ISOCHRONS REVISITED: A NEW APPROACH TO DEAL WITH EXCESS SCATTER

The isochron technique for dating geologic events has lost favor in the past few decades, in part due to the realization that many samples do not behave in an isochronous manner and often show excess scatter around a regression line. Here we introduce a technique to deal with excess scatter in isochron space by assuming that this scatter is due to mixing of multiple discrete geological events that might be resolved from one another. We provide a new model to mathematically determine the mixture of ages and initial daughter isotope compositions that may be present within a dataset without any prior knowledge of the number of mixtures. Additionally, and crucially, our model reports the proportion of each sample that belongs to each of the modeled isochrons allowing the user to return to the samples and independently test the results with other techniques. We demonstrate our new model using some synthetic datasets to show that it can successfully determine the number of isochrons in a complex mixture as well as output their ages and initial daughter isotope compositions. We further validate our approach using two previously published datasets, one using the ¹⁸⁷Re-¹⁸⁷Os system in bitumen, and the other using the ⁸⁷Rb-⁸⁷Sr system in chondritic meteorites. We discuss the implications of treating these scattered datasets as complex mixtures for the formation of bitumen and solar system evolution, respectively.