地幔矿物中CO_2流体包裹体的微量元素特征

Trace elements in CO2 fluid inclusions in mantle Iherzolite.

本文在镜下观测基础上,挑选富含CO_2包裹体的地幔橄榄岩的橄榄石、斜方辉石和单斜辉石样品,运用热爆法提取和ICP-MS技术,直接测定了单矿物中CO_2流体包裹体的稀土和微量元素含量。在同样条件下,采用四极质谱法测定了CO_2等气相成分,并以CO_2含量为参考值,得出流体中的微量元素相对含量。热爆样品的显微镜观测表明,靠近颗粒表面的较大的CO_2包裹体在1000℃爆裂时均已破裂,所测定的REE和其他微量元素主要来自CO_2包裹体。CO_2流体/球粒陨石标准化的REE数据表明,地幔CO_2流体相对于地幔岩或地幔矿物而言,富集稀土元素,特别是轻稀土元素,LREE/HREE为1.53～11.96,REE分馏程度较大,(La/Yb)_N一般大于1。CO_2流体/上地幔岩石标准化值研究表明,除Co、Ni外,铁族元素在地幔流体中趋于贫化;Cu、Mo、W、Bi、Ag等热液矿床的重金属成矿元素在地幔CO_2流体中富集。

Trace elements including REEs in fluid inclusions in olivine, orthopyroxene and clinopyroxene of lherzolite, have been determined by heating-decrepitation and ICP-MS method, based on careful observation under microscope. Quadrupole mass spectrum method was also done for obtaining CO2 and other gas components in fluid inclusions 9 in order to understand the components of trace elements in CO2 fluid. Observation under microscope show that most larger CO2 inclusions near the surfaces of the sample grains were decrepitated at 1000degreesC, indicating contribution of CO2 inclusions to the components of trace elements. Normalized CO2 fluid/chondrite data show that mantle fluids are rich in REEs, especially LREEs, with LREE/HREE ranging from 1.53 to 11.96. The fractionation of REEs is high, with (La/Yb)(N) larger than 1. The values of normalized CO2 fluids/upper mantle show that Fe group elements except Co and Ni tend to be poor in mantle fluids, while heavy metal elements in hydrothermal deposits, such as Cu, Mo, W, Bi and Ag, tend to be rich.