Mantle wedge olivine modifies slab-derived fluids: Implications for fluid transport from slab to arc magma source
Mantle wedge olivine modifies slab-derived fluids: Implications for fluid transport from slab to arc magma source
复制标题
地幔楔橄榄石改变了板片衍生的流体：对从板片到弧岩浆源的流体输送的影响
作者：
Hoog J
硼是俯冲带流体过程的有效示踪剂。弧岩浆中的高 B 和 δ11B 需要从板片到岩浆源区的有效 B 转移。日本三波川高压带的东赤石变橄榄岩体由俯冲通道中出土的局部蛇纹石化地幔楔橄榄岩组成。粗粒原生地幔橄榄石的核心具有 1–4 µg/g B，与典型地幔橄榄石相比富集，δ11B 为 -10‰ 至 -1‰，这与来自约 90–120 km 深度脱水板片的流体的掺入一致。由于在约 70-100 km 深度处掺入了板片流体，原生地幔橄榄石的边缘以及橄榄石新生细胞具有更高的 B（5-20 µg/g）和更高的 δ11B（-8‰ 至 +2‰）。叶蛇纹石形成于 650 °C 以下，其 δ11B 和 B 含量与橄榄石边缘相当。数据表明，橄榄石能够通过在弧下压力和温度下的扩散和重结晶从流体中清除大量的硼。考虑到地幔楔中存在大量橄榄石，将板片衍生的物质输送到岩浆源需要与地幔橄榄岩相互作用最小的过程，例如强烈通道化的流体流动或混杂底辟的上升，以及有限的多孔流体流动。
Boron is an effective tracer of fluid processes in subduction zones. High B and δ11B in arc magmas require efficient B transfer from the slab to magma source regions. The Higashi-akaishi metaperidotite body in the Sanbagawa high-pressure belt, Japan, is composed of locally serpentinized mantle wedge peridotites exhumed in a subduction channel. Cores of coarse-grained primary mantle olivine have 1–4 µg/g B, enriched compared to typical mantle olivine, and δ11B of −10‰ to −1‰, consistent with incorporation of fluids from dehydrating slab at ~90–120 km depth. Rims of primary mantle olivine as well as olivine neoblasts have even higher B (5–20 µg/g) and higher δ11B (−8‰ to +2‰) due to incorporating slab fluids at depths of ~70–100 km. Antigorite, formed below 650 °C, shows comparable δ11B and B contents as olivine rims. The data show that olivine is capable of scavenging significant amounts of B from fluids by diffusion and recrystallization at sub-arc pressures and temperatures. Considering the large amount of olivine in the mantle wedge, transport of slab-derived material to magma sources requires processes with minimal interaction with mantle peridotite, such as intensely channelized fluid flow or ascent of mélange diapirs, and limited porous fluid flow.