Ages of pristine noritic clasts from lunar breccias 15445 and 15455

Abstract

Rb-Sr and Sm-Nd isotopic studies were undertaken of two Apollo 15 breccias, 15445 and 15455, collected near Spur Crater on the Apennine Front. Seven pristine lithic clasts including four norites, one anorthositic norite, one troctolitic anorthosite and one spinel troctolite, as well as a matrix sample of 15445, were analysed. In addition, a K-Ar age determination was also made for a plagioclase separate from a different anorthositic norite clast. Sm-Nd isotopic data of a pristine Mg-norite sample, 15445,17, from the large white Clast B of breccia 15445 yielded a precise internal isochron age of 4.46 ± 0.07 Ga for λ( 147 Sm) = 0.00654 Ga −1 and an initial 143Nd 144Nd of 0.506058 ∓ 0.000078 (normalized to 148Nd 144Nd = 0.24308) corresponding to an initial ϵ Nd of +0.71. However, Sm-Nd isotopic results of another Mg-norite sample, 15445,247, chipped from the same Clast B, about 1 cm away from 15445,17, yielded a significantly younger isochron age of 4.28 ± 0.03 Ga and a higher initial 143Nd 144Nd of 0.506246 ∓ 0.000037 corresponding to an initial ϵ Nd of −0.35. The Sm-Nd data indicate that Clast B is heterogeneous and contains at least two similar lithologies. The Rb-Sr isotopic systems were severely disturbed for both norites. No useful isochron ages were observed. The Sm-Nd age and ϵ Nd data for A-15 Mg-norites 15445,17 and 15445,247, A-17 Mgnorite 78236, and two Mg-gabbronorites 73255,27,45 and 67667 are clearly resolved from each other. Four distinct parental magmas are required for derivation of these two major groups of Mg-suite noritic rocks. The Rb-Sr isotopic system of an anorthositic norite clast, 15455,228, was slightly disturbed; and isotopic data defined an isochron age of 4.59 ± 0.13 Ga for λ( 87Rb) = 0.0139 Ga −1 or 4.55 ± 0.13 Ga for λ( 87Rb) = 0.01402 Ga −1 . The corresponding initial 87Sr 86Sr for the rock is 0.69899 ∓ 0.00006 and is in satisfactory agreement with the LUNI value of Nyquist et al. (1974). The Sm-Nd system is more disturbed than the Rb-Sr system for this rock. A tie-line between whole-rock samples and a pyroxene separate gives a Sm-Nd age of 4.53 ± 0.29 Ga, which is in nominal agreement with the Rb-Sr isochron age. The 39Ar 40Ar age of a plagioclase separate of the rock has a weighted average value of ~ 3.83 Ga, suggesting that the age was reset by the Imbrium impact event. Probably, the Sm-Nd and Rb-Sr systems of the clast were also partly reset by this event. Our age results indicate that some Mg-suite rocks are as old as ferroan-anorthosite-suite rocks. Furthermore, age data of three major crustal rocks (a Mg suite, a ferroan-anorthosite suite, and an evolved suite) show that they all have variable ages. These observations are difficult to explain by the “magma ocean” hypothesis alone. Petrogenetic processes such as production of Mg-suite rocks by “serial magmatism” ( Walker, 1983), of ferroan-anorthosite-suite rocks by “multiple diapiric intrusions” ( Longhi and Ashwal, 1985), and evolved-suite rocks by “continuous magmatism” ( Meyer et al., 1988, 1989) probably all played a role. These processes seem to be able to accommodate the wide ranges of ages and different origins observed for lunar crustal rocks.