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Phase Transformations and Thermodynamic Properties of Constituent Minerals of Slabs in the Lower Mantle

下地幔板片组成矿物的相变和热力学性质

基本信息

批准号：
11440162
负责人：
AKAOGI Masaki
金额：
$ 1.73万
依托单位：
Gakushuin University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2001
项目状态：
已结题

项目摘要

High-pressure phase relations of minerals consisting of MORE and pyrolite in slabs subducting into the lower mantle were investigated up to pressure-temperature conditions of the uppermost lower mantle. Calorimetric measurements were also performed, and phase relations were determined using the obtained thermodynamic data. Main results are as follows.(1) High-pressure phase relations in the systems MgAl_2O_4-NaAlSiO_4 and MgAl_2O_4-CaAl_2O_4 were clarified up to 30 Gpa, and a new phase with hexagonal symmetry was found. Structural analysis of the hexagonal phase showed that this phase has a new structure related to calcium-ferrite structure. This phase may be an important host of alkali elements and aluminum in the lower mantle. Phase transitions in NaAlSiO_4 were also examined by high-pressure experiments and calorimetry, and the phase boundary for transition to NaAlSiO_4 calcium ferrite was determined.(2) Phase relations in the system Mg_4Si_4O_<12>-Mg_3Al_2Si_3O_<12> were examined at 1000 ℃ up to 27 Gpa. It was clarified that garnet solid solution transforms to ilmenite solid solution which further transforms to perovskite solid solution. Enthalpies of the garnet-ilmenite-perovskite transitions were measured, and the phase boundaries were calculated. Based on the results, it was established that the garnct-ilmcnitc-pcrovskitc transitions occur in pyrolite in subducting slabs at the uppermost lower mantle.(3) Enthalpies of perovskite solid solutions in the systems CaSiO_3-CaTiO_3 and CaSiO_3-CaGeO_3 were measured, and the enthalpy was estimated for CaSiO_3 perovskite which cannot be recovered at ambient conditions. Phase relations of CaSiO_3 perovskite, one of major constituent minerals of the lower mantle, were thermodynamically calculated.

根据上、下地幔的压力-温度条件，研究了俯冲下地幔板块中MORE和软锰矿矿物的高压相关系。还进行了量热测量，并根据所得的热力学数据确定了相关系。主要结果如下：(1) MgAl_2O_4-NaAlSiO_4和MgAl_2O_4-CaAl_2O_4体系的高压相关系在30gpa下得到澄清，并发现了具有六方对称的新相。对六方相的结构分析表明，该相具有一种与铁酸钙结构有关的新结构。该相可能是下地幔中碱元素和铝元素的重要寄主。用高压实验和量热法研究了NaAlSiO_4的相变，确定了向NaAlSiO_4铁酸钙过渡的相界。(2)在1000℃、27 Gpa温度下，研究了Mg_4Si_4O_<12>-Mg_3Al_2Si_3O_<12>体系的相关系。阐明了石榴石固溶体向钛铁矿固溶体转变，钛铁矿固溶体向钙钛矿固溶体转变。测量了石榴石-钛铁矿-钙钛矿相变焓，并计算了相界。在此基础上，确定了在上、下地幔俯冲板块的软锰矿中，存在着横切—横切—横切的转变。(3)测定了CaSiO_3- catio_3和CaSiO_3- cageo_3体系中钙钛矿固溶体的焓，并对环境条件下不能回收的CaSiO_3钙钛矿的焓进行了估算。对下地幔主要组成矿物之一CaSiO_3钙钛矿的物相关系进行了热力学计算。