Nd Isotopic variations and petrogenetic models

Abstract

^(147)Sm ɑ-decays to ^(143)Nd so that ^(143)Nd/^(144)Nd reflects the time-integrated Sm/Nd environment of a sample. The increase in 143/144 in a reservoir with chondritic Sm/Nd is 1.2% in 4.5AE. There exists sufficient variation of Sm/Nd to cause sizeable effects in 143/144. Young samples were measured to elucidate the nature of their source regions. An oceanic high Fe, Ti basalt (113152) and alk. basalt (113031), a continental alk. besalt (BCR-l), an apatite (Khibiny massif) and two reagent "normals", NN1 and NN2, were analyzed. Isotopic ratios of NN2 and BCR-1, normalized to 148/144=0.241572 are tabulated. Following the pioneering work of Lugmair, et al. (EPSL 27, 79) our 143/144 data are presented relative to the total rock value for Juvinas (0.51278). the present value of a chondritic reservoir. Data are given as deviations from this value in parts in 10^4 (є) and show a wide range. Nd in the source regions of the rock samples evolved in an environment of approximately chondritic Sm/Nd (±5%) over the history of the earth. Small variations exist, reflecting long time scale differences of Sm/Nd in the source regions. The low Sm/Nd observed in alkali basalts cannot reflect an ancient source region with low Sm/Nd as є is near zero. REE patterns of alkali basalts must thus reflect relatively recent fractionation from a source with essentially chondritic relative abundances. Study of initial ^(143)Nd/^(144)Nd in conjunction with REE patterns promises to contribute important petrogenetic information.

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