HF Leaching of Volcanic Groundmass in an Effort to Reduce 39Ar Recoil in 40Ar/39Ar Dating
Abstract
Recoil of 39Ar in geological samples during irradiation by fast neutrons is a significant problem in the dating of certain volcanic samples by 40Ar/39Ar techniques. In this induced process 39Ar is ejected from high-K (low temperature) domains in samples and transferred to low-K (high temperature) domains or lost as a free gas in the irradiation tubes. 39Ar recoil makes dates apparently too old (when 39Ar is lost or redistributed) resulting in age spectra that cannot easily or reliably be interpreted to determine crystallization ages. In this methods study, an HF leaching step was added to Oregon State University's Argon Geochronology Laboratory's standard HCl and HNO3 acid leaching procedures for volcanic groundmass prior to analysis. Two size fractures per sample and five levels of HF acid leaching strength for each size fraction were performed on seven samples from both terrestrial and submarine lavas to test the ability of HF on remediating 39Ar recoil. Preliminary results suggest samples leached with HF result in flatter age plateaus and inverse isochrons with significantly increased linearity in the data arrays. At the same time, the samples lose a significant fraction of their radiogenic argon from recoil-affected domains and thus gain greater atmospheric 40Ar and 36Ar components. We infer that HF leaching removes a significant fraction of the total K in samples, likely from the K-enriched, previously glassy, but now altered domains within the groundmass. These sites are also the most likely to loosely retain their neutron-produced 39Ar because of the higher abundance of (ultra) fine-grained secondary clay mineral phases. The potential removal of recoil-prone sites within groundmass grains will be examined by SEM imaging and electron microprobe imaging with element mapping to quantify the differences between unleached and variously leached grains. Our preliminary results on both terrestrial and submarine basaltic groundmass samples improve the methodology of high-precision 40Ar/39Ar geochronological studies by determining how to reduce the effects of 39Ar recoil with the use of hydrofluoric acid (HF). This has the potential to provide geologically meaningful ages for previously undatable suites of rocks.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11D0108K
- Keywords:
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- 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRY;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 1094 Instruments and techniques;
- GEOCHEMISTRY