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

W standard Nd standard OIB -7 -6 -5 -4 -3 -2 -1 0 1 2 3 142 Nd OIB MORB 0.0 0.2 0.4 0.6 0.8 1.0 Probability density Probability density -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 -10 -9 -8New 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 μ 142 Nd and μ 182 W values of the depleted mantle source of mid-ocean ridge basalts known as DMM.The PAR basalts yield a mean μ 142 Nd = -1.6 ± 5.0 (2 s.d.) and μ 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 μ 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.