The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: Experiments and models
The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: Experiments and models
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DOI：
10.1016/s0016-7037(98)00224-5
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发表时间：
1998-10-01
影响因子：
5
通讯作者：
Shaw, HF
中科院分区：
文献类型：
--
作者：
Brenan, JM;Ryerson, FJ;Shaw, HF
低原子质量元素B、Be和Li被认为是俯冲物质参与岛状岩浆成因的敏感示踪物。为了更好地评估在俯冲过程中这些元素的板幔转移中致密含水流体的作用，测量了B、Be和Li在含水流体和可能存在于下行板玄武质部分的矿物（即单斜辉石和石榴石）之间的分配系数。在900 ℃和2.0 GPa下的实验表明，Be的平均单斜辉石-流体分配系数（类似于2）超过Li（类似于0.2）或B（类似于0.02）的平均单斜辉石-流体分配系数，并且该值比石榴石的分配系数大100倍（B，Li）至1000倍（Be）。单斜辉石-流体分配系数随单斜辉石中氧化铝含量的变化而变化，但这种变化被解释为反映了控制这些元素进入辉石结构的特定交换反应，而不是矿物-流体不平衡。元素对B-Be，B-Nb和Li-Yb被认为在部分熔融过程中基本上未分馏，作为证据，他们的连贯性行为显然同生熔岩和相似的测量矿物熔体分配系数。单斜辉石-流体分配系数与单斜辉石-硅酸盐熔体值的比较表明，B/Be，B/Nb和Li/Y B b比将显着分馏在共存的含水流体相对于残留的固体。岛状熔岩相对于MORE的B/Be、B/Nb和Li/YB b比值的升高被认为与流体介导的板幔搬运的起源相一致。板脱水伴随着渐进的水分流失和残留矿物模式的变化的定量模型表明，源区B/Be和B/Nb适合生产伊豆和千岛IA B套房可以产生使用现有的估计组成的蚀变洋壳，虽然B丰度在高端公布的值是必需的。由于B/Be和B/Nb的最高值产生于相对较浅深度的地幔楔中，因此需要一些额外的过程，例如俯冲引起的水合地幔楔流动，以便将富集的物质转移到适合于在火山前缘下方形成岩浆的深度。计算表明，当板片达到200公里深度时，脱水残渣中的B/Be和B/Nb已降低到接近初始值的5-12%。因此，脱水过程中B的优先损失被认为是防止洋壳近地表蚀变过程中获得的过量B循环进入地幔的可行机制，从而维持了地幔和地壳储层中B/Be和B/Nb的区别。版权所有（C）1998 Elsevier Science Ltd.
The low atomic mass elements B, Be, and Li are viewed as sensitive tracers of the involvement of subducted materials in the genesis of island are magmas. In order to better assess the role of dense aqueous fluids in the slab-to-mantle transfer of these elements during subduction, measurements have been made of partition coefficients for B, Be, and Li between aqueous fluid and minerals likely to be present in the basaltic portion of the downgoing slab, namely clinopyroxene and garnet. Experiments at 900 degrees C and 2.0 GPa reveal that the average clinopyroxene-fluid partition coefficient for Be (similar to 2) exceeds that for either Li (similar to 0.2) or B (similar to 0.02) and values are 100X (B,Li) to 1000X (Be) larger than partition coefficients for garnet. Clinopyroxene-fluid partition coefficients were found to vary with the alumina content of run-product clinopyroxenes, but this variation is interpreted to reflect the specific exchange reaction that governs the incorporation of these elements into the pyroxene structure, and not mineral-fluid disequilibrium.The element pairs B-Be, B-Nb, and Li-Yb are considered to be essentially unfractionated during the partial melting process, as evidence by their coherent behaviour in apparently cogenetic lavas and the similarity in their measured mineral-melt partition coefficients. A comparison of clinopyroxene-fluid partition coefficients for these elements with clinopyroxene-silicate melt values reveals that B/Be, B/Nb, and Li/Yb ratios will be significantly fractionated in coexisting aqueous fluid with respect to the residual solid. The elevated ratios of B/Be, B/Nb, and Li/Yb in island are lavas relative to MORE are thus considered to be consistent with an origin by fluid-mediated slab-to-mantle transport. A quantitative model of slab dehydration accompanied by progressive water loss and changes in residual mineral mode reveals that source regions with B/Be and B/Nb appropriate for producing the Izu and Kurile IAB suites can be generated using available estimates for the composition of altered oceanic crust, although B abundances at the high end of published values are required. Because the highest values of B/Be and B/Nb are produced in the mantle wedge at relatively shallow depths, some additional process, such as subduction-induced flow of a hydrated mantle wedge, is required in order to transfer enriched material to depths appropriate for the formation of magmas beneath the volcanic front.Calculations indicate that by the time the slab reaches a depth of 200 km, B/Be and B/Nb in the dehydration residue has been reduced to similar to 5-12% of initial values. Thus, the preferential loss of B during dehydration is viewed as a viable mechanism to prevent the excess B acquired during near-surface alteration of oceanic crust from being cycled into the mantle, thereby maintaining the distinction in B/Be and B/Nb for mantle and crustal reservoirs. Copyright (C) 1998 Elsevier Science Ltd.