Abstract
We compare quartz zonation and diffusion timescales of crystal-rich rhyolitic ignimbrites and crystal-poor rhyolitic lava flows from the Jurassic Chon Aike Province as exposed in Patagonia (Argentina). The timescales are assessed by using diffusion modelling based on nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of titanium (Ti) concentration profiles in quartz crystals oriented by image analysis using micro-tomography. Quantitative Ti-data were acquired by SIMS to estimate crystallization temperatures. The textural and geochemical analysis revealed clear differences between crystal-poor rhyolitic lava flows and crystal-rich rhyolitic ignimbrites. Quartz crystals from rhyolitic lava flows display simple oscillatory cathodoluminescence (CL) zoning interpreted to be magmatic and diffusion chronometry suggest a short timescale for quartz crystallization from 5.6 ± 2.2 yr to 41.6 ± 9.8 yr. Resorption textures are rare, and hence crystals in rhyolitic lava flows recorded a simple, rapid extraction, transport and eruption history for these crystal-poor melts. Rhyolitic ignimbrites, in contrast, reveal complex zoning patterns, reflecting several episodes of partial resorption and growth throughout their crystallization history. The complex quartz zoning textures together with longer diffusion times (< 350 yr), rather suggest a storage in a mush with fluctuating pressure and temperature conditions leading to intermittent resorption. Yet, a final quartz overgrowth rim occurred over a much shorter timescale in the order of years (< 3 yr), which implies that crystal-rich ignimbrites can be re-mobilized very fast.
Highlights
The understanding of silicic volcanic systems is constantly evolving
The majority of studies used either grayscale profiles, which were acquired from backscattered secondary electron (BSE) and cathodoluminescence (CL) images, or element profiles measured by electron microprobe (EPMA)
The intriguing results of these recent studies are the short timescale they report with the implication that large volumes of silicic melt may accumulate over a short period of time, in centuries or less, and that late stage crystallization immediately prior to eruption may occur at even shorter timescale of decades, years or months (Turner and Costa, 2007)
Summary
The understanding of silicic volcanic systems is constantly evolving. many important questions as to the nature of crustal magma storage, including magma chamber geometry and dynamics, the “trigger” of large eruptions and the time of magma accumulation before eruption are still under discussion. Numerical models predict different timescales on which crystal-rich mushes and crystal-poor magmas can be erupted (Huber et al, 2012), e.g., 100–1,000 years for crystalrich mushes and a few years for crystal-poor magmas These short times cannot be resolved with conventional, absolute dating techniques at present, despite the remarkable advances made in the last decade (Michel et al, 2008; Leuthold et al, 2012; Barboni and Schoene, 2014; Wotzlaw et al, 2014). Studies from the Taupo Volcanic Zone (New Zealand) report slightly shorter timescales of 10–85 years (Ti diffusion in quartz, Saunders et al, 2010; Matthews et al, 2012) for the magma recharge event before eruption. That the advances in new analytical techniques with a sub-micrometer spatial resolution, e.g., NanoSIMS, TOF-SIMS, and FEG-EPMA are crucial to these studies (e.g., Hellebrand et al, 2005; Charlier et al, 2012; Lloyd et al, 2014; Saunders et al, 2014; Ferry et al, 2015; Till et al, 2015; Seitz et al, 2016)
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