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

Abstract

Combined U-Pb-Hf-O isotopic analyses of zircons are useful probes to investigate the origin and temporal evolution 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 Quaternary. The new isotopic data differ slightly within and between eruptive units, with the majority of zircons having ε Hf values between −8 to −5 and δ 18 O values between 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 δ 18 O values varying from ~6 to 8, with low ε Hf values extending to −14, indicating shallow-level crustal contamination. The largely negative correlations between ε Hf and δ 18 O for zircons from the MTT is indicative of contamination of high-δ 18 O crustal materials, whereas those of the positive correlations for zircons from the Youngest Toba Tuff (YTT) implies contamination of hydrothermally altered low-δ 18 O 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-δ 18 O or high-δ 18 O materials during their ascent to the surface or storage in the shallow magma reservoirs. The increase of zircon δ 18 O values towards eruptions in both the MTT and YTT suggests that the two eruptions may be accompanied by roof collapse that added high-δ 18 O materials to the magma. Thus, assimilation of high-δ 18 O crustal materials could be a possible trigger for the MTT and YTT eruptions. • Toba volcanic rocks were derived from an isotopically enriched and heterogenous mantle source with significant crustal contamination. • Hf-O isotopes of MTT zircon crystals exhibit characteristics of shallow-level crustal contamination. • The YTT magma reservoir was contaminated by both high- and low-δ 18 O crustal materials. • Assimilation of high-δ 18 O materials by roof collapse could be a likely eruption trigger for the MTT and YTT eruptions.