Abstract A number of possible hypotheses have been proposed to explain the origin of mid-lithospheric discontinuities (MLDs), typically characterized by ∼2-6% reductions in seismic shear wave velocity (VS) at depths of 60 km to ∼150 km in the cratonic sub-continental lithospheric mantle (SCLM). One such hypothesis is the presence of low-shear wave velocity, hydrous and carbonate mineral phases. Although, the presence of hydrous silicates and carbonates can cause a reduction in the shear wave velocity of mantle domains, the contribution of volatile metasomatism to the origins of MLDs has remained incompletely evaluated. To assess the metasomatic origin of MLDs, we compiled experimental phase assemblages, phase proportions, and phase compositions from the literature in peridotite + H2O, peridotite + CO2, and peridotite + H2O + CO2 systems at P-T conditions where hydrous silicate and/or carbonate minerals are stable. By comparing the experimental assemblages with the compiled bulk peridotite compositions for cratons, we bracket plausible proportions and compositions of hydrous silicate and carbonate mineral phases that can be expected in cratonic SCLMs. Based on the CaO and K2O contents of cratonic peridotite xenoliths and the estimated upper limit of CO2 content in SCLM, ≤∼10 vol.% pargasitic amphibole, ≤∼2.1 vol.% phlogopite and ≤∼0.2 vol.% magnesite solid solution can be stable in the SCLM. We also present new elasticity data for the pargasite end member of amphibole based on first principles simulations for more accurate estimates of aggregate VS for metasomatized domains in cratonic mantle. Using the bracketed phase compositions, phase proportions, and updated values of elastic constants for relevant mineral end members, we further calculate aggregate VS at MLD depths for three seismic stations in the northern continental U.S. Depending on the choice of background wave speeds of unmetasomatized peridotite and the cratonic geotherm, the composition and abundance of volatile-bearing mineral phases bracketed here can explain as much as 2.01 to 3.01% reduction in VS. While various craton formation scenarios allow formation of the amphibole and phlogopite abundances bracketed here, presence of volatile-bearing phases in an average cratonic SCLM composition cannot explain the entire range of velocity reductions observed at MLDs. Other possible velocity reduction mechanisms thus must be considered to explain the full estimated range of shear wave speed reduction at MLD depths globally.

中文翻译：

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评估克拉通地幔中含挥发分矿物相的存在，作为中岩石圈不连续性的可能原因

摘要 已经提出了许多可能的假设来解释中岩石圈不连续性 (MLD) 的起源，其典型特征是地震横波速度 (VS) 在 60 公里至约 150 公里的深度处降低约 2-6%。克拉通次大陆岩石圈地幔（SCLM）。其中一种假设是存在低剪切波速度、含水和碳酸盐矿物相。尽管含水硅酸盐和碳酸盐的存在会导致地幔域的剪切波速度降低，但挥发性交代作用对 MLD 起源的贡献仍未得到完全评估。为了评估 MLD 的交代起源，我们从橄榄岩 + H2O、橄榄岩 + CO2、和橄榄岩 + H2O + CO2 系统在 PT 条件下，其中水合硅酸盐和/或碳酸盐矿物是稳定的。通过将实验组合与已编译的克拉通块状橄榄岩成分进行比较，我们确定了在克拉通 SCLM 中可以预期的水合硅酸盐和碳酸盐矿物相的合理比例和组成。根据克拉通橄榄岩捕虏体的 CaO 和 K2O 含量以及 SCLM 中 CO2 含量的估计上限，≤ ～ 10 vol.% 长闪石、≤ ～ 2.1 vol.% 金云母和 ≤ ～ 0.2 vol.% 菱镁矿固溶体在 SCLM 中稳定。我们还基于第一性原理模拟为角闪石的 pargasite 末端成员提供了新的弹性数据，以便更准确地估计克拉通地幔交代域的总 VS。使用括号内的相组成，相比例和相关矿物端部弹性常数的更新值，我们进一步计算了美国大陆北部三个地震台站 MLD 深度处的总 VS 取决于未交代橄榄岩和克拉通地温的背景波速的选择，组成此处包含的大量含挥发物的矿物相可以解释 VS 减少 2.01% 至 3.01% 之多。虽然各种克拉通形成场景允许形成此处包含的角闪石和金云母丰度，但在平均克拉通 SCLM 组成中存在挥发性相不能解释在 MLD 观察到的整个速度降低范围。