Thermal history recorded by the Apollo 17 impact melt breccia 73217

Abstract

Lunar breccia 73217 is composed of plagioclase and pyroxene clasts originating from a single gabbronorite intrusion, mixed with a silica-rich glass interpreted to represent an impact melt. A study of accessory minerals in a thin section from this breccia (73217,52) identified three different types of zircon and anhedral grains of apatite which represent distinct generations of accessory phases and provide a unique opportunity to investigate the thermal history of the sample. Equant, anhedral zircon grains that probably formed in the gabbronorite, referred to as type-1, have consistent U-Pb ages of 4332 ± 7 Ma. A similar age of 4335 ± 5 Ma was obtained from acicular zircon (type-2) grains interpreted to have formed from impact melt. A polycrystalline zircon aggregate (type-3) occurs as a rim around a baddeleyite grain and has a much younger age of 3929 ± 10 Ma, similar to the 3936 ± 17 Ma age of apatite grains found in the thin section. A combined apatite-type-3 zircon age of 3934 ± 12 Ma is proposed as the age of the Serenitatis impact event and associated thermal pulse. X-ray mapping and electron probe analyses showed that Ti is inhomogeneous in the zircon grains on the sub-micrometer scale. However, model temperatures estimated from SHRIMP analyses of Ti-concentration in the 10 lm diameter spots on the polished surfaces of type-1 and type-2 zircons range between about 1300 and 900°C respectively, whereas Ti-concentrations determined for the type-3 zircon are higher at about 1400-1500°C. A combination of U-Pb ages, Ti-concentration data and detailed imaging and petrographic studies of the zircon grains shows that the gabbronorite parent of the zircon clasts formed shortly before the 4335 ± 5 Ma impact, which mixed the clasts and the felsic melt and projected the sample closer to the surface where fast cooling resulted in the crystallization of acicular zircon (type-2). The 3934 ± 12 Ma Serenitatis event resulted in partial remelting of the glass and formation of polycrystalline zircon (type-3). This event also reset the U-Pb system of apatite, formed merrillite coronas around some apatite grains, and probably re-equilibrated some pyroxenes in the clasts. Although there have been arguments for pre-3.9 Ga impacts based on other types of samples, the age of the acicular zircon at 4335 ± 5 Ma provides the first evidence of impact melt significantly predating the lunar cataclysm. Our data, combined with other chronological results, demonstrate the occurrence of pre-3.9 Ga impacts on the Moon and suggest that the lunar impact history consisted of a series of intense bombardment episodes interspersed with relatively calm periods of low impact flux.

Illusory Late Heavy Bombardments

“…Restated, monotonically declining impact histories yield apparent age peaks that could be misinterpreted as LHB-type events. We further conclude that the assignment of apparent 40 Ar/ 39 Ar plateau ages bears an undesirably high degree of subjectivity. When compounded by inappropriate interpretations of histograms constructed from plateau ages, interpretation of apparent, but illusory, impact spikes is likely.…”

“…A significant portion of the evidence for the existence of the LHB comes from histograms of 40 Ar/ 39 Ar "plateau" ages (i.e., regions selected on the basis of apparent isochroneity). However, due to lunar magmatism and overprinting from subsequent impact events, virtually all Apollo-era samples show evidence for arth contributes relatively little to our knowledge of the early impactor flux to the inner solar system, due to its constant resurfacing by the combined effects of erosion and cratonic growth.…”

“…However, due to lunar magmatism and overprinting from subsequent impact events, virtually all Apollo-era samples show evidence for 40 Ar/ 39 Ar age spectrum disturbances, leaving open the possibility that partial 40 Ar* resetting could bias interpretation of bombardment histories due to plateaus yielding misleadingly young ages. We examine this possibility through a physical model of 40 Ar* diffusion in Apollo samples and test the uniqueness of the impact histories obtained by inverting plateau age histograms. Our results show that plateau histograms tend to yield age peaks, even in those cases where the input impact curve did not contain such a spike, in part due to the episodic nature of lunar crust or parent body formation.…”

“…planetary science | Late Heavy Bombardment | geochronology | 40 Ar/ 39 Ar | impacts E arth contributes relatively little to our knowledge of the early impactor flux to the inner solar system, due to its constant resurfacing by the combined effects of erosion and cratonic growth. Although the Moon's longstanding stability and relatively short duration of crustal growth in principle transcends these terrestrial limitations, after nearly 50 y of lunar sample analysis, our understanding of the Earth−Moon impact history remains limited (1-3); reasons for this include the relatively small area of the lunar surface from which we have documented sample locations and the potentially cryptic nature of impact thermal signatures (4).…”

“…The existence of an LHB-type event has broader implications to other planets, and its origin has been linked to dramatic changes in giant planet orbital dynamics (17) and ejected debris from a large Mars impact (18). The 40 Ar/ 39 Ar data are not the only source of evidence that has been used to support the LHB hypothesis. Indeed, the original proposal of a "terminal lunar cataclysm" (6) was based on the observation of widespread U−Pb fractionation at ca.…”