Assessing the presence of volatile-bearing mineral phases in the cratonic mantle as a possible cause of mid-lithospheric discontinuities
Assessing the presence of volatile-bearing mineral phases in the cratonic mantle as a possible cause of mid-lithospheric discontinuities
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DOI：
10.1016/j.epsl.2020.116602
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发表时间：
2021
影响因子：
5.3
通讯作者：
S. Saha;Ye Peng;R. Dasgupta;M. Mookherjee;K. Fischer
中科院分区：
文献类型：
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作者：
S. Saha;Ye Peng;R. Dasgupta;M. Mookherjee;K. Fischer
中岩石圈不连续（MLDs）的典型特征是在60 ~ 150 km深度处，地震横波速度（VS）降低了102 ~ 6%。一个这样的假设是存在低剪切波速度，含水和碳酸盐矿物相。虽然，含水硅酸盐和碳酸盐的存在下，可以导致地幔域的剪切波速度的降低，挥发交代的MLD的起源的贡献仍然没有完全评估。为了评估MLDs的交代起源，我们从文献中编译了实验相组合，相比例和相组成，在P-T条件下，在橄榄岩+ H2O，橄榄岩+ CO2和橄榄岩+ H2O + CO2系统中，含水硅酸盐和/或碳酸盐矿物是稳定的。通过比较的实验组合与编译散装橄榄岩组合物的peconons，我们支架合理的比例和组合物的含水硅酸盐和碳酸盐矿物相，可以预期在pecononic SCLM。根据橄榄岩捕虏体的CaO和K2 O含量以及SCLM中CO2含量的估算上限，部分气化角闪石，≤ 2.1 vol. %金云母和≤ 0.2 vol. %菱镁矿固溶体在SCLM中稳定存在。我们还提出了新的角闪石的角闪石端元的弹性数据的基础上第一性原理模拟更准确地估计聚合VS的交代域的地幔。使用括号中的相组成、相比例和相关矿物端元弹性常数的更新值，我们进一步计算了美国大陆北方三个地震台站的MLD深度。含挥发分矿物相的组成和丰度可以解释2.01%~ 3.01%虽然各种克拉通形成情景允许形成角闪石和金云母丰度，在一个平均的超声波SCLM组合物中存在的挥发物轴承相不能解释在MLD处观察到的速度降低的整个范围。因此，必须考虑其他可能的速度降低机制，以解释全球MLD深度处剪切波速度降低的全部估计范围。
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 + CO2systems atP-Tconditions 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 CO2content 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 onfirst principlessimulations for more accurate estimates of aggregate VSfor 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 VSat 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.