Origin of ultra-low velocity zones at the core mantle boundary

核幔边界超低速带的起源

• 批准号: NE/H021027/1
• 负责人: John Brodholt
• 金额: $ 19.66万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2011
• 资助国家: 英国
• 起止时间: 2011 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FH021027%2F1
• 关键词: Origin; ultra; low; velocity; zones

Using new seismological observations together with mineral physics constraints, we will test hypotheses as to the origin of small scale (10 to 100 kms) heterogeneities called ultra-low velocity zones (ULVZs) at the core mantle boundary (CMB). The lowermost mantle is host to a wide variety of structures and shows heterogeneities on many scale lengths. Arguably the most intriguing seismic discovery of the last 15 years regarding the lowermost mantle is that of intermittent thin patches of extremely reduced seismic velocities near the CMB dubbed ULVZs, indicative of the existence of a 10 to 40 km basal mantle layer. ULVZs influence many aspects of mantle dynamics and it has been speculated they are the roots of mantle plumes, areas of core material entering the mantle, remnants of a global magma ocean, an influence on the path of the magnetic poles during polar reversals, and chemically distinct exotic material. Therefore, understanding the origin and properties of ULVZs is not of just academic interest, but impacts on a wide range of first-order Earth issues. Nonetheless, the origin of ULVZs remains unsolved and fundamental questions such as partial melt vs. chemical heterogeneity as source for ULVZs are still debated. To test hypotheses on the origin of ULVZs we will use a combined mineral-physical and seismological approach. Each of the proposed ULVZ models will lead to specific velocity changes, P-wave to S-wave velocity ratios and density changes for ULVZs. For instance, a partial melt origin of ULVZs will lead to a P-to-S wave velocity ratio of 1:3, while a compositional origin creates VP/VS~1-2. Currently our knowledge about ULVZ structure and lower mantle material properties is not sufficient to differentiate between these models and we will employ new seismological probes to better resolve the existent velocity and density structure of ULVZ. Furthermore, we will determine the elastic properties of perovskite and post-perovskite, as function of composition, pressure and temperature from first principle calculations to understand the elastic properties of potential ULVZ material. Identification of regions devoid of ULVZs is crucial to understand the connection between mantle flow and ULVZs. Improving the seismic coverage we will obtain a map of the CMB indicating ULVZ regions, their seismic velocities and densities. Using forward modeling based on the mineral-physics results we will be able to thoroughly test different models of origin for ULVZs.

使用新的地震观测与矿物物理学的限制，我们将测试假设的起源小规模（10至100公里）的不均匀性称为超低速区（ULVZs）在核幔边界（CMB）。最低地幔是各种各样的结构的宿主，并在许多尺度长度上显示出不均匀性。可以说，过去15年来关于最低地幔的最有趣的地震发现是在被称为ULVZ的CMB附近的地震速度极低的间歇性薄斑，表明存在10至40公里的基底地幔层。ULVZ影响了地幔动力学的许多方面，据推测它们是地幔柱的根源，核心物质进入地幔的区域，全球岩浆海洋的残余，在极地逆转期间对磁极路径的影响，以及化学上独特的外来物质。因此，了解ULVZ的起源和性质不仅具有学术意义，而且会对广泛的一阶地球问题产生影响。尽管如此，超低容量区的起源仍然没有解决，一些基本问题，如部分熔融与化学异质性作为超低容量区的来源，仍然存在争议。为了检验关于ULVZ起源的假设，我们将使用矿物物理和地震学相结合的方法。每一个建议的ULVZ模型将导致特定的速度变化，P波到S波的速度比和密度变化的ULVZ。例如，ULVZ的部分熔融起源将导致P波与S波速度比为1：3，而成分起源产生VP/VS~1-2。目前我们对ULVZ结构和下地幔物质性质的了解还不足以区分这些模型，我们将采用新的地震学探测器来更好地解决ULVZ存在的速度和密度结构。此外，我们将根据第一原理计算确定钙钛矿和后钙钛矿的弹性性质，作为组成，压力和温度的函数，以了解潜在ULVZ材料的弹性性质。识别没有ULVZ的区域对于理解地幔流和ULVZ之间的联系至关重要。通过提高地震覆盖率，我们将获得一张CMB图，显示ULVZ区域及其地震速度和密度。使用基于矿物物理结果的正演建模，我们将能够彻底测试ULVZ的不同起源模型。

项目成果

Helium diffusion in olivine based on first principles calculations

• DOI: 10.1016/j.gca.2015.01.023
• 发表时间: 2015-05
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: Kai Wang;J. Brodholt;Xiancai Lu

Elastic properties of ferrous bearing MgSiO3 and their relevance to ULVZs

• DOI: 10.1093/gji/ggv045
• 发表时间: 2015-04
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: J. Muir;J. Brodholt

Elastic properties of MgSiO3-perovskite under lower mantle conditions and the composition of the deep Earth

下地幔条件下MgSiO3-钙钛矿的弹性特性及地球深部的成分

• DOI: 10.1016/j.epsl.2013.07.034
• 发表时间: 2013-10
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Zhigang Zhang;Zhigang Zhang;Lars Stixrude;Lars Stixrude;John Brodholt;John Brodholt
• 通讯作者: John Brodholt

Water distribution in the lower mantle: Implications for hydrolytic weakening

• DOI: 10.1016/j.epsl.2017.11.051
• 发表时间: 2018-02
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: J. Muir;J. Brodholt

Elastic properties of ferropericlase at lower mantle conditions and its relevance to ULVZs

• DOI: 10.1016/j.epsl.2015.02.023
• 发表时间: 2015-05
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: J. Muir;J. Brodholt