Fossil Meteorites of Sweden: What Are Their Cosmic-ray Exposure Ages?

Introduction: More than 80 L-chondritic fossil meteorites were found in Middle Ordovician limestone in the Thorsberg quarry at Kinnekulle in southern Sweden [1, 2]. All meteorite-bearing sediment beds at Thorsberg contain sediment-dispersed extraterrestrial chromite (SEC) grains of L chondrite composition [3]. Similar SEC grains were also found in several correlated sediment beds of other quarries and outcrops in Sweden and in China [3-5]. Detection solar wind noble gases in the SEC grains (Fig. 1) has unequivocally and convincingly allowed to identify these grains as micrometeorites which have been generated as dust during the disruption of the L chondrite parent body in the asteroid belt ~470 Ma and arrived on all Earth over a timescale of 1-2 Myr [6, 7]. According to [5] "sediment-dispersed EC grains are two orders-of-magnitude more abundant over a part, representing a few million years, of mid-Ordovician strata compared to background levels." Fossil meteorites. The similar conclusion about increase of L-chondrite flux during the same time onto all Earth is represented less proved. Results of precise measurements of the noble gas content in chromite grains of the fossil meteorites have allowed to calculate their cosmic-ray exposure ages [7-9.] All these ages are close to or less than a million years. It is accepted that these ages generally increased according to their position in the sediment column and the difference between the highest and lowest ages is consistent with presumed total deposition time of the sediments of 1-2 Myr. According to [7-9], these data provide evidence of a long-lasting rain of meteorites following the destruction of the L-chondrite parent asteroid ~470 Ma ago. However, a clear negative correlation between the exposure ages and mass of samples (batches) of chromite grains from fossil meteorites has been found [10-12]. At that, higher exposure ages (T 21 ~0.6-0.8 Myr) were inherent to the meteorites with chromite grains of bad safety (with preservation states 4 and 5, according to classification [8]), whereas meteorites with well preserved grains (preservation states 1 and 2) had mainly low exposure ages (~0.2-0.3 Myr). These features are shown on Fig. 2 for meteorites of non-Ark (a) and Ark (b) groups. Here and below, the T 21 values of all samples were calculated according to [7-9] for P 21 =7.04×10-10 cm 3 g-1 Myr-1. It has been assumed that the found negative correlation is caused by underestimation of the nucleogenic 21 Ne nucl contribution in the chromite grains …