Ancient solar wind in lunar microbreccias

The Apollo 11 soil breccias are samplers of the ancient lunar environment due to their history in the regolith and their efficient closure to addition of recent solar wind upon compaction. These breccias contain the lowest¹⁵N/¹⁴N isotopic ratio yet reported for any lunar sample (in fact, for any natural sample). This extends the range of variation of¹⁵N/¹⁴N of the solar wind to greater than 30%, from a δ¹⁵N of ?190‰ in the past to +120‰ at present. No mechanism is yet known that is capable of accounting for such a large change in the¹⁵N/¹⁴N ratio without producing a substantial concomitant change in the¹³C/¹²C ratio, although some sort of nuclear reaction in the sun appears to be required. Apollo 11 soil breccias and 15086 are all formed by meteoritic impacts which compact the lower regolith against the basement rock without much heating. Rock 15086 formed from the layer of regolith between 100 and 200 cm depth, as shown by the close agreement between the nitrogen content and isotopic ratios of 15086 and those of the Apollo 15 deep drill core. Cosmic ray exposure ages, based on spallation-produced¹⁵N, are 2.3 ± 0.4 b.y. for Apollo 11 breccias. This age is much greater than the estimate from cosmogenic²¹Ne, presumably due to diffusive loss of neon.