The trace element and U-Pb systematics of metamorphic apatite and its application in provenance studies

Abstract

Apatite is a common accessory mineral in igneous, sedimentary and metamorphic rocks. It has potential as a provenance indicator in clastic sedimentary systems, as it can host a wide variety of trace elements in its crystal structure and can yield thermochronological age information. However, the processes controlling the trace element and U-Pb systematics of metamorphic apatite remain poorly understood, and metamorphic apatite remains significantly under-represented in compositional provenance databases linking apatite trace-element chemistry to its corresponding parent rock type. In this thesis, I investigate the trace-element and U-Pb systematics of metamorphic apatite from a suite of twenty-two bedrock samples of diverse metamorphic grade and protolith type, sampled from a variety of metamorphic terranes (Chapter 2); nineteen samples of metapelites and calcareous metapelites from two progressive metamorphic sequences from the Central Alps (Chapter 3); and a modern river sediment sample from the Central Alps which sourced a variety of metamorphic units (Chapter 4). U-Pb and trace element data were acquired from apatite mineral separates and for some samples in situ on thin sections by LA-Q-ICPMS. The trace element data were explored using chondrite- and whole-rock normalized multi-element plots, PCA plots and SVM biplots. The results show that metamorphic apatite from low- to medium-grade metapelites and metabasites can be easily distinguished from all types of igneous apatite as it is significantly depleted in Th, REE, and Y. Depletion in Th and REE+Y is attributed to growth of co-genetic epidote, which is the dominant carrier phase of the REE+Y, Th, and U in all the low- to medium-grade samples. LA- Q-ICP-MS imaging demonstrates that low REE+Y, Th, and U metamorphic apatite rims can nucleate on detrital igneous apatite precursors characterised by high REE+Y, Th, and U. With increasing metamorphic grade, 1) relict detrital apatite is consumed, 2) the coherence of the U- Pb concordia systematics and the age precision improve, and 3) the degree of dispersion on metamorphic apatite multi-element plots decreases. These results can be readily employed in provenance studies as demonstrated in Chapter 4 of this thesis, where metamorphic apatite detritus from a modern river sediment sample from the Alps was classified by both U-Pb age and rock type information, using apatite trace elements as a lithology classification tool.