Trace elements in igneous apatite: A case study of redox tracers in felsic plutons

Apatite, a common accessory mineral in igneous rocks, can incorporate various trace elements, making it a powerful petrogenetic indicator. The abundances of redox-sensitive elements in apatite offer a valuable probe for assessing magmatic oxygen fugacity (fO₂). Previous studies have explored the dependence of S, V, Mn, Fe, As, and Eu concentrations in apatite on magmatic fO₂. While the use of S in apatite as a redox proxy is well-established, the reliability of other elements for estimating magmatic fO₂ remains controversial.

In this study, we present new geochemical data for apatite grains separated from Miocene granitoids in Japan, analyzed using laser ablation-inductively coupled plasma-tandem mass spectrometry (LA-ICP-MS/MS). Based on the iron content in plagioclase, the analyzed plutons are classified into oxidized and reduced types. Apatites crystallized in oxidized magmas are enriched in S (> 80 μg/g) and V (> 6 μg/g), whereas those in reduced magmas had low S (< 80 μg/g) and V (< 5 μg/g) contents, irrespective of their crystallization timing within the magma. This enrichment clearly reflects the fO₂ of their parental magmas. Conversely, the Mn and Fe contents of apatite crystallized in oxidized magmas overlap with those in reduced magmas. The apparent partition coefficients of Mn and Fe between apatite and melt are strongly correlated with the ratio of non-bridging oxygen to tetrahedrally coordinated cations in the host rocks, rather than with fO₂. Thus, Mn and Fe contents in apatite do not record magmatic fO₂. The As content in apatite from oxidized magmas is generally higher (typically >1 μg/g) than that in apatite from reduced magmas; however, some overlaps exist between the two types. As a result, arsenic in apatite is not a reliable oxybarometer for reasons that remain unclear. Based on our dataset, only sulfur and vanadium in apatite record magmatic ƒO₂.