The influence of pressure and composition on the viscosity of andesitic melts
The influence of pressure and composition on the viscosity of andesitic melts
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DOI：
10.1016/s0016-7037(02)01139-0
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发表时间：
2003-02
影响因子：
5
通讯作者：
C. Liebske;H. Behrens;F. Holtz;R. Lange
中科院分区：
文献类型：
--
作者：
C. Liebske;H. Behrens;F. Holtz;R. Lange
用平行板粘度计研究了在108~1011.5Pa.S粘度范围内，压力和组成对无水和无水安山岩熔体粘度的影响。用高P-T粘度计测量了三种合成的、不含铁的液体(安山岩类似物)的粘度与压力的关系，这些液体分别含有0.0wt%、1.06wt%和1.96wt%H2O，范围为100-300兆帕。这些结果与Richet等人的结果相结合。(1996)，表明无水安山岩熔体的粘度与压力无关，而含水熔体的粘度随着压力的增加而略有增加。这一趋势与含水熔体中解聚程度的增加是一致的。通过比较解聚程度相近的无铁组分和含铁组分，研究了组成对粘度的影响。在常压和高压(100~300 Mpa)下进行实验时，空气中预平衡的含铁无水熔体的粘度不断增加，样品变得顺磁性。通过透射电子显微镜对这些样品的分析表明，晶体(可能是磁铁矿)分布均匀，尺寸在10到50 nm之间。在970~1122K的温度范围内，经170~830min的热处理后，样品中晶体的体积分数没有显著差异。在内热压力容器中，在固有fO2条件下合成的含铁安山岩，其粘度行为与无水熔体相似。在恒温下粘度的持续增加归因于富铁相的析出引起熔体结构的变化。通过外推粘度的时间演化到达到运行温度的时间，对于无水(1055K)和含水(860K)含铁安山岩，粘度比Richet等人的模型预测的低0.7个对数单位。(1996年)。这可能是由于模拟组合物中使用的Fe2+和Fe3+及其替代物Mg2+、Ca2+和Al3+的结构性质不同所致。用膨胀仪在常压下研究了铁的氧化还原状态对无水合成安山岩熔体粘度的影响。通过在Ar/CO气氛中对石墨炉中的熔体进行不同时间的退火，获得了还原的含铁样品。与氧化样品相比，还原程度最高的玻璃没有观察到粘度随时间的变化，也没有观察到氧化铁相的析出。这表明三价铁促进了氧化铁在过冷熔体中的析出。当Fe~(3+)/ΣFe比从0.58降至0.34时，玻璃的粘度在964~1098K范围内下降了1.6log∼单位。当Fe~(3+)/ΣFe=0.21时，玻璃的粘度没有进一步降低。我们从这些结果得出的结论是，当使用从空气中合成的样品获得的数据时，天然熔体的粘度可能被高估了很多。
The effect of pressure and composition on the viscosity of both anhydrous and hydrous andesitic melts was studied in the viscosity range of 108to 1011.5Pa · s using parallel plate viscometry. The pressure dependence of the viscosity of three synthetic, iron-free liquids (andesite analogs) containing 0.0, 1.06, and 1.96 wt.% H2O, respectively, was measured from 100 to 300 MPa using a high-P-T viscometer. These results, combined with those from Richet et al. (1996), indicate that viscosities of anhydrous andesitic melts are independent of pressure, whereas viscosities of hydrous melts slightly increase with increasing pressure. This trend is consistent with an increased degree of depolymerization in the hydrous melts. Compositional effects on the viscosity were studied by comparing iron-free and iron-bearing compositions with similar degrees of depolymerization. During experiments at atmospheric and at elevated pressures (100 to 300 MPa), the viscosity of iron-bearing anhydrous melts preequilibrated in air continuously increased, and the samples became paramagnetic. Analysis of these samples by transmission electron microscopy showed a homogeneous distribution of crystals (probably magnetite) with sizes in the range of 10 to 50 nm. No significant difference in the volume fractions of crystals was found in samples after annealing for 170 to 830 min at temperatures ranging from 970 to 1122 K. An iron-bearing andesite containing 1.88 wt.% H2O, which was synthesized at intrinsic fO2conditions in an internally heated pressure vessel, showed a similar viscosity behavior as the anhydrous melts. The continuous increase in viscosity at a constant temperature is attributed to changes of the melt structure due to exsolution of iron-rich phases. By extrapolating the time evolution of viscosity down to the time at which the run temperature was reached, for both the anhydrous (at 1055 K) and the hydrous (at 860 K) iron-bearing andesite, the viscosity is 0.7 log units lower than predicted by the model of Richet et al. (1996). This may be explained by differences in structural properties of Fe2+and Fe3+and their substitutes Mg2+, Ca2+, and Al3+, which were used in the analogue composition. The effect of iron redox state on the viscosity of anhydrous, synthetic andesite melts was studied at ambient pressure using a dilatometer. Reduced iron-bearing samples were produced by annealing melts in graphite crucibles in an Ar/CO atmosphere for different run times. In contrast to the oxidized sample, no variation of viscosity with time and no exsolution of iron oxide phases was observed for the most reduced glasses. This indicates that trivalent iron promotes the exsolution of iron oxide in supercooled melts. With decreasing Fe3+/ΣFe ratio from 0.58 to 0.34, the viscosity decreases by ∼1.6 log units in the investigated temperature range between 964 and 1098 K. A more reduced glass with Fe3+/ΣFe = 0.21 showed no additional decrease in viscosity. Our conclusion from these results is that the viscosity of natural melts may be largely overestimated when using data obtained from samples synthesized in air.