Department of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
Proc Natl Acad Sci U S A. 2012 Feb 14;109(7):2275-9. doi: 10.1073/pnas.1109204109. Epub 2012 Feb 1.
The discovery of a phase transition in Mg-silicate perovskite (Pv) to postperovskite (pPv) at lowermost mantle pressure-temperature (P - T) conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core-mantle boundary, hereafter the D('') discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe(2+) and Al for pyrolite increase the thickness (width of the mixed phase region) of the Pv - pPv boundary (400-600 km) to much larger than the D('') discontinuity (≤ 70 km). These results challenge the assignment of the D('') discontinuity to the Pv - pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv → pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv → pPv transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P - T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv - pPv discontinuity. Whereas the pyrolitic mantle may not have a seismologically detectable Pv → pPv transition due to the effect of Al, harzburgitic compositions have detectable transitions due to low Al content. In contrast, Al-rich basaltic compositions may have a detectable Pv - pPv boundary due to their distinct mineralogy. Therefore, the observation of the D('') discontinuity may be related to the Pv → pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.
在最低地幔压力-温度 (P-T) 条件下，镁硅酸盐钙钛矿 (Pv) 向后钙钛矿 (pPv) 的相变的发现，可能为某些地区在核幔边界以上 200 至 400 公里深度范围内发现的剪切波速度的不连续增加提供了解释，此后称为 D('')间断。然而，最近对二元和三元体系的研究表明，对于原始地幔组成的橄榄石，合理含量的 Fe(2+) 和 Al 会使 Pv-pPv 边界的厚度（混合相区域的宽度）增加到远大于 D('')间断（≤70km）的程度。这些结果对 D('')间断在原始地幔（均质地幔组成）中分配给 Pv-pPv 边界提出了挑战。此外，能够容纳可检测到的 Pv→pPv 边界的岩石的矿物学和组成仍不清楚。在这里，我们报告了在最低地幔 P-T 条件下多相体系（圣卡洛斯橄榄石、原始地幔和中洋脊玄武岩组成）中 Pv→pPv 转变的深度和厚度的原位测量结果，寻找具有尖锐 Pv-pPv 间断的候选岩石。由于 Al 的影响，原始地幔可能没有地震学上可检测到的 Pv→pPv 转变，而富含 Al 的地幔组成由于 Al 含量低而具有可检测到的转变。相比之下，富含 Al 的玄武质成分可能具有可检测到的 Pv-pPv 边界，因为它们具有独特的矿物学。因此，D('')间断的观测可能与俯冲板块输送到最低地幔的分异大洋岩石圈物质中的 Pv→pPv 转变有关。
