南阿尔金地区含铀-富铀锆钛石的发现及其新成因机制

Discovery of uranium-bearing srilankite and a new genetic mechanism in South Altyn Tagh, China

锆钛石(ZrTi2O6)为高级变质岩、交代地幔岩和岩浆岩中的一种罕见副矿物,是研究高场强元素迁移与富集机理的理想对象.中国南阿尔金高压-超高压变质带的高压基性麻粒岩中发现了含铀-富铀锆钛石,其主要化学组成为TiO2,ZrO2和UO2,但不同颗粒或同一颗粒不同区域成分变化较大,UO2含量最高可达20.83 wt%,且U与Zr具类质同象替代关系,晶体化学式可简写为(Zr,U)Ti2O6.低-中等UO2含量的含铀锆钛石与不含铀锆钛石相比晶体结构未发生显著变化,高UO2含量的富铀锆钛石由于放射性损伤已发生蜕晶化.矿物显微结构及成分分析表明南阿尔金含铀-富铀锆钛石及其伴生矿物组合为:金红石-钛铁矿-含铀-富铀锆钛石-白钨矿或钛铁矿-含铀-富铀锆钛石-白钨矿,其中钛铁矿为金红石退变形成,而含铀-富铀锆钛石和白钨矿是金红石退变分解过程中释放的Zr,Ti,W等元素与变质流体中的U,Ca结合直接结晶形成.该研究对矿物尺度高场强元素迁移富集机理具有重要的指示意义.

The mineral srilankite(Zr Ti2O6)is a rare accessory phase in high-grade metamorphic,metasomatic mantle and igneous rocks.Due to being highly enriched in high field strength elements(HFSE),it provides an ideal basis to study the migration and enrichment mechanism of HFSE during geologic processes.Here,we present analyses of a cluster of uranium-bearing srilankite with UO2 up to 20.83 wt%discovered in the mafic granulite from the high-pressure(HP)to ultrahigh-pressure(UHP)belt in South Altyn Tagh,China.A new genetic mechanism of metamorphic srilankite is proposed based on the study of the mineralogical characteristics of the uranium bearing srilankite and its associated minerals through various methods such as scanning electron microscope(SEM),electron probe microanalysis(EPMA),focused ion beam(FIB)and transmission electron microscope(TEM).The uranium-bearing srilankite occurs as irregular to subhedral grains,up to 30μm in size,characterized by relatively light backscattered electron image and no cathodoluminescence under the electron beam in SEM.The results of EPMA show that the main chemical components of srilankite was Ti O2,Zr O2 and UO2,but with dramatic variation among different grains or in different compositional domains of one individual crystal.For example,the content of the Zr O2 ranges from 27.03 wt%to 42.31 wt%,TiO2 from 49.39 wt%to 56.55 wt%and UO2 from 0.65 wt%to 20.83 wt%.Characteristic variation of the chemical composition indicates the isomorphous substitution of U and Zr.Therefore,the crystallochemical formula of the uranium-bearing srilankite could be simply written as(Zr,U)Ti2O6.TEM analysis of the mineral slice prepared by FIB shows that the crystal structure of srilankite with low to medium UO2 content did not change significantly compared to those containing no uranium,whereas srilankite with high UO2 had been metamictized due to radioactive damage.Microstructure and chemical composition analysis show that the uranium-bearing srilankite and associated mineral assemblages are rutile-ilmenite-uranium-bearing srilankite-scheelite or ilmenite-uranium-bearing srilankite-scheelite.The ilmenite was all formed from consumption of the rutile during retrogression of the host rock.During this process,HFSE such as Zr and W would be released from the rutile.For freed Zr,it has three distinct fates,as described below.(1)Part of the Zr would enter into the crystal lattice of ilmenite as it crystallized.(2)Some would be back-diffused into the residual rutile,giving significant increase in Zr where the rutile is in contact with ilmenite.(3)The remaining Zr could combine with Ti and U to form the independent compound uranium-bearing srilankite.By contrast,scheelite is the final destination for the released W when Ca is available.We infer that most U and Ca that constitute uranium-bearing srlankite and scheelite come from the metamorphic fluid.This study is a significant contribution to understanding the mechanism of migration and enrichment of HFSE at the mineral scale.