The issue of the terrestrial record of Sm-146

• Published in: Geochimica et cosmochimica acta 1996-06, Vol.60 (11), p.2037-2047
• Main Authors: Sharma, M, Papanastassiou, D A, Wasserburg, G J, Dymek, R F
• Format: Article
• Language: English
• ISSN: 0016-7037
• DOI: 10.1016/0016-7037(96)00082-8

Harper and Jacobsen (1992) reported a 33 mu Nd-142 excess in a 3.8 Ga felsic gneiss (IE 715-28) from Isua, West Greenland. This paper addresses the question of how well a 30 mu effect can be resolved in the Nd-142 excess, whether the effect observed in the IE 715-28 is true, and whether there are Nd-142 excesses in other rocks. Data were obtained on the Caltech nNdbeta standard and two enriched gravimetric standards, the latter prepared with 30 and 57 mu excesses in Nd-142 relative to nNdbeta. The data were collected on an instrument identical to that used by Harper and Jacobsen. The Nd-142/Nd-144 ratio was measured in two Isua felsic gneisses in addition to IE 715-28. The results suggest that reliable measurements of Nd-142-Nd-144 ratios to the 10 mu level as needed to resolve 30 mu excesses are not yet possible by the authors. It is demonstrated that although claims of well-resolved, about 30 mu Nd-142 excesses cannot be unambiguously confirmed, there is a strong hint of the effect claimed by Harper and Jacobsen. Moreover, another felsic gneiss from Isua examined here may also have an excess of Nd-142. It is concluded that further refinements in the experimental techniques are needed to obtain reliable ultrahigh precision measurements. This is necessary before the issue of the terrestrial record of Sm-146 is resolved. (AIAA)