Melt extraction in the Earth's mantle: Constraints from U–Th–Pa–Ra studies in oceanic basalts
Melt extraction in the Earth's mantle: Constraints from U–Th–Pa–Ra studies in oceanic basalts
复制标题
DOI：
10.1016/j.epsl.2006.01.057
复制
发表时间：
2005-12
影响因子：
5.3
通讯作者：
A. Stracke;B. Bourdon;D. McKenzie
中科院分区：
文献类型：
--
作者：
A. Stracke;B. Bourdon;D. McKenzie
大洋玄武岩中的铀系研究对于了解地幔熔融和熔体提取至关重要。在冰岛北方的Theistareykir，熔融和熔体提取的详细地球化学研究的综合结果，提供了一个强有力的情况下，熔体提取通过渠道熔体流在一个活跃的扩张脊。然而，人们常常认为，广泛使用的熔融和熔体提取模型，模拟通道熔体提取（即分数和/或动态熔融），只能部分解释全球大洋玄武岩的U系列系统学。因此，更复杂的模型已经被调用，这表明在地幔中不同的深度/压力，所谓的“双孔隙度模型”的不同风格的熔体提取。或者，已经提出了扩散控制机制。在这里，我们表明，U-Th-Pa-Ra系统在大洋玄武岩中确实可以解释的模型，熔体运输发生时没有熔体和固体之间的化学平衡时，在所有三个关键的熔融参数（残余孔隙度，固体地幔的上涌速率和熔体速度）的变化被考虑在内。熔融脊需要系统的变化，至少有两个关键的熔融参数，最有可能的上升流和熔体提取率。随着到脊轴的横向距离的增加而产生的熔体以较慢的上涌速率产生，并且以较低的速度提取，而不是更靠近脊轴的熔体。另一方面，海洋岛屿的融化只能用上升流速率的变化来解释。OIB的全球U系列系统学起源于由于不同浮力通量导致的上升流速度的叠加全球变化，以及上升流速度作为到羽流中心的径向距离的函数的局部变化。这里提出的模型与其他海洋玄武岩的地球化学数据是一致的，并强烈支持熔体提取通过高孔隙度的通道作为一种通用的手段，从地球上地幔熔体提取。
U-series studies in oceanic basalts are critical for understanding melting and melt extraction in the Earth's mantle. The combined results of a detailed geochemical study of melting and melt extraction at Theistareykir, northern Iceland, provide a strong case for melt extraction via channeled melt flow at an active spreading ridge. It has often been argued, however, that widely used melting and melt extraction models, which simulate channeled melt extraction (i.e. fractional and/or dynamic melting), can only partially explain the global U-series systematics in oceanic basalts. As a consequence, more complicated models have been invoked, which suggest different styles of melt extraction at different depths/pressures in the mantle, so-called “two-porosity models”. Alternatively, diffusion-controlled mechanisms have been proposed. Here we show that U–Th–Pa–Ra systematics in oceanic basalts can indeed be explained by models where melt transport occurs without chemical equilibrium between melt and solid when variations in all three critical melting parameters (residual porosity, upwelling rate of the solid mantle and melt velocity) are taken into account. Melting at ridges requires systematic variation of at least two critical melting parameters, most likely upwelling and melt extraction rate. Melts generated with increasing lateral distance to the ridge axis are generated with slower upwelling rates and are extracted with lower velocities than melts created closer to the ridge axis. Melting at ocean islands, on the other hand, can successfully be explained by variations in upwelling rate only. Global U-series systematics in OIB originate from superimposed global variations in upwelling velocity due to different buoyancy fluxes and from local variation in upwelling velocity as a function of radial distance to the plume center. The model proposed here is consistent with other geochemical data for oceanic basalts and strongly supports melt extraction via high-porosity channels as a general means of melt extraction from the Earth's upper mantle.