Plagioclase growth rates control three-grain junction geometry in dolerites and gabbros
Plagioclase growth rates control three-grain junction geometry in dolerites and gabbros
复制标题
斜长石生长速率控制辉长岩和辉长岩中的三晶粒连接几何形状
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
M. Holness
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文献类型：
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作者：
M. Holness
对辉长岩中两粒斜长石和一粒另一种矿物的三粒结的二面角的测量表明，任何样品中所有矿物的中位二面角基本上是相同的。少数例外情况可归因于在高度演化的液体固化的最后阶段发生反应或停止生长斜长石，这些液体不结晶体积上重要的斜长石。因此，二面角主要由斜长石在最后剩余液体中的生长行为控制。二面角的最终值由斜长石在(010)面上生长的程度控制：当(010)面上的生长与其他生长面上的生长相比较小时，形成低角度，当(010)面适应显著生长时，形成高角度。因此，二面角对结晶时间变化的响应可以用斜长石生长对降温速度的响应来解释，在快速冷却时(010)面上的生长有限(导致低的二面角)，而在缓慢冷却时更显著的生长(导致高的二面角)。二面角和斜长石颗粒形状之间的对应关系(通过在薄片中观察到的平均表观长宽比来量化)对于未分离的物体，如辉绿岩岩床，是明显的。尽管斯卡尔加德侵入体底板堆积物中斜长石颗粒总体形状的地层变化与在岩床中观察到的大致相似，但与观察到的Augite-��斜长石-��斜长石二面角没有对应关系，后者显示出阶梯式的地层变化，与液线组合的变化相对应。这种解耦的发生是因为层状侵入体中的斜长石生长分两个阶段发生，第一阶段在岩浆-��泥浆界面或接近岩浆界面，第二阶段在岩浆泥浆内。辉石-二面角阶梯两侧样品的化学图谱表明，在第二阶段生长的斜长石的长径比呈阶梯式变化，该阶段的长径比与从二面角预测的长径比相对应。因此，层状侵入体中的斜长石形状记录了两个不同的热状态，总体形状受侵入体的全球冷却速度控制，糊状层中的第二阶段(次要)反映了受块状岩浆液相线组合控制的局部热缓冲。层状侵入体中的二面角记录了第二热体制。
Measurements of dihedral angles at three-grain junctions in gabbros, involving two grains of plagioclase and one grain of another mineral, demonstrate that the median dihedral angle is generally the same for all minerals in any sample. The few exceptions to this can be attributed to reaction or to the cessation of growth of plagioclase during the last stages of solidification of highly evolved liquids that do not crystallize volumetrically important amounts of plagioclase. The dihedral angle is therefore primarily controlled by the growth behavior of plagioclase in the last remaining liquid. The final value of the dihedral angle is controlled by the extent to which plagioclase growth is accommodated on the (010) faces: low angles form when growth on the (010) faces is minor compared with that on the other growth faces, and high angles form when the (010) faces accommodate significant growth. The response of dihedral angles to changes in crystallization time is therefore explained by the changing response of plagioclase growth to cooling rate, with limited growth on (010) faces during rapid cooling (leading to a low dihedral angle) and more significant growth at slow cooling (leading to high dihedral angle). The correspondence between dihedral angle and plagioclase grain shape (as quantified by the average apparent aspect ratio observed in thin section) is clearly evident for non-fractionated bodies such as dolerite sills. Although the stratigraphic variation of the overall plagioclase grain shape in the floor cumulates of the Skaergaard Intrusion is broadly similar to that observed in sills, there is no correspondence to observed augite�plagioclase�plagioclase dihedral angles, which show a step-wise stratigraphic variation, corresponding to changes in the liquidus assemblage. This decoupling occurs because plagioclase growth in layered intrusions occurs in two stages, the first at, or close to, the magma�mush interface and the second within the mush. Chemical maps of samples on either side of the augite-in dihedral angle step demonstrate a step-wise change in the aspect ratio of the plagioclase grown during the second stage, with the aspect ratio of this stage corresponding to that predicted from the dihedral angles. Plagioclase shape in layered intrusions thus records two separate thermal regimes, with the overall shape controlled by the global cooling rate of the intrusion, and the second (minor) stage within the mushy layer reflecting local thermal buffering controlled by the liquidus assemblage of the bulk magma. Dihedral angles in layered intrusions record the second thermal regime.
影响因子：
3.9
作者：
O. Namur;M. Humphreys;M. Holness
通讯作者：
O. Namur;M. Humphreys;M. Holness
露营地堤坝：了解东格陵兰斯卡尔加德侵入早期凝固后历史的窗口
影响因子：
3.5
作者：
Holness M
通讯作者：
Holness M