The Gefion Family as the Probable Source of the L Chondrite Meteorites

We show that fragments from the Gefion family-forming event are the probable source of the shocked L chondrite meteorites, which represent about 2/3 of all L chondrite falls. The 100-150 km diameter Gefion parent body, which had semi-major axis a ≈ 2.8 AU, experienced a super-catastrophic disruption ∼ 470 Ma. This event injected tiny fragments immediately into the 5:2 mean motion resonance with Jupiter at 2.823 AU (J5:2) that quickly evolved onto Earth-crossing orbits. They most likely produced the fossil L chondrite meteorites and iridium enrichment found in an ≈467 Ma old marine limestone quarry in Sweden. Since that time, meteoroid precursors from Gefion have collisionally and dynamically evolved down to the J3:1 resonance at 2.5 AU, a better location for meteorite delivery than the J5:2. This would allow meteoroid-sized fragments from the Gefion family to make up a large fraction of all meteorite falls. Introduction. Nearly 80% of all meteorite falls are ordinary chondrites (OCs) and about 40% of those are L chondrites. Because 2/3 of L chondrites were heavilyshocked and degassed 470 Ma [1], it has been suggested that the L chondrite parent body, which was larger than 100 km, catastrophically disrupted at this time [e.g., 2]. Interestingly, the timing of this shock event coincides with the stratigraphic age (467 ± 2 Ma) of the midOrdovician strata where abundant fossil L chondrites, meteorite-tracing chromite grains, and iridium enrichment were found in Swedish marine limestone quarry [e.g., 3]. These shocked and fossil L chondrites likely record the same event and can be used to constrain the identify and location of L chondrite remnants in the main asteroid belt. We analyzed all existing asteroid families for their potential relationship to this 470 Ma event. Most can be ruled out because they have Yarkovsky/YORP effectderived ages older than 1 Ga or they have taxonomic signatures incompatible with L chondrites [4]. Further constraints come from the short Cosmic Ray Exposure (CRE) ages of fossil L chondrites and the narrow interval of sediment depths in which they were found. Their derived delivery times of < 1-2 Myr [5] indicate that the breakup event had to occur close enough to a powerful resonance that fragments could be transferred to Earth on this timescale. Flora Family. Our two main candidates for the source of the L chondrites are the Flora and Gefion families. Flora, however, has many problems. It is located near the ν6 secular resonance in the inner main belt. To produce the most extreme CRE ages of fossil meteorites (∼50-200 kyr; [5]), meteoroid-sized fragments would have to be launched from the Flora parent body with ejection speeds > 1 km/s [6]. Its olivine/pyroxene composition is also most consistent with LL chondrites [7]. Gefion Family. The Gefion family, on the other hand, is a better fit. We have identified 2240 dynamical family members to date that are located near the J5:2. The age of the Gefion family, according to Yarkovsky/YORP models of family member evolution [8], is 485 −10 My (assuming that family members have bulk densities of 2.0 g cm and albedos of 0.2). This formation time is consistent with the 470 Ma shock event of L chondrites, though we caution that our age estimate is sensitive to the precise location of the parent body. Gefion family members have estimated olivine fractions between 6268%, very consistent with L chondrite values [7]. Finally, the size frequency distribution (SFD) of the Gefion family starts at D ≈ 15 km, steeply rises down to D ≈ 3 km and flattens for D < 3 km from observational incompleteness. Using SPH/N -body results [e.g., 9], we estimate that the parent body was 100-150 km in diameter and that it experienced a super-catastrophic disruption event. These results are consistent with the above constraints. Figure 1. The impact rate of meteorites from the 5:2 resonance matches the CRE age range of fossil L chondrites (0.05-1.5 Myr; shown by the two-headed arrow). On average, one out of ∼500 fragments inserted into the resonance impacts Earth within 2 Myr after the Gefion