Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden, and new evidence for the L‐chondrite parent body breakup event

Abstract

Abstract-We present noble gas analyses of sediment-dispersed extraterrestrial chromite grains recovered from ~470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment-dispersed grains and the meteorite Gullhögen 001 were generated in the Lchondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment-dispersed chromite grains have partly been retained during ~470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sedimentdispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting-Robertson (P-R) drag. The fossil meteorite Gullhögen 001 has a low cosmic-ray exposure age of ~0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L-chondrite parent body breakup.

The origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid‐Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)

“…The small decrease in EC content of the limestone over about 5 million years may reflect a slight tailing-off in the flux of micrometeorites to Earth, however, it is difficult to determine variations by a factor of a few in sedimentation rates, and uncertainty exists as to the significance of the small decline in EC content. Measurements of Ne isotopes in sedimentdispersed EC grains from the mid-Ordovician show high amounts of solar-wind implanted gases, indicating that most grains arrived on Earth as parts of unmelted micrometeorites (Heck et al 2008). …”

“…The meteorites represent a range of petrographic types, 3/4 to 6, and are definitely not just one fall that has been reworked in some way ). Additional support comes from sediment-dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains (>63 µm) from decomposed micrometeorites and meteorites in condensed (i.e., very slowly deposited) limestone in southern Sweden and central China (Schmitz et al 2003;Schmitz and Häggström 2006;Häggström and Schmitz 2007;Heck et al 2008;Schmitz et al 2008). Condensed limestone of mid-to late Arenigian age contains only rare EC grains, on the order 0.01-0.02 grain kg −1 , whereas latest Arenigian-early Llanvirnian limestone, formed at the same rate, contains 1-3 EC grains kg −1 .…”

Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material

“…The lesser occurrence of inclusions in the sediment-dispersed grains in comparison to the fossil meteorite grains is most likely the effect of more extensive alteration in the former. The sediment-dispersed grains have been interpreted as originating mainly from micrometeorites (Heck et al, 2008), thus possibilities for external influences and alteration is likely to be greater than in the more sheltered chromite of the proper fossil meteorites. In a couple of instances alteration has only affected the inclusions partially, i.e., along the boundary, resulting in a chemical zoning (see below).…”

“…An analogue to the well established classification system for ordinary chondrites, founded on the fact that olivine and Ca-poor pyroxene in equilibrated (type 4-6) ordinary chondrites have specific ranges of chemical composition depending on group (H, L, LL; e.g., Mason, 1963;Keil and Fredriksson, 1964;Gomes and Keil, 1980;Rubin 1990), can then be established based on the inclusions. This would create an independent tool in the classification of fossil chondrites and sediment-dispersed extraterrestrial chromite grains originating from decomposed micrometeorites (Heck et al, 2008) and chondritic fragments related to impact events (Alwmark and Schmitz, 2007), when no matrix silicates are preserved or present.…”

Determining the impactor of the Ordovician Lockne crater: Oxygen and neon isotopes in chromite versus sedimentary PGE signatures

“…For further details on the instrument, analytical procedures 127 and calculations, see Heck et al (2004Heck et al ( , 2008 and Meier et al (2010). 128…”