The Alnö complex: A messenger from the Earth’s mantle

Carbonatites and kimberlites are believed to be “messengers from the Earth’s mantle”. Carbonatites are rare igneous rocks that form intrusive and extrusive suites and usually contain at least 50% carbonate minerals. Globally, there is only one known active carbonatite volcano, located in northern Tanzania. Carbonatites and associated alkaline igneous rocks, due to their rareness and the information they may carry about the mantle, deserve special attention. The Alnö complex in central Sweden is presumably one of the largest of the few known alkaline and carbonatite intrusions in the world. Alnöites are melilite- and carbonate-bearing ultramafic lamprophyres that were first described from the island of Alnö in central Sweden. Since its identification over a century ago, the Alnö complex has been extensively studied for its origin, mantle composition and diamond exploration. The c. 30 km² Alnö alkaline and carbonatitic complex is a Late Precambrian intrusion in Early Proterozoic gneisses and migmatites, located in the northwest of Alnö island. It is one of two similar complexes in Scandinavia, the other being the Fen Complex in southeastern Norway. The inferred explosive origin for the Sälskär vent, that forms part of the complex, is a unique example of a large fossil carbonatite eruption. Despite its unique and globally significant nature, the structural relationships between the Alnö complex and associated lithological units of the surrounding rocks are poorly known at depth. For example, the depth extent, dip and dip direction of carbonatitic rocks have only been extrapolated from surface geological observations and, hence, requires robust constraints. The intrusion mechanism(s) is a matter of controversy and of great interest to geoscientists worldwide.
By drilling in two or three key locations within the Alnö complex (e.g., central and marginal parts of the complex), we aim to unravel a series of key scientific questions that have for a long time engaged the geoscientific community. These are: (i) what is the intrusion mechanism(s), (ii) what is the spatial, geochronological and compositional relationship between the main Alnö intrusion and the inferred Sälskär carbonatite breccia vent, (iii) what is the origin of the carbonate minerals and its implications for mantle processes and (iv) what are the petrophysical properties of the main lithological units in the Alnö complex at depth.

Principal investigators: Alireza Malehmir, Valentin Troll, Uppsala University.