What event occurred at 0 years on a horizontal axis on a graph of the decay of strontium go in an igneus rock?

At 0 years on a horizontal axis of a graph depicting the decay of strontium in igneous rock, the concentration of strontium-87 (the stable isotope) would represent the initial amount present in the rock. This point marks the starting measurement before any decay has occurred, establishing a baseline for subsequent measurements of strontium-87 and strontium-86 isotopes over time as radioactive parent isotopes decay into daughter isotopes.