CCSD主孔榴辉岩中磷灰石微区微量元素和Sr同位素组成研究

Spatial variations of trace element and Sr isotopic compositions of apatite in eclogite from the CCSD main hole.

本文利用LA-ICP-MS和LA-MC-ICP-MS对CCSD-MH中一件强退变多硅白云母榴辉岩中一粗粒(～2mm×6.5mm)磷灰石进行了详细的微区微量元素和Sr同位素组成研究。线扫描剖面分析和单点分析结果共同表明该磷灰石颗粒的微量元素和Sr同位素组成不均一。总体上Na、Sr、LREE、MREE、U、Th和Pb等元素具有从中心到两侧含量逐渐降低的特征。磷灰石主体部分HREE含量均一,但和石榴石紧密相邻的边部(～400μm)HREE显著富集。Na、Sr、LREE和MREE等元素在个别区域明显具有先升高、然后再降低的变化规律。磷灰石最边部(<200μm)相对于核部区域显著高^(87)Sr/^(86)Sr比值。结合磷灰石中微量元素分配和扩散行为的实验研究以及磷灰石晶体化学特征,磷灰石微量元素组成变化应主要记录了磷灰石复杂的生长过程。Sr、LREE和MREE从核部到边部逐渐降低的总体特征指示磷灰石从中心向两侧的生长过程,而局部出现的次级先升高—再降低的变化规律则反映了磷灰石经历的多次溶解和再生长作用。HREE含量均一的磷灰石主体形成于石榴石稳定存在的超高压变质作用阶段。和石榴石紧密相邻的磷灰石边部显著富集HREE的特征则是折返过程中短时增温作用导致石榴石释放HREE和/或退变质阶段石榴石分解释放HREE共同作用的结果。磷灰石最边部显著高^(87)Sr/^(86)Sr的特征则记录了角闪岩相退变质阶段多硅白云母的分解作用。

Trace element and Sr isotope compositions of an apatite grain (similar to 2mm x 6.5mm) in a strongly retrograded eclogite from the CCSD-MH were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Both line-scanning and single spot analyses reveal that the apatite is heterogeneous in trace element and Sr isotope compositions. Na, Sr, LREE, MREE, U, Th and Pb gradually decrease from the core to the rim. Remarkable secondary changes of Na, Sr, LREE and MREE occur in some local parts. HREE are generally homogeneous in the main part of the apatite, but enriched in the outmost rim (similar to 400 mu m) close to garnet. The outmost rim (< 200 mu m) has the highest Sr-87/Sr-86 ratios. Combining trace elemental partitioning and diffusion, and crystal-chemical characteristics of apatite, we suggest that the trace element variations reflect the complicated growth of apatite. The gradually decreasing from the core to the rim for Na, Sr, LREE, MREE, U, Th and Pb indicate apatite growth, while the secondary changes in local parts imply multi-stage dissolution and re-growth. The relatively homogeneous HREE compositions of the main part of the apatite suggest that apatite growth was mainly associated with the UHP metamorphism. HREE enrichments in the outmost rim of apatite could have resulted from garnet breakdown in amphibolite-facies retrograde metamorphism and/or HREE-releasing of garnet due to a short-lived heating during the exhumation. The high-Sr-87/Sr-86 rim could indicate apatite growth coupled with phengite breakdown in the amphibolite-facies retrograde metamorphism.