The Effect of Cr 2 O 3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO- Al 2 O 3 -SiO 2 -Cr 2 O 3 at 1 1 GPa
The Effect of Cr 2 O 3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO- Al 2 O 3 -SiO 2 -Cr 2 O 3 at 1 1 GPa
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DOI：
10.1093/petrology/egh055
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发表时间：
2004-11
影响因子：
3.9
通讯作者：
Xi Liu;H. O’Neill
中科院分区：
文献类型：
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作者：
Xi Liu;H. O’Neill
铬取代了上地幔矿物中的八面体配位铝，从而降低了体系中Al_2O_3的活度，从而降低了部分熔体中Al_2O_3的浓度。在CaO-MgO-Al_2O_3-SiO_2-CrO_2O_3系中，用尖晶石(Cr#sp)中100Cr/(CrAl)的函数定量研究了Cr2O_3对饱和尖晶石二辉橄榄岩相组合的熔体成分的影响。随着Cr#Sp的增加，熔体中的Al_2O_3减少，而CaO几乎保持不变。因此，熔体中的CaO/Al_2O_3比值随着Cr#Sp的增加而增大，熔体中的标准透辉石、透辉石和石英变得更加丰富。这一效应可以解释某些具有异常高CaO/Al_2O_3比值的地幔熔体。即使在高Cr#sp时，熔体中Cr2O3的浓度仍然很低，这意味着从熔体本身的浓度来看，Cr2O3对部分熔融平衡的强烈影响并不明显。在玄武岩成岩实验研究中，由于部分熔体的成分中没有足够的信息来重建熔融条件，Cr2O3等难熔主组分的存在使玄武岩成岩实验研究中“逆方法”得出的简化结论无效。
Chromium as Cr3þ substitutes for octahedrally coordinated Al in upper-mantle minerals, thereby reducing the activity of Al2O3 in the system and hence the concentration of Al2O3 in partial melts. The effect of Cr2O3 on melt compositions multiply saturated with the spinel lherzolite phase assemblage has been quantified in the system CaO–MgO–Al2O3–SiO2–Cr2O3 at 1 1 GPa as a function of 100 Cr/(Cr þ Al) in the spinel (Cr#sp). The decrease of Al2O3 in the melt with increasing Cr#sp is accompanied by increasing MgO and SiO2, whereas CaO remains almost constant. Consequently, the CaO/Al2O3 ratio of the melt increases with Cr#sp, and the melt becomes richer in normative diopside, hypersthene and quartz. The effect may explain certain mantle melts with unusually high CaO/Al2O3 ratios. The concentration of Cr2O3 in the melt remains low even at high Cr#sp, which means that the strong effect of Cr2O3 on partial melting equilibria is not readily apparent from its concentration in the melt itself. The existence of a highly refractory major component such as Cr2O3 nullifies simplified conclusions from the ‘inverse approach’ in the experimental study of basalt petrogenesis, as there is insufficient information in the composition of the partial melt to reconstruct the conditions of melting.