Magma Storage, Recharge and the Caldera Cycle at Rabaul, Papua New Guinea

Abstract:

Many calderas have a history of repeated caldera-forming eruptions, interspersed with periods of more minor activity. Rabaul, for instance, has had at least 11 ignimbrite-forming eruptions over the last 200 ky. The most recent of these was the ‘1400 BP’ eruption, which led to caldera collapse. Since then, there has been multiple smaller eruptions, including the ongoing activity from Tavurvur and Vulcan. An important question facing volcanology today is what controls the size of eruptions at calderas such as Rabaul.

Detailed stratigraphic sampling of the 1400BP eruption reveals that prior to eruption, the magma reservoir below Rabaul contained a well-mixed dacite with whole-rock SiO₂ contents of 65.0–66.4 wt%. The dacite contains a single phenocryst assemblage of plag (An_44–52), cpx (En_43–46Fs_13–15Wo_40–41), opx (En_69–71Fs_25–28Wo₃) and magnetite, along with minor apatite. The homogeneity of the dacite is underscored by the narrow range of compositions of both the matrix glass and the melt inclusions (67.8–69.0 wt% SiO₂). The only exception to this is at the top of the ignimbrite, representing some of the last magma to have been withdrawn. Dispersed throughout the dacitic pumices are darker, more mafic blebs. Streaks of mingled magma with a range of SiO₂ contents, down to 59.9 wt% SiO₂ are also found in the pumice, suggesting that a mafic recharge magma was intruded into the base of the reservoir shortly before eruption. High TiO₂ contents rule out the direct involvement of basalt, and instead imply the magma that intruded into the reservoir was an andesite with at least 56 wt% SiO₂. Phenocrysts related to this recharge magma are rare, and the crystals found in the dark blebs are identical in composition to those found in the dacite, indicating that the recharge was aphyric.

The present-day, post-caldera recharge magma is different to the pre-1400 BP recharge magma: it is basaltic. This suggests that the plumbing system of Rabaul is different during the pre-caldera and post-caldera stages of the caldera cycle. Currently, basaltic magma can reach shallow levels of the plumbing system, and even make it to the surface as enclaves. Prior to a large, caldera-forming eruption, however, any basaltic magma that enters the system gets trapped at depth. This allows a large reservoir of evolved magma to develop.