WHY ARE SOME YOUNG VOLCANIC ROCKS UNDATEABLE: CHEMISTRY, ENVIRONMENT, OR INSTRUMENTATION?

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.