The effect of high-pressure metasomatism on the boron isotope signature of subducted oceanic crust in the Raspas Complex (Ecuador)

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Jie Dodo Xu, Horst R. Marschall, Axel Gerdes, Alexander Schmidt, Timm John
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Abstract

Boron abundances and B isotopic compositions of well-characterized blueschists and eclogites from the Raspas Complex (Ecuador) were analyzed to improve the use of boron as a tracer for recycling at convergent margins. The MORB-type eclogite interacted with internally-derived fluids released from metabasalt during the transition from blueschist to eclogite, with input from sediments. During metasomatism, B was gradually leached from the MORB-type eclogites (decrease from 6\(\upmu \)g/g to 1.5\(\upmu \)g/g), and their B isotopic composition was driven to isotopically heavier values in the range of \(-\)7.4\(\permille \) to \(-\)3.4\(\permille \). The B isotopic composition of the metasomatic fluid is estimated between \(-3\) and +1\(\permille \). The isotopic composition of the least metasomatized MORB-type eclogite samples (\({-7.4\pm 0.7}{\permille }\)) is considered close to the B isotopic composition of the dehydrated AOC in the case of Raspas at the stage of deepest subduction and most extensive dehydration. This constitutes a decrease in \(\delta ^{11}\text {B}\) of approximately 10\(\permille \) from its likely pre-subduction AOC protolith. The blueschist experienced a type of high-pressure metasomatism that is distinct from the one that affected the MORB-type eclogites. The metasomatic fluids were internally-derived and released by metabasalt as well, but with more input from sediments. The metasomatic fluid had a B isotope signature of approximately \(-\)5.2\(\permille \). The zoisite eclogite samples show a very distinct mineralogical and geochemical composition that records the highest degree of high-pressure metasomatic overprint. Their elemental and isotopic composition was thereby set to \(\text {[B]}={2.1\pm 0.3}\upmu \hbox {g/g}\) and \(\delta ^{11}\text {B}={-5.8\pm 1.8}{\permille }\). As demonstrated in previous studies, the high-pressure metasomatic fluid that caused the metasomatic overprint was mainly derived from– or interacted with– serpentinite, but had admixed components from metabasalts and metasediments. The B isotopic composition of the respective fluid is estimated at \({-2.6} {\permille }\), which overlaps with the composition of most volcanic arc basalts. This study, therefore shows, that metasomatic fluids that migrated through the Raspas slab at a depth of 50–70km had a B isotopic composition between \(-5.2\) to +1\(\permille \) and were, thus, significantly heavier than that of the mantle.

高压交代作用对厄瓜多尔拉斯帕斯杂岩俯冲洋壳硼同位素特征的影响
分析了厄瓜多尔拉斯帕斯杂岩蓝片岩和榴辉岩的硼丰度和B同位素组成,以改善硼作为示踪剂在会聚边缘的回收利用。morb型榴辉岩在蓝片岩向榴辉岩转变过程中与变质玄武岩释放的内源流体相互作用,并有沉积物的输入。在交代过程中,B从morb型榴辉岩中逐渐浸出(从6 \(\upmu \) g/g减少到1.5 \(\upmu \) g/g),其B同位素组成在\(-\) 7.4 \(\permille \) ~ \(-\) 3.4 \(\permille \)范围内被驱动到同位素较重的值。交代流体的B同位素组成估计在\(-3\) ~ +1 \(\permille \)之间。变质程度最低的morb型榴辉岩样品(\({-7.4\pm 0.7}{\permille }\))的同位素组成被认为接近于拉斯巴斯俯冲最深、脱水最广泛阶段的脱水AOC的B同位素组成。这使得\(\delta ^{11}\text {B}\)比其可能的俯冲前AOC原岩减少了约10 \(\permille \)。蓝片岩经历了一种不同于morb型榴辉岩的高压交代作用。交代流体也由变质玄武岩内部衍生和释放,但更多的是来自沉积物的输入。交代流体B同位素特征约为\(-\) 5.2 \(\permille \)。黝帘榴辉岩样品显示出非常独特的矿物学和地球化学组成,记录了最高程度的高压交代叠印。它们的元素和同位素组成由此确定为\(\text {[B]}={2.1\pm 0.3}\upmu \hbox {g/g}\)和\(\delta ^{11}\text {B}={-5.8\pm 1.8}{\permille }\)。前人研究表明,造成交代套印的高压交代流体主要来源于蛇纹岩或与蛇纹岩相互作用,但也有变质玄武岩和变质沉积岩的混合成分。各流体的B同位素组成估计为\({-2.6} {\permille }\),与大多数火山弧玄武岩的组成重叠。因此,这项研究表明,在深度为50-70km的拉斯帕斯板块中迁移的交代流体的B同位素组成在\(-5.2\)到+1 \(\permille \)之间,因此明显比地幔重。
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来源期刊
Contributions to Mineralogy and Petrology
Contributions to Mineralogy and Petrology 地学-地球化学与地球物理
CiteScore
6.50
自引率
5.70%
发文量
94
审稿时长
1.7 months
期刊介绍: Contributions to Mineralogy and Petrology is an international journal that accepts high quality research papers in the fields of igneous and metamorphic petrology, geochemistry and mineralogy. Topics of interest include: major element, trace element and isotope geochemistry, geochronology, experimental petrology, igneous and metamorphic petrology, mineralogy, major and trace element mineral chemistry and thermodynamic modeling of petrologic and geochemical processes.
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