Textures and Geochemistry of Authigenic Albite from Miocene Sandstones, Louisiana Gulf Coast

Abstract

Feldspars in Louisiana Miocene sandstones, at depths of 2,400 to 6,000 m (80 to 150 degrees 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 degrees 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 degrees C) and for the most part ends by 5,000 m (130 degrees C). Albite replacement of plagioclase also occurs primarily between 3,600 and 5,000 m (100 to 130 degrees C) but continues to the limit of sample control at 6,000 m (150 degrees C). K-feldspar dissolution is negligible until 4,800 to 5,600 m (125 to 140 degrees 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 (An (sub 10-30) ) interspersed with domains of nearly pure authigenic albite (An (sub 0-2) ). 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 fluid 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.