The trace element composition of apatite and its application to detrital provenance studies

Abstract

Apatite's ubiquity in crystalline rocks, variable trace element contents (particularly with regard to the REE, actinides and Sr), and amenability to various dating techniques based on the decay of the radioisotopes U and Th, permit specific provenance determinations. In this study, we first present a comprehensive description of the trace element behaviour of apatite in various kinds of bedrocks (igneous rocks from felsic through to ultramafic compositions, metamorphic rocks from low to high grades and of diverse protolith composition, and authigenic apatite) in which we explain why apatite is so highly diverse in terms of its trace element composition. Next, we present a synthesis of bedrock apatite trace-element compositional data from previous work, assembling a library of apatite compositions that includes the most abundant apatite-bearing lithologies in the Earth's crust, and many other less abundant rock types. Compositional statistics, classification, and a machine learning classifier are then applied to this dataset to generate biplots that can be used to determine the broad source lithology of detrital apatite, with misclassification averaging 15%. This methodology is tested in three case studies to demonstrate its utility. In these examples, detrital apatite can be convincingly linked to different lithology types, and combined apatite trace-element and UPb data can determine the terranes from which individual apatites were likely derived. The addition of apatite trace-element information therefore enables the determination of the source lithology, making the extraction of novel information and more specific provenance determinations possible, and opening up new avenues in source-to-sink modelling.