Grain growth systematics for forsterite ± enstatite aggregates: Effect of lithology on grain size in the upper mantle
Grain growth systematics for forsterite ± enstatite aggregates: Effect of lithology on grain size in the upper mantle
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DOI：
10.1016/j.epsl.2009.12.026
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发表时间：
2010-03-01
影响因子：
5.3
通讯作者：
Sano, Satoru
中科院分区：
文献类型：
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作者：
Hiraga, Takehiko;Tachibana, Chihiro;Sano, Satoru
研究了镁橄榄石（Fo）和顽火辉石（En）在Fo - En细粒集合体中的晶粒生长动力学，考察其作为En体积分数（f(En)=0.00到0.42）的函数。生长速率符合dⁿ∝k·t（d：平均晶粒直径；n：晶粒生长指数；k：生长常数；t：时间），在含顽火辉石的样品中，镁橄榄石和顽火辉石晶粒的n均约为5。k(Fo)和f(En)之间的负相关关系可表示为k(Fo)≈0.06 exp[30f(En)(f(En) - 1.1)]，而对于不同的f(En)，k(En)几乎取恒定值。此外，在晶粒生长过程中d(Fo)/d(En)几乎恒定，其值随An增加而变小，即d(Fo)/d(En)≈0.74/f(En)⁰.⁵⁹。我们获得的晶粒生长参数以及实验样品和天然样品中的微观结构特征都表明，在实验和自然界中，晶粒生长的速率控制过程是Si通过橄榄石晶界的晶界扩散。因此，我们能够提出一个半经验关系d(Ave)/d(dun)≈{exp[30f(px) - 1.1)]}¹/⁵·(1 + 1.35f(px)¹.⁵⁹ - f(px))（d(Ave)：岩石中的平均晶粒尺寸；d(dun)：纯橄榄岩中的平均晶粒尺寸；f(px)：辉石的分数），它可以预测岩性如何影响上地幔中的晶粒尺寸。二辉橄榄岩（辉石含量为40%）中的晶粒尺寸可能比纯橄榄岩（辉石含量为5%）中的小约4倍。这种通过岩性变化导致的晶粒尺寸减小可通过晶粒尺寸敏感蠕变显著影响地幔变形的粘度。（c）2009 Elsevier B.V.保留所有权利。
Grain growth kinetics of forsterite (Fo) and enstatite (En) in fine grain aggregates of Fo En are examined as a function of volume fraction of En (f(En) = 0.00 to 0.42). Growth rates fit d(n)proportional to k . t (d: mean grain diameter; n: grain growth exponent: k: growth constant; t: time), where n similar to 5 for both forsterite and enstatite grains in the enstatite-bearing samples. A negative correlation between k(Fo) and f(En) can be expressed as k(Fo)approximate to 0.06 exp [30 f(En) (f(En) - 1.1)], whereas k(En) takes almost constant values for different f(En). In addition, the ratio of d(Fo)/d(En) is almost constant during grain growth, and its value becomes smaller with increasing An, such that d(Fo)/d(En)approximate to 0.74/f(En)(0.59). Our obtained grain growth parameters and the microstructural characteristics in experimental and natural samples all indicate that the rate-controlling process for grain growth in both experiments and in nature is grain boundary diffusion of Si through grain boundaries of olivine. Thus, we are able to propose a semi-empirical relationship of d(Av)e / d(dun) approximate to {exp[30f(px)-1.1)]}(1/5).(1 + 1.35f(px)(1.59)-f(px)) (d(Ave), : average grain size in the rock; d(dun): average grain size in dunite:f(px):fraction of pyroxenes), which can predict how lithology affects grain size in the upper mantle. Grain size in lherzolite (pyroxenes content of 40%) can be similar to 4 times smaller than that in dunite (pyroxenes content of 5%). Such grain size reduction through change in lithology can significantly affect viscosity of mantle deformation via grain-size sensitive creep. (c) 2009 Elsevier B.V. All rights reserved.