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    Accretionary lapilli cores and laminae: chemical tracers and 'time capsules' of a pyroclastic eruption?

    Drake, Simon and Brown, D.J. and Beard, Andy (2014) Accretionary lapilli cores and laminae: chemical tracers and 'time capsules' of a pyroclastic eruption? In: Volcanic and Magmatic Studies Group: Annual Meeting, 2014, Edinburgh, UK. (Unpublished)

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    Abstract

    A variety of field studies, laboratory experiments and modelling investigations have been undertaken to determine the processes of ash aggregation during eruptions; however, there is a paucity of data on the chemistry of the aggregates and the role of eruption chemistry in their formation. Here we present electron microprobe analyses of accretionary lapilli (ash aggregates with a massive pellet at their core and a cortex of multiple concentric fine ash laminae) collected from a variety of pyroclastic density current deposits (Scafell Caldera, English Lake District; Kilchrist Caldera, Isle of Skye, NW Scotland; Poris Formation, Tenerife) These rocks span the absolute time period ~460 Ma-273 ka and formed in a variety of tectonic and environmental settings. In all cases there are significant geochemical variations between cores and laminae of individual accretionary lapilli and the matrix of the tuffs/lapilli-tuffs in which they are found. We suggest that ash pellets form in co-ignimbrite plumes and on reaching a critical density fall under gravity into the underlying turbulent pyroclastic density current. Here the pellets accrete laminae that reflect the chemistry of the pulse of the density current surrounding it at that time, and therefore, the laminae of the growing accretionary lapilli progressively record chemical flux during the eruption. Complex chemical zoning of the laminae may record repeated lofting and settling of the growing lapilli into chemically diverse parts of the density current. The accretionary lapilli therefore behave as “time capsules” that provide a high-resolution record of the chemical evolution of the eruption. These subtle changes may not be recorded by the density current deposits themselves due to the effects of current bypassing and/or erosion.

    Metadata

    Item Type: Conference or Workshop Item (Poster)
    School: Birkbeck Schools and Departments > School of Law
    Depositing User: Sarah Hall
    Date Deposited: 05 May 2016 16:00
    Last Modified: 05 May 2016 16:00
    URI: http://eprints.bbk.ac.uk/id/eprint/15115

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