image: Laser 40Ar/39Ar isochron ages and age-probability diagrams (p) probability statistics for all ages; (q) probability statistics for the first peak; (r) probability statistics for the second peak; (s) probability statistics for the third peak. view more
Credit: ©Science China Press
Reconstruction of Quaternary environments, late Cenozoic geodynamics and evaluation of volcanic hazards, all depend on the precise delineation of eruptive stages. To date, it has been difficult to achieve high-precision dating of young volcanic rocks.
A research entitled "Using a Gaussian mathematical model to define eruptive stages of young volcanic rocks in Tengchong based on laser 40Ar/39Ar dating", Zhao Xinwei as the first author, Zhou Jing, Ma Fang, Ji Jianqing and Alan Deino as co-authors, published in Science China Earth Sciences.
As one of the youngest volcanic areas in Southwest China, Tengchong volcanoes are characterized by multi-stage eruptions and complex overlapping relationships. However, the stages of volcanic eruption within this area are controversial. This paper applied high-precision laser 40Ar/39Ar dating to the main volcanic units in the Tengchong area and obtained ages in the range of 0.025~5.1 Ma using conventional data processing methods. But the dating of young volcanic rocks is very difficult and requires high precision. Any tiny factors, such as very low radiogenic 40Ar content, inconsistency in the initial 40Ar/36Ar ratios, instrumental precision levels, errors from preparation and measurement methods, contribute to great age deviations. Moreover, lacking a unified timescale, conventional methods were unable to strictly define the stages of the Tengchong volcanic eruptions. Therefore, there is such great uncertainty related to using a single age as an eruption age in the absolute chronological study of volcanic eruptions; it can even be misleading.
To solve the above issues, researchers applied a Gaussian mathematical model to deal with all 378 original ages from 13 samples (Figure 1). An apparent age-probability diagram, consisting of three independent waveforms, have been obtained. The corresponding isochron ages of these three waveforms suggest there were three volcanic eruptive stages, namely during the Pliocene 3.78±0.04 Ma, early Middle Pleistocene 0.63±0.03 Ma and late Middle Pleistocene to early Late Pleistocene 0.139±0.005 Ma. The new method for identifying stages proposed in this paper within the same timescale has the advantages of controlling variables, reducing deviations, approaching the true age value, weakening the influence of subjective factors and precisely defining the eruptive stages in young volcanic rocks. In addition, by increasing the number of testing data, the final result will be more representative of the real volcanic history, with reproducible and verifiable eruptive stages.
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This research was funded by the Geological Survey Project of the China Geological Survey Bureau (Grant No.1212113013700).
See the article: http://engine.scichina.com/publisher/scp/journal/SCES/63/5/10.1007/s11430-019-9569-9?slug=fulltext
Journal
Science China Earth Sciences