Mineralogy and temporal relations of coexisting authigenic minerals in altered silicic tuffs and their utility as potential low-temperature dateable minerals

Coexisting fine-grained (0.1–20 μm) authigenic silicate minerals separated from altered tuffs in Miocene and Plio-Pleistocene lacustrine deposits were characterized petrographically and using X-ray powder diffraction. The authigenic minerals are dominated by clinoptilolite, erionite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristobalite, and barite are present with the major alteration minerals. Authigenic minerals from altered tuffs were dated using the KAr method to evaluate the utility of these minerals for determining the time of alteration in low-temperature diagenetic environments. The eruption ages of some of these zeolite-rich tuffs were determined using the 40Ar39Ar method on single sanidine and plagioclase minerals. The KAr isotopic ages of the fine-grained K-feldspar show minimal variation compared with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites (16.-6.7 Ma) are similar to the eruption ages of the tuffs and indicate early alteration. Despite their open-framework structure, zeolites apparently can retain part or all of their radiogenic argon under favorable conditions (e.g., saturated environment). How much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the KAr method. Although zeolite isotopic ages should be interpreted with caution, they may be useful to constrain temporal relations of low-temperature diagenetic processes when used in conjunction with other dateable minerals.