The 1300AD dacite pumice eruption, Rinjani Volcano, East Sunda Arc: petrology, petrogenesis and plumbing

Abstract

Physical controls on fractional crystallisation in subduction settings around the globe are the focus of much discussion within academia. It is frequently observed that the typical magma to be erupted in arc settings is andesite from the differentiation of partially melted peridotite mantle wedge. It is less common in these settings to see more felsic end member magmas such as dacite or rhyolite being erupted in cataclysmic events. Discussion has arisen into what physical process or processes can drive a volcano, or set of volcanoes, within an arc to produce dacite or rhyolite magmas where fractional crystallisation is known to be the process of initial crystallisation. Research into these processes can be applied to Mount Rinjani (Lombok) within the Sunda Arc system, where the normally composite andesite volcano produced a cataclysmic eruption of dacite magma known as the Rinjani Pumice at 1300AD. Data is presented here in order to investigate processes within the sub-volcanic magma plumbing at Rinjani, primarily using volatiles from melt inclusions, coupled with petrology, pressure, temperature and water saturation estimates. It is theorised that ascent-driven crystallisation from a parent basalt drove fractional crystallisation to andesite composition beneath Rinjani volcano. Water undersaturated conditions induced ascent of the andesite magma to a shallow reservoir at ~3 kbar at temperatures ~1015 °C, with water saturation ~3.7 weight percent. Melt inclusion data from these same crystals reveals the interstitial liquid in the andesite magma to be of evolved dacite composition (~66 wt % SiO2) compared to the andesite (~50-54 wt % SiO2).

Critical crystallinity is the physical process believed to have acted upon fractional crystallisation where the percentage of crystals forming exceeded the ability of the magma chamber to convect within this reservoir and therefore physical separation of the evolved dacite liquid from the andesite magma occurred. The buoyant dacitic liquid ascends to another shallow reservoir directly beneath Rinjani volcano and reaches its saturation pressure at 1.8 kbar at a temperature of ~900 °C, with water saturation increased to ~5 wt %.

Sub-plinian cataclysmic eruption of the 1300AD Rinjani Pumice occurred due to pressure increase involved in saturation of the dacite, and syn-eruptive degassing aids in excavating the western flank of the Rinjani stratocone leaving a caldera.

Subsequent volcanism occurs where the magma ascent path migrates west producing more cataclysmic eruptions increasing the volume of the edifice. Today, resurgent volcanism can be seen in the central eastern part of Segara Anak lake at Gunung Baru within the Rinjani caldera to produce basalt and andesite lavas.

The application of physical mechanisms acting upon fractional crystallisation proposed here may be applied to other arc settings around the globe, where fractional crystallisation is found to be the primary driving force of felsic end member cataclysmic eruptions at what is typically an andesitic arc.