Petrographic classification of Middle Ordovician fossil meteorites from Sweden

Abstract

Abstract-The maximum diameter of chromite (FeCr 2 O 4 ) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite D max = 34-50 μm, L4 = 87-150 μm, L5 = 76-158 μm, and L6 = 253-638 μm. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism.We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4-0.6 mm) is consistent with measurements made in the same way on recent L chondrites.

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

“…In summary, all inclusions plot within the L-chondritic field, which is in accordance with the previous classifications for the Ö sterplana fossil meteorites and the sediment-dispersed EC grains (Schmitz et al, 2001(Schmitz et al, , 2003Schmitz and Häggströ m, 2006;Bridges et al, 2007;Greenwood et al, 2007) as well as the revised classification of the Brunflo meteorite (Alwmark and Schmitz, 2008). The overall concordance in classification of the fossil meteorites and the sediment-dispersed grains with previous studies implies that the composition of inclusions of olivine and Ca-poor pyroxene in chromite from ordinary equilibrated chondrites can be used in determining meteorite group with no or very little overlap.…”

“…The samples are listed in Table 1. The Thorsberg quarry meteorites have previously been classified as L chondrites, based on their chromite element composition, chondrule sizes and in one case oxygen isotopic composition (Schmitz et al, 2001;Bridges et al, 2007;Greenwood et al, 2007). The Brunflo meteorite has formerly been assigned to the H-chondrite group (Thorslund et al, 1984), but a recent study by Alwmark and Schmitz (2008) shows that it belongs to the L group, founded, for example, on mean chondrule size.…”

Asteroid breakup linked to the Great Ordovician Biodiversification Event

“…Ordovician limestone in southern Sweden show that the meteorite flux was enhanced by one to two orders of magnitude for at least a few million years after the disruption event 7,8 . The L chondritic origin of the fossil meteorites is demonstrated by element and oxygen isotope analyses of relict chromite grains as well as by petrographic studies of chondrule textures [7][8][9] .…”

“…The L chondritic origin of the fossil meteorites is demonstrated by element and oxygen isotope analyses of relict chromite grains as well as by petrographic studies of chondrule textures [7][8][9] . Chromite is the only common mineral in chondrites that survives extensive weathering on the wet Earth surface.…”

Concentration of Platinum Group Elements during the Early Earth Evolution: A Review*

“…Analysis of presence of chromite in carbonaceous and ordinary chondrites shows that: the Ivuna (CI1) carbonaceous chondrite contain chromite and olivine [304] and serpentine [305]; chromite and olivine and chromite-olivine pairs are presented in type II chondrules in CM, CO, CV, CR, H, L, LL, and two ungrouped chondrites [306]; the Mokoia CV matrix contains chromite and chromites presented in partially characterized phases [307]; dissolution of samples from two Ivuna phases left behind a tiny acid-resistant residue highly enriched in Cr, most likely a chromite-spinel phase [308]; chromites were found in the Murchison CM2 chondrite [309]; chromite was found in powdered meteoritic material of carbonaceous chondrites [310]; in the Dhofar 225 CM chondrite olivine aggregates and chondrule-like olivine objects are common and the chondrules consist of olivine with minor chromite and sulfides [311]; chromite is presented in the carbonaceous chondrites but is considerably smaller than chromite in the ordinary chondrites (~25 μm compared to ~270 μm) [312]; Ramdohr [313] has investigated chromite in the Jilin chondrite which shows mostly coarse chromite (exsolution chromite), but with some grains too small to be included in the study; in NWA 4428 (CM2) chondrite grains as small as 11 μm, 16 -26 μm were reported [312]; all chromite with a diameter larger than 10 μm in the Allende (CV3) chondrite is situated in porphyritic olivine chondrules, and Chromite grains smaller than 10 μm (typically 1 -3 μm) are also occurred in the porphyritic olivine chondrules [312]; the maximum diameter of chromite in the L3 chondrites is 34 -50 μm, in the L4-84 -150 μm, in the L5-76 -158 μm and in the L6-253 -638 μm [314]; olivine and chromite are presented in the Tafassasset (CR?) chondrite [315]; the H chondrites contain ~0.22 wt% chromite and the L chondrites contain about ~0.27 wt% chromite [314]; chromite is the most common spinel group mineral in the ordinary chondrites [316]; chromite is more common in type 4 -6 ordinary chondrites than in type 3 ordinary chondrites [317].…”