Boron isotope variations of Franciscan serpentinites, northern California
Boron isotope variations of Franciscan serpentinites, northern California
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DOI：
10.1016/j.lithos.2019.02.004
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发表时间：
2019-06
期刊：
影响因子：
3.5
通讯作者：
C. Yamada;T. Tsujimori;Q. Chang;J. Kimura
中科院分区：
文献类型：
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作者：
C. Yamada;T. Tsujimori;Q. Chang;J. Kimura
美国北加州科迪勒拉的方济各杂岩中赋存着不同构造成因的蛇纹岩和蛇纹化地幔橄榄岩。蛇纹岩的硼同位素与流体介质作用相分化，在会聚边缘具有重要的地质标志潜力。为了了解Franciscan消减带系统中的硼同位素行为，我们采用了一种新开发的烧蚀体积校正（AVC）方法，利用激光烧蚀多收集器电感耦合等离子体质谱（LA-MC-ICPMS）对从八个不同区域收集的17种不同的Franciscan蛇纹岩（sensulato）进行了原位同位素/元素分析。硼同位素丰度和组成蛇纹岩研究显示出较大的差异B = 1.6 -239 μg·g−1,δ11 B = −12.0 + 24.4‰,允许歧视的蛇纹岩分为两组:(1)一个打火机δ11 B−12.0 + 8.8‰,小B < ~ 56 μg·g−1和(2)一个重δ11 B + 7.2 + 24.4%更大B ~ 34 - 239 μg·g−1。这些组在岩性上分别对应于伴生蓝片岩相变质岩的存在或不存在。含蓝片岩和/或与蓝片岩相关的蛇纹岩可能受到> ~2 GPa深弧前板块流体的影响。11b优先分配到浅叶俯冲板块释放的流体中，板块中的δ11B较轻，导致深层板块流体中的δ11B较轻。不含蓝片岩、δ11B较重的蛇纹岩可能形成于具有较重δ11B的浅板块或热液的浅层环境。深层和浅层蛇纹岩中B含量的减少和增加也与板岩中B的脱水特征相一致。研究结果表明，硼同位素和硼组成在太平洋型造山带蛇纹岩成因鉴定中的多功能性。
Serpentinites and serpentinized mantle peridotites with various tectonic origins occur in the Franciscan Complex of the Northern California Cordillera, USA. Boron isotopes of serpentinites differentiate with fluid-mediated processes, and have great potential for key geologic markers in convergent margins. To understand boron isotope behavior within the Franciscan subduction zone system, we apply a newly developed ablation volume correction (AVC) method for in-situ isotope/elemental analyses using a laser-ablation multiple collector inductively-coupled-plasma mass spectrometry (LA-MC-ICPMS) on seventeen different Franciscan serpentinites (sensulato) collected from eight separate areas. Boron abundances and isotope compositions of the studied serpentinites show large variations B = 1.6–239 μg·g−1, δ11B = −12.0 to +24.4‰, which allow to discriminate the serpentinites into two groups: (1) a lighter δ11B of −12.0 to +8.8‰ with a lesser B < ~56 μg·g−1and (2) a heavier δ11B of +7.2 to +24.4% with a greater B ~34–239 μg·g−1. These groups lithologically correspond to the presence or absence of associated blueschist-facies metamorphic rocks, respectively. The blueschist-bearing and/or blueschist-associated serpentinites might have been affected by a deep forearc slab fluids in the depth of > ~2 GPa. Preferential partitioning of11B into fluids released from the subducted slab at shallow leaves lighter δ11B in the slab resulting in lighter δ11B in the deep slab fluids. In contrast, the blueschist-absent serpentinites with heavier δ11B may have formed at a shallow environment where shallow slab or hydrothermal fluids with heavier11B were present. Lesser versus greater amounts of B in the deep versus shallow serpentinites are also consistent with the dehydration profile of B from a slab. Our results show the versatility of boron isotopes and composition for identification of the origin of serpentinite in Pacific-type orogenic belts.