Importance of permeability and deep channel network on the distribution of melt, fractionation of REE in abyssal peridotites, and U-series disequilibria in basalts beneath mid-ocean ridges: A numerical study using a 2D double-porosity model
Importance of permeability and deep channel network on the distribution of melt, fractionation of REE in abyssal peridotites, and U-series disequilibria in basalts beneath mid-ocean ridges: A numerical study using a 2D double-porosity model
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渗透率和深水通道网络对熔体分布、深海橄榄岩中稀土元素分馏以及大洋中脊下玄武岩 U 系不平衡的重要性：使用二维双孔隙模型的数值研究
DOI：
10.1016/j.epsl.2019.115788
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发表时间：
2019
影响因子：
5.3
通讯作者：
Liang, Yan
中科院分区：
文献类型：
--
作者：
Liu, Boda;Liang, Yan
在大洋中脊和蛇绿岩的地幔部分进行了大量的观测：存在板状粗面纯橄榄岩，残余深海橄榄岩中的LREE高度贫化，新鲜玄武岩中的U系列不平衡，地壳厚度，以及扩展中心下方的高度衰减的地震和大地电磁结构。这些独立的观察结果如果在一个自洽模型中进行联合分析，可能会变得更加强大。该模型的难点在于如何在构造尺度的地球动力学模型中求解和评价纯橄榄岩通道等细尺度岩石学特征的影响。本文提出了一个由低孔隙度残余二辉橄榄岩和方辉橄榄岩基质和高孔隙度连通通道网络组成的二维双孔隙山脊模型。地球动力学模拟的特点是空间分布的通道网络和各向异性渗透率，逐渐发展的上涌地幔变形。我们计算了孔隙率，熔体和固体流场的通道分布和渗透率模型的几个选择。利用计算得到的孔隙度分布和速度场模拟了残留地幔中稀土元素的变化和熔体中铀系的不平衡，强调了深通道网络和渗透率模型在解释大洋中脊扩张中心一级地球物理和地球化学观测中的重要性。通道的各向异性渗透率可以使熔体聚集增加60%，从而导致地壳变厚。衰减的地震和大地电磁结构需要一个从山脊轴下60公里深处开始的通道网络。亏损的稀土模式在残留的深海橄榄岩单斜辉石也是一致的熔体提取到通道开始从60公里的深度。虽然深通道有利于产生U系列不平衡在喷发熔体，目前的模型仍然不能解释所观察到的全部范围内的U系列不平衡的MORB样品的数据。其他因素，过程和模型进行了讨论。最后，我们发现，在渗透率的不确定性范围内，孔隙度变化了三倍，230 Th过剩量变化了两倍。
A plethora of observations have been made at mid-ocean ridges and mantle sections of ophiolites: presence of tabular replacive dunites, highly depleted LREE in residual abyssal peridotites, U-series disequilibria in fresh basalts, crustal thickness, and highly attenuated seismic and magnetotelluric structures beneath spreading centers. These independent observations can become more powerful if they are jointly analyzed in a self-consistent model. The difficulty for such a model is to resolve and evaluate the effect of fine scale petrologic feature such as dunite channels in a tectonic scale geodynamic model. Here we present a two-dimensional double-porosity ridge model that consists of low-porosity residual lherzolite and harzburgite matrix and high-porosity interconnected channel network. The geodynamic simulation features a spatially distributed channel network and anisotropic permeability that gradually develops as the upwelling mantle is deformed. We compute the porosity, melt and solid flow fields for several choices of channel distribution and permeability model. We use the calculated porosity distribution and velocity fields to model the variations of REE in residual mantle and U-series disequilibria in melts.The present study underscores the importance of deep channel networks and permeability model to the interpretation of first order geophysical and geochemical observations at mid-ocean ridge spreading centers. The anisotropic permeability of channels can enhance melt focusing by 60%, resulting in thicker crust. The attenuated seismic and magnetotelluric structures require a channel network starting from 60 km depth beneath the ridge axis. The depleted REE patterns in clinopyroxenes in residual abyssal peridotites are also consistent with melt extraction into channels starting from 60 km depth. Although deep channels are conducive to producing U-series disequilibria in eruptible melts, the present model still cannot explain the full ranges of the observed U series disequilibria data in MORB samples. Additional factors, processes, and models are discussed. And finally, we found that, within the uncertainty of the permeability, the porosity varies by three folds and the excess of230Th varies by up to two folds.
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