Analysis of manganese crusts from Galapagos

Manganese oxide crusts similar to those reported from the Mid-Atlantic Ridge rift valley by Scott et al., 1974 (https://doi.pangaea.de/10.1594/PANGAEA.848726) were dredged at two sites near the Galapagos spreading axis on ocean floor estimated from magnetic anomalies to be 2.4 and 0.3 m.y. old. Compared to the typical ocean-floor manganese deposits attributed to precipitation from seawater, the 2-6 cm thick manganese crusts reported here exhibit very low Fe/Mn and low 232Th/238U ratios, as well as lower transition metal and higher manganese concentrations. The manganese crusts were deposited several orders of magnitude faster than the more common hydrogenous nodules; this fact together with other geochemical characteristics and the geophysical environment suggests the manganese deposits reported here are of hydrothermal origin.本研究在加拉帕戈斯扩张轴附近的两个洋底站位拖网采获了氧化锰结壳（manganese oxide crusts）样品，该洋底经磁异常估算形成年龄分别为2.4百万年和0.3百万年；这些结壳与Scott等人1974年报道的大西洋中脊裂谷产出的同类样品相似，相关文献链接为https://doi.pangaea.de/10.1594/PANGAEA.848726。相较于常规由海水沉淀形成的海底锰矿床，本次报道的2~6厘米厚氧化锰结壳展现出极低的铁锰比（Fe/Mn）与钍232/铀238（232Th/238U）比值，同时过渡金属含量更低、锰元素浓度更高。该类氧化锰结壳的沉积速率比更为普遍的水成结核（hydrogenous nodules）快数个数量级，结合其他地球化学特征与地球物理环境，可推断本次报道的锰矿床属于热液成因（hydrothermal origin）。