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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >On the conditions for lower crustal convective instability
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On the conditions for lower crustal convective instability

机译：下地壳对流不稳定的条件

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Continental crust is thought to be formed as a result of are magmatism, but many of the lavas produced in these settings are basaltic, while those that are silicic are commonly evolved, with lower Mg #s than the continental crust. The bulk composition of continental crust can be produced by mixing of end-member basaltic and silicic compositions, via magma mixing or in mechanical, tectonic juxtaposition, but some process is required to remove the cumulates and residues formed during generation of the silicic, "granitic" end-member. We consider convective instability of dense mafic and ultramafic lower crust as a means to remove mafic residues of basalt differentiation in order to produce end-member compositions that can mix to form the bulk composition of the continental crust. Using a range of lower crustal and mantle bulk compositions, ranging from mafic and ultramafic cumulates to primary liquid compositions, we calculated the subsolidus phase assemblage and resulting density. The results show that densities of likely lower crustal lithologies can exceed those of the mantle (by similar to 50-250 kg m(-3)) I but the density contrast is a strong function of composition, temperature, and pressure. For a "cold" geotherm with a Moho temperature of 300 degreesC, relevant to cratonic settings, densities of all of the lower crustal compositions that we considered, except granulite, exceed the density of the underlying mantle at pressures as low as 0.8 GPa. For a "hot" geotherm with a Moho temperature in the range of 800-1000 degreesC, the density of the lower crust is much more variable, with gabbroic and granulite compositions having lower densities than the mantle, while "arc gabbronorite" and ultramafic cumulate compositions having higher densities than the mantle at pressures similar to that for the cold geotherm. Instability times calculated for a two-dimensional Rayleigh-Taylor convective instability, where a dense lower crustal layer sinks into a lower-density mantle, show that high temperatures (>700 degreesC, or >500 degreesC with a background strain rate) are required for this process to occur on a timescale of 10 Myr with theological parameters expected for the crust and mantle. The high temperature required for dense lower crustal mafic-ultramafic cumulates to sink into the mantle suggests that this process is restricted to arcs, volcanic rifted margins, and continental regions that are undergoing extension, are underlain by a mantle plume, or have had part of the conductive upper mantle removed. [References: 72]
机译：大陆壳被认为是岩浆作用的结果，但是在这些环境中产生的许多熔岩是玄武质的，而硅质的则通常被演化出来，其Mg #s低于大陆壳。大陆壳的整体成分可以通过岩浆混合或机械，构造并置混合端元玄武质和硅质成分而产生，但是需要一些工艺来去除硅质“粒状”生成过程中形成的堆积物和残渣。 “最终成员。我们考虑将致密的镁铁质和超镁铁质下地壳的对流不稳定性作为去除玄武岩分化的镁铁质残留物的一种手段，以便产生可以混合形成大陆壳主体成分的端部成分。使用从镁铁质和超镁铁质累积物到初级液体成分的一系列下部地壳和地幔整体成分，我们计算了亚固相相组合及其密度。结果表明，可能较低的地壳岩性的密度可以超过地幔的密度（与50-250 kg m（-3）相似），但密度对比是组成，温度和压力的强函数。对于Moho温度为300摄氏度（与克拉通环境有关）的“冷”地热，在低至0.8 GPa的压力下，我们考虑的所有下部地壳成分（粒粒除外）的密度都超过了下地幔的密度。对于Moho温度在800-1000摄氏度范围内的“热”地热，下地壳的密度变化更大，辉长岩和花岗石成分的密度低于地幔，而“弧辉长石”和超镁铁质的累积在类似于冷地热的压力下，这些组合物的密度高于地幔。为二维瑞利-泰勒对流不稳定性计算的不稳定性时间，其中密集的下地壳层沉入了较低密度的地幔中，这表明需要高温（> 700摄氏度，或具有背景应变率的> 500摄氏度）这个过程发生在10 Myr的时间尺度上，预期地壳和地幔的神学参数。致密的下地壳铁质-超镁铁质颗粒沉入地幔所需的高温表明，这一过程仅限于弧形，火山裂谷边缘和正在伸展的大陆区域，被地幔柱包裹着或有一部分导电的上地幔被拆除。 [参考：72]

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