Department of Earth Sciences, CEMPEG, Uppsala University, Sweden.
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy.
Sci Rep. 2017 Jan 25;7:40624. doi: 10.1038/srep40624.
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 δO values. Overall, Toba quartz crystals exhibit comparatively high δO 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 δO values in outermost growth zones compared to their cores, with values as low as 6.7‰ (maximum ∆ = 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-δO component into the magma reservoir just prior to eruption. Here we argue that this late-stage low-δO 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.
石英是高硅质火成岩中的常见相，且不易受喷发后蚀变的影响，因此能可靠记录硅酸岩浆系统中岩浆的演化过程。在此，我们以 75ka 的多巴超级火山喷发为例，证明了石英可以解析岩浆 δO 值在后期的时间变化。总体而言，由于岩浆在当地花岗岩基底内的停留和同化作用，多巴石英晶体的 δO 值较高，最高可达 10.2‰。然而，约 40%的分析石英晶体的最外层生长带的 δO 值相对于其核心有所降低，最低值低至 6.7‰（最大 ∆=1.8‰）。这些较低的值与多巴可用的有限锆石记录一致，且多巴石英的结晶历史表明，在喷发前，有一个低 δO 成分的物质流入了岩浆库。在此，我们认为这个晚期的低 δO 成分来源于水热蚀变的顶盖物质。我们的研究表明，石英同位素地层学可以解析可能被全岩或锆石同位素研究遗漏的岩浆事件，且蚀变顶盖物质的同化可能代表大型多巴式岩浆系统中一种可行的喷发触发机制。
