A Branched Magma Feeder System during the 1669 Eruption of Mt Etna: Evidence from a Time-integrated Study of Zoned Olivine Phenocryst Populations

Abstract

The 1669 eruption of Mt Etna was one of the most voluminous and devastating of its flank eruptions in historical times. Despite a large body of relevant research, knowledge of the timing and duration of magma transfer and magma recharge through the internal plumbing system preceding and during the eruption is still limited. To address that lack of knowledge, we apply a three-way integrated method, linking systems analysis of crystals, a time-integrated study of zoned olivine populations, and a forward-modelling approach using thermodynamic calculations. Analysis of 202 olivine crystals erupted during the initial (pre-March 20, i.e. SET1) and the final (post-March 20; i.e. SET2 and MtRs) stages of the eruption reveals the existence of three magmatic environments (MEs) in which the majority of the olivine cores [M1 (= Fo75–78)] and rims [i.e. M5 (= Fo51–59) and M3 (= Fo65–69)] formed. Application of the rhyolite-MELTS software allowed us to constrain the key intensive variables associated with these MEs. We find that temperature, water content and oxidation state vary between these MEs. Application of diffusion modelling to the zoned olivine crystals allowed us to reconstruct the timing and chronology of melt and crystal transfer prior to and during the 1669 flank eruption. We find that, following the formation of the olivine cores [M1 (= Fo75–78)], the reservoir M1 was intruded by batches of more evolved, degassed and possibly aphyric M5-type magma, commencing 1·5 years prior to eruptive activity. This is the origin of the SET1 olivine rims (i.e. Fo51–59). In the months prior to eruption, timescale data show that recharge activity along the newly established pathway M1–M5 increased notably. Starting in November 1668, only a few weeks after the first intrusive episode into the M1 reservoir, a second pulse of magma injections (M3-type magma) occurred and a new pathway M1–M3 opened; this is how the SET2 olivine rims (i.e. Fo65–69) formed. For several weeks a bifurcated transport system with two dominant magma pathways developed along M1–M5 and M1–M3 dyke injections. Accompanied by vigorous seismicity, in the days immediately before eruption the local magma transfer dynamics changed and the M1–M5 recharge activity slowed down, as shown by a relative lack of crystals recording shorter timescales. M1–M3 recharge, however, remained high and persisted following the eruption onset on March 11, during which the SET1 lavas were drained. We propose that the change of the local magma transfer dynamics might be linked to changes in the local stress field brought on during eruption. This may potentially have been due to repeated dyke injections into Etna’s shallow plumbing system disrupting the early M1–M5 pathway and at the same time stabilizing the M1–M3 route as a dominant feeder. This transfer of system feeding would reproduce the observed syn-eruptive recharge and mixing in the weeks following eruption onset, culminating in the eruption of the later SET2 lavas.