A mechanism for low‐extent melts at the lithosphere‐asthenosphere boundary
A mechanism for low‐extent melts at the lithosphere‐asthenosphere boundary
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DOI：
10.1029/2010gc003234
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发表时间：
2010-10
期刊：
影响因子：
3.7
通讯作者：
C. Till;L. Elkins‐Tanton;K. Fischer
中科院分区：
文献类型：
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作者：
C. Till;L. Elkins‐Tanton;K. Fischer
最近的研究显示，岩石圈-软流圈边界(LAB)的剪切波速度垂直急剧下降。在一些地区，实验室中负速度梯度的大小太大，不能仅用温度变化来解释。这项研究表明，浅层软流层中的少量部分熔融是这种尖锐地震边界的可行模型。特别是，我们研究了厚克拉通岩石圈边缘上软流层中的熔融，使用了北美东部的例子，在那里观察到了明显的实验室速度梯度。岩石圈厚度横向骤减时软流圈流动的有限元模拟表明，这种几何形状与板块横向运动一起在大陆边缘产生边缘驱动对流和软流圈上涌。这项工作的一个关键组成部分是对不同H2O含量的橄榄岩固相线的不同模型产生的熔融位置和程度进行比较。此外，我们发展了一个简化的参数化水不饱和橄榄岩固相水在名义上无水矿物中的恒定的水饱和度。我们的数值模拟和各种固相线参数所产生的地幔流动模式预测，对于地幔潜在温度为1350℃和150ppm H2O的软流圈，在102-126公里深度的熔融不到0.1wt%到2.8wt%(0.01-3.3vol%)，对于1350℃和450ppm H2O的地幔，熔化在91公里到最大200公里之间。如果软流圈的地幔潜在温度为≤1340°C，或者在3 Gpa时H2O含量低于150ppm，则不会发生熔融。这种在软流圈产生熔体以在实验室产生急剧垂直速度梯度的过程在其他浅层软流层发生对流上涌的地方也是可行的，尽管它取决于软流圈的潜在温度、成分和H2O含量。由于软流圈在成分和H2O含量上可能是不均匀的，实验室下方的融化起点可能会随着时间和空间的变化而波动，实验室的剪切速度下降的幅度也可能会波动。
Recent studies have imaged sharp vertical drops in shear wave velocity at the lithosphere‐asthenosphere boundary (LAB). In some regions, the magnitude of the negative velocity gradient at the LAB is too large to be explained by changes in temperature alone. This study demonstrates that small amounts of partial melt in the shallow asthenosphere are a viable model for this sharp seismic boundary. In particular, we examine melting in the upper asthenosphere at the edge of thick cratonic lithosphere, using the example of eastern North America where a sharp LAB velocity gradient has been observed. Finite element modeling of asthenospheric flow at an abrupt lateral decrease in lithosphere thickness indicates that this geometry, together with lateral plate motions, produces edge‐driven convection and asthenospheric upwelling at the continental margin. A key component of this work is a comparison of the locations and extents of melting produced by using different models for the depression of the peridotite solidus with varying H2O content. In addition, we develop a simplified parameterization of the H2O‐undersaturated peridotite solidus for a constant degree of H2O saturation in nominally anhydrous minerals. The patterns of mantle flow produced by our numerical modeling and various solidus parameterizations predict less than 0.1 wt % to 2.8 wt % (0.01–3.3 vol %) melting at depths between 102 and 126 km for an asthenosphere with a mantle potential temperature of 1350°C and 150 ppm H2O, or between 91 km and a maximum of 200 km for a mantle at 1350°C and 450 ppm H2O. If the asthenosphere has a mantle potential temperature ≤1340°C or contains less than 150 ppm H2O at 3 GPa, no melting will occur. This process of generating melt in the asthenosphere to produce a sharp vertical velocity gradient at the LAB is viable in other locations where convective upwelling occurs in the shallow asthenosphere although it is dependent on asthenospheric potential temperature, composition, and H2O content. Because the asthenosphere may be heterogeneous in composition and H2O content, the onset of melting below the LAB may fluctuate with time and space, as may the magnitude of the shear velocity drop at the LAB.