HR: 0800h
AN: V11A-1410    [Abstracts]
TI: Allanite and apatite from the Sulu UHP eclogites (CCSD): light rare earth elements and thorium carriers in subducted crust
AU: * Wang, R
EM: rcwang@nju.edu.cn
AF: Department of Earth Sciences, Nanjing University, 22 Hankou Road, Nanjing, 210093 China
AU: Wang, S
EM: wansuo@yahoo.com.cn
AF: Department of Earth Sciences, Nanjing University, 22 Hankou Road, Nanjing, 210093 China
AU: Qiu, J
EM: jsqiu@nju.edu.cn
AF: Department of Earth Sciences, Nanjing University, 22 Hankou Road, Nanjing, 210093 China
AU: Ni, P
EM: peini@nju.edu.cn
AF: Department of Earth Sciences, Nanjing University, 22 Hankou Road, Nanjing, 210093 China
AB: Light rare earth elements (LREE) and Th are ones of important elements for the characterization of the components in subduction zones. Therefore, a profound understanding of the behaviour of these elements in subduction zone processes is only possible if we know in which phases LREE and Th are incorporated, how stable these phases are and how they interact with subduction zone liquids. Allanite and apatite are considered two accessory minerals incorporating important LREE and Th in subduction-zone rocks. Chinese Continental Scientific Drilling (CCSD) provides us an opportunity to make a systematic study on distribution of REE-bearing accessory minerals in the Sulu UHP eclogites. The rocks contain garnet, omphacite, quartz (probably pseudomorph after coesite), phengite and rutile. Accessory allanite and apatite often form aggregates. The following features can be taken for preliminarily characterizing the aggregates. (1) allanite grains are cored by apatite, and mantled by epidote; (2) micro-inclusions of thorite are found within the apatite core or allanite, or at the contact between them; (3) allanite transforms progressively to epidote; (4) epidote reveals zoned texture (e.g., lower right part in Fig. 1a), which is represented by variations in Fe and Al contents. Electron-microprobe analyses reveal that the apatite contains up to 1 wt%(LREE)2O3 and 0.5 wt% ThO2, particularly allanite incorporates not only LREE, but also as high as 2.1 wt% ThO2. The above observations indicate that there is a continuous transition from epidote to allanite, which is stabilized due to the presence of LREE, deriving possibly from the breakdown of primary LREE- and Th-bearing apatite in the course of UHP metamorphism. This work was financially supported by Chinese Ministry of Science and Technology (2003CB716507).
DE: 3620 Crystal chemistry
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting