辽宁宽甸地幔矿物三价铁的穆斯堡尔谱测定及意义

FERRIC IRON CONTENTS OF MANTLE XENOLITH MINERALS BY MSSBAUER SPECTROSCOPY AND ITS IMPLICATIONS TO CALCULATED TEMPERATURE, PRESSURE AND OXYGEN FUGACITY

利用穆斯堡尔谱仪对辽宁宽甸地幔岩包体中的矿物进行了三价铁的测定，测试结果与国内外已发表的数据接近。Ｆｅ３＋／∑Ｆｅ比值分别为：斜方辉石００６６～０１９６，单斜辉石０２１６～０３４４，尖晶石０２８３～０２９９，石榴石００５７～０２２３。依据电价平衡原理计算的Ｆｅ３＋含量受Ｓｉ４＋、Ａｌ３＋和Ｃｒ３＋的电子探针分析误差影响很大。根据地幔矿物的电子探针分析结果计算平衡温度、压力和氧逸度时，二辉石Ｅｎ、Ｃａ转移反应温度计和石榴石斜方辉石Ａｌ压力计的计算结果不超过温压计的一般误差范围，而石榴石单斜辉石Ｆｅ２＋Ｍｇ交换反应温度计在忽略Ｆｅ３＋时计算的温度值可偏高１００℃以上；采用橄榄石／斜方辉石／尖晶石组合氧压力计计算的氧逸度值，误差一般约在±０７个对数单位。

erric iron contents of coexisting minerals from four mantle xenoliths, collected from Kuandian, Liaoning Province, were measured by Mssbauer spectroscopy and found to be similar to the published data from domestic and overseas. The ratios of Fe 3+ /∑Fe are 0066～0196 in orthopyroxenes, and 0216～0344, 0283～0299 and 0057～0223 in clinopyroxenes, spinels and garnets respectively. Comparison of Mssbauer and microprobe analytical results shows that Fe 3+ values calculated from microprobe data are significantly effected by analytical uncertainty of Si 4+ ,Al 3+ and Cr 3+ . In terms of equilibrium temperature, pressure and oxygen fugacity calculated from electron microprobe analyses of mineral compositions, the results estimated from orthopyroxene clinopyroxene En and Ca transfer reaction geothermometers and garnet orthopyroxene Al geobarometers are reliable with uncertaines less than 30℃ for temperature and 01～04GPa for pressure. Garnet clinopyroxene Fe 2+ -Mg exchange reaction geothermometers usually overestimate temperatures as higher as 89～166℃. The comparatively reliable oxygen fugacity values can be obtained from olivine orthopyroxene spinel oxybarometers with uncertainties around±07 log units.