Aragonite and magnesite in eclogites from the Jæren nappe, SW Norway: disequilibrium in the system CaCO₃–MgCO₃ and petrological implications

Abstract

Eclogites from the Jaeren 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 + clinozoisite + 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.

Subduction, peak and multi-stage exhumation metamorphism: Traces from one coesite-bearing eclogite, Tso Morari, western Himalaya

“…6 from this study closely resemble Fig. 7a in Smit et al (2008), and that Fig. 10B from this study apparently resemble Fig.…”

“…10B from this study apparently resemble Fig. 10 in Smit et al (2008) with the exception that aragonite is present in the Garbon eclogites of Norway but calcite crosscuts magnesite in our studied Tso Morari eclogite and that goethite formed always alongside the first generation of calcite (Point 3 in Fig. 6 and Table 1).…”

Provenance and exhumation of an exotic eclogite-bearing nappe in the Caledonides: a U–Pb and Rb–Sr study of the Jæren nappe, SW Norway

“…1b), which hosts an eclogite occurrence that is somewhat enigmatic in the context of recent geodynamic models. Recently, Lu-Hf dating of garnet from sporadic eclogite outcrops yielded an age of 471 AE 1 Ma, which was interpreted to closely approximate the time when peak-metamorphic conditions of c. 2.6 GPa and 585-655 8C were reached (Smit et al 2008(Smit et al , 2010. The data showed that HP metamorphism was protracted, lasting at least until c. 458 Ma.…”

Aragonite-calcite-dolomite relationships in UHPM polycrystalline carbonate inclusions from the Kokchetav Massif, northern Kazakhstan

“…Aragonite and magnesite present clear disequilibrium textures, indicating the replacement of magnesite by aragonite (Smit et al, 2008). They are inferred to have occurred via a coupled dissolution/precipitation reaction not related to the breakdown of dolomite.…”

“…Aragonite inclusions in diamond have recently been interpreted as evidence of very deep subductions of roughly 280 km (Dobrzhinetskaya et al, 2006) although P-T conditions for the main eclogite stage in the eclogites from the relevant Jaeren nappe in the Norwegian Caledonian orogenic belt, have been estimated at 2.3-2.8 GPa and 585-655 C (Smit et al, 2008). Aragonite and magnesite present clear disequilibrium textures, indicating the replacement of magnesite by aragonite (Smit et al, 2008).…”