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

Vittorio Scribano,Serafina Carbone

doi:10.3390/geosciences10090378

Abstract

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.

Highlights

Garnet-bearing pyroxenites from ultramafic massifs and xenolith suites the world over can be due to crystal accumulation from mafic magmas at mantle depths, together with variable amounts of trapped interstitial magma [1]
Mineral chemistry information was obtained by scanning electron microscope (SEM) fitted with energy dispersion system (EDS), microprobe fitted with wavelength dispersion system (WDS) and Laser-Ablation inductively coupled plasma mass spectrometry (LA-ICPMS)
Experimental results in the simplified CaO–MgO–Al2 O3 –SiO2 (CMAS) system conducted by different authors over time, indicates that, at high pressure conditions (P ≥ 2 GPa), an Al-rich, Ca-Si-poor clinopyroxene first segregates from the liquid

Summary

Introduction

Garnet-bearing pyroxenites from ultramafic massifs and xenolith suites the world over can be due to crystal accumulation from mafic magmas at mantle depths, together with variable amounts of trapped interstitial magma [1]. Garnet is either considered a primary igneous phase [2,3,4] or product of reaction between pyroxene and spinel, due to variations in pressure and/or temperature conditions in the system [5,6]. Generally called "recycled" [7], are instead thought to have origin from high-pressure metamorphism of crustal igneous mafic rocks during subduction events. The Hyblean Plateau (Sicily, Southern Italy: Figure 1) consists of a tectonically uplifted sequence of deep-sea clayey and carbonate rocks, Upper Triassic to Miocene in age and open-shelf clastics. Plio–Pleistocene in age, with intercalations of mafic volcanic rocks. The oldest sedimentary and volcanic rocks outcropping in this region date back to the Upper Cretaceous. The main tectonic feature of the Hyblean Plateau consists of a NE-SW oriented system of extensional faults accommodated along a N-S trending right-lateral transform fault zone (Figure 1). An active system of normal faults originated a steep submarine slope, the “Hybla-Malta Escarpment,” that separates the Eastern Sicily from the Ionian abyssal plain

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