Abstract

Ferromanganese (FeMn) crusts with oxides ranging from a thin coating to a maximum of 5 cm thickness were recovered from the South West Indian Ridge (SWIR) and Central Indian Ridge (CIR) segments. The substrate for the oxides are altered mafic, ultramafic, and serpentinite rocks. Twenty FeMn crusts were studied to determine their mineralogical compositions, microstructures, and chemical compositions. The crusts display rough botryoidal or smooth surface textures. Todorokite, hematite, goethite, and nontronite are the major minerals followed by vernadite, magnetite, and maghemite. Electron micrographs show framboids of FeMn oxides, fossil worms, foraminifera nuclei, and euhedral mineral crystals. The oxides have higher FeO wt% (27–47; average ∼43) than MnO (13–45, average ∼30), and the average wt% of SiO₂ is ∼40, Al₂O₃ ∼3, MgO ∼2, CaO ∼4, Na₂O ∼0.4, K₂O, NiO ∼0.5, CuO ∼0.3, CoO ∼0.8, and P₂O₅ ∼1.5. Our studies indicate a hydrothermal origin for the crusts and this fact is corroborated by low MnO/FeO ratio (≤ 1) and occurrence of framboids and hydrothermal minerals. Negative Ce anomaly and the plot of Mn–Fe–10×(Ni + Co + Cu) indicate characteristic signatures of hydrogenous-hydrothermal origin for most of the studied FeMn crusts.

摘要

从西南印度洋脊 (SWIR) 和中印度洋海脊 (CIR) 区段回收了具有从薄涂层到最大 5 厘米厚的氧化物的锰铁 (FeMn) 结壳。氧化物的基质是蚀变的镁铁质、超镁铁质和蛇纹岩。研究了 20 个 FeMn 结壳以确定它们的矿物组成、微观结构和化学组成。结痂表现出粗糙的葡萄状或光滑的表面纹理。Todorokite、赤铁矿、针铁矿和绿脱石是主要矿物，其次是蛭石、磁铁矿和磁赤铁矿。电子显微照片显示了 FeMn 氧化物、蠕虫化石、有孔虫核和自面体矿物晶体的 fmboids。氧化物的FeO wt%（27-47；平均~43）高于MnO（13-45，平均~30），SiO _{2的平均wt%}为～40、Al ₂ O ₃～3、MgO ～2、CaO ～4、Na ₂ O ～0.4、K ₂ O、NiO ～0.5、CuO ～0.3、CoO ～0.8、P ₂ O ₅～1.5。我们的研究表明地壳的热液起源，这一事实得到了低 MnO/FeO 比 (≤ 1) 以及 framboids 和热液矿物的出现的证实。负 Ce 异常和 Mn–Fe–10×(Ni + Co + Cu) 曲线表明大多数研究的 FeMn 结壳的氢热液成因的特征特征。