Discussion
The depleted MORB Mantle (DMM) reservoir shows following Sr-Nd isotopic compositions :
Answer: C
DMM has low 87Sr/86Sr and high 143Nd/144Nd.
The depleted MORB mantle has lost incompatible elements to the continental crust over Earth history. Rb is more incompatible than Sr, so the residue has a low Rb/Sr ratio and has produced little radiogenic 87Sr. Nd is more incompatible than Sm, so the residue has a high Sm/Nd ratio and has grown more radiogenic 143Nd, giving positive \(\varepsilon_{\text{Nd}}\).
High Sr with low Nd ratios describes enriched sources, such as continental crust and EM2-type mantle. The other two combinations do not match the coupled Rb-Sr and Sm-Nd behaviour of a depleted reservoir.
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