The SPICE project: Calibrated cosmogenic 26Al production rates and cross-calibrated 26Al /10Be, 26Al/14C, and 26Al/21Ne ratios in quartz from the SP basalt flow, AZ, USA

Abstract

The formally named SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt flow that is preserved in the desert climate of northern Arizona, USA. The flow is independently dated with an 40 Ar/ 39 Ar age of 72 ± 4 ka (2σ) and has undergone negligible erosion and/or burial, making its surface an ideal site for direct calibration of cosmogenic nuclide production rates. Production rates for cosmogenic 26 Al have been determined from SP flow quartz in this study and are combined with production rates for 10 Be, 14 C, and 21 Ne (Fenton et al., 2019) to yield a suite of production rate ratios. The error-weighted mean, sea-level, high latitude (SLHL) total reference production rate of 26 Al is 25.8 ± 2.5 at/g/yr (2 σ x ‾ ; standard error) using time-independent Lal (1991)/Stone (2000) ( St ) scaling factors. The St scaled spallogenic 26 Al rate is 25.0 ± 2.4 at/g/yr integrated over the past 72 ka. This rate overlaps within 2σ uncertainty with other St -scaled production rates in the literature. SLHL spallogenic 26 Al production rates in SPICE quartz ( S P Flow P roduction-Rate I nter-Calibration Site for C osmogenic-Nuclide E valuations) are nominally lower if time- dependent Sf , Sa , and Lm scaling factors are used, yielding values of 22.9 ± 2.2 at/g/yr, 22.6 ± 2.2 at/g/yr, and 24.1 ± 2.2 at/g/yr (2 σ x ‾ ), respectively. All 26 Al production rates in SP flow quartz overlap within 2σ uncertainty, regardless of time independent or time dependent scaling. Production rate ratios for cosmogenic 26 Al/ 10 Be, 26 Al/ 14 C, and 26 Al/ 21 Ne are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2 σ x ‾ ; standard error) of 6.7 ± 0.6, 2.23 ± 0.20, and 1.51 ± 0.13, respectively. This study suggests that, similar to cosmogenic 21 Ne and 10 Be production rates in SP flow quartz, production rates of cosmogenic 26 Al in quartz do not significantly increase when integrated over 72 ka, a time span which includes the period of decreased magnetic strength from 20 to 50 ka. • Total reference production rates at SLHL are calculated in 72 ka quartz. • Cosmogenic 26 Al production rate ( St scaling): 25.8 ± 2.5 at/g/yr (2σ). • This rate agrees with St scaled production rates over past 20 ka in literature. • The unscaled 26 Al/ 10 Be production ratio in 72 ka quartz is 6.7 ± 0.6 (2 σ x ‾ ). • Unscaled 26 Al/ 14 C and 26 Al/ 21 Ne production ratios are 2.23 ± 0.20 and 1.51 ± 0.13, respectively.