几个重要Ge同位素平衡分馏参数的理论预测

本研究基于Urey模型(或称Bigeleisen和Mayer公式),结合量子化学计算的方法,在B3LYP/6-311+G(d, p)理论水平下,计算了Ge在类似石英(包括蛋白石)、钠长石、钾长石、橄榄石结构以及水溶液(包括海水)中Ge(OH)4和GeO(OH)3-之间的Ge同位素平衡分馏系数.其中,溶液效应用"水滴法"处理,矿物结构用簇合物方法模拟.结果显示这些基本分馏参数的精度约±0.3‰;类石英(或蛋白石)结构最可能富集重Ge同位素,在25 ℃,几个Ge同位素分馏系数分别约为:Δ石英-Ge(OH)4=0.9‰、ΔGe(OH)4-GeO(OH)3-=0.3‰(海水中)、Δ石英-钠长石=0.6‰、Δ石英-钾长石=0.4‰、Δ橄榄石-Ge(OH)4=-1.2‰.类石英与类橄榄石结构之间存在较大的分馏,Δ石英-橄榄石=2.1‰.这些具有重要地质意义的基本分馏参数可以为探索未知的Ge同位素地球化学应用领域打下基础.此外,本文还用所得的分馏参数定量地解释了Siebert等(2006)和Rouxel等(2006)的一些工作,说明了这些参数的可靠性及其在地学中的重要应用意义.

THEORETICAL PREDICTION FOR SEVERAL IMPORTANT EQUILIBRIUM Ge ISOTOPE FRACTIONATION FACTORS

This study provided a number of theoretical equilibrium isotope fractionations between several Ge species, including quartz- (or opal-), albite-, K-feldspar- and olivine-like structures, as well as the dominant Ge(OH)4 and GeO(OH)3- species in aqueous solutions. They are basic parameters for exploring the unknown application of Ge isotope in the field of geochemistry. Our method is based on the Urey model (or the Bigeleisen-Mayer equation) and quantum chemistry calculation. We estimated that the accuracy of the parameters could be around 0.3‰, with the calculations made at B3LYP/6-311+G(d,p) level. Solvent effect was handled using explicit solvent model ("water droplet" method), and the minerals were simulated by cluster model. Our results showed that quartz- or opal-like structure can enrich most heavy Ge isotopes, and at 25℃, the fractionations are around Δquartz-Ge(OH)4=0.9‰, ΔGe(OH)4-GeO(OH)3-=0.3‰ (in seawaters), Δquartz-albite=0.6‰, Δquartz-K-feldspar=0.4‰, Δolivine-Ge(OH)4=-1.2‰ and Δquartz-olivine=2.1‰. These data could give better explanations to the experimental observations of Siebert et al. (2006) and Rouxel et al. (2006), suggesting that our theoretical prediction is accurate and important for real geochemical problems.