Disequilibrium dihedral angles in dolerite sills: A new proxy for cooling rate
Disequilibrium dihedral angles in dolerite sills: A new proxy for cooling rate
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DOI：
10.1130/g33119.1
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发表时间：
2012-09
期刊：
影响因子：
5.8
通讯作者：
M. Holness;C. Richardson;R. T. Helz
中科院分区：
文献类型：
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作者：
M. Holness;C. Richardson;R. T. Helz
镁铁质岩石中单斜辉石-斜长石-斜长石交界处的几何形状（通过中位二面角Θcpp测量）是在凝固过程中产生的。在凝固的基拉韦厄伊基（夏威夷）熔岩湖中，斜长石颗粒之间较宽的交界处首先被辉石填充，随后是较窄的交界处。当所有的单斜辉石-斜长石-斜长石交界处形成时，最终的Θcpp在湖的上地壳为78°，在下部的凝固前沿为85°。3.5 m厚Traigh巴恩na Scurra岩床（内赫布里底群岛）的Θcpp到处都是78°。在Whin Sill（英格兰北方，38米厚）和Portal Peak Sill（南极洲，129米厚），Θcpp呈对称变化，在边缘处最低。266 m厚的基底岩床（南极洲）具有不对称的Θcpp变化，归因于复杂的填充历史。地下室窗台的冷却边缘部分质地平衡，具有高Θcpp。在两个最宽的岩床中，斜长石的粒度变化不对称，在上三分之一处发现了粗粒岩。Θcpp和平均晶粒尺寸都是模型结晶时间的函数。当结晶时间从1年增加到500年时，Θcpp从78°增加到最大100°。由于使用粒度作为结晶时间的量度取决于晶体生长速率的估计，二面角提供了一个更直接的代理冷却速率在白云石。
The geometry of clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, measured by the median dihedral angle Θcpp, is created during solidification. In the solidifying Kilauea Iki (Hawaii) lava lake, the wider junctions between plagioclase grains are the first to be filled by pyroxene, followed by the narrower junctions. The final Θcpp, attained when all clinopyroxene-plagioclase-plagioclase junctions are formed, is 78° in the upper crust of the lake, and 85° in the lower solidification front. Θcpp in the 3.5-m-thick Traigh Bhan na Sgurra sill (Inner Hebrides) is everywhere 78°. In the Whin Sill (northern England, 38 m thick) and the Portal Peak sill (Antarctica, 129 m thick), Θcpp varies symmetrically, with the lowest values at the margins. The 266-m-thick Basement Sill (Antarctica) has asymmetric variation of Θcpp, attributed to a complex filling history. The chilled margins of the Basement Sill are partially texturally equilibrated, with high Θcpp. The plagioclase grain size in the two widest sills varies asymmetrically, with the coarsest rocks found in the upper third. Both Θcpp and average grain size are functions of model crystallization times. Θcpp increases from 78° to a maximum of ∼100° as the crystallization time increases from 1 to 500 yr. Because the use of grain size as a measure of crystallization time is dependent on an estimate of crystal growth rates, dihedral angles provide a more direct proxy for cooling rates in dolerites.