Deformation beneath Gakkel Ridge, Arctic Ocean: From mantle flow to mantle shear in a sparsely magmatic spreading zone
Deformation beneath Gakkel Ridge, Arctic Ocean: From mantle flow to mantle shear in a sparsely magmatic spreading zone
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北冰洋加克尔海脊下方的变形：稀疏岩浆扩散区从地幔流到地幔剪切
DOI：
10.1016/j.tecto.2021.229186
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发表时间：
2022
期刊：
影响因子：
2.9
通讯作者：
Snow Jonathan E.
中科院分区：
文献类型：
--
作者：
Harigane Yumiko;Michibayashi Katsuyoshi;Morishita Tomoaki;Tamura Akihiro;Hashimoto Satoshi;Snow Jonathan E.
由于大多数深海橄榄岩的高度蛇纹化和风化状态，导致海底扩张的地幔变形过程往往难以推断。通过对采自两个采泥点（相距<1 km）的12个地幔橄榄岩的分析，研究了Gakkel海岭系低速扩张岩浆带中不均匀地幔中相对新鲜（<50%模式蛇纹岩）和超新鲜（<1%蛇纹岩）地幔橄榄岩的高温结晶塑性变形和低温糜棱岩化过程。在显微结构上，这12个橄榄岩包括7个高T变形样品和5个糜棱岩。在形态上，这12个样品包括方辉橄榄岩、二辉橄榄岩、橄榄石二辉橄榄岩和含斜长石的二辉橄榄岩。根据矿物主量和微量元素组成，二辉橄榄岩、方辉橄榄岩和橄榄二辉橄榄岩为残余橄榄岩。含单斜辉石的二辉橄榄岩具有平坦的稀土模式，可能经历了高熔体流入的再受精作用。含斜长石的二辉橄榄岩可能是部分熔融后的亚固相线反应形成的。显微构造观测支持高T晶体塑性变形（最有可能在超过1000 °C的温度下）在高T变形组的橄榄岩中活跃，容纳了加克尔岭下的地幔流动。已确定的熔体再受精过程可能有助于形成[010]-纤维橄榄石织物在这些橄榄岩。糜棱岩显微结构、组构强度降低和橄榄石粒度减小表明糜棱岩化作用发生在相对低温的地幔条件下（约800 °C），可能适应了应变局部化。在剪切带发育过程中，水对橄榄岩的影响不大，但在糜棱岩化后的一个糜棱岩橄榄岩中发育了具有“尘”带的角闪石，表明剪切带内局部发生了晚期交代作用。这种低Tmylonitization很可能已经影响了独立于成岩过程的地幔橄榄岩。这些变形过程在加克尔岭的发展表明，从流动的最上层地幔剪切带形成裂谷壁的转变。
Mantle deformation processes leading to seafloor spreading are often difficult to infer due to the highly serpentinized and weathered state of most abyssal peridotites. We investigated the development of high-temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high-Tdeformed samples and five mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with flat REE patterns likely underwent refertilization with a high influx of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high-Tcrystal-plastic deformation (most likely at temperatures exceeding 1000 °C) was active in the peridotites of the high-Tdeformation group, accommodating mantle flow beneath the Gakkel Ridge. The identified melt refertilization process may have contributed to the formation of [010]-fiber olivine fabrics in these peridotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (~800 °C) and probably accommodated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with “dusty” zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low-Tmylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The development of these deformation processes in Gakkel Ridge suggests a shift from flow in the uppermost mantle to shear zone formation in the rift valley walls.