800MPa下泥质岩部分熔融实验研究:残留相及熔体相REE地球化学特征

Parital melting experimental study of pelite at 800 MPa:REE geochemical characteristics of melts and residual phase

利用JL-3600t压机实验研究了800MPa、不同温度条件下泥质岩部分熔融过程,利用EMPA和LA-ICPMS分别测定了熔体相和残留相中主要化学组成以及微量元素(包括REE)组成。实验结果表明,泥质岩低程度部分熔融(<25%)形成的熔体中REE含量分布于308.8～3565μg/g较大范围内,显示较大的不均匀性,其REE球粒陨石标准化分布模式显示弱的M型REE"四分组效应",而残留相矿物石榴子石中REE含量变化于167.5～1008μg/g范围,也显示有明显的不均匀性,其REE球粒陨石标准化分布模式显示明显的W型REE"四分组效应",尤以第一段La-Nd最为显著;随着部分熔融程度的增加(>30%),其形成的熔体中REE集中在523.2～1130μg/g范围,残留相石榴子石中REE集中在288.6～512.7μg/g范围,均显示相对均匀;熔体相和残留相石榴子石矿物的REE球粒陨石标准化分布模式不发育REE"四分组效应"。实验前后Cl质量平衡计算的结果表明该实验过程中并没有产生岩浆挥发分相。上述特征表明S型花岗岩中的REE"四分组效应"现象很可能与泥质岩低程度部分熔融具有成因联系。

Partial melting experiments of pelite were conducted at 800 MPa and different temperatures by JL-3600t pressure vessel. The main chemical compositions and trace element （including REE） concentrations of the quenched melts and residual minerals were determined using EMPA and LA-ICPMS. It shows that REE contents in melt, formed at low degree of partial melting （〈 25% ）, range from 308.8 to 3565 μg/g, indicating REE is significantly heterogeneously distributed in the melt. The REE chondrite-normalized pattern shows slight M-type ＂tetrad effect＂. In the residual garnets, REE contents vary from 167.5 to 1008 μg/g, also showing heterogeneous distribution of REE. It＇s characterized by obvious W-type REE ＂tetrad effect＂ as normalized to chondrite, especially in the first segment from La to Nd. At more than 30% of partial melting, REE is relatively homogeneously distributed in the melt and residual garnets, ranged from 523.2 to 1130 μg/g and 288.6 to 512.7 μg/g, respectively. In the chondrite-normalized patterns, REE ＂tetrad effect＂ is not shown either in the melt or in the residual garnets. According to the mass balance calculation of chlorine before and after experiments, it indicates that the exsolution of magmatic volatile phase didn＇t occur during the experiments. Therefore, we deduce that＂ the origin of REE ＂tetrad effect＂ in the S-type granite may be associated with the low degree of partial melting of pelite.