Philpotts AR, Dickson LD
Department of Geology and Geophysics, University of Connecticut, Storrs 06269, USA.
Nature. 2000 Jul 6;406(6791):59-61. doi: 10.1038/35017542.
The basaltic rock in the lower part of the thick Holyoke lava flow in Connecticut and Massachusetts has been shown to have a remarkable texture, with crystals of feldspar linked together in a continuous three-dimensional network of chains. Heating experiments have revealed that this network persists to temperatures where the rock is 75% liquid, and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow. Despite the texture's importance to our understanding of how such basalt flows form, the origin of the texture has remained uncertain. Here we show that, although the network is present in the lower third of the flow, it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush. Convection of this type has been postulated for intrusive magma chambers, but corroborative field evidence has been equivocal, especially in lava lakes and flows. Preservation of the roof-generated texture in the lower part of a thick flood-basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet.
康涅狄格州和马萨诸塞州厚层霍利奥克熔岩流下部的玄武岩显示出一种非凡的纹理，长石晶体以连续的三维链状网络连接在一起。加热实验表明，这种网络在岩石75%为液态的温度下依然存在，因此该网络被解释为在结晶早期形成，并在熔岩流下部的晶体糊压实过程中发挥了重要作用。尽管这种纹理对于我们理解此类玄武岩流如何形成至关重要，但其纹理的起源仍不确定。我们在此表明，尽管该网络存在于熔岩流下部三分之一处，但实际上是在上部凝固前沿形成，并通过密集晶体糊的羽流向下传输。这种对流类型已被假定存在于侵入性岩浆房中，但确凿的野外证据一直模棱两可，尤其是在熔岩湖和熔岩流中。因此，厚层溢流玄武岩流下部保存的源自顶部的纹理构成了对流在简单岩浆层凝固和分异过程中所起作用的重要证据。
