The northwestern caldera wall of Brothers volcano in the southern Kermadec arc features several clusters of hydrothermal venting in a large area that extends from near the caldera floor (~1700 mbsl) almost up to the crater rim (~1300 mbsl). Abundant black smoker-type hydrothermal chimneys and exposed stockwork mineralization in this area provide an excellent archive of hydrothermal processes that form seafloor massive sulfide deposits. Using sulfate precipitates from chimneys and stockwork recently recovered by remotely operated vehicles, we conducted fluid inclusion microthermometry and Sr isotope studies to determine the role of phase separation and mixing between vent fluid and seawater. The variability in the vast majority of fluid inclusion salinities (i.e., 0.1–5.25 wt.% NaCl eq.) and entrapment temperatures of up to 346°C are indicative of phase-separated hydrothermal fluids. Large salinity variations in samples with entrapment temperatures mostly below the boiling temperature for the sample’s depth show that the majority of fluids ascending below the NW Caldera are phase separating in the subsurface and cooling, prior to discharge. In several samples, entrapment temperatures of over 343°C suggest that phase-separating fluids have at least sporadically exited the seafloor at the NW Caldera site. Isobaric-isenthalpic mixing trends between coexisting phase-separated vapors and brines with seawater are consistent with phase-separated fluids at near-seafloor pressures of ~170 bar and suggest that the vast majority of the ascending fluids continue to phase separate to within tens to hundreds of meters below seafloor prior to mixing with seawater. A small subset of the most saline fluid inclusions (up to 18.6 wt.% NaCl eq.) is unlikely formed by near-seafloor phase separation and is considered to be produced either by supercritical phase separation or by the contribution of a magmatic brine from near the magmatic-hydrothermal interface. 87Sr/86Sr values of sulfate samples range from 0.7049 (i.e., near hydrothermal end-member) to 0.7090 (i.e., near seawater) and show that the crystals grew from vapor- and brine-derived fluids in a hydrothermally dominated mixing regime. Our work provides new insights into mineral growth conditions, mixing regimes, and in particular, the extent and character of subseafloor phase separation during the formation of hydrothermal vents and their underlying stockwork in seawater-dominated, arc-related hydrothermal systems.

中文翻译：

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NW Caldera Vent Field, Brothers Volcano 中深层盐水的亚临界相分离和发生：来自热液沉淀物中流体包裹体的证据

位于 Kermadec 弧南部的 Brothers 火山的西北部火山口壁具有大面积的几个热液喷口群，从火山口底部附近（~1700 mbsl）几乎延伸到火山口边缘（~1300 mbsl）。该地区丰富的黑烟型热液烟囱和裸露的网状矿化提供了形成海底块状硫化物矿床的热液过程的绝佳档案。使用最近由遥控车辆回收的烟囱和木制品中的硫酸盐沉淀物，我们进行了流体包裹体显微测温和 Sr 同位素研究，以确定排放流体和海水之间的相分离和混合的作用。绝大多数流体包裹体盐度的变异性（即 0.1-5.25 wt.% NaCl eq. ) 和高达 346°C 的截留温度表明存在相分离的热液流体。夹带温度大多低于样品深度沸腾温度的样品中的大盐度变化表明，在 NW Caldera 下方上升的大多数流体在地下相分离并在排放之前冷却。在几个样本中，超过 343°C 的截留温度表明相分离流体至少偶尔会离开 NW Caldera 站点的海底。共存的相分离蒸气和盐水与海水之间的等压-等焓混合趋势与近海底压力约 170 巴下的相分离流体一致，并表明绝大多数上升流体继续相分离到几十到几百在与海水混合之前，海底以下几米。大多数含盐流体包裹体的一小部分（高达 18.6 wt.% NaCl 当量）不太可能由近海底相分离形成，被认为是通过超临界相分离或近海岩浆盐水的贡献产生的。岩浆-热液界面。硫酸盐样品的 87Sr/86Sr 值范围从 0.7049（即接近热液端元）到 0.7090（即，靠近海水）并表明晶体从蒸汽和盐水衍生的流体中以热液为主的混合方式生长。我们的工作为矿物生长条件、混合方式，特别是海底热液喷口形成过程中相分离的程度和特征及其在以海水为主的弧相关热液系统中的底层网状结构提供了新的见解。