The hydrothermally active Brothers volcano on the Kermadec arc, New Zealand, hosts two geochemically distinct hydrothermal systems within a single caldera. At the NW Caldera, metal-sulfide–rich black smoker spires form on the caldera wall. In contrast, Fe-rich crusts and native sulfur-rich chimneys occur at the resurgent central Upper Cone. Previous studies have revealed that the contrasting styles of hydrothermalism relate to the variable contribution of magmatic volatiles between these sites, with the Upper Cone experiencing relatively higher amounts of magmatic volatile influx. We present results of a study of the hydrothermal alteration within Brothers volcano based on core samples to a depth of 453 meters below sea floor (mbsf) from both the Upper Cone (Site U5128) and NW Caldera sites (Site U1527 and U1530), drilled by the International Ocean Discovery Program. The dacitic to rhyolitic breccias that make up the volcano are variably altered to alteration mineral assemblages consisting of chlorite + quartz, illite + pyrophyllite, natroalunite + pyrophyllite, and smectite-rich assemblages. The distribution and textures of the alteration minerals within and between different sites at Brothers volcano reflect variations in temperature, fluid pH, and fluid flux. We find that natroalunite only occurs at the Upper Cone, while alteration at the NW Caldera is more diverse and is characterized by both chlorite and pyrophyllite-rich alteration, indicating that seawater-derived hydrothermal fluids overprinted earlier magmatic volatile-influenced alteration. Our data indicate that in magmatic volatile-dominated systems, the alteration mineralogy transitions from natroalunite to pyrophyllite-rich with increasing age or maturity. This is accompanied by a distinct change in sample texture from dominantly bleached selvages to a more massive, equigranular texture.

中文翻译：

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新西兰克马德克弧兄弟海底弧火山内的热液蚀变

新西兰克马德克弧上的兄弟火山热液活跃，在一个破火山口内拥有两个地球化学上截然不同的热液系统。在火山口西北部，火山口壁上形成富含金属硫化物的黑色烟囱尖顶。相比之下，富铁地壳和天然富硫烟囱出现在复兴的中央上锥体处。先前的研究表明，热液作用的对比风格与这些地点之间岩浆挥发物的不同贡献有关，上锥体经历了相对较高量的岩浆挥发物流入。我们展示了兄弟火山内热液蚀变的研究结果，该研究基于从上锥体（U5128 站点）和西北火山口遗址（U1527 和 U1530 站点）到海底以下 453 米深度 (mbsf) 钻探的岩芯样本由国际海洋探索计划资助。构成火山的英安岩到流纹岩角砾岩被不同程度地改变，改变了由绿泥石+石英、伊利石+叶腊石、硝明矾+叶蜡石和富含蒙脱石组合组成的矿物组合。兄弟火山不同地点内部和之间的蚀变矿物的分布和结构反映了温度、流体 pH 值和流体通量的变化。我们发现钠明矾石仅出现在上锥体，而西北火山口的蚀变更加多样化，并且以富含绿泥石和叶蜡石的蚀变为特征，这表明海水衍生的热液流体覆盖了早期岩浆挥发性影响的蚀变。我们的数据表明，在以岩浆挥发物为主的系统中，随着年龄或成熟度的增加，蚀变矿物从钠明矾转变为富含叶腊石。这伴随着样品纹理的明显变化，从主要漂白的布边到更厚重、等颗粒的纹理。