Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks
Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks
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DOI：
10.1016/j.gca.2009.12.003
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发表时间：
2010-03
影响因子：
5
通讯作者：
R. Halama;G. Bebout;T. John;V. Schenk
中科院分区：
文献类型：
--
作者：
R. Halama;G. Bebout;T. John;V. Schenk
本文首次测量了高压和超高压镁铁质榴辉岩的氮 (N) 浓度和同位素组成，旨在表征深俯冲蚀变洋壳 (AOC) 中氮的俯冲输入通量。研究的样本来自 Raspas Complex（厄瓜多尔）、Lago di Cignana（意大利）、Zambezi Belt（赞比亚）和 Cabo Ortegal（西班牙），共同代表俯冲深度为 50-90 公里。榴辉岩含有 2–20ppm N，δ15Nair 值范围为 -1 至 +8‰。这些值与改变的洋壳的值重叠，但与新鲜 MORB 的值不同（后者为 ∼1.1ppm N 和 δ15Nair∼−4‰）。根据氮数据与其他微量元素数据相结合，榴辉岩套可细分为那些与其可能的原岩 AOC 无法区分的、有或没有脱挥发分叠加效应的榴辉岩套（Lago di Cignana、Cabo Ortegal），以及那些在俯冲带变质作用期间经历过交代作用添加的榴辉岩套（赞比西带、Raspas）。对于前一组，与各种改变的 MORB 成分相比，榴辉岩中没有可检测到的氮损失，这表明氮保留在深俯冲的洋壳中。影响后一组的交代效应可以通过与（偏）沉积成分混合来最好地解释，导致 N 和其他微量元素（特别是 Ba 和 Pb）的相关富集，这些元素被认为是在 HP/UHP 变质作用期间移动的。蛇纹石化和高压变质橄榄岩，与 Raspas 和 Cabo Ortegal 的榴辉岩有关，含有 3–15ppm N，δ15Nair 值范围为 +3 至 +6‰，显着高于 MORB 地幔普遍接受的值 (δ15Nair∼−5‰)。基于其相对较高的氮含量及其均匀且正的 δ15N 值，蛇纹石化橄榄岩也表明存在沉积氮的混合。将沉积物衍生的氮添加到榴辉岩和橄榄岩中的一种可能途径涉及沿板片-地幔楔界面与流体混合。或者，沉积的氮可以在外隆起弯曲相关断层的蛇纹石化过程中并入橄榄岩中，并在后来的去蛇纹石化过程中释放到流体中，然后渗透到上覆岩石中。在任何试图平衡俯冲输入与弧火山气体形式输出的尝试中，都应考虑在俯冲洋壳中深度保留氮。如果这些榴辉岩和蛇纹石化橄榄岩等物质最终俯冲到弧下深度以外，进入更深的地幔，其中包含弧前-弧下氮库存的一部分（此处记录），它们可以将同位素重氮输送到地幔中，从而可能被与地幔柱相关的岩浆取样。
This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50–90km depths. The eclogites contain 2–20ppm N with δ15Nairvalues ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1ppm N and δ15Nair∼−4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3–15ppm N with δ15Nairvalues ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ15Nair∼−5‰). Based on their relatively high N contents and their homogeneous and positive δ15N values, admixing of sedimentary N is also indicated for the serpentinized peridotites. One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab–mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc–subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.