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 k elyphite. Many authors have identified orthopyroxene, spinel, clinopyroxene and amphibole in kelyphite. They have emphasized the common existence of t wo concentric coronae: an outer spinel-poor corona on the olivine side and an inner spinel-rich corona on the garnet side. Chlorite and plagioclase ha ve also been reported in low-grade kelyphites, whereas more complex kelyphi te-forming associations, including phlogopite, ilmenite, calcite and perovs kite, have been observed in kimberlite xenoliths and interpreted as resulti ng from metasomatic action of the kimberlite magma or deep fluids. Three di fferent origins have been proposed for kelyphite: magmatic origin, exsoluti on from orthopyroxene and metamorphic reaction between garnet and olivine d uring retrogression. This last interpretation is by far the most common. Fi nally, we discuss the origins of other pyroxenes + spinel symplectites and clusters that occur in peridotite. In the second part, we study various kel yphites observed in the Ulten peridotites (Italian Alps), They are composed of two concentric coronae: (i) a narrow outer spinel-free corona, in conta ct 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 o r clinopyroxene. An attempt to balance the kelyphite-forming reaction is ma de through a material transfer study, in order to determine element mobilit ies and to evaluate the opening of the system during the reaction. Two diff erent quantitative treatments are used: the first approach consists in esti mating the stoichiometric coefficients by the least square method; the seco nd is based on measurements of the stoichiometric coefficients of the kelyp hitisation products by analysis of back-scattered electron and X-ray images . This study shows that the kelyphite-forming reactions range between two e xtremes: garnet + olivine --> orthopyroxene + spinel + clinopyroxene and ga rnet + olivine + H2O --> orthopyroxene + spinel + amphibole. This accounts for strong variations in the relative abundance of amphibole and clinopyrox ene, which were correlated with local fluctuations of P-H2O It is demonstra ted 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. M oreover, the whole kelyphite system was slightly open, with gains in alkali s, water and Mg, and losses of Fe and Al. The behaviour of Mg and Fe is lik ely explained by their incipient redistribution between ferromagnesian phas es during kelyphitisation, in relation with the temperature decrease. Chang e in garnet composition close to kelyphite strongly supports this hypothesi s. Incipient Tschermakitic substitution in neighbouring minerals could also explain the slight opening to Si and Al. Kelyphile formation has many char acteristics of diffusion metasomatism at microscale. mu-mu diagrams are use d to illustrate some aspects of this metasomatism. Finally, pressure-temper ature conditions of kelyphite formation in the evolution of peridotite duri ng exhumation are discussed. Kelyphitisation is mainly related to a drop in pressure, as attested by a strong volume increase (Delta V/V= 7.1). (C) 20 00 Elsevier Science Ltd. All rights reserved.