A. Hall et al., AMMONIUM ENRICHMENT ASSOCIATED WITH DIAGENETIC ALTERATION IN TERTIARYPYROCLASTIC ROCKS FROM GREECE, Chemical geology, 118(1-4), 1994, pp. 173-183
The ammonium ion has been measured in fresh volcanic rocks, in vitric
tuffs, and in tuffs which have undergone secondary alteration. The amm
onium content of fresh lavas of a wide range of composition is shown t
o be negligible. In contrast, high concentrations of ammonium are foun
d in pyroclastic rocks that have undergone diagenetic alteration. Data
are given for three types of diagenetic alteration, namely (1) hydrat
ion, (2) zeolitization, and (3) alteration to secondary K-feldspar. Di
fferent levels of ammonium enrichment are associated with each type. H
ydrated vitric tuffs from Kozani in northern Greece and Heraklion in C
rete contain from 158 to 390 ppm NH4+. Zeolitized ash and lapilli tuff
s from Santorini and from Lefkimi, NE Greece are much lower in their a
mmonium content, ranging from 12 to 56 ppm. Tuffs containing secondary
K-feldspar in addition to zeolites, from Metaxades, N.E. Greece, have
ammonium contents from 51 to 512 ppm. Measurements of fixed and excha
ngeable ammonium were used to distinguish between the ammonium held by
zeolites and that held by K-feldspars. In the clinoptilolite- and mor
denite-bearing tuffs from Santorini and Lefkimi, most of the ammonium
is in cation-exchangeable form and must be held by the zeolites. By co
ntrast, in the feldspar-bearing tuffs from the Metaxades area almost a
ll the ammonium is fixed, and it is inferred to be held mainly by seco
ndary K-feldspar.