Why dunite has no volcanic equivalent?

Answer 1

Dunite is composed predominantly of olivine. A volcanic rock of dunite composition would need to form from a lava/magma of dunnite composition. A magma of dunite i.e. olivine composition would never be produced from melting of mantle or crustal rocks because olivine melts/crystallises at higher temperatures than most other minerals. Therefore, melting any rock other than dunite itself would produce a magma with a composition less Fe-Mg-rich than dunnite. Dunite forms in 2 ways: When olivine is concentrated by accumulation at the base of a magma chamber (cumulate layaered gabbro); and by chemical reaction between an exsolved basaltic melt and residual peridotite in the lithospheric mantle. Pyroxene in the mantle dissolves and olivine is left behind as a residual material.

Answer 2

Dunite has no volcanic equevalent simply because the magma that have dunite composition does not reach or go out in the surface.