麦哲伦海山富钴结壳的稀有气体丰度及He、Ar同位素组成  

The noble gas contents and helium and argon isotopic compositions in the cobalt-rich crusts from the Magellan Seamounts

作　　者：叶先仁[1] 方念乔[2] 丁林[3] 张铭杰[4] 

机构地区：[1]中国科学院气体地球化学实验室,兰州730000 [2]中国地质大学地质科学院,北京100083 [3]中国科学院青藏高原研究所,北京100083 [4]兰州大学资源环境学院,兰州730000

出　　处：《岩石学报》2008年第1期185-192,共8页Acta Petrologica Sinica

基　　金：国家自然科学基金(批准号:40573012;40273009);中国科学院"西部之光"人才培养计划资助

摘　　要：使用MM5400质谱计对麦哲伦海山富钴结壳和结壳基岩进行稀有气体同位素分析,以此示踪结壳形成期间的海洋环境。富钴结壳的He、Ne、Ar、Kr、Xe稀有气体同位素丰度模式与ASW(air-saturated water,饱和空气的海水)的变化趋势一致,说明结壳是在与海水交换平衡的流体环境中形成的。结壳的分馏因子F_3比饱和空气的海水的F_3高2～3个数量级,表明其相对于海水而言是富集~3He的。其相对于空气亏损~3He、富集Kr和Xe的模式与高度富集~3He、亏损Kr和Xe的星际尘粒的模式完全不同,暗示地球外物质并不是支配富钴结壳稀有气体同位素丰度模式的主要因素。结壳中高的R值甚至超过上地幔代表MORB(大洋中脊玄武岩)的典型值8R_a(R_a为地球大气中~3He/~4He比值,R为样品中~3He/~4He比值),揭示了热点型地幔热液对西太平洋海洋环境的局部贡献。圈层结壳的各结壳层之间的R值差异明显,各结壳层之间的尺值的变化趋势是"较高→最高→较高",结壳中层具有高达15.60R_a的R值,明显有深部地幔流体的加入,这进一步表明滋养结壳生长的热液环境本质上是源于地幔柱型的热点热液。玄武质基岩具有较低的R值,说明该类基岩在形成时严重混染有地壳物质,也说明结壳的物源并非来自玄武质基岩。结壳的^(40)Ar/^(36)Ar比值接近于饱和空气的海水值,也表明结壳的成矿流体与周围海水发生了充分的交换。The noble gas contents and isotopic compositions in the cobalt-rich crusts and basement rocks from the Magellan Seamounts had been measured using MM5400 mass spectrometry to trace the ocean condition. The abundance patterns of the noble gases in the crusts were consistent with the one of air-saturated water （ASW）. It indicated that the fluid formed crusts was balanceable with surrounding seawater, All of the F3 fractionation factor of the crusts were higher 2 - 3 magnitude degree than one of ASW, this suggested that the crusts were rich in 3 He relative to ASW. The patterns of crusts, which were depleted in 3 He and rich in Kr and Xe, were completely different with the one of interplanetary dust particles （IDP）, which were strongly rich in 3He and depleted in Kr and Xe. The fact implied that the noble gases budgets of the crusts were not essentially dominated by extraterrestrial matters as IDP. The fact that the higher R （ R = 3 He/4 He in a sample） values even in excess of 8R, （ R. = 3 He/4 He in air）, a typical value of MORB, were found in the crusts revealed the contributions from mantle fluid to the Western Pacific seawater. The R values between each and every layer in a layered crust were clearly different. The value much higher as 15.60Ra was found in the middle layer of the crust, and it further suggested that the fluid included into the crusts was derived from a plume-type hydrothermal. The R values of the basaltic basement rocks were less than Ra, A possible reason was that basaltic magmas had been contaminated by the earth＇s crust-derived matter, and also, this result showed the materials formed the crusts was not directly originated from the basement rocks. The 40 Ar/36Ar values of the crusts were closed to the one of ASW, and it suggested the exchange between the ore-formed fluids and surrounding seawater was sufficient.

关 键 词：麦哲伦 富钴结壳 稀有气体同位素 丰度模式 

分 类 号：P597.2[天文地球—地球化学]