Discussion of Blundy and Holland's (1990) “Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer”
Discussion of Blundy and Holland's (1990) “Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer”
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Blundy 和 Holland (1990)“钙质角闪石平衡和新型角闪石-斜长石地温计”的讨论
DOI：
10.1007/bf00348957
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发表时间：
1992
影响因子：
3.5
通讯作者：
E. Zen
中科院分区：
文献类型：
--
作者：
J. Hammarstrom;E. Zen
Blundy 和 Holland (1990) 最近提出了一种基于钙质角闪石的四面体铝含量和共存斜长石的钠长石含量的压力相关地温计。他们将这些平衡公式化为气压计，当设置在 700°C 的固定温度时，给出的方程与我们提出的经验地理气压计方程非常相似（Hammarstrom 和 Zen 1986）。 Blundy 和 Holland 认为，之前提出的角闪石地质气压计，包括我们的以及 Hollister 等人 (1987) 的经验版本以及 Johnson 和 Rutherford (1988、1989) 的实验校准版本，都是不可靠的，除非我们能够假设岩浆温度并排除次固相线再平衡。这些假设很重要，因为：（I）钙质角闪石中的四面体铝与温度密切相关；（2）四面体铝占石英饱和岩石中遇到的钙质角闪石中存在的大部分铝。与 Blundy 和 Holland 的推论相反，我们并没有忽略钙角闪石中四面体铝的温度依赖性，事实上，我们的图 7b 与 Blundy 和 Holland 的图 6a 非常相似。我们表明，对于我们考虑的数据集，四面体和总铝的变化遵循正相关线性趋势，并且对于我们认为适合结晶最后阶段的约 100°C 的限制温度范围，实验研究中遇到的钙角闪石的总铝含量表现出压力依赖性。 Johnson 和 Rutherford (1989) 最近证实了适合在 740 至 800 ℃下应用角闪石地质气压计的三种散装成分的压力依赖性~我们对来自我们原始数据集的英闪长岩和花岗闪长岩的代表性样品测试了 Blundy 和 Holland 地温计，我们对其进行了共存斜长石分析。我们用来校准方程的三个“8-kbar”岩体的就位压力已通过独立压力验证
Blundy and Holland (1990) recently proposed a pressuredependent geothermometer based on the tetrahedral aluminum content of calcic amphibole and the albite content of coexisting plagioclase. Their formulation of these equilibria as barometers, when set at a fixed temperature of 700~ gave an equation quite similar to the equation that we proposed as an empirical geobarometer (Hammarstrom and Zen 1986). Blundy and Holland suggest that the previously proposed hornblende geobarometers, including ours as well as the empirical version of Hollister et al.(1987) and the experimentally calibrated version of Johnson and Rutherford (1988, 1989), are unreliable unless one can assume a magmatic temperature and rule out subsolidus reequilibration. These assumptions are important because:(I) tetrahedral aluminum in calcic amphibole is strongly temperature dependent;(2) tetrahedral aluminum accounts for most of the aluminum present in calcic amphibole encountered in quartz-saturated rocks. Contrary to Blundy and Holland's inference, we did not ignore the temperature dependence of tetrahedral aluminum in calcic amphibole, and, in fact, our Fig. 7b is remarkably similar to Blundy and Holland's Fig 6a. We showed that changes in both tetrahedral and total aluminum followed a positively correlated linear trend for the data set we considered and that for a restricted temperature range of about 100~ which we consider appropriate for the final stages of crystallization, the total aluminum content of calcic amphiboles encountered in experimental studies exhibits a pressure dependence. Johnson and Rutherford (1989) recently confirmed this pressure dependence for three bulk compositions appropriate for application of the hornblende geobarometer at 740 to 800~We tested the Blundy and Holland geothermometer on representative samples of tonalites and granodiorites from our original data set for which we had coexisting plagioclase analyses. The pressures of emplacement of the three" 8-kbar" plutons that we used to calibrate our equation since have been verified by independent pressure