Holocene mush mobilization timescales in the Bárðarbunga-Veiðivötn volcanic system, Iceland: application of olivine diffusion chronometry

Abstract:

Compositional zoning patterns in crystals can be exploited to extract timescales of magmatic processes. Here, we use multi-element diffusion chronometry in olivine crystals to yield information about the time that has elapsed between crystal mush disaggregation and eruption. Studied samples consist of temporally diverse Holocene formations erupted in the Bárðarbunga-Veiðivötn volcanic system in Iceland’s Eastern Volcanic Zone. Early-Holocene olivine crystals have core compositions in the range Fo_84-87, whilst middle-Holocene and historical samples register a wider core compositional range (Fo_80-86.5). Olivine rims, found to be in chemical equilibrium with the carrier liquid, are more evolved in early-Holocene units (Fo_76-81) compared to middle-Holocene (Fo_78-80) and historical (Fo_81-83) units. Diffusion modelling results reveal that olivine residence time in the carrier melt has changed throughout the Holocene. Early-Holocene units record most recurrent timescales between 200-400 days, whereas middle-Holocene and historical units record timescales of about 70 and 60 days, respectively. Furthermore, barometry constraints suggest that crystals and melts were stored and gradually transported upwards throughout a multi-tiered system until they ultimately accumulated in a mid-crustal reservoir(s) at about 6.8-7.5± 2.5 km depth, where final disaggregation by the carrier liquid took place. The variability of timescales over time can be associated with the occurrence of a more compact mush fabric in the early-Holocene, as evidenced by the large macrocryst content and the presence of gabbroic nodules. Conversely, in the middle-Holocene and historical time, the system became characterized by a less compact mush fabric, which required less energy and time to be loosened and converted into an eruptible magma.