60025: relict of primitive lunar crust?

Abstract

New isotopic analyses are presented for 3 plagioclase-rich fractions and one mafic fraction from ferroan anorthosite 60025. The observed 206Pb/ 204Pb ratios vary between 52.5 and 60.5, all much higher than the ratio for terrestrial contamination. In a 207Pb/ 206Pb 204Pb/ 206Pb correlation diagram, the plagioclase data define a model Pb Pb age of 4.520 ± 0.007 AE using meteoritic primordial lead for the non-radiogenic component. In the concordia diagram the plagioclase data yield intersections at 4.503 ± 0.007 and 0.28 AE. The meaning of the lower intercept is obscure. The earlier 60025 analysis of Tera and Wasserburg [1], with an observed 206Pb/ 204Pb of 23.0, agrees closely with the new plagioclase data in the isotope correlation and concordia diagrams. Since the apparent age does not correlate with the 206Pb/ 204Pb ratios and U contents of the samples, it does not appear to be controlled by terrestrial lead contamination. The time-averaged μ values for the plagioclase leads are exceptionally low, 16–55, and agree within factors of 2 with the observed μ values in the samples. These are much lower than the values observed for mafic rocks or their sources, showing that the anorthosite lead has never been associated for a substantial length of time with any high μ source. In this way the 60025 data differ substantially from U Pb data for two other lunar anorthosites, 15415 and 60015. The results suggest that the averaged model Pb ages of 4.51 ± 0.01 AE closely approximate the crystallization age for the plagioclase fraction of the anorthosite, and that it dates back to an early phase in lunar history. One sample from the mafic fraction of 60025 yields a younger model Pb age of 4.42 AE. The age may have been lowered by post-crystallization disturbances or perhaps this fraction is not coeval with the plagioclase fraction.