Multiple Active Volcanoes in the Northeast Lau Basin

The northeast Lau Basin occupies a complex geological area between the Tafua arc front, the E-W trending Tonga Trench, and the Northeast Lau Spreading Center. These boundaries create multiple zones of extension and thus provide abundant opportunities for magma to invade the crust. The 25-km-long chain of “Mata” volcanoes lies near the center of this area, separated from both the arc front and the spreading ridge. In 2008 we discovered hydrothermal venting on the largest and most southerly of these volcanoes, W and E Mata. In 2010 we visited the 7 smaller volcanoes that form a 15-km-long arcuate sweep to the north from W and E Mata (the “North Matas”). We also revisited W and E Mata. Over each volcano we conducted CTD tows to map plumes and collect water samples. Based on the CTD results, camera tows searched for seafloor sources on three volcanoes. The N Mata volcanoes, extending from Mata Taha (1) in the south to Mata Fitu (7) in the north, lie within a prominent gap in the shallow bathymetry along the southern border of the Tonga trench. Northward from E Mata the Mata volcanoes degrade from large symmetrical cones to smaller and blocky volcanic edifices. Summit depths range from 1165 m (W Mata) to 2670 m (Mata Nima (5)). The most active volcano in the chain is the erupting W Mata, with an intense plume that extended 250 m above the summit. Hydrothermal temperature anomalies (Δθ, corrected for hydrographic masking effects) reached ∼1.7°C, with light-scattering values as high as 2-5 ΔNTU. The 2010 surveys now show that 6 of the 7 N Mata volcanoes are also hydrothermally active. Along the N Matas, Δθ and ΔNTU signals ranged from robust to weak, but distinct oxidation-reduction potential (aka Eh) anomalies confirmed active venting in each case. The most concentrated plumes were found near Mata Ua (2) and Mata Fitu (7), with Δθ and ΔNTU maxima of 0.1-0.17°C and 0.3, respectively. Despite the variability in plume strength, however, ΔNTU/Δθ ratios were mostly similar throughout the entire Mata chain. Mata Taha, Ua, Fa (4), and Ono (6) had ΔNTU/ΔT ratios between 3/°C and 4.4/°C. W Mata and Mata Fitu were lower at 1.9/°C. The ratio at Mata Tolu (3) was much lower (0.35/°C) and implies a diffuse, low particle source. (E Mata is also likely a diffuse source, but the plume was too weak to calculate a reliable ΔNTU/Δθ ratio.) These inferences will be evaluated by calculating 3He/heat, Fe/heat, and Mn/heat ratios in plume samples from each volcano. Camera tows confirmed the location of active vents fields on the SE flank of Mata Fitu (~2600 m) and the summit of Mata Tolu (∼1800 m). Including the 9 Matas, a total of 15 volcanoes and 3 ridge segments have been examined during several expeditions since 2008 in this small (∼70x70 km) study area. Active hydrothermal fields occur on all 3 ridge segments and 12 of the volcanoes, making this region one of the most intensely active, and volcanically diverse, yet identified.