Authors: Sriparna Saha, Ye Peng, Rajdeep Dasgupta, Mainak Mookherjee, Karen M. Fischer
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 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 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.
Saha, S., Peng, Y., Dasgupta, R., Mookherjee, M. & Fischer, K. M. (2021). Assessing the presence of volatile-bearing mineral phases in the cratonic mantle as a possible cause of mid-lithospheric discontinuities. Earth and Planetary Science Letters 553, 116602. doi:10.1016/j.epsl.2020.116602