T. Neil Irvine and Hatten S. Yoder, Jr.

Metasomatic rocks are formed when an original rock is transgressed by mineralized solutions (liquid and/or vapor) that react in passing to produce material of new composition. Commonly the new rock replaces the original on an approximately volume-for-volume basis. Metasomatic rocks are widespread in the Earth's crust and upper mantle, and they frequently are exotic in mineralogy and texture. Many feature monomineralic zones; some are economically important as ore deposits; almost all are problematic in origin. The nature and origin of the fluids and the mechanics of the reaction processes are especially difficult to define.

The Skaergaard intrusion is situated in East Greenland, just north of the Arctic Circle. It solidified from a body of magma (molten rock) roughly 10 km long, 7 km wide, and 3.5 km thick. The intrusion is renowned for spectacular primary layering, and it is by virtue of this layering that the secondary metasomatic bodies considered here are recognized (below). These bodies are mostly anorthosite, a white rock composed largely of the mineral plagioclase (a solid solution of CaAl₂Si₂O₈ and NaAlSi₃O₈), but many have thin basal rims of peridotite, a brown rock rich in olivine (a solid solution of Mg₂SiO₄ and Fe₂SiO₄). The anorthosite bodies typically wend through their layered host rock without appreciably displacing its stratification, this being prime evidence that volume-for-volume replacement, and the bodies are ideal for study because they are well exposed, convenient in size (1-10 m long), and distinctively simple. It is reasonably established that the host rock (called gabbroic troctolite) originated by sedimentation of plagioclase and olivine crystals on the floor of the original magma body as it cooled and solidified, and that the metasomatism was imposed as the pore liquid between the accumulated crystals gradually solidified. Water was evidently important to the metasomatism, because the anorthosite plagioclase contains tiny, H₂O-rich vapor bubbles.

The metasomatism is further explored here through a new experimental observation that addition of H₂O to olivine-plagioclase cotectic melts in a model system Mg₂SiO₄-CaAl₂Si₂O₈-SiO₂ strongly shifts their compositions toward plagioclase.

The diagram below illustrates schematically how such shifting might induce mineral exchange reactions in troctolite, and the drawing (right) interprets the Skaergaard metasomatic processes using these reactions. The dry liquid, L^O, is portrayed as original pore liquid being gradually filter pressed upward through the layered crystal pile by compaction. Hydrous liquid L¹ forms in the lower part of the intrusion where H₂O entering along fractures from the floor or wall rocks interacts with the first L^O it encounters; then, being the more fluid and buoyant liquid, L¹ preferentially rises along permeable channels to local sites of metasomatism near the top of the crystal pile. Metasomatic anorthosite forms where L¹ dehydrates by boiling; the peridotite rim at its base develops concurrently below a density graded L¹/L^O diffusive interface.

Fig. 11. Metasomatic anorthosite formed by replacement of layered gabbroic troctolite in the Skaergaard intrusion is shown here. A thin, dark rim of peridotite follows the lower edge of the anorthosite. The layering was almost horizontal originally, and all the rocks have been tilted to the right.

Fig. 12. This image shows possible interactions of olivine-plagioclase cotectic liquids with troctolite (plagioclase-olivine rock) from gains or losses of H₂O and heat, based on newly defined relations for the system Mg₂SiO₄-CaAl₂Si₂O₈-SiO₂-H₂O at 5 kilobars pressure.

Fig. 13. Physical processes are inferred for the formation of Skaergaard metasomatic anorthosite and peridotite in this diagram.