Mixed glass compositions in the ~230 AD Taupō Tephra: Multiple magma types or particle recycling from earlier deposits?

https://cot.iavceivolcano.org/first-circular-cot-intimate-thm-2024-international-conference-2-2/(external link)

Abstract:

The c. 230 AD Taupō Tephra is a widespread marker bed in terrestrial, lacustrine and marine deposits around the North Island of New Zealand. The eruption comprised six phases, generating a characteristic and distinctive suite of fall deposits and ignimbrite. In proximal deposits, crystal-poor juvenile pumices from all fall and ignimbrite units display a very narrow chemical compositional range throughout the eruption sequence. These compositions are distinct from older eruption units, including the 25.5 ka Oruanui supereruption, having lower SiO2, (73.5–75 wt %) with restricted mineralogy and reverse crystal zonation, suggesting a hotter and less evolved magma source. Distal tephra deposits, however, variably contain glass shards that span a wider range of compositions, from ~74 to 79 wt% SiO2, with a distinct cluster at 78 ± 0.7 wt % SiO2. This observation raises the question: did the Taupō eruption actually have multiple, more evolved magma types, not represented in single pumice populations?

Here we investigate the relative content and origins of higher- SiO2 glass in the Taupo Tephra from different depositional settings and geographic locations around the North Island. We infer that the published Taupō Tephra glass field contains a significant proportion of glass shards recycled from earlier eruptions and, in particular, the 25.5 ka Oruanui supereruption that was sourced from a broad vent region within which lie the 230 AD Taupō vent sites. The recycling of glass shards in the 230 AD Taupō event was facilitated by the fine-grained nature of the phreatomagmatic Oruanui eruption deposits and their kilometre-scale intracaldera thicknesses, coupled with the outstandingly explosive nature of the 230 AD Taupō eruption. The mixed modal compositions found in documented locations (including Antarctica), represent a unique double fingerprint for this event and highlight that caution must be taken when interpreting compositional fields and magmatic processes from distal tephra deposits.