Magma reservoir dynamics at Toba caldera, Indonesia, recorded by oxygen isotope zoning in quartz

Abstract

Quartz is a common phase in high-silica igneous rocks and is resistant to post-eruptive alteration, thus offering a reliable record of magmatic processes in silicic magma systems. Here we employ the 75 ka Toba super-eruption as a case study to show that quartz can resolve late-stage temporal changes in magmatic δ18O values. Overall, Toba quartz crystals exhibit comparatively high δ18O values, up to 10.2‰, due to magma residence within, and assimilation of, local granite basement. However, some 40% of the analysed quartz crystals display a decrease in δ18O values in outermost growth zones compared to their cores, with values as low as 6.7‰ (maximum ∆core−rim = 1.8‰). These lower values are consistent with the limited zircon record available for Toba, and the crystallisation history of Toba quartz traces an influx of a low-δ18O component into the magma reservoir just prior to eruption. Here we argue that this late-stage low-δ18O component is derived from hydrothermally-altered roof material. Our study demonstrates that quartz isotope stratigraphy can resolve magmatic events that may remain undetected by whole-rock or zircon isotope studies, and that assimilation of altered roof material may represent a viable eruption trigger in large Toba-style magmatic systems.

Isotopic and Petrologic Investigation, and a Thermomechanical Model of Genesis of Large-Volume Rhyolites in Arc Environments: Karymshina Volcanic Complex, Kamchatka, Russia

“…Zirconium concentrations in whole rocks and computed saturation temperatures using the Watson and Harrison (1983) formulation show a slight decrease with time, and are relatively low for all rocks: from 804 ± 20 • C for precaldera magmas, to 771 ± 12 • C in the 1.78 Ma ignimbrites, to 779 ± 20 • C in post caldera intrusions. Given the presence of amphibole and biotite in most magmas and crystal contents in 30-45% range, this agrees with the "cold and wet" magma definition pertinent to continental volcanic arcs (Miller et al, 2003;Loewen and Bindeman, 2016), and matches phase equilibria near-wet solidus conditions described for such systems worldwide, for example in Toba caldera in Indonesia (Chesner, 1998;Budd et al, 2017) or the intracontinental cold and wet Fish Canyon Tuff of the western United States (Bachmann and Bergantz, 2004), which has both oxygen isotopic and zircon saturation temperatures of 750 • C.…”

“…These authors interpreted zircon diversity as reflecting assembly of diversein-δ 18 O magma batches in petrogenesis of the Karymshina ignimbrite. Such processes are much more prominent in hot and dry intraplate low-δ 18 O rhyolites elsewhere in the world, and are somewhat more muted in cold and wet arc magmas such as the Fish Canyon and Toba Tuffs (Bindeman and Simakin, 2014;Colón et al, 2015Colón et al, , 2018cBudd et al, 2017 (Table 3), indicative of a predominance of depleted mantle-derived material within the magma (Figures 7, 8). The range of values overlaps with isotopic compositions for neighboring Late Pleistocene-Holocene volcanoes and other volcanoes from the Eastern Volcanic Zone of Kamchatka (Hawkesworth et al, 1997;Churikova et al, 2001;Duggen et al, 2007;Seligman et al, 2014;Simon et al, 2014), with the surprising exception of very radiogenic Nd isotopes in three studied post-caldera high-silica rhyolitic intrusions that have ε Nd values of +11 to +13 (Table 3, Figures 7, 8), the highest in the region.…”

Recovering the past history of natural recording media by Bayesian inversion

“…The P -T path can be used to infer movements of mass and heat at depths of up to a couple of hundred kilometers, leading to the construction of tectonic models of plate subduction and collision. Likewise, igneous petrologists have estimated the processes and conditions (pressure, temperature, and volatile content) present in the magma chambers of volcanoes [22][23][24][25]. This can be used to infer information about movements, compositional evolution, and recharge events in magma chambers, and may contain clues to the mechanisms that trigger eruptions.…”

Derivation of intermediate to silicic magma from the basalt analyzed at the Vega 2 landing site, Venus

“…Pressure is an important parameter for the genesis of silicic magmas by fractional crystallization. Numerical modeling, geological, and seismic studies indicate that mafic magmas undergo polybaric differentiation within the crust before eruption [ 57 – 64 ]. Assuming that either of the Vega 2 compositions used for the modeling is a close approximation of the silicate liquid that erupted, then the CaO content is too low and the Al 2 O 3 is too high for a primary melt [ 65 ].…”