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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Textures and Geochemistry of Authigenic Albite from Miocene Sandstones, Louisiana Gulf Coast
Paul B. Gold (2)
ABSTRACT
Feldspars in Louisiana Miocene sandstones, at depths of 2,400 to 6,000 m (80 to 150°C), show progresssive reaction with increasing burial. Plagioclase undergoes both dissolution and replacement by albite, whereas K-feldspar undergoes only dissolution. Reactions do not reach completion by 6,000 m (150°C), where a small amount of oligoclase and K-feldspar remain within an assemblage of nearly pure authigenic albite.
Dissolution of plagioclase gradually becomes volumetrically important near 3,600 m (100°C) and for the most part ends by 5,000 m (130°C). Albite replacement of plagioclase also occurs primarily between 3,600 and 5,000 m (100 to 130°C) but continues to the limit of sample control at 6,000 m (150°C). K-feldspar dissolution is negligible until 4,800 to 5,600 m (125 to 140°C). Below 5,600 m, K-feldspar is nearly absent in the rocks.
Discontinuous zones of blocky to tabular extinction characterize grains of partially albitized plagioclase. Backscattered electron images of these grains, coupled with electron microprobe analyses, indicate that these zones represent domains of unreacted calcic plagioclase (An10-30) interspersed with domains of nearly pure authigenic albite (An0-2). SEM images show that some albite domains form from precipitation of small albite overgrowths on nucleation sites within secondary intragranular megascopic pores. In thin section, the overgrowths are optically clear. Other highly vacuolized albite domains may develop from direct replacement of calcic domains by dissolution-reprecipitation in submicroscopic pores without the formation of megascopic pores. Here, pore flu d may have been trapped between large numbers of subcrystals that grew together with imperfect boundaries. Operating on both megascopic and submicroscopic scales, albite replacement of plagioclase grains can be completed.
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