Si-metasomatism in serpentinized peridotite: The effects of talc-alteration on strontium and boron isotopes in abyssal serpentinites from Hole 1268a, ODP Leg 209
Si-metasomatism in serpentinized peridotite: The effects of talc-alteration on strontium and boron isotopes in abyssal serpentinites from Hole 1268a, ODP Leg 209
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DOI：
10.1016/j.gca.2013.10.035
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发表时间：
2014-02-01
影响因子：
5
通讯作者：
Walshaw, Richard
中科院分区：
文献类型：
--
作者：
Harvey, Jason;Savov, Ivan P.;Walshaw, Richard
在大西洋中部海脊15°20‘N断裂带以南的1268a孔中发现的超镁铁质岩石，在不同的温度和pH条件下，经历了一段复杂的熔体枯竭和随后与一系列流体相互作用的历史。在强烈的熔体耗尽之后，在100-200摄氏度发生了不同程度的蛇纹岩作用，最初是在类似海水的pH条件下。随后，与温度较高(300-350℃)、低(4-5)pH和低MgO/SiO_2的流体相互作用，导致这些蛇纹岩不均匀地蚀变为含滑石的超镁铁质岩性。位于大西洋中部海脊对面的当前活跃的高温Logatchev热液区域的接近表明，与在ODP209航次期间采样的更远的位置不同，1268a孔经历了类似Logatchev的热液流体的硅交代作用(即滑石蚀变)。蛇纹岩的锶同位素比值不会因为与随后的高T流体的相互作用而发生实质性的变化，尽管这种流体可能与大洋中脊辉长岩发生了局部的相互作用。1268a孔超镁铁质岩性中的锶-87/锶-86与海水中的锶-87/锶-86接近(约0.709)，即使是酸浸蛇纹岩也保留了超过0.707的锶-87/锶-86，与洛加切夫热液没有区别。另一方面，硼同位素比值似乎已从蛇纹岩中类似海水的值(增量B-11=c.+千分之40)转变为滑石蚀变蛇纹岩中轻得多的值(增量B-11=+千分之9至+20ppm)。这很可能是在从蛇纹石到滑石的矿物学转变过程中环境pH和温度变化的结果。不均匀的硼同位素系统学对俯冲返回对流地幔的岩石圈超镁铁质部分的组成产生了影响。三角洲B-11、[B]和矿物学的不均一性给俯冲早、中期释放的板岩流体成分的预测带来了很大的不确定性。(C)2013爱思唯尔有限公司。保留所有权利。
Ultramafic rocks recovered from Hole 1268a, Ocean Drilling Program Leg 209, to the south of the 15 degrees 20'N Fracture Zone on the Mid-Atlantic ridge have experienced a complex history of melt depletion and subsequent interaction with a series of fluids under varying temperature and pH conditions. After intense melt depletion, varying degrees of serpentinization at 100-200 degrees C took place, initially under seawater-like pH conditions. Subsequently, interaction with a higher temperature (300-350 degrees C) fluid with low (4-5) pH and low MgO/SiO2 resulted in the heterogeneous alteration of these serpentinites to talc-bearing ultramafic lithologies. The proximity of the currently active, high temperature Logatchev hydrothermal field, located on the opposite flank of the Mid-Atlantic ridge, suggests that unlike more distal localities sampled during ODP Leg 209, Hole 1268a has experienced Si-metasomatism (i.e. talc-alteration) by a Logatchev-like hydrothermal fluid. Serpentinite strontium isotope ratios were not materially shifted by interaction with the subsequent high-T fluid, despite the likelihood that this fluid had locally interacted with mid-ocean ridge gabbro. Sr-87/Sr-86 in the ultramafic lithologies of Hole 1268a are close to that of seawater (c.0.709) and even acid leached serpentinites retain Sr-87/Sr-86 in excess of 0.707, indistinguishable from Logatchev hydrothermal fluid. On the other hand, boron isotope ratios appear to have been shifted from sea-water-like values in the serpentinites (delta B-11 = c.+40 parts per thousand) to much lighter values in talc-altered serpentinites (delta B-11 = +9 to +20 parts per thousand). This is likely a consequence of the effects of changing ambient pH and temperature during the mineralogical transition from serpentine to talc. Heterogeneous boron isotope systematics have consequences for the composition of ultramafic portions of the lithosphere returned to the convecting mantle by subduction. Inhomogeneities in delta B-11, [B] and mineralogy introduce significant uncertainties in the prediction of the composition of slab fluids released during the early-to mid-stages of subduction. (C) 2013 Elsevier Ltd. All rights reserved.