角闪石族矿物的氧同位素分馏

利用增量方法和同位素交换技术,对角闪石族矿物的氧同位素分馏进行了理论计算和实验测定。理论结果表明,不同化学成分的角闪石之间存在一定的氧同位素分馏,其13O富集顺序为:钠闪石>蓝闪石>铁闪石>阳起石=镁铁门石≥直闪石≥透闪石>普通角闪石>铝直闪石>韭闪石。高温条件下(>500℃),角闪石相对于水亏损¹⁸O达1‰至3‰。实验进行在有少量流体存在的条件下,温度为520℃至680℃。所确定的方解石-透闪石氧同位素分馏系数与理论计算值在误差范围内完全一致。理论和实验确定的石英-透闪石分馏曲线均显着低于已知的经验校准曲线,反映了变质岩中含角闪石矿物集合体内部的退化同位素再平衡。

OXYGEN ISOTOPE FRACTIONATION IN AMPHIBOLES

Oxygen isotope fractionation in amphiboles was determined by theoretical calculation using the increment method and by experimental measurement using isotope exchange technique. The theoretical results suggest that there are given oxygen isotope fractionations among chemically distinct amphiboles. The obtained order of ¹⁸O——enrichment is as follows: riebeckite M glaucophane> grunerite>actinolite= cummingto’lite > anthophyllite > tremolite De hornblende> gedrite> pargasite. At temperatures above 500t, the amphiboles are depleted in ¹⁸O (up to 1-3‰) relative towater.The experiments to determine oxygen isotope fractionation between calcite and tremolite were carried out in the presence of small amounts of a supercritical CO₂ H₂O fluid under the following conditions: (1) 680 and 640℃, 10kb and X_co2= 0.2, (2) 580 and 560℃, 5kb and X_co2 = 0.4, (3) 520℃ 3kb and X_co2=0.4. All of the experiments was performed within the T-P-X_co2 stability field of the assemblage calcite-tremolite based on the thermodynamic phase equilibrium data, so that thedecomposition of tremolite was avoided under the experimental conditions. The isotopic equilibrium was approached from two directions by varying the isotopic compositions of starting materials. A data processing method for three——phase systemisotope exchange was applied to extrapolate partial equilibrium data to equilibrium values. The experimentally calibrated fractionations are in good agreement with the theoretical calculations. However, both theoretical and experimental data are significantly lower than the known empirical calibrations. This is consistent with oxygen diffusion data on amphiboles. Probably retrograde isotopic reequilibration may be common for amphibole relative to diopside in metamorphic rocks.