Kruger Willem, Latypov Rais
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa.
Sci Rep. 2021 Apr 1;11(1):7341. doi: 10.1038/s41598-021-86724-y.
An understanding of magma chamber dynamics relies on answering three important yet highly controversial questions: where, why, and how magma chambers crystallize and differentiate. Here we report on a new natural phenomenon-the undercut-embayed chamber floor in the Bushveld Complex-which allows us to address these questions. The undercut-embayed floor is produced by magmatic karstification (i.e. erosion by dissolution) of the underlying cumulates by replenishing magmas that form basal flows on the chamber floor. This results in a few metres thick three-dimensional framework of spatially interconnected erosional remnants that separate the floor cumulates from the overlying resident melt. The basal flow in this environment is effectively cooled through the floor, inducing heterogeneous nucleation and in situ growth against much of its three-dimensional framework. The solidification front thus propagates in multiple directions from the surfaces of erosional remnants. Fractional crystallization may occur within this environment by convective removal of a compositional boundary layer from in situ growing crystals and is remarkably efficient even in very confined spaces. We propose that the way magma crystallizes and differentiates in the undercut-embayed chamber floor is likely common for the evolution of many basaltic magma chambers.
岩浆房在何处结晶和分异、为何结晶和分异以及如何结晶和分异。在此，我们报告一种新的自然现象——布什维尔德杂岩体中出现的底部被侵蚀且呈港湾状的岩浆房底部——这使我们能够回答这些问题。底部被侵蚀且呈港湾状的底部是由补给岩浆对下伏堆积物的岩浆岩溶蚀作用（即通过溶解进行侵蚀）形成的，这些补给岩浆在岩浆房底部形成底部流。这导致形成了一个几米厚的三维框架，其中包含空间相互连接的侵蚀残余物，将底部堆积物与上覆的驻留熔体分隔开来。在这种环境下，底部流通过底部有效地冷却，促使异质形核并在其大部分三维框架上原位生长。因此，凝固前沿从侵蚀残余物的表面向多个方向传播。通过对流去除原位生长晶体的成分边界层，在这种环境中可能会发生分离结晶，而且即使在非常狭窄的空间中也非常有效。我们认为，在底部被侵蚀且呈港湾状的岩浆房底部，岩浆结晶和分异的方式可能在许多玄武质岩浆房的演化过程中很常见。
