A Lunar Cataclysm at ~3.95 AE and the Structure of the Lunar Crust

Fouad Tera, D. A. Papanastassiou and G. J. Wasserburg, Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif. 9 1 109 ABSTRACT: Detailed Rb-Sr and U-Th-Pb analyses on two Apollo 16 rocks are presented and compared with results from Apollo 12,14 and 15 samples. These data point to a period of extensive melting and metamorphism at -3.95AE (possibly produced by the Imbrium event at 3.95-4.00AE? ) which covered a large fraction of the lunar surface with materials rich in almost magic component. A Rb-Sr internal isochron for the plagioclase rich basalt 68415 yields T = 3.84k0. 01AE and initial Is,=0.69920~3. The model age is T B A B I = ~ . ~ ' ~ . ~ A E . This igneous rock could have been produced from a magma generated either by impact melting or by internal heat sources (1). Additional (2) analyses of 65015 which is a complex, metamorphosed, multilithic rock are shown in Table 1. These include a single plagioclase phenocryst; a composite of plag phenocrysts; pure pyroxene; pyroxene with 4% whitlockite; an ilmenite rich fraction; and quintessence (Q) separates. A line drawn through the total rock (TR) and the Q data (Fig. 1) corresponds to T=3.92AE and IS,= 0.70030. This line passes through all data except all the plagioclases. The bulk of the plag in the rock lies close to, but below, this line. All plagioclases lie on a mixing line with a low end member phenocryst near BABI and the bulk plag (with higher K, Rb contents) as the high end member. The 3.92AE line is interpreted to be an isochron which represents the time of recrystallization of 65015. The plagioclase phenocrysts are relict phases which are often clasts and clearly were not isotopically or chemically equilibrated, whereas the finer grained euhedral plag came very close to equilibration. If this rock were originally igneous and if the relict crystals are cognate, then the plag phenocrysts and TR determine the time of primary crystallization. This time is nearly equal to T B A B I = ~ . ~ ~ A E . If the relict plag are xenocrysts, picked up in a magma, or if the whole rock is a polymict agglomerate, then they do not represent the initial 87Sr/ 86Sr. In the latter case, the Rb-Sr TBABI age is strongly suggestive of a "magic" component which was mobilized and transported at -3.9AE. A summary of all Rb-Sr ages from this laboratory is shown in Fig. 2. Note the following major points: 1) Two Apollo 16 rocks have the same age as Apollo 14; 2) The high IS, of 650 15 (a high K rock) is identical to that for 143 10 type rocks although the ages are not identical; 3) The remobilization age of 120 13 (3) overlaps the age of some Apollo 14 rocks and 650 15; 4) There is a great gap between 4.OAE and the formation of the moon; 5) Rb-Sr model ages for soils and many total rocks (including mare basalts) commonly lie near -4.6AE. The extent of the temporal and isotopic similarity of 6841 5 and 650 15 and the Apollo 14 rocks may be seen from comparing Pb-U-Th analyses. Plag separates were leached in 1N HN03 and then dissolved. 204 Pb blanks were 0.02-0.04 picomoles. The picomoles of isotopes in each sample, the daughter-parent ratios and model ages are given in Table 3. Entries are corrected for chemistry blanks. The leaches are dominated by common Pb from mineral separation steps. Leaching apparently does not cause serious differential loss of parent and daughter. High 208Pb and Th suggest phosphate dissolution. The systematics displayed by the TR and leached plag are approximately obeyed by the leaches within the uncertainty in a of contaminant Pb from mineral seperations. A suggestion of differential leaching of Pb-U is seen for 650 15 PI. In general the inherent dangers of leaching require carefully controlled procedures. Concentrations and atomic ratios are given in Table 2 and include the anorthosite 60025(4). The large difference in 207Pb/206Pb for the TR and leached plag in 68415 shows that there is initial radiogenic Pb in the rock. Thus, the essentially concordant Pb-U-Th ages of -4.5AE (note T B A B I = ~ . ~ * O . ~ A E ) do not correspond to the crystallization age but reflect the possibility that this rock was a very early lunar differentiate which has not been significantly fractionated during later melting. The TR and plag data are shown on the diagram of our choice. An isochron through these data intersects concordia at 3.9450. 05 and 4.47'0.02AE with an initial radiogenic 207Pb/206Pb, Ipiy1.45. The data for 6501 5 TR are essentially concordant at 4.00AE for all Pb-U-Th systems but distinct from TBABI=~. SAE. This shows that this total rock cannot be a closed system of ancient lunar crust although this was our previously preferred hypothesis from Rb-Sr data (2). The 65015 Pb-U TR age is very close to the Rb-Sr recrystallization age (3.92AE). Because of the lower age resolution possible for these Pb-U systems, a best fit isochron has been drawn (Fig. 3) for the