中国大陆科学深钻主孔岩屑中含钠和锌的尖晶石的发现

Discovery of Na- and Zn bearing spinel in the rock cuttings from the main hole, Chinese Continental Scientific Drilling (CCSD) project.

一种含钠和锌的尖晶石矿物产于中国大陆科学深钻主孔超镁铁岩中,以出溶的方式存在于钛铁矿和磁铁矿中,长5～60μm,宽2—3μm,化学组成(wB％)为MgO 17．143％-22．337％,Al2O3 63．646％-66．492％,FeO 9．784％-13．924％, ZnO 1．649％-6．386wt％,Na2O 0．029％-0．219wt％。Na2O与ZnO在尖晶石中的含量呈正相关关系。经同步辐射测得其为尖晶石单晶,空间群为Fm3m,晶胞参数为a=0．83781nm。根据尖晶石与钛铁矿、石榴石(pyr=56—58mol％)共生和石榴石退变为顽火辉石和镁绿泥石的特点,表明该尖晶石来自超镁铁岩,它们一起经历了至少为角闪岩相的变质作用。广泛的混溶间断现象及其所造成的出溶结构出现在氧化物中,表现为Mt-Usp、Mt-Spl及Mt—Ilm之间的交生。根据前人有关Mt-Usp和Ilm-Gk-Hem固溶体系的实验,表明主孔岩石的氧逸度相对较低,形成温度应在720℃以上。这些早期在岩浆分离结晶过程中形成的铁钛氧化物出溶体,在后来的超高压变质作用下可能发生再次固溶和因降温减压造成的出溶。

In the rock cuttings from the main hole, CCSD, Na- and Zn bearing spinel occurred as exsolution lamella in ilmenite and magnetite. The mineral is about 5 - 60μm in width and 2 - 3μm in length, composed of MgO 17. 143 - 22. 337wt%, Al2O363.646 - 66. 492wt%, FeO 9. 784 - 13. 924wt%, ZnO 1. 649 -6. 386wt%, Na2O 0.029 -0. 219wt%, where Na2O and ZnO are positively in different grains. By x-ray analysis of synchrotron light source, the spinel is cubic symmetry with space group of Fm3m and crystal cell parameter of a =0. 83781nm. With the fact that the spinel is associated with ilmenite, magnetite, garnet （ pyr = 50 - 58mol% ） and the garnet is retrograded into orthopyroxene and Mg-chlorite, the spinel should be derived from ultramafic, and experienced amphibolite facies metamorphism. The phenomenon of miscibility gap and exsolution texture are widely occurred in the oxide minerals between Mt- Usp, Mt-Spl and Mt-Ilm. According to the known petrologic experiments of Mt-Usp binary system and Ilm-Gk-Hem ternary system, the oxygen fugacity of the rocks in the main hole is low and forming temperature is at least over 720-C. The ferro-titanium oxides exsoluted during uhramafic magma crystallization, but later in ultrahigh pressure metamorphism may be formed solid solutions and exsoluted again after temperature decrease.