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

Probability density Probability density .0 0.8 0.6 0.4 0.2 0.0 0.6 0.4 0.2 0.0 142 Nd-10-9-8 182 W MORB Nd standard OIB-7-6-5-4-3-2-1 0 1 2 3 OIB MORB W standard-10-9-8-7-6-5-4-3-2-1 0 1 2 3 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 mu 142 Nd and mu 182 W values of the depleted mantle source of mid-ocean ridge basalts known as DMM. The PAR basalts yield a mean mu 142 Nd = - 1.6 +/- 5.0 (2 s.d.) and mu 182 W = - 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 mu 182 W 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.