Geochemistry of Ocean Floor and Fore-arc Serpentinites: Constraints on the Ultramafic Input to Subduction Zones
Geochemistry of Ocean Floor and Fore-arc Serpentinites: Constraints on the Ultramafic Input to Subduction Zones
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DOI：
10.1093/petrology/egr058
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发表时间：
2012-02-01
影响因子：
3.9
通讯作者：
Gmeling, Katalin
中科院分区：
文献类型：
--
作者：
Kodolanyi, Janos;Pettke, Thomas;Gmeling, Katalin
我们基于从深海钻探项目和海洋钻探计划的钻芯上收集的块体岩石和原位矿物主量和微量元素成分数据，对大洋中脊（大西洋中脊和赫斯深海）、被动边缘（伊比利亚深海平原和纽芬兰）和弧前（马里亚纳和危地马拉）蛇纹岩的地球化学提供了新的见解。这些数据对于限制俯冲带蛇纹岩微量元素库存非常重要。相对于地幔熔融过程中相容性相似的元素，块状蛇纹岩的 Cl、B、Sr、U、Sb、Pb、Rb、Cs 和 Li 富集程度高达几个数量级，对应于大洋岩石圈俯冲岩性（蛇纹岩、沉积物和蚀变火成岩）中最高的原始地幔归一化 B/Nb、B/Th、U/Th、Sb/Ce、Sr/Nd 和 Li/Y。洋壳）。在相对富集的元素中，Cl和B的丰度最高，总体浓度大多分别在1000μg g(-1)和30μg g(-1)以上。除含碳酸盐蛇纹岩中的 Sr（数千 μ g g(-1)）外，所有其他显示相对富集的微量元素通常以低浓度（μ g g(-1) 水平）存在。原位数据表明，蛇纹石化作用会增加 Cl、B、Sr、U、Sb、Rb 和 Cs 的浓度，并且 Li 的浓度也会增加。这些元素主要存在于蛇纹石（蜥蜴石和温石棉，但不是叶蛇纹石）中。文石沉淀导致 Sr、U 和 B 显着富集，而方解石仅作为 Sr 主体而重要。通常观察到的水镁石缺乏微量元素。来自大洋中脊、被动边缘和弧前的蛇纹岩的总体富集模式具有可比性，而富集程度通常取决于地球动力学环境。特定环境（和地点）内相对微量元素富集的变化可能有几个数量级。除了普遍存在的 Cl、B 和 Sr 富集之外，大洋中脊蛇纹岩通常表现出明显的块岩 U 富集。它们还在球粒陨石标准化稀土元素图上表现出正 Eu 异常。被动边缘蛇纹岩的总体不相容微量元素含量往往高于洋中脊和弧前蛇纹岩，并且在所有研究的蛇纹岩中显示出最高的 B 富集度。弧前蛇纹岩的特点是微量元素总体含量较低，Cl 最低，但 Rb、Cs 和 Sr 富集度最高。根据我们的数据，俯冲脱水蛇纹岩可能释放出高B/Nb、B/Th、U/Th、Sb/Ce和Sr/Nd的流体，使它们成为弧岩浆一些特征微量元素指纹（例如高B/Nb、高Sr/Nd、高Sb/Ce）的潜在来源之一。然而，尽管蛇纹岩是全球俯冲带化学循环的重要组成部分，但由于其总体微量元素含量较低（B和Cl除外），它们在弧岩浆源上的地球化学印记（除了添加H2O、B和Cl）可能会被俯冲地壳成分的微量元素信号相当大地掩盖。
We provide new insights into the geochemistry of serpentinites from mid-ocean ridges (Mid-Atlantic Ridge and Hess Deep), passive margins (Iberia Abyssal Plain and Newfoundland) and fore-arcs (Mariana and Guatemala) based on bulk-rock and in situ mineral major and trace element compositional data collected on drill cores from the Deep Sea Drilling Project and Ocean Drilling Program. These data are important for constraining the serpentinite-hosted trace element inventory of subduction zones. Bulk serpentinites show up to several orders of magnitude enrichments in Cl, B, Sr, U, Sb, Pb, Rb, Cs and Li relative to elements of similar compatibility during mantle melting, which correspond to the highest primitive mantle-normalized B/Nb, B/Th, U/Th, Sb/Ce, Sr/Nd and Li/Y among subducted lithologies of the oceanic lithosphere (serpentinites, sediments and altered igneous oceanic crust). Among the elements showing relative enrichment, Cl and B are by far the most abundant with bulk concentrations mostly above 1000 mu g g(-1) and 30 mu g g(-1), respectively. All other trace elements showing relative enrichments are generally present in low concentrations (mu g g(-1) level), except Sr in carbonate-bearing serpentinites (thousands of mu g g(-1)). In situ data indicate that concentrations of Cl, B, Sr, U, Sb, Rb and Cs are, and that of Li can be, increased by serpentinization. These elements are largely hosted in serpentine (lizardite and chrysotile, but not antigorite). Aragonite precipitation leads to significant enrichments in Sr, U and B, whereas calcite is important only as an Sr host. Commonly observed brucite is trace element-poor. The overall enrichment patterns are comparable among serpentinites from mid-ocean ridges, passive margins and fore-arcs, whereas the extents of enrichments are often specific to the geodynamic setting. Variability in relative trace element enrichments within a specific setting (and locality) can be several orders of magnitude. Mid-ocean ridge serpentinites often show pronounced bulk-rock U enrichment in addition to ubiquitous Cl, B and Sr enrichment. They also exhibit positive Eu anomalies on chondrite-normalized rare earth element plots. Passive margin serpentinites tend to have higher overall incompatible trace element contents than mid-ocean ridge and fore-arc serpentinites and show the highest B enrichment among all the studied serpentinites. Fore-arc serpentinites are characterized by low overall trace element contents and show the lowest Cl, but the highest Rb, Cs and Sr enrichments. Based on our data, subducted dehydrating serpentinites are likely to release fluids with high B/Nb, B/Th, U/Th, Sb/Ce and Sr/Nd, rendering them one of the potential sources of some of the characteristic trace element fingerprints of arc magmas (e.g. high B/Nb, high Sr/Nd, high Sb/Ce). However, although serpentinites are a substantial part of global subduction zone chemical cycling, owing to their low overall trace element contents (except for B and Cl) their geochemical imprint on arc magma sources (apart from addition of H2O, B and Cl) can be masked considerably by the trace element signal from subducted crustal components.