Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling
Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling
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超越石榴石等梯度：QuiG 气压测量法和热力学模型的比较
DOI：
10.1007/s00410-014-1059-6
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发表时间：
2014
影响因子：
3.5
通讯作者：
Hallett, Benjamin W.
中科院分区：
文献类型：
--
作者：
Spear, Frank S.;Thomas, Jay B.;Hallett, Benjamin W.
通过比较在超越P-T条件下形成的石榴石的成分(超越或“OS”模型)与P-T条件(假定石榴石在等温线上与基质组合成核平衡(平衡或“EQ”模型)而推断的P-T条件，研究了超越石榴石等值线反应的后果。在超越条件下形成的石榴石核成分被计算为在所研究的主体成分中，从平衡的、不含石榴石的基质组合中产生最大吉布斯自由能下降的成分。然后计算了这种石榴石核成分的同位素，假定与基质组合(EQ模型)是平衡的。将实际成核的P-T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。一般来说，推测的石榴石成核P-T条件(EQ模型)比实际的(OS模型)成核条件更低的温度和更高或更低的压力(取决于共存的钙相(S))。此外，差异程度随着越界程度的增加而增加。利用石榴石中石英包裹体的拉曼位移(石英石或奎格气压法)对石榴石的成核压力进行了独立的估计。为了验证这种方法的有效性，在800℃，20kbar的温度下进行了石榴石包裹体的实验合成，测量的拉曼位移重现了合成条件在120Kbar以内。然后利用三个天然样品的拉曼频移来计算石榴石的等温线，推测石榴石沿着该等温线成核。在等温线上的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的计算条件(EQ模型)进行了比较，以估计这些样品所显示的超越程度。佛蒙特州东部石榴石等值线的样本与超越约10度和0.6kbar(亲和力约为2kJ/摩尔石榴石)是一致的。同一地体中的柱石-蓝晶石带的样品需要超过约50摄氏度和2-5千巴(亲和力约为10-18千焦耳/摩尔石榴石)。对于来自希腊Sifnos的蓝片岩样本，也推断出了类似的超越程度。这些结果表明，在区域变质地体中，石榴石成核反应的超越性可能是常见的和明显的，而从石榴石分带研究中推断的P-T条件和路径可能是错误的。
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-garnet 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 to have 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.
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