40Ar/39Ar geochronology of Holocene volcanic activity at Changbaishan Tianchi volcano, Northeast China

• We report 40Ar/39Ar results on millennial volcanic ejecta from Changbaishan volcano.
• We establish eruptive history since 20 ka by dating on sanidine from pumice and lavas.
• 40Ar/39Ar ages are close to the previous U-series ages from the same section.
• We suggest that it was far more active and dangerous than previously thought.

We undertook a detailed 40Ar/39Ar study of the youngest eruption material from the top of the Tianchi volcano cone, dating from the last 20 ka, in order to better understand the frequency of recent eruptions. The results show that the multi-aliquot step-lasing 40Ar/39Ar method can yield meaningful ages at this young time, even in Na sanidine (Or40 An60). Xenocrystic contamination can potentially be recognized by the significant scatter in apparent ages between runs of different step-lasing experiments on sanidine derived from pumice. Sanidine that originates from the alkali lava flows is free of excess argon and reflects several reliable plateau ages from 20 ka to 5 ka in sequence. We cannot obtain a plateau age from the pumices of more recent eruptions, but with analysis of a large amount of data, we are able to derive reliable inverse isochron ages of 1 ka and 4 ka, respectively. Our research shows that the Changbaishan Volcano has been more active in the last 20 ka than previously thought; and therefore, the volcano may pose hazards that have previously been underappreciated. The small difference between the 40Ar/39Ar ages calculated here and previous Uranium-series ages obtained from the same section suggests a short residence time for the Tianchi volcano, in which the time between crystallization and eruption was brief.