Hydrothermal activity is abundant in the area between the North Eastern Lau Spreading Center and the Tofua intra-oceanic island arc with multiple active sites in the rear-arc at the Mata and Niuatahi volcanoes. We report geochemical data for high-temperature vent fluids sampled from within the caldera of Niuatahi volcano. Hydrothermal fluids were sampled from three vent sites: South Central, Southwestern Cone and Northern Cone located in water depths between 1607 and 1699 m. Maximum temperatures of 334 °C were measured and pH values were as low as 2.8. The vent fluids were characterized by depletions in Mg, SO₄ and U as well as an enrichment of (trace) metals (e.g., Fe, Mn, K, Li) and dissolved gases (e.g., H₂S, CO₂, H₂) relative to seawater. Water-rock ratios calculated based on concentrations (K, Li, Rb, Cs, REE) and isotope ratios (δ⁷Li, δ¹¹B, ⁸⁷Sr/⁸⁶Sr) suggest fluid-rock interactions under rock-dominated conditions at all three vent sites.

The South Central vents lie closest to the site of most recent volcanic activity in the Niuatahi caldera. Vent fluids are characterized by relatively low Cl concentrations (as low as 292 mmol/kg) that are indicative of sub-critical phase separation. These fluids also had the lowest pH values (2.8-3.1), highest H₂S and lowest H₂, CH₄ and CO₂ concentrations of the three sites. The δD and δ¹⁸O values suggest that H₂O and CO₂ were added in small amounts by subduction-related volcanic vapors. However, there was no evidence for magmatic SO₂ input in the vent fluids at the time of sampling in 2018. Vent fluids from the Northern and Southwestern Cone sites on the caldera ring fault had a similar chemical composition, despite being situated at opposite sides of the caldera. Fluids from these sites had lower Fe/Mn ratios (<1) and H₂S concentrations than those from South Central suggesting that they were affected by subsurface cooling and sulfide precipitation. This study indicates variations of the Niuatahi hydrothermal vent sites depending on the location within the caldera with variable effects of fluid-rock interaction and magmatic input on fluid compositions in agreement with previous work on fluid S isotopes and sulfides.

东北刘扩散中心和托富阿洋内岛弧之间的地区热液活动丰富，在马塔和纽阿塔希火山的弧后有多个活动地点。我们报告了从纽瓦塔希火山的破火山口内取样的高温喷口流体的地球化学数据。热液流体的样本来自三个喷口：位于 1607 至 1699 米水深的中南部、西南锥体和北锥体。测得的最高温度为 334 °C，pH 值低至 2.8。排放流体的特征在于 Mg、SO ₄和 U 的消耗以及（痕量）金属（例如 Fe、Mn、K、Li）和溶解气体（例如 H ₂ S、CO ₂、H _{2 ）的富集}) 相对于海水。基于浓度（K、Li、Rb、Cs、REE）和同位素比（δ ⁷ Li、δ ¹¹ B、⁸⁷ Sr/ ⁸⁶ Sr）计算的水-岩比表明在所有三个岩石主导条件下的流体-岩石相互作用发泄场所。

中南部喷口最靠近纽瓦塔希火山口最近的火山活动地点。排放流体的特点是氯浓度相对较低（低至 292 毫摩尔/千克），这表明亚临界相分离。这些流体还具有三个站点的最低pH值（2.8-3.1）、最高H ₂ S和最低H ₂、CH ₄和CO ₂浓度。δD 和δ ¹⁸ O 值表明H ₂ O 和CO ₂是由与俯冲相关的火山蒸气少量添加的。然而，没有证据表明岩浆 SO ₂在 2018 年采样时输入喷口流体。尽管位于火山口的相对两侧，但来自火山口环断层的北部和西南锥体位置的喷口流体具有相似的化学成分。来自这些地点的流体比来自中南部的流体具有较低的 Fe/Mn 比率 (<1) 和 H ₂ S 浓度，这表明它们受到地下冷却和硫化物沉淀的影响。这项研究表明，纽瓦塔希热液喷口位置的变化取决于火山口内的位置，流体-岩石相互作用和岩浆输入对流体成分的不同影响与之前关于流体 S 同位素和硫化物的工作一致。