Composition and origin of sediments at DSDP Site 54-424

The sediments recovered on Deep Sea Drilling Project Leg 54 appear to be mixtures of the normal pelagic sediments of the area and hydrothermally produced manganese and iron phases. The latter are mineralogically and chemically very similar to phases recovered from surficial sampling of the mounds. The hydrothermal nontronite which is approximately 15 meters thick in the three holes is essentially free of carbonate or detrital contaminants. The basal sediments are similar to the carbonate oozes presently being deposited in the region, but are enriched in Mn and Fe. This enrichment appears to be the result of hydrothermal deposition that took place at or near the spreading center and may not be associated with the mounds formation. Three different hypotheses for the formation of the nontronite layer and the mounds deposits are considered. An initial deposition of a widespread nontronite layer and subsequent diapiric-like movement of the layer into carbonates could account for the observed stratigraphy; however, if this be correct, analogous deposits should be present in other DSDP sites. The second hypothesis - replacement of the normal sediments by nontronite - may be feasible, but the high purity of the nontronite requires dissolution and removal of refractory elements. The third hypothesis, metal deposition in an advancing oxidation gradient, is compatible with submersible observations of the mounds; however, it can account only for the high purity of the nontronite by very rapid deposition of the hydrothermal phases.深海钻探计划（Deep Sea Drilling Project, DSDP）第54航次所获取的沉积物，实为该区域正常远洋沉积物与热液成因锰铁相的混合体。后者在矿物学与地球化学特征上，与海山浅表采样获取的物相极为相似。在三个钻探孔中厚度约15米的热液成因绿脱石（nontronite），基本不含碳酸盐或陆源碎屑污染物。基底沉积物与该区域当前沉积的碳酸盐软泥（carbonate oozes）性质相近，但锰、铁元素含量显著富集。这种富集现象应是扩张中心（spreading center）附近发生的热液沉积作用所致，或许与海山的形成并无关联。本次研究针对绿脱石层与海山沉积的成因，提出了三种不同假说。第一种假说认为，先广泛沉积一层绿脱石，随后该层发生类似底辟的运动并侵入碳酸盐地层，该机制可解释观测到的地层序列；但若该假说成立，类似的沉积物应当在其他DSDP钻孔位点中也有分布。第二种假说——绿脱石交代正常沉积物——具备一定可行性，但要形成高纯度的绿脱石，需要将难熔元素（refractory elements）溶出并移除。第三种假说认为，金属在持续推进的氧化梯度中沉积，该机制与载人潜水器（submersible）对海山的观测结果相符；不过该假说仅能通过热液物相的快速沉积，来解释绿脱石的高纯度特征。