Spinel–garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited: an in situ X-ray study
Spinel–garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited: an in situ X-ray study
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DOI：
10.1016/s0016-7037(02)00845-1
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发表时间：
2002-06
影响因子：
5
通讯作者：
M. Walter;T. Katsura;A. Kubo;T. Shinmei;O. Nishikawa;E. Ito;C. Lesher;K. Funakoshi
中科院分区：
文献类型：
--
作者：
M. Walter;T. Katsura;A. Kubo;T. Shinmei;O. Nishikawa;E. Ito;C. Lesher;K. Funakoshi
在CaO-MgO-Al_2O_3-SiO_2(CMAS)系统中，尖晶石-石榴石二辉橄榄岩的近固相线、压力-温度位置一元化曲线有很大的差异。之前从活塞-气缸装置获得的实验数据表明，实验室间的压力差最高可达30%的相对值。为了研究这种差异--因为这种反应对理解上地幔岩石学至关重要--用一种独立的方法确定了相边界。在6-8型多顶压机上用原位X射线衍射法研究了该反应。压力是用氧化镁作为内标测定的，并用新开发的氧化镁高温状态方程由测得的晶胞体积计算出来。实时和猝灭样本观测的组合被用来支持相变。在1350℃到1500℃之间的转变是颠倒的，反应在1207℃到1545℃之间进一步受到限制，在这个温度范围内，转变的平均dt/dp斜率为∼40±10℃/kbar，这与以前几个活塞-气缸研究一致。将我们的曲线外推到1575°C，即P-T不变点的既定温度，得到的压力为25.1±1.2kbar。我们还获得了1357℃下30.5±2.3kbar的石英-柯石英转变的实时反转，这比以前在活塞-圆柱体装置中测得的压力低了约2-4kbar。
Large discrepancies are reported for the near-solidus, pressure–temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al2O3-SiO2(CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz–coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.