Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism
Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism
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DOI：
10.1016/j.epsl.2005.11.063
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发表时间：
2006-02-28
影响因子：
5.3
通讯作者：
Shanks, WC
中科院分区：
文献类型：
--
作者：
Alt, JC;Shanks, WC
西太平洋的马里亚纳弧和伊豆-博宁弧以弧前的蛇纹岩海山为特征，这些海山提供了进入地幔楔形的独特窗口。我们提供了马里亚纳弧前锥形海山蛇纹岩的稳定同位素(O、H、S和C)数据和伊豆-博宁前弧Torishima海山的S同位素数据，以了解地幔楔体中流体的组成和蛇纹岩的温度，并研究硫从板块到地幔楔体的输运。六个蛇纹石矿物的有限范围为O-18(千分之6.5-8.5)。安提戈尔岩分离物的增量D值为-29.5‰至-45.5‰，反映地幔楔体内的蛇纹岩化作用，而温石棉具有较低的Delta D值(-51.8‰至-84.0‰)，这是与流体在低温下重新平衡的结果。蛇纹石和磁铁矿之间的氧同位素分馏表明蛇纹岩的形成温度为300-375℃。两个晚期的交叉纤维温石棉矿脉具有较高的增量O-18值，在千分之8.9到千分之10.8之间，形成于较低的温度(低至100摄氏度)。在300-375摄氏度与蛇纹石平衡的水溶液中，增量O-18=6.5-9‰，增量D=-4‰-26‰，与俯冲板岩中的沉积物脱水反应一致，而不是玄武岩板源。蛇纹岩流中变质玄武岩和粉砂岩碎屑中的三个文石矿脉的增量O-18=16.7-24.5‰，与250℃下的蛇纹岩流体一致。0.1-2.5‰的Delta C-13值表明，俯冲碳酸盐沉积是一种物源。锥形海山蛇纹岩中硫化物的S-34增量值(-6.7‰~9.8‰)是板片沉积物中硫酸盐水溶液(S-34增量=14‰)发生交代作用的结果。尽管有硫交代作用，蛇纹岩的硫含量很低(一般为164ppm)，这反映了地幔楔体的高度贫化性质。蛇纹岩主要富集于S-34(中位数为S-34(硫化物)=4.5ppm)，与由弧状熔岩推断的S-34富集地幔楔体一致。(C)2006爱思唯尔B.V.保留所有权利。
The Mariana and Izu-Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu-Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of delta O-18 (6.5-8.5 parts per thousand). Antigorite separates have delta D values of -29.5 parts per thousand to -45.5 parts per thousand that reflect serpentinization within the mantle wedge whereas chrysotile has low delta D values (-51.8 parts per thousand to -84.0 parts per thousand) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300-375 degrees C. Two late cross-fiber chrysotile veins have higher delta O-18 values of 8.9 parts per thousand to 10.8 parts per thousand and formed at lower temperatures (as low as similar to 100 degrees C). Aqueous fluids in equilibrium with serpentine at 300-375 degrees C had delta O-18=6.5-9 parts per thousand and delta D = -4 parts per thousand to -26 parts per thousand, consistent with sediment dehydration reactions at temperatures < 200 degrees C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have delta O-18 = 16.7-24.5 parts per thousand, consistent with the serpentinizing fluids at temperatures < 250 degrees C. delta C-13 values of 0.1-2.5 parts per thousand suggest a source in subducting carbonate sediments. The delta S-34 values of sulfide in serpentinites on Conical Seamount (-6.7 parts per thousand to 9.8 parts per thousand) result from metasomatism through variable reduction of aqueous sulfate (delta S-34 = 14 parts per thousand) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in S-34 (median delta S-34(sulfide) = 4.5 parts per thousand), consistent with a S-34-enriched mantle wedge as inferred from arc lavas. (c) 2006 Elsevier B.V. All rights reserved.