Geochemistry of agates: a trace element and stable isotope study

Agate samples of acidic, intermediate and basic volcanics from 18 localitie s around the world and of different age (Precambrian to Tertiary) were stud ied by trace element and stable isotope analysis to provide information abo ut 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 slept: 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 agate s 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 fr om feldspar alteration. Deuterium and oxygen isotope analyses of agates and associated quartz incru stations (delta D: -44 parts per thousand to -130 parts per thousand; delta O-18: + 16.4 parts per thousand to 33.4 parts per thousand) reveal variati ons in isotopic composition between samples of different localities but als o within single agate samples (up to 10 parts per thousand for delta O-18). In general, oxygen isotope compositions become heavier as volcanic host ro cks grow more acidic. Furthermore, agates have higher delta O-18 values tha n associated quartz incrustations. Variations within single agate samples c an be explained either by kinetic effects during isotope fractionation (e.g ., the formation of agate from a noncrystalline precursor) or by mixing pro cesses of meteoric and magmatic fluids. Remarkably high delta O-18 values o f the parent volcanic rocks (up to + 19.5 parts per thousand) suggest that the circulation of O-18 enriched hydrothermal fluids originated from heated meteoric water and/or residual magmatic fluids. This conclusion is support ed by delta C-13 and delta O-18 data of paragenetic calcite. The temperatur e of agate formation was calculated for different fluid compositions, and t hey indicate a temperature range of ca. 50 degreesC to 250 degreesC. (C) 20 01 Elsevier Science B.V. All rights reserved.