活度在矿物温度计与压力计中的作用——以GB温度计与GASP压力计为例

浓度是溶液中客观存在的物理性质,活度则指非理想溶液(包括非理想固溶体)中组分的"有效浓度"。本文以石榴子石-黑云母(GB)温度计、石榴子石-Al2SiO5矿物-斜长石-石英(GASP)压力计为例,探讨了矿物活度模型对于温度计与压力计的影响。将石榴子石和黑云母作为理想或非理想固溶体,根据实验数据建立的不同版本的石榴子石-黑云母温度计,都能较好地重现实验温度,还能识别递增变质带、倒转变质带、热接触变质晕中,不同地带变质温度的系统性变化。但是,将石榴子石和黑云母同时作为理想固溶体的温度计,系统误差明显较大,计算温度系统性偏低。同时采用石榴子石和黑云母非理想活度模型,得到的温度计精确度高。将石榴子石和斜长石都作为理想固溶体建立的GASP压力计,计算压力明显偏高,不能准确识别Al2SiO5矿物的稳定域,有明显的系统误差。如果同时采用石榴子石与斜长石的非理想活度模型,即便不同活度模型之间热力学参数有很大差别,得到的GASP压力计准确度都较高,还能准确识别Al2SiO5矿物的稳定域。迄今为止,还严重缺乏适用于多种岩石类型和宽广温度-压力范围的通用型矿物活度模型。此外,虽然矿物温度计、压力计不涉及流体、熔体,但是其计算结果的确能准确反映客观地质事实,并不受流体、熔体的影响。

Effect of activity on geothermobarometry: Case study of the GB geothermometer and GASP geobarometer

Concentration is the real physical property of a solution, whereas activity is the "effective concentration" of non-ideal solutions including solid solutions. Taken the garnet-biotite (GB) geothermometer and garnet-Al₂SiO₅-plagioclase-quartz (GASP) geobarometer as examples, the effect of activity on geothermobarometry has been discussed in this contribution. Using experimental data and adopting different activity models of garnet and biotite, the yielded different versions of GB geothermometers can rebuild the experimental temperatures to different precision, and furthermore, they all can discern the gradual temperature change of different metamorphic zones of either prograde sequences, or inverted metamorphic terranes or thermal contact aureoles. But, larger systematic errors can be obtained when taking both garnet and biotite as ideal solid solutions, and the calibrated GB geothermometer always underestimate temperature conditions. Meanwhile, the GASP geobarometer always overestimates metamorphic pressure conditions, when taking both garnet and plagioclase as ideal solid solutions. When adopting the non-ideal activity models of both garnet and plagioclase, the derived different GASP geobarometers can successfully plot the Al₂SiO₅-bearing metapelite into the correct stability field of Al₂SiO₅ phases. Up till now, universal activity models of minerals suitable to different rock types as well as wide P-T conditions are extremely scarce. In addition, P-T results yielded from geothermobarometers calibrated solely by solid phases, can truthfully reflect the geologic P-T realities, albeit fluid or melt phases are not considered.