Widespread slab melting in modern subduction zones
Widespread slab melting in modern subduction zones
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DOI：
10.1016/j.epsl.2023.118544
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发表时间：
2024-01
影响因子：
5.3
通讯作者：
M. Klaver;Gene Yogodzinski;Capucine Albert;M. Camejo-Harry;M. Elburg;K. Hoernle;Colin Macpherson;Geoff Nowell;Tracy Rushmer;Helen Williams;M. Millet
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文献类型：
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作者：
M. Klaver;Gene Yogodzinski;Capucine Albert;M. Camejo-Harry;M. Elburg;K. Hoernle;Colin Macpherson;Geoff Nowell;Tracy Rushmer;Helen Williams;M. Millet
俯冲板块的部分熔融在多大程度上促进了现代俯冲带的弧岩浆作用，这仍然是一个激烈的争论。特别是，很难区分由板块部分熔融形成的硅酸盐熔体和作为板块到地幔楔形传质的主要介质的固相线下脱水过程中释放的水溶液。在这里，我们使用δ49/47Ti(样品的49Ti/47Ti与OL-Ti标准物质的偏差)作为板坯熔化的可靠的地球化学示踪剂。俯冲洋壳和上覆沉积层的含水部分熔融与残留的金红石形成平衡的硅质熔体。模拟表明，这种硅质板片熔体的δ49/47Ti(+0.24±0.06‰)明显高于其原岩，这是由于金红石对钛的较轻同位素有强烈的偏好。相反，即使是高盐度的流体也不能从板岩中携带钛，因此含水橄榄岩部分熔体的δ49/47Ti类似于大洋中脊玄武岩(‰；约为0)。8个俯冲带中未受Fe-Ti氧化物分离结晶影响的原始(Mg#≥60)弧状熔岩显示，δ49/47Ti的变化比MORb中的变化大10倍以上。尤其是原始弧熔岩的SiO_2含量与δ_(49/47)Ti具有显著的相关性，范围从与δ重叠的岛弧玄武岩到阿留申西部喷发的原始流纹英安岩(‰_(49/47)Ti)。这些原始弧状熔岩的δ49/47Ti的升高提供了确凿的证据，证明该板块的部分熔融是俯冲带内物质传递的关键介质。阿留申流纹岩代表了一种罕见的板岩熔体，它只需最小的修改就穿过了地幔楔形。更常见的是，板片熔体与地幔楔体相互作用，形成一系列原生弧状岩浆，它们是板片和橄榄岩来源的熔体的混合体。我们在所有八个俯冲带地点识别出具有明显可分辨的板片熔融特征的原始弧熔岩，证实了板片熔融在现代俯冲带中普遍存在。
It is still a matter of intense debate to what extent partial melting of the subducting slab contributes to arc magmatism in modern subduction zones. In particular, it is difficult to differentiate between silicate melts formed by partial melting of the slab, and aqueous fluids released during subsolidus dehydration as the main medium for slab-to-mantle wedge mass transfer. Here we use δ49/47Ti (the deviation in49Ti/47Ti of a sample to the OL-Ti reference material) as a robust geochemical tracer of slab melting. Hydrous partial melting of subducted oceanic crust and the superjacent sedimentary layer produces silicic melts in equilibrium with residual rutile. Modelling shows that such silicic slab melts have notably higher δ49/47Ti (+0.24 ± 0.06 ‰) than their protolith due to the strong preference of rutile for the lighter isotopes of Ti. In contrast, even highly saline fluids cannot carry Ti from the slab and hence hydrous peridotite partial melts have δ49/47Ti similar to mid-ocean ridge basalts (MORB; ca. 0 ‰).Primitive (Mg# ≥60) arc lavas from eight subduction zones that are unaffected by fractional crystallisation of Fe-Ti oxides show a more than tenfold larger variation in δ49/47Ti than found in MORB. In particular, primitive arc lavas display a striking correlation between SiO2content and δ49/47Ti that ranges from island arc basalts overlapping with MORB, to primitive rhyodacites with δ49/47Ti up to 0.26 ‰ erupted in the western Aleutian arc. The elevated δ49/47Ti of these primitive arc lavas provides conclusive evidence for partial melts of the slab as a key medium for mass transfer in subduction zones. The Aleutian rhyodacites represent a rare example of slab melts that have traversed the mantle wedge with minimal modification. More commonly, slab melts interact with the mantle wedge to form an array of primary arc magmas that are a blend of slab- and peridotite-derived melt. We identify primitive arc lavas with a clearly resolvable slab melt signature in all eight subduction zone localities, confirming that slab melting is prevalent in modern subduction zones.