Sakamaki Tatsuya, Suzuki Akio, Ohtani Eiji
Department of Earth and Planetary Material Sciences, Tohoku University, Sendai 980-8578, Japan.
Nature. 2006 Jan 12;439(7073):192-4. doi: 10.1038/nature04352.
Seismological observations have revealed the existence of low-velocity and high-attenuation zones above the discontinuity at 410 km depth, at the base of the Earth's upper mantle. It has been suggested that a small amount of melt could be responsible for such anomalies. The density of silicate melt under dry conditions has been measured at high pressure and found to be denser than the surrounding solid, thereby allowing the melt to remain at depth. But no experimental investigation of the density of hydrous melt has yet been carried out. Here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410-km discontinuity. We infer that hydrous magma formed by partial melting above the 410-km discontinuity may indeed be gravitationally stable, thereby supporting the idea that low-velocity or high-attentuation regions just above the mantle transition zone may result from the presence of melt.
地震学观测揭示，在地球上地幔底部410千米深度的间断面上方存在低速和高衰减区域。有人认为，少量的熔体可能是造成这种异常现象的原因。在高压下测量了干燥条件下硅酸盐熔体的密度，发现其比周围的固体更致密，从而使得熔体能够留在深部。但是，尚未对含水熔体的密度进行实验研究。在此，我们给出了在压力下限制含水玄武质熔体密度的数据，以检验410千米间断面上方熔体的稳定性。我们推断，在410千米间断面上方通过部分熔融形成的含水岩浆可能确实在重力作用下是稳定的，从而支持了这样一种观点，即地幔过渡带正上方的低速或高衰减区域可能是熔体存在的结果。
