Frontier of the K–Ar (⁴⁰Ar/³⁹Ar) Geochronology

Dear Colleagues,

Radiogenic ⁴⁰Ar was discovered from natural minerals in 1948, and the K–Ar dating method has been developed since the 1950s. Subsequently, the ⁴⁰Ar/³⁹Ar dating method was established in the 1960s, and further developments in the application of the ⁴⁰Ar/³⁹Ar led to improvement of the in situ dating technique. Thus far, this K–Ar (⁴⁰Ar/³⁹Ar) method has been applied to many varieties of geological materials as the most approachable radiometric dating method. However, the method often yields some certain geological inconsistency and/or anomalously old ages, especially for high- and ultrahigh pressure (HP–UHP) metamorphic rocks in collisional orogenic belts. For example, some micas in UHP-metamorphosed granite give unusual apparent K–Ar (⁴⁰Ar/³⁹Ar) ages, significantly older than the host granite. Such observation infers the presence of extremely high excess argon. This problem has been discussed by many geochronologists for the last 40 years. Although the reconnaissance of radiogenic argon-loss or -gain is difficult, multidisciplinary approaches have the potential to understand the physicochemical behavior of argon in nature and to improve the reliability of K–Ar (⁴⁰Ar/³⁹Ar) dating of metamorphic processes.

Understanding argon behavior also enables more accurate K–Ar (⁴⁰Ar/³⁹Ar) dating for young volcanic rocks which have experienced Ar isotope mass fractionation and for fluid-induced gold mineralization with excess argon. Furthermore, it would improve authigenic illite and smectite K–Ar (⁴⁰Ar/³⁹Ar) dating for fault gouge rocks, which has been applied to determine fault movements.

This Special Issue invites submissions from K–Ar (⁴⁰Ar/³⁹Ar) geochronology and geochemistry within a multidisciplinary scope, including field observations, petrology, mineralogy, structural geology, and numerical modeling. Studies that help to better understand argon behavior in nature are particularly encouraged.

Prof. Dr. Tetsumaru Itaya
Prof. Dr. Tatsuki Tsujimori
Guest Editors

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