Physical properties of the 1980 Mount St. Helens cryptodome magma

 Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log₁₀ η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T>900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log₁₀ η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log₁₀ η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 10^10.8 Pa s. Received: 25 August 1996 / Accepted: 19 July 1997     

Viscous effects on penetrating shafts in clays

The penetration of rigid objects such as piles and penetrometers into soils creates a zone of soil disturbance around them. The extent of this disturbed zone influences the resistance of the moving rigid body. This paper presents a theoretical framework to analyze the resistance in the disturbed zone created by a shaft penetrating a clay soil. The soil is modeled as a viscous material after it reaches failure [critical state (CS)]. The results of this analysis show that the viscous drag stress component on the shaft surface is influenced by the size of disturbed zone that has reached CS around the shaft, the shear viscosity of the soil and the velocity profile (or strain rate) in the CS zone around the shaft. The size of CS zone, the velocity profile and the viscosity of soil are interdependent. Large variation in viscous drag occurs when the size of the CS soil zone is less than four times the shaft’s radius. Limiting drag occurs when the size of the CS soil zone exceeds six times the shaft’s radius. The theoretical velocity distribution of the movement of soil in the CS zone shows that the soil is dragged along with shaft in the near field (close to the shaft surface) and moves upwards in the far field.     

Water partitioning between mantle minerals from peridotite xenoliths

The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H₂O. Orthopyroxene, olivine and garnet contain 169–201, 3–54 and 0 to <3 ppm H₂O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D _water^cpx/opx = 2.1 ± 0.1). The total water contents of each xenolith are remarkably similar (113 ± 14 ppm H₂O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D _water^cpx/oliv(gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D _water^cpx/oliv(sp) = 88.1 ± 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2–0.7 log units and a reduction of its electrical resistivity by a factor of 0.5–0.8 log units.     

Grain growth in the forsterite-diopside system

Grain growth experiments in dunite, clinopyroxenite, and wehrlites with various forsterite/diopside ratios were performed to investigate the effect of modal composition on grain growth kinetics in the Earth's uppermost mantle. The experiments were conducted using a piston-cylinder apparatus at 1200 °C and 1.2 GPa for 2-763 h under dry conditions. Normal grain growth (NGG) occurred in dunite, clinopyroxenite, and relatively forsterite-poor wehrlites (≤70 vol.% of forsterite). Grain growth rates of forsterite and diopside in relatively forsterite-poor wehrlites were much slower than those in dunite and clinopyroxenite. In the forsterite-rich wehrlites (≥80 vol.% of forsterite), NGG of diopside and abnormal grain growth (AGG) of forsterite occurred. The growth rate of diopside was significantly slower than that in clinopyroxenite, while the growth rate of forsterite by AGG was found to be similar to that by NGG in dunite. The presence of ≤20 vol.% diopside had the effect of inhibiting the forsterite grain growth during the initial stage, resulting in AGG of forsterite, thus overtaking the growth rate in dunite. Our experimental results suggest that there would be a significant difference in grain growth rate and consequently in effective viscosity between olivine-rich peridotites (depleted mantle) and relatively olivine-poor peridotites (fertile mantle) in the case of grain size-sensitive creep. Variation of mean grain sizes in the upper mantle would result not only from differences in temperature and phase assemblage, but also from the variation of modal compositions.     

The viscosity of hydrous dacitic liquids: implications for the rheology of evolving silicic magmas

The viscosity of a series of six synthetic dacitic liquids, containing up to 5.04 wt% dissolved water, was measured above the glass transition range by parallel-plate viscometry. The temperature of the 10¹¹ Pa s isokom decreases from 1065 K for the anhydrous liquid, to 864 K and 680 K for water contents of 0.97 and 5.04 wt% H₂O. Including additional measurements at high temperatures by concentric-cylinder and falling-sphere viscometry, the viscosity (η) can be expressed as a function of temperature and water content w according to: where η is in Pa s, T is temperature in K, and w is in weight percent. Within the conditions of measurement, this parameterization reproduces the 76 viscosity data with a root-mean square deviation (RMSD) of 0.16 log units in viscosity, or 7.8 K in temperature. The measurements show that water decreases the viscosity of the dacitic liquids more than for andesitic liquids, but less than for rhyolites. At low temperatures and high water contents, andesitic liquids are more viscous than the dacitic liquids, which are in turn more viscous than rhyolitic liquids, reversing the trend seen for high temperatures and low water contents. This suggests that the relative viscosity of different melts depends on temperature and water content as much as on bulk melt composition and structure. At magmatic temperatures, rhyolites are orders of magnitude more viscous than dacites, which are slightly more viscous than andesites. During degassing, all three liquids undergo a rapid viscosity increase at low water contents, and both dacitic and andesitic liquids will degas more efficiently than rhyolitic liquids. During cooling and differentiation, changing melt chemistry, decreasing temperature and increasing crystal content all lead to increases in the viscosity of magma (melt plus crystals). Under closed system conditions, where melt water content can increase during crystallization, viscosity increases may be small. Conversely, viscosity increases are very abrupt during ascent and degassing-induced crystallization.     

四川龙门山断裂带高精度地壳/岩石圈黏度结构及其动力学意义

岩石圈黏度是大陆动力学研究中一个重要参数,但是岩石圈黏度,尤其是横向小尺度(<100 km)黏度结构的确定是一个挑战.本文根据电阻率和黏度与它们控制因素的相似关系,直接把一条跨过青藏高原东缘和四川龙门山断裂带的大地电磁(MT)探测的电阻率剖面转换成黏度结构作为输入,在GPS速度和地表地形数据的约束下,利用地球动力学数值模拟获得了该剖面的二维地壳/岩石圈黏度结构.本文推断的黏度与前人获得的区域尺度的黏度值一致,但揭示出了更多的细节.本文的黏度结构揭示出研究区域内的地壳/岩石圈黏度存在较大的空间变化范围(约5量级),黏度值分布在1.48×10^17~8.44×10^22 Pa·s之间;龙门山断裂带下的黏度存在强烈的小尺度横向变化,其中、下地壳的黏度分别为1.99×10^18~8.21×10^20 Pa·s(平均1.17×10^20 Pa·s)和4.09×10^19~7.08×10^20 Pa·s(平均1.77×10^20 Pa·s).基于该黏度结构的地球动力学模型表明驱动青藏高原中-下地壳物质流动的可能是热-化学浮力,以及上地壳和中-下地壳可能处于解耦状态.本文获得的黏度结构可以为龙门山断裂带地震成因和机制、岩石圈小尺度变形和构造应力状态的深入研究提供重要的帮助.     

大地测量约束下的阿尔泰山岩石圈流变结构

本文根据大地测量数据得到过去50年左右阿尔泰山富蕴区域形变场,破裂带中段的相对位移最大,平均速率达6mm/a,总体上表现为沿断裂的走滑运动特征．该形变场可以用1931年富蕴8级地震的震后黏弹性松弛模型进行模拟,反演得到下地壳黏滞系数为1.6×10¹⁹～7.9×10¹⁹Pa·s,上地幔黏滞系数为16×1018～63×1019Pa·s,与华北、Nevada等地区利用震后变形资料推算的黏度基本一致．根据该地区最佳黏弹性分层模型,最近五十年由于岩石圈下部应力松弛引起的地震破裂带两侧最大水平速率约为4mm/a． 我们的研究表明：大陆7～8级大震在几十年后仍可能有可观的地表变形,GPS监测得到的现今变形场可能包含震后变形成分．     

Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms

 Lithic megablocks ranging from <1 to 50 m in diameter occur in the Yardea Dacite, a widespread (12,000 km²), thick (>200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic with 30–40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained and effusively erupted. Received: 11 November 1998 / Accepted: 18 April 1999     

一种计算NaAlSi3O8熔体粘度的理论方法

硅酸岩熔体的结构特征是制约熔体粘度的主要因素，熔体结构的变异是其中粘流作用发生的原因，化学成分对熔体粘度的控制是通过改变熔体结构而实现的。ＳｉＯ２熔体中仅存在Ｓｉ－Ｏ键，而ＮａＡｌ－Ｓｉ３Ｏ８熔体中存在Ｓｉ－Ｏ键，Ａｌ－Ｏ键和Ｎａ－Ｏ键，Ｎａ－Ｏ键在熔体结构中通过Ｏ与Ｓｉ－Ｏ键相联结，并且使与之相联的Ｓｉ－Ｏ键的键强变弱。因此，熔体结构单元中与Ｎａ－Ｏ键相联结的Ｓｉ－Ｏ键最易断开，并因此导致流变作用的发生。文中计算了不同温度条件下ＮａＡｌＳｉ３Ｏ８熔体中Ｓｉ－Ｏ键的键强，以ＳｉＯ２熔体中的Ｓｉ－Ｏ键的键强和ＳｉＯ２熔体的粘度为标准，建立了计算了ＮａＡｌＳｉ３Ｏ８熔体粘度的理论模型，依此理论模型求得的熔体粘度在合理的地质温度范围内与实验结果完全吻合     

岩浆作用的物理过程研究进展

概略地介绍了８０年代以来硅酸盐熔体及硅酸盐晶－液悬浮体的密度、粘度、熔体结构、流全动力学等方面的研究动向,及其对岩浆作用、岩浆运移、岩浆侵位机制的动力学约束条件,硅酸盐熔体的结构是制约熔粘度的主导因素,化学成分对熔体匠控制是通过改变熔体结构而实现的,粘度在一定程度上决定着岩浆的迁移、侵痊和喷发方式。密度和浮力是岩浆上升侵位的重要约束,地壳是岩交涉升的一个密度过滤器岩浆最终由于浮和的消失而停止上升。