HR: 0800h
AN: MR11A-0918    [Abstracts]
TI: Na- and Zn- bearing (Mg,Fe)Al2O4 spinel exsolution from ilmenite and magnetite of eclogite in the CCSD Main Hole
AU: * Ren, Y
EM: yfren@ccsd.org.cn
AF: Key Laboratory for Continental Dynamics of the Ministry of Land and Resources, Institute of Geology, CAGS, Baiwanzhuang 26, Beijing, Beijing, 100037 China
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AF: Key Laboratory for Continental Dynamics of the Ministry of Land and Resources, Institute of Geology, CAGS, Baiwanzhuang 26, Beijing, Beijing, 100037 China
AU: Shu, J
EM: shu@gl.ciw.edu
AF: Geophysical Laboratory & Brookheaven Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington D. C., Washington, 20015-1305 United States
AU: Mao, H
EM: mao@gl.ciw.edu
AF: Geophysical Laboratory & Brookheaven Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington D. C., Washington, 20015-1305 United States
AB: In the eclogite of the main hole, CCSD, there were found ilmenite\- titanomagnetite and ilmenite\-magnetite oxide series, each of which is formed intergrowth textures. Titanomagnetite takes near 40%, ilmenite 30% and magnetite 30% in volume proportion. In those two series, many micro-exsoluted lamellas are found in ilmenite and magnetite and are never in titanomagnetite. By synchrotron light source analyses, those minerals are spinel with space group of Fd3m and unit cell parameter of a=8.3781$\AA$. The spinel exsolutions are mostly aligned and parallel to the long axis of lensoidal ilmenite, about several $\mu$m to 30$\mu$m long and several $\mu$m wide, the largest found to be about 30$\mu$m$\times$60$\mu$m. The exsoluted spinel lamellas in magnetite are 5$\sim$100$\mu$m long and several $\mu$m wide, oriented in three $<$111$>$ crystallographic planes of magnetite. Microprobe analyses show that spinel exsoluted in ilmenite is composed of Na$_{2}$O 0.091$\sim$0.241, MgO 18.617$\sim$22.337, ZnO 1.649$\sim$6.386, Al$_{2}$O$_{3}$ 64.514$\sim$66.492, FeO 9.105$\sim$10.639, TiO$_{2}$ 0.035$\sim$0.127, with total of 99.696$\sim$100.472wt%. Its chemical formula is (Na$_{0.008}$Mg$_{0.8}$Fe$_{0.14}$Zn$_{0.06}$)(Al$_{1.94}$Fe$_{0.06}$)$_{2}$O$_{4}$. The ilmenite is composed of MgO 3.604$\sim$4.826, TiO$_{2}$ 50.376$\sim$51.820, FeO 41.862$\sim$44.727, Cr$_{2}$O$_{3}$ 0.050$\sim$0.263, Al$_{2}$O$_{3}$ 0.025, with total of 98.641$\sim$99.047wt%. The titanomagnetite is MgO 0.923, Al$_{2}$O$_{3}$ 0.124, TiO$_{2}$ 12.430, FeO 79.306, Cr$_{2}$O$_{3}$ 0.035, with total of 92.819wt%. Magnetite contains MgO 0.377, Al$_{2}$O$_{3}$ 0.157, TiO$_{2}$ 0.015, FeO 92.049, Cr$_{2}$O$_{3}$ 0.053, with total of 92.688wt%. According to the intergrowth and exsolution textures, there widely exists a miscibility gap in (Mg,Fe)TiO$_{3}$-Fe(Ti,Fe)$_{2}$O$_{4}$\-(Na,Mg,Fe,Zn)(Al,Fe)$_{2}$O$_{4}$-Fe$_{3}$O$_{4}$ series and (Mg,Fe)(Al,Fe)$_{2}$O$_{4}$-Fe$_{3}$O$_{4}$ series. In light of the FeO-Fe$_{2}$O$_{3}$-MgO-TiO$_{2}$ join experiment (Pinckney and Lindsley, 1976), the coexisting temperatures for titanomagnetite occurrence is around 750$\sim$800$\deg$C, when magnetite takes 30% mole ratio in Ni-NiO buffer. All of those spinel exsolutions as well as their host ilmenite-titanomagnetite and magnetite are associated with garnet and omphacite. According to main and trace element compositions of the eclogite in the main hole, Na$_{2}$O and Zn (Zhang et al., 2004) are relatively abundant. Na and Zn occurrence in the spinel is closely related with the formation of the eclogite. The peak metamorphic temperature of the eclogite is 700$\sim$850$\deg$C (You et al., 2004), which is close to the miscibility temperature of the magnetite and imenite.
DE: 3625 Descriptive mineralogy
DE: 3660 Metamorphic petrology
DE: 3665 Mineral occurrences and deposits
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting