The Rb-Sr age of a crystalline rock from Apollo 16

Abstract

A precise internal isochron was determined for 68415, a plagioclase rich basalt or anorthosite returned by the Apollo 16 mission. The age and initial ⁸⁷Sr/⁸⁶Sr are T = 3.84 ± 0.01AE, andI = 0.69920 ∓ 3. This rock appears to have crystallized almost contemporaneously with the younger basalts found at the Apollo 14 site. If we associate the dates of the Apollo 14 rocks with the Imbrium event, then 68415 was formed by an event which followed closely after or during that episode of basin formation. If this rock represents lunar magmatic activity from internal heat sources, then it is likely that the lunar highlands and possibly older mare basins were intruded and flooded by leucocratic basalt flows or by plagioclase rich differentiates just before the major basaltic mare flows preserved in the Sea of Tranquility. If the parent magma for this rock was produced by impact melting, presumably of an anorthositic basement, then this datum suggests an extremely intense period of bombardment at that time. There is as yet no direct evidence of lunar rocks crystallizing in the interval 4.0 to 4.6 AE, or of the location of the ancient radioactive lunar crust (or crustal layers) hypothesized by us. The initial ⁸⁷Sr/⁸⁶Sr of lunar basalts and anorthosites are typically very primitive and show that these rocks are not related to chondrites or to lunar materials with high Rb/Sr such as 12013. The time of formation of the lunar highlands remains undetermined. The possibility exists that the moon underwent a major cataclysmic differentiation at ∼ 4.0 AE during which the highlands were formed.

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