Samarium-neodymium evolution of meteorites

Abstract

We have obtained Sm-Nd data on two differentiated meteorites, Ibitira, a eucrite with distinct basaltic texture and with evidence of crystallization; and Morristown, a group 3A mesosiderite; as well as on Acapulco, an unclassified meteorite with chondritic chemical composition and a highly recrystallized texture. We have demonstrated the presence of in situ decay of short-lived ^(146)Sm in these meteorites with initial abundance of ^(146)Sm/^(144)Sm from 0.009 to 0.007 for the different meteorites. Precisely defined ^(147)Sm-^(143)Nd internal isochrons were obtained yielding ages of 4.60 ± 0.03 AE for Acapulco, 4.47 ± 0.02 AE for Morristown and 4.46 ± 0.02 AE for Ibitira. The corresponding initial ^(143)Nd/^(144)Nd values obtained were 0.2 ± 0.9, 2.1 ± 0.7 and 1.6 ± 0.8 ϵu (epsilon units). Detailed consideration of the coupled ^(147)Sm-^(143)Nd and ^(146)Sm-^(142)Nd systematics and of the elevated initial ^(143)Nd/^(144)Nd values indicates the presence of inconsistencies in the data for the two Sm-Nd parent-daughter pairs in spite of the precise isochrons. We present a model which provides an interpretation of the paired Sm-Nd systematics in terms of an impulsive disturbance. The abundance of phosphate, with large concentrations of REEs, controls the REE economy. Plagioclase, with low REE concentrations is assumed to be susceptible to exchange. Pyroxene is considered to be undisturbed because of the low diffusion coefficients of trace elements. The experimental results can be reasonably explained by this model. The results indicate that three meteorites studied, some with very low REE concentrations, including a mesosiderite, are relatively ancient objects, formed within the first 50 to 100 m.y. of the solar system, by planetary differentiation and impact processes, and were subjected to late metamorphism. The time of late metamorphism is substantially younger than 4.4 AE and possibly as recent as ~3.9 AE. This metamorphism has resulted in partial, very limited reequilibration between plagioclase and phosphate, resulting in the observed shifts in initial ^(143)Nd/^(144)Nd, Sm-Nd model ages, and the ^(142)Nd/^(144)Nd abundances, but preserving the major temporal structures of early original formation and later disturbances. These results show the need for care in the interpretation of the parent-daughter systematics, even when they appear well behaved and the mineral phases are viewed as highly resistant to mild metamorphism.