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Traces in Earth's geological record of the break-up of the L-chondrite parent body 470 Ma

by Alwmark, Carl, PhD

Abstract (Summary)
This thesis deals with signatures in sediments on Earth related to the L-chondrite parent body break-up event at 470 Ma. The aim of this thesis is twofold: (1) investigate if the increased flux of extraterrestrial material to Earth, following the disruption of the L-chondrite parent body included larger, asteroid-sized bodies (2) try to develop new methods and tools for finding and classifying relict cosmic material in sediments. The material studied originates from Middle and Late Ordovician sedimentary rocks from different localities within Sweden and in one case from Estonia. Traces of the L-chondrite break-up found and studied include extraterrestrial chromite grains, relict silicate inclusions in extraterrestrial chromite grains, pseudomorphosed chondrules in fossil meteorites and shock metamorphic features in quartz. Studies of extraterrestrial chromite grains in the resurge deposit of the early Late Ordovician (458 Ma) Lockne impact crater in central Sweden, show that the structure was formed by an L-chondritic impactor. This together with the fact that vast amounts of chromite with an L-chondritic composition is found in the 466 Ma Osmussaar Breccia in Estonia, suggesting yet another L-chondritic impact event, imply that the increased flux of extraterrestrial material to Earth, following the disruption of the L-chondrite parent body included larger bodies. This is further corroborated by the verification of the Granby structure as being impact derived, based on the findings of PDFs in quartz in the infill breccias. Although the projectile type is unknown, the timing of the impact (467 Ma) raises the suspicion that it too originates from the L-chondrite parent body. The finding of abundant sediment-dispersed extraterrestrial chromite grains (0.45 grain kg?1) with an L-chondritic composition in limestone in the mid-Ordovician Gärde quarry in central Sweden, suggests that the enhanced flux of L-chondrites prevailed at the time the Brunflo meteorite fell ca. 5 Ma after the meteorites from the Thors¬berg quarry settled on the seafloor. New chondrule-size measurements for the Brunflo mete¬orite indicate that it too is an L-chondrite, and thus most likely a part of this increased flux of L-chondritic matter. Furthermore, the low TiO2 (1.8 wt%), the Dmax of relict chromite and the relatively sharp chondrule definition imply that Brunflo is of petrographic type 4. The discov¬ery of extraterrestrial chromite grains in impact related material shows that physical pieces of larger projectiles can survive the impact process. Chemical analyses of chromite show that the majority of the recovered grains have retained their primary composition and that they thus can be used to classify the impacting projectile. Primary inclusions of olivine, pyroxene, merrillite and plagioclase have been identified in chromite of extraterrestrial origin, both in recent and fossil meteorites, as well as in sediment-dispersed extraterrestrial chromite grains. The systematic compositional difference in Fa in the chromite-hosted olivine and the more or less analogous Fs-content in Ca-poor pyroxene, compared to matrix phases, makes it pos¬sible to establish ranges for inclusions analogous to the well established classification system based on Fa in olivine and Fs in Ca-poor pyroxene, for ordinary equilibrated chondrites. Thus, making this a good tool in classification of fossil meteorites, as well as the origin of sediment-dispersed chromite grains from decomposed meteorites and larger impacts, where no other matrix minerals have survived, or are present.
Bibliographical Information:

Advisor:

School:Lunds universitet

School Location:Sweden

Source Type:Doctoral Dissertation

Keywords:NATURAL SCIENCES; Earth sciences; Middle Ordovician; L chondrite; extraterrestrial material; meteorite flux; Sweden

ISBN:978-91-86746-59-9

Date of Publication:01/01/2009

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