sentence: The youngest homogeneous population of sanidine crystals was calculated when the weighted mean age of these crystals has the following statistical characteristics : Mean Square Weighted Deviation ( MSWD ) < 1.5 , Probability of fit > 0.1.The probability diagram obtained for the crystals from the U4T tephra layer is simple , dominated by primary crystals ( Fig 4 ) allowing us to calculate a precise weighted mean age of 586 ± 1 ka ( 1 σ analytical uncertainty , J = 0.00035760 + / - 0.00000122 ) .