...
  • 主题
高级搜索  >
历史搜索 清空历史
首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Thermal constrains on the survival of primitive blobs in the lower mantle
【24h】

Thermal constrains on the survival of primitive blobs in the lower mantle

机译：下地幔原始斑点存活的热约束

获取原文
获取原文并翻译 | 示例
           
页面导航
  • 摘要
  • 著录项
  • 相似文献
  • 相关主题

摘要

Geochemical models have frequently divided the mantle into depleted upper and undepleted lower mantle reservoirs, usually taken as indication for a layered style of convection. This is difficult to reconcile with seismological and geodynamical evidence for substantial mass flux between lower and upper mantle. Various models have been proposed to jointly interpret the evidence, including that of G.F. Davies [J. Geophys. Res. 89 (1984) 6017-6040] in which the author suggested that lumps of primitive material may exist in the lower mantle, representing reservoirs for undepleted basalts. Mixing calculations have suggested, however, that such blobs could not survive 4 Ga of convection. Calculations by M. Manga [Geophys. Res. Lett. 23 (1996) 403-406] on the other hand showed that high-viscosity blobs could persist in convective cells for geologically long times without being substantially deformed and mixed with the surrounding flow. We investigate a blob model of convection based on these ideas and consider dynamical, thermal, geochemical and rheological consequences. The radiogenic heat production in the primitive blobs would lead to higher temperatures. However, these would be modest (#DELTA#T < 300 K) for sufficiently small blobs (radius < 800 km). The resulting thermal buoyancy can be offset by a small intrinsic density excess (< 1%) so that blob material is hidden from the ridges but sampled by rising plumes. To satisfy geochemical constraints, blobs would have to fill 30% to 65% of the mantle (less if they are taken to be enriched rather than primitive). Thermal considerations require that they be surrounded by depleted material of lower viscosity that would convectively transport heat to the surface. The thermal decrease in blob viscosity would be about one order of magnitude but constrained to the interior; the stiffer 'shell' can then be expected to control the dynamical mixing behavior. On average, the viscosity of the lower mantle would be increased by the presence of the blobs; if they were 100 times more viscous than the surrounding mantle the net effect would be to increase the effective viscosity approximately 5-fold. The origin of the proposed blobs is an unresolved problem. We suggest that perovskite/magnesiowustite ratio variations could be the reason, which would yield an intrinsic density contrast as well. Blob geometries are at the current resolution limit of global tomographic models, and the trade-off between temperature and compositional effect on seismic wave speeds tends to blur the signal. However, joint P- and S-wave inversions and scattering studies may ultimately approach the necessary precision to detect blobs. Under the simplifying assumptions employed in this paper, we find that the viscous blob model is internally self-consistent and feasible. The model may explain the outstanding problem of incongruous geochemical and geophysical data.
机译：地球化学模型经常将地幔划分为贫化的上地幔储层和未贫化的下地幔储层，通常被用作分层对流形式的指示。这很难与上下地幔之间大量传质的地震学和地球动力学证据相一致。已经提出了各种模型来联合解释证据，包括G.F.的证据。戴维斯[J.地理学。 Res。 89（1984）6017-6040]，其中作者认为下地幔中可能存在原始物质团块，代表未耗尽玄武岩的储层。但是，混合计算表明，此类斑点无法在4 Ga对流中幸存。 M. Manga的计算[地球物理学报。 Res。来吧另一方面，例如，J.Biol.Chem.23（1996）403-406]显示，高粘度斑点可以在对流细胞中在地质上长期存在，而基本上不变形并与周围流混合。我们基于这些思想研究了对流的斑点模型，并考虑了动力学，热学，地球化学和流变学的后果。原始斑点中产生的放射热会导致温度升高。但是，对于足够小的斑点（半径<800 km），这些值将是适中的（＃DELTA＃T <300 K）。产生的热浮力可以被少量的固有密度过高（<1％）所抵消，因此斑点材料从山脊中隐藏起来，但通过上升的羽状流进行采样。为了满足地球化学的限制，斑点必须填充地幔的30％到65％（如果认为是富集而不是原始的话，则更少）。出于热方面的考虑，要求它们被低粘度的耗尽材料包围，该材料会通过对流将热量对流传输到表面。团块粘度的热降低将约为一个数量级，但仅限于内部。然后可以预期更坚硬的“外壳”将控制动态混合行为。平均而言，斑点的存在会增加下地幔的粘度;如果它们的粘性比周围的地幔高100倍，则净效应将是使有效粘度增加大约5倍。所提出的斑点的来源尚未解决。我们建议钙钛矿/镁硅钙石比率的变化可能是原因，这也会产生固有的密度对比。 Blob几何形状处于当前全球层析成像模型分辨率的极限，并且温度和成分对地震波速度的影响之间的权衡往往会使信号模糊。但是，联合的P波和S波反演和散射研究最终可能会达到检测斑点所需的精度。在本文采用的简化假设下，我们发现粘性斑点模型在内部是自洽的且可行的。该模型可以解释地球化学和地球物理数据不一致的突出问题。

著录项

相似文献

  • 外文文献
  • 中文文献
  • 专利
获取原文

客服邮箱：kefu@zhangqiaokeyan.com

京公网安备：11010802029741号 ICP备案号：京ICP备15016152号-6 六维联合信息科技 (北京) 有限公司©版权所有

  • 客服微信

  • 服务号