A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab
A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab
复制标题
DOI：
10.1016/j.chemgeo.2010.12.001
复制
发表时间：
2011-02
期刊：
影响因子：
3.9
通讯作者：
R. Halama;T. John;P. Herms;F. Hauff;V. Schenk
中科院分区：
文献类型：
--
作者：
R. Halama;T. John;P. Herms;F. Hauff;V. Schenk
两种不同类型的榴辉岩的Raspas复杂的（厄瓜多尔），这可以区分的基础上岩相学和微量元素地球化学，分析其稳定的（Li，O）和放射性（Sr，Nd）同位素的签名，以约束交代的变化，由于流体叠印在高压条件下的变质玄武岩，并确定流体来源。MORB型榴辉岩具有类似于MORB的LREE相对亏损特征。这类榴辉岩的高压矿物氧同位素组成变化很大（石榴石：+4.1 ~+9.8‰;绿辉石：+6.1 ~+11.0‰;多硅白云母：8.7 ~ 10.4‰;角闪石：6.2 ~ 10.1‰），一般表现出平衡氧同位素分馏。初始87 Sr/86 Sr同位素比值也是可变的（0.7037-0.7063），而ε Nd 130 Ma值（+8.3至+11.0）相对相似。岩石之间的Sr和O同位素组成的差异露头规模，保存的低温蚀变洋壳的O同位素组成，和Sr-Nd同位素的趋势，典型的海底蚀变建议继承从地幔蚀变洋壳。然而，δ 7 Li值（-0.5至-12.9 ‰）随Li浓度（11- 94 ppm）的增加而降低，表明Li同位素分馏是通过与流体-岩石相互作用有关的扩散进行的。Li同位素被证明是一个非常敏感的交代示踪剂，虽然小的影响，Sr-Nd-O同位素系统表明，在MORB型榴辉岩的流体诱导的交代事件是小规模的低水/岩比。这种交代流体被认为主要来源于MORB型岩石的原位脱水。与MORB型榴辉岩相比，Raspas杂岩中的第二类榴辉岩--带状榴辉岩以富含不相容微量元素和带状榴辉岩为特征。榴辉岩具有均一的Sr-Nd同位素特征（初始87 Sr/86 Sr =0.7075-0.7081，ε Nd 130 Ma =−6.7-−8.7），被解释为反映了交代叠加。同位素特征可以归因于交代形成的带状疱疹，因为相关带状疱疹静脉的同位素相似。石榴子石、绿辉石、角闪石和绿柱石的氧同位素值相对均一（10.9-12.3‰、9.4 - 10.8‰、10.0-10.1‰和10.5-11.9‰），矿物间氧同位素不平衡与开放体系流体输入的交代叠加作用一致。榴辉岩的Li含量（46- 76 ppm）和δ 7 Li值与MORB型榴辉岩的范围重叠。同位素均一化所需的大量流体，结合流体包裹体研究的结果，表明，在产生交代流体，改变了闪长榴辉岩的沙漠化发挥了重要作用。然而，（Meta）沉积物源的影响，需要基于Sr-Nd同位素数据和微量元素富集。在试图限制会聚边缘的再循环时，必须考虑到与交代流体相互作用所产生的各种榴辉岩的显著地球化学变化。
Two distinct types of eclogites from the Raspas Complex (Ecuador), which can be distinguished based on petrography and trace element geochemistry, were analyzed for their stable (Li, O) and radiogenic (Sr, Nd) isotope signatures to constrain metasomatic changes due to fluid-overprinting in metabasaltic rocks at high-pressure conditions and to identify fluid sources. MORB-type eclogites are characterized by a relative LREE depletion similar to MORB. High-pressure (HP) minerals from this type of eclogite have highly variable oxygen isotope compositions (garnet: +4.1 to +9.8‰; omphacite: +6.1 to +11.0‰; phengite: 8.7 to 10.4‰; amphibole: 6.2 to 10.1‰) and generally show equilibrium oxygen isotope fractionation. Initial87Sr/86Sr isotope ratios are also variable (0.7037–0.7063), whereas εNd130Mavalues (+8.3 to +11.0) are relatively similar. Sr and O isotopic compositional differences among rocks on outcrop scale, the preservation of O isotopic compositions of low-temperature altered oceanic crust, and Sr–Nd isotopic trends typical for seafloor alteration suggest inheritance from variably altered oceanic crust. However, decreasing δ7Li values (−0.5 to −12.9‰) with increasing Li concentrations (11–94ppm) indicate Li isotope fractionation by diffusion related to fluid–rock interaction. Li isotopes prove to be a very sensitive tracer of metasomatism, although the small effects on the Sr–Nd–O isotope systems suggest that the fluid-induced metasomatic event in the MORB-type eclogites was small-scale at low-water/rock ratios. This metasomatic fluid is thought to predominantly derive from in situ dehydration of MORB-type rocks. Zoisite eclogites, the second eclogite type from the Raspas Complex, are characterized by the presence of zoisite and enrichment in many incompatible trace elements compared to the MORB-type eclogites. The zoisite eclogites have a homogenous Sr–Nd isotopic signature (initial87Sr/86Sr=0.7075–0.7081, εNd130Ma=−6.7 to −8.7), interpreted to reflect a metasomatic overprint. The isotopic signature can be attributed to the metasomatic formation of zoisite because associated zoisite veins are isotopically similar. Relatively homogenous O isotope values for garnet (10.9–12.3‰), omphacite (9.4 to 10.8‰), amphibole (10.0–10.1‰) and zoisite (10.5–11.9‰) and inter-mineral O isotopic disequilibria are consistent with a metasomatic overprint via open-system fluid input. Li concentrations (46–76ppm) and δ7Li values of the zoisite eclogites overlap the range of the MORB-type eclogites. The large amount of fluid required for isotopic homogenization, combined with the results from fluid inclusion studies, suggests that deserpentinization played a major role in generating the metasomatic fluid that altered the zoisite eclogites. However, influence of a (meta)sedimentary source is required based on Sr–Nd isotope data and trace element enrichments. The significant geochemical variation in the various eclogites generated by interaction with metasomatic fluids has to be considered in attempts to constrain recycling at convergent margins.