The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan |
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| Authors: | H Paulick G Franz |
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| Institution: | (1) GeoForschungsZentrum Potsdam, Projektbereich 4.1, Postfach 600751, D-14407 Potsdam, Germany, DE;(2) CODES-Special Research Centre, Department of Geology, University of Tasmania, GPO Box 252–79, Hobart, Tasmania 7001, Australia, AU;(3) TU Berlin, Fachgebiet Petrologie, Strasse des 17. Juni 135, D-10623 Berlin, Germany, DE |
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| Abstract: | This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition
and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black
pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits
of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence
of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical
spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the
pumice clasts are related to the size and distribution of Fe3+-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with
very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in
thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin
section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting
of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred
from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high
water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and
low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption
probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.
Received: 3 February 1997 / Accepted: 28 July 1997 |
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| Keywords: | Holocene volcanism in Sudan Meidob volcanic field Trachytic pumice deposit Color of pumices Black pumice Streaky pumice Brown pumice Buff pumice Microcrysts |
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