Comparative 142Nd and 182W study of MORBs and the 4.5 Gyr evolution of the upper mantle

New high precision Nd and W isotopic compositions were obtained on the same basalt samples from the Pacific-Antarctic Ridge. These provide the best estimate so far for the μ¹⁴²Nd and μ¹⁸²W values of the depleted mantle source of mid-ocean ridge basalts known as DMM. The PAR basalts yield a mean μ¹⁴²Nd = −1.6 ± 5.0 (2 s.d.) and μ¹⁸²W = −1.9 ± 3.5 (2 s.d.), which together with the literature data allow the isotope composition of the DMM to be constrained. The present-day DMM μ¹⁸²W is 10–20 ppm lower than that of the Archean mantle. This decrease could be related to the broad incorporation of mantle plume material into the upper mantle, starting between 2.4 and 3 billion years ago, due to the onset of deep cold slab subduction, and its attendant return mantle flow.