Discussion on aluminium loss during sandstone diagenesis

Abstract

Norman Oxtoby & Jon Gluyas write: Wilkinson & Haszeldine (1996) have added to the lively debate on the mobility of elements during the diagenesis of silicate minerals in sandstones. They concluded from a petrographic study of the Fulmar Formation (Jurassic, North Sea) that large quantities of aluminium (Al) have been lost from the sandstone during the course of its burial history. While we have no objection to the notion that large volumes of a variety of elements can be moved in or out of sandstones (cf. Evans 1990 ; Gluyas & Coleman 1992*), we believe that the evidence presented by Wilkinson & Haszeldine is ambiguous. We develop equally plausible interpretations of their data which lead to conclusions of aluminium content stasis or even import! According to our reading of their paper, Wilkinson & Haszeldine (1996) present the argument that, if the initial composition of the Fulmar Formation was >35% feldspar, then a mass balance of Al-bearing minerals quantified from petrographic studies shows that c. 1500 μmol cm– 3 Al has been exported from the most deeply buried sandstones in the studied area. We show below that we need to be more certain about the validity of the 'if condition, before accepting many of the conclusions of this paper, including major Al loss. We do this by commenting on the statements made in the conclusions, shown in italics in the following. (1) The Upper Jurassic Fulmar Formation sandstones show a steady decline of K-feldspar abundance with increased burial. This is partially correct