Search for chemical effect on partitioning of rare-earth elements in the crystallization process of basaltic systems at 20kb, with some applications.

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.

Rectilinear partition coefficient function of REE pertaining to the formation of basaltic rocks from the New Georgia Group, the Solomon Islands.

“…Four REE patterns of KB 309, -343, -356 and -602 could be satisfactorily in terpreted to have started from the initial melt represented by KO-4, and to correspond to the varying F values with the rectilinear partition coefficient function presumably having a break at Ho. Moreover, it is worth noting that the ob tained partition coefficient function is terrace shaped (Masuda et al, 1977) and is essentially similar to those experimentally obtained by Shimizu (1980) and Shimizu et al (1982).…”

“…Moreover, he has main tained a unique viewpoint by suggesting that REE patterns of whole rocks are characterized by a process or an effect of separating "single phase solid precursors" from the true melt. The linear REE partition patterns were obtained ex perimentally for clinopyroxene-melt system at 10, 20 and 30 kb (Masuda and Kushiro, 1970;Tanaka and Nishizawa, 1975;Masuda et al, 1977;Shimizu, 1980;Shimizu et al, 1982). Shimizu (1980) and Shimizu et al (1982) em phasized that, in partial solidification of melt, the formation of clinopyroxene essentially free from other minerals prevails in wide ranges of high pressures and temperatures.…”

Experimental study on rare-earth element partitioning in olivine and clinopyroxene formed at 10 and 20kb for basaltic systems.

“…Incidentally, 82, on the other hand, the segment between Tb if the possible effect of olivine phenocryst is and Tm seems to be slightly inclined. These RO (1970), TANAKA and NISHIZAWA (1975), MASUDA et al (1977) and SHIMIZU (1980. SHIMIZU (1980) This observation holds good for the clinopyroxene of this study.…”

Light REE inclination and distance from volcanic front; a case of volcanic rocks in northeastern Japan.

“…And the fractional solidification starting from such a melt proceeds under operation of the bulk parti tion coefficients of REE which show rectilinear function corresponding to "terrace-shaped" one (TANAKA and NISHIZAWA, 1975;MASUDA et al, 1977;SHIMIZU, 1980). In the course of solidifi cation, concentration of a certain element in the liquid phase varies according to the follow ing equation: log (CL /CO) = (k 1) log fW (1) where fW = ratio of mass of residual liquid rela tive to the initial liquid mass (fW is designated as residual liquid fraction value), k = bulk parti tion coefficient of a given element, CL = its concentration in the residual liquid, and Co = concentration of the same element in the "initial" melt hypotethically with chondritic REE abundance ratios.…”