Ultra-Low Velocity Zone Beneath the Atlantic Near St. Helena

IF 2.9 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Felix Davison, Carl Martin, Rita Parai, Sanne Cottaar
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Abstract

There are various hotspots in the Atlantic Ocean, which are underlain by mantle plumes that likely cross the mantle and originate at the core-mantle boundary. We use teleseismic core-diffracted shear waves to look for an Ultra-Low Velocity Zone (ULVZ) at the potential base of central Atlantic mantle plumes. Our data set shows delayed postcursory phases after the core-diffracted shear waves. The observed patterns are consistent in frequency dependence, delay time, and scatter pattern with those caused by mega-ULVZs previously modeled elsewhere. Synthetic modeling of a cylindrical structure on the core-mantle boundary below St. Helena provides a good fit to the data. The preferred model is 600 km across and 20 km high, centered at approximately 15° South, 15° West, and with a 30% S-wave velocity reduction. Significant uncertainties and trade-offs do remain to these parameters, but a large ULVZ is needed to explain the data. The location is west of St. Helena and south of Ascension. Helium and neon isotopic systematics observed in samples from this region could point to a less-outgassed mantle component mixed in with the dominant signature of recycled material. These observations could be explained by a contribution from the Large Low Shear Velocity Province (LLSVP). Tungsten isotopic measurements would be needed to understand whether a contribution from the mega-ULVZ is also required at St. Helena or Ascension.

Abstract Image

圣赫勒拿附近大西洋海底的超低流速区
大西洋有多个热点,其下层是地幔羽流,这些羽流很可能穿过地幔,发源于地核-地幔边界。我们利用远震岩心衍射剪切波来寻找大西洋中部地幔羽流潜在底部的超低速度区(ULVZ)。我们的数据集显示了岩心衍射剪切波之后的延迟后发阶段。观测到的模式在频率依赖性、延迟时间和散射模式上与之前在其他地方建模的超大型低速区所造成的模式一致。在圣赫勒拿岛下方的地核-地幔边界上建立一个圆柱形结构的合成模型,可以很好地拟合数据。首选模型宽 600 千米,高 20 千米,中心大约在南纬 15°,西经 15°,S 波速度降低 30%。这些参数仍存在很大的不确定性和权衡,但需要一个大的超低压区来解释数据。该地点位于圣赫勒拿岛以西、阿森松以南。在这一地区的样本中观测到的氦和氖同位素系统学可能表明,在回收物质的主要特征中混杂着气体较少的地幔成分。这些观测结果可以用大低剪切速度区(LLSVP)的贡献来解释。需要进行钨同位素测量,以了解在圣赫勒拿或阿森松是否也需要巨型低剪切变速带的贡献。
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来源期刊
Geochemistry Geophysics Geosystems
Geochemistry Geophysics Geosystems 地学-地球化学与地球物理
CiteScore
5.90
自引率
11.40%
发文量
252
审稿时长
1 months
期刊介绍: Geochemistry, Geophysics, Geosystems (G3) publishes research papers on Earth and planetary processes with a focus on understanding the Earth as a system. Observational, experimental, and theoretical investigations of the solid Earth, hydrosphere, atmosphere, biosphere, and solar system at all spatial and temporal scales are welcome. Articles should be of broad interest, and interdisciplinary approaches are encouraged. Areas of interest for this peer-reviewed journal include, but are not limited to: The physics and chemistry of the Earth, including its structure, composition, physical properties, dynamics, and evolution Principles and applications of geochemical proxies to studies of Earth history The physical properties, composition, and temporal evolution of the Earth''s major reservoirs and the coupling between them The dynamics of geochemical and biogeochemical cycles at all spatial and temporal scales Physical and cosmochemical constraints on the composition, origin, and evolution of the Earth and other terrestrial planets The chemistry and physics of solar system materials that are relevant to the formation, evolution, and current state of the Earth and the planets Advances in modeling, observation, and experimentation that are of widespread interest in the geosciences.
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