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Effect of chromium on the silicate systems

铬对硅酸盐体系的影响

基本信息

批准号：
60540512
负责人：
ONUMA Kosuke
金额：
$ 1.47万
依托单位：
TOHOKU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1985
资助国家：
日本
起止时间：
1985 至 1986
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-60540512/
关键词：
chromium spinel phase equilibrium diagram spinel lherzolite plagioclase lherzolite

项目摘要

The effect of Cr on the phase equilibrium diagram of the silicate system has been studied in air at 1 atm by adding a small amount of Mg <Cr_2> <O_4> (0.3 wt.%) to the system <Mg_2> <SiO_4> (forsterite)- <CaAl_2> <Si_2> <O_8> (anorthite)-CaMg <Si_2> <O_6> (diopside) - <SiO_2> , which is an important petrologic model to the formation of basaltic magma. The spinel primary field is enlarged compared with that of the Cr-free system at the expense of the anorthite field, and as a result forsterite and anorthite do no coexist at the liquidus temperature. Two invariant points have been confirmed: one shows liquid coexisiting with forsterite, protoenstatite, diopside and spinel, and the other shows the assemblage of anorthite, protoenstatite, diopside, silica mineral and liquid. The mineral assemblage at the first invariant point correponds to that of spinel lherzolite. In the Cr-free system this assemblage is not stable at 1 atm and an invariant point having plagioclase lherzolite mineral assemblage is present. It is generally said that the transformation from spinel lherzolite to plagioclase lherzolite is about 8 kbar at 1000゜C. However, the result obtained in the present study indicates that the presence of small amount of Cr in magma significantly reduces this transformation pressure. Another important aspect of the result obtained is that the position of the invariant point is possibly moved by the cange of bulk Cr content in the system, because the spinel primary field becomes large with increasing Cr content in the system. Therefore, if the liquid generated by the partial melting of the spinel lherzolite mantle has the composition of this point, the Cr content of the mantle would give significant effect to the composition of magma thus formed.

本文研究了在1atm空气中加入少量Mg（0.3wt%）后，Cr对硅酸盐体系相平衡图的影响<Cr_2><O_4>（<Mg_2><SiO_4>镁橄榄石）-（<CaAl_2><Si_2><O_8>钙长石）-CaMg<Si_2><O_6>（透辉石）-<SiO_2>体系是玄武岩浆形成的重要岩石学模式。与无铬体系相比，尖晶石初生场增大，钙长石初生场减小，镁橄榄石和钙长石在液相线温度下不共存。确定了两个不变点：一是镁橄榄石、原顽火辉石、透辉石、尖晶石的液相共生;二是钙长石、原顽火辉石、透辉石、硅质矿物和液相的组合。第一不变点的矿物组合与尖晶石二辉橄榄岩的矿物组合相对应。在无铬系统中，该组合在1个大气压下不稳定，并且存在具有斜长石二辉橄榄岩矿物组合的不变点。一般认为，在1000 ℃时，尖晶石二辉橄榄岩向斜长石二辉橄榄岩的转变约为8千巴。然而，在本研究中获得的结果表明，在岩浆中的少量Cr的存在显着降低这种转换压力。另一个重要的方面是不变点的位置可能是移动的范围内的体Cr含量的系统，因为尖晶石的初级场变大，在系统中的Cr含量的增加。因此，如果尖晶石二辉橄榄岩地幔部分熔融产生的液体具有该点的成分，则地幔中的Cr含量将对所形成的岩浆成分产生重要影响。