含水对富铁橄榄石流变性的影响

橄榄石是下地壳上地幔最丰富的造岩矿物,橄榄石集合体的高温高压流变性对地幔动力学过程有着很大的影响。以往对橄榄石单晶和集合体进行过的流变性实验研究,涉及了温度、压力、氧逸度、含铁量以及含水等因素的影响,本文研究了含水对富铁橄榄石集合体流变性能的影响,首先,利用高精度 Paterson 气体介质变形装置对含水富铁橄榄石集合体(Mg_(0.9),Fe_(0.1))_2SiO_4、(Mg_(0.7),Fe_(0.3))_2SiO_4和(Mg_(0.5),Fe_(0.5))_2SiO_4进行了高温三轴压缩蠕变试验,变形试验条件为:温度1323～1473K,围压300MPa,差应力10～300MPa,应变率10~(-7)～10~(-4)s~(-1),每个试件的最大变形量为20%。利用三维非线性拟合方法对蠕变试验数据进行分析,结果表明,含水富铁橄榄石集合体的微观变形机制为扩散域和位错域,对三种铁含量,扩散域的应力指数为1。位错蠕变域中,Fo_(50)应力指教为3.8,活化能为444kJ/mol;Fo_(70)应力指数为3.7,活化能为479kJ/mol,Fo_(90)应力指数为3.6,活化能为514kJ/mol。将实验结果与不含水富铁橄榄石集合体的强度进行对比,发现含水使富铁橄榄石的强度明显下降,舍铁量相同时,含水橄榄石集合体的流变强度比干燥时小至少一个数量级。从而得到了含水对含铁量不同的橄榄石集合体粘性(强度)影响的初步试验结果。     

铁橄榄石高温高压流变性的实验研究

通过高温高压实验研究了铁橄榄石集合体的流变性.首先,利用高精度的Paterson气体介质变形装置对铁橄榄石集合体Fe2SiO4进行了高温三轴压缩蠕变试验.变形试验条件为:温度1273～1423K,围压300MPa,差应力10～250MPa,应变率10-7～10-4s-1,试件的最大变形量不超过20%.利用三维非线性拟合方法对蠕变试验数据进行分析,得到铁橄榄石集合体的微观变形机制为扩散域和位错域,扩散域的应力指数为1左右.位错蠕变域中,干燥铁橄榄石集合体的应力指数为5.4.活化能为781kJ/mol;含水铁橄榄石集合体的应力指数为3.4,活化能为516kJ/mol.实验结果表明,含水时铁橄榄石的流变强度比干燥时小一个数量级.将实验结果与铁橄榄石单晶的强度进行对比,发现铁橄榄石集合体的流变强度比铁橄榄石单晶的强度高.从而得到了铁橄榄石集合体高温高压流变性(强度)的初步试验结果.     

橄榄石集合体扭转大变形实验研究

对(Mg0.5,Fe0.5)2SiO4和(Mg0.7、Fe0.3)2SiO4橄榄石集合体进行了高温高压扭转大变形实验研究.试验在高精度和高分辨率Paterson气体介质变形试验机上进行,试验温度为1473K,围压为300MPa,应变率为10-4～10-5S-1,剪切应力为82～124MPa,剪切应变为～400%,将变形后试件沿轴向切开进行了显微结构分析,确定了不同纵截面上橄榄石的颗粒尺寸.由蠕变数据拟合出的流动律中,应力指数为与三轴压缩实验给出的应力指数吻合.利用EBSD方法分别对变形前后试件中橄榄石的晶格最优取向(LPO)进行了测定.在富铁橄榄石样品中观测到的最优晶格取向表明滑动属于(okl)[100]滑动系统.这个结果与Bystficky等人在Fo90种观测到的最优晶格取向特征是一致的.     

大变形橄榄石动态重结晶的实验观测研究

本文利用高温高压大变形扭转实验,对富铁橄榄石大变形情况下的动态重结晶和颗粒尺度变化的特点进行了实验观测分析.富铁橄榄石集合体由纯铁橄榄石Fa100和圣卡罗橄榄石Fa10混合物经人工合成得到,试件的几何形状为圆柱体,直径9.6mm,高度4.86mm.实验在常角速率条件下进行,温度为1473K,围压为300MPa,剪应力为72 ～ 99MPa,剪应变率为1.35×10-4～3.11×10-4s-1,累积剪应变为3.98.对变形后样品的微结构进行了光学和扫描电镜的观测分析,得到橄榄石颗粒随应变增加而产生动态重结晶的具体物理图像,由EBSD观测结果给出了晶格取向随外加应变增加而产生的变化过程,利用两种统计方法得到了颗粒尺寸随应变增加而变化的过程,探讨了橄榄石动态重结晶的微观机制.与已有三轴压缩、简单剪切以及圣卡罗橄榄石的扭转变形结果进行了对比分析,同时对实验结果在地球物理方面的应用进行了讨论.     

玄武质岩石中橄榄石斑晶的NiO-Fo关系—原始岩浆组分的指示剂

橄榄石是幔源原始岩浆中最早结晶的一种矿物,而且其化学成分可以提供源区橄榄岩性质的信息。地幔橄榄岩中橄榄石的100Mg/(Mg+Fe)值(Fo)范围较宽,从94到85,而NiO的含量约为0.4％(wt,以下同)。橄榄石Fo值的微小差异导致对原始岩浆MgO含量估计的较大不同。例如,原始的洋中脊玄武岩中橄榄石的Fo值相当于92、90和88,而MgO的含量分别约为16、12.5和10％。因此,在估计原始岩浆组分时,关键在于得到源区中橄榄石Fo的精确值。     

压力对橄榄石流变性的影响

本文给出细粒橄榄石集合体在位错蠕变域中压力效应的实验结果。对干燥、人工合成的橄榄石集合体进行了三轴压缩实验,试验在一台高分辨率的气体介质试验机上进行。典型的常载荷蠕变试验温度为1473K或1523K,围压为400MPa、200MPa、400MPa、200MPa。当差压力和温度给定时,试件在400MPa围压下,变形速度较200MPa围压下要慢。实验得出的细粒橄榄石集合体在位错蠕变域中的活化体积值为(23±3)×10~(-6)m~3/mol。最后,对实验结果在地幔动力学中的应用进行了讨论。     

赣杭构造带龙游地区早白垩世基性岩矿物化学、岩石地球化学特征及其地质意义

通过龙游晚中生代基性岩岩相学观察、全岩主量、微量元素和Sr-Nd同位素及矿物电子探针分析,对岩石成因、岩浆演化和构造环境进行探讨。龙游基性岩岩性为橄榄辉长岩,Sr-Nd同位素显示为幔源特征;主量、微量元素特征显示其经历了以橄榄石、辉石为主的分离结晶作用,且未发生明显的地壳物质混染。橄榄石颗粒具核-边结构,富Mg贫Fe的核部Fo值为90.1～91.8,指示原始岩浆是软流圈地幔,富Fe贫Mg的边部显示橄榄石Fo值为77.4～85.3,且核-边两部分的Fo值相差较大,显示原始橄榄石形成之后受到地幔熔体/流体的交代作用。辉石斑晶大多属于透辉石,在部分斑晶的边部发育少量霓辉石。透辉石斑晶普遍具有核-幔-边结构,从核部到边部的SiO_2含量降低,TiO_2、Al_2O_3含量升高,结晶温度升高,显示原始辉石形成之后受到温度更高的地幔熔体/流体的交代作用。研究认为,古太平洋板块后撤造成岩石圈地幔拉伸作用并形成赣杭构造带深部断裂后,部分软流圈物质受到地幔流体/熔体的交代作用,并沿这些深部断裂向上侵位,经历了以橄榄石和辉石矿物为主的分离结晶作用和微弱的地壳物质混染,最终形成龙游橄榄辉长岩。     

新疆东天山黄山东岩体橄榄石成因意义探讨

黄山东岩体位于新疆东天山造山带中段,由四次岩浆侵入形成:第一次侵入形成了岩体上部的橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体;第二次侵入形成辉长苏长岩,呈岩墙状分布于岩体西端和西北部;第三次侵入岩石为斜长二辉橄榄岩,位于岩体下部,为主要的赋矿岩石;第四次侵入岩石为底部角闪辉长岩。橄榄石为第三次侵入的斜长二辉橄榄岩和第一次侵入的橄榄辉长岩主要造岩矿物之一,橄榄石的镁橄榄石牌号(Fo)值介于68.5～82.5之间。其中含硫化物斜长二辉橄榄岩中的橄榄石具有较高的Fo值(79.7～82.5);斜长二辉橄榄岩中橄榄石的Fo值为78.3～79.9;而基性程度较低的橄榄辉长岩中橄榄石具有较低的Fo值(68.5～72.2)。利用橄榄石矿物成分计算得出黄山东岩体母岩浆Mg#(Mg2+/(Mg2++Fe2+))为0.59,为原生玄武质岩浆经历结晶分异作用形成。模拟计算结果显示黄山东岩体不含矿岩石中橄榄石是母岩浆经过2%的橄榄石结晶分异且硫达到饱和后,在硫化物熔离的同时岩浆发生橄榄石结晶而形成,并且橄榄石︰硫化物≈50︰1,部分橄榄石成分投点在橄榄石结晶和硫化物熔离的模拟曲线右下侧,指示它们可能受到晶间硅酸盐熔浆作用的影响。含硫化物斜长二辉橄榄岩中Fo值与Ni含量呈负相关关系,说明橄榄石与硫化物熔体之间发生了Fe-Ni交换反应。     

锗橄榄石-尖晶石扭转大变形实验研究

对(Mg，Ni)2GeO4尖晶石一橄榄石集合体进行了高温高压扭转大变形实验研究。试验在一台高分辨率的气体介质试验机上进行，试验温度为1473K，围压为300MPa，应变率为10^-4～10^-5S^-1，剪切应力为80～250MPa，剪切应变为30％～700％。将变形后试件沿轴向切开进行了显微结构分析，确定了两相成分的比值。利用EBSD方法分别对变形前后试件中的橄榄石和尖晶石中的晶格最优取向(LPO)进行了测定，由微结构的差异确定了变形机制及两相材料的相对强度。     

一种改进的简单的估算原始岩浆的方法

利用橄榄石和熔浆的Fe—Mg分配系数,制作了MgO—FeO演化图解。根据MgO—FeO体系中Fo橄榄石的Fo值:Fo=n(Mg)÷n(Mg)+n(Fen(Mg)/n(Mg)+n(Fe))等值线与Fe—Mg演化曲线交点确定原始岩浆成分。该图解优点在于可以更简单、直观地判别原始岩浆成分,适用于橄榄石分离结晶体系。同时介绍了该图解的使用方法,并以峨眉山大火成岩省丽江地区的大具与仕满两个剖面的苦橄岩为例,说明如何应用该图解来恢复原始岩浆以及解释岩石的成因。研究结果表明,仕满地区原始岩浆MgO、FeO含量分别为23.5%和12.8%,部分熔融程度较高,SM-14和SM-15是由仕满原始岩浆经过轻度橄榄石分离结晶形成的,基本可以代表原始岩浆成分。大具地区原始岩浆可分为两类,一类部分熔融程度较低,MgO、FeO含量分别为19.8%和11.3%;另一类部分熔融程度较高,MgO、FeO含量分别为23%和13.3%,与仕满原始岩浆成分类似。大具地区的岩石样品成分均不能代表该地区的原始岩浆成分,而是经历了明显的橄榄石分离结晶作用。另外,峨眉山大火成岩省中绝大多数的玄武岩中CaO含量不符合橄榄石分离结晶关系,并且MgO含量一般都低于8%,以及岩石中普遍出现单斜辉石和斜长石矿物等都表明这些玄武岩不可能由地幔部分熔融形成的原始岩浆直接通过橄榄石分离结晶作用形成。     

Extracting olivine (Fo–Fa) compositions from Raman spectral peak positions

The dominant feature of the olivine Raman spectrum is a doublet that occurs in the spectral region of 815–825 cm⁻¹ (DB1) and 838–857 cm⁻¹ (DB2). These features arise from coupled symmetric and asymmetric stretching vibrational modes of the constituent SiO₄ tetrahedra. The frequencies of both peaks show monotonic shifts following cation substitution between forsterite and fayalite. We present a calibration for extracting olivine Fo contents (Fo = Mg/(Mg + Fe) molar ratio; Fo_0–100) from the peak positions of this doublet, permitting estimates of chemical composition from Raman spectra (acquired in the laboratory or field) as well as providing information on crystal structure (distinction of polymorphs). Eight samples spanning the compositional range from forsterite to fayalite were used to develop the calibration equations for the DB1 and DB2 peaks individually and together. The data indicate that the DB1 peak is more reliable for calculating the compositions of Fe-rich olivine but that the DB2 peak is better for magnesian compositions. The two-peak calibration overcomes the limitations of the single-peak calibrations and is capable of calculating olivine compositions to within ±10 Fo units.     

Partition of Ni between olivine and sulfide: the effect of temperature,f_{{\text{O}}_{\text{2}} } and f_{{\text{S}}_{\text{2}} }

The experimental distribution coefficient for Ni/ Fe exchange between olivine and monosulfide (K_D3) is 35.6±1.1 at 1385° C, \(f_{{\text{O}}_{\text{2}} } = 10^{ - 8.87} ,f_{{\text{S}}_{\text{2}} } = 10^{ - 1.02} \) , and olivine of composition Fo96 to Fo92. These are the physicochemical conditions appropriate to hypothesized sulfur-saturated komatiite magma. The present experiments equilibrated natural olivine grains with sulfide-oxide liquid in the presence of a (Mg, Fe)-alumino-silicate melt. By a variety of different experimental procedures, K _D3 is shown to be essentially constant at about 30 to 35 in the temperature range 900 to 1400° C, for olivine of composition Fo97 to FoO, monosulfide composition with up to 70 mol. % NiS, and a wide range of \(f_{{\text{O}}_{\text{2}} } \) and \(f_{{\text{S}}_{\text{2}} } \) .     

High-temperature creep in a Ni2GeO4: a contribution to creep systematics in spinel

 High-temperature creep behavior in Ni₂GeO₄ spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4 microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to 1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following flow laws: where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion (Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less than 2 μm. Received: 18 June 2000 / Accepted: 3 April 2001     

橄榄石集合体的简单剪切形变实验研究

本文利用高精度Paterson气体介质变形装置对富铁橄榄石集合体(Mg_(0.5),Fe_(0.5))_2SiO_4进行了高温简单剪切变形试验.试验在温度1473K和围压300MPa的条件下进行,差应力为64～153MPa,应变率为10~-5～10~-3s~-1.一共进行了三组试验,试件的剪切变形量分别为89%,131%和200%,通过对变形后试件的反射光学显微结构分析,得到富铁橄榄石集合体动态重结晶的微观机制,由结晶各向异性分析给出晶格最优取向和波速各向异性的分布及随应变的增加而表现出的演化特征.     

Development of orthopyroxene-Fe/Mg ferrite symplectites by continuous olivine oxidation

The development of orthopyroxene-Fe/Mg ferrite symplectites associated with olivine is discussed with respect to the chemical reactions by which they form. Previously proposed reactions are presented graphically and the differences between them are reviewed. With the exception of exsolution, these are all discontinuous reactions in the sense that olivine is replaced by the two-phase symplectite assemblage.Olivine-hosted symplectites developed in the margins of lherzolite xenoliths from Kauai, Hawaii, demonstrate a reaction mechanism which has not been previously documented from natural samples. Original Fo₉₀ olivine in these samples oxidized to a new assemblage consisting of orthopyroxene (En_92–95)-Fe/Mg ferrite (Mf_35–50) symplectites developed within more magnesian olivine (Fo_92–96) hosts. Thus, by this mechanism, olivine of a different composition persists as part of a final three-phase assemblage. As oxidation advanced, the compositions of all three product phases became continuously more magnesian and the stoichiometric coefficients of the orthopyroxene and Fe/Mg ferrite continuously increased, whereas those of the product olivine decreased in the mass-balance equations. These characteristics demonstrate that the reaction was controlled by oxygen diffusion into the xenoliths from the highly oxidized alkali picrite melt in which they were entrained. Thermodynamic calculations suggest that a gradient in oxygen fugacity of 10^0.9 bars existed across the xenolith rims and resulted in compositional gradients of 4 mol% fayalite and ferrosilite and 15 mol% magnetite.     

The rheology of olivine and spinel magnesium germanate (Mg2GeO4): TEM study of the defect microstructures

Synthetic polycrystals of α-Mg₂GeO₄ (with the olivine structure) and γ-Mg₂GeO₄ (with the spinel structure) deformed at high temperature and pressure in their respective stability fields were investigated by analytical transmission electron microscopy. Specimens with a mean grain size of 20–30 µm deform by dislocation glide and/or climb. The predominance of glide versus climb depends on stress and grain orientation. The defect microstructures of both polymorphs are very similar to those observed in their respective silicate analogues, α- and γ-(Mg,Fe)₂SiO₄, and, in the case of the spinel phase, very similar to those observed in magnesium aluminate spinels. These observations suggest that Mg₂GeO₄ is a good rheological analogue for the Earth’s upper mantle. A spinel specimen deformed under the same conditions of temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. In specimens of both phases deformed at or above 1400 K, a thin amorphous film composed of Mg, Ge, and O was detected along some grain boundaries. Grains ≤10 µm diameter surrounded by a film of amorphous phase (>10 nm thick) exhibited low dislocation densities, and deformation appeared to have occurred by grain boundary sliding.     

Thermal infrared (vibrational) spectroscopy of Mg–Fe olivines: A review and applications to determining the composition of planetary surfaces

The reststrahlen features in thermal infrared, or vibrational, spectra of Mg-Fe olivines ((Mg,Fe)₂SiO₄) exhibit trends in position, strength, and number of features that are diagnostic of the relative proportions of the Mg and Fe cations in the minerals. Although band positions move to lower wavenumbers (longer wavelengths) across the forsterite–fayalite compositional binary in a generally linear manner, specific feature shifts in transmittance data are described best by two linear fits with a break in slope near Fo₇₀. The break in slope may be accompanied by an offset as well; both traits are attributed to structural changes in olivine brought about by distortion of the crystal lattice by Fe. Reflectance and emissivity spectra exhibit similar trends in band position with composition, and all three types of data demonstrate that some olivine band strengths change across the Mg–Fe solid solution series and also are diagnostic of composition. Olivines have been identified in a wide array of thermal infrared spectra of planetary materials and have been interpreted as being present on the surfaces of Mercury, the Moon, Mars, and a number of asteroids based on the analysis of thermal infrared spectra. New linear least squares models of the emissivity spectra of olivine-bearing Martian meteorites enable a preliminary estimation of the accuracy with which quantitative estimates of olivine abundance and solid solution composition can be derived from the spectra of mixtures.     

The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep

We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800°C and at strain rates between 10⁻⁶ and 10⁻³ s⁻¹. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation.