With ( 87 Sr/ 87 Rb) Sr,initial,corrected being the true 87 Sr/ 87 Rb ratios corrected for the initial Sr-isotope composition ( 87 Sr/ 86 Sr) initial . To calculate ( 87 Sr/ 87 Rb) Sr,initial,corrected a "ratio of means" approach is applied. To obtain the random uncertainty (s random ) on each data point, the standard error of ( 87 Sr/ 86 Sr) true and ( 87 Rb/ 86 Sr) ratios (denoted s m ) and the excess variance (e) were added in quadrature.