Phase Relations in the System CaO-MgO-Al_2O_3-Cr_2O_3-SiO_2 at high pressures and temperatures.

本研究取得了以下结果。(1)本文研究了Cr对Mg_2SiO_4-CaAl_2Si_2O_8-CaMgSi_2O_6-SiO_2系统相关系的影响。添加少量Cr（Cr_2O_3= 0.24 wt.%）对相位关系有很大的影响。观察到橄榄石+斜方辉石+单斜辉石+尖晶石的相组合，这在无铬体系中是不稳定的。该组合与尖晶石二辉橄榄岩的组合相对应。结果表明，微量Cr的存在降低了斜长二辉橄榄岩向尖晶石二辉橄榄岩的转变温度。(2)由于MgSiO_3-CaAl_2Si_2O_8系统的相关系存在问题，在研究Cr在该系统中的作用之前，进行了详细的重新研究。该体系中的斜方辉石不具有MgSiO_3-CaAl_2Si_2O<@23的组成，橄榄石+斜方辉石+钙长石+液体的组合具有一定的温度范围，表明该体系不是三元体系，而是四元体系。(3)在10 kbar压力下，对CaMgSi_2O_6-Mg_2SiO_2-CaAl_2Si_2O_8- 0.2%MgCr_2O_4体系进行了高压实验，研究了Cr对该体系的影响。与无铬体系相比，单斜辉石的原生场增大，尖晶石的原生场减小。这种现象可能是由于微量元素在晶相和液相中的分配造成的。需要更详细的研究来了解这个问题。

In the present study following resutls have been obtained.(1) The effect of Cr on phase relations in the system Mg_2SiO_4-CaAl_2Si_2O_8-CaMgSi_2O_6-SiO_2, an important petrologic model, has been studied at 1 atm in air. Addition of minor amount of Cr (Cr_2O_3= 0.24 wt.%) gives a significant effect on the phase relations. The phase assemblage olivine + orthopyroxene + clinopyroxene + spinel, which is not stable in the Cr-free system, is observed. This assemblage corresponds to that of spinel lherzolite. The facts obtained indicate that the presence of very small amount of Cr lowers the transformation temperature from plagioclase lherzolite to spinel lherzolite.(2) Since the phase relations of the system MgSiO_3-CaAl_2Si_2O_8 has been problematical, detailed reexamination was carried out, before the study of the effect of Cr in this system. The orthopyroxene present in this system does not have a composition on join MgSiO_3-CaAl_2Si_2O<@23 and the assemblage of olivine + orthopyroxene + anorthite + liquid has some temperature range, indicating that the syste is not ternary but belongs to quaternary system.(3) The experiments on the join CaMgSi_2O_6-Mg_2SiO_2-CaAl_2Si_2O_8-0.2% MgCr_2O_4 has been performed at 10 kbar to understand the effect of Cr on the silicate system under high pressures. The primary field of clinopyroxene is enlarged and that of spinel is decreased compared with that in the Crfree system. This phenomena are probably due to the partitioning of trace element among the crystalline phases and liquid. More detailed studies are needed to understand this problem.