TRACE-ELEMENT ANALYSIS OF DETRITAL APATITE

Apatite is almost always present in crystalline rocks, as it is usually the only mineral capable of incorporating a significant proportion of the whole rock P-budget. As a result apatite crystallises in rocks with vastly different chemistries. Reflecting this, the trace-element composition of apatite is inherently diverse.

We have assembled a database of published apatite trace element compositions from the literature. These LA-ICPMS data comprise c. 360 separate bedrock samples from all the common lithologies present on the Earth’s surface. When data from this database are plotted on biplots we are capable of reliably separating different apatite lithological classes using a machine-learning classifier. This in turn enables the determination of general source-rock type for detrital apatite, which permits very specific provenance determinations when combined with detrital apatite U-Pb dating. These provenance determinations are further aided by the fact that apatite is a relatively labile mineral, and thus non-diagnostic multi-cyclic detritus is unlikely to comprise a significant portion of a detrital apatite dataset.

We demonstrate the utility of combined trace element and U-Pb analysis of detrital apatite in three provenance case studies. In the first study, from modern sediment in central France, we prove the utility of this methodology in a well-controlled natural laboratory, wherein the simple arrangement of lithologies permits the tracing of almost every apatite grain back to a specific source. In the second study, of modern sediment sampling a more complex array of lithologies in the French Broad River of North Carolina, detrital apatite trace element composition and U-Pb age can be reliably linked to the portion of the Appalachian orogen sampled. In the third study, of c. 1 Ga detritus in NW Scotland, the U-Pb ages of apatite of different composition record different events. This allows us to de-convolute broad apatite U-Pb peaks on kernel density (KDE) plots into source-diagnostic KDEs that are specific to each lithology, which can be used to constrain the separate phases of an orogenic cycle recorded in those sediments.