Flow strengths of quartz aggregates: Grain size and pressure effects due to hydrolytic weakening
Flow strengths of quartz aggregates: Grain size and pressure effects due to hydrolytic weakening
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DOI：
10.1029/jb089ib06p04281
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发表时间：
1984-06
影响因子：
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通讯作者：
A. Kronenberg;J. Tullis
中科院分区：
文献类型：
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作者：
A. Kronenberg;J. Tullis
为了确定粒度和压力对石英岩流变学的影响，粒度在1.2 - 211μm范围内的均密石英岩和石英岩在350 - 1620 MPa的围压下，在恒定位移速率和恒定偏应力条件下进行了实验变形。改变样品可获得的水量，以便在几种名义上相等的水浓度下比较整套样品的流动强度；样品在800°C下真空干燥12小时，保持原样，或者密封在添加了0.03 - 0.4 wt%水的铂套中。在800°C和10⁻⁶/s⁻¹且存在0.4 wt%水的条件下变形的均密石英岩，在350 - 1590 MPa范围内，随着围压的增加，流动强度持续降低。在高围压950 - 1600 MPa下，恒定位移速率实验显示出三种不同的粒度效应，对应于三种水浓度水平：（1）真空干燥样品的“晶界硬化”，（2）原样样品的粒度无关强度，（3）在有水存在下变形的样品的“晶界弱化”。尽管晶界 - 位错相互作用可能导致晶界硬化，晶界滑动导致弱化，但力学数据的细节，结合微观结构观察，与现有的本征晶界效应模型不一致。相反，强度 - 粒度关系被解释为是由于结构结合水的变化引起的，而这种变化又由水在晶界的进出扩散所控制。最后，恒定应力实验表明，蠕变的活化能Q和应力指数n不依赖于粒度，因此观察到的不同强度必须包含在指前项中。然而，Q和n都随着可获得水量的增加而持续降低，从真空加热样品的Q = 300 kJ/mol和n≃4，到添加水的样品的Q = 130 kJ/mol和n = 2.6。
Novaculites and quartzite ranging in grain size from 1.2–211 μm have been experimentally deformed at confining pressures of 350–1620 MPa under conditions of constant displacement rate and constant deviatoric stress in order to determine the effects of both grain size and pressure on the rheology of quartzite. The amount of water available to the samples was varied so that flow strengths for the entire suite of samples could be compared at several, nominally equal water concentrations; samples were vacuum dried at 800°C for 12 hours, left as is, or sealed in Pt jackets with 0.03–0.4 wt % water added. Novaculites deformed at 800°C and 10−6 /s−1 in the presence of 0.4 wt % water show a continuous decrease in flow strength with increasing confining pressure over the range 350–1590 MPa. At high confining pressures 950–1600 MPa, constant displacement rate experiments show three distinct grain size effects, corresponding to the three levels of water concentration: (1) “grain boundary hardening” for vacuum-dried samples, (2) grain size independent strength for as is samples, and (3) “grain boundary weakening” for samples deformed in the presence of water. Although grain boundary-dislocation interactions may lead to grain boundary hardening and grain boundary sliding to weakening, the details of the mechanical data, in combination with microstructural observations, are inconsistent with existing models of intrinsic grain boundary effects. Instead, the strength-grain size relations are interpreted as resulting from variations in structurally incorporated water, which, in turn, are controlled by diffusion of water to and from the grain boundaries. Finally, constant stress experiments show that the activation energy Q and stress exponent n for creep do not depend on grain size, so that the different strengths observed must be incorporated in the preexponential terms. However, both Q and n show a continuous decrease with increasing amounts of available water, from Q = 300 kJ/mol and n ≃ 4 for vacuum-heated samples, to Q = 130 kJ/mol and n = 2.6 for water-added samples.