The 1994–2001 eruptive period at Rabaul, Papua New Guinea: Petrological and geochemical evidence for basalt injections into a shallow dacite magma reservoir, and significant SO2 flux

Abstract

The eruptions that began at Rabaul Caldera on 19 September 1994 had two focal points, the vents Tavurvur and Vulcan, located 6 km apart on opposing sides of the caldera. Vulcan eruptives define a tight cluster of dacite compositions, whereas Tavurvur eruptives span an array from equivalent dacite compositions to mafic andesites. The eruption of geochemically and mineralogically identical dacites from both vents indicates sourcing from the same magma reservoir. This, together with previously reported H2O-CO2 volatile contents of dacite melt inclusions, a caldera-wide seismic low-velocity zone, and a seismically active caldera ring fault structure are consistent with the presence at 3-6 km depth of an extensive, tabular dacitic magma body having volume of about 15-150 km 3. The Tavurvur andesites form a linear compositional array and have strongly bimodal phenocryst assemblages that reflect dacite hybridisation with a mafic basalt. The moderately large volume SO2 flux documented in the Tavurvur volcanic plume (and negligible SO2 flux in the Vulcan plume) combined with high dissolved S contents of basaltic melt inclusions trapped in olivine of Tavurvur eruptives, indicate that the amount of degassed basaltic magma was ~0.1 km 3 and suggest that the injection of this magma was confined to the Tavurvur-side (eastern to northeastern sector) of the caldera. Circumstantial evidence suggests that the eruption was triggered and evolved in response to a series of basaltic magma injections that may have commenced in 1971 and continued up until at least the start of the 1994 eruptions. The presence of zoned plagioclase phenocrysts reflecting older basalt-dacite interaction events (i.e. anorthite cores overgrown with thick andesine rims), evaluation of limited available data for the products of previous eruptions in 1878 and 1937-1943, and the episodic occurrence of major intra-caldera seismodeformational events indicates that the shallow magma system at Rabaul Caldera is subjected to repeated mafic magma injections at intervals of several years to several decades.

Detecting exchange networks in New Britain, Papua New Guinea: geochemical comparisons between axe‐adze blades and in situ volcanic rock sources

“…), 54 specimens from Rabaul (Heming ; McKee ; Patia et al . ; Wood et al . ), 22 from the Manus Island Arc (Johnson et al .…”

Morphotectonic Analysis of the East Manus Basin, Papua New Guinea

“…(3) Volcanic edifices that are conical volcanoes and calderas: examples include North Su, and the collapsed vent structure of the South Su volcano; and Tuvai Caldera, offshore New Britain (e.g., Patia et al, 2017).…”

“…Remnant arc crust extension, incipient rifting (e.g., Parson and Wright, 1996) Silicic (Patia et al, 2017) SFD 1-relatively shallow seafloor, lack of clear normal faults, or lineations Conical volcanoes (e.g., North Su and Nimab/Kaia Natai) caldera (Tuvai)…”

“…Rabaul Caldera on the Gazelle Peninsula is an active volcano (Nairn et al, 1995). Tuvai caldera is silicic (Patia et al, 2017). Nimab/Kaia Natai hydrothermally active (Dekov et al, 2016) Current transition to nascent rifting from Phase 1 to Phase 2…”

The Temporal Variation of Magma Plumbing System of the Kattadake Pyroclastics in the Zao Volcano, Northeastern Japan

“…In most cases, the intrusion of deeper and hotter magma into a shallow magma chamber is the cause of such a process, which may evolve from mingling to mixing depending on the time the two magmas are in contact before the eruption (Murphy et al 2000 [3]; Plail et al 2018 [4]). In some cases, the overpressure caused by the intrusion of the new magma into the magma chamber is sufficient to trigger the eruption (Izbekov et al 2004 [5]; Patia et al 2017 [6]), while in others, an additional contribution (e.g., exsolution of volatiles) is required to reach the eruption conditions (Eichelberger 1980 [7]; Synder 2000 [8]). This may require keeping the two magmas in contact a certain time, during which they may allow chemical diffusion and facilitate magma mixing (Cooper 2018 [9]; Nishi et al 2019 [10]).…”