Utility of trace elements in apatite for discrimination and correlation of Quaternary ignimbrites and co-ignimbrite ashes, Japan

Apatite is a common accessory mineral in intermediate and felsic igneous rocks. Because apatite has a wide range of trace-element compositions as well as strong resistance to diagenetic alteration, the trace-element composition of apatite has been used for tracing petrogenetic processes of plutonic bodies as well as the tephrochronology of Paleozoic tephras. However, it has been little used in Quaternary volcaniclastic deposits. Our studies of apatite trace-element compositions in representative Quaternary ignimbrites and their corresponding co-ignimbrite ashes in Japan demonstrated that such compositions are not affected by welding and can be successfully used for discrimination of, and correlation between, tephras and ignimbrites. We also could distinguish different tephra deposits and ignimbrites from the same caldera. Although most apatite trace-element compositions did not change throughout the succession of ejecta from a single eruption, the Fe contents in apatite varied across stratigraphic horizons. Consequently, we were able to correlate the Fe contents of apatite phenocrysts in specific ignimbrite horizons and their corresponding co-ignimbrite ashes and thus identify the phase of the eruption during which the major part of a co-ignimbrite ash was released.