Superplasticity in hydrous melt-bearing dunite: implications for shear localization in Earth's upper mantle
Superplasticity in hydrous melt-bearing dunite: implications for shear localization in Earth's upper mantle
复制标题
含水熔体纯晶岩的超塑性：对地球上地幔剪切局部化的影响
DOI：
10.1016/j.epsl.2012.04.032
复制
发表时间：
2012
影响因子：
5.3
通讯作者：
E.
中科院分区：
文献类型：
--
作者：
Ohuchi;T.;Nishihara;Y.;Kawazoe;T.;Spengler;D.;Shiraishi;R.;Suzuki;A.;Kikegawa;T.;Ohtani;E.
在压力1.3-5.7GPa、温度1270- 1490 K条件下，对含水熔体纯橄榄岩（橄榄石+4vol%斜方辉石+4vol%单斜辉石，熔体含量小于2.5vol%）进行了变形实验，以探讨粒间流体对地球上地幔橄榄石塑性流动的影响。应变速率与稳态蠕变强度的2.1次方成正比，蠕变强度随晶粒尺寸的增大而显著增大。即使样品缩短33-55%，也几乎没有观察到橄榄石晶体学择优取向的发展和橄榄石晶粒的扁平化。这些观察结果表明，晶界滑动（GBS）主导了橄榄石的变形（即，超塑性）。含水熔体纯橄榄岩的蠕变强度比不含熔体纯橄榄岩的蠕变强度低2-5倍。蠕变速率对熔体分数的依赖性已知经验地表示为ε今（ε）=ε今（0）exp（α），其中α是常数，α是熔体分数。实验获得的α值在150-230的范围内，相当于0.3GPa下橄榄石-玄武岩系统的报告值的5-7倍（即，纯橄榄岩的蠕变强度被含水熔体有效地降低）。超塑性是含流体细粒橄榄岩中橄榄石在低温和高应力条件下（即，上地幔橄榄岩剪切带）。地质流体诱发的超塑性在上地幔剪切局部化过程中起着重要作用。
Deformation experiments on hydrous melt-bearing dunite (olivine+4vol% orthopyroxene+4vol% clinopyroxene with less than 2.5vol% of the melt phase) were conducted at pressures of 1.3–5.7GPa and temperatures of 1270–1490K in order to explore the effect of intergranular fluids on the plastic flow of olivine in Earth’s upper mantle. The strain rate was proportional to steady-state creep strength to the 2.1 power, and the creep strength markedly increased with increase in grain size. Developments of the crystallographic preferred orientation of olivine and flattening of olivine grains were hardly observed even after 33–55% shortening of the samples. These observations show that grain boundary sliding (GBS) dominated the deformation of olivine (i.e., superplasticity). The creep strength of hydrous melt-bearing dunite was 2–5 times lower than that of melt-free dunite. The dependence of creep rate on melt fraction is known to be expressed empirically as ε̇(ϕ)=ε̇(0)exp(αϕ), where α is a constant and ϕ is the melt fraction. The experimentally obtained value of α was in the range of 150–230, corresponding to 5–7 times the reported values for the olivine–basalt system at 0.3GPa (i.e., creep strength of dunite was efficiently reduced by the hydrous melt). Superplasticity is the dominant creep mechanism of olivine in fluid-bearing fine-grained peridotites under low-temperature and high-stress conditions (i.e., peridotite shear zones in the upper mantle). Superplasticity induced by geological fluids would play an important role in the shear localization (and thus initiation of subduction) in the upper mantle.