Large-Scale Thermo-chemical Structure of the Deep Mantle: Observations and Models
Large-Scale Thermo-chemical Structure of the Deep Mantle: Observations and Models
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DOI：
10.1007/978-3-319-15627-9_15
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发表时间：
2015-01-01
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影响因子：
--
通讯作者：
Tackley, P. J.
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文献类型：
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作者：
Deschamps, Frederic;Li, Yang;Tackley, P. J.
地震层析成像表明，最低的地幔，从2400公里到核幔边界（CMB）是强烈的不均匀性在大波长。最引人注目的特征是两个大的低剪切波速度区（LLSVP），与平均地幔相比，剪切波速度下降了几个百分点。几次地震观测进一步表明，最低地幔地震异常不可能是纯粹的热成因。成分异常需要充分解释的意见，如剪切和体积声速之间的反相关性，和正常模式映射的密度分布。与此同时，热化学对流模型表明，致密的、化学差异物质的储层可以长期保持在地幔的最低层，在这些储层的表面产生了上升到地表的热羽流。模型参数搜索表明，保持这样的储层需要在致密和常规材料之间的适度密度对比和大的热粘度对比。目前的热化学对流模型还解释了旅行时间和地震波形数据所揭示的细节，特别是LLSVP的锐边，以及LLSVP表面羽流的分布。一个遗留的问题是下地幔大规模化学不均匀性的详细性质。致密物质的储集可能是地幔早期部分分异或洋壳再循环的结果。从一个连贯的矿物物理数据库推断的地震敏感性表明，LLSVP更好地解释了温暖的物质富含铁和硅酸盐，比高压MORB。相比之下，如果比周围的地幔温度低400 K，高压MORB可以很好地解释古板块可能存在的地区的地震速度异常。例如，在一个实施例中，在日本俯冲带和中南美洲之下。在解释地震观测，特别是D "不连续性方面，后渗透相当然起着重要作用，但不可能单独解释所有的地震观测。
Seismic tomography indicates that the lowermost mantle, from 2400 km down to the core-mantle boundary (CMB) is strongly heterogeneous at large wavelengths. The most striking features are two large low-shear-wave velocity provinces (LLSVPs), where shear-wave velocity drops by a few percent compared to averaged mantle. Several seismic observations further show that lowermost mantle seismic anomalies cannot be purely thermal in origin. Compositional anomalies are required to fully explain observations like the anti-correlation between shear-and bulk-sound velocities, and the distribution of density mapped by normal modes. In the meantime, models of thermo-chemical convection indicate that reservoirs of dense, chemically differentiated material can be maintained in the lowermost mantle over long periods of time and that thermal plumes rising up to the surface are generated at the surface of these reservoirs. Model parameter searches indicate that maintaining such reservoirs requires a moderate density contrast between dense and regular material and a large thermal viscosity contrast. Current models of thermo-chemical convection also explain details revealed by travel time and seismic waveform data, in particular the LLSVP sharp edges, and the distribution of plumes at the surface of LLSVPs. A remaining question is the detailed nature of the lower mantle large-scale chemical heterogeneities. Reservoirs of dense material may result either from early partial differentiation of the mantle or recycling of oceanic crust (MORB). Seismic sensitivities inferred from a coherent mineral physics database suggest that LLSVPs are better explained by warm material enriched in iron and silicate, than by highpressure MORB. By contrast, if colder than the surrounding mantle by similar to 400 K, high-pressure MORB explains well seismic velocity anomalies in regions where ancient slabs are expected to rest, e. g., beneath the Japan subduction zones and beneath Central and South America. The post-pervoskite phase certainly plays a significant role in explaining seismic observations, in particular the D '' discontinuity, but is unlikely to explain all seismic observations alone.