September 22, 2026 - 8AM, 231 Trowbridge Hall (TH)
The general audience will be invited to leave after the presentation and Q & A session.
Title: Eruptive Chronology and Plagioclase Geochemistry of the Saunders Ash, Mt. Taranaki, New Zealand
Abstract: The Saunders Ash is an anomalously thick volcanic deposit preserved more than 12 km from its inferred source at Mt. Taranaki, New Zealand. Its age, eruptive origin and relationship to Taranaki’s evolving magmatic plumbing system have remained uncertain. Stratigraphy, radiocarbon dating and componentry analysis were used to establish the age and eruptive character of the deposit. Plagioclase textures and compositions were investigated using backscattered electron (BSE) imaging, and electron microprobe (EMP) analysis, together with in situ laser ablation multi collector inductively coupled mass spectrometry (LA-MC-ICP-MS) Sr-isotope analysis. In situ laser ablation inductively coupled mass spectrometry (LA-ICP-MS) was used to characterize anorthite and trace-element variability within plagioclase domains and across high-resolution rim transects, while solution ICP-MS provided major and trace-element compositions of lithic fractions from the Saunders Ash.
New radiocarbon ages place the Saunders Ash eruption at approximately 21.6 ka cal BP and, together with componentry and stratigraphic relationships demonstrate that it was distinct from the older Poto Tephras and major sector collapse events at Mt. Taranaki. Plagioclase textures, major and trace-element compositions and Sr-isotope ratios reveal a diverse crystal cargo assembled through magma storage, recycling, recharge and partial re-equilibration within a heterogeneous magmatic system. Lithic geochemistry further records compositional variability related to different mineral components incorporated into the eruption. Comparison with younger Taranaki eruptive products indicates that the dominant Sr-isotopic and broader geochemical characteristics of the magmatic system were already present by the time of the Saunders Ash eruption and persisted through construction of the modern edifice (<14 ka). Together, these results refine the late-Pleistocene eruptive history of Mt. Taranaki, provide insight into the long-term evolution of the magmatic plumbing system and demonstrate that Mt. Taranaki can generate laterally extensive pyroclastic density currents independent of sector collapse.