AMMONIUM ENRICHMENT ASSOCIATED WITH DIAGENETIC ALTERATION IN TERTIARYPYROCLASTIC ROCKS FROM GREECE

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.