Xu Man, Jing Zhicheng, Bajgain Suraj K, Mookherjee Mainak, Van Orman James A, Yu Tony, Wang Yanbin
Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, OH 44106.
Department of Earth and Space Sciences, Southern University of Science and Technology, 518055 Shenzhen, Guangdong, China.
Proc Natl Acad Sci U S A. 2020 Aug 4;117(31):18285-18291. doi: 10.1073/pnas.2004347117. Epub 2020 Jul 20.
Deeply subducted carbonates likely cause low-degree melting of the upper mantle and thus play an important role in the deep carbon cycle. However, direct seismic detection of carbonate-induced partial melts in the Earth's interior is hindered by our poor knowledge on the elastic properties of carbonate melts. Here we report the first experimentally determined sound velocity and density data on dolomite melt up to 5.9 GPa and 2046 K by in-situ ultrasonic and sink-float techniques, respectively, as well as first-principles molecular dynamics simulations of dolomite melt up to 16 GPa and 3000 K. Using our new elasticity data, the calculated V/V ratio of the deep upper mantle (∼180-330 km) with a small amount of carbonate-rich melt provides a natural explanation for the elevated V/V ratio of the upper mantle from global seismic observations, supporting the pervasive presence of a low-degree carbonate-rich partial melt (∼0.05%) that is consistent with the volatile-induced or redox-regulated initial melting in the upper mantle as argued by petrologic studies. This carbonate-rich partial melt region implies a global average carbon (C) concentration of 80-140 ppm. by weight in the deep upper mantle source region, consistent with the mantle carbon content determined from geochemical studies.
深度俯冲的碳酸盐岩可能导致上地幔发生低度熔融，从而在深部碳循环中发挥重要作用。然而，由于我们对碳酸盐熔体弹性性质了解不足，在地球内部直接通过地震探测碳酸盐诱发的部分熔融受到阻碍。在此，我们分别通过原位超声技术和沉浮法，首次实验测定了白云石熔体在高达5.9吉帕和2046开尔文条件下的声速和密度数据，以及通过第一性原理分子动力学模拟得到了白云石熔体在高达16吉帕和3000开尔文条件下的数据。利用我们新得到的弹性数据，计算出深部上地幔（约180 - 330千米）含有少量富碳酸盐熔体时的V/V比值，这为全球地震观测中上地幔V/V比值升高提供了一种自然解释，支持了存在普遍的低度富碳酸盐部分熔融（约0.05%）的观点，这与岩石学研究中提出的上地幔挥发性诱导或氧化还原调节的初始熔融相一致。这个富碳酸盐部分熔融区域意味着深部上地幔源区的全球平均碳（C）浓度按重量计为80 - 140 ppm，与地球化学研究确定的地幔碳含量一致。
