美国南卡罗莱纳大学Owen Jensen团队最新的一项研究，利用硼同位素识别出了阿留申弧岩浆源区的深部板状蛇纹岩。该研究于2024年10月30日发表于国际一流学术期刊《地质学》杂志上。

据悉，阿留申岛弧西部的海底熔岩具有同位素重硼（δ¹¹B达+13.4‰），与B含量（ppm）呈负相关的特征。

数据显示，端元样品为原始英安岩和流纹岩(δ¹¹B>+10‰，SiO₂=63%7-0%, Mg#>0.60)，具有埃达克质微量元素和同位素特征，需要残余石榴子石和金红石在其形成过程中发挥作用。同位素重B的存在可能是俯冲板块地幔部分的蛇纹岩，脱水成倒转的地热梯度，在先前海水蚀变影响最小的深度处推动上覆火山部分的熔融。

阿留申群岛位置的大多数火山岩具有10-30ppm的B，平均δ¹¹B值为约+1.0‰±1.3‰，反映了以俯冲沉积物为主的混合源。

一部分样品的子集具有同位素较轻的B (δ¹¹B<- 2.4‰)和低浓度（<11.0ppm），这是全球弧后背景下弧熔岩的典型特征，弧后背景下弧到板的深度高，δ¹¹B可以解释为反映脱水（同位素轻）蚀变的洋壳中的来源。

质量平衡模型表明，来自深板蛇纹岩的同位素重B在阿留申岛弧范围内存在，但在岛弧最西段外的火山中心通常被沉积物来源的B所掩盖。

附：英文原文

Title: Boron isotopes identify deep-slab serpentinite in the source of Aleutian arc magma

Author: Owen Jensen, Ryan Waldman, Gene Yogodzinski, Michael Bizimis, Jeffrey Ryan, William Leeman, Howie Scher, Kaj Hoernle

Issue&Volume: 2024-10-30

Abstract: Seafloor lavas of the Western Aleutian arc have isotopically heavy boron (δ¹¹B to +13.4‰) that is negatively correlated with B content (ppm). Endmember samples are primitive dacites and rhyodacites (δ¹¹B > +10‰, SiO₂ = 63%-70%, Mg# > 0.60) with adakitic trace-element and isotopic characteristics that require roles for residual garnet and rutile in their formation. The source of isotopically heavy B is likely serpentinite in the mantle section of the subducting plate, which dewaters into an inverted geothermal gradient and drives melting within the overlying volcanic section at depths where prior effects of seawater alteration were minimal. Most volcanic rocks from the Aleutian Island locations have 10-30 ppm B with an average δ¹¹B of ～+1.0‰ ± 1.3‰, reflecting a mixed source dominated by subducted sediment. A subset of island samples has B that is isotopically light (δ¹¹B < 2.4‰) and at low concentrations (<11.0 ppm), which is typical of arc lavas globally from rear-arc settings where depth-to-slab is high, and where δ¹¹B may be interpreted to reflect a source in dehydrated (isotopically light) altered oceanic crust. Mass balance modeling indicates that isotopically heavy B from deep-slab serpentinite is present in the Aleutian source arc-wide but is typically masked by sediment-derived B at volcanic centers outside of the westernmost segment of the arc.

DOI: 10.1130/G52419.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G52419.1/649800/Boron-isotopes-identify-deep-slab-serpentinite-inredirectedFrom=fulltext