橄榄岩中尖晶石的氧同位素继承性与晶体结构转变

OXYGEN ISOTOPE INHERITANCE AND STRUCTURAL TRANSITION OF SPINEL IN PERIDOTITES

根据激光探针分析含尖晶石橄榄岩中矿物的氧同位素组成，应用增量法计算矿物对氧同位素分馏曲线，得到正尖晶石－矿物对氧同位素温度高于含尖晶石橄榄岩的相平衡温度，而用反尖晶石－矿物对计算的氧同位素温度则很接近含尖晶石橄榄岩的相平衡温度，较为合理。因此，地幔橄榄岩中镁铝尖晶石的氧同位素组成继承了其母体反尖晶石特征，即与橄榄石、单斜辉石和斜方辉石平衡的是反结构尖晶石，在其发生相变作用变成正结构尖晶石时未发生氧同位素再平衡。对镁铝尖晶石高温下Mg2＋－Al3＋非趋同有序的热力学分析表明，当s＝1／3时是正尖晶石和反结构尖晶石的分界线，相变温度在1550—1782℃之间，与应用反尖晶石结构分馏系数计算的同位素温度结果一致。

Using oxygen isotope data offered by laserprobe and fractionation curves for mantle minerals, oxygen isotope geothermometry has been carried out for mineral pairs containing spinel from mantle-derived peridotites. Using increment method of the mineral pair O-isotope fractionation, the calculated temperatures between the normal spinel and coexisting minerals are above the equilibria temperature of peridotite assemblages, whereas the temperatures calculated by using inverse spinel-mineral pairs are approximated to the equilibria temperature of the peridotite assemblages. Thus, it is inferred that the oxygen isotope feature of the spinel from mantle peridotite shows the nature of its precursor inverse spinel. It is the inverse spinel that has equilibrated oxygen isotope with coexisting minerals in the peridotites. Although the inverse spinel was transformed to the normal spinel afterwards, the oxygen isotopes of inverse spinel was not reequilibrated. A thermodynamic analysis of the non-convergent order of Mg2+and Al3+ in MgAl2O4 shows that the normal spinel and the inverse spinel have their stable fields at different temperatures. The calculated discontinuity s=1/ 3 is the critical point between the two polymorphs. The corresponding temperature is estimated between 1550℃ and 1782℃, which is in accordance with the oxygen isotope temperatures calculated from fractionations of the inverse spinel-mineral pair.