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首页> 外文期刊>Contributions to Mineralogy and Petrology >Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling
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Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling

机译：超越石榴石isograd：QuiG气压计和热力学模型的比较

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The consequences of overstepping the garnet isograd reaction have been investigated by comparing the composition of garnet formed at overstepped P-T conditions (the overstep or "OS" model) with the P-T conditions that would be inferred by assuming garnet nucleated in equilibrium with the matrix assemblage at the isograd (the equilibrium or "EQ" model). The garnet nucleus composition formed at overstepped conditions is calculated as the composition that produces the maximum decrease in Gibbs free energy from the equilibrated, garnet-absent, matrix assemblage for the bulk composition under study. Isopleths were then calculated for this garnet nucleus composition assuming equilibrium with the matrix assemblage (the EQ model). Comparison of the actual P-T conditions of nucleation (the OS model) with those inferred from the EQ model reveals considerable discrepancy between the two. In general, the inferred garnet nucleation P-T conditions (the EQ model) are at a lower temperature and higher or lower pressure (depending on the coexisting calcic phase(s)) than the actual (OS model) nucleation conditions. Moreover, the degree of discrepancy increases with the degree of overstepping. Independent estimates of the pressure of nucleation of garnet were made using the Raman shift of quartz inclusions in garnet (quartz-in-gar-net or QuiG barometry). To test the validity of this method, an experimental synthesis of garnet containing quartz inclusions was made at 800 °C, 20 kbar, and the measured Raman shift reproduced the synthesis conditions to within 120 bars. Raman band shifts from three natural samples were then used to calculate an isochore along which garnet was presumed tohave nucleated. Model calculations were made at several temperatures along this isochore (the OS model), and these P-T conditions were compared to those computed assuming equilibrium nucleation (the EQ model) to estimate the degree of overstepping displayed by these samples. A sample from the garnet isograd in eastern Vermont is consistent with overstepping of around 10 degrees and 0.6 kbar (affinities of around 2 kJ/mole garnet). A sample from the staurolite-kyanite zone in the same terrane requires overstepping of around 50 °C and 2-5 kbar (affinities of around 10-18 kJ/mole garnet). A similar amount of overstepping was inferred for a blueschist sample from Sifnos, Greece. These results indicate that overstepping of garnet nucleation reactions may be common and pronounced in regionally metamorphosed terranes, and that the P-T conditions and paths inferred from garnet zoning studies may be egregiously in error.
机译：通过比较在超量PT条件（超量或“ OS”模型）下形成的石榴石的成分与假设石榴石成核并与基质组合成平衡时所推断的PT条件，研究了石榴石等级反应的后果。 isograd（平衡或“ EQ”模型）。在超步条件下形成的石榴石核成分的计算方法是：对于所研究的总体成分，从平衡的，缺少石榴石的基质组合物中产生的吉布斯自由能最大降低的成分。然后，假设该石榴石核组成与基质组合平衡，则计算出等值线（EQ模型）。将实际的P-T成核条件（OS模型）与从EQ模型推断的条件进行比较，发现两者之间存在相当大的差异。通常，推断的石榴石成核条件（EQ模型）处于比实际（OS模型）成核条件更低的温度和更高或更低的压力（取决于共存的钙化相）。此外，差异程度随着步调程度的增加而增加。石榴石成核压力的独立估计是使用石榴石中石英夹杂物的拉曼位移（quar石英或QuiG气压计）得出的。为了测试该方法的有效性，在800°C，20 kbar的压力下进行了石榴石含石英夹杂物的实验合成，所测得的拉曼位移将合成条件重现至120 bar以内。然后使用来自三个自然样品的拉曼谱带位移来计算等时线，推测石榴石会沿着该等时线成核。沿着该等时线在几个温度下进行了模型计算（OS模型），并将这些P-T条件与假设平衡成核（EQ模型）所计算的条件进行了比较，以估计这些样品所显示的超步程度。来自佛蒙特州东部的石榴石等值线的样品与大约10度和0.6 kbar（亲和力约为2 kJ /摩尔石榴石）的超步相符。来自同一地层的辉石-蓝晶石带的样品需要超过50°C和2-5 kbar（亲和力约为10-18 kJ /摩尔石榴石）的超载。从希腊锡弗诺斯岛（Sifnos）的blueschist样品推断出类似的超标量。这些结果表明，在区域变质的地层中，石榴石成核反应的超速可能是普遍现象，并且很明显，并且从石榴石分区研究中推断出的P-T条件和路径可能会出现严重错误。

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