磷灰石的稀土组成及其示踪意义

REE Content and Distribution in Apatite and Its Geological Tracing Significance

磷灰石是一种分布很普遍的含稀土矿物。作者测定并收集了产于各类岩石及某些矿石中磷灰石的稀土元素组成,绘制了磷灰石的稀土元素球粒陨石标准化分布模式的8种典型代表。据此,磷灰石可以作为一种示踪矿物来应用。

Apatite is one of the most widespread minerals in nature and it can be found almost in all types of igneous rocks, as well as in metamorphic rocks, sedimentary, and some ore deposits. The chemical formula of apatite is A_5(XO_4)_3(F,OH,Cl),where A represents Ca, REE, Mn, Sr, etc.; X stands for P, As, V, Si, S, C, etc. More or less REE are contained in apatite, with the maximum value up to 11.14%. The REE in the crystal structure of apatite replace Ca while Ca occupies two sites in the lattice (nine coordinated and seven coordinated, respectively). So the apatite can host various rare earth element,As compared with the selective-assemblage minerals apatite is more suitable to use as a indicator mineral to reflect the REE characteristics of rock- and ore-forming geological environments. Summarized in this paper are the REE compositions and Sr isotopic values (^(87)Sr/^(86)Sr)_I of apatites separated from ultrabasic rocks, basic rocks, intermediate rocks, acid rocks, alkaline rocks, carbonatite, granopegmatite, phosphorite and some ore deposits. The results have shown that there are significant differences in chondrite-normalized REE distribution patterns for apatites differing in the mode of occurrence and origin. They can be distinguished from one another by plotting (La/Yb)_N against δEu. For instance, apatites in alkaline rocks and carbonatite possess the highest (La/Yb)_N and δEu values wheres those occurring in granites (especially those granites associated with mineralization of Nb, Ta, W, Sn, Li and Be) have the lowest values. Both (La/Yb)_N and δEu values of apatites from basic and intermediate rocks lie between the above two types. In combination with Sr isotopic measurements in apatite, it is possible to acquire more geological information.