The Formation of Dunite Channels within Harzburgite in the Wadi Tayin Massif, Oman Ophiolite: Insights from Compositional Variability of Cr-Spinel and Olivine in Holes BA1B and BA3A, Oman Drilling Project

Cocomazzi Cocomazzi,Bussolesi Bussolesi,Crispini Crispini,Grieco Grieco,Zaccarini Zaccarini,Science Team Science Team,Tartarotti Tartarotti

doi:10.3390/min10020167

Cocomazzi Cocomazzi, Bussolesi Bussolesi + Show 5 more

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https://doi.org/10.3390/min10020167

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Abstract

Holes BA1B and BA3A were drilled into the Wadi Tayin Massif, southern ophiolite complex of Oman, a fragment of the Tethyan oceanic lithosphere obducted onto the Arabian continent. Within the sequence, we have studied a portion of the shallow mantle, composed mainly of strongly serpentinised harzburgite that embeds dunitic levels, the biggest being over 150 m thick. The formation of thick dunitic channels, already approached via published structural and mathematical models, is here investigated with a mineral chemistry approach. We focused on Cr-spinel, the only widespread phase preserved during serpentinization, whose TiO2 content displays a wide variability from low in harzburgite, (TiO2 < 0.25 wt. %), typical of non-metasomatised ophiolite mantle, to moderately high in dunite (TiO2 < 1.10 wt. %) characterizing a rock/melt interactions. The high variability of TiO2, accompanied by similar patterns of Cr# and Mg# is observed, in a fractal pattern, at all scales of investigation, from the whole channel scale to the single thin section, where it affects even single grain zonings. Our results suggest that the over 150 m thick dunite channel here investigated was formed by coalescence of different scale melt channels and reaction zones with different sizes, confirming the published structural model.

Highlights

The study area is of great interest, as it is occupied by one of the most complete and best-preserved ophiolite sequences in the world: the Semail ophiolite complex, a group of several massifs cropping out in a NW–SE trending and 500 km long belt along the coast of Oman (Figure 1)
The Semail ophiolite, to which the Wadi Tayin Massif belongs, represents a fragment of the Tethyan oceanic lithosphere obducted onto the Arabian continent and is the largest, best exposed and most studied ophiolite complex in the world (e.g., [7,8,9,10])
Microchemical analyses were on two different sets of samplesFor by samples using two different electron microprobes (EMPs) withperformed a carbon-coating used as electrical conductor

Summary

Introduction

The study area is of great interest, as it is occupied by one of the most complete and best-preserved ophiolite sequences in the world: the Semail ophiolite complex, a group of several massifs cropping out in a NW–SE trending and 500 km long belt along the coast of Oman (Figure 1). Harzburgite and dunite occur as alternate levels of various thickness, intruded at places by minor gabbroic and pyroxenitic dykes. A relevant feature of the drilled ophiolitic sequence is the presence of thick dunitic levels within the harzburgitic mantle rocks, such as the one located at the top of the Hole BA1B, with an apparent thickness of over 160 m. These dunitic levels are interpreted as channels resulting from melt (mid ocean ridge basalt, MORB) percolation through the upper mantle in an ocean ridge geodynamic environment [1,2]

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