The influence of T, a(SiO2), and f(O2) on exsolution textures in Fe-Mg olivine: An example from augite syenites of the Ilimaussaq Intrusion, South Greenland

Two samples from the augite syenite unit of the 1.13 Ga alkaline to peralka line Illimaussaq intrusion in South Greenland contain olivine (Fa(75.85))Wi th micrometer-sized exsolution lamellae of intergrown augite and magnetite. The exsolved olivine contains only about 0.6 wt% CaO, whereas the reintegr ated magmatic composition contained about 1.4% CaO corresponding to about 3 mol% of a larnite (La) component. The exsolved olivine bears no measurable Na, Al, or Ti, but the reintegrated magmatic olivine contained up to 0.4 w t% Na2O, 0.2 wt% Al2O3, and 0.3 wt% TiO2. Magmatic baddeleyite in the same samples indicates an extremely low silica activity of below 0.4 during the early stages of magmatic crystallization at > 900 degreesC and 1 kbar. Acco rding to the equilibrium Ca2SiO4 (La in Ol) + SiO2 = 2 CaSiO3 (Wo in Cpx), the low silica activity is thought to be responsible for the unusually Ca-r ich compositions of the primary olivines, which, in these samples, formed a t 875 +/- 15 degreesC and at oxygen fugacities between 1.2 and 2.3 log unit s below the QFM buffer. The composite exsolution lamellae formed during coo ling due to an overstep of the schematic reaction Fa + Kst (Ca-rich compone nt in olivine) + 1/3 O-2 = Hd + 2/3 Mt. Phase equilibria indicate temperatu res between 300 and 750 degreesC and oxygen fugacities close to QFM for thi s exsolution process. The lamellae only developed in samples where the oliv ines were both especially high in Ca and presumably rapidly cooled (althoug h not chilled!), as the lamella-bearing samples were collected at the outer most rim of the augite syenite unit and close to the top of the Ilimaussaq intrusion. Schreinemakers analysis shows that Ca-rich olivine of the kirsch steinite-monticellite solid solution series [Ca(Fe,Mg)SiO4] would exsolve f rom Ca-bearing olivine of the fayalite-forsterite series only at unusually low silica activities. whereas at higher silica activities, the observed ex solutions of clinopyroxene with magnetite are more stable. Hence, both cool ing history and evolution of intensive variables during: cooling are import ant for the formation of olivine exsolution textures.