Quantifying the volume increase and chemical exchange during serpentinization
Quantifying the volume increase and chemical exchange during serpentinization
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量化蛇纹石化过程中的体积增加和化学交换
作者：
Klein, F;LeRoux, V
量化的岩石体积和流体成分在蛇纹石化过程中的并发变化仍然是一个重大的挑战，在大陆裂谷，海底扩张和俯冲过程中评估其物理化学效应。在这里，我们进行了一系列的11个热液实验室实验，其中天然纯橄榄岩，方辉橄榄岩和辉石岩的圆柱形岩心与水溶液在300 °C和35 MPa下反应长达18个月。使用三维显微计算机断层扫描和热重分析，我们表明，岩石体积随时间和反应程度系统地增加，导致在蛇纹石化10-18个月后，蚀变岩域的体积增加了44%（±8%）。体积膨胀伴随着Mg-Ca交换，而Fe和Si在很大程度上是守恒的。我们发现，原岩组成（橄榄石/斜方辉石比）起着重要的作用，在控制流体化学和含水次生矿物的比例在蛇纹石化。蚀变矿物学，反应流体的组成，和测得的体积变化之间的协议表明，静态条件下的蛇纹石化是一个体积增加的过程，尽管明显的质量转移。体积膨胀意味着单位质量原岩的载水量和浮力增加，而蛇纹岩化过程中的Mg-Ca交换可能会影响地球和其他海洋世界海水的Mg/Ca比。
Quantifying the concurrent changes in rock volume and fluid composition during serpentinization remains a major challenge in assessing its physicochemical effects during continental rifting, seafloor spreading, and subduction. Here we conducted a series of 11 hydrothermal laboratory experiments where cylindrical cores of natural dunite, harzburgite, and pyroxenite were reacted with an aqueous solution at 300 °C and 35 MPa for up to 18 months. Using three-dimensional microcomputed tomography and thermogravimetry, we show that rock volume systematically increased with time and extent of reaction, leading to a volume increase of 44% (±8%) in altered rock domains after 10–18 months of serpentinization. The volume expansion was accompanied by Mg-Ca exchange between fluid and rock, while Fe and Si were largely conserved. We find that the protolith composition (olivine/orthopyroxene ratio) plays a significant role in controlling the fluid chemistry and the proportions of hydrous secondary minerals during serpentinization. Agreement between alteration mineralogy, composition of reacting fluids, and measured volume changes suggests that serpentinization under static conditions is a volume-increasing process in spite of demonstrable mass transfer. Volume expansion implies an increased water carrying capacity and buoyancy force of serpentinite per unit mass of protolith, while Mg-Ca exchange during serpentinization may affect the Mg/Ca ratio of seawater on Earth and possibly other ocean worlds.
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