Diffusion of Chemical Elements in Some Naturally Occurring Silicate Inclusions

Abstract

In the York area, Maine, the Agamenticus alkaline ring-dike complex cuts the enclosing Kittery formation. The rock types of the ring-dike complex are, from oldest to youngest, hybrid alkaline granite, alkaline syenite (nordmarkite), alkaline granite, and granodiorite. The nordmarkite and the alkaline granite contain numerous inclusions of the Kittery formation. These inclusions show mineralogical and chemical zoning caused by the influence of the surrounding melt. The Kittery formation is a rhythmically bedded sequence of alternating light-colored and dark-colored beds. The light-colored beds were originally of impure limestones and tuffaceous limestones, which have now been metamorphosed to a fine-grained rock containing diopside-hedenbergite, tremolite-actinolite, plagioclase, and orthoclase. The dark-colored beds were originally subgraywackes and tuffaceous subgraywackes which have now been metamorphosed to a rock containing quartz, biotite, muscovite, orthoclase, and andesine. When fragments of these two rock types were introduced to the ring-dike melt, there occurred a reaction, caused by a concentration gradient which existed between inclusions and melt. Mineralogical and chemical changes were produced by this reaction in both the inclusions and the melt. During the early stages these changes are represented within the inclusions by concentric zones of contrasting mineralogy and chemistry. The mineralogy of each of the early-formed zones is characterized by development of the following minerals: aegirinaugite (exterior zone), microcline, hastingsite, typical Kittery formation (at center). The chemistry varies greatly, depending on which zone is sampled. Generally speaking, the oxides of the elements show one of the following trends: geochemical depression in a specific zone, geochemical culmination in a specific zone, a gradual increase from the center of the inclusion outward, a gradual decrease from the center of the inclusion outward. During the later stage of alteration the earlier-formed zones are obliterated, and the inclusion approaches the mineralogical and chemical composition of a syenite. Throughout the history of alteration the basic constituents and silicon move outward toward the borders of the inclusions, and the alkalies all move into the inclusions from the surrounding melt.