Abstract
ABSTRACT Iron-rich oolites from a Middle Ordovician (Llanvirn-Llandeilo) sequence in south-central Sweden are associated with volcanic ash beds, clear evidence that volcanic activity was coeval with a period of extensive iron ooid formation in Baltoscandia. Relict texture of the ash is preserved as illitical rims on primary glass shards that underwent rapid calcitization in a marine environment. XRD and Mossbauer spectroscopy reveal that the ooids are composed mainly of chamosite with a Fe2+/Fe3+ ratio of 6.3. It is suggested that authigenic chamosite was formed from elements released into the sediment when the ash dissolved on the sea bottom. Chamosite precipitated on skeletal grains, and repeated reworking of the sediment oxidized the outer surface of the grains to goet ite/hematite, thus creating the typical concentric ooid lamination. The presence of Fe2+ in ooids is in contrast to the agitated environment in which they are formed. Fresh volcanic ash, however, usually contains more Fe2+ than Fe3+, which can explain the high Fe2+/Fe3+ ratio in the ooid chamosite, if the iron is derived from the volcanic ash.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
