Nature2014-06-18 5:08 PM

地幔熔融物有利于构造板块运动 Electrical conductivity during incipient melting in the oceanic low-velocity zone

论文摘要 

以确定坚硬的构造板块怎样在较软的地幔之上穿过地球表面运动为目的的研究工作,关注岩石圈-软流圈边界上发生的事件以及一个低粘度层是怎样在那里产生的。在这篇论文中,David Sifré等人发表了对富含二氧化碳和水的硅酸盐熔融物(它们被认为是在地幔熔化过程开始时产生的)的电导性所做的测量。他们发现,当水和二氧化碳含量适中时,电导性会稍有增加,但当熔融物中二氧化碳含量的重量比超过6%时,电导性会急剧增加。他们的结论是,这种熔融物对在地球海洋软流圈上层观测到的低地震波速度和高电导性都有可能做出解释.

Abstract 

The low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics. The seismic low velocities and the high electrical conductivities of the asthenosphere are attributed either to subsolidus, water-related defects in olivine minerals or to a few volume per cent of partial melt, but these two interpretations have two shortcomings. First, the amount of water stored in olivine is not expected to be higher than 50 parts per million owing to partitioning with other mantle phases(including pargasite amphibole at moderate temperatures) and partial melting at high temperatures9. Second, elevated melt volume fractions are impeded by the temperatures prevailing in the asthenosphere, which are too low, and by the melt mobility, which is high and can lead to gravitational segregation. Here we determine the electrical conductivity of carbon-dioxide-rich and water-rich melts, typically produced at the onset of mantle melting. Electrical conductivity increases modestly with moderate amounts of water and carbon dioxide, but it increases drastically once the carbon dioxide content exceeds six weight per cent in the melt. Incipient melts, long-expected to prevail in the asthenosphere, can therefore produce high electrical conductivities there. Taking into account variable degrees of depletion of the mantle in water and carbon dioxide, and their effect on the petrology of incipient melting, we calculated conductivity profiles across the asthenosphere for various tectonic plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (more than five million years old), incipient melts probably trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas in young plates, where seamount volcanism occurs, a higher degree of melting is expected.

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