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An investigation of multiphase solid inclusion assemblages inside a garnet websterite sample from Fjørtoft, W. Norway

By G.P. Sypkens


Multiphase solid inclusions are found in a garnet websterite from Bardane, Fjørtoft Island, W. Norway. This garnet websterite consists out of large crystals of orthopyroxene + clinopyroxene + garnet + olivine + Cr-spinel and comes from a pod inside an orogenic garnet peridotite body inside a continental gneiss complex. The websterite pod crystallized in original minimal mid-Proterozoic depleted mantle peridotite and got metasomatised during a subsequent Caledonian to Scandian subduction cycle. The subducting slab released COH-rich subduction zone fluids concentrated and along into the slab-mantle interface, responsible for the precipitation of multiphase solid inclusions inside the primary minerals. Previous work on a garnet websterite from the same peridotite body in Bardane indicated the presence of microdiamonds in multiphase solid inclusions giving evidence for UHP (>130 km) deep subduction. A comparison with this previously studied garnet websterite will be made with an in-situ garnet websterite to confirm the presence of in-situ microdiamonds in multiphase solid inclusions. Back-scattered electron SEM images have identified multiphase solid inclusions with mineral assemblages of phlogopite, BaMg-carbonate, dolomite, magnesite, Cl-apatite, monazite, rutile, Cr-spinel (Cr#=58-91), (Fe,Ni,Cu) sulfides and elemental C inside M2 orthopyroxene and clinopyroxene crystals. This means introduction of at least the elements C, K, Ti, Ba, Cl, P, Ni, Cu, S, Nb, La, Ce and H2O by pervasive subduction zone fluids. Inclusions in M2 spinel grains (Cr#=55) contain OH-apatite, orthopyroxene, clinopyroxene and Al-oxides. Several elemental carbon inclusions in orthopyroxene and clinopyroxene have an euhedral shape similar to those already identified as microdiamonds in the previously studied websterite. The current study unfortunately lacks Raman confirmation to positively identify the carbon inclusions as microdiamonds, but taken the similarities between the two websterites into account it is very likely that the euhedral shaped carbon inclusions found in this study are microdiamonds or are graphitized microdiamonds. Electron microprobe data confirmed the similarities between the two garnet websterites by elemental wt. % oxide analysis where the chemical compositions of the main M2 minerals are within 1 wt. % oxide difference from each other. The high occurrence of elemental carbon in the multiphase solid inclusions gives evidence for a reducing character of the subduction zone fluid, although there is also a fair amount of carbonate present in the multiphase assemblages. Oxygen fugacity was therefore not high enough to break up all carbonate. This leads to the conclusion that the subduction zone fluid was an immiscible C-saturated C-O-H-, silicate- and sulfide-rich (supercritical) fluid. Where LILE, HFSE and LREE were released from the subducting slab by dehydration and decarbonization reactions, forming fluids rich in solutes which were pervasive and precipitated inside recrystallizing minerals as MSI (in the garnet websterite) in a reducing environment

Year: 2016
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