Pb isotopes in the impact melt breccia 66095: Association with the Imbrium basin and the isotopic composition of lithologies at the Apollo 16 landing site

Abstract

Recent in situ Secondary Ion Mass Spectrometry (SIMS) Pb isotope analyses of lunar basalts have provided precise crystallisation ages and initial Pb isotopic compositions for these samples. In this study, the same approach has been tested in the Apollo 16 impact melt breccia 66095, referred to as the "Rusty Rock" due to its enrichments in volatile elements, including Pb. Based on these analyses of the breccia, a Pb-Pb isochron age of 3909±17 Ma (at the 95% confidence level) and an initial Pb composition for 66095 have been determined. This age is interpreted as representing the time of breccia formation that, when combined with recent studies of lunar breccias, can be linked to the Imbrium basin forming impact. The directly measured initial Pb composition of the breccia from this work is similar a modelled compositions presented previously, and likely reflects an average value for the lithologies present at the Apollo 16 landing site at the time that the Imbrium ejecta was emplaced. The 66095 initial Pb isotopic composition is compared with the compositions in other lunar samples and the nature of the endmember lithologies in this mixture has been discussed within the framework of a multiple stage model of Pb isotope evolution on the Moon. This study demonstrates the effectiveness of this technique beyond its application in crystalline basalts, opening up the possibility of obtaining precise geochronological and Pb isotopic compositions from a broader sample set than was previously recognised.

Chlorine isotopic compositions of apatite in Apollo 14 rocks: Evidence for widespread vapor-phase metasomatism on the lunar nearside ∼4 billion years ago

“…Vapor condensation has also been proposed as a mechanism to explain volatile contents in lunar glass beads (74220, 15426) and 66095 (Day et al, 2017). The two samples, 66095 and 79215, have yielded dates around 3.9 Ga (Fischer-Gödde and Becker, 2012;Norman et al, 2006;Snape et al, 2017), similar to the Apollo 14 samples studied here (Table 1). Altogether, these samples hint at the widespread occurrence of vapor-related metasomatism during a period when the impact flux on the lunar surface may have been particularly high (Gomes et al, 2005;Morbidelli et al, 2012;Tera et al, 1974).…”

Imbrium Age for Zircons in Apollo 17 South Massif Impact Melt Breccia 73155

“…The ages and petrography of these breccias have been used as references to constrain the age of the Imbrium basin formation. Recent U‐Pb isotopes of lunar zircons, phosphates, and whole‐rock melts proposed an Imbrium age within the period of 3.91 to 3.94 Ga (Gnos et al, ; Lin et al, ; Liu et al, ; Merle et al, ; Nemchin et al, ; Snape et al, ; Snape, Nemchin, Bellucci, et al, ; Thiessen et al, ; Thiessen et al, ; Hopkins & Mojzsis, ).…”

Post-Imbrium Pb–Pb isochron ages for Apollo basaltic impact melt samples 14078 and 68415

“…The majority of these compositions can be explained by a similar source µ-value of approximately 2400. ( c ) Modelled initial Pb composition of 68415 compared with the measured composition of impact melt breccia 66095 [78]. The composition of 66095 appears to indicate a slightly lower contribution of KREEP than is present in 68415, given that a growth curve extrapolated from the age of 66095 (3909 Ma) until that of 68415 (3834 Ma) fails to intercept the 68415 axis.…”

“…A slightly higher proportion of the KREEPy Pb component appears to have been incorporated into 68415, as a Pb growth curve between 3909 and 3834 Ma, starting from the 66095 initial Pb composition with a µ-value of 1000, fails to intercept the 68415 isochron (figure 4c). [79]; and the 3.9 Ga Apollo 16 impact melt breccia 66095 [78].…”

“…Plot of the best estimates for the initial208 Pb/204 Pb compositions for 14078 and 68415 plotted against X ¼ ðe l 232 t 1 À e l 232 t i =e l 238 t 1 À e l 238 t i Þ Â ( 206 Pb= 204 Pb i À 206 Pb= 204 Pb 1 ), such that the slope of the trend is equal to the κ-value of the system, given that: 208 Pb= 204 Pb i ¼ kðe l 232 t 1 À e l 232 t i =e l 238 t 1 À e l 238 t i Þ Â ( 206 Pb= 204 Pb i À 206 Pb= 204 Pb 1 ) þ 208 Pb= 204 Pb 1 .Here l 232 and l 238 are the 232 Th and 238 U decay constants; t 1 = time at which the source formed (defined by model differentiation age determined by[34]); t i = age of the sample; 20x Pb= 204 Pb i = initial Pb isotope composition of sample; 20x Pb= 204 Pb 1 ¼ Pb isotope composition of source at t 1 . The trend is defined by all of the points plotted, including the 14078 and 68415 data together with compositions from: 3.1-3.9 Ga Apollo 11, 12, 14, 15 and 17 basalts[52]; 2.0 Ga Chang'e-5 basalt[35]; 4.3 Ga basaltic clasts in the lunar meteorite MIL 13317[79]; and the 3.9 Ga Apollo 16 impact melt breccia 66095[78].…”