We determined the single-crystal elasticity of Fe3+-bearing grossular using Brillouin scattering up to 22 GPa and 1,000 K. Using the obtained results, we modeled mantle velocity profiles to investigate the origin of low-velocity zones (LVZs) in the bottom 100 km of the mantle transition zone. Our modeling revealed that although a water-saturated transition zone could be seismically detected as the LVZs, it would also cause a negative velocity jump across the 520-km discontinuity, which has not been seismically detected. However, mid-ocean ridge basalts (MORB) with an FeO content of 10–17 wt.% in garnet will have VS 2.0(5)%–3.0(5)% lower than that in the pyrolitic mantle. Oxidizing (FeO = 17 wt.%, Fe3+/(Fe2+ + Fe3+) = 0.3–0.8) or lowering the Ca content of garnet to 1.4 wt.% can further decrease the VS of MORB. Such Fe-enriched, oxidized MORB or Ca-depleted, oxidized MORB, with a 3.5(5)%–7.6(5)% lower VS and distinct dln ρ/dln VS ratio, have a seismic signature consistent with the observed LVZs in the region.

Wei, W.Mao, Z.Sun, N.Sun, D., & Tkachev, S. N. (2021). High pressure-temperature single-crystal elasticity of grossular: Implications for the low-velocity layer in the bottom transition zoneGeophysical Research Letters48, e2021GL093540. abstract

Elastic moduli of grossular at high pressure and temperature. Blue: 300 K; green: 750 K; red: 1,000 K; solid circles: experimental data; solid lines: fitting results; blue dashed, dotted, short dashed, and dashed dotted lines: high pressure measurements of grossular (Conrad et al., 1999; Gwanmesia et al., 2014; Jiang, Speziale, & Duffy, 2004; Kono et al., 2010); blue solid squares and triangles: literature data at 300 K and 1 bar (Bass, 1989; Isaak et al., 1992). Uncertainties of the elastic moduli are smaller than the symbols if not shown.