Impact ages of meteorites: A synthesis

Abstract

Abstract— Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the 39Ar‐40Ar technique, but several Rb‐Sr, Pb‐Pb, and Sm‐Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K‐Ar (easiest), Rb‐Sr, Pb‐Pb, and Sm‐Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward. Essentially all 39Ar‐40Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 109 years). Several disturbed Rb‐Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large‐scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal. Although a few chondrites show evidence of age resetting ∼3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ∼4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of 39Ar‐40Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it. The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ∼3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K‐Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic‐ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional di...

Constraints on the thermal and mixing history of lunar surface materials and comparisons with basaltic meteorites

“…Most significant is the lack of material corresponding to the top comer of the ternary. Ga, a range which is not unlike that of lunar samples, 3.7-4.1 Ga [Bogard, 1995] 2. Rego!ith maturation processes affect the TL and the CL properties of the soils and therefore provide a new means of exploring these processes.…”

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

“…Although eucrites are commonly shocked, 12 of the 13 in Table 5 are unshocked; QUE 97053 is the only one with pervasive evidence for shock. The Ar-Ar ages of the eucrites in Table 5 are also unusual: all but one exceeds 4.45 Gyr, although the vast majority of HEDs have Ar-Ar ages of 3.4 to 4.1 Gyr (Bogard, 1995;. Possible implications of these Ar-Ar ages are discussed below.…”

“…Even allowing for possible 40 K half-life errors that might increase the Ar-Ar age by ~30 Myr, this age is still ~50 Myr after the eucrites crystallized (e.g., Sanders and Scott, 2007b). Brecciated eucrites, which are commonly shocked, show consistently younger Ar-Ar ages which are mostly between ~3.4 and 4.1 Gyr, overlapping the Late Heavy Bombardment of the Moon (Bogard, 1995). inferred that the 4.48 Gyr Ar-Ar ages of the 11 eucrites may have resulted from a major impact on Vesta that excavated deeply-buried cumulate eucrites and unbrecciated and metamorphosed basaltic eucrites from the hot crust, allowing them to cool rapidly.…”

“…Although many smaller asteroids would have been easily demolished during the Late Heavy Bombardment, those that survived would not have contained many shocked rocks. Bogard (1995) argued from the Ar-Ar ages of HEDs and ordinary chondrites that only bodies as large as Vesta or the Moon could acquire a crust largely composed of rocks and debris that were impact-heated during the Late Heavy Bombardment. Scott and Bottke (2007) used similar arguments to those of to explain why angrites are unshocked and unbrecciated unlike eucrites, which are commonly shocked and brecciated, even though many angrites have textures indicating they crystallized rapidly near the surface.…”

40Ar/39Ar impact ages and time–temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite

“…As such, the K/Ar system on basaltic asteroids is perhaps not best suited to record the age of large impact events, but rather the youngest recorded ages provide information about the timing of the cessation of any significant bombardment activity on an asteroid parent body(?). This problem is similar for instance to the occurrence of the HED unbrecciated eucrite population that shows distinct 40 Ar/ 39 Ar ages no younger than ~4.5 Ga (Bogard, 1995;Bogard, 2011;Kennedy et al, 2013), but which is virtually unperturbed by any secondary impact events after that.…”