Petrogenesis of kelyphites in garnet peridotites: a case study from the ulten zone, italian Alps

Spinel-bearing symplectites around garnet (kelyphite) record the transition from high-pressure garnet peridotite to low-pressure spinel peridotite. In the first part, we review the previous studies that have been devoted to kelyphite. Many authors have identified orthopyroxene, spinel, clinopyroxene and amphibole in kelyphite. They have emphasized the common existence of two concentric coronae: an outer spinel-poor corona on the olivine side and an inner spinel-rich corona on the garnet side. Chlorite and plagioclase have also been reported in low-grade kelyphites, whereas more complex kelyphite-forming associations, including phlogopite, ilmenite, calcite and perovskite, have been observed in kimberlite xenoliths and interpreted as resulting from metasomatic action of the kimberlite magma or deep fluids. Three different origins have been proposed for kelyphite: magmatic origin, exsolution from orthopyroxene and metamorphic reaction between garnet and olivine during retrogression. This last interpretation is by far the most common. Finally, we discuss the origins of other pyroxenes + spinel symplectites and clusters that occur in peridotite. In the second part, we study various kelyphites observed in the Ulten peridotites (Italian Alps). They are composed of two concentric coronae: (i) a narrow outer spinel-free corona, in contact with olivine, made of orthopyroxene ± amphibole ± clinopyroxene; (ii) a wide inner corona, in contact with garnet, which consists of vermicular spinel forming various symplectites with either orthopyroxene, amphibole or clinopyroxene. An attempt to balance the kelyphite-forming reaction is made through a material transfer study, in order to determine element mobilities and to evaluate the opening of the system during the reaction. Two different quantitative treatments are used: the first approach consists in estimating the stoichiometric coefficients by the least square method; the second is based on measurements of the stoichiometric coefficients of the kelyphitisation products by analysis of back-scattered electron and X-ray images. This study shows that the kelyphite-forming reactions range between two extremes: garnet + olivine → orthopyroxene + spinel + clinopyroxene and garnet + olivine + H2O → orthopyroxene + spinel + amphibole. This accounts for strong variations in the relative abundance of amphibole and clinopyroxene, which were correlated with local fluctuations of PH2O. It is demonstrated that the boundary between the two concentric coronae coincides with the former garnet–olivine interface. These coronae did not maintain the garnet and olivine compositions; they exchanged components during their growth. Moreover, the whole kelyphite system was slightly open, with gains in alkalis, water and Mg, and losses of Fe and Al. The behaviour of Mg and Fe is likely explained by their incipient redistribution between ferromagnesian phases during kelyphitisation, in relation with the temperature decrease. Change in garnet composition close to kelyphite strongly supports this hypothesis. Incipient Tschermakitic substitution in neighbouring minerals could also explain the slight opening to Si and Al. Kelyphite formation has many characteristics of diffusion metasomatism at microscale. μ–μ diagrams are used to illustrate some aspects of this metasomatism. Finally, pressure–temperature conditions of kelyphite formation in the evolution of peridotite during exhumation are discussed. Kelyphitisation is mainly related to a drop in pressure, as attested by a strong volume increase (ΔV/V=7.1).