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

Budd, DA, Troll VR, Deegan FM, Jolis EM, Smith VC, Whitehouse MJ, Harris C, Freda C, Hilton DR, Halldorsson SA, Bindeman IN.  2017.  Magma reservoir dynamics at Toba caldera, Indonesia, recorded by oxygen isotope zoning in quartz. Scientific Reports. 7

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canary-islands, gran-canaria, hydrogen isotope, hydrothermal alteration, igneous rocks, low-o-18 silicic magmas, merapi volcano, origin, pb isotope, sunda arc


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 delta O-18 values. Overall, Toba quartz crystals exhibit comparatively high delta O-18 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 delta O-18 values in outermost growth zones compared to their cores, with values as low as 6.7% (maximum Delta(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-delta O-18 component into the magma reservoir just prior to eruption. Here we argue that this late-stage low-delta O-18 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.






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