Pre-eruptive storage, evolution, and ascent timescales of a high-Mg basaltic andesite in the southern Cascade Arc

Abstract

Crustal storage and transit play a critical role in the compositional evolution of arc magmas; however, the enigmatic nature of lower crustal magma storage and early differentiation limit our understanding of the connections between the physical processes of subduction zones and the architecture of the arc crust. We present new geochemical data and applications of existing barometric and chronometric tools to interrogate the mantle source compositions, crustal storage depths, and ascent timescales of a primitive, high-Mg, clinopyroxene-bearing cinder cone, the basaltic andesite of Box Canyon, located in the vicinity of Lassen Volcanic National Park, CA, in the southern Cascades. Petrographic examination in addition to bulk and in situ geochemical analyses (XRF, LA-ICP-MS, and EPMA) of tephra and lava-derived samples reveals co-crystallization of clinopyroxene and olivine as phenocrysts and glomerocrysts with ~ Mg# 80 and > Fo85, respectively. Phase equilibria experiments of analogous Cascade Arc magma compositions estimate crustal storage of the observed phase assemblage at pressures in the lower crust > 700 MPa. Reverse zonation in olivine and clinopyroxene phenocryst interiors from core-rim analytical profiles record a lower crustal mafic mixing event. Results from one-dimensional, multi-elemental olivine and clinopyroxene diffusion models fit to these interior mixing zones provide an assessment of available trace element (Ti, La, Yb, Ce, and Nd) diffusion chronometers in clinopyroxene by considering multiple element profiles within two phases that experienced the same pre-eruptive conditions. While multi-elemental and multi-phase diffusion timescales span two to three orders of magnitude, Ni in olivine profiles provide the most-robust estimate of 19.1 ± 8.6 years from mixing-to-eruption. Our results provide new constraints on arc crustal differentiation processes, indicate rapid crustal transit timescales in agreement with a growing database of diffusion-based ascent timescales of basaltic magmas, and demonstrate significant, systematic deviation of diffusion timescales calculated from natural zonation of rare earth elements in clinopyroxene and Ti in clinopyroxene and olivine.