Multiple thermal events during 4350–3920 ma recorded in a single lunar meteorite Northwest Africa 4485
Tomoko Arai, Takayuki Tomiyama, Takafumi Niihara, Tatsunori Yokoyama, Miwa Yoshitake, Hiroshi Kaiden, Keiji Misawa, Anthony J. Irving
Icarus, in Press, Journal Pre-proof, Available online 22 June 2025
“Highlights: We have undertaken mineralogical and petrological studies of NWA 4485, a KREEP-rich polymict breccia, along with U-Pb and 207Pb-206Pb dating of zircon and baddeleyite in it, and mainly focus on the mineralogy and chronology of that clast and related mineral fragments to further constrain the nature and the timing of KREEP magmatism and a relationship between pre-mare and mare volcanisms. The presence of a 4160 Ma zircon in a millimeter-sized lithic clast, a core of 4210 Ma in a detrital zircon with multiple rims of ~3960 Ma, and an individual zircon grain of 4350 Ma in the matrix clearly indicate that they are products of pre-Mare multiple KREEP-related magmatism, predating the lunar cataclysm. The multiple rims in the overgrown zircon in the matrix could have recorded later thermal events, possibly by the lunar cataclysm.”
“A lunar meteorite Northwest Africa (NWA) 4485 is a KREEP (potassium, rare earth elements and phosphorus)-rich polymict breccia, likely paired with NWA 4472. NWA 4485 includes mm-sized lithic clasts with variable textures and modal abundances. The lithic clasts share features with KREEP basalts, and consist dominantly of moderately Mg-rich pyroxene and less calcic plagioclase than those in the Apollo 17 KREEPy basalt with zircon and phosphates. Uranium‑thorium‑lead isotopic studies on zircon and baddeleyite in the lithic clasts and matrix of NWA 4485 revealed that the 207Pb-206Pb age spectrum (4350–3920 Ma) is compatible with that for apatite in the paired NWA 4472, broadly covering the ages of zircons in Apollo polymict breccia samples from multiple landing sites. The presence of a 4160 Ma zircon in a millimeter-sized lithic clast, a core of 4210 Ma in a detrital zircon with multiple rims of ~3960 Ma, and an individual zircon grain of 4350 Ma in the matrix clearly indicates that they are products of pre-mare multiple KREEP-related magmatism, predating the lunar cataclysm (~3900–4000 Ma).”
