CONSTRAINING THE AGE AND DEPOSITIONAL CHARACTERISTICS OF THE SAUNDERS ASH, MT. TARANAKI, NEW ZEALAND

The Saunders Ash is pyroclastic density current (PDC) unit located on the NW flank of Mt. Taranaki, New Zealand, for which the precise timing and methods of emplacement remain to be determined. The deposit is believed to represent an unusually explosive eruption as one of a rare set of long-runout PDCs in the region, however the position of the Saunders Ash in the regional stratigraphy as well as the timing and relation to volcanic collapse events is uncertain.

New radiocarbon dating has been accomplished to improve upon the previously existing debated age of the Saunders Ash. Two new radiocarbon ages indicate that original age estimates were too young, leading to the ability to pinpoint the deposit more accurately within regional stratigraphy through reevaluation and recalibration of existing ¹⁴C ages.

Focus has been placed on plagioclase crystals present throughout the Saunders Ash sample suite, as the slow diffusion of major elements due to coupled substitution make it a strong recorder of magmatic processes. SEM investigations of individual grain mounted plagioclase crystals were conducted to determine if chemical and/or mineralogical temporal variations occurred spanning the ~3.5 m thick outcrop located 13 km from source.

SEM back scatter imagery results indicate that crystal textures which preserve evidence of multiple dissolution/growth episodes, vary throughout the section. Crystals were visually sorted into four categories based on specific rim/core, oscillatory zoning, sieved texture, and dissolution texture characteristics. SEM imagery will be paired with EMP analysis to quantify anorthite variations across representative crystals of each textural type and determine minor element (Fe,Mg) contents. Transects will be utilized to further understand crystal chemical/textural variation to determine potential changes in magmatic plumbing system and eruptive characteristics.

This work aims to contribute to the effort of understanding the details and progressive development of the Mt. Taranaki magmatic plumbing system to aid in forecasting future potential eruptive scenarios.