Approximately two-thirds of Earth's outermost shell is composed of oceanic plates that form at spreading ridges and recycle back to Earth's interior in subduction zones. A series of physical and chemical changes occur in the subducting lithospheric slab as the temperature and pressure increase with depth. In particular, olivine, the most abundant mineral in the upper mantle, progressively transforms to its high-pressure polymorphs near the mantle transition zone, which is bounded by the 410 km and 660 km discontinuities. However, whether olivine still exists in the core of slabs once they penetrate the 660 km discontinuity remains debated. Based on SKS and SKKS shear-wave differential splitting times, we report new evidence that reveals the presence of metastable olivine in the uppermost lower mantle within the ancient Farallon plate beneath the eastern United States. We estimate that the low-density olivine layer in the subducted Farallon slab may compensate the high density of the rest of the slab associated with the low temperature, leading to neutral buoyancy and preventing further sinking of the slab into the deeper part of the lower mantle.

中文翻译：

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美国东部下地幔最上层海洋岩石圈内的亚稳橄榄石

大约三分之二的地球最外壳由海洋板块组成，这些板块在扩张的山脊处形成，并在俯冲带中循环回地球内部。随着温度和压力随深度的增加，俯冲岩石圈板片发生一系列物理和化学变化。特别是，橄榄石是上地幔中最丰富的矿物，在地幔过渡带附近逐渐转变为高压多晶型物，该过渡带以 410 公里和 660 公里不连续面为界。然而，一旦穿过 660 公里的不连续性，橄榄石是否仍然存在于板片的核心仍然存在争议。基于 SKS 和 SKKS 横波差分分裂时间，我们报告了新的证据，表明在美国东部下方的古法拉隆板块的最上层下地幔中存在亚稳态橄榄石。我们估计俯冲的 Farallon 板片中的低密度橄榄石层可以补偿与低温相关的板片其余部分的高密度，导致中性浮力并防止板片进一步下沉到下地幔的较深部分.