Polymineralic inclusions in eclogite-facies metapelites from Tanzania – insights on partial melting and geodynamic processes in a 2 Ga old subduction zone
Final Report Abstract
Polymineralic inclusions found in garnet porphyroblasts of eclogite-facies metapelites of the Usagaran belt (Tanzania) indicate the unique case of partial melting of a subducted oceanic crust 2 Ga ago. Their investigation by Micro Raman spectroscopy, La-ICP-MS and piston cylinder experiments combined with thermodynamic modeling (using Theriak-Domino) enable to constrain the composition of the melt and the influence of a C–O–H fluid. Furthermore, the time and process of melt formation in relation to the P–T path is evaluated. Two types of polymineralic inclusions (silicate-rich and carbonate-rich) occurring side by side together with CO2±N2 fluid inclusions in the intermediate growth zone of the garnet porphyroblasts imply a simultaneous formation. While the silicate-rich inclusions, containing quartz, feldspar, biotite, kyanite, dolomite (<10 vol%), represent the trapped melt, the carbonate-rich inclusions (10–40 vol% dolomite, quartz, kyanite) can be regarded as a reaction product of the host garnet with the CO2 fluid. This interpretation is supported by the exceptional positive trend of the LREE spectra of dolomite within the carbonate-rich polymineralic inclusions, which mirrows the LREE spectra of the garnet. By replacement of garnet by dolomite, the immobile REE could be incorporated into the dolomite. A relationship with carbonatite-like melts can be excluded for the carbonaceous inclusions because of the low LREE content of the dolomite. Fluid inclusion investigations indicate immiscibility of an original H2O–CO2±N2 fluid into a CO2–N2-bearing H2O fluid and a CO2–N2 fluid with variable N2 content. The variable N2 content supports an internal origin of the fluid phase. As the silicate-rich melt inclusions also contain dolomite, the melt should have been saturated with CO2. Graphite, only determined in garnet, probably was not present in the rock prior to metamorphism, but precipitated inside the Fe2+-rich garnet by reduction of the CO2 fluid through oxidizing the Fe2+ of the garnet to Fe3+. The highly ordered graphite structure indicates a graphite formation at high temperatures of about 700°C. Melting experiments at pressures of 1.5 GPa, 750–900°C and 2.5 GPa, 950–1050°C show that the silicate-rich inclusions homogenized at the higher P–T conditions, although reactions with the garnet host indicated by halos and newly formed minerals are often observed. The melt composition of homogenized inclusions without obvious reaction halo or secondary minerals is rhyolitic/granitic, peraluminous with a high K/Na ratio. A similar melt composition is found by thermodynamic modeling just above the phengite-out line in the stability field of the assemblage melt+garnet+alkali feldspar+biotite+quartz+kyanite+rutile. CO2 inclusions and dolomite-bearing inclusions suggest that CO2 was present in addition. The suggested P–T path during uplift of the subducted rocks from a depth of 60-70 km starts from P–T conditions of 1.8–1.9 GPa / 700–800°C to the stage of partial melting at 1.3–1.6 GPa / 820–870°C. The P–T path continues with decompression and cooling below solidus, thereby remaining in the kyanite stability field at pressures below phengite stability (0.8–0.9 GPa / 700–750°C). The investigation of the melt inclusions from the old subduction environment demonstrates, that partial melting of the metasedimentary cover of the oceanic crust was possible at a moderate depth of about 50–60 km. The temperatures at this depth must have been much higher than for most younger oceanic crusts, where temperatures near the slab interface are too cold for melting processes. Melt formation in the 2 Ga old subducted oceanic crust was possible in spite of the presence of CO2, which lowers the H2O activity in the metapelites.
Publications
- (2018). Melting in high-pressure granulitefacies metapelites from a 2 Ga old subducted oceanic crust: evidence from polymineralic inclusions. GeoBonn 2018, Abstract book p.54, Bonn
P. Herms, C. Giehl, P. Appel, A. Möller, P. Raase
- (2019). Polymineralic silicateand carbonate-rich inclusions in eclogite-facies metapelites: indication for immiscible melts in the subducted oceanic crust. European Current Research on Fluid Inclusions, Acta Mineralogica-Petrographica, Abstract Series, Vol.10, 2019, p.54, Budapest
P. Herms, C. Giehl, A. Rohrbach, P. Appel, L. Aradi, P. Raase
