Zircon Hf-O isotopic constraints on the origin and temporal evolution of the Toba volcanic system, Indonesia

Combined U-Pb-Hf-O isotopic analyses of zircons are useful probes to investigate the origin and temporal evo-lution of long-lived magma reservoirs feeding large eruptions. This study presents a complete dataset of zircon Hf-O isotopes of the latest five Toba eruptions in the Quaternary. The new isotopic data differ slightly within and between eruptive units, with the majority of zircons having epsilon Hf values between-8 to-5 and 818O values be-tween 6 and 6.8. These Hf-O isotopic compositions of zircons were considered to inherit from an enriched and slightly heterogenous magma source. Specifically, zircons of the Oldest Toba Tuff (OTT) have rather restricted and constant Hf-O isotopes, which may crystallize from a reservoir protractedly supplied from the same magma source. Zircons of the Middle Toba Tuff (MTT) have a large range of 818O values varying from-6 to 8, with low epsilon Hf values extending to-14, indicating shallow-level crustal contamination. The largely negative correlations between epsilon Hf and 818O for zircons from the MTT are indicative of contamination of high-818O crustal materials, whereas those of the positive correlations for zircons from the Youngest Toba Tuff (YTT) imply contamination of hydrothermally altered low-818O crustal materials. It is important that the Hf-O isotopes of all the zircons do not record any single batch of magma that was derived from partial melting of depleted mantle peridotites, but are consistent with an enriched and heterogenous mantle source hybridized by the subducted sediments, followed by contamination of low-818O or high-818O materials during their ascent to the surface or storage in the shallow magma reservoirs. The increase of zircon 818O values towards eruptions in both the MTT and YTT suggests that the two eruptions might be accompanied by roof collapse that added high-818O materials to the magma. Thus, assimilation of high-818O crustal materials could be a possible trigger for the MTT and YTT eruptions.