REVISED 40AR/39AR GEOCHRONOLOGY OF THE JEMEZ AND CERROS DEL RIO VOLCANIC FIELDS, NM

Volcanism in the Jemez volcanic field began in the middle Miocene and culminated in the two major Quaternary ash-flow tuff eruptions that formed the Valles caldera. As one of the three largest Quaternary silicic calderas in North America, the Jemez has received great scrutiny by volcanologists. The Jemez Mountains is also a key locality where the plate-tectonic connection between seafloor spreading and isotopic age determination of the Jaramillo magnetic polarity chron was first established. Advances in ⁴⁰Ar/³⁹Ar isotopic-age determination have rendered old conventional K-Ar ages obsolete due to their imprecision and inaccurate young ages caused by incomplete degassing of sanidine. About 400 ⁴⁰Ar/³⁹Ar ages in the past two decades have been obtained on the rhyolites, dacites, andesites, basalts, and basanites of the Miocene to Quaternary Jemez volcanic field, and the interfingering Pliocene Cerros del Rio volcanic field to the east. In coordination with geologic mapping and hydrogeologic investigations of the Los Alamos-Espanola Basin area, a new synthesis of ⁴⁰Ar/³⁹Ar age determinations has been compiled.

Individual ⁴⁰Ar/³⁹Ar age determinations may be highly precise, with relative analytical uncertainty as good as 0.1% for laser fusion analysis of sanidine. Yet the actual accuracy of age determination (for instance, relative to the Geomagnetic Polarity Time Scale) may be inadequately resolved due to the need to calculate ages relative to the age of a secondary monitor mineral. Ages reported for the commonly used monitors MMhb-1 hornblende (513.9 Ma-523.1 Ma) and Fish Canyon Tuff sanidine (27.57-28.27 Ma) vary as much as 2%, and ⁴⁰Ar/³⁹Ar data for the Jemez volcanic field have been published using this range of different monitor values. The argon geochronologic community has recently adopted a middle value of 28.02 Ma on Fish Canyon sanidine in order to consistently report data from labs using different internal standards. For the first time, we report on the ⁴⁰Ar/³⁹Ar geochronologic framework of the Jemez and Cerros del Rio volcanic field with all age determinations normalized to the same monitor value. New observations and interpretations on the timing and pattern of Jemez volcanism and its tectonic setting result from these recalculations.