Maturation and rejuvenation of a silicic magma reservoir: High-resolution chronology of the Kneeling Nun Tuff

Abstract

Knowledge of the conditions of magma storage prior to volcanic eruptions is key to their forecasting, yet little is known about how melt compositions, crystallinity and intensive parameters within individual magma reservoirs evolve over time. To address this, we studied the Kneeling Nun Tuff, a voluminous (>900 km3) deposit of an Eocene caldera-forming eruption from the Mogollon–Datil volcanic field in New Mexico, USA. Whole-rock, feldspar and amphibole compositions were combined with zircon trace-element geochemistry and precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) U–Pb zircon crystallisation ages to arrive at a detailed, time-resolved record of chemical and physical changes within the voluminous, upper-crustal (∼2.2 kbar) magma reservoir. Chemical compositions and zircon ages from the Kneeling Nun Tuff and from co-magmatic clasts hosted within it reveal prolonged (>1.5 million years) growth and maturation of the magma reservoir that was heterogeneous in terms of temperature, melt composition and crystallinity. This protracted storage at a dominant crystallinity in excess of 50% culminated in a period of ca. 50 ky of increase in recharge heat supply and related homogenisation, decrease in crystallinity to 40–50%, and potential increase in average melt temperature, leading up to eruption at 35.299 ± 0.039 Ma. Sampling of co-magmatic lithic clasts derived from early-cooled domains of the reservoir shows that the long, million year-scale maturation time is shared across all erupted domains of the magmatic system, irrespective of their final cooling history. This study provides key observations from a natural system against which thermal and mechanical models of upper-crustal magma reservoir construction can be validated.