摘要
对中国大陆科学钻探(CCSD)主孔200~1005m范围内8件榴辉岩样品的金红石进行了LA-TCPMS原位微区微量元素分析,结合前人已发表的全岩和金红石分析数据,研究结果发现:在不同类型榴辉岩中,金红石的微量元素与其全岩成分具有不同的相关关系。金红石中的Nh和Ta元素含量不同程度地受控于全岩Nb和Ta含量。在高钛和低镁钛榴辉岩中,金红石的Cr与全岩Cr/TiO_2,正相关;在富镁榴辉岩中,金红石的Cr含量受全岩MgO含量的控制;在高钛和富镁榴辉岩中,全岩成分明显影响着金红石的Zr含量,金红石Zr温度计可能不适用。低镁钛榴辉岩的金红石的平衡温度普遍低于榴辉岩峰期变质温度,可能是变质流体参与下的扩散作用和退变质作用所致;多数情况下,单个样品中大部分金红石颗粒的Zr含量是均匀的,金红石Zr温度计所给出的温度可能代表着退变质再平衡的温度;CCSD榴辉岩的全岩Nb/Ta比值普遍低于其中金红石的Nb/Ta比值,不支持金红石榴辉岩可能是地球上超球粒陨石Nb/Ta比值储库的观点。
Trace elements of rutiles in eight eclogites from 200 ~ 1005m of CCSD main-hole were analyzed in-situ by LA-ICPMS. Combined our data with data of eclogites and rutiles from the literature, it is found that trace elements in rutiles from various types of eclogites have different correlations with the whole-rock chemistry. Nb and Ta contents in rutiles are significantly controlled by whole-rock compositions to degrees which in turn depend on the rock types. Cr contents in rutiles from high-Ti as well as low-Mg-Ti eclogites are positively correlated to whole-rock Cr/TiO_2 ratios. In contrast, Cr contents in rutiles from Mg-rich eclogites are controlled by whole-rock MgO contents. In high-Ti and Mg-rich eclogites, whole-rock compositions have strong effects on the Zr contents in rutiles, therefore, the Zr-in-rutile thermometer does not yield meaningful temperature information in these rock types. Temperatures from Zr-in-rutile thermometer for the low-Mg-Ti eclogites are usually lower than those in peak metamorphism, which might be caused by diffusion and retrogression with the aid of fluids. In most cases, Zr contents in rutile grains from single sample are homogeneous, Zr-in-rutile temperatures represent the re-equilibrium temperature during retrograde reactions. Nb/Ta ratios of eclogites are generally lower than those in rutiles, which does not support the hypothesis that rutile-eclogite is the hidden reservoir with a superchondritic Nb/Ta ratio within the interior of the earth.