金红石榴辉岩——一个可能的超球粒陨石Nb/Ta储库

Nb,Ta的硅酸盐地球质量不平衡问题争论由来已久,备受关注。近年来研究发现,含金红石的榴辉岩Nb/Ta往往高于球粒陨石值(Nb/Ta=17.5),暗示其可能是平衡地球Nb亏损的独立储库。而洋壳玄武岩部分熔融实验表明Ta比Nb更倾向进入金红石晶格,这意味着作为俯冲洋壳部分熔融残留相的榴辉岩Nb/Ta不可能高于原岩。为了解释地质观察和实验结果之间的矛盾,系统分析了中国大陆科学钻探工程(CCSD)主孔、先导孔及附近地表榴辉岩的矿物微量元素。结果发现:榴辉岩中的Nb,Ta主要存在于金红石之中,其他矿物中含量极少;Nb,Ta之间存在着强烈分异(Nb/Ta=5.3～96.2),并总体上具有超球粒陨石的特征;韭闪石和多硅白云母的Nb/Ta平均分别为48.6,21.8,显示了很强的Nb,Ta分异能力;其他矿物如石榴石、绿辉石、绿帘石、磷灰石等的Nb、Ta含量及Nb/Ta都很低,对Nb-Ta分异不造成影响。认为导致Nb-Ta分异的不是金红石,而应出现在洋壳俯冲过程中金红石相出现之前的脱水和部分熔融阶段。富含Ti的角闪石(韭闪石)和白云母可能对Nb-Ta分异起到了决定性的作用。等金红石相出现之后,由于其对Nb,Ta的绝对控制作用,此前阶段的分异结果便被固定在金红石中而继承下来。因此,含金红石的榴辉岩常常表现出超球粒陨石Nb/Ta的特征,与金红石不能有效地分异Na,Ta的实验结果之间并不矛盾。在不均匀的上地幔中含金红石的榴辉岩是可能的超球粒陨石Nb/Ta储库之一。

Rutile-bearing Eclogite: One of the Possible Reservoirs Balancing the Nb-depleted Silicate Earth

The mass imbalance of Nb-Ta in the silicate Earth is a significant geochemical issue which has long been concerned as a paradox. Weather is rutile-bearing eclogite the bridge of continental crust and the depleted mantle with regard to the Nb depletion，and how to explain the controversy between the oceanic basalt partial melting results (rutile favors Ta over Nb) and the naturally observed results (rutile bearing eclogites commonly have superchondritic Nb/Ta ratios)? The present work analyzed the trace elements of minerals in eclogites from the main hole of Chinese Continental Scientific Drilling Project (CCSD) and some outcrops nearby by LA-ICP-MS. The results showed that: (1) Rutile predominantly controls the distribution of Nb, Ta in eclogite, with extremely minor concentrations of the two elements in other minerals; (2) Significant fractionation between Nb and Ta presents in eclogites (Nb/Ta =5.3 ~96.2), with a overall superchondritic ratio (the mean Nb/Ta is 25.9); (3) The ratios of Nb/Ta in amphibole and phengite are unexpectedly high, 48.6 and 21.8 respectively, exhibiting stronger fractionation between Nb and Ta in the two minerals; (4) Both the concentrations of Nb, Ta and the ratios of Nb/Ta in the minerals such as garnet, omphacite, apatite, phengite, epidote, etc., in eclogite are extremely low. Based on these results, we argued it is likely that Ti-bearing minerals, such as pargasite and phengite fractionated Nb from Ta significantly before rutile occurring steadily, and the results of fractionation were fixed and inherited by rutile subsequently because of its absolutely control for Nb and Ta. Therefore, we propose that rutile-bearing eclogite be one of the reservoirs in deep mantle which can balance the depletion of Nb in the silicate Earth.