Geochemistry

Emulsifying the Crust

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Science  09 Dec 2005:
Vol. 310, Issue 5754, pp. 1587
DOI: 10.1126/science.310.5754.1587a

Not quite 2 billion years ago, a large asteroid stuck itself into what is now eastern central Ontario, forming the Sudbury impact crater. The energy of the impact melted a large amount of the continental crust, producing a thick melt sheet that was initially about 1700°C, well above the liquidus for norite (~56% silica) and for granophyre (~70% silica).

Zieg and Marsh describe the subsequent evolution and cooling of this molten body as a natural experiment that can be compared to the formation of magma bodies in igneous intrusions such as those underlying volcanoes. The superheated Sudbury melt sheet began as an emulsion containing droplets of silica-rich and silica-poor magma; the less dense, silica-rich drops separated within months and coalesced into an upper melt sheet. Vigorous convection in both sheets occurred until they cooled to the liquidus, at which time crystals began to form and convection ceased. The combined melt layers solidified from the top and bottom. Aside from the initial separation of the two liquids, the solidified sheet shows little compositional gradations. Early formed crystals are dispersed throughout, and layers are not apparent. These textures contrast with those of many igneous magma bodies, suggesting that the latter may not have originated as large hot chambers at an instant in time. — BH

Geol. Soc. Am. Bull. 177, 1427 (2005).

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