Modelling komatiitic melt accumulation and segregation in the transition zone
Modelling komatiitic melt accumulation and segregation in the transition zone
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模拟过渡区的科马提质熔体堆积和偏析
DOI：
10.1016/j.epsl.2017.05.021
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发表时间：
2017
影响因子：
5.3
通讯作者：
N. Arndt
中科院分区：
文献类型：
--
作者：
Schmeling;N. Arndt
科马提岩可能产生于非常热的地幔隆起或地幔柱中。在这种条件下，熔融将发生在上地幔深处，甚至在地幔过渡带。由于在这种压力下的可压缩性，熔体的密度可能比橄榄石高，但由于其较高的温度和更致密的石榴石的存在，熔体在橄榄石-华兹华斯岩相界之上和之下的橄榄岩地幔中仍然具有浮力。我们研究了熔融和熔融分离的物理热上涌地幔通过过渡区，特别强调熔体和环境地幔之间的深度依赖的密度对比的效果。假设一个一维羽流，我们解决了两相流方程的熔体-基质系统占基质压实和孔隙率依赖的剪切和体积粘度。我们假设一个恒定的上升速度和熔体生成速率。在第一个模型系列中，假设中性浮力Z neutr的水平位于熔化开始的深度之上，即存在一个区域，在该区域中，致密熔体可能落后于上升羽流中的固相。根据两个无量纲数（积累数Ac，压实阻力数Cr），我们发现四个制度：1）时间依赖性熔体积累的孔隙驻波的规模与压实长度。这些波的最低处随时间变宽，直到在稳定状态下形成高熔体聚集区。在这一瞬态过程中，孤立孔隙波可能穿过中性密度层并逃逸。2)在zneutr附近稳态弱熔体积聚，3）由于小密度对比没有熔体积聚，或4）高基质粘度。在制度4地幔粘度高，防止开放的孔隙空间积累熔体。在第二个系列中，上升的地幔穿过橄榄石-华兹斯利岩相界，这对熔体和环境地幔之间的密度差造成了跳跃。一个尖锐的熔体孔隙度对比度的发展和大的熔体孔隙度积累紧接相边界上方。这两个模型系列表明：1）熔体密度和孔隙度相关的基质粘度都控制着熔体的上升或累积; 2）存在着可能导致非常大的熔体孔隙度累积的参数范围和物理条件（>熔融程度），3）尽管熔体在某些深度比橄榄石密度大，一般来说，这些熔融物会逃离这些区域，并继续以比上升的地幔更快的速度向上渗透。熔融和熔体输送的数值模拟预测的条件下，能够再现的主要类型的科马提岩的组成。因此，大熔体馏分的积累，并连续逃逸的熔体孔隙度波，解释了几个令人困惑的特点科马提岩的地球化学成分。
Komatiites are probably produced in very hot mantle upwellings or plumes. Under such conditions, melting will take place deep within the upper mantle or even within the mantle transition zone. Due to its compressibility at such pressures, melt might be denser than olivine, but would remain buoyant with respect to a peridotitic mantle both above and below the olivine–wadsleyite phase boundary because of the presence of its higher temperature and denser garnet. We studied the physics of melting and melt segregation within hot upwelling mantle passing through the transition zone, with particular emphasis on the effect of depth-dependent density contrasts between melt and ambient mantle. Assuming a 1D plume, we solved the two-phase flow equations of the melt-matrix system accounting for matrix compaction and porosity-dependent shear and bulk viscosity. We assumed a constant ascent velocity and melt generation rate. In a first model series, the level of neutral buoyancy z neutr is assumed to lie above the depth of onset of melting, ie there exists a region where dense melt may lag behind the solid phases within the rising plume. Depending on two non-dimensional numbers (accumulation number Ac, compaction resistance number Cr) we find four regimes: 1) time-dependent melt accumulation in standing porosity waves that scale with the compaction length. The lowermost of these waves broadens with time until a high melt accumulation zone is formed in steady state. During this transient solitary porosity waves may cross the depth of neutral density and escape. 2) steady-state weak melt accumulation near z neutr, 3) no melt accumulation due to small density contrast or, 4) high matrix viscosity. In regime 4 the high mantle viscosity prevents the opening of pore space necessary to accumulate melt. In a second series, the rising mantle crosses the olivine–wadsleyite phase boundary, which imposes a jump in density contrast between melt and ambient mantle. A sharp melt porosity contrast develops and a large melt porosity accumulates immediately above the phase boundary. Both model series show 1) that not only melt density, but also porosity-dependent matrix viscosity controls the melt ascent or accumulation, 2) that there are parameter ranges and physical conditions which may lead to the accumulation of very large melt porosities (> degree of melting), 3) that in spite of melt being denser than olivine at some depths, in general these melts escape these regions and continue to percolate upward faster than the rising mantle. Melting and melt transport under the conditions predicted by the numerical modelling is able to reproduce the compositions of the main types of komatiite. Thus, the accumulation of large melt fractions, and sequential escape of melt from porosity waves, explains several puzzling features of the geochemical compositions of komatiites.
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