SECTOR-ZONED AEGIRINE FROM THE ILIMAUSSAQ ALKALINE INTRUSION, SOUTH GREENLAND - IMPLICATIONS FOR TRACE-ELEMENT BEHAVIOR IN PYROXENE

Sector-zoned aegirine crystals from a peralkaline nepheline syenite in Ilimaussaq, South Greenland, have been analyzed for major and trace e lements using combined electron microprobe and secondary-ion mass spec trometry techniques. Unlike in calcic clinopyroxene, the faster growin g basal sector of the sodic clinopyroxene is enriched in incompatible elements such as Sr, REE, and Zr relative to the slower growing prism sectors. The prism sectors are enriched in Al and Ti relative to the b asal sector, but the Al-Ti enrichment patterns in the prism sectors co ntrast with those in calcic clinopyroxene. Zr, REE, and Sr exhibit a p ositive correlation to Ca in the M2 site and a negative correlation to Na in the M2 site. The contrasts we report between aegirine and augit e demonstrate that site characteristics and the ability of a site to a ccommodate a cation are important factors for controlling sector enric hments. The addition of an acmite component [Ca (M2) + Fe2+ (M1) <-- - -> Na (M2) + Fe3+ (M1)) to the pyroxene results in a shift of the opti mum potential radius for cations accommodated in the M1 and a change i n the ideal charge characteristic of the M2 site. Those cations with r adii that deviate the farthest from the optimum M1 and M2 site radii a nd ideal charge for a particular pyroxene composition have a greater p robability of being retained in a faster growing basal sector than in the more slowly growing prism sector. These observations do not, howev er, eliminate a model in which adsorption and desorption kinetics play s an important role in the differences among sectors in major componen ts [i.e., Na/(Na + Ca) and Fe3+/(Fe2+ + Fe3+)]. This mechanism may the n ultimately be responsible for the differences in crystal chemistry o r geometric factor among sector growth surfaces.