Aragonite and magnesite in eclogites from the Jæren nappe, SW Norway: disequilibrium in the system CaCO 3 –MgCO 3 and petrological implications M. A. SMIT, M. BRO ¨ CKER AND E. E. SCHERER Institut fu ¨ r Mineralogie, Westfa ¨lische Wilhelms-Universita ¨t, Corrensstrasse 24, D-48149 Mu ¨ nster, Germany (m.a.smit@uni-muenster.de) ABSTRACT Eclogites from the Jæren nappe in the Caledonian orogenic belt of SW Norway contain aragonite, magnesite and dolomite in quartz-rich layers. The carbonates comprise composite grains that occur interstitially between phases of the eclogite facies assemblage: garnet + omphacite + zoisite + clino- zoisite + quartz + apatite + rutile ± dolomite ± kyanite ± phengite. Pressure and temperature conditions for the main eclogite stage are estimated to be 2.3–2.8 GPa and 585–655 °C. Published ultrahigh pressure (UHP) experiments on CaO-, MgO- and CO 2 -bearing systems have shown that equilibrium assemblages of aragonite and magnesite form as a result of dolomite breakdown at pressures >5 GPa. As a result, recognition of magnesite and aragonite in eclogite facies rocks has been used as an indicator for UHP conditions. However, petrological testing showed that the samples studied here have not experienced such conditions. Aragonite and magnesite show disequilibrium textures that indicate replacement of magnesite by aragonite. This process is inferred to have occurred via a coupled dissolution–precipitation reaction. The formation of aragonite is constrained to eclogite facies conditions, which implies that the studied rocks have experienced metasomatic, reactive fluid flow during their residence at high pressure (HP) conditions. During decompression, the bimineralic carbonate aggregates were overgrown by rims of dolomite, which partially reacted with aragonite to form Mg-calcite. The well-preserved carbonate assemblages and textures observed in the studied samples provide a detailed record of the reaction series that affected the rocks during and after their residence at P–T conditions near the coesite stability field. Recognition of the HP mechanism of magnesite replacement by aragonite provides new insight into metasomatic processes that occur in subduction zones and illustrates how fluids facilitate HP carbonate reactions that do not occur in dry systems at otherwise identical physiochemical conditions. This study documents that caution is warranted in interpreting aragonite-magnesite associations in eclogite facies rocks as evidence for UHP metamorphic conditions. Key words: Aragonite; eclogite; magnesite; Norway; ultrahigh pressure metamorphism. INTRODUCTION The reaction dolomite M aragonite + magnesite (R1) occurs at a pressure higher than 5 GPa (Liu & Lin, 1995; Martinez et al., 1996; Luth, 2001; Sato & Kat- sura, 2001; Shirasaka et al., 2002; Buob et al., 2006; Morlidge et al., 2006). This observation was corro- borated by findings of aragonite + magnesite as by-products in diamond-synthesis experiments on dolomite-bearing systems at pressures higher than 5 GPa (Sokol et al., 2001; Dobrzhinetskaya et al., 2004). Coexisting magnesite, dolomite and (magnesian-) calcite have been found in eclogites from several high pressure (HP) and ultrahigh pressure (UHP) meta- morphic terranes (e.g. Wang & Liou, 1993; Shatsky et al., 1995; Zhang et al., 2003). Aragonite and pseudomorphs after magnesite have been reported from UHP rocks of the Kokchetav Massif (Zhu & Ogasawara, 2002). Because of incomplete mineral assemblages or textural complications, geological interpretation of these carbonate assemblages is often highly controversial (e.g. Hermann, 2003; Klemd, 2003). The assumption that coexisting aragonite and magnesite in an eclogite represent an equilibrium assemblage is not self-evident (e.g. Proyer et al., 2008) and requires verification by detailed petrological study. The aragonite and magnesite inclusions in diamond and in coesite-bearing zircon that were reported in Dobrzhinetskaya et al. (2006) and Liu et al. (2006) respectively, represent the only well-documented occurrences of an UHP aragonite + magnesite para- genesis. In the present study, an association of arago- nite, magnesite and dolomite has been identified in eclogites from the southern part of the Scandinavian Caledonides. These eclogites occur outside the well- established UHP terrane of the Western Gneiss Region (WGR). Do these carbonate associations imply the J. metamorphic Geol., 2008, 26, 959–979 doi:10.1111/j.1525-1314.2008.00795.x Ó 2008 Blackwell Publishing Ltd 959