A GEOCHEMICAL AND GEOCHRONOLOCIAL DATABASE FOR THE YELLOWSTONE PLATEAU VOLCANIC FIELD: IMPLICATIONS FOR THE ORIGIN OF POST-CALDERA BASALT AND THE FUTURE OF THE YELLOWSTONE MAGMATIC SYSTEM

We have compiled a database of geochemical and geochronological data for basaltic rocks in the Yellowstone Park area that contains over 150 analyses and 20 dates. Our database includes samples from modern studies (1963-1995) in addition to 39 new chemical analyses and 9 ⁴⁰Ar/³⁹Ar dates. Basaltic eruptions were coeval with the three Yellowstone caldera cycles (2.1-0.6 Ma) and may preserve the long-term record of their mantle source and any crustal interaction. The most recent basalts (<0.6 Ma) occur in the Norris-Mammoth corridor (NMC) north of the Yellowstone Caldera. Our first goal was to determine if the most recent basaltic volcanism within the NMC represents the initiation of a new caldera cycle or the dying phases of the last cycle. Isotopic ratios of the <0.6 Ma basalts suggest a relationship of eNd and ⁸⁷Sr/⁸⁶Sr to periods of caldera formation. ⁸⁷Sr/⁸⁶Sr changed from 0.707 to 0.703 and eNd from -7 to -0.4 at the inception of the first caldera cycle (2.1 Ma) suggesting injection of asthenospheric basalt into the crust. This injection may have triggered partial melting of the crust that ultimately led to generation of a rhyolitic magma chamber and caldera formation. The isotopic signatures of the <0.6 Ma basalts show both high and low values of ⁸⁷Sr/⁸⁶Sr and eNd; however, ages of the basalts are not precise enough to determine a chronology. There are two scenarios: 1) a new injection of basalt has occurred and a new caldera cycle has begun or; 2) the system is in a post-caldera phase. To determine which of these scenarios is correct, high precision ⁴⁰Ar/³⁹Ar dates must be determined for the basalts. This work is currently underway. Our second goal was to answer questions about the mantle source for the Yellowstone basalts and their degree of lithospheric contamination. Although, many previous studies suggested that the basalts were contaminated and provided little information about their mantle source, our new data and the re-plotting of older data shows that most of the basalt is similar to OIB in character. Many samples are similar to typical Great Basin OIB-type basalt in the Lunar Crater field of central Nevada that are interpreted as melts of the asthenospheric mantle. The Yellowstone database may provide important information about the source of basalts and the future of Yellowstone eruptions.