Partial melting of eclogite, Tromso, Norway

Continental eclogites in the Tromso Nappe, Norway exhibit partial melting textures consisting of peritectic grains amidst quartzofeldspathic leucosomes. There are three types of partial melting textures, where melt reactions have resulted in (1) peritectic garnet formation; (2) peritectic amphibole formation, with preservation of eclogitic garnet is preserved; and (3) peritectic amphibole formation, where eclogitic garnet is depleted and biotite inclusions exist within amphibole. Each of these reactions is analyzed collectively using multivariate statistical means, while individual textures, each containing three to five samples, are also analyzed using geochemical mass balance techniques. Thermobarometry is also carried out on each reaction, showing that reaction (1) occurred at 2.0-2.2 GPa and 700-750°C, whereas reactions (2) and (3) occurred at 1.1-1.2 GPa and 720-800°C. The melt forming reactions are shown to be approximately univariant, with the first principal component of their composition matrices defining variance caused by the melt-forming reaction. A technique is developed to determine the bulk-rock partitioning behavior of each element from its factor loading in the principal component of melting. Inferred bulk partitioning behavior agrees with melt composition estimates yielded from mass balance of residual rock and inferred protolith compositions. For reactions (1) and (2), where garnet either grows or is preserved, strong partitioning of Ti, heavy rare earth elements, Y, Zr, and Fe into residue is observed, whereas Sr, light rare earth elements, and Mg partition into melt. In reaction (3) the amount of garnet is depleted relative to the starting material, and heavy rare earth elements partition weakly into melt, whereas light rare earth elements partition into residue, producing the opposite rare earth element fractionation to the other reactions. Melts for reactions (1) and (2) are adakitic, whereas reaction (3) lacks the high La/Lu of adakites. Reactions (1) and (2) produce high Mg# (Mg# = MgO/(FeO total+MgO) melts, showing that high Mg# adakites can be produced in the lower continental crust, where opportunities for mantle wedge interactions traditionally thought to cause high Mg# melts are lacking. Based on mass balance between residual rock and protoliths, we infer degrees of melting of around 50-55% for reaction (1), 45-55% for reaction (2), and 35-40% for reaction (3). Partitioning behavior of H2O, alkali elements, U, Th, and Pb suggest that influx of a fluid rich in these elements caused the high-degree melting.