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A New Type of Solar-System Material Recovered From Ordovician Marine Limestone


A New Type of Solar-System Material Recovered From Ordovician Marine Limestone

B Schmitz et al. Nat Commun.


From mid-Ordovician ∼470 Myr-old limestone >100 fossil L-chondritic meteorites have been recovered, representing the markedly enhanced flux of meteorites to Earth following the breakup of the L-chondrite parent body. Recently one anomalous meteorite, Österplana 065 (Öst 65), was found in the same beds that yield L chondrites. The cosmic-ray exposure age of Öst 65 shows that it may be a fragment of the impactor that broke up the L-chondrite parent body. Here we show that in a chromium versus oxygen-isotope plot Öst 65 falls outside all fields encompassing the known meteorite types. This may be the first documented example of an 'extinct' meteorite, that is, a meteorite type that does not fall on Earth today because its parent body has been consumed by collisions. The meteorites found on Earth today apparently do not give a full representation of the kind of bodies in the asteroid belt ∼500 Myr ago.


Figure 1
Figure 1. The Österplana 065 fossil meteorite from the Glaskarten 3 bed.
The meteorite is 8 × 6.5 × 2 cm large. It is surrounded by a grey reduction halo, in the otherwise red limestone. Oxygen was consumed when the meteorite weathered on the sea floor. The coin in the image has a diameter of 2.5 cm.
Figure 2
Figure 2. Thorsberg quarry and cosmic-ray exposure ages of fossil meteorites.
(a) The quarried profile in the Thorsberg quarry with the names of individual beds. (b) Cosmic-ray exposure ages for some of the mid-Ordovician fossil meteorites. Meteorite cosmic-ray exposure ages increase with stratigraphic height, in accordance with sedimentation rates as established from conventional biochronology. The data indicate that all the fossil meteorites, including Öst 65, originate from a breakup event that took place about 200 Ka before the lowermost beds in the quarry formed. The details and the analytical background of the 21Ne exposure ages are discussed in ref. . In the figure the Österplana 065 meteorite is represented by its ‘field' name, Gla3 003, at the top of the section studied. Our fossil meteorites obtain official names, approved by the Meteoritical Society, following the convention of naming meteorites after the locality at which they have been found, that is, in this case Österplana. But in order to emphasize that meteorites in different beds fell at different times they also receive an informal ‘field' name. For Öst 65 the name Gla3 003, implies that it is the third meteorite found in the bed Glaskarten 3.
Figure 3
Figure 3. Oxygen and chromium isotopic composition of meteorites.
Comparison of Δ17O versus ɛ54Cr of achondrites, and ordinary chondrites fallen on Earth in recent time with Österplana 065 (Öst 65) and fossil L-chondritic meteorites Österplana 018, 029 and 032 (Öst 18, Öst 29 and Öst 32 in figure). The fields for ordinary chondrites (OC), Mars (SNCs), Earth and earth-likes, Vesta (HEDs), brachinites, ureilites, winonaites (win), acapulcoites (acp)/lodranites/ungrouped achondrites (ung) and a newly identified unique metal-rich chondrite are marked with select representative samples with available data. Symbol colours indicate meteorite type or grouping. Note that the carbonaceous chondrites and affiliated achondrites plot outside the field, with highly positive ɛ54Cr. The ɛ54Cr literature values are from refs , , , , , , , , , , , , , and Δ17O literature values are from refs , , , , , , , , , , , , , .
Figure 4
Figure 4. Back-scattered electron images of mineral grains from Österplana 065.
(a) Chrome-spinel grain with inclusion of a TiCr-mineral, possibly olkhonskite (TiO2=56 wt%, Cr2O3=30 wt%, FeO=9 wt%). (b) Rutile and chrome spinel in polished section of Öst 65. (c) Chrome-spinel grain with apparent shock deformation lamellae. Scale bars, 50 μm.

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