VARIATION IN OLIVINE TRACE ELEMENT CONTENTS REVEAL THE SIGNATURE OF DEEP MANTLE MELTING DURING THE ERUPTION OF THE MCCARTYS FLOW, ZUNI-BANDERA VOLCANIC FIELD, NEW MEXICO

Monogenetic volcanoes are the most abundant volcanic landforms on Earth but are some of the most poorly understood systems. Their short durations, small volumes and lack of reoccurrence make them difficult to monitor and assess volcanic hazards. Additionally, the architecture of sub-volcanic plumbing systems of these volcanoes is often difficult to resolve. Studies of magma storage and residence times reveal a relationship between asthenospheric mantle composition, varying degrees of partial melting and controlling the scale of eruption. The Zuni-Bandera volcanic field in western New Mexico contains monogenetic volcanoes erupting tholeiitic to alkalic basalts. Evidence shows no correlation of magma composition with eruption age, location or volumetric output, prompting questions about magma ascent rates, magma storage conditions and mantle source.

Here, we present electron microprobe olivine mineral chemistry from the 3200-year-old McCartys flow, the youngest tholeiite basalt in the volcanic field. Olivine displays three populations with unique textures and/or major and minor element compositions. Type 1 crystals are euhedral, and contain cores with Fo_72-86, moderate CaO (0.23-0.31 wt%) and Ni (380-2150 ppm) contents. Crystals contain a rim with lower forsterite, Ni and CaO content. Crystals contain only a single rim with lower forsterite contents. Type 2 crystals are anhedral to skeletal with identifiable cores, mantles and rims. Cores range in forsterite from Fo_67-82, and Ni from 740-3220 ppm. Mantles and rims are more variable ranging from Fo_31-80. In each crystal, there is a general decrease in forsterite and Ni from core to rim. Type 3 olivine are euhedral. Forsterite (Fo_73-90) and Ni (860-3760 ppm) contents vary widely between crystals but lack variation within a single grain. A poorly developed plumbing system can be inferred by three unique olivine populations, suggesting they were in contact with different local magma bodies, with unique chemical environments. Long residence times can be inferred by the extensive time required for olivine to re-equilibrate in the three different chemical environments. Three different zoned olivine populations, if from the same source, must have been stored in magma bodies influenced by magma mixing to be able to develop different local chemical environments.