The 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10^–4 to 10^–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (>300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

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兄弟火山 IODP 376 探险队热液蚀变和矿化对岩石古磁特性的影响

海底热液系统的 3D 海底结构在很大程度上是未知的，特别是在弧火山。这些系统中火山岩的变化导致其磁性发生巨大变化。在这里，我们首次对来自兄弟火山（克马德克弧）的热液蚀变英安岩定向样本进行古地磁测量的全面研究，这些样本是在国际海洋发现计划 (IODP) 第 376 次探险期间钻探的。这些数据使人们能够深入了解磁性矿物的逐渐演化。在受不同类型和程度的热液蚀变影响的海底火山岩中，以响应不同的流体温度、化学成分和相关矿化；从典型的相对低温与海水相互作用的初始绿泥石化到受高温、强酸性岩浆流体影响的极度蚀变岩石。热液蚀变样品显示自然剩磁强度显着降低（10 –4 至 10 –2 ^A / ^m） ）与未蚀变样品（1-10 A/m）相比，表明原生钛磁铁矿晶粒在热液蚀变过程中被破坏。除了靠近矿化网状带的小区域外，没有观察到与热液蚀变相关的化学剩磁，这与黄铁矿、金红石和白二烯等抗磁性和/或顺磁性矿物的广泛形成一致，这些矿物不携带退磁实验表明，大多数取向样品都具有稳定的特征剩磁，这是由海底岩石冷却时形成的残余原生磁性矿物引起的。然而，部分绿泥石化英安岩的特点是磁化率大、柯尼斯堡比低、磁矫顽力极低，与较小的单畴磁性颗粒的初始溶解一致，表明强烈热液蚀变的岩石比最初绿泥石化的样品是更好的古地磁指标。除了在兄弟火山观察到的厚层（> 300 m）退磁岩石之外，新鲜岩石和热液蚀变岩石之间的显着磁力对比证实了经验结果，即磁异常是确定磁异常的重要地球物理工具。海底弧火山热液系统的几何形状。