanoes is fundamental to understanding magmatic processes, natural hazards assessment, investigating climatic-volcanic interactions, and archaeology pursuits. 40Ar/39Ar geochronology is the preferred method for dating Pleistocene and historic volcanic rocks to facilitate these research pursuits. Young volcanic rocks are often deemed “undateable” as we approach the edge of time applying the 40Ar/39Ar method.

It is not fully clear what are and how to address the controlling factors that lead to 40Ar/39Ar undateable young volcanic rock samples, which often provide negative apparent age results. Chemistry likely plays a role, whereas young (<~100 ka) volcanics with high Ca and low K content may have unresolvable apparent excess 36Ar derived from 40Ca during irradiation. High 36Cl content could also be responsible for actual excess 36Ar through radioactive decay. 36Ar derived from Cl during irradiation may also be a factor contributing to >100 % atmospheric content measurements and apparent negative ages. Magma chamber environment maybe responsible for natural excess 36Ar, whereas kinetic mass fractionation might be responsible for light isotope enrichment favoring magma absorption of 36Ar over 40Ar. The resolution, sensitivity and background of instrumentation may also be a factor controlling the young limits of 40Ar/36Ar geochronology. Apparent excess 36Ar from H35Cl and 12C3 may be resolvable with high-resolution instrumentation. High sensitivity and high precision instrumentation may also limit negative measurements for low abundance Ar isotopes as measurements tend towards zero. Low volume extraction and mass spectrometers under high-vacuum also can decrease background levels leading to overall measurement improvement.

We will present preliminary results from an inter-laboratory collaboration between Oregon State University Corvallis (OSU) and the University of Alaska Fairbanks (UAF) noble gas geochronology facilities in pursuit of dating the edge of time, delineating the factors that lead to undateable young volcanic rocks and our attempts to better account for negative apparent ages. Natural and potentially reactor produced excess 36Ar can result in confounding negative age determinations when attempting to date young volcanic rocks.