EXTRACTION TO ERUPTION: COMPARING THE CRUSTAL RESIDENCE OF LARGE-VOLUME MAGMAS AT THE TAUPŌ VOLCANIC CENTER (NEW ZEALAND) (Invited Presentation)

The 25.5 ka, 530 km³ Oruanui and 1.72 ka, 35 km³ Taupō eruptions (Taupō Volcanic Center [TVC], New Zealand) are two of the most recent silicic caldera-forming eruptions known on Earth, occurred in one of the most volcanically active regions of the world, and are the largest volumes and most evolved magmas to erupt from the TVC. They are a well-suited pair of eruptions to study in tandem, as aspects of their products suggest similarities and differences in the crustal residence of the magmas. This offers a rare opportunity for direct comparison studies of caldera-forming eruptions from the same volcanic center.

We use rhyolite-MELTS geobarometry, mineral compositions, and 3D textures of pumice clasts to assess the longevity (phenocryst textures, diffusion chronometry) and conditions of extraction (whole-pumice extraction pressures), storage (glass crystallization pressures), and eruption (microlite textures) of the eruptible magmas in these systems.

We reach several important results and conclusions:

1. The longevity of the eruptible Oruanui and Taupō magmas was similarly short (10²-10³ a) despite the large difference in their erupted volumes. This suggests eruption size is not a function of residence time for large-volume systems.

2. Rhyolite-MELTS geobarometry suggests both systems were extracted from a relatively oxidized source crystallizing plagioclase+orthopyroxene±quartz, but the eruptible magmas were stored at notably different crustal depths. The Taupō magma was extracted from its source at depth (320-375 MPa), stored at nearly the same level (275-360 MPa), and erupted with little-to-no shallow residence. The Oruanui magma was extracted over a wide range of pressures (250-380 MPa), extending to the storage and extraction depths of the Taupō system, but it incurred storage at much shallower depths (50-200 MPa). Our results are consistent with the fact that Oruanui glass and whole-pumice compositions vary widely, while those of Taupō products are tightly constrained (<1 wt. % variation in SiO₂).

3. Microlites are absent in Oruanui pumice clasts but abundant in Taupō samples. This likely reflects differences in the eruptive process for the two systems due to differences in the storage depths, ascent times, ascent rates, and/or ascent paths (e.g., stalling) of the magmas.

This work adds to our understanding of silicic magmatism and the structure of the crust at the TVC, and it highlights the power in combining information from multiple methodologies.