1kbar、800℃下REE在富磷过铝质熔体/流体相间分配的实验研究

An experimental study of REE partitioning between phosphorus-rich peraluminous melt and coexisting aqueous fluid at 1 kbar and 800 ℃

利用"RQV-快速内冷淬火"(或称之为"外加热冷封式")高温高压实验装置,实验研究了1kbar、800℃条件下12个REE+Y在富磷过铝质熔体/含水流体相间的分配,并利用EMP、LA-ICPMS和ICP-MS分析技术分别测定了实验初始物、实验产物玻璃中主要化学组成以及熔体相和流体相中REE含量。实验结果表明,REE元素(La,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb和Lu)在流体/熔体相间的分配系数(Dfluid/melt)在(0.1～19.9)×10-4范围,DfYluid/melt在(0.2～7.8)×10-4范围,指示REE和Y强烈趋向于在熔体中富集。REE在流体/熔体相间的分配系数(Dfluid/melt)与体系中P2O5含量变化呈近抛物线状分布,其最大值对应于残余熔体中w(P2O5)为1.44%处。REE在流体/熔体相间的分配系数(Dfluid/melt)随REE的原子序数增大而逐渐降低,构成右倾的平滑曲线,总体上显示出DLREE>DMREE>DHREE的趋势。Y与Ho在流体/熔体相间分配系数的比值(DY/DHo)约为1(0.91～1.28),不随体系中P2O5变化而变化的特征。上述特征表明熔体-流体作用不会导致Y-Ho及REE间的分异,因此,可推断熔体-流体作用过程不可能是过铝质岩浆体系中产生稀土"四重效应"机制。

Fluid-saturated experiments were conducted to investigate the partitioning of 12 REE and Y between the phosphorus-rich peraluminous melt and coexisting aqueous fluid at 1 kbar and 800 ℃. All runs were conducted in cold-seal Rapid-Quench pressure Vessels （RQV, a kind of ＂externally heated cold-seal pressure vessels＂） using water as a pressure medium for durations of 144 hours. The major chemical compositions of the initial and run-product glasses were determined by EMPA; REE contents of the initial and run-product glasses were determined by LA-ICPMS; REE contents of aqueous fluids were analyzed by ICP-MS. It shows that the partitioning coefficients of REE （including La, Nd, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu） between aqueous fluid and hydrous melt （D^fluid/melt） vary from 0. 1 × 10^-4 to 19.9 × 10^-4 , and DY^fluid/melt range from 0.2× 10^-4 to 7.8× 10^-4 , indicating that REE and Y are preferentially partitioned into the hydrous melts. The relationship of the fluid-melt partitioning coefficients of REEs to the phosphorus contents in the run-product glasses shows a near-normal distribution as the contents of P2O5 increasing from 0.19 wt% to 6.09 wt %, with the maximum D^fluid/melt corresponding to 1.44 wt/00 of P2O5. The fluid-melt REE partitioning coefficients decrease as increasing the atomic numbers of REE from La to Lu, showing DLREE〉 DMREE〉DHREE generally. The ratios of DY to DHo show no obvious variations with increasing P2 O5 contents in the run-product glasses from 0.19 wt% to 6.09 wt%, keeping a nearly constant value of -1 （in the range of 0. 91-1.28）. The experimental results demonstrate that the interaction of phosphorus-rich peraluminous melt with coexisting aqueous fluid could not result in the fractionation between Y and Ho, and among the REEs. Therefore, it is implied that the melt-fluid interaction cannot be a candidate for the mechanism of REE tetrad effect during the evolution of peraluminous magmatic system.