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 SO 2 flux

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 km³. 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 km³ 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 seismo-deformational 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.We thank Shane Nancarrow, formerly of Geoscience Australia, for making many aspects of this project possible and AusAID for providing financial support to HP to undertake research at the Australian National University (ANU) into the 1994 and historical eruptions at Rabaul

Niveles de concentración anómalos de C02 en los suelos y su relación con la actividad sísmica de la caldera de Rabaúl, Papua Nueva Guinea

Medidas de C02 en los suelos de la caldera de Rabaul, Papua Nueva Guinea, reflejan niveles de concentración altos, hasta cerca de un 20%, en relación con un margen activo de la caldera definido por sismicidad. Estos niveles anómalos del CO, en los suelos fueron detectados lejos de los aparatos volcánicos activos. La caracterización isotópica de carbono refleja un origen principalmente biogénico para el C02, pero aquellas muestras que presentan un mayor contenido de C02 muestran una firma isotópica relativamente más pesad

Temporal variations in U-series disequilibria in an active caldera, Rabaul, Papua New Guinea

Intra-caldera eruptions have been speculated to sample the last batches of magma remaining from earlier caldera-forming eruptions. Rabaul Caldera, New Britain, Papua New Guinea has erupted several times since the last caldera-forming eruption in ad 640, with the most recent intra-caldera eruptions in 1878, 19371941 and 1994present from the Tavurvur and Vulcan vents. U-series isotopes, in conjunction with Sr-87/Sr-86 and Nd-143/Nd-144, were analyzed on 16 samples collected from 1994 to 2001 to monitor short-term changes in magma composition to model magmatic processes and to test whether there is evidence of recent fresh magma input. Inflections on MgO diagrams imply that fractional crystallization is an important process in long-term magma evolution, and the homogeneity in Sr-87/Sr-86 and Nd-143/Nd-144 shows that assimilation of isotopically distinct material has not occurred. A vertical array on a (ThU)-Th-230-U-238 isochron diagram requires open-system behavior and could support a model of differentiation of multiple magma batches over 26 kyr. However, the presence of (Ra-226/Th-230) excesses requires introduction of new magma within the past 8000 years and is permissible of a model in which the currently erupting magmas were emplaced at or since the last caldera-forming event. Other than the presence of mafic enclaves in the 1878 and 1937 eruptions, no evidence exists to suggest open-system magma injection. Systematic variation in U-series disequilibria between 1994 and 2001 is lacking, which may indicate that the system is broadly in steady state or that the processes acting to produce the limited compositional variation have time scales that are too short to be resolved by Ra isotopes (i.e. are less than a few hundred years)

( 210 Pb/ 226 Ra) variations during the 1994-2001 intracaldera volcanism at Rabaul Caldera

Determining the timing and source of gas transfer during intermittent intracaldera volcanism can aid in our understanding of degassing in these large systems. Using (210Pb/226Ra) ratios, (parentheses denote activity ratios) as a time-sensitive tracer, injections of 222Rn and the subsequent time scales of gas accumulation and loss can be determined. Variations in (210Pb/226Ra) have been measured for 15 volcanic products erupted at Rabaul Caldera over the period 1994 to 2001. In addition, one basaltic enclave from the 1937 eruption was also analyzed. Water and carbon dioxide contents determined from olivine hosted melt inclusions erupted in 1997 are < 1% and suggest extensive shallow-level degassing. Both 210Pb excesses and deficits are found in andesites and dacites, whereas the basaltic enclave displays an (210Pb/226Ra)0 ratio of 7. Between 1994 and 1997 three samples with (210Pb/226Ra) deficits were erupted which indicate open system gas loss since 1992 and 1994. No correlation exists between (210Pb/226Ra) and lava chemistry, eruptive style or date. 210Pb excesses are more common than deficits in Rabaul samples but cannot be explained by plagioclase feldspar accumulation, Pb sublimate accumulation or differentiation. Instead, a model of intra-magma 222Rn transfer can produce 210Pb excesses of the appropriate magnitude if gas transfer occurs over 1-5 years from an underlying body of magma that is 2-10 times larger than the volume of erupted material and that is consistent with geophysical estimates. Although intermittent gas transfer events can be inferred by the development of 210Pb excess, there is no evidence at Rabaul for a direct link between eruptive style, gas flux and (210Pb/226Ra)

Temporal variations in U-series disequilibria in an active caldera, Rabaul, Papua New Guinea

Intra-caldera eruptions have been speculated to sample the last batches of magma remaining from earlier caldera-forming eruptions. Rabaul Caldera, New Britain, Papua New Guinea has erupted several times since the last caldera-forming eruption in ad 640, with the most recent intra-caldera eruptions in 1878, 1937-1941 and 1994-present from the Tavurvur and Vulcan vents. U-series isotopes, in conjunction with 87Sr/86Sr and 143Nd/144Nd, were analyzed on 16 samples collected from 1994 to 2001 to monitor short-term changes in magma composition to model magmatic processes and to test whether there is evidence of recent fresh magma input. Inflections on MgO diagrams imply that fractional crystallization is an important process in long-term magma evolution, and the homogeneity in 87Sr/86Sr and 143Nd/144Nd shows that assimilation of isotopically distinct material has not occurred. A vertical array on a 230Th-238U isochron diagram requires open-system behavior and could support a model of differentiation of multiple magma batches over 26 kyr. However, the presence of (226Ra/230Th) excesses requires introduction of new magma within the past 8000 years and is permissible of a model in which the currently erupting magmas were emplaced at or since the last caldera-forming event. Other than the presence of mafic enclaves in the 1878 and 1937 eruptions, no evidence exists to suggest open-system magma injection. Systematic variation in U-series disequilibria between 1994 and 2001 is lacking, which may indicate that the system is broadly in steady state or that the processes acting to produce the limited compositional variation have time scales that are too short to be resolved by Ra isotopes (i.e. are less than a few hundred years)

Mafic magma replenishment, unrest and eruption in a caldera setting: insights from the 2006 eruption of Rabaul (Papua New Guinea)

<p>Understanding the magmatic processes that drive unrest at silicic calderas remains a major goal in Volcanology. Rabaul in Papua New Guinea is an exceptional location because after two decades of unrest and a peak in seismicity and deformation in 1983–85, eruptive activity began in 1994 and is still ongoing. A particularly large sub-Plinian eruption occurred from Tavurvur in October 2006. Whole-rock compositions are andesitic and reflect mixing/mingling between basaltic and dacitic magmas from the same system. The magmas that fed the 2006 eruption were stored at about 930°C, with 1–3 wt% H₂O, 25–520 ppm CO₂, and 50–2500 ppm SO₂ in the melt. Melt inclusions hosted in pyroxene, and plagioclase phenocrysts record fractional crystallization at ≤200 MPa under relatively dry and poorly oxidizing conditions. Magma mixing/mingling is expressed as heterogeneous glass compositions, strongly zoned phenocrysts, and mafic crystal aggregates. A textural maturation from fine, acicular to large, blocky crystal clots implies different relative ages of formation. Modelling the chemical zoning of plagioclase shows that mafic–silicic interactions started a couple of decades prior to the 2006 eruption and continued until days to weeks prior to eruption. Basaltic replenishments have been driving unrest and eruption at the Rabaul caldera since the 1970s. </p

(²¹⁰Pb/²²⁶Ra) variations during the 1994-2001 intracaldera volcanism at Rabaul Caldera

Determining the timing and source of gas transfer during intermittent intracaldera volcanism can aid in our understanding of degassing in these large systems. Using (²¹⁰Pb/²²⁶Ra) ratios, (parentheses denote activity ratios) as a time-sensitive tracer, injections of ²²²Rn and the subsequent time scales of gas accumulation and loss can be determined. Variations in (²¹⁰Pb/²²⁶Ra) have been measured for 15 volcanic products erupted at Rabaul Caldera over the period 1994 to 2001. In addition, one basaltic enclave from the 1937 eruption was also analyzed. Water and carbon dioxide contents determined from olivine hosted melt inclusions erupted in 1997 are < 1% and suggest extensive shallow-level degassing. Both ²¹⁰Pb excesses and deficits are found in andesites and dacites, whereas the basaltic enclave displays an (²¹⁰Pb/²²⁶Ra)0 ratio of 7. Between 1994 and 1997 three samples with (²¹⁰Pb/²²⁶Ra) deficits were erupted which indicate open system gas loss since 1992 and 1994. No correlation exists between (²¹⁰Pb/²²⁶Ra) and lava chemistry, eruptive style or date. ²¹⁰Pb excesses are more common than deficits in Rabaul samples but cannot be explained by plagioclase feldspar accumulation, Pb sublimate accumulation or differentiation. Instead, a model of intra-magma ²²²Rn transfer can produce ²¹⁰Pb excesses of the appropriate magnitude if gas transfer occurs over 1–5 years from an underlying body of magma that is 2–10 times larger than the volume of erupted material and that is consistent with geophysical estimates. Although intermittent gas transfer events can be inferred by the development of ²¹⁰Pb excess, there is no evidence at Rabaul for a direct link between eruptive style, gas flux and (²¹⁰Pb/²²⁶Ra).11 page(s