Abstract

We present the results of analogue experiments carried out in a piston–cylinder apparatus at 750–900°C and 2.9 GPa aimed to simulate metasomatic transformation of the fertile mantle caused by fluids and melts released from the subducting sediment. A synthetic H₂O- and CO₂-bearing mixture that corresponds to the average subducting sediment (GLOSS, Plank and Langmuir (1998)) and mineral fractions of natural lherzolite (analogue of a mantle wedge) were used as starting materials. Experiments demonstrate that the mineral growth in capsules is controlled by ascending fluid and hydrous melt (from 850°C) flows. Migration of the liquids and dissolved components develops three horizontal zones in the sedimentary layer with different mineral parageneses that slightly changed from run to run. In the general case, however, the contents of omphacite and garnet increase towards the upper boundary of the layer. Magnesite and omphacite (±garnet ± melt ± kyanite ± phengite) are widespread in the central zone of the sedimentary layer, whereas SiO₂ polymorph (± kyanite ± phengite ± biotite ± omphacite ± melt) occurs in the lower zone. Clinopyroxene disappears at the base of lherzolite layer and the initial olivine is partially replaced by orthopyroxene (± magnesite) in all experiments. In addition, talc is formed in this zone at 750°C, whereas melt appears at 850°C. In the remaining volume of the lherzolite layer, metasomatic transformations affect only grain boundaries where orthopyroxene (± melt ± carbonate) is developed. The described transformations are mainly related to a pervasive flow of liquids. Mineral growth in the narrow wall sides of the capsules is probably caused by a focused flow: omphacite grows up in the sedimentary layer, and talc or omphacite with the melt grow up in the lherzolite layer. Experiments show that metasomatism of peridotite related to a subducting sediment, unlike the metasomatism related to metabasites, does not lead to the formation of garnet-bearing paragenesis. In addition, uprising liquid flows (fluid, melt) do not remove significant amounts of carbon from the metasedimentary layer to the peridotite layer. It is assumed that either more powerful fluxes of aqueous fluid or migration of carbonate-bearing rocks in subduction melanges are necessary for more efficient transfer of crustal carbon from metasediments to a mantle in subduction zones.

中文翻译：

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2.9 GPa的俯冲沉积物-锂沸石相互作用：交代作用和部分熔融的影响

摘要

我们介绍了在750-900°C和2.9 GPa的活塞-缸装置中进行的模拟实验的结果，目的是模拟由俯冲沉积物释放的流体和熔体引起的肥沃地幔的交代转化。合成的H ₂ O-和CO ₂对应于平均俯冲沉积物的含碳混合物（GLOSS，Plank和Langmuir（1998））和天然锂铁矿的矿物级分（地幔楔的类似物）被用作起始材料。实验表明，胶囊中矿物质的生长受到上升流体和含水熔体（从850°C开始）流动的控制。液体和溶解成分的迁移在沉积层中形成了三个水平带，不同的矿物共生物之间存在着微小的变化。然而，在一般情况下，绿霞石和石榴石的含量朝着该层的上边界增加。菱镁矿和绿辉石（±石榴石±熔体±蓝晶石±锂铁矿）广泛分布在沉积层的中心区域，而SiO ₂在下部区域中出现了多晶型物（±蓝晶石±变色石±黑云母±绿沸石±熔体）。在所有实验中，斜生辉石在锂铁矿层的底部消失，并且最初的橄榄石部分被原辉石（±菱镁矿）替代。另外，在750℃在该区域中形成滑石，而在850℃出现熔体。在锂铁矿层的剩余体积中，交代转换仅影响晶格界线，而邻苯二茂（±熔体±碳酸盐）会发育。所描述的转换主要与普遍的液体流动有关。胶囊的窄壁侧面的矿物质生长可能是由集中的流动引起的：绿泥石在沉积层中长大，滑石或绿泥石与熔体在长石岩层中长大。实验表明，橄榄岩的交代作用与俯冲沉积物有关，与代谢物有关的交代作用不同，不会导致带有石榴石的共生物的形成。另外，起伏的液体流（流体，熔体）不会从次沉积层到橄榄岩层中去除大量的碳。人们认为，更有效的含水流体通量或俯冲混杂岩中含碳酸盐岩石的运移对于将地壳碳从变质沉积物更有效地转移到俯冲带中的地幔是必要的。