Petrogenesis of an early Archaean (3.4 Ga) norite dyke, Isua, West Greenland: evidence for early Archaean crustal recycling?

An unaltered norite cumulate dyke from the central part of the Isua area, West Greenland, has been studied in an attempt to determine the isotopic composition of early Archaean mantle. A mineral (plagioclase–orthopyroxene) and whole rock Sm–Nd isochron yields an age of 3413±57 Ma, which is slightly younger than a previously published U–Pb zircon SHRIMP age of 3512±6 Ma (Nutman et al., 1997). While the mineralogy (opx+plag+ol+cr-sp) and mineral chemistry of the dyke (opx: En90–70, plag: An73–54, ol: Fo90–87, and Cr-sp: Cr66–56) are consistent with crystallization from a near primary melt, a strong crustal signature characterizes the trace element and isotopic composition of the dyke i.e. (Nb/La)PM=0.20–0.25; εNd=−1.3±1; (La/Yb)N=4.8–4.1. Hydrothermally altered samples up to 5 cm distant from a cross-cutting quartz-vein exhibit systematic changes in trace element and isotopic composition approaching the quartz-vein (a total variation of ∼2.5 εNd-units; (La/Yb)N=5.5–4.1). This illustrates one danger of simple interpretations of isotope compositions of early Archaean whole rocks. Modeling the chemical and isotopic signature of the dyke either requires unrealistically large amounts (>20%) of crustal assimilation or introduction of smaller amounts (∼5%) of crustal material directly into a mantle reservoir prior to melting. Therefore, we suggest a petrogenetic model involving early Archaean crustal recycling processes.