Metasomatic Reaction Phenomena from Entrainment to Surface Cooling: Evidence from Mantle Peridotite Xenoliths from Bulgaria

Abstract

We present a detailed study of the mineralogical and chemical modifications of peridotite xenoliths during magma transport and cooling at the Earth’s surface. The xenoliths are entrained in three small-volume (<1 km3), monogenetic, Miocene–Oligocene, basanite domes in the Moesian Platform, north Bulgaria, arranged along a NNE-directed right-lateral strike-slip fault. The domes show symmetrically decreasing volumes of erupted magma correlated with the size and quantity of the entrained xenoliths, according to their position along the fault. The xenoliths exhibit different degrees of mineralogical and chemical interaction with their host rocks, with the extent of interaction strongly depending on their position in the dome. Xenoliths from the fine-grained brecciated carapaces of the domes show very thin, fine-grained, reaction rims around orthopyroxene and spinel, and thin diffusion zones around olivine, limited mostly to the rims of the xenoliths. Clinopyroxene shows almost no visible reaction and is always strongly depleted in light rare earth elements (LREE) and Sr. Melt pockets in the xenoliths are small, composed mostly of fine-grained olivine and clinopyroxene. Modelling of Fe–Mg zoning in olivine suggests a very short residence time of a few days in the magma during transport and fast cooling at the Earth’s surface. In contrast, xenoliths from the interior of the domes, hosted in holocrystalline groundmass, are much more strongly affected by the host basanite magma. Their constituent minerals have wider reaction rims around orthopyroxene, sometimes leading to its entire consumption, and show transformation of spinel into chromite. Fe–Mg diffusion profiles in olivine are up to 400 μm wide and calculations indicate diffusion times up to 200 days, recording protracted cooling in the inner part of the dome. Melt pockets are much larger and coarser-grained, composed of minerals identical to the host groundmass. With few exceptions, clinopyroxene is sieve-textured and shows variable enrichment in LREE and Sr, ranging from several times higher than in the depleted xenoliths to complete equilibration with the host basanite. Strongly veined xenoliths show stronger chemical modifications, facilitated by infiltrated melt, which also progressively increase depending on the position of the xenoliths in the dome. The most enriched xenoliths from the core of the dome exhibit large inter- and intra-grain variations in Sr and LREE. Our study demonstrates that chemical and mineral modifications, although starting at the time of entrainment of the xenoliths at mantle depths, were completed mostly during their residence in the magma at the surface. The reaction phenomena are the result of post-entrainment partial melting, and reactions between xenolith minerals and infiltrated fluids and melts from the host basalts. The large inter- and intra-crystal chemical variations in a single xenolith suggest that reactions strongly depend on the access of fluids and melts (permeability) in different parts of the xenoliths. The results of this study allow us to introduce the term ‘host basalt metasomatism’ for those mantle xenoliths that have undergone chemical alteration at or near the surface during cooling of the host magma. Comparison with xenoliths stored in large-scale magmatic systems under La Palma (Canary Islands) shows that, although the products of interaction between xenoliths and host rocks are similar, there are considerable differences in the mechanism of entrainment, depth and longevity of the reactions between small-volume and large-scale magmatic systems.