The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)
The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)
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DOI：
10.2138/am-2020-6994
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发表时间：
2020-03
影响因子：
3.1
通讯作者：
B. Urann;V. Roux;T. John;G. Beaudoin;J. Barnes
中科院分区：
文献类型：
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作者：
B. Urann;V. Roux;T. John;G. Beaudoin;J. Barnes
摘要我们提出了在现场二次离子质谱（西姆斯）和电子探针分析共存的石榴石，绿辉石，多硅白云母，角闪石，磷灰石，结合热解块体岩石分析，以限制分布，丰度和行为的卤素（F和Cl）在六个MORB样榴辉岩的Raspas复杂（厄瓜多尔南部）。在所有关于晶格中的卤素的情况下，F相容性从磷灰石（1.47-3.25重量%）到角闪石（563-4727 μg/g）、多硅白云母（610-1822 μg/g）、绿辉石（6.5-54.1 μg/g）和石榴石（1.7-8.9 μg/g）降低。Cl的相对相容性以磷灰石最高（192-515 μg/g），其次为角闪石（0.64-82.7 μg/g）、多硅白云母（1.2-2.1 μg/g）、绿辉石（<0.05-1.0 μg/g）和石榴石（<0.05 μg/g）。SIMS重建的F体丰度和产量校正的批量热解分析之间的一致性表明，F主要是托管在榴辉岩矿物的晶格内。然而，SIMS重建的Cl丰度是一个因素的5低，平均而言，比热解衍生的散装浓度。这种差异的结果，从流体包裹体的贡献，其中可能主机至少80%的散装岩石Cl。西姆斯和热解的组合是高度互补的。而西姆斯是非常适合于确定散装F丰度，热解更好地量化散装Cl浓度，其中包括流体包裹体托管Cl的贡献。拉斯帕斯榴辉岩含F 145-258 μg/g，Cl至少7-11 μg/g。我们估计，约95%的F保留在板通过榴辉岩化和俯冲过程中返回到上地幔，而至少95%的俯冲氯从岩石中删除的时候板平衡在榴辉岩相条件。我们的计算提供了进一步的证据，在高压变质作用在俯冲带的F从Cl的分馏。虽然HIMU（高U/Pb）地幔源（脱水洋壳）往往与富集Cl/K和F/Nd，Raspas榴辉岩显示出相对较低的卤素比相同的不确定性亏损MORB地幔（DMM）。因此，所观察到的卤素富集在HIMU洋岛玄武岩需要进一步分馏过程中地幔加工或回收的卤素富集载体岩性，如蛇纹岩到地幔。
Abstract We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (<0.05–1.0 μg/g), and garnet (<0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.