大别山榴辉岩中磷灰石的结构碳酸根

STRUCTURAL CARBONATE WITHIN APATITE FROM ECLOGITES IN THE SOUTHEASTERN DABIE MOUNTAINS

在大别山东南部的高压和超高压榴辉岩中发现磷灰石含有结构碳酸根。磷灰石主要以包裹体的形式存在于筛状变晶结构的石榴石中 ,其形成时间早于石榴石和榴辉岩的矿物组合。X射线衍射和傅立叶变换红外光谱研究表明 ,磷灰石中结构碳酸根替代 [PO4 ]3- ,表现为在 14 5 5～ 14 2 2cm- 1的吸收带 ,以及在 15 0℃以下温度难以消除的在 1630cm- 1的吸附水 ,但常规X射线衍射和傅立叶变换红外光谱难以给出其含量。用EA MS连线技术测量给出结构碳酸根的含量在 0 95～ 5 2 4CO2 wt%。榴辉岩磷灰石中结构碳酸根的鉴定和碳含量的定量测定证明 ,在超高压变质过程中存在含CO2 的峰变质流体。

Structural carbonate is found within apatite from eclogites in the southeastern part of the Dabie Mountains. The apatite is mainly hosted in the diablastically textured garnet. Euhedral hexagonal or needle like apatite occur at an earlier stage of metamorphism than the garnet and the eclogitic assemblages. Apatite has a small diameter between 0.02 and 0.07mm, which is averaged 65 times smaller than the host garnet. X ray diffraction and Faurier transform infrared spectroscopic studies show that all of the apatite has detectable amount of structural carbonate. It is identified to substitute [PO 4] 3- , but its amount cannot be measured by the traditional spectroscopic methods. An EA MS online measurement gives a wide range of structural carbonate content (0 95～5 24 CO 2wt%). The high C content is attributed to the contribution from the other kinds of carbon, such as the fluid inclusion or the carbonaceous film in the micro crack. As a result, the spectroscopic calibrated values are always lower than the EA MS online ones because significant amounts of structural carbonate show no signal. Both identification of structural carbonate within apatite from eclogites and the quantitative measurenment of bulk carbon concentration in the apatite demonstrate the presence of CO 2 bearing fluid during ultrahigh pressure metamorphism in the Dabie Mountains.