Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics
Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics
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DOI：
10.1016/j.epsl.2022.117970
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发表时间：
2023-02
影响因子：
5.3
通讯作者：
Ekaterina Rojas-Kolomiets;O. Jensen;M. Bizimis;G. Yogodzinski;L. Ackerman
中科院分区：
文献类型：
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作者：
Ekaterina Rojas-Kolomiets;O. Jensen;M. Bizimis;G. Yogodzinski;L. Ackerman
钼同位素组成已越来越多地用作岩浆系统的源示踪剂。本文研究了俯冲洋壳、富流体源（如蛇纹岩、蚀变洋壳- aoc）、俯冲沉积物和上地幔在阿留申弧岩浆化学组成中的相对作用。从东部的Okmok火山到最西部的Piip海山，阿留申群岛（n= 59）熔岩的元素和Mo同位素组成（δ 98/95 Mo）和B浓度表明岩浆分异过程中没有Mo同位素分异。此外，我们还报道了阿留申弧外的西南印度洋脊（SWIR）（n= 6）、AOC （n= 2）和太平洋沉积物（DSDP 183, ODP 886）（n= 5）的蛇纹岩化榴石的Mo同位素系统。钼同位素组成和B富集模式（如B/Ce）沿弧呈阶梯递增，在弧西段表现为低的、类似morb的δ 98/95 Mo和低的B/Ce值（B/Ce= 0.15 ~ 1.07; δ 98/95 Mo= - 0.38 ~ + 0.01‰），在科罗温火山、西古姆火山和尤纳斯卡火山中东部陡增(B/Ce= 1.20 ~ 2.60；δ 98/95 Mo=+ 0.03 ~ + 0.30‰)在Amlia断裂带（AFZ）与海沟交点附近，但在更东的Okmok （B/Ce= 0.76, δ 98/95 Mo= - 0.12‰）处再次下降。这些数据模式被解释为反映了阿留申岩浆的沿弧变化源。AOC和太平洋沉积物以δ 98/95 Mo低（分别为- 0.47 ~ - 0.32和+ 0.17 ~ - 1.9‰）为主，而蛇纹岩具有极高的δ 98/95 Mo（高达+ 1.09‰）和高B/Ce（~ 22000）。基于沉积物和AOC中δ 98/95 Mo较低，以及沿弧δ 98/95 Mo与放射性成因示踪剂缺乏相关性，俯冲沉积物和AOC对Mo同位素组成没有一级控制作用。相反，西部样品中δ 98/95 Mo（类似morb），但Mo富集（如Mo/Ce）较高，与含金红石榴辉岩相下的板岩熔融一致，从板岩到弧源几乎没有Mo同位素分异。而弧区附近熔岩中δ 98/95 Mo和B/Ce的突然增加，最好的解释是弧区附近的蛇纹岩端元（可能是脱水流体），而弧区其他地方并不明显。研究结果表明，蛇纹岩是俯冲带中另一种重Mo同位素特征组分，高δ 98/95 Mo伴B富集是追踪俯冲带蛇纹岩流体的有效标志。
Mo isotope compositions have been increasingly used as a source tracer in magmatic systems. Here, we investigate the relative role of subducting oceanic crust, fluid-rich sources (eg, serpentinite, altered oceanic crust–AOC), subducted sediments and upper mantle in the chemical composition of Aleutian arc magmas with Mo and B systematics. We present elemental and Mo isotope compositions (δ 98/95 Mo) and B concentrations on Aleutian lavas (n= 59), from Okmok Volcano in the east to the westernmost seamount Piip, showing absence of Mo isotope fractionation during magmatic differentiation. Additionally, we report Mo isotope systematics for serpentinized peridotites from the South-West Indian Ridge (SWIR)(n= 6), AOC (n= 2) and Pacific sediments (DSDP 183, ODP 886)(n= 5) outboard the Aleutian arc. Molybdenum isotope composition and B enrichment (eg, B/Ce) patterns display a step-function increase along the arc, with low, MORB-like, δ 98/95 Mo and low B/Ce values in the western section of the arc (B/Ce= 0.15–1.07; δ 98/95 Mo=− 0.38 to+ 0.01‰), that abruptly increase in the central-eastern volcanoes Korovin, Seguam and Yunaska (B/Ce= 1.20–2.60; δ 98/95 Mo=+ 0.03 to+ 0.30‰) near the intersection of the Amlia Fracture Zone (AFZ) with the trench, but decrease again farther east at Okmok (B/Ce= 0.76 and δ 98/95 Mo=− 0.12‰ on average). These data patterns are interpreted to reflect an along-arc changing source in the Aleutian magmas. AOC and Pacific sediments have predominantly low δ 98/95 Mo (− 0.47 to− 0.32 and+ 0.17 to− 1.9‰, respectively), while serpentinites have extremely high δ 98/95 Mo (up to+ 1.09‰) and high B/Ce (∼ 22000). Based on the low δ 98/95 Mo in sediments and AOC, and lack of correlation between along-arc δ 98/95 Mo and radiogenic sediment tracers, subducted sediments and AOC do not exert first-order controls on the observed Mo isotope compositions. Rather, low, MORB-like, δ 98/95 Mo but high Mo enrichments (eg, Mo/Ce) in the western samples are consistent with slab melting under rutile-bearing eclogitic facies with near absent Mo isotope fractionation from the slab to the arc sources. In turn, the abrupt increase of δ 98/95 Mo and B/Ce in lavas near the AFZ are best explained by a serpentinite endmember (likely dehydration fluids) at the AFZ that is not evident elsewhere along the arc. Results from this study provide evidence for serpentinites as an additional heavy Mo isotope signature component in subduction zones and demonstrate that high δ 98/95 Mo coupled with B enrichments are a useful proxy for tracing serpentinite fluids in subduction zones.