Chemistry of altered basaltic glass shards and zeolites of volcaniclastic sediments from ODP Hole 157-953C

Trace element concentrations of altered basaltic glass shards (layer silicates) and zeolites in volcaniclastic sediments drilled in the volcanic apron northeast of Gran Canaria during Ocean Drilling Program (ODP) leg 157 document variable element mobilities during low-temperature alteration processes in a marine environment. Clay minerals (saponite, montmorillonite, smectite) replacing volcanic glass particles are enriched in transition metals and rare earth elements (REE). The degree of retention of REE within the alteration products of the basaltic glass is correlated with the field strength of the cations. The high field-strength elements are preferentially retained or enriched in the alteration products by sorption through clay minerals. Most trace elements are enriched in a boundary layer close to the interface mineral–altered glass. This boundary layer has a key function for the physico-chemical conditions of the subsequent alteration process by providing a large reactive surface and by lowering the fluid permeability. The release of most elements is buffered by incorporation into secondary precipitates (sodium-rich zeolites, phillipsite, Fe- and Mn-oxides) as shown by calculated distribution coefficients between altered glasses and authigenic minerals. Chemical fluxes change from an open to a closed system behavior during prograde low-temperature alteration of volcaniclastic sediments with no significant trace metal flux from the sediment to the water column.

大洋钻探计划（Ocean Drilling Program, ODP）第157航次在大加纳利岛东北部火山裙（volcanic apron）钻取的火山碎屑沉积物（volcaniclastic sediments）中，蚀变玄武岩玻璃碎屑（层状硅酸盐（layer silicates））与沸石（zeolites）的微量元素浓度数据，阐明了海洋环境下低温蚀变（low-temperature alteration）过程中元素活动性的变化特征。
替换火山玻璃颗粒的黏土矿物（clay minerals）（皂石（saponite）、蒙脱石（montmorillonite）、蒙皂石（smectite））富集过渡金属（transition metals）与稀土元素（rare earth elements, REE）。
玄武岩玻璃蚀变产物中稀土元素的保留程度与阳离子场强（field strength of the cations）呈显著相关。
高场强元素（high field-strength elements）通过黏土矿物的吸附作用（sorption），优先在蚀变产物中留存或富集。
多数微量元素在靠近矿物-蚀变玻璃界面（mineral–altered glass interface）的边界层（boundary layer）内发生富集。
该边界层可提供大面积反应表面（reactive surface）并降低流体渗透率（fluid permeability），对后续蚀变过程的物理化学条件（physico-chemical conditions）具有关键调控作用。
通过计算蚀变玻璃与自生矿物（authigenic minerals）间的分配系数（distribution coefficients）可知，多数元素的释放会被次生沉淀矿物（secondary precipitates）（富钠沸石（sodium-rich zeolites）、片沸石（phillipsite）、铁锰氧化物（Fe- and Mn-oxides））的固溶作用所缓冲。
在火山碎屑沉积物的递进式低温蚀变（prograde low-temperature alteration）过程中，化学通量（chemical fluxes）呈现从开放体系（open system）向封闭体系（closed system）的转变特征，且未出现显著的痕量金属从沉积物向水体水柱（water column）的迁移。