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首页> 外文期刊>Contributions to Mineralogy and Petrology >Paragonite stability at 700 degrees C in the presence of H2O-NaCl fluids: constraints on H2O activity and implications for high pressure metamorphism
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Paragonite stability at 700 degrees C in the presence of H2O-NaCl fluids: constraints on H2O activity and implications for high pressure metamorphism

机译：H2O-NaCl流体存在下在700摄氏度下的仿石稳定性：对H2O活性的限制及其对高压变质作用的影响

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摘要

We carried out reversed piston-cylinder experiments on the equilibrium paragonite = jadeite + kyanite + H2O at 700degreesC, 1.5-2.5 GPa, in the presence of H2O-NaCl fluids. Synthetic paragonite and jadeite and natural kyanite were used as starting materials. The experiments were performed on four different nominal starting compositions: X(H2O)=1.0, 0.90, 0.75 and 0.62. Reaction direction and extent were determined from the weight change in H2O in the capsule, as well as by optical and scanning electron microscopy (SEM). At X(H2O)=1.0, the equilibrium lies between 2.25 and 2.30 GPa, in good agreement with the 2.30-2.45 GPa reversal of Holland (Contrib Miner Petrol 68:293-301, 1979). Lowering X(H2O) decreases the pressure of paragonite breakdown to 2.10-2.20 GPa at X(H2O)=0.90 and 1.85-1.90 GPa at X(H2O)=0.75. The experiments at X(H2O) = 0.62 yielded the assemblage albite + corundum at less than or equal to1.60 GPa, and jadeite + kyanite at greater than or equal to1.70 GPa. This constrains the position of the isothermal paragonite-jadeite-kyanite-albite-corundum-H2O invariant point in the system Na2O-Al2O3-SiO2-H2O to be at 1.6-1.7 GPa and X(H2O)similar to0.65+/-0.05. The data indicate that H2O activity, a(H2O), is 0.75-0.86, 0.55-0.58, and <0.42 at X(H2O)=0.90, 0.75, and 0.62, respectively. These values approach X(H2O)(2), and agree well with the a(H2O) model of Aranovich and Newton (Contrib Miner Petrol 125:200-212, 1996). Our results demonstrate that the presence or absence of paragonite can be used to place limits on a(H2O) in high-pressure metamorphic environments. For example, nearly pure jadeite and kyanite from a metapelite from the Sesia Lanzo Zone formed during the Eo-Alpine metamorphic event at 1.7-2.0 GPa, 550-650degreesC. The absence of paragonite requires a fluid with low a(H2O) of 0.3-0.6, which could be due to the presence of saline brines.
机译：在H2O-NaCl流体存在的情况下，我们在700摄氏度，1.5-2.5 GPa的平衡方钠石=翡翠+蓝晶石+ H2O上进行了反向活塞缸实验。合成的agon石和硬玉和天然蓝晶石被用作起始原料。实验是在四种不同的标称起始组成下进行的：X（H2O）= 1.0、0.90、0.75和0.62。反应方向和程度由胶囊中H2O的重量变化以及光学和扫描电子显微镜（SEM）确定。在X（H2O）= 1.0时，平衡值介于2.25和2.30 GPa之间，与Holland的2.30-2.45 GPa逆转很好地吻合（Contrib Miner Petrol 68：293-301，1979）。降低X（H2O）可使方解石分解的压力在X（H2O）= 0.90时降至2.10-2.20 GPa，在X（H2O）= 0.75时为1.85-1.90 GPa。在X（H2O）= 0.62的实验中，合成的钠长石+刚玉小于或等于1.60 GPa，而翡翠+蓝晶石的聚集体大于或等于1.70 GPa。这将Na2O-Al2O3-SiO2-H2O系统中等温文石-翡翠-蓝晶石-蓝晶石-刚玉-H2O不变点的位置限制在1.6-1.7 GPa和X（H2O）近似于0.65 +/- 0.05 。数据表明，在X（H2O）= 0.90、0.75和0.62时，H2O活性a（H2O）分别为0.75-0.86、0.55-0.58和<0.42。这些值接近X（H2O）（2），并且与Aranovich和Newton的a（H2O）模型非常吻合（Contrib Miner Petrol 125：200-212，1996）。我们的结果表明，在高压变质环境中，是否存在方解石均可用于限制a（H2O）。例如，在Eo-Alpine变质事件中，在550-650°C的Eo-Alpine变质事件中，形成了来自Sesia Lanzo带的变质岩的近乎纯净的翡翠和蓝晶石。不含石楠石要求流体的a（H2O）较低，为0.3-0.6，这可能是由于盐水的存在所致。

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