Effects of magma mixing on crystal residence times
Magma mixing is a ubiquitous process in silicic magmatic systems. My recent study on uranium-series residence ages of plagioclase phenocrysts in mixed and unmixed magmas shows that individual mixing episodes can shift apparent crystal residence to significantly older ages.
We present pre- and post-mixing time scales for a single magma mixing event associated with the 1846-47 eruption of Volcan Quizapu. We compare and contrast 226Ra-230Th crystal residence ages between the mixed lavas of the 1846-47 effusive eruption and the largely homogenous plinian dacite eruption of 1932. Both eruptions are dominated by the same dacite magma, while the earlier eruption contains evidence for a single episode of mafic recharge and mixing.
The use of uranium-series mineral separate dating on a mixed magma requires deconvolution of different crystal populations. We present a four component mixing model for the crystal separates to obtain dacite-derived plagioclase model ages as well as andesite-derived (recharge) plagioclase ages. Plagioclase model ages average ~ 3.6 kyr for dacite-derived plagioclase from the mixed (1846-47) eruption, ~ 1.4 kyr older than average plagioclase model ages of the 1932 eruption, suggesting that magma mixing and reheating led to some resorption of young crystal rims in the 1846-47 magmatic episode and/or that some growth of plagioclase rims occurred during the repose time of 86 yr. The andesite-derived plagioclase residence ages are younger than the corresponding dacite-derived plagioclase residence ages, but they are still on the order of thousands of years. In addition, we use Mg diffusion in plagioclase to constrain time scales of magma mixing. We show that the 1846-47 eruption was triggered by large-volume mafic recharge magma that mixed and mingled with the resident dacite within, likely days to weeks of the ensuing eruption.
collaborator and Ph.D. advisor: Kari M. Cooper
Plagioclase residence ages for the 1846-47 eruption that was effected strongly by magma mixing (dark lava) are more than 1000 years older than residence ages obtained the same type of plagioclase in the 1932 eruption (light colored pumice) suggesting that individual mixing events can alter residence ages significantly.
Deposits fom the 1846-47 and 1932 Quizapu eruptions
Figure
Crystal residence ages for dacite-derived plagioclase phenocrysts are obtained using 226Ra-230Th disequilibrium dating. Plagioclase populations in both eruptions are identical compositionally and suggest the same origin from the shallow dacite magma storage system. The shift in crystal residence ages can be best explained by partial resorption of the young (potentially zero age) rims during the magma mixing episode and the significant reheating.
