Geochemistry of Fe-Mn crusts in sediments of the Central Indian Ridge

Layered Fe-Mn crusts from the off-axis region of the first segment of the Central Indian Ridge north of the Rodrigues Triple Junction were studied geochemically and mineralogically. Vernadite (delta-MnO2) is the main mineral oxide phase. 230Thxs and Co concentrations suggest high growth rates of up to 29 mm/Myr and a maximum age of the basal crust layer of 1 Ma. Whereas most of the major and minor elements show concentrations which are typical of hydrogenetic formation, Co, Pb, Ni and Ti concentrations are strikingly lower. Concentrations and distribution of the strictly trivalent rare-earths and yttrium (REY) are typical of hydrogenetic ferromanganese oxide precipitates, but in marked contrast, the crusts are characterized by negative CeSN (shale normalized) anomalies and (Ce/Pr)SN ratios less than unity. Profiles through the crusts reveal only minor variations of the REY distribution and (Ce/Pr)SN ratios range from 0.45 to 0.68 (compared to ratios of up to 2 for typical hydrogenetic crusts from the Central Indian Basin). The apparent bulk partition coefficients between the crusts and seawater suggest that for the strictly trivalent REY the adsorption-desorption equilibrium has been reached. Positive Ce anomalies in the partition coefficient patterns reveal preferential uptake of Ce, but to a lesser extent than in normal hydrogenetic crusts. A new parameter (excess Ce, Cexs) to quantify the degree of decoupling of Ce from REY(III) is established on the basis of partition coefficients. Cexs/Cebulk ratios suggest that the CIR crusts formed by precipitation of Fe-Mn oxides from a hydrothermal plume and that in hydrothermal plumes and normal seawater the enrichment of Ce results from the same oxidative sorption process. The growth rates, calculated with 230Thxs data as well as with the Co formula, are inversely related to Cexs.

针对罗德里格斯三联点以北中印度洋海岭第一段轴外区域产出的层状铁锰结壳，开展了地球化学与矿物学研究。水锰矿（δ-MnO₂）为该类结壳的主要矿物氧化物物相。基于过剩²³⁰Th（²³⁰Thxs）与钴（Co）的浓度数据，推算得到该结壳最高可达29 mm/Myr的快速生长速率，且结壳基底层的最大年龄为1 Ma。尽管多数主量与微量元素的浓度均符合水成成因铁锰结壳的特征，但钴（Co）、铅（Pb）、镍（Ni）与钛（Ti）的浓度却显著偏低。严格三价稀土元素与钇（REY）的浓度及分布特征均与水成因铁锰氧化物沉淀物的典型特征一致，然而该结壳却呈现出显著差异：其具有负的CeSN（页岩标准化）异常，且(Ce/Pr)SN比值小于1。对结壳剖面的分析显示，REY的分布仅存在微小变化，(Ce/Pr)SN比值介于0.45至0.68之间（而中印度洋盆地典型水成结壳的该比值最高可达2）。结壳与海水之间的表观整体分配系数表明，对于严格三价的REY而言，其吸附-解吸过程已达到平衡状态。分配系数图谱中的正Ce异常表明，铈（Ce）存在优先吸附现象，但其吸附程度弱于常规水成结壳。研究基于分配系数建立了一项新参数——过剩铈（Cexs），用于量化Ce与REY(III)之间的解耦程度。Cexs/Cebulk比值表明，中印度洋海岭（CIR）结壳是由热液羽流中铁锰氧化物沉淀形成，且热液羽流与正常海水中Ce的富集均源自同一氧化吸附过程。基于过剩²³⁰Th数据与钴浓度公式计算得到的生长速率，与Cexs呈负相关关系。