苏北盆地新生代玄武岩中橄榄岩包体的含水性

对来自苏北盆地盘石山、练山和方山地区新生代玄武岩中的50个橄榄岩包体矿物进行了系统的微区傅立叶变换红外光谱(Micro-FTIR) 分析.结果显示, 所有的单斜辉石和斜方辉石颗粒都含有以OH形式存在的结构水, 盘石山、练山和方山橄榄岩的单斜辉石水含量分别为64×10-6~183×10-6、37×10-6~102×10-6和41×10-6~177×10-6; 斜方辉石水含量分别为16×10-6~61×10-6、13×10-6~45×10-6和21×10-6~74×10-6.几乎所有的橄榄石都没有检测到明显的OH吸收峰, 暗示其水含量低于仪器的检出限(~2×10-6).根据矿物水含量(假设橄榄石的水含量为2×10-6) 和它们的体积分数计算的盘石山、练山和方山橄榄岩全岩的水含量分别为12×10-6~52×10-6、7×10-6~25×10-6和13×10-6~44×10-6.结合已经发表的橄榄岩包体的数据来看, 在岩石圈地幔的物理化学条件下, 单斜辉石与斜方辉石之间水的平衡分配系数大约为2.2±0.4.结合已经发表的安徽女山和河北汉诺坝的橄榄岩数据, 对比世界上其他地区橄榄岩的数据来看, 华北的岩石圈地幔具有低的水含量: 华北橄榄岩的单斜辉石水含量多 < 200×10-6, 而世界上其他地区(包括南非克拉通、美国新墨西哥地区、美国Colorado高原、美国盆岭省地区、墨西哥南部、法国中央地体以及加拿大WestKettle地区) 橄榄岩的单斜辉石水含量多 > 200×10-6; 华北的斜方辉石水含量多 < 100×10-6, 而世界上其他地区多 > 100×10-6; 华北的橄榄岩全岩水含量多 < 50×10-6, 而世界上其他地区多 > 100×10-6.华北岩石圈地幔的低水含量有可能是由于上升软流圈的热侵蚀造成的, 因此目前的华北岩石圈地幔可能大部分都是中生代岩石圈减薄后的残余, 而不是新生地幔. 

Water Contents in Peridotite Xenoliths from Subei Basin,Eastern China

Nominally anhydrous minerals, clinopyroxene (Cpx), orthopyroxene (Opx) and olivine (Ol), in peridotite xenoliths hosted by Cenozoic basalts from Panshishan, Lianshan and Fangshan of the Subei basin, eastern China have been investigated by Micro-FTIR for their water contents.All Cpx and Opx grains contain a certain amount of "water" as hydroxyl defect in the crystal structure.Water contents (expressed as H₂O wt.) of Cpx and Opx from Panshishan, Lianshan and Fangshan peridotites are 64×10-6-183×10-6, 37×10-6-102×10-6, 41×10-6-177×10-6 and 16×10-6-61×10-6, 13×10-6-45×10-6, 21×10-6-74×10-6, respectively.OH peaks can not be detected for almost all olivine grains, indicating that the water content is below the detection limit (~2×10-6) of FTIR.The whole rock water contents calculated according to the water contents of minerals and their volume proportions are 12×10-6-52×10-6, 7×10-6-25×10-6 and 13×10-6-44×10-6 for Panshishan, Lianshan and Fangshan respectively.Based on the new results from the Subei basin and the data reported in literatures, the partition coefficient of H₂O between Cpx and Opx in the continental lithosphere mantle is estimated to be 2.2±0.4.Peridotite xenoliths from Nüshan, Hannuoba and the Subei basin of the North China block have much lower water contents than those from other localities of the world (South Africa craton, New Mexico, Colorado plateau and basin and Range province of USA, South Mexico, French Central Massif and West Kettle of Canada).This difference is probably induced by the thermal erosion of asthenosphere during Mesozoic-Cenozoic period in the North China block.If this explanation is correct, the present lithosphere mantle in the North China block is mainly the relict after the Mesozoic lithospheric thinning, rather than the new accreted mantle.