V53H-07
40Ar/39Ar Interlaboratory Calibration into the Holocene.

Friday, 18 December 2015: 15:10
310 (Moscone South)
Matthew Thomas Heizler1, Brian Jicha2, Anthony A P Koppers3 and Daniel P Miggins3, (1)New Mexico Institute of Mining and Technology, Bureau of Geology and Mineral Resources, Socorro, NM, United States, (2)University of Wisconsin Madison, Department of Geoscience, Madison, WI, United States, (3)Oregon State University, Corvallis, OR, United States
Abstract:
Advances in 40Ar/39Ar analytical precision for very young rocks requires collaborative efforts amongst argon geochronology labs to demonstrate age reproducibility commensurate with high precision. NM Tech (NMT), the University of Wisconsin (UW) and Oregon State University (OSU) have each dated Quaternary flux monitor standard AC-2 sanidine (~1.185 Ma), a blind sanidine described as being 50-100 ka (BS) and sanidine from the Qixiangshan (QIX) flow (~10 ka), Changbaishan volcano, China.

The samples were irradiated in a single package with FC-2 sanidine (28.201 Ma) as the flux monitor and the irradiated material was distributed amongst the labs. Heizler was present during analysis at both OSU and UW and Jicha attended OSU during analysis. Physical presence was key towards gaining understanding of individual protocols and prompted valuable discussions.

Analyses were carried out on single crystals using total fusion and/or step heating approaches. Age agreement was achieved within 2s uncertainty that ranged between (0.03-0.3%, 0.13-0.37% and 1.8-2.6%) for AC-2, BS and QIX, respectively. Each lab found AC-2 to vary somewhat beyond a normal distribution and to yield an age relative to FC-2 of ~1.185 Ma that is ~1.3% (~5-10 sigma) lower than some published estimates. A key cause of the variation between this study and previous results may be variable gas pressure equilibration times between extraction line and mass spectrometer coupled with variable choices to estimate time zero by other laboratories. The majority of our efforts concentrated on the QIX sanidine where prior data obtained by our labs revealed a factor of two spread in age (~11 and 23 ka) based on experiments carried out by total fusion and bulk incremental heating. By conducting single crystal age spectrum analysis we were able to mitigate effects of melt inclusion hosted excess argon and xenocrystic contamination towards obtaining analytical agreement with apparent ages near 10 ka. However, philosophical differences in choosing data subsets leads to variable eruption age estimates between about 7.6±0.2 and 10.5±0.5 ka.

Our collaborative efforts continue to show that overall the argon community is making very good progress towards achieving the Earthtime goal of high precision interlaboratory calibration.

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