40AR/39AR ANALYSES OF QUATERNARY SAMPLES AND SANIDINE STANDARDS WITH A SMALL VOLUME, SINGLE-COLLECTOR SECTOR MASS SPECTROMETER IN THE ANIMAL FACILITY

Low-volume mass spectrometers offer the ability to measure larger signals for a given number of ions and permit analysis of smaller and younger samples. The Auburn Noble Isotope Mass Analysis Lab (ANIMAL) has been operating for over 10 years with the ‘GLM-110,’ a custom 10-cm, 90° sector instrument with extended geometry, a single ETP electron multiplier, and a Nier-type source; the spectrometer and total system volumes are ~ 400 cc and 800 cc. The instrument’s mass resolution is ~170 and typical sensitivity for argon is estimated to be ~ 2x10^-15 moles/volt; typical blanks during this study were 4x10^-17 moles for m/e=40, 6x10^-18 moles for 39, and 3x10^-19 moles for 36. Samples for the present study were irradiated for 1 hour in the USGS TRIGA facility with ACS-2 (distributed by Nomade et al., 2005) as the primary monitor, with the age of 1.1864 Ma proposed by Jicha et al. (2016). These samples included one sample of transitionally magnetized, whole-rock basaltic andesite from the Tatara-San Pedro volcanic complex of the Chilean Andes (sample QTW-10-3, from the same sample studied by Brown et al., 2004, JGR, v. 109). Incremental heating of four matrix aliquots from this sample with a CO₂ laser yielded plateau ages with a mean of 796±11 ka (2σ) and a MSWD of 0.93; regression of the isotopic data for the same analyses yields four isochrons with a mean age of 772±11 ka, a MSWD of 0.4, and initial ⁴⁰Ar/³⁶Ar ratios that are generally above that of modern atmosphere. The spectra give indication of extraneous argon that is enriched in ⁴⁰Ar in the first steps, and this is confirmed by initial intercepts of the regressions with ⁴⁰Ar/³⁶Ar ratios up to 313. Based on these results, the age of this sample is interpreted to be 772 Ma, in agreement with results for the astrochronologic age estimates for the Matuyama-Brunhes event and the results for transitionally magnetized basalts reported for Haleakala crater by Coe et al. (2004). These results also lead to the suggestion that earlier analyses of this sample by Brown et al. (2004), obtained with resistance furnace analyses and measured on MAP instruments more than a dozen years ago, may be biased by unresolved components of extraneous argon. These results are presented to encourage discussion of potential factors affecting the accuracy of ⁴⁰Ar/³⁹Ar ages, and to strive toward improvements for the ANIMAL facility.