Precambrian to Pleistocene 40Ar/39Ar dating of clinopyroxene-hosted melt inclusions

Clinopyroxene is a rock-forming mineral that commonly hosts melt inclusions in mafic to intermediate composition volcanic and plutonic rocks. It is highly resistant to alteration compared to other co-existing phenocrysts such as plagioclase. Several recent studies have Ar-40/Ar-39 dated clinopyroxene in Neoproterozoic to Miocene basalts and dolerites. To assess the viability of the technique at the youngest end of the geologic time scale, we performed Ar-40/Ar-39 incremental heating experiments on clinopyroxene-hosted melt inclusions from a variety of mafic lithologies and tectonic settings. Most samples produced precise plateau ages including several Quaternary basalts to andesites as young as 0.6 Ma. All data are indistinguishable from new and/or published Ar-40/Ar-39 ages on groundmass or plagioclase from the same samples. The source of potassium (K) and resulting Ar-40* within clinopyroxene has been debated, but thus far has only been inferred based on Ar-40/(3)9Ar data. Using electron probe microanalysis (EPMA) we show that there is negligible K in the clinopyroxene host, but substantial K (e.g., 1-4 wt%) in trapped melt inclusions and minor amounts in plagioclase inclusions. Thus, melt inclusions, which are common in phenocrysts in basaltic magmas, can be used to obtain accurate and precise Ar-40/Ar-39 ages for difficult-to-date volcanic and plutonic rocks from the Precambrian to the Pleistocene.