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3 years ago in Mantle Processes , StructuralGeology By Walterwig
Why do some authors consider Dunite as a transitional zone while the underlying Harzburgite is the residual peridotite in some ophiolite sequences?
I'm studying the Samail ophiolite mantle section, and the standard model is that harzburgite is the residue after melt extraction. But directly above it, I have a thick dunite zone. If we've already extracted melt from the harzburgite, where does this dunite come from? It seems counterintuitive that a more refractory rock sits below a more fertile one. Is this a reaction or a chanIneling feature?
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By Shahruddin Jafri Answered 1 year ago
You've hit on a key insight dunite in this context is not a simple melting residue. From my work on mantle sections, I interpret these dunite zones as high-permeability melt channels. They form when ascending silica-undersaturated melts dissolve orthopyroxene from the surrounding harzburgite, leaving behind a purified olivine rock. So, the harzburgite is the regional residue from ~15% melting, but the dunite represents the fossilized plumbing system where the final melts were focused and chemically equilibrated on their way to form the overlying crust. It’s a reaction feature, not a stratigraphic layer in the traditional sense.
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