The comparative behavior of apatite‐zircon U‐Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation

Abstract

Abstract-We report secondary ion mass spectrometry (SIMS) U-Pb analyses of zircon and apatite from four breccia samples from the Apollo 14 landing site. The zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two. SIMS U-Pb analyses show that the ages of zircon grains range from 4023 ± 24 Ma to 4342 ± 5 Ma, whereas all apatite grains define an isochron corresponding to an age of 3926 ± 3 Ma. The disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite U-Pb systems have been completely reset at 3926 ± 3 Ma, whereas the U-Pb system of zircon has not been noticeably disturbed at this time. The apatite U-Pb age is slightly older than the ages determined by other methods on Apollo 14 materials highlighting need to reconcile decay constants used for the U-Pb, Ar-Ar and Rb-Sr systems. We interpret the apatite age as a time of formation of the Fra Mauro Formation. If the interpretation of this Formation as an Imbrium ejecta is correct, apatite also determines the timing of Imbrium impact. The contrast in the Pb loss behavior of apatite and zircon places constraints on the temperature history of the Apollo 14 breccias and we estimate, from the experimentally determined Pb diffusion constants and an approximation of the original depth of the excavated samples in the Fra Mauro Formation, that the breccias experienced an initial temperature of about 1300-1100 °C, but cooled within the first five to ten years.

Petrogenesis and chronology of lunar meteorite Northwest Africa 4472: A KREEPy regolith breccia from the Moon

“…Pb, U and Th concentrations in Matrix Apatites 1-3 range from 53 to 471 ppm, 36 to 373 ppm and 44 to 298 ppm (Table 3), respectively, and have Th/U ratios in the range 0.8-1.34. U concentrations in the KREEP (apatite-rich) basaltic assemblage range from 258 to 529 ppm (average 373 ppm), which are notably higher than apatites (<212 ppm and typically <60 ppm) reported by Nemchin et al (2009) (Fig. 13) and similar Th/U ratios (12.5-13.8) suggesting that they are possibly from the same parent lithology.…”

“…However, as has been recently demonstrated by Nemchin et al (2009), phosphates have low-closure temperatures ($500°C: Cherniak et al, 1991) that may make them more susceptible to metamorphic thermal resetting than co-existing zircon phases (see also Vaughan et al, 2006;Arai et al, 2010).…”

“…As discussed in Section 3.2.1, U-Pb and Pb-Pb age dates of lunar phosphates has in the past been interpreted to represent both magmatic (Vaughan et al, 2006;Terada et al, 2007aTerada et al, ,b, 2008Joy et al, 2009) and impact thermal metamorphic reset age events (Nemchin et al, 2009;Arai et al, 2010). As the majority of the phosphates we have analysed are matrix grains with little or no petrographic context it is difficult for us to definitively conclude if the NWA 4472 matrix apatites and merrillites represent all magmatic episodes or subsequent impact events.…”

Uranium–Lead, Extraterrestrial, Planetary Materials

“…Nevertheless, only a limited number of U-Pb ages of lunar phosphates are published. These published results indicate a profound resetting of the system during the 3.9 Ga cataclysm (Grange et al 2009;Nemchin et al 2009b). Rasmussen et al (2008) demonstrated that at least some lunar basalts contain a suite of Zr minerals, such as zirconolite, tranquillityite, and baddeleyite.…”

Degassing Processes and Chemistry

“…Obviously there would be some delay in forming the lunar highlands to provide for accretion of the Moon after the MFI, but this is presumed to be short given the ages of the oldest lunar rocks. However, the MFI dates from about 4.5 BYBP, while the oldest lunar rocks are at least 50 MY and perhaps more than 100 MY younger (Halliday 2008;Nemchin et al 2009Nemchin et al , 2012. Figure 2.1 illustrates that this is precisely the time interval during which the late veneer would have been added to Earth.…”