The SPICE Project: Calibrated production rates of cosmogenic 3He and 21Ne in olivine and pyroxene from the 72 ka SP basalt flow, Arizona, 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 has an ⁴⁰Ar/³⁹Ar 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 ³He (³He_c) and ²¹Ne (²¹Ne_c) have been determined from SP flow olivine and pyroxene in this study. The error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of ³He in olivine and pyroxene have identical values of 135 ± 8 at/g/yr (2${\sigma }_{\bar{x}}$; standard error) using time-independent Lal (1991)/Stone (2000) (St) scaling factors. These production rates decrease to identical values of 130 ± 8 at/g/yr (2${\sigma }_{\bar{x}}$; standard error) when ³He measurements are standardized to the CRONUS-P pyroxene standard. The St-scaled, error-weighted mean, total reference production rates of ²¹Ne in olivine and pyroxene are 48.4 ± 2.9 at/g/yr and 26.5 ± 1.7 at/g/yr (2${\sigma }_{\bar{x}}$; standard error), respectively, increasing to 49.3 ± 3.0 at/g/yr and 27.0 ± 1.7 at/g/yr (2${\sigma }_{\bar{x}}$; standard error), respectively, when standardized to the CREU-1 quartz standard. ³He and ²¹Ne production rates (St) overlap within 2σ uncertainty with other St-scaled production rates in the literature. SLHL ³He and ²¹Ne production rates in SP flow olivine and pyroxene are nominally lower if time-dependent Lm and Sa scaling factors are used. Olivine and pyroxene both have identical, error-weighted mean SLHL production rates of 127 ± 8 at/g/yr (2${\sigma }_{\bar{x}}$; standard error) using Lm scaling factors and CRONUS-P standardized ³He measurements. These production rates decrease to identical values of 110 ± 7 at/g/yr (2${\sigma }_{\bar{x}}$; standard error) for olivine and pyroxene when using Sa scaling factors. The Lm-scaled, error-weighted mean, total reference production rates of ²¹Ne in olivine and pyroxene are 48.1 ± 2.8 at/g/yr and 26.4 ± 1.7 at/g/yr (2${\sigma }_{\bar{x}}$; standard error), respectively, when standardized to the CREU-1 quartz standard. The error weighted mean, local ²¹Ne/³He production rate ratio in olivine is 0.358 ± 0.009 (2${\sigma }_{\bar{x}}$; standard error), which increases to 0.378 ± 0.012 when using CREU-1 standardized ²¹Ne production rates and CRONUS-P standardized ³He production rates. The error weighted mean, local ²¹Ne/³He production rate ratio in pyroxene is 0.197 ± 0.006, or 0.208 ± 0.008 when ²¹Ne and ³He are standardized to CREU-1 and CRONUS-P, respectively. The updated, CREU-1 standardized ²¹Ne_c rate (St) in SPICE quartz is 16.5 ± 1.1 at/g/yr. Production of ²¹Ne in coexisting SPICE olivine (ol), pyroxene (px), and quartz (qz) (standardized to CREU-1; Fenton et al., 2019; this study) yields error-weighted mean, local production rate ratios of 3.00 ± 0.13 (2${\sigma }_{\bar{x}}$) and 1.64 ± 0.08 (2${\sigma }_{\bar{x}}$) for ²¹Ne_ol/²¹Ne_qz and ²¹Ne_px/²¹Ne_qz, respectively. This study suggests that production rates of ³He and ²¹Ne in SPICE olivine and pyroxene agree well with St- and Lm-scaled global mean production rates in the literature. It also indicates that CRONUS-P and CREU-1 standardizations yield production rates in even stronger agreement with these global mean rates.