The Ordovician chondrite from Brunflo, central Sweden, I. General description and primary minerals

Geochimica et Cosmochimica Acta

2009

Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1-10 lm) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found.Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 lm) in chromite from the Ordovician fossil chondritic material plot within the Lchondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts.

Section: Methodsmentioning

confidence: 99%

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

Alwmark

Geochimica et Cosmochimica Acta

2009

“…This does not apply, however, to fossil meteorites in Swedish Middle Ordovician limestone, because the primary, mainly silicate, minerals have been replaced during diagenesis by low-temperature secondary minerals, particularly carbonates, sulfates, and phyllosilicates. The only common surviving mineral is chromite (Thorslund et al 1984;Nyström et al 1988;Schmitz et al 1996). Many meteorites with chondrule structures have been found in the 470 Ma Middle Ordovician marine limestone in the Thorsberg Quarry, southern Sweden.…”

Section: Introductionmentioning

confidence: 99%

Petrographic classification of Middle Ordovician fossil meteorites from Sweden

Bridges

Meteorit & Planetary Scien

Hutchison

et al. 2007

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.

“…It is not possible to rule out the AlC grains as being extraterrestrial since ordinary chondrites contain also chromian spinels of very varying character. These are, however, rare making up for only 1-2% of the total Cr-rich oxides in chondrites [24].…”

Section: Chemical Composition and Properties Of The Spinelsmentioning

confidence: 99%

“…The search has focused on chromite (FeCr 2 O 4 ), a common accessory mineral in many meteorites [23]. Chromite has a high resistance against weathering and diagenesis and is often the only mineral surviving in chondritic fossil meteorites [24,25]. Chromite is also a Fig.…”

Section: Introductionmentioning

confidence: 99%

Extraterrestrial chromite in the resurge deposits of the early Late Ordovician Lockne crater, central Sweden

Alwmark

Earth and Planetary Science Letters

2007

The distribution of extraterrestrial chromite grains (N63 μm) has been studied in the resurge deposit of the early Late Ordovician (458 Ma) Lockne impact structure in central Sweden. A 1-kg-sample of resurge sediment from the rim of the crater is extremely rich in extraterrestrial chromite. In total the sample contains N 125 chromium spinel grains kg − 1 , most grains being chromite. Due to post-depositional alterations, the origin of several of the grains is dubious, although it is clear that the major part of the chromites is of extraterrestrial origin (N 75 grains kg − 1 ), most likely derived from the impactor. The alterations are primarily due to the hydrothermal system induced by the impactor, with zinc enrichment in the chromites as the most common characteristic. The element chemistry of the least altered chromite grains indicates that the impactor was an ordinary L chondrite. This concurs with a suggested increase in the flux of L chondritic asteroids to Earth, following the disruption of the L chondrite parent body at ca. 470 Ma. More than 170 impact craters are known from around the Earth, but only few of the impactors have been identified, mainly due to the low survival rate of projectile material. Further studies of impact craters will reveal if survival of extraterrestrial chromite is unique for the Lockne structure.