An Integrated Apatite Geochronology and Geochemistry Tool for Sedimentary Provenance Analysis

Sediment provenance studies utilizing detrital geochronology are often limited by source-rock non-uniqueness with respect to mineral age. Thus, the development of integrated techniques which permit identification of source rock age and lithology are highly desirable. In this case study from the southern Massif Central, France, we target modern-river sediment from the river Tarn to assess the utility of combined U-Pb and multiple trace-element geochemical analysis of detrital apatite as a provenance tool. The study area was chosen because the sediment source areas chiefly comprise a relatively simple mix of medium- to low-grade Variscan metasediments and late Variscan granitoids, which should yield detrital apatite readily distinguishable by age, trace-element chemistry, or both. Based on comparison with previously published apatite trace-element data from metasedimentary rocks and granitoids, pelitic apatite in the river Tarn detritus is primarily distinguished by high Sr/Mn, light rare-earth element depletion (LREE) and low actinide contents, whereas granitic apatite is characterized by much lower Sr/Mn, and high LREE and actinide abundances. These source rock determinations are highly consistent with apatite trace-element data from Tarn tributaries that drain either predominantly metapelitic or granitoid catchments. U-Pb analysis of detrital rutile was also undertaken on those catchments for comparative purposes. As pelitic and granitic apatite can be readily distinguished, samples that have experienced downstream sediment mixing can then be comprehensively characterized. Using this method we identify a source lithology for nearly every analyzed apatite grain in the river sediment, even though 59% of the analyzed grains do not yield reliable U-Pb ages. Plain Language Summary Determining the origin of the detritus that makes up sediments is critically important to the understanding of ancient sediment-pathways, and therefore past tectonic and mountain-building events, as the rocks from which sediments are derived are by definition no longer in existence. These determinations are complicated, however, as detritus in sediments may often provide ambiguous (i.e., non-unique) answers as to the possible original source. The tool investigated here involves combining dating by U-Pb with trace-element fingerprinting of apatite, a common accessory mineral in a compositionally wide range of igneous and metamorphic source rocks, and sediments derived from them. Our results demonstrate that this tool is able to give highly-specific determinations as to the origin of single grains; including information on the rock-type, the grain age, or in many cases both. This tool will therefore be able to expand the range of information geologists are able to extract from ancient sedimentary rocks.