Internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet–orthopyroxene–plagioclase–quartz assemblages and their application to the South Indian granulites
Internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet–orthopyroxene–plagioclase–quartz assemblages and their application to the South Indian granulites
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DOI：
10.1111/j.1525-1314.1993.tb00195.x
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发表时间：
1993-11
影响因子：
3.4
通讯作者：
R. K. Lal
中科院分区：
文献类型：
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作者：
R. K. Lal
对石榴石-正辉石-斜长石-石英组合的三种反应进行了标定，分别为：1/2硅铁石+ 1/3白云石±1/2顽辉石+ 1/3铝镁石(A)；硅铁石+钙长石±2/3铝镁石+ 1/3铝镁石+石英(B)；顽辉石+钙长石±2/3长锰矿+ 1/3长锰矿+石英(C)。基于(A)、(B)和(C)反应的内部一致地温计仅根据实验数据进行校准。反应(A)的热力学参数来源于已发表的FMAS系统（n= 104）在700 ~ 1400℃和5 ~ 50 kbar范围内的实验数据，而反应(B)的热力学参数来源于已有的矿物平衡的反向实验数据的总和：硅铁石±费雅石+石英(D)和钙长石+费雅石±2/3 almandine + 1/3 grossularite (E)。(A)、(B)和(C)反应的热力学参数分别为：（ΔH0, cal） -3367±209、-2749±350和+3985±545；（ΔS0, cal K−1）-1.634±0.163,-8.644±0.298和-5.376±0.391；（ΔV01,298, cal bar−1）-0.024,-0.60946和-0.5614。在单阳离子基础上，得到了石榴石中三元Ca-Fe-Mg的Margules参数：WFe-Mg = -1256 + 1.0 (~ 0.23) T(K)、WMg-Fe = 2880 -1.7 (~ 0.13) T(K)、WCa-Mg = 4047 (~ 77) -1.5 T(K)、WMg-Ca = 1000 (~ 77) -1.5 T(K)、WCa-Fe = -723 + 0.332 (~ 0.02) T(K)、WFe-Ca = 1090 （cal）和三元常数C123= -4498 + 1.516 (~ 0.265) T(K) cal（非理想混合的亚规则溶液模型）；正晶硅中的Fe-Mg-Al: WFe-Mg = 948 (~ 200) -0.34 (~ 0.10) T(K), WFe-Al = -1950（~ 500）和WMg-Al = 0 (cal)（非理想混合的规则溶液模型）。斜长石中的钙长石活度是根据(B)和(C)反应计算的，采用常用于地压测量的亚规则Ca-Na混合“Al-avoidance”模型。对南印度前寒武纪麻粒岩中的石榴石-正辉石-斜长石-石英组合（n= 45）进行地温压测量，得到温度范围为690 ~ 860℃（X= 760±45℃），压力范围为5 ~ 10 kbar。同时估计了P-T值，从PMg（反应C）和PFe（反应B）计算的压力没有差异。在现有的校准中，这个差异是1kbar或更多。此外，在不同温度下，FMAS （n= 104）和CFMAS （n= 78）体系中实验数据的lnk与南印度麻粒岩的估计温度没有组分依赖性。
Abstract Three reactions are calibrated as geothermobarometers for garnet–orthopyroxene–plagioclase–quartz assemblages, namely: 1/2 ferrosilite + 1/3 pyrope ± 1/2 enstatite + 1/3 almandine (A): ferrosilite + anorthite ± 2/3 almandine + 1/3 grossularite + quartz (B); and enstatite + anorthite ± 2/3 pyrope + 1/3 grossularite + quartz (C). The internally consistent geothermobarometers based on reactions (A), (B) and (C) are calibrated from experimental data only. The thermodynamic parameters of reaction (A) are derived from published experimental data in the FMAS system (n= 104) in the range 700–1400°C and 5–50 kbar, while those for reaction (B) are derived by summation of the existing reversed experimental data of the mineral equilibria: ferrosilite ± fayalite + quartz (D) and anorthite + fayalite ± 2/3 almandine + 1/3 grossularite (E). The retrieved thermodynamic parameters for reactions (A), (B) and (C) are, respectively: (ΔH0, cal) -3367 ± 209, -2749 ± 350 and +3985 ± 545; (ΔS0, cal K−1) -1.634 ± 0.163, -8.644 ± 0.298 and -5.376 ± 0.391; and (ΔV01,298, cal bar−1) -0.024, -0.60946 and -0.5614. On a one-cation basis, the derived Margules parameters of the ternary Ca–Fe–Mg in garnet are: WFe–Mg= -1256 + 1.0 (∼0.23) T(K), WMg–Fe= 2880 -1.7 (∼0.13) T(K), WCa–Mg= 4047 (∼77) -1.5 T(K), WMg–Ca= 1000 (∼77) -1.5 T(K), WCa–Fe= -723 + 0.332 (∼0.02) T(K), WFe–Ca= 1090, (cal) and the ternary constant C123= -4498 + 1.516 (∼0.265) T(K) cal (subregular solution model of non-ideal mixing); and Fe–Mg–Al in orthopyroxene: WFe–Mg= 948 (∼200) -0.34 (∼0.10) T(K), WFe–Al= -1950 (∼500) and WMg–Al= 0 (cal) (regular solution model of non-ideal mixing). The anorthite activity in plagioclase is calculated by the ‘Al-avoidance’model of subregular Ca–Na mixing commonly used for geobarometry based on reactions (B) and (C). When the geothermobarometers are applied to garnet–orthopyroxene–plagioclase–quartz assemblages (n= 45) of wide compositional range from the Precambrian South Indian granulites, temperature ranges of 690–860°C (X= 760 ± 45°C) and pressure ranges of 5–10 kbar were obtained. The P–T values were estimated simultaneously and there is no difference in the pressure calculated from PMg (reaction C) and PFe (reaction B). In the existing calibrations this difference is 1 kbar or more. Furthermore, there is no compositional dependence of the ln K of the experimental data in the FMAS (n= 104) and the CFMAS (n= 78) systems at different temperatures and the estimated temperatures of the South Indian granulites.