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 > Fo 85 , 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.

Geophysical and Geochemical Constraints on Magma Storage Depths Along the Cascade Arc: Knowns and Unknowns

“…This discrepancy makes the inference from Hollyday et al. (2020) of lower crustal storage difficult to validate.…”

“…Cpx-based barometry in the Cascades has utilized a variety of models to convert measured phase compositions into pressures (and temperatures). Hollyday et al (2020) and Scruggs and Putirka (2018) used the Cpx-Liq barometer of Neave and Putirka (2017) iterated with the Cpx-Liq thermometer of Putirka (2008, Equation 33). Sas et al (2017) used the Cpx-Liq barometer from Putirka (2008, Equation 32c) with an unspecified thermometer.…”

“…MI saturation pressure and mineral barometers yield pressures, which are then converted to depths (H) using assumptions about crustal density (e.g., P = ρ gH) or a crustal density model. Cascade MI and thermobarometry studies have used a wide variety of crustal densities to convert pressures into depths in the crust ( ρ = 2,200 kg/m 3 by Bacon et al., 1992, ρ = 2,700 kg/m 3 by Hollyday et al., 2020, ρ = 2,800 kg/m 3 by Johnson & Cashman, 2020, ρ = 2,200 kg/m 3 for the first 2 km and ρ = 2,800 below that by Gardner et al., 1995). Here, we convert pressures to depths using a uniform crustal density of ρ = 2,700 kg/m 3 .…”

“…Considering all PEC-corrected MIs from BBL, we instead obtain pressures distributed between −0.5 and ∼2.5 kbar (regardless of the exact equilibrium tests used). This discrepancy makes the inference from Hollyday et al (2020) of lower crustal storage difficult to validate.…”

Barometers behaving badly II: A critical evaluation of Cpx-only and Cpx-Liq thermobarometry in variably-hydrous arc magmas

“…Cpx is a stable phase over a very wide range of pressures, temperatures, melt compositions and oxygen fugacities (e.g., Costa, 2004;Nandedkar et al, 2014;Ulmer et al, 2018), meaning Cpx-based thermobarometry has broad utility and has been applied in a wide variety of tectonic settings. In this contribution we specifically focus on the use of Cpx-based thermobarometer to investigate storage conditions in magmas erupted in subduction zones (see Table . 1 for studies performing such calculations; Auer et al, 2013;Belousov et al, 2021;Cassidy et al, 2015;Caulfield et al, 2012;Cigolini et al, 2018;Dahren et al, 2012;Deegan et al, 2016;Freundt and Kutterolf, 2019;Geiger et al, 2018;Hollyday et al, 2020;Jeffery et al, 2013;Lai et al, 2018;Lormand et al, 2021;Moussallam et al, 2021Moussallam et al, , 2019Namur et al, 2020;Preece et al, 2014;Romero et al, 2022;Ruth and Costa, 2021;Sas et al, 2017;Scruggs and Putirka, 2018;Sheehan and Barclay, 2016).…”