资源环境科技发展态势分析平台

We determined the single‐crystal elasticity of Fe³⁺‐bearing grossular using Brillouin scattering up to 22 GPa and 1000 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 hasn't been seismically detected. However, MORB with an FeO content of 10‐17 wt.% in garnet will have V_S 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 V_S of MORB. Such Fe‐enriched, oxidized MORB or Ca‐depleted, oxidized MORB, with a 3.5(5)‐7.6(5)% lower V_S and distinct dlnρ/dlnV_S ratio, have a seismic signature consistent with the observed LVZs in the region.