He, Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nuclide abundances and isotopic ratios can be classified into two types: low He-3/He-4 type and high He-3/He-4 type. The low He-3/He-4 type is characterized by high He-4 abundances of 191x10(-9) cm(3.)STP(.)g(-1) on average, with variable He-4, Ne-20 and Ar-40 abundances in the range (42.8-421)x10(-9) cm(3.)STP(.)g(-1), (5.40-141)x10(-9)cm(3.)STP(.)g(-1), and (773-10976)x10(-9) cm(3.)STP(.)g(-1), respectively. The high He-3/He-4 samples are characterized by low He-4 abundances of 11.7x10(-9) cm(3.)STP(.)g(-1) on average, with He-4, Ne-20 and Ar-40 abundances in the range of (7.57-17.4)x10(-9) cm(3.)STP(.)g(-1), (110.4-25.5)x10(-9) cm(3.)STP(.)g(-1) and (5354-9050)x10(-9) cm(3.)STP(.)g(-1), respectively. The low He-3/He-4 samples have He-3/He-4 ratios (with RIRA ratios of 2.04-2.92) which are lower than those of MORB (R/R-A=8 +/- 1) and Ar-40/Ar-36 ratios (447-543) which are higher than those of air (295.5). The high He-3/He-4 samples have He-3/He-4 ratios (with R/R-A ratios of 10.4-12.0) slightly higher than those of MORB (R/R-A=8 +/- 1) and Ar-40/Ar-36 ratios (293-299) very similar to those of air (295.5). The Ne isotopic ratios (Ne-20/Ne-22 and Ne-21/Ne-22 ratios of 10.3-10.9 and 0.02774-0.03039, respectively) and the Ar-38/Ar-36 ratios (0.1886-0.1963) have narrow ranges which are very similar to those of air (the Ne-20/Ne-22, Ne-21/Ne-22, Ar-38/Ar-36 ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low He-3/He-4 type and high He-3/He-4 type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)- and EM (Enriched Mantle)-type mantle material, respectively. The low He-3/He-4 type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts whereas the high He-3/He-4 type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with variations in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantle surce may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation.

Os isotope dating and growth hiatuses of Co-rich crust from central Pacific

Up to now, accurate determination of the growth age and hiatuses of the Co-rich crust is still a difficult work, which constrains the researches on the genesis, growth process, controlling factors, regional tectonics, paleo-oceanographic background, etc. of the Co-rich crust. This paper describes our work in determining the initial growth age of the Co-rich crust to be of the late Cretaceous Campanian Stage (about 75-80 Ma), by selecting the Co-rich crust with clear multi-layer structures in a central Pacific seamount for layer-by-layer sample analysis and using a number of chronological methods, such as Co flux dating, dating by correlation with Os-187/Os-188 evolution curves of seawater, and stratigraphic division by calcareous nannofossils. We have also discovered growth hiatuses with different time intervals in the early Paleocene, middle Eocene, late Eocene and early-middle Miocene, respectively. These results have provided an important age background for further researches on the Co-rich crust growth process and the paleo-oceanographic environment evolution thereby revealed in the said region.

中太平洋海区富钴结壳地球化学特征及成因机制

本文利用ICP-AES、EPMA、X-ray衍射等测试技术以及聚类分析和因子分析等多种数理统计方法，系统地对中太平洋海区富钴结壳的元素地球化学特征、矿物组成和微观组构进行了研究，并探讨了其成因机制，获得以下主要认识： 1中太平洋海区富钴结壳类型多样，均为水成成因，其矿物相主要由锰矿物相、铁矿物相和非金属矿物相组成；富钴结壳壳层发育多种原生和次生构造类型。 2磷酸盐化作用不仅强烈改变富钴结壳元素初始含量，而且造成富钴结壳某些元素间的相关性发生改变，这些敏感型元素对可用于指示富钴结壳是否发生磷酸盐化。在不同水深段内，未磷酸盐化型富钴结壳的主要元素随经向、纬向的变化趋势相似，表明其受水体化学障、表面生产力和物质来源等环境参数控制；而随水深的变化则具有区域一致性，表明水体化学具有区域成层性。 3未磷酸盐化富钴结壳稀土元素含量和轻重稀土分馏程度随水深发生规律变化，这种变化不仅与它们在海洋中的含量和行为有关，也与海洋背景颗粒的吸附有关；铈(Ce)在富钴结壳中基本上呈4价，且动力学因素控制了其富集过程，因此Ce异常不能用于指示富钴结壳形成环境的氧化程度。 4基于富钴结壳微层呈锯齿状且同一微层生长速率不同，提出了富钴结壳在各种基底表面生长以及后继发育过程受固液界面双电层控制的发育模式。在富钴结壳整个发育过程中，经历了从贫氧环境向富氧环境的转变，但微环境则呈富氧-低氧过程的交替。 关键词： 富钴结壳；中太平洋；元素地球化学；界面双电层