Convective Instability in Intraplate Oceanic Mantle Caused by Amphibolite-Derived Garnet-Pyroxenites—A Xenolith Perspective (Hyblean Plateau, Sicily)

Geochemical characteristics of middle ocean ridge basalts (MORBs) testify partial melting of spinel‐peridotite mixed with a few amounts of garnet‐pyroxenite. The latter can be considered either autochthonous products of the crystallization of partial melts in the sub‐oceanic mantle or allocthonous recycled crustal materials originated in subduction contexts. Here we suggest the “autocthnous recycled” origin for garnet‐pyroxenites. Such a hypothesis derives from the study of garnet‐bearing pyroxenite xenoliths from the Hyblean Plateau (Sicily). These consist of Al‐diopside, pyralspite‐series garnet, Al‐spinel and Al‐rich orthopyroxene. Trace element distribution resembles an enriched MORB but lower chromium. Major‐element abundances closely fit in a tschermakitic‐horneblende composition. Assuming that a high‐Al amphibolite was formed by hydrothermal metasomatism of a troctolitic gabbro in a slow‐spreading ridge segment, a transient temperature increasing induced dehydroxilization reaction in amphiboles, giving Al‐spinel‐pyroxenite and vapor as products. Garnet partially replaced spinel during an isobaric cooling stage. Density measurements at room conditions on representative samples gave values in the range 3290–3380 kg m–3. In general, a density contrast ≥300 kg m–3 can give rise to convective instability, provided a sufficient large size of the heavy masses and adequate rheological conditions of the system. Garnet‐pyroxenite lumps can therefore sink in the underlying mantle, imparting the “garnet geochemical signature” to newly forming basaltic magma.