Nonspiked ⁴⁰Ar and ³⁶Ar quantification using a quadrupole mass spectrometer: A potential for K–Ar geochronology

详细信息    查看全文

• 作者：Virgile Rouchon ; Jean-Claude Lefè ; vre ; Xavier Quidelleur ; Gilles Gué ; rin ; Pierre-Yves Gillot
• 关键词：Quadrupole mass spectrometer ; K– ; Ar geochronology ; Volcanism ; Geochronological standard
• 刊名：International Journal of Mass Spectrometry
• 出版年：2008
• 出版时间：15 February 2008
• 年：2008
• 卷：270
• 期：1-2
• 页码：52-61
• 全文大小：840 K

文摘

Potassium–argon geochronology relies on the absolute quantification of radiogenic ⁴⁰Ar (⁴⁰Ar*) produced by the natural decay of ⁴⁰K in K-bearing rocks and minerals. It implies the precise determination of the ⁴⁰Ar and ³⁶Ar contents to account for atmospheric contamination, which, for very young (<100 kA) and/or K-poor material (K < 0.2 wt. % ), may represent over 99 % of their argon budget. We have tested the applicability of a standard resolution quadrupole mass spectrometer (QMS) for the determination of ⁴⁰Ar* in a set of K–Ar and ⁴⁰Ar/³⁹Ar international geochronological standards.

The analytical apparatus is designed to perform in-line extraction, purification and measurement of 10¹² to 10¹⁴ atoms of Ar from various K-bearing minerals. The sensitivity of the mass spectrometer is calibrated against the international K–Ar standard GL-O and determined with a 0.6 % (1σ) uncertainty. The average ⁴⁰Ar/³⁶Ar ratio of repeated air-standards through a 1-month period is 262.5 ± 0.36 (1σ), and its daily standard deviation never exceeds 1‰.

Geochronological standards MMhb-1 (520.4 ± 1.7 Ma, hornblende), LP-6 (127.8 ± 1.4 Ma, biotite), Taylor Creek Rhyolite (27.92 ± 0.04 Ma, TCR sanidine), Alder Creek Rhyolite (1.193 ± 0.001 Ma RAC-20 sanidine) and RMF96 (0.331 ± 0.002 Ma, sanidine) dated here at 529.7 ± 7.2, 130.1 ± 1.0, 29.13 ± 0.4, 1.189 ± 0.022 and 0.340 ± 0.006 Ma, respectively, are within or close to their published reference ages at the 1σ confidence level.

Recent high-sensitivity and high-resolution quadrupole mass filters, equipped with appropriate and high stability electronic equipments, provide compact and fast analytical alternatives to expensive magnetic-sector-type mass spectrometers for applications such as K–Ar geochronology.