USING PYROCLASTIC TEXTURES TO UNDERSTAND DYNAMIC ERUPTION PROCESSES IN THE AUCKLAND VOLCANIC FIELD, NEW ZEALAND

During the transition from phreatomagmatic to predominately magmatic eruptions, the alleviation of external water from the process changes the eruption style. Characteristics resulting from these transitions can be observed by the textures within the erupted pyroclasts. Groundmass from juvenile pyroclasts collected from phreatomagmatic and magmatic deposits from Motukorea (Brown’s Island), and Maungauika (North Head volcano), New Zealand, can yield insight into the availability of external water, complexity of eruption processes, and changing eruptions styles.

We have specifically targeted a limited number of surge/fall deposits for analysis which track changing interactions with external water to test the impact on clast characteristics. Sample cleaning and separation is particularly challenging for surge deposits where materials are well cemented, requiring multiple round of ultrasonic cleaning, scraping, and treatment in weak HCl solutions- this intricate cleaning procedure however is necessary for isolation of the juvenile clasts for analysis. Juvenile clasts were characterized texturally and analysed by X-ray diffraction (XRD) to determine mineralogy and crystallinity. XRD diffraction patterns were analysed using the AMORPH software for determining the crystallinity and characteristics of the amorphous material. Sample sizes range from <250 micrometers to ~ 2cm. Multiple clasts were analysed per sample (up to 15) to better characterize the heterogeneity of each sample. All analyses for both Motukorea and North Head indicate that total crystallinity values for North Head (30% to 55%) are lower than those from Motukorea (60% to 85%). Higher crystallinities observed in Motukorea explosive juvenile material may be explained by differences in eruption styles and their respective processes, including variations in the amount of external water-magma interaction, magma ascent rate, and/or degassing histories.