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High Pressure‐Temperature Single‐Crystal Elasticity of Grossular: Implications for the Low‐Velocity Layer in the Bottom Transition Zone
Geophysical Research Letters ( IF 5.2 ) Pub Date : 2021-04-18 , DOI: 10.1029/2021gl093540
Wei Wei 1 , Zhu Mao 1, 2 , Ningyu Sun 1, 2 , Daoyuan Sun 1, 2 , Sergey N. Tkachev 3
Affiliation  

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.

中文翻译:

高压的高温单晶弹性:对底部过渡带低速层的影响

我们使用高达22 GPa和1,000 K的布里渊散射确定了含Fe 3+的颗粒的单晶弹性。根据获得的结果,我们对地幔速度剖面进行了建模,以研究底部的低速带(LVZs)的成因。地幔过渡带100公里。我们的模型显示,尽管可以通过地震将水饱和过渡带检测为LVZ,但它还会在520 km不连续处引起负速度跳跃,而这在地震中还没有被检测到。然而,石榴石中FeO含量为10–17 wt。%的中洋脊玄武岩(MORB)的V S将比热释岩地幔的V S 2.0(5)%– 3.0(5)%低。氧化(FeO = 17 wt。%,Fe 3+ /(Fe 2+  + Fe3+)= 0.3-0.8)或将石榴石的Ca含量降低至1.4 wt。%可以进一步降低MORB的V S。这种铁富集,氧化MORB或Ca贫化,氧化MORB,具有3.5(5)% - 7.6(5)%降低V小号和不同的d LN  ρ / d LN  V小号比,具有与一致的地震特征在该区域观察到的LVZ。
更新日期:2021-05-05
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