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

作者：Budd David A; Troll Valentin R^*; Deegan Frances M; Jolis Ester M; Smith Victoria C; Whitehouse Martin J; Harris Chris; Freda Carmela; Hilton David R; Halldorsson Saemundur A; Bindeman Ilya N

来源：Scientific Reports, 2017, 7(1): 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 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.

• 出版日期2017-1-25