From mush to eruption in centuries: assembly of the super-sized Oruanui magma body

摘要

The magmatic systems that give rise to volu minous crystal-poor rhyolite magma bodies can be con sidered to operate on two contrasting timescales: Those governed by longer-term processes by which a magma acquires its chemical and isotopic characteristics (e.g., fractional crystallisation and assimilation), and those operating at shorter timescales during the physical accu mulation of the melt-dominant magma body that finally erupts. We explore the compositional and textural rela tionships between amphibole and orthopyroxene crystals from the 25.4 ka, 530 km~3 (magma) Oruanui eruption products (Taupo volcano, New Zealand) to investigate how processes related to the physical assembly of the pre emptive magma body are represented in the crystal record. Over 90 % of orthopyroxenes from the volumetrically dominant high-SiO_2 (>74 wt%) rhyolite pumices record textural evidence for a significant disequilibrium event(partial dissolution ± resorption of cores and interiors) prior to the growth of 40-500 urn thick rim zones. This dissolution/regrowth history of orthopyroxene is recorded in the chemistry of co-crystallising amphiboles as a prominent inflection in the concentrations of Mn and Zn, two elements notably enriched in orthopyroxene relative to amphibole. Textural and chemical features, linked with in situ thermobarometric estimates, indicate that a major decompression event preceded the formation of the melt-dominant body. The decompression event is inferred to represent the extraction of large volumes of melt plus crystals from the Oruanui crystal mush/source zone at pressures of 140-300 MPa (～6-12 km depth). Orthopy roxene underwent partial dissolution during ascent before reestablishing in the melt-dominant magma body at pres sures of 90-140 MPa (～ 3.5-6 km). We model Fe-Mg diffusion across the core-rim boundaries along the crys-tallographic a or b-axes to constrain the timing of this decompression event, which marked establishment of the melt-dominant magma body. Maximum modelled ages indicate that this event did not begin until ～ 1,600 years before eruption, consistent with constraints from zircon model-age spectra. Once extraction began, it underwent runaway acceleration with a peak extraction age of ～230 years, followed by an apparent period of stasis of ～60 years prior to eruption. The rapidity of the extraction and accumulation processes implies the involvement of a dynamic driving force which, in the rifted continental arc setting of the Taupo Volcanic Zone, seems likely to be represented by magma-assisted extensional tectonic processes.