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 ?¹⁸O values. Overall, Toba quartz crystals exhibit comparatively high ?¹⁸O 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 ?¹⁸O 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-?¹⁸O component into the magma reservoir just prior to eruption. Here we argue that this late-stage low-?¹⁸O 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.

SUBMITTER: Budd DA 

PROVIDER: S-EPMC5264179 | biostudies-literature | 2017 Jan

REPOSITORIES: biostudies-literature