Subaqueous cryptodome eruption, hydrothermal activity and related seafloor morphologies on the andesitic North Su volcano

Abstract

North Su is a double-peaked active andesite submarine volcano located in the eastern Manus Basin of the Bismarck Sea that reaches a depth of 1154 m. It hosts a vigorous and varied hydrothermal system with black and white smoker vents along with several areas of diffuse venting and deposits of native sulfur. Geologic mapping based on ROV observations from 2006 and 2011 combined with morphologic features identified from repeated bathymetric surveys in 2002 and 2011 document the emplacement of a volcanic cryptodome between 2006 and 2011. We use our observations and rock analyses to interpret an eruption scenario where highly viscous, crystal-rich andesitic magma erupted slowly into the water-saturated, gravel-dominated slope of North Su. An intense fragmentation process produced abundant blocky clasts of a heterogeneous magma (olivine crystals within a rhyolitic groundmass) that only rarely breached through the clastic cover onto the seafloor. Phreatic and phreatomagmatic explosions beneath the seafloor cause mixing of juvenile and pre-existing lithic clasts and produce a volcaniclastic deposit. This volcaniclastic deposit consists of blocky, non-altered clasts next, variably (1-100 %) altered clasts, hydrothermal precipitates and crystal fragments.The usually applied parameters to identify juvenile subaqueous lava fragments, i.e. fluidal shape or chilled margin, were not applicable to distinguish between pre-existing non-altered clasts and juvenile clasts. This deposit is updomed during further injection of magma and mechanical disruption. Gas-propelled turbulent clast-recycling causes clasts to develop variably rounded shapes. An abundance of blocky clasts and the lack of clasts typical for the contact of liquid lava with water is interpreted to be the result of a cooled, high-viscosity, crystal-rich magma that failed as a brittle solid upon stress. The high viscosity allows the lava to form blocky and short lobes. The pervasive 3 volcaniclastic cover on North Su is partly cemented by hydrothermal precipitates. These hydrothermally-cemented breccias, crusts and single pillars show that hydrothermal circulation through a thick layer of volcaniclastic deposits can temporarily increase slope stability through precipitation and cementation.

Application of B, Mg, Li, and Sr Isotopes in Acid‐Sulfate Vent Fluids and Volcanic Rocks as Tracers for Fluid‐Rock Interaction in Back‐Arc Hydrothermal Systems

“…Instead, water-rock interaction with a less altered (fresher) basement (and thus higher Li concentrations) at North Su best explains the observed offsets. This hypothesis is supported by evidence for ongoing volcanic activity at North Su (Thal et al, 2016), which indicates the near-seafloor emplacement of very recent (i.e., post-2006) fresh rock. In contrast, there was no conspicuous evidence for recent volcanic activity in the caldera wall site of DESMOS, where activity has persisted in a relatively localized area at the Onsen site from 1995 to 2006, albeit with an apparent gradual increase in SO 2 emitted in acid-sulfate fluids there (Gamo et al, 1997;Seewald et al, 2015).…”

“…Similar to the 2006 fluids, fluids venting at North Su can be categorized as acid‐sulfate (NS9), hybrid smoker (NS8 and NS11), and black smoker (NS7 and NS10) fluids. The most vigorously venting site sampled in 2006 (comprising vents NS1 and NS2) was buried prior to the 2011 cruise by products of a volcanic cryptodome eruption (Thal et al, ). In 2011, a previously unexplored vent area, named Sulfur Candles, was also sampled.…”

“…Geochemistry, Geophysics, Geosystems named Sulfur Candles, was also sampled. This area is characterized by meter-thick flows of molten sulfur, with white smoke and liquid CO 2 bubble venting (Bach et al, 2011;Reeves et al, 2015;Thal et al, 2016).…”

Constraints on the source of Cu in a submarine magmatic-hydrothermal system, Brothers volcano, Kermadec island arc

“…In addition to a magmatic volatile signature, acid-sulphate (advanced argillic) alteration, sulphosalts (e.g., enargite, Cu 3 AsS 4 ) typical for high-sulphidation conditions (Einaudi et al 2003) and high contents of economically important metals (e.g., Cu and Au) in some submarine arc and back-arc hydrothermal systems imply that they may be comparable to epithermal-porphyry deposits on land, such as the worldclass Far Southeast-Lepanto Cu-Au deposits (Philippines; Hedenquist et al 1998). This includes, for example, the magmatic-hydrothermal systems of Brothers volcano (Kermadec arc; de Ronde et al 2005;Berkenbosch et al 2012) and Kolumbo volcano (Hellenic arc; Kilias et al 2013Kilias et al , 2016, as well as SuSu Knolls in the Manus back-arc basin Yeats et al 2014;Thal et al 2016). Hence, arc-hosted hydrothermal systems are characteristically distinct in terms of their chemical and mineralogical composition compared to those along midocean ridges, which were interpreted to be the classic modern analogues of volcanic-hosted massive sulphide (VHMS) deposits mined on land (Hannington et al 1998de Ronde et al 2014;Keith et al 2016b).…”

Morphotectonic Analysis of the East Manus Basin, Papua New Guinea

“…Intermediate to silicic (Sinton et al, 2003;Beier et al, 2015;Thal et al, 2016;Siegburg et al, 2018) The transition between SFD1 and SFD2. Transfer structures, steep scarps, transfer fault-oriented depressions Conical volcano with pyroclastic deposits, shallow knolls with unknown lithofacies.…”