Organic, Gas, and Element Geochemistry of Hydrothermal Fluids of the Newly Discovered Extensive Hydrothermal Area in the Wallis and Futuna Region (SW Pacific)

Abstract

Two newly discovered hydrothermal vent fields of the Wallis and Futuna region, Kulo Lasi and Fatu Kapa, were sampled for fluid geochemistry. A great geochemical diversity was observed and assigned to the diversity of lithologies as well as the occurrence of various processes. Kulo Lasi fluids likely formed by interaction with fresh volcanic rocks, phase separation, and mixing with magmatic fluid. Conversely, the geochemistry of the Fatu Kapa fluids would be mostly due to water/felsic lavas reactions. In terms of organic geochemistry, fluids from both fields were found to be enriched in formate, acetate, and semivolatile organic compounds (SVOCs): n-alkanes, n-fatty acids, and polyaromatic hydrocarbons (PAHs). Concentrations of SVOCs reached a few ppb at most. The distribution patterns of SVOCs indicated that several processes and sources, at once of biogenic, thermogenic, and abiogenic types, likely controlled organic geochemistry. Although the contribution of each process remains unknown, the mere presence of organics at the M level has strong implications for metal dispersion (cycles), deposition (ore-forming), and bioavailability (ecosystems), especially as our fluxes estimations suggest that back-arc hosted vent fields could contribute as much as MOR to the global ocean heat and mass budget.

Volcanic and hydrothermal processes in submarine calderas: The Kulo Lasi example (SW Pacific)

“…Thus, variations in chlorinity, from 34% higher to 9% lower than that of the seawater, can be explained by a process of phase separation (Bischoff and Rosenbauer, 1987). An adiabatic decompression of supercritical seawater Pitzer, 1989, Foustoukos andSeyfried, 2007a) would cause it to separate, at about 320-350 bars and 415-420°C (Konn et al , 2018) into two phases having the NaCl percentages observed at Kulo Lasi.…”

“…The high temperature fluids (>300°C) from the Kulo Lasi (Table 3) have unusually high concentrations of Mg (24.6 to 34.9 mM) and SO 4 2-(6.2 to 12.0 mM). However, the low pH (2.24 to 3.32) and the high temperature (338-343°C) of these fluids indicate that significant mixing with cold seawater at subsurface or during sampling is rather unlikely (Konn et al , 2018). Mixing with cold seawater would not maintain the temperature of 343°C and the low pH while increasing the concentration of Mg from 0 to ~50% of the seawater concentration.…”

“…The low CH 4 concentration in the fluids collected in 2010 cannot account for the elevated concentration of CH 4 in the plume existing above the caldera at the same time. The strong difference in the CH 4 /Mn ratios between the plume (0.7 -4.5) (Konn et al , 2016) and the sampled fluids (0.001 -0.01) (Konn et al , 2018) also indicate that the methane plume in 2010 is not generated by the hot fluids venting in 2010. In addition, the elevated CH 4 levels in the plume had vanished in 2011.…”

“…Aliquots for silica determination were immediately diluted 100-to 200-fold and analyzed on board using the silicomolybdate automatic colorimetric method. In addition, dissolved organic compounds were recovered from fluids on board using stir-bar sorptive extraction and then qualitatively analyzed by gas chromatographymass spectrometry on a 6890 -5973N GC-MS from Agilent (Konn et al , 2009, Konn et al , 2018.…”

“…(Honsho et al , 2015, Iizasa et al , 2004; 2: (Gamo et al , 1996, Seewald et al , 2015; 3: (Berkenbosch et al , 2012, Kim et al , 2009, Lupton et al , 2015; 4: (Falloon et al , 1992, Lisitsyn et al , 1992; 5: (Ligi et al , 2014); 6: This paper; 7,8,9,10: (Lupton et al , 2012); 11: (Anderson et al , 2015); 12: (Iizasa et al , 1999, Tanahashi et al , 2008; 13: (Iizasa et al , 1992); 14: (Tsunogai et al , 1994); 15: (Urabe et al , 1987); 16: (Gamo et al , 2004); 17: (Hein et al , 2014); 18: (Stern et al , 2008); 19: (Brounce et al , 2016); 20: (Lupton et al , 2008); 21: (Stuben et al , 1992) 22-23: (Stoffers et al , 2006); 24: (de Ronde et al , 2005, de Ronde et al , 2011; 25: (de Ronde et al , 2001; 26: (Leybourne et al , 2012b); 27: (Boschen et al , 2013); 28: (CASM, 1985, Hannington andScott, 1988); 29: (Haymon et al , 2008). (Konn et al , 2018). Samples TiG3 and TiD3 correspond to the chimney sample FU-PL-06-02 and samples TiG1 and TiD1 correspond to the chimney sample FU-PL-06-03.…”