The Creston, California, meteorite fall and the origin of L chondrites

Abstract

It has been proposed that all L chondrites resulted from an ongoing collisional cascade of fragments that originated from the formation of the ~500 Ma old asteroid family Gefion, located near the 5:2 mean‐motion resonance with Jupiter in the middle Main Belt. If so, L chondrite pre‐atmospheric orbits should be distributed as expected for that source region. Here, we present contradictory results from the orbit and collisional history of the October 24, 2015, L6 ordinary chondrite fall at Creston, CA (here reclassified to L5/6). Creston's short 1.30 ± 0.02 AU semimajor axis orbit would imply a long dynamical evolution if it originated from the middle Main Belt. Indeed, Creston has a high cosmic ray exposure age of 40–50 Ma. However, Creston's small meteoroid size and low 4.23 ± 0.07° inclination indicate a short dynamical lifetime against collisions. This suggests, instead, that Creston originated most likely in the inner asteroid belt and was delivered via the ν6 resonance. The U‐Pb systematics of Creston apatite reveals a Pb‐Pb age of 4,497.1 ± 3.7 Ma, and an upper intercept U‐Pb age of 4,496.7 ± 5.8 Ma (2σ), circa 70 Ma after formation of CAI, as found for other L chondrites. The K‐Ar (age ~4.3 Ga) and U,Th‐He (age ~1 Ga) chronometers were not reset at ~500 Ma, while the lower intercept U‐Pb age is poorly defined as 770 ± 320 Ma. So far, the three known L chondrites that impacted on orbits with semimajor axes a <2.0 AU all have high (>3 Ga) K‐Ar ages. This argues for a source of some of our L chondrites in the inner Main Belt. Not all L chondrites originate in a continuous population of Gefion family debris stretching across the 3:1 mean‐motion resonance.

The CM carbonaceous chondrite regolith Diepenveen

“…That means Diepenveen's origin site in the asteroid belt is not constrained by the lifetime against collisions (Jenniskens et al. ).…”

“…At UC Davis, ultrahigh‐precision Cr isotope ratio measurements were completed in a powdered, bulk rock aliquot of Diepenveen using methods described before (e.g., Jenniskens et al. ; Unsalan et al. ).…”

“…At the University of California at Davis (UCD), the major, minor, and trace element abundances were determined in a homogenized powder of a 13.80 mg rock aliquot of Diepenveen using a Thermo Element XR high resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) and methods described in Jenniskens et al (2019) and Unsalan et al (2019). A calibration curve for HR-ICP-MS was generated using a series of Allende Smithsonian reference powder solutions.…”

“…Oxygen isotope studies were performed at the University of New Mexico using a dual inlet Thermo Finnigan MAT 253 mass spectrometer and methods described before (e.g., Popova et al 2013;Jenniskens et al 2019;Unsalan et al 2019). Several aliquots of Diepenveen were measured against San Carlos olivine standards (~1-2 mg), which were analyzed daily.…”

“…Values are normalized to standard mean ocean water (SMOW). Excess d 17 O 0 from the Terrestrial Fractionation Line D 17 O 0 = d 17 O 0 À 0.528 d 18 O 0 , using a slope of 0.528 in the calculation (Barkan and Luz 2005), with the various notations described in, e.g., Jenniskens et al (2019).…”