MSWD and n Value in Laser Ablation 40Ar/39ar Isochron Dating for Young Volcanic Rocks

HU Yan†, JI Jianqing, ZHOU Jing, TU Jiyao

The Key Laboratory of Orogenic Belts and Crustal Evolution (MOE), School of Earth and Space Sciences, Peking University, Beijing 100871; † E-mail: yan_hu@hotmail.com

Abstract The laser ablation 40Ar/39ar method makes it possible to date quanterary volcanic rocks precisely. So far, it lacks an assessment criterion that can be used to estimate the reliability of test data and geochronological information. The mean square weighted deviation (MSWD) and the number of measurement (n value) are used to be the crucial parameter to evaluate sample properties and experimental standard. When applying laser ablation 40Ar/39ar method to date young volcanic rocks, a best-fit 40Ar/39ar isochron diagram requires isotopic homogeneity of the sample and permanent closure of the isotope system at the same time. While MSWD, which is related to n value, is a parameter to determine whether the data are located in the same normal distribution, or in other words, whether the sample has a unified true age, it is applicable to use MSWD and n value to estimate the level of confidence placed on the computed mean. The critical values of MSWD for the 99.7% confidence interval using differing degrees of freedom are calculated in this paper. It is important to note that MSWD is available in evaluating dating result of both apparent and isochron ages. For the samples which tend to have heterogeneous initial 40Ar/36ar ratios or different closure times, MSWD combined with the probability density of apparent ages can be used to distinguish different groups of samples, calculate the isochron ages respectively, and interpret the