This interval was held constant for each individual spot during a particular session. No cut-off was needed during line scan analyses. Calibration factors are derived based on replicate line scan analyses on NIST610 glass to correct for mass bias. "Raw" isotope ratios (e.g. ~ 87 Sr/~8 8 Sr and ~85 Rb/~8 8 Sr) were transferred into "true" isotope ratios (e.g. 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios by applying the following and analogous equations: