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The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles

机译：水热蚀变的海洋地壳对地幔卤素和稀有气体循环的贡献

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Recent studies suggest that seawater-derived noble gases and halogens are recycled into the deep mantle by the subduction of oceanic crust. To understand the processes controlling the availability of halogens and noble gases for subduction, we determined the noble gas elemental and isotopic ratios and halogen (Cl, Br, I) concentrations in 28 igneous samples from the altered oceanic crust (AOC) from 5 ODP sites in the Eastern and Western Pacific Ocean. Crushing followed by heating experiments enabled determination of noble gases and halogens in fluid inclusions and mineral phases respectively. Except for He and Ar, Ne, Kr and Xe isotopic ratios were all air-like suggesting that primary MORB signatures have been completely overprinted by air and/or seawater interaction. In contrast, 3He/4He ratios obtained by crushing indicate that a mantle helium component is still preserved, and 40Ar/36Ar values are affected by radiogenic decay in the mineral phases. The 130Xe/36Ar and 84Kr/36Ar ratios are respectively up to 15 times and 5 times higher than those of seawater and the highest ratios are found in samples affected by low temperature alteration (shallower than 800-900 m sub-basement). We consider three possible processes: (i) adsorption onto the clays present in the samples; (ii) fluid inclusions with a marine pore fluid composition; and (iii) fractionation of seawater through phase separation caused by boiling. Ninety percent of the Cl, Br and I were released during the heating experiments, showing that halogens are dominantly held in mineral phases prior to subduction. I/Cl ratios vary by 4 orders of magnitude, from 3 × 10-6 to 2 × 10-2. The mean Br/Cl ratio is 30% lower than in MORB and seawater. I/Cl ratios lower than MORB values are attributed to Cl-rich amphibole formation caused by hydrothermal alteration at depths greater than 800-900 m sub-basement together with different extents of I loss during low and high temperature alteration. At shallower depths, I/Cl ratios higher than MORB values can be explained by the addition of organic-rich sediments or the presence of organic detritus, both known to efficiently sequester I. Concentrations of 36Ar of the pre-subducting materials are sufficient to account for the 36Ar and composition of the mantle in the context of existing subduction-flux models. We find the Cl subduction flux of the oceanic crust to be about three times higher than the previous estimates and that sufficient Cl and Br can potentially be delivered by subduction over the last 3 Ga to account for mantle source compositions.
机译：最近的研究表明，海水的稀有气体和卤素通过俯冲大洋地壳而再循环到深层地幔中。为了了解控制俯冲过程中卤素和稀有气体的可用性的过程，我们确定了来自5个ODP站点改变后的地壳（AOC）的28个火成岩样品中的稀有气体元素和同位素比以及卤素（Cl，Br，I）浓度在东太平洋和西太平洋。压碎后再进行加热实验，可以分别测定流体包裹体和矿物相中的稀有气体和卤素。除He和Ar外，Ne，Kr和Xe的同位素比均呈空气状，这表明主要的MORB信号已完全被空气和/或海水相互作用覆盖。相反，通过压碎获得的3He / 4He比表明，地幔氦成分仍然保留，而40Ar / 36Ar值受矿物相中放射性成因的影响。 130Xe / 36Ar和84Kr / 36Ar的比率分别比海水高15倍和5倍，并且在受低温变化影响（小于800-900 m的地下室）的样品中比率最高。我们考虑了三个可能的过程：（i）吸附到样品中存在的粘土上； （ii）具有海洋孔隙流体成分的流体包裹体； （iii）通过沸腾引起的相分离来分离海水。在加热实验中释放出90％的Cl，Br和I，这表明在俯冲之前，卤素主要保留在矿物相中。 I / Cl比变化4个数量级，从3×10-6到2×10-2。 Br / Cl的平均比率比MORB和海水低30％。 I / Cl比值低于MORB值的原因归因于地下800到900 m深度处热液蚀变引起的富含Cl的闪石形成，以及低温和高温蚀变过程中I损失的程度不同。在较浅的深度处，I / Cl比值高于MORB值可以通过添加富含有机物的沉积物或存在有机碎屑来解释，这两者都可以有效地隔离I。预俯冲物质中36 Ar的浓度足以说明在现有俯冲-通量模型的背景下分析36Ar和地幔组成。我们发现大洋地壳的Cl俯冲通量比以前的估计高大约三倍，并且在最后3 Ga内通过俯冲可以潜在地输送足够的Cl和Br以解释地幔源的组成。

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