A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Experiments
A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Experiments
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DOI：
10.1029/2022jb024638
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发表时间：
2022-08
期刊：
影响因子：
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通讯作者：
A. Okamoto;T. Hiraga
中科院分区：
文献类型：
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作者：
A. Okamoto;T. Hiraga
我们对细粒镁橄榄石（Mg2SiO4）+ 10体积%方镁石（MgO）聚集体进行了单轴压缩和晶粒生长实验。这种聚集体是一种独特的多矿物系统，其中次级相中的所有组成元素构成初级矿物相，除了Si，它是初级相中扩散最慢的物种。多矿物聚集体中的晶粒生长，其中第二相的存在导致原生相的晶界钉扎，通过第二相的奥斯特瓦尔德熟化进行。分析了晶粒生长和扩散蠕变速率，发现这两种速率都受到相同晶界扩散率的限制。对于方镁石熟化，包围方镁石颗粒的镁橄榄石颗粒必须变形，使得Si以及Mg和O的扩散对于方镁石熟化是必需的，类似于聚集体的扩散蠕变。大多数岩石是多矿物的，通常含有硅酸盐矿物作为其主相，其中Si是扩散最慢的物质。这导致预测，一个单一的扩散率，这是硅扩散率在大多数情况下，决定了在地球内部的大部分晶粒生长和扩散蠕变率。作为晶粒生长结果的晶粒尺寸以及达到该尺寸所需的时间指示扩散率。扩散蠕变期间的粘度由这些晶粒尺寸和扩散率值确定。我们认为这些发现是一个独特的机会，以估计从地壳到下地幔的粘度。
We conducted uniaxial compression and grain growth experiments on fine‐grained forsterite (Mg2SiO4) + 10 vol% periclase (MgO) aggregates. This aggregate is a unique polymineralic system in which all of the constituent elements in the secondary phase constitute the primary mineral phase, except for Si, which is the slowest diffusing species in the primary phase. Grain growth in polymineralic aggregates, where the presence of a secondary phase results in grain boundary pinning of the primary phase, proceeds via Ostwald ripening of the secondary phase. Grain growth and diffusion creep rates were analyzed, and both rates were found to be limited by the same grain‐boundary diffusivity. For periclase ripening, forsterite grains surrounding the periclase grains must be deformed, such that diffusion of Si as well as Mg and O is necessary for periclase ripening, similarly to diffusion creep of the aggregates. Most rocks are polymineralic and usually contain a silicate mineral as their main phase, in which Si is the slowest diffusing species. This leads to the prediction that a single diffusivity, which is Si diffusivity in most cases, determines the rates of grain growth and diffusion creep in much of the Earth's interior. The grain size, which is the result of grain growth, and the time it takes to achieve that size, indicate the diffusivity. Viscosity during diffusion creep is determined by these grain size and diffusivity values. We view these findings as a unique opportunity to estimate the viscosity from the Earth's crust to the lower mantle.