New developments in two-feldspar thermometry
New developments in two-feldspar thermometry
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DOI：
10.2138/am-2004-1018
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发表时间：
2004-10
影响因子：
3.1
通讯作者：
A. Benisek;H. Kroll;L. Cemič
中科院分区：
文献类型：
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作者：
A. Benisek;H. Kroll;L. Cemič
摘要对双长石温度计的热力学模型进行了修正。从最近的焓和体积测量（Na，Ca）-和（K，Ca）-长石二进制，新的相互作用参数已被导出，以前的已经更新。用Seck（1971）、埃尔金斯和格罗夫（1990）的相平衡数据拟合了熵参数。这两个数据集可以适当地合并为一个。理想的抗体，或，和一个活动已表示在分子混合和铝避免模型。在干燥条件下缓慢冷却的高级变质岩中的双长石对遭受了一种独特的逆行复位。而斜长石和碱性长石中的原始Au含量由于晶间Ca + Al Particles（Na，K）+ Si扩散缓慢而得以保留，Na和K可在相间自由交换。在常数Δ n下的Na-K交换的数学反演得到了两种长石最初共存的温度。从反转中获得的Ab和Or含量的变化反映了在薄片中观察到的相对斜长石/碱性长石比例。从斯里兰卡麻粒岩长石对计算和测量的比率之间的良好协议。该观察结果代表了对计算的Ab-Or位移的可靠性的成功测试。与干燥的变质岩相反，在火山岩的情况下，化学约束的类似应用没有被指出。然后，双长石温度计提供三个通常不一致的温度：T（Ab），T（Or）和T（An）。根据温度的丰富性，Fuhrman和Lindsley（1988年）建议在假设的化学不确定性范围内调整成分（例如，± 2mol%），从而可以获得一致的温度。然而，结果并非独一无二。取决于起始组合物的微小变化，温度可以变化几十度。此外，温度根据搜索算法的类型在类似程度上变化。因此，我们建议用户完全放弃这种做法。相反，一个统计程序建议：两个长石组合物随机生成的高斯分布，其平均值在观察到的组合物和标准误差选择根据化学分析的质量。此过程返回正态分布温度[T（Ab），T（Or），T（An）]以及平均值和标准差。从三条高斯曲线的重叠可以解决长石对的平衡或非平衡结晶的问题。
Abstract The thermodynamic model of the two-feldspar thermometer has been revised. From recent enthalpy and volume measurements in the (Na,Ca)- and (K,Ca)-feldspar binaries, new interaction parameters have been derived and previous ones have been updated. Entropy parameters have been fitted to the phase equilibrium data of Seck (1971) and Elkins and Grove (1990). The two data sets could be suitably combined into one. Ideal Ab, Or, and An activities have been expressed in terms of both the molecular mixing and Al-avoidance models. Two-feldspar pairs from high-grade metamorphic rocks that cooled slowly under dry conditions suffer from a distinct type of retrograde resetting. Whereas the original An content in both the plagioclase and the alkali feldspar is preserved because the intercrystalline Ca + Al ↔ (Na,K) + Si diffusion is sluggish, Na and K may be freely exchanged between phases. Mathematical reversal of the Na-K exchange at constant An yields the temperature at which the two feldspars originally coexisted. The shifts in Ab and Or contents obtained from the reversal reflect the relative plagioclase/alkali feldspar proportions observed in thin sections. Good agreement between calculated and measured ratios was found for feldspar pairs from Sri Lankan granulites. This observation represents a successful test of the reliability of the calculated Ab-Or shifts. In contrast to dry metamorphic rocks, similar application of chemical constraints is not indicated in the case of volcanic rocks. Then the two-feldspar thermometer delivers three, usually incongruent temperatures: T(Ab), T(Or), and T(An). From the abundance of temperatures, Fuhrman and Lindsley (1988) suggested adjusting compositions within assumed chemical uncertainties (e.g., ±2 mol%) so that congruent temperatures could be obtained. However, the result is not unique. Depending on minute variations in the starting compositions, the temperatures may vary by several tens of degrees. In addition, temperatures vary to a similar extent depending on the type of search algorithm. Therefore, we advise users to completely abandon this practice. Instead, a statistical procedure is suggested: Two-feldspar compositions are randomly generated according to Gaussian distributions with their means at the observed compositions and standard errors chosen according to the quality of the chemical analysis. This procedure returns normally distributed temperatures [T(Ab), T(Or), T(An)] together with means and standard deviations. From the overlap of the three Gaussian curves the question of equilibrium or non-equilibrium crystallization of feldspar pairs may be addressed.