A Note on the Origin of Ophitic Texture in the Chilled Olivine Gabbro of the Skaergaard Intrusion

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Abstract
The textures of the chilled marginal gabbro of the Skaergaard intrusion and of the coarser marginal gabbros 25 yards further in are described and related to counts of the number of separate crystals of the chief mineral species per cubic centimetre of the rock. An explanation of the textures and number of crystal individuals is attempted in terms of various degrees of supersaturation in the magma. In the chilled rock abundant nucleation of plagioclase and olivine, presumably at the middle labile stage of supersaturation, is inferred. This is followed by ophitic or poikilitic crystallization of pyroxene and ilmenite about scarce nuclei, produced at lower temperatures, presumably during the early labile stage of supersaturation for these two minerals. The coarser gabbro, further in from the margin, has about half the number of plagioclase crystals per cm.3 compared with the chilled rock and only one twenty-fifth the number of olivine crystals, and these have a tendency to be ophitic in texture. Under these slower cooling conditions, supersaturation apparently still reached into the middle labile region for plagioclase, giving many nuclei, while olivine, like pyroxene, separated under early labile supersaturation conditions and only relatively few nuclei formed. The contrasting textures of the plateau magma-type basalts of Mull and the tholeiitic basalts of Northern Ireland are explained along similar lines.

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