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High Pressure‐Temperature Single‐Crystal Elasticity of Grossular: Implications for the Low‐Velocity Layer in the Bottom Transition Zone
Author(s) -
Wei Wei,
Mao Zhu,
Sun Ningyu,
Sun Daoyuan,
Tkachev Sergey N.
Publication year - 2021
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2021gl093540
Subject(s) - grossular , geology , mantle (geology) , transition zone , basalt , mineralogy , hydrothermal circulation , analytical chemistry (journal) , geophysics , geochemistry , chemistry , seismology , metamorphic rock , chromatography
We determined the single‐crystal elasticity of Fe 3+ ‐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 V S 2.0(5)%–3.0(5)% lower than that in the pyrolitic mantle. Oxidizing (FeO = 17 wt.%, Fe 3+ /(Fe 2+ + Fe 3+ ) = 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 d ln ρ / d ln V S ratio, have a seismic signature consistent with the observed LVZs in the region.