ABSTRACT

Employing two kinds of basalts, i.e., alkali olivine basalt and high-alumina basalt, the partitioning of REE was investigated for partial crystallization at 20kb. Even with the same alkali olivine basalt, the peak-shaped and the terrace-shaped partition coefficient functions were obtained, although both of them belong to a major “rectilinear” partition coefficient group. This difference in the shape appears to be associated with the relative content of Ca to Mg and Fe in crystal phase. Meanwhile, use of a high-Al basalt yielded a new partition coefficient function (schanze-shaped) quite distinct from rectilinear functions and their varieties. Intriguingly, addition of MgO (5%) to the high-Al basalt powder (95%) brought about a drastic change of partition coefficient function, resulting in the terrace-shaped one. It is evinced that the higher values of atomic ratios, Al/(Mg+Fe+Ca) and Si/(Mg+Fe+Ca), in the starting material are favorable to the appearance of schanze-shaped partition coefficient function. Besides, REE in island arc tholeiite from New Britain were accurately determined. REE abundances in this sample have demonstrated to imply that this basalt has a history of temporary separation from melt with 6-8 times chondritic REE abundances through the regulation of the terrace-shaped partition coefficient function. It was also attempted to use the schanze-shaped partition coefficient function in order to explain the genesis of felsic potassic basalt (KAY and GAST, 1973) based on two-stage fractional crystallization model. The REE pattern of the material removed during the second stage was calculated together with the “liquid-type” pattern corresponding to the switching point between two stages in question.