压力对稀土元素分配系数相对变化的影响——以宽甸、汉诺坝玄武岩中巨晶矿物为例

EFFECT OF PRESSURE ON THE PEAK POSITION IN THE REE PARTITION PATTERN: EVIDENCE FROM MEGACRYST MINERALS IN KUANDIAN AND HANNUOBA BASALTS

通过对与寄主岩(碱性玄武岩)具有化学平衡或近于化学平衡关系的巨晶(石榴石、辉石和角闪石)的REE分配系数的研究,发现其REE分配系数的对数值相对于元素半径呈抛物线性变化。抛物线的顶点位置不但随矿物种类的不同而不同,而且对同一矿物也是变化的。随压力增大,同一种矿物的分配系数抛物线顶点向离子半径小的元素方向偏移,可以认为在不同深度生成或演化的玄武岩岩浆的REE分布模式受压力的影响较大。

Major element compositions, REE abundances, and Sr and Nd isotope ratios were measured for three megacrysts of garnet, five of clinopyroxene, one of amphibole and the host alkali basalts from two volcanoes in norhteastern China. Sr and Nd isotpic data and distribution of major elements (Fe and Mg) between megacrysts and host lavas indicate a cognate origin of megacrysts and host basalts. Probably, these megacrysts of garnet, clinopyroxene and amphibole have been in equilibrium with their host magmas at high pressures.Logarithmic values of REE partition coefficients for these minerals were found to vary parabolically with inoic radius, and REE substitution into these three crystalline silicates is explicable in terms of crystal structure. It is shown that the shift of ionic radius at peak position of the parabolas toward the smaller ions is accompanied by an increase in Na2O and Al2O3 contents in the case of clinopyroxene and with an increase in MgO content in the case of garnet. Because the contents of these major elements are functions mainly of pressure, the peak position shift is considered to be mainly due to pressure change in the basaltic system. Since the pressure, among other factors, significantly affects the REE partitioning between crystal and melt, it is suggested that, in use of REE partition coefficients for modeling the petrogenesis of igneous rocks, the effects of not only chemical composition but also pressure on the REE partition coefficients should be taken into account.