Jellinek A Mark, Manga Michael
Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA.
Nature. 2002 Aug 15;418(6899):760-3. doi: 10.1038/nature00979.
Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers that are laterally heterogeneous. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earth's outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain. Observed zones of 'ultra-low' seismic velocity suggest that this dense layer may contain metals or partial melt, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years.
地震学观测提供了证据，表明下地幔包含叠加的热边界层和成分边界层，这些边界层在横向上是不均匀的。热边界层是由地球外核的热通量形成的，而（本质上致密的）化学边界层的起源仍然不确定。观测到的“超低”地震波速区域表明，这个致密层可能含有金属或部分熔体，因此可以合理地预期致密层具有相对较低的粘度。此外，人们认为热边界层中的不稳定性可能导致地幔柱间歇性形成和上升。流入上升地幔柱的物质会使致密层变形，进而导致致密层逐渐被卷入。在这里，我们通过模拟实验表明，下地幔底部存在致密层会引起温度和粘度的横向变化，进而决定地幔柱的位置和动力学。致密层会使地幔柱在空间上固定下来，而低粘度流体的卷入使地幔柱能够在地球内部持续存在数亿年。
