Preservation of inherited argon in plagioclase crystals and implication for residence time after reservoir remobilization

Description

We compare K-Ar ages obtained on groundmass and plagioclase from lava domes that erupted after flank collapse events in the Lesser Antilles and Ecuador. All samples contain plagioclase with distinct zoning patterns, as well as inclusion-rich zones, that reveal one or more crystal resorption events due to rapid temperature changes. In these samples, plagioclase crystals yield ages 2 to 3 times older than the groundmass due to a partial retention of inherited 40Ar. We investigate textural and compositional zoning in plagioclase phenocrysts using backscattered electron images, electron microprobe and scanning electron microscope analysis of major and trace elements. Age and zoning patterns are coupled to modelling of Ar and Sr diffusion to calculate residence time of crystals at magmatic temperature after reservoir remobilization. Combined data suggest that crystals were remobilized after a magma mixing event related to a flank collapse event. In order to account for the age difference, we have modeled the residence time of plagioclase using magma temperature conditions and possible inherited crystal initial ages. We have calculated that the age differences observed require residence times of tens to a few hundred years. This suggests that eruptions studied here have been triggered by reservoir remobilization in less than 100 years. This can be related to changes in the volcano's morphology due to large scale flank collapse having affected the plumbing system relatively quickly. Based on similar features observed in different settings, it can be proposed that similar processes are common at arc volcanoes. Our approach should help us to better constrain the timing between magmatic intrusion, mixing, flank collapse and eruptions.

Abstract

International audience

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