Pa. Schroeder et Aa. Mclain, Illite-smectites and the influence of burial diagenesis on the geochemicalcycling of nitrogen, CLAY MINER, 33(4), 1998, pp. 539-546
Fixed nitrogen in illite-smectites (I-S) has been measured for Miocene shal
es from a Gulf of Mexico oil well. Fixed N values for the <0.2 mu m fractio
n increase with depth from 150 ppm (1000 m) to a maximum of 360 ppm (3841 m
). This increase is coincident with illitization from 41% I in I-S to 75% I
in I-S. Below 3841 m, fixed N values decrease to 190 ppm (4116 m) while I-
S is maintained with a slight increase from 77 to 82%, The changes in fixed
N with increasing illitization are consistent with the nation that illitiz
ation proceeds via both transformation and dissolution/precipitation reacti
on mechanisms. The trend of decreasing fixed N in illitic I-S is compatible
with surface-controlled crystal growth and Ostwald ripening mechanisms for
illitization. The trend may also be linked to the timing of maximum NH4+ r
elease from kerogen maturation during oil generation. The changing rate of
NH4+ liberation from organic matter and multiple illitization reaction mech
anisms can result in complex N geochemical cycling pathways throughout earl
y diagenesis to metamorphism.