Geochemistry of agates: a trace element and stable isotope study

Abstract

Agate samples of acidic, intermediate and basic volcanics from 18 localities around the world and of different age (Precambrian to Tertiary) were studied by trace element and stable isotope analysis to provide information about the process of agate formation and the origin of mineral-forming fluids. Trace element data are similar for agates from acidic and basic volcanics. The general chondrite-normalized REE distribution pattern is characterized by a slope from La to Lu with enriched LREE and a positive Eu anomaly in some samples. The similarity in the shape of the REE patterns between agates and the parent volcanic rocks suggests that the elements are mobilized by circulating fluids during syn- and postvolcanic alteration of the volcanic wall rocks. Observed positive Eu anomalies in agates probably originate from feldspar alteration. Deuterium and oxygen isotope analyses of agates and associated quartz incrustations ( δD: −44‰ to −130‰; δ 18O: +16.4‰ to 33.4‰) reveal variations in isotopic composition between samples of different localities but also within single agate samples (up to 10‰ for δ 18O). In general, oxygen isotope compositions become heavier as volcanic host rocks grow more acidic. Furthermore, agates have higher δ 18O values than associated quartz incrustations. Variations within single agate samples can be explained either by kinetic effects during isotope fractionation (e.g., the formation of agate from a noncrystalline precursor) or by mixing processes of meteoric and magmatic fluids. Remarkably high δ 18O values of the parent volcanic rocks (up to+19.5‰) suggest that the circulation of 18O enriched hydrothermal fluids originated from heated meteoric water and/or residual magmatic fluids. This conclusion is supported by δ 13C and δ 18O data of paragenetic calcite. The temperature of agate formation was calculated for different fluid compositions, and they indicate a temperature range of ca. 50°C to 250°C.