Fe2+ -Mg partitioning between garnet, magnesiowüstite, and (Mg,Fe)2SiO4 phases of the transition zone
Fe2+ -Mg partitioning between garnet, magnesiowüstite, and (Mg,Fe)2SiO4 phases of the transition zone
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Fe2+ -Mg 在过渡区的石榴石、镁方石和 (Mg,Fe)2SiO4 相之间分配
DOI：
10.2138/am-2003-2-315
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发表时间：
2003
影响因子：
3.1
通讯作者：
D. Frost
中科院分区：
文献类型：
--
作者：
D. Frost
在9 ~ 19 GPa和1400-1700 ℃条件下，测定了Fe、Mg在石榴子石与橄榄石、华氏体、菱锰矿和镁华石固溶体之间的分配。除了研究涉及的交换反应的辉石-铁铝榴石石榴石固溶体，在天然橄榄岩块体组合物中形成的多角石榴石也已被检查。将金属Fe添加到所有本体组合物中以将Fe 3+浓度缓冲在其最低可能水平。结合辉石-铁铝榴石石榴石和林伍德石之间的分配数据与石榴石-镁钨铁矿和林伍德石-镁钨铁矿分配数据，以细化这些固溶体在18 GPa和1400 ℃下的热力学混合性质。采用这3种固溶体之间的阳离子交换数据的多元非线性回归得到以下对称固溶体相互作用参数的差异的良好约束值：WmwFeMg-WringFeMg = 8.8（2）kJ/mol，WmwFeMg-WgtFeMg = 12.8（5）kJ/mol，WringFeMg-WgtFeMg = 4.0（4）kJ/mol。然后同时求解这些差异，得到基于单位点的WmwFeMg = 13.2（3）kJ/mol，WringFeMg= 4.4（2）kJ/mol，WgtFeMg = 0.3（3）kJ/mol。在橄榄岩组合物中，镁铝榴石和钙铝榴石石榴石成分的出现对Fe-Mg分配产生了可测量的影响，然而，组合效应相对较小，因为每个单独的成分影响相反方向的分配。这些数据，结合先前确定的Mg_2SiO_4-Fe_2SiO_4系统中的相关系，被用来计算石榴石Fe-Mg配分对过渡区中二元（Mg，Fe）_2SiO_4相变的压力间隔的影响。结果表明，石榴子石的存在下减少的压力区间（Mg0.9，Fe0.1）2SiO 4橄榄石到wadsleyite转换发生在橄榄岩组合物为0.35 GPa（10公里）相比，在一个无石榴子石系统的0.5 GPa。然而，这一宽度仍然大于从高频反射和转换地震波推断的在地球某些地区观察到的410公里不连续性的估计宽度4公里。因此，在地幔中橄榄岩成分中石榴石的存在不能成为唯一的解释，为什么在地球的某些地区，410公里的不连续性明显比实验估计的（Mg0.9，Fe0.1）2SiO 4橄榄石到wadsleyite转变更尖锐。的wadsleyite到ringwoodite转换减少到0.8 GPa（25公里）相比，1 GPa的石榴石的系统。
Abstract The partitioning of Fe and Mg between garnet coexisting with olivine, wadsleyite, ringwoodite and magnesiowüstite solid solutions has been measured between 9 and 19 GPa and 1400-1700 ℃. In addition to studying exchange reactions involving the pyrope-almandine garnet solid solution, majoritic garnets forming in a natural peridotite bulk composition have been also examined. Metallic Fe was added to all bulk compositions to buffer the Fe3+ concentration at its lowest possible level. Partitioning data between pyrope-almandine garnet and ringwoodite were combined with data for garnet-magnesiowüstite and ringwoodite-magnesiowüstite partitioning to refine thermodynamic mixing properties for these solid solutions at 18 GPa and 1400 ℃. A multiple non-linear regression employing cation exchange data between these 3 solid solutions yielded the following well-constrained values for the differences in symmetric solid-solution interaction parameters: WmwFeMg - WringFeMg = 8.8(2) kJ/mol, WmwFeMg - WgtFeMg = 12.8(5) kJ/mol,WringFeMg - WgtFeMg = 4.0(4) kJ/mol. These differences were then solved simultaneously to give, on a single site basis: WmwFeMg = 13.2(3) kJ/mol, WringFeMg= 4.4(2) kJ/mol, WgtFeMg = 0.3(3) kJ/mol. In a peridotite composition, the occurrence of majorite and grossular garnet components produced a measurable influence on the Fe-Mg partitioning, however, the combined effect was relatively small because each individual component affected the partitioning in an opposite direction. These data, combined with previously determined phase relations in the Mg2SiO4-Fe2SiO4 system, were used to calculate the influence of garnet Fe-Mg partitioning on the pressure intervals of divariant (Mg,Fe)2SiO4 phase transformations in the transition zone. Results show that the presence of garnet reduces the pressure interval over which the (Mg0.9,Fe0.1)2SiO4 olivine to wadsleyite transformation occurs in a peridotite composition to 0.35 GPa (10 km) in comparison to 0.5 GPa in a garnet-free system. This width, however, is still greater than the estimated width of 4 km for the 410 km discontinuity observed in some regions of the Earth as inferred from high frequency reflected and converted seismic waves. The existence of garnet in a peridotite composition in the mantle cannot, therefore, be the only explanation as to why the 410 km discontinuity is, in some regions of the Earth, apparently sharper than experimental estimates for the (Mg0.9,Fe0.1)2SiO4 olivine to wadsleyite transformation. The wadsleyite to ringwoodite transformation is reduced to 0.8 GPa (25 km) compared to 1 GPa in the garnet-free system.