Polymineralic inclusions as tracers of multistage metasomatism in a paleo mantle wedge

Abstract

Paleosubduction zones are key areas where slab-derived fluids interact with the overlying mantle wedge. Slab fluids play therefore an important role in the deep element cycle and can metasomatize ultramafics showing particular mineral assemblages. The nature of slab fluids is usually investigated from fluid inclusions and/or multiphase solid inclusions, as well as from incompatible elements in mantle minerals. We have made a study on a garnet-peridotite from the Variscan Ulten Zone (Eastern Alps, Italy) in which peculiar polymineralic inclusions (PI) in garnet provide a wealth of information on the interaction between mantle wedge and slab fluids. The selected sample shows a fine-grained amphibole-olivine-orthopyroxene-spinel-clinopyroxene matrix and a fractured large garnet porphyroclast, surrounded by a kelyphitic corona. Garnet core and rim host randomly distributed PI, which may be as large as 284,000 μm2. The PI show irregular, fracture-controlled shapes and often contain unusual mineral associations. Many PI are directly associated with Mg-hornblende- or chlorite-sealed micro-fractures crosscutting garnet. The most abundant PI contain spinel and amphibole as the main minerals. Spinel-bearing PI consists of high-Cr spinel (Cr# = 0.31–0.41) surrounded by amphiboles (magnesio-hornblende), locally associated with dolomite, sapphirine, calcite, sulphides and kinoshitalite (i.e. a Ba-rich phlogopite). Amphibole-bearing PI contain zoned amphiboles (tschermakitic cores surrounded by Mg-hornblende, which is rimmed by tremolite and Mg-amphibole), dissakisite (an REE-bearing epidote), apatite, dolomite, calcite, spinel, sapphirine, chlorite, kinoshitalite, and sulfides. The retrieved mineral reaction history and the major and some incompatible elements (Cl, Ba, Sr) of PI minerals provide clues about the composition of metasomatic fluids. The studied PI formed in garnet fractures during exhumation by reactions between garnet and (1) slab-derived COH fluids and (2) late-stage saline brines. Metasomatic fluids/brines were rich in volatile elements and had a crustal signature (enrichment in LILE and LREE) with variable Cl contents and CO2/H2O ratios. Products of early garnet-fluid reactions became reactants of later reactions, thus generating complex textures, including replacement textures of sapphirine over spinel, calcite over dolomite and chlorite over Mg-hornblende. Textures and compositions of the PI testify to a multistage metasomatic overprint at peak P-T conditions above 2 GPa (M1), during the first exhumation stage at 775–1000 °C, below 2 GPa (M2) and finally at 775 °C and 1 GPa (M3). In this study we demonstrate that Gibbs free energy minimization may aid in distinguishing between multiphase solid inclusions and polymineralic inclusions; the latter do provide valuable information on the evolution of retrograde P-T paths in the paleosubduction channel, the relative chronology of metasomatic events and clues on the chemical signature of the fluids.