Fingerprinting fluid sources in Troodos ophiolite complex orbicular glasses using high spatial resolution isotope and trace element geochemistry
Fingerprinting fluid sources in Troodos ophiolite complex orbicular glasses using high spatial resolution isotope and trace element geochemistry
复制标题
使用高空间分辨率同位素和微量元素地球化学对特罗多斯蛇绿岩复合球状玻璃中的流体源进行指纹识别
DOI：
10.1016/j.gca.2016.12.012
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发表时间：
2017
影响因子：
5
通讯作者：
Lenting C.
中科院分区：
文献类型：
--
作者：
Fonseca R.O.C;Kirchenbaur M;Ballhaus C;Münker C;Zirner A;Gerdes A;Heuser A;Botcharnikov R;Lenting C.
摘要塞浦路斯特罗多斯火成杂岩是一个典型的火山岩。90 Ma，保存完好的俯冲带上蛇绿岩。特罗多斯的独特之处在于，它在整个复合体中显示出流体饱和的证据，从其底部（即豆荚状铬铁矿）到其最上面的单元--上部枕状熔岩（UPL）。然而，目前尚不清楚UPL拉斑玄武岩中溶解水的来源，可能包括浅层海水渗透，蚀变Troodos洋壳的同化，再循环的蛇纹石化洋壳或俯冲的远洋沉积物。为了识别和表征这些成分，我们对特罗多斯UPL的球状结构和玻璃上的拉斑玄武岩熔岩进行了详细的高分辨率研究。用LA-MC-ICP-MS测量了玄武岩球的Sr-Nd-Hf-Pb同位素组成，并原位测量了它们的B同位素组成。UPL球显示出非常窄的Nd和Hf的范围（分别为+ 7到+ 8和+ 13到+ 15），表明亏损地幔源的熔融。铅同位素，特别是207 Pb/204 Pb与206 Pb/204 Pb，与远洋沉积物形成混合阵列。此外，高分辨率表征的个别圆形显示，UPL拉斑玄武岩显示出强烈的变化，在87 Sr/86 Sr（0.7039-0.7060）在露头规模。样品的δ 11 B介于− 8.2（±0.5）‰和+ 5.9（±1.1）‰之间，平均B含量约为1.5 ‰。5 μg/g。与预期相反，蚀变球体及其相关的玻璃质碎屑岩基质显示出较低的δ 11 B（低至-10 ‰）和较高的B含量（最大值）。200 μg/g）。此外，这里研究的圆形物很少或没有与海水相互作用的证据，这是由它们的微量元素含量和同位素组成支持的。当所有的同位素系统都考虑在内，UPL熔岩反映熔融的耗尽地幔源，是叠印的含水沉积物熔体，并可能，流体状俯冲成分，部分源于蛇纹洋壳。随后的低温蚀变使δ 11 B降低，同时由于吸附到斜长岩中而使B吸收增加。
Abstract The Troodos igneous complex (Cyprus) is a ca. 90 Ma old, well preserved supra-subduction zone ophiolite. Troodos is unique in that it shows evidence of fluid-saturation throughout the complex, from its base (ie podiform chromitites) to its uppermost units–the upper pillow lavas (UPL). However, it is unclear what the source of dissolved water in UPL tholeiites is, with possibilities including shallow seawater infiltration, assimilation of altered Troodos oceanic crust, recycled serpentinized oceanic crust, or subducted pelagic sediments. In order to identify and characterize these components we have carried out a detailed high-resolution study on tholeiitic lavas on orbicular structures and glasses from the UPL in Troodos. Basaltic orbicules were measured for their Sr–Nd–Hf–Pb isotope compositions, and in situ for their B isotopes using LA-MC-ICP-MS. UPL orbicules display a very narrow range in∊ Nd and∊ Hf (+ 7 to+ 8 and+ 13 to+ 15, respectively) indicating melting of a depleted mantle source. Lead isotopes, specifically 207 Pb/204 Pb vs. 206 Pb/204 Pb, form a mixing array with pelagic sediments. Furthermore, high-resolution characterization of individual orbicules revealed that UPL tholeiites display strong variability in 87 Sr/86 Sr (0.7039–0.7060) at the outcrop scale. Samples display δ 11 B between− 8.2 (±0.5)‰ and+ 5.9 (±1.1)‰ with an average B content of ca. 5 μg/g. Contrary to expectation, altered orbicules and their associated hyaloclastite matrixes display lower δ 11 B (down to− 10‰) and higher B contents (max. 200 μg/g) when compared to fresh glass. Furthermore, the orbicules studied here show little or no evidence of interaction with seawater, which is supported by their trace element contents and isotope compositions. When all isotope systems are taken into account, UPL lavas reflect melting of a depleted mantle source that was overprinted by hydrous sediment melts, and potentially, fluid-like subduction components that in part originate from serpentinized oceanic crust. Subsequent low-temperature alteration then drove δ 11 B to lower values coupled with increased B uptake due to its adsorption into palagonite.
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作者：
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