THE COLOR OF PUMICE - CASE-STUDY ON A TRACHYTIC FALL DEPOSIT, MEIDOB VOLCANIC FIELD, SUDAN

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 distin ct subunits with brown, buff, and black pumices was deposited during t he 5000-B.P. eruption of a tuff ring in the central Meidob volcanic fi eld (Sudan). Subunits of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evi dence of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical spectrosco py, transmission electron microscopy, and magnetic susceptibility meas urements show that color variations of the pumice clasts are related t o the size and distribution of Fe3+-rich oxide microcrysts. Buff pumic e and light-gray domains of streaky pumice have a colorless, transpare nt 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 i s most likely caused by submicroscopic magnetite microcrysts. Brown pu mice clasts have a mixed groundmass consisting of brown domains and do mains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred from lithological observations. S ubunits of black pumices are related to eruption pulses with low magma discharge and high water/magma mass ratio, whereas subunits of buff p umice were deposited during eruption pulses with high magma discharge and low water/magma mass ratio. Brown pumices represent the top part o f the magma body, and the initial stage of the eruption probably had a low magma discharge. Streaky pumices are interpreted as the product o f syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.