Premium
A new unspiked K‐Ar dating approach using laser fusion on microsamples
Author(s) -
Wang Fei,
Shi Wenbei,
Guillou Hervé,
Zhang Weibin,
Yang Liekun,
Wu Lin,
Wang Yinzhi,
Zhu Rixiang
Publication year - 2019
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.8385
Subject(s) - isochron , chemistry , isochron dating , k–ar dating , isotopes of argon , hornblende , radiochemistry , geology , radiometric dating , mineralogy , muscovite , biotite , argon , analytical chemistry (journal) , geochemistry , quartz , environmental chemistry , paleontology , organic chemistry
Rationale Issues induced by neutron irradiation makes 40 Ar/ 39 Ar dating inapplicable in some cases. The first issue is 37 Ar and 39 Ar recoil effects during irradiation that affect fine‐grained minerals (<50 μm), such as lunar rocks, glassy groundmass, supergene minerals (e.g., illite, glauconite, Mg‐oxide, etc.). The second issue from neutron irradiation is the high radioactivity gain of iron‐rich samples such as pyrite, and the third is the production of interference nuclides during irradiation. The inherent drawbacks of conventional K‐Ar and current unspiked K‐Ar dating make it difficult to assess the reliability of age results. Methods A new approach is proposed using well‐calibrated 40 Ar/ 39 Ar standard minerals to directly quantify 40 Ar, 38 Ar and 36 Ar. Fish Canyon sanidine (FCs), B4M muscovite and MMhb‐1 hornblende, the widely used international standard minerals, were analyzed as unknowns to test the approach. Argon isotope analyses were carried out on a noble‐gas mass spectrometer using laser fusion on microsamples (n × 0.01 to n × 0.2 mg). A new isochron – an “inverse isochron” for K‐Ar dating – was designed. Results FCs and B4M yielded apparent and inverse isochron ages of 28.1 ± 0.1 and 28.0 ± 0.3 Ma, 18.2 ± 0.1 and 18.2 ± 0.5 Ma, which are consistent with the recommended ages, while the MMhb‐1 presented lower apparent and inverse isochron ages (510.8 ± 4.8 and 512.3 ± 17.0 Ma) than the recommended ones. The initial argon compositions for the three standard minerals are 299.2 ± 205.3 (FCs), 294.0 ± 16.4 (B4M) and 290.9 ± 203.1 (MMhb‐1), agreeing with that of air. Conclusions The proposed approach potentially overcomes the issues of 40 Ar/ 39 Ar rising from irradiation and the drawbacks of K‐Ar. By using laser fusion on multiple microaliquots from a same sample, this approach can produce accurate and precise K‐Ar ages and give an inverse isochron. This new approach may provide an alternate dating method of geochronology based on argon isotopes.