钇(Y)在铁钛氧化物中的地球化学行为——以峨眉山大火成岩省为例

Geochemical Behavior of Yttrium in Fe—Ti Oxides——An Example Inferred from the Emeishan Large Igneous Province

钛铁矿和钛磁铁矿是火成岩中最常见的副矿物,查明钇(Y)及Ti/Y值与铁钛氧化物的分离结晶/堆积作用的关系对于利用微量元素来探讨岩石成因具有重要的科学意义。峨眉山大火成岩省中的溢流玄武岩及与其具有成因联系的层状岩体均有不同含量的钛磁铁矿或钛铁矿,本文通过对其TiO2含量与FeO*(FeO*为全铁含量,FeO*=FeO+0.9Fe2O3)、Y与Ti/Y的相关性研究,发现与前人的实验成果并不完全一致,在TiO2含量较低时(TiO2<7%),Y在铁钛氧化物中具有一定的相容性,而在大量铁钛氧化物存在时(TiO2>7%),则表现为不相容性。但是岩石中存在一定量的铁钛氧化物时,其Ti/Y值则随TiO2含量的增高而增高,所以该比值不能反映源区的特征。在铁钛氧化物含量较低或基本不含时,Ti/Y值则与TiO2的含量无关,可能对源区具有指示意义。

Ilmenite and titanomagnetite are the most common accessory minerals in igneous rocks. Understanding the relationship between yttrium, Ti/Y and fractional crystallization/accumulation of Fe—Ti oxides will help to discuss the petrogenesis by using trace elements. The various amount of titanomagnetite or ilmenite is generally hosted in the flood basalts and the genetically related layered intrusions within the Emeishan large igneous province （ELIP）. In this paper, the investigation on the correlation of TiO2 contents with FeO, Y and Ti/Y for the ELIP has shown that our conclusion is not completely consistent with the previous experimental work. yttrium appears to be of compatible element in Fe—Ti oxides at TiO2〈～7%, but of incompatible elements at TiO2〉～7%. However, Ti/Y ratios increase with increasing TiO2 if there are some Fe—Ti oxides in the rocks, and therefore they cannot be considered as an indicator of the source. In contrast, Ti/Y ratios might be the indicator of the source if there are little or no Fe—Ti oxides, or absence of the correlation between Ti/Y ratios and TiO2 contents.