APATITE AS AN INDICATOR OF MAGMATIC VOLATILE EVOLUTION IN THE CAETANO CALDERA SYSTEM, NEVADA

Magmatic volatile contents exert major controls on the eruptibility, explosivity, and crystallization of silicic magmas. However, volatile evolution in magmatic source regions (MSRs) can be difficult to constrain, particularly in plutonic and devitrified volcanic samples. In this study, we assess the potential to use major, minor, and halogen concentrations in apatite as a proxy for magmatic volatile and halogen contents. Our case study investigates the late-Eocene Caetano caldera in north-central Nevada, where crustal extension has exposed cogenetic volcanic, plutonic, and shallow intrusive igneous rocks. Previous work on the Caetano caldera suggests that the intrusive rocks are genetically equivalent to the least evolved volcanic rocks and formed from magma that remained in the MSR after eruption and caldera collapse (Watts et al., 2016). Comparison of volcanic and plutonic apatite from the same MSR offers the potential to expand upon the understanding of how magma differentiation and eruption processes impact the storage and release of volatiles.

Apatite from 17 samples collected throughout the intrusive and extrusive units of the Caetano Caldera were analyzed using FTIR (OH, CO₂), SIMS (OH, F, Cl, Si, P), and EPMA (F, Cl, Ca, P, Na, Mg, Al, Si, S, K, Ti, Mn, Fe, Sr) techniques. Volcanic samples were collected from the upper and lower Caetano Tuff, a phenocryst-rich, rhyolite ash-flow; plutonic rocks were sampled from the Carico Lake pluton, and the shallow Redrock Canyon porphyry, a ring fracture intrusion, and a lava plug. SIMS results indicate that all analyzed Caetano apatites are fluorapatites, with > 3.0 wt% F, < 0.2 wt% OH, and < 0.4 wt% Cl. Apatite volatile and halogen chemistry from the Caetano Tuff shows little variation with stratigraphic position, suggesting there was limited differentiation and degassing of magmas in the MSR between the eruptions of the lower and upper tuff. Preliminary FTIR data demonstrate that intrusive apatite OH concentrations are generally higher (mean= 0.22 wt% OH) than volcanic equivalents (mean= 0.10 wt% OH), either because of degassing during eruption of the Caetano tuff or post-eruption volatile enrichment because of continued magmatic differentiation. Further in-situ analyses will constrain the processes affecting apatite chemistry in the Caetano caldera MSR over time.