Chemical composition of altered volcanic rocks from ODP Sites 193-1188 and 193-1189

The conventional model of leaching volcanic rocks as a source of metals in a seafloor hydrothermal systems has been tested by examining the behavior of Pb and other trace elements during hydrothermal alteration. ODP Leg 193 drill sites 1188 (Snowcap) and 1189 (Roman Ruins) on Pual Ridge in the eastern Manus Basin offshore eastern Papua New Guinea provide a unique three-dimensional window into an active back-arc hydrothermal system. We investigate by means of a LA-ICP-MS microbeam technique the capacity of Pb to be leached from a host volcanic rock exposed to various types and intensities of alteration. Our results are in general agreement with previous studies that utilized bulk analytical techniques but provide a more detailed explanation of the processes.
Fresh representative dacitic lavas from the Pual Ridge have an average whole rock Pb content of 5.2 ppm, an average interstitial glass Pb content of 5.6 ppm and an average plagioclase Pb content of 1.0 ppm. Altered matrix samples have highly variable Pb values ranging from 0 to 52.4 ppm. High Pb values in altered samples are associated with a low temperature chlorite and clay mineral assemblage, in some cases overprinted by a high temperature (up to 350°C) silica-rich "bleaching" alteration. Only the most highly altered matrix samples have REE patterns that differ from the fresh Pual Ridge dacite. This may represent either different lava histories or alteration characteristics that have affected normally immobile REEs. Altered samples with the highest Pb values have similar REE patterns to those of the local unaltered lavas. They are compositionally similar to typical Pual Ridge dacites indicating a genetic relationship between the main regional volcanic suite and the subseafloor hydrothermally altered, Pb-enriched material.
Relative loss/gain for Pb between the analyzed altered samples and a calculated precursor show a maximum relative gain of 901%. Samples with relative Pb gain from both drill sites are associated with lower temperature alteration mineral assemblages characterized by pervasive chloritization. The related lower temperature (220-250°C) neutral to slightly acidic fluids have been ascribed by others to return circulation of hydrothermal fluids that did not interact with seawater. Because altered samples have a higher Pb content than the fresh precursor, leaching of fresh volcanic rocks cannot be the source of Pb in the hydrothermal systems.

以海底热液系统中火山岩作为金属来源的传统淋滤模型，已通过考察热液蚀变过程中铅（Pb）及其他微量元素的行为得以验证。位于巴布亚新几内亚东部近海马努斯海盆东部保罗海岭的大洋钻探计划（Ocean Drilling Program, ODP）第193航次钻探站位1188（雪顶）与1189（罗马遗迹），为了解活跃的弧后热液系统提供了独特的三维窗口。本研究借助激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）微束技术，探究了暴露于不同类型与强度蚀变作用的宿主火山岩中铅的淋滤能力。本研究结果与既往采用整体分析技术的研究总体一致，但为相关过程提供了更为细致的解释。

取自保罗海岭的新鲜典型英安岩熔岩，其全岩铅平均含量为5.2 ppm，粒间玻璃质铅平均含量为5.6 ppm，斜长石铅平均含量为1.0 ppm。蚀变基质样品的铅含量波动范围极大，介于0至52.4 ppm之间。蚀变样品中的高铅含量与低温绿泥石-黏土矿物组合相关，部分样品还叠加了高温（最高达350℃）富硅“漂白”蚀变。仅蚀变程度最高的基质样品，其稀土元素（REE）配分模式与新鲜保罗海岭英安岩存在差异，这可能代表两种情况：一是熔岩成因历史不同，二是蚀变作用影响了通常不易活动的稀土元素。铅含量最高的蚀变样品，其稀土元素配分模式与当地未蚀变熔岩相似；它们的化学成分与典型保罗海岭英安岩一致，表明区域主要火山岩组合与海底以下经热液蚀变且富铅的物质之间存在成因联系。

在所分析的蚀变样品与计算得到的原始岩样之间，铅的相对损失/增益量最高可达901%的相对富集。两个钻探站位中铅相对富集的样品，均与以普遍绿泥石化为特征的低温蚀变矿物组合相关。相关的低温（220~250℃）中性至弱酸性流体，被其他学者归因于未与海水发生相互作用的热液流体循环回流。由于蚀变样品的铅含量高于原始新鲜岩样，因此新鲜火山岩的淋滤作用不可能是海底热液系统中铅的来源。