Fluids in the Siilinjärvi carbonatite complex, eastern Finland: Fluid inclusion evidence for the formation conditions of zircon and apatite

Abstract

In the studied zircon and apatite crystals, data recorded two different compositional types of fluid inclusions: Type 1 H,0-C0, low salinity inclusions (XCO, = 0.42 to 0.87; XNaCl = 0.001 to 0.005) with bulk densities of 0.73 to 0.87 g/cm 3 , and Type 2 H,0 moderate salinity (XNaCl = 0.03 to 0.06) inclusions with densities of 0.83 to 1.02 g/cm 3. The Type 1 inclusions are not present in apatite. In zircon, the observed fluid inclusion types occur in separate domains: around (Type 1) and outside (Type 2) the apparent core. Fluid inclusions are further subdivided into pseudosecondary and secondary inclusions. Using a combination of SEM-EDS, optical characteristics and crushing-stage, various daughter and captive minerals were identified. The fluid inclusion data suggest that the pseudosecondary Type 1 and Type 2 inclusions in zircon and apatite were trapped during the pre-emplacement evolution of the carbonatite at mid-crustal conditions (P>4 kbar, T> 625°C). The Type 1 fluid was depleted in CO, during crystal fractionation and cooling leading to a fluid phase enriched in water and alkalies. Fenitization was obviously induced by these saline aqueous fluids. During emplacement of the carbonatite to the present level, zircon phenocrysts were intensively fractured, some Type 1 inclusions were re-equilibrated, and multiphase Type 2 inclusions were trapped. It is assumed that all these inclusions in zircon and the pseudosecondary Type 2 inclusions in apatite have a magmatic origin. In apatite, calcite inclusions occur side-by-side with the secondary Type 2 inclusions. These calcites co-existed with the aqueous fluid during fracturing and metamorphic re-crystallization of apatites. Probably, this metamorphic fluid also is responsible for the transport and deposition of at least some of the calcite at low temperatures (200-350°C).

Geochemical and spectroscopic investigation of apatite in the Siilinjärvi carbonatite complex: Keys to understanding apatite forming processes and assessing potential for rare earth elements

“…Studies focusing on the mineralogy and chemistry of apatite from Siilinjärvi are scarce. Apart from the analyses referred to above, a microthermometric study by Poutiainen (1995) corroborated a primary igneous origin of the apatite and the involvement of an aqueous fluid during fracturing and recrystallization of this mineral. A few isotopic studies on apatite separates from Siilinjärvi have given more information about the origin of this mineral.…”

The Archean Siilinjärvi Carbonatite Complex

“…Thus, the development of H 2 O-and alkali-rich late-stage fluids that formed the fenite halo was a direct consequence of the early crystallization of predominantly carbonate and apatite (Poutiainen, 1995). Thus, the development of H 2 O-and alkali-rich late-stage fluids that formed the fenite halo was a direct consequence of the early crystallization of predominantly carbonate and apatite (Poutiainen, 1995).…”

“…Compositions of the fluids that produced these fenites have been determined from fluid inclusions within magmatic zircon and apatite (Poutiainen, 1995). The main minerals in the fenites are microcline, amphibole, and pyroxene, but there exists a wide variety of fenite types including: pyroxene, amphibole, carbonate, quartz, aplitic, and quartz-aegirine varieties.…”

Integrated Geological and Geophysical Mapping of a Carbonatite-Hosting Outcrop in Siilinjärvi, Finland, Using Unmanned Aerial Systems

“…The carbonatite intrusion was emplaced into basement gneiss and deformed by the Svecofennian orogeny at 1.8 Ga [53]. Local rock types are fenite, gneiss, carbonatite-glimmerite, diabase, and other dykes (e.g., local diorites).…”