Microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers

Flow laws for high-temperature creep of olivine, plagioclase, and diabase are used to place constraints on the rheology of partially molten lower oceanic crust. This analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene. Extrapolation of experimental flow laws indicates that at temperatures above 1100°C and stresses less than 10 MPa, olivine may be the weakest phase in rocks with gabbroic composition. By accounting for variations in the melt fraction (0–10%) and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep. Calculated effective viscosities range from 10¹⁵ to 10¹⁹ Pa s; based on observations of the geometry of the partially molten zone beneath the East Pacific Rise and the microstructural and experimental constraints we favor a value of ∼10¹⁸ Pa s. This value approaches estimates for the viscosity of the upper mantle beneath ridge axes, but is significantly higher than previously suggested for the partially molten lower crust. Such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution. However, the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2D, ‘sheet-like’ fashion.     

Experimental investigation of melt topology in partially molten quartzo-feldspathic aggregates under hydrostatic and non-hydrostatic stress

Continental collision results in deep burial of crustal rocks and their subsequent partial melting. Field relations of melt along zones of intense deformation suggest that partially molten rocks may play an important role in regional tectonics. However, subsequent deformation may erase the microstructures produced by the earlier deformation mechanisms, inhibiting our understanding of the rheology of partially molten crustal rocks. Thus, in this paper, we report the results of an experimental study of the distribution of 2–5 vol% melt in quartzo-feldspathic aggregates of various grain sizes: 2–5, 5–10, 10–16 and 26–31 μm. Three types of samples were examined, all with the composition of 60 wt% albite, 25 wt% potassium feldspar, 10 wt% quartz and 5 wt% biotite. The first group included mineral powders annealed at 1000 °C, 1.0 GPa, for c. 100 h. The second group included commercially hot-pressed mineral powders which yielded c. 25 vol% glass; cores of this material were also annealed at 1000 °C, 1.0 GPa, for c. 100 h. The third group included cores of hot-pressed material that were annealed at 1000 °C, 1.0 GPa, for c. 45 h, then deformed. All samples were quenched rapidly in order to examine the melt distribution. Wetting angles are very similar in both the hydrostatically annealed and the deformed samples. Analysis of melt pool orientations reveal that melt migrates away from grain boundaries normal to the maximum compressive stress direction in response to the applied non-hydrostatic stress. This response is easily seen in the coarser-grained samples in which melt pools elongated parallel to the maximum compressive stress direction formed during deformation. If these results extrapolate to naturally deformed rocks, it will be important to consider the orientation of the state of stress in a region during syn-magmatic deformation because of its effect on the melt distribution.     

The Electrical Conductivity of Gabbro at High Temperature and High Pressure

The electric conductivity of gabbro has been measured at 1.0 - 2.0 GPa and 320-700℃, and the conduction mechanism has been analyzed in terms of the impedance spectra.Experimental results indicated that the electric conductivity depends on the frequency of alternative current. Impedance arcs representing the conduction mechanism of grain interiors are displayed in the complex impedance plane, and the mechanism is dominated at high pressure.These arcs occur over the range of 102 - k× 105 Hz (k is the positive integer from 1 to 9). On the basis of our results and previous work, it is concluded that gabbro cannot form any high conductivity layer (HCL) in the middle-lower crust.     

Mechanisms and orientation of melt segregation paths during pure shearing of a partially molten rock analog (norcamphor–benzamide)

In-situ deformation experiments were performed on partially molten analog materials (norcamphor in the presence of a benzamide–norcamphor melt) undergoing pure shearing at a constant melt fraction of 0.13. Melt in the samples induces a strain-dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding (GBS). Intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries. Along such boundaries, grain-boundary migration recrystallization is inhibited, while GBS occurs. Intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction, but melt must have migrated parallel to the elongation direction of the samples, as indicated by melt accumulations at both extruding ends of the sample. Intergranular melt pockets parallel to the elongation direction were only rarely observed, because melt was rapidly expelled from these sites. Nevertheless, these grain boundaries are the pathways of melt segregation in the samples.     

煤体复电性频散响应实验研究

为研究煤体复电性频散响应特征,实测不同测量面积,不同方向煤体的复电阻实部R和虚部X,分析其频散特征,并用不同经典模型进行数据反演对比。结果表明：任意方向煤体的复电参数（R、X的绝对值）与测量面积成反比,频散曲线的特征频率点不发生偏移,X频散度随测量面积增大而减小;相同测量面积,不同方向煤体的复电性频散响应不同,复电频散曲线发生偏移,平行层理方向煤体X的频散度大于垂直层理方向;Debye模型与Cole-Cole模型对煤体复电性频散曲线均可高精度拟合,但前者模型参数精简,物理意义明确,与煤体复电频散曲线开合情况吻合度较高。本研究可为复电性评价法监测煤层裂隙发育及评价煤层渗透率提供实验基础。     

低频信息对阻抗反演的影响分析

在地震波阻抗成像和岩石物理解释中,低频信息是非常重要的。缺失低频会导致地震波频带变窄,时间域分辨率降低,最终会使波阻抗成像错误或造成虚假成像,这将会造成储层预测和岩石物理解释不正确。针对同一工区的两块资料ZJNew和ZJOld进行了波阻抗反演,含较多低频成分的ZJNew资料的反演结果分辨率较高。通过楔形模型反演结果的比较可知,低频缺失会导致在储层位置出现虚假薄夹层,造成储层预测解释错误。     

Petrologic evolution of partially molten cumulate: the Atok section of the Bushveld Complex

The postcumulus evolution of a portion of the Bushveld Complex that includes the Merensky reef is inferred from the study of a continuous 56 m drill core. The core penetrated the basal orthopyroxenites of the Merensky and Bastard units, the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the Merensky pyroxenite. Detailed profiles of major, minor and rare earth element (REE) contents of clinopyroxene and orthopyroxene were determined by electron and ion probe. Good correlations exist between textural and lithological variations and the REE contents of the pyroxenes. Specifically, enrichments in pyroxene REE abundances are observed in the basal pyroxenites of the Merensky and Bastard units relative to the underlying and overlying rocks. In the pyroxenites the Nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm (≈90 × chondrite), and Nd/Yb ratio is in the range 8 to 25. These extreme enrichments in REE are not accompanied by large variations in major element contents. Computations of the compaction parameters relevant to the conditions of crystallization of the Bushveld Complex combined with a consideration of cooling history confirm the importance of compaction as a post-cumulus process. This analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile. A model for the development of the Atok section is developed. The Merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late-stage, hydrous and incompatible-element enriched melt, which was thus trapped in the underlying Merensky pyroxenite. The flow was driven by compaction of the 350+ meter-thick section of predominantly norite beneath the anorthosite. The introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution, recrystallization and REE enrichment of the rock forming minerals, and in the formation of the main lithologic features of the Merensky pyroxenite. Further upward percolation of the interstitial melt through the Merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite. This melt accumulated in and metasomatized the Bastard pyroxenite in the same manner as the Merensky pyroxenite, having again been trapped by the overlying Bastard anorthosite. Received: 10 July 1996 / Accepted: 27 February 1997     

熔体的形态与分布特征对岩石流变的影响

熔体的形态与分布研究表明,在静态条件下,熔融程度比较低时,熔体主要分布于三个矿物颗粒之间,形成三角形状熔体结构,熔体二面角在0°~60°;熔融程度比较高时,熔体沿多个颗粒边界形成孤立的三角形或四边形结构,熔体三联点的二面角接近60°或大于60°。在动态条件下,在部分或全部矿物颗粒边界出现熔体薄膜,把熔体三角形连通,形成局部熔体网络,熔体三联点的二面角接近0°。如果熔体呈孤立的三角形或四边形结构时,熔体对岩石流变的影响比较小:当熔体含量小于2%~3%,熔体对岩石流变基本没有影响;只有熔体含量接近或超过3%~5%,熔体对流变强度的弱化作用才出现,当熔体含量达到10%时,流变强度弱化增加3倍左右。如果矿物颗粒边界出现熔体薄膜,微量熔体(小于1%)就对岩石流变强度有显著的弱化作用。流变实验表明,在颗粒边界含有小于1%的熔体时,熔体对流变强度的弱化达到4倍,当颗粒边界含有3%的熔体时,这种弱化作用达到10倍。     

A possible effect of melt structure on the Mg-Fe partitioning between olivine and melt

Partitioning of Mg and Fe²⁺ between olivine and mafic melts has been determined experimentally for eight different synthetic compositions in the temperature range between 1335 and 1425°C at 0.1 MPa pressure and at fo₂ ∼1 log unit below the quartz-fayalite-magnetite buffer. The partition coefficient [K_D = (Fe²⁺/Mg)^ol/(Fe²⁺/Mg)^melt] increases from 0.25 to 0.34 with increasing depolymerization of melt (NBO/T of melt from 0.25-1.2), and then decreases with further depolymerization of melt (NBO/T from 1.2-2.8). These variations are similar to those observed in natural basalt-peridotite systems. In particular, the variation in NBO/T ranges for basaltic-picritic melts (0.4-1.5) is nearly identical to that obtained in the present experiments. Because the present experiments were carried out at constant pressure (0.1 MPa) and in a relatively small temperature range (90°C), the observed variations of Mg and Fe²⁺ partitioning between olivine and melt must depend primarily on the composition or structure of melt. Such variations of K_D may depend on the relative proportions of four-, five-, and six-coordinated Mg²⁺ and Fe²⁺ in melt as a function of degree of NBO/T.     

The effect of storms on Finnish dairy farms: electrical outage statistics and the effect on milk production

Natural Hazards - Due to technological advancement, agricultural production is increasingly dependent on electricity. At the same time, power delivery infrastructures are challenged by extreme...     

压力对辉长岩矿物反应和部分熔融影响的实验研究

本文在高温高压条件下,开展了辉长岩矿物反应与部分熔融实验,利用偏光显微镜与扫描电镜对实验样品微观结构观察,研究实验中的新生矿物与熔体的分布;通过电子探针分析熔体成分特征。实验结果表明,在低压(300MPa)条件下,静压和塑性变形实验样品中,单斜辉石以固体反应方式生成橄榄石,在高压(1300MPa)塑性实验中所有实验样品都没有发现新生矿物颗粒,这与相图中低压条件下斜长石与橄榄石稳定共存,而高压下斜长石-辉石稳定共存相吻合。高压塑性变形条件下,单斜辉石和黑云母首先发生部分熔融,随着温度增高,斜长石逐渐参与熔融,熔体呈薄膜状分布在矿物颗粒边界,熔体成分依赖于参与熔融的矿物成分,表明出现的熔体为非平衡熔融结果。     

The effect of melt composition on the wetting angle between silicate melts and olivine

The wetting angle between silicate melts containing Ca, Li, Na, or K and olivine single crystals have been measured as part of an investigation of the dependence of the solid-liquid interfacial energy on melt composition and olivine orientation. The wetting angle increases with increasing silica content of the melt on (100) surfaces, but decreases with increasing silica content on (010) and (001) surfaces. For a given silica content, the wetting angle on (100) decreases in going from Ca to Li to Na to K, while the wetting angle on (010) and (001) increases in going from Ca to K-bearing melts. Based on published values for liquid-vapor interfacial energies, the observed changes in wetting angle with changes in melt composition indicate that the solid-liquid interfacial energy increases with increasing silica content of the melt for the (100) surface. However, for (010) and (001) surfaces, the variation of the solid-liquid interfacial energy with silica content depends upon whether Ca or K is present in the melt. In addition, the solid-liquid interfacial energy depends upon the orientation of the olivine in the following manner: _sl ⁽⁰¹⁰⁾ _sl ⁽⁰⁰¹⁾ _sl ⁽¹⁰⁰⁾ .     

The effect of Ca-Tschermaks component on trace element partitioning between clinopyroxene and silicate melt

We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients (D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452–454] we obtain results for the strain-free partition coefficients of theoretical cations (D₀), with site radius r₀, and for the site's Young's Modulus (E).

In agreement with earlier data our results show that increasing ^ivAl concentration in cpx is matched by increasing D, E_M1, E_M2 and D₀ for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing ^ivAl in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D₀, r₀ and E. We have found that D₀ and r₀ decrease with increasing cation charge, e.g. r₀=0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E_M1 and E_M2 increase with cation charge as well as with increasing ^ivAl content. The increase in E_M2 is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation–anion polyhedra. J. Geophys. Res. 84 (10) 6723–6728] for oxides. E_M1 values are much higher and do not fit the trend predicted by the Hazen and Finger relationship.     

The electrical conductivity of some hydrous and anhydrous molten silicates as a function of temperature and pressure

Electrical conductivity of the following molten silicate systems (in mol%): 60SiO₂-40Na₂O; 65SiO₂-35Na₂O; 75SiO₂-25Na₂O; 78SiO₂-22Na₂O; 72SiO₂-24Na₂O-4CaO; 66SiO₂-19Na₂O-15H₂O; and an anhydrous and hydrous (4 wt% H₂O) Mt. Erebus lava, have been measured as a function of temperature (to 1000°C) and pressure (to 1.3 kbar). The anhydrous soda-rich melts have a positive pressure coefficient of conductivity to ~200 bars and beyond this pressure the pressure coefficient is small and negative. Addition of water lowers the conductivity and gives rise to a negative pressure coefficient at the highest temperatures. The conductivity of hydrous Mt. Erebus lava passed through a maximum with increasing temperature at constant pressure. These phenomena are interpreted in terms of explanations of similar phenomena found in molten salt and aqueous electrolyte solutions.     

The effect of liquid composition on the partitioning of Ni between olivine and silicate melt

We report the results of experiments designed to separate the effects of temperature and pressure from liquid composition on the partitioning of Ni between olivine and liquid, \(D_{\text{Ni}}^{\text{ol/liq}}\). Experiments were performed from 1300 to 1600 °C and 1 atm to 3.0 GPa, using mid-ocean ridge basalt (MORB) glass surrounded by powdered olivine in graphite–Pt double capsules at high pressure and powdered MORB in crucibles fabricated from single crystals of San Carlos olivine at one atmosphere. In these experiments, pressure and temperature were varied in such a way that we produced a series of liquids, each with an approximately constant composition (~12, ~15, and ~21 wt% MgO). Previously, we used a similar approach to show that \(D_{\text{Ni}}^{\text{ol/liq}}\) for a liquid with ~18 wt% MgO is a strong function of temperature. Combining the new data presented here with our previous results allows us to separate the effects of temperature from composition. We fit our data based on a Ni–Mg exchange reaction, which yields \(\ln \left( {D_{\text{Ni}}^{\text{molar}} } \right) = \frac{{ -\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{RT} + \frac{{\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{R} - \ln \left( {\frac{{X_{\text{MgO}}^{\text{liq}} }}{{X_{{{\text{MgSi}}_{ 0. 5} {\text{O}}_{ 2} }}^{\text{ol}} }}} \right).\) Each subset of constant composition experiments displays roughly the same temperature dependence of \(D_{\text{Ni}}^{\text{ol/liq}}\) (i.e.,\(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\)) as previously reported for liquids with ~18 wt% MgO. Fitting new data presented here (15 experiments) in conjunction with our 13 previously published experiments (those with ~18 wt% MgO in the silicate liquid) to the above expression gives \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 3641 ± 396 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 1.597 ± 0.229. Adding data from the literature yields \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 4505 ± 196 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 2.075 ± 0.120, a set of coefficients that leads to a predictive equation for \(D_{\text{Ni}}^{\text{ol/liq}}\) applicable to a wide range of melt compositions. We use the results of our work to model the melting of peridotite beneath lithosphere of varying thickness and show that: (1) a positive correlation between NiO in magnesian olivine phenocrysts and lithospheric thickness is expected given a temperature-dependent \(D_{\text{Ni}}^{\text{ol/liq}} ,\) and (2) the magnitude of the slope for natural samples is consistent with our experimentally determined temperature dependence. Alternative processes to generate the positive correlation between NiO in magnesian olivines and lithospheric thickness, such as the melting of olivine-free pyroxenite, are possible, but they are not required to explain the observed correlation of NiO concentration in initially crystallizing olivine with lithospheric thickness.     

Structural control on bulk melt properties: Single and double quantum Si NMR spectroscopy on alkali-silicate glasses

The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15-50 mol% alkali oxide) was analyzed by ²⁹Si single quantum and double quantum MAS NMR spectroscopy. Their glass transition temperatures (T_g) were measured by calorimetry. The chemical shifts and the relative abundance of Qⁿ species correlate with the cationic field strength (Z/r) of the network modifier. A correlation is observed between T_g and the inverse of the entropy of mixing of the different Qⁿ species, which is explained in the framework of the Adam-Gibbs relaxation theory. At high alkali content, up to 44% of the SiO₄ tetrahedra are part of three-membered rings. At a given alkali content, the abundance of these rings increases with increasing cation size. The abundance of three-membered rings in K-silicate melts correlates with a temperature and a non-linear composition dependence of the heat capacity. It is also a possible cause for the anomalous volumetric behavior of potassium silicate glasses.     

The effect of water activity on the oxidation and structural state of Fe in a ferro-basaltic melt

Experimental investigations have been performed at T = 1200°C, P = 200 MPa and fH₂ corresponding to H₂O-MnO-Mn₃O₄ and H₂O-QFM redox buffers to study the effect of H₂O activity on the oxidation and structural state of Fe in an iron-rich basaltic melt. The analysis of Mössbauer and Fe K-edge X-ray absorption nearedge structure (XANES) spectra of the quenched hydrous ferrobasaltic glasses shows that the Fe³⁺/ΣFe ratio of the glass is directly related to aH₂O in a H₂-buffered system and, consequently, to the prevailing oxygen fugacity (through the reaction of water dissociation H₂O ↔ H₂ + 1/2 O₂). However, water as a chemical component of the silicate melt has an indistinguishable effect on the redox state of iron at studied conditions. The experimentally obtained relationship between fO₂ and Fe³⁺/Fe²⁺ in the hydrous ferrobasaltic melt can be adequately predicted in the investigated range by the existing empiric and thermodynamic models. The ratio of ferric and ferrous Fe is proportional to the oxygen fugacity to the power of ∼0.25 which agrees with the theoretical value from the stoichiometry of the Fe redox reaction (FeO + ¼ O₂ = FeO_1.5). The mean centre shifts for Fe²⁺ and Fe³⁺ absorption doublets in Mössbauer spectra show little change with increasing Fe³⁺/ΣFe, suggesting no significant change in the type of iron coordination. Similarly, XANES preedge spectra indicate a mixed (C3h, Td, and Oh, i.e., 5-, 4-, and sixfold) coordination of Fe in hydrous basaltic glasses.