Very high CO₂ cordierite from Norwegian Lapland: Mineralogy, petrology, and carbon isotopes

Abstract

The most CO₂-rich cordierite thus far encountered in nature with about 2.2 wt.% CO₂ and 0.3 wt.% H₂O occurs as large poikiloblasts in a strange non-foliated “reaction rock” that dissects well-foliated granulites being part of the classical Lapland granulite area described by Eskola. The cordierite is optically positive with the highest optic angle 2V _x (∼106°) and birefringence (γ−α = 0.017) ever measured on natural cordierites, but it is also optically very heterogeneous due to secondary loss of CO₂ along fractures and zones paralleling the fluid-bearing channels. Based on the optical properties of the degassed Lapland cordierite and on literature data a ternary diagram is given, which shows the variations of this cordierite in 2V _x and birefringence as a function of channel-filling with both CO₂ and H₂O.

Following Losert (1971) the cordierite coexists with calcite, a thus far unique mineral assemblage that is probably only stable at very high CO₂ pressures. In the present case, the $X_{{\text{CO}}_{\text{2}} }$ of the cordierite (∼0.75) indicates, on the basis of literature data, a coexisting fluid with $X_{{\text{CO}}_{\text{2}} }$>0.95.

The carbon isotope composition δ ¹³C of CO₂ in cordierite lies near −7‰, that of the calcite is slightly lighter than about −9‰. Thus, at least for the CO₂ in cordierite, a deep-seated origin may be possible.

Based on the geologic occurrence it is speculated that the cordierite-bearing “reaction rock” could perhaps represent an annealed channel of late degassing in the granulitic lower crust.