The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation
The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation
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DOI：
10.1007/s00410-009-0454-x
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发表时间：
2010-06
影响因子：
3.5
通讯作者：
B. Wunder;F. Deschamps;A. Watenphul;S. Guillot;A. Meixner;R. Romer;R. Wirth
中科院分区：
文献类型：
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作者：
B. Wunder;F. Deschamps;A. Watenphul;S. Guillot;A. Meixner;R. Romer;R. Wirth
在P、T = 0.2-4.0 GPa、200-500°C条件下，测定了含锂蛇纹石相利蛇纹石、纤蛇纹石、叶蛇纹石和水相流体的锂同位素分馏。对于蜥蜴石-流体和安替哥石-流体系统中的实验，7 Li优先分配到流体中，Δ 7 Li值遵循含Li云母-流体的T依赖性分馏（Wunder等人，2007）。相比之下，对于温石棉流体实验，7 Li弱分配到温石棉。这种对比鲜明的行为可能是由于不同的锂环境中的三个蛇纹石品种：在利蛇纹石和叶蛇纹石锂是六重协调，而在纤蛇纹石锂被纳入两种方式，八面体和锂轴承水簇填充的纳米管芯。温石棉的低温红外光谱测量表明，大量的水，其凝固点被抑制由于锂含量和有限的几何形状的流体内管。纤蛇纹石流体的Li同位素分馏是由于八面体Li[6]的晶内Li同位素分馏优先于6Li和纤蛇纹石通道Li[Ch]的晶内Li同位素分馏优先于7 Li。纤蛇纹石纳米管可能是锂的重要载体，也可能是其他流体活动元素在蛇纹化洋壳中的重要载体。这可能解释了相对于高温石棉蛇纹岩，低温石棉蛇纹岩具有更高的Li丰度。同位素重含锂流体的纤蛇纹石纳米管可以在俯冲过程中释放在相对较浅的深度，完成纤蛇纹石反应之前，形成叶蛇纹石。在进一步的俯冲过程中，蛇纹石相分解过程中产生的流体将耗尽7 Li。这种行为可以解释一些观察到的蛇纹橄榄岩锂同位素的不均匀性。
We determined the lithium isotope fractionation between synthetic Li-bearing serpentine phases lizardite, chrysotile, antigorite, and aqueous fluid in theP,Trange 0.2–4.0 GPa, 200–500°C. For experiments in the systems lizardite-fluid and antigorite-fluid,7Li preferentially partitioned into the fluid and Δ7Li values followed theT-dependent fractionation of Li-bearing mica-fluid (Wunder et al. 2007). By contrast, for chrysotile-fluid experiments,7Li weakly partitioned into chrysotile. This contrasting behavior might be due to different Li environments in the three serpentine varieties: in lizardite and antigorite lithium is sixfold coordinated, whereas in chrysotile lithium is incorporated in two ways, octahedrally and as Li-bearing water cluster filling the nanotube cores. Low-temperature IR spectroscopic measurements of chrysotile showed significant amounts of water, whose freezing point was suppressed due to the Li contents and the confined geometry of the fluid within the tubes. The small inverse Li-isotopic fractionation for chrysotile-fluid results from intra-crystalline Li isotope fractionation of octahedral Li[6]with preference to6Li and lithium within the channels (Li[Ch]) of chrysotile, favoring7Li. The nanotubes of chrysotile possibly serve as important carrier of Li and perhaps also of other fluid-mobile elements in serpentinized oceanic crust. This might explain higher Li abundances for low-Tchrysotile-bearing serpentinites relative to high-Tserpentinites. Isotopically heavy Li-bearing fluids of chrysotile nanotubes could be released at relatively shallow depths during subduction, prior to complete chrysotile reactions to form antigorite. During further subduction, fluids produced during breakdown of serpentine phases will be depleted in7Li. This behavior might explain some of the Li-isotopic heterogeneities observed for serpentinized peridotites.