The Full Form of DMM is‍ Depleted Morb Mantle.

Depleted mantle: The depleted mantle is the part of Earth’s mantle from which basaltic melt has been extracted in one or multiple melting events at, for example, mid-ocean ridges, hot spots, or island arcs. Elements that do not fit into the crystal lattice of mantle minerals, the so-called incompatible elements, are preferentially incorporated into the melt and thus removed from the mantle, leaving the latter depleted with respect to these elements. This residual depleted mantle amounts to anywhere between 30 % and 100 % of Earth’s mantle.