Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization
Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization
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DOI：
10.1016/j.epsl.2014.07.004
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发表时间：
2014-10-01
影响因子：
5.3
通讯作者：
Brunet, Fabrice
中科院分区：
文献类型：
--
作者：
Malvoisin, Benjamin;Brunet, Fabrice
一系列在高压和高温下烧结的具有两种不同孔隙率（约1%和10%）和粒度（1-5 μ m和0-38 μ m）的San卡洛斯橄榄石聚集体在300 ℃和500巴下在纯水存在下反应。反应过程用磁力监测，在每次实验结束时分析反应产物的组成和分布。水镁石的形成主要发生在骨料表面作为一个结果，既一致的橄榄石溶解和水溶液中的Mg和Si的反应产物，即水镁石和利蛇纹石缓冲。在290天后测量的反应进程不超过2.6%，这与先前对San卡洛斯橄榄石粉末进行的研究（即，水溶液中分离的颗粒）。因此，有限的水运输通过粒间区域的聚合物急剧减少了橄榄石表面积的水，从而减缓了整个蛇纹石化过程。当外推到橄榄岩相关的橄榄石晶粒尺寸，我们的实验结果表明，水的扩散速率将变得如此之慢，暴露于水的第一层原生矿物的网状结构必须充分反应之前，下一个矿物层开始反应（“逐层”机制）。这种类型的反应-传输机制显然是不符合微尺度的蛇纹石分布在大洋橄榄岩样品的网格，因此，额外的水传输路径是必需的。拉伸热应力下形成的裂缝是很好的候选人，因为相比，在晶界或反应引起的骨折，他们是足够宽，大大提高海洋橄榄岩中的水运输，因此占观察到的纹理。这里推断的“逐层”机制只能为蛇纹化作用的完成设定一个较低的时间界限。假设筛孔尺寸为1 mm，初始粒度为100 μ m，考虑到温度范围为100 - 300 ℃，永久水饱和，应在100-1000年内完成。令人惊讶的是，这一持续时间仅占天然蛇纹石化过程估计时间尺度的1%至10%，强调了水的可用性在自然反应过程中发挥的核心作用。(C)2014爱思唯尔有限公司版权所有。
A series of San Carlos olivine aggregates, sintered at high pressure and high temperature, with two different porosities (around 1 and 10%) and grain sizes (1-5 mu m and 0-38 mu m) were reacted at 300 degrees C and 500 bars in the presence of pure water. The reaction progress was monitored magnetically and the composition and distribution of the reaction products were analyzed at the end of each experiment. Brucite formation mainly occurred at the aggregate surface as a result of both congruent olivine dissolution and aqueous Mg and Si buffering by the reaction products, i.e. brucite and lizardite. The measured reaction progress did not exceed 2.6% after 290 d, which strongly contrasts with previous studies performed on San Carlos olivine powders (i.e., isolated grains in aqueous solution). Hence, limited water transport through the intergranular region of the aggregate drastically decreased the olivine surface area accessible to water and thus slowed down the whole serpentinization process. When extrapolated to peridotite relevant olivine grain sizes, our experimental results indicate that the water diffusion rate will become so slow that the first layer of primary minerals exposed to water within a mesh structure must fully react before the next mineral layer starts reacting ("layer by layer" mechanism). This type of reaction-transport mechanism is obviously not consistent with the micro-scale serpentine distribution in the mesh of oceanic peridotite samples, therefore additional water transport pathways are required. Cracks formed under extensional thermal stresses are good candidates since, in comparison to grain boundary or reaction-induced fractures, they are wide enough to drastically enhance water transport in oceanic peridotites and therefore account for the observed textures. The 'layer by layer' mechanism inferred here can only set a lower time bound for serpentinization completion. Assuming a mesh size of 1 mm and an initial grain size of 100 pm and considering a temperature ranging from 100 to 300 degrees C with permanent water saturation, completion should take place within 100-1000 yr. Surprisingly, this duration represents only 1 to 10% of the estimated timescale of the natural serpentinization process, emphasizing the central role played by water availability in the natural reaction process. (C) 2014 Elsevier B.V. All rights reserved.