In situ cosmogenic ¹⁰Be in pyroxene with an application to surface exposure dating

• Main Author: Collins, Julia Anne
• Other Authors: Norton, Kevin, Mackintosh, Andrew
• Format: Master Thesis
• Language: English
• Published: Victoria University of Wellington 2015
• Subjects: Cosmogenic; ¹⁰Be; Pyroxene; Mackay; Mackay Glacier; Mount Gran; New Zealand; Antarc*; Antarctica
• Online Access: http://researcharchive.vuw.ac.nz/handle/10063/4851

Cosmogenic nuclides are an important tool in quantifying many Earth-surface processes. Beryllium-10 (¹⁰Be) is commonly extracted out of the mineral quartz; however many landscapes lack quartz bearing rocks. In order to establish a new chronometer based on ¹⁰Be in pyroxene for use in New Zealand and Antarctica, it is necessary to verify cleaning protocols and determine a local production rate. In this study, I have tested and modified an existing pyroxene decontamination procedure in order to further develop the use of ¹⁰Be in pyroxene as a chronometer. This method successfully removes the meteoric component of ¹⁰Be in pyroxene, allowing only the concentration of in situ produced ¹⁰Be to be measured. Additionally, production rates for ¹⁰Be in pyroxene have been determined empirically for New Zealand using cross-calibration with measured ³He concentrations and an independent radiocarbon age of the Murimotu debris avalanche in the central North Island, New Zealand of 10.6 ± 1.1 ka. Theoretical ¹⁰Be pyroxene production rates were also determined, based on the composition of the Murimotu pyroxene. The best estimate for the 10Be pyroxene production rate is 3.4 ± 0.8 atoms g⁻¹ yr⁻¹ at sea-level high latitude, which was determined via cross-calibration with the radiocarbon age for the deposit. This work shows that production rates for ¹⁰Be in pyroxene are both empirically and theoretically 8-27% lower than in quartz. The ³He/¹⁰Be ratio in the Murimotu pyroxene is 34.5 ± 9.9; this is indistinguishable from global ³He-pyroxene/¹⁰Be-quartz production ratios. In a case study surface exposure ages were determined for bedrock samples and cobble erratics collected in a vertical transect on Mount Gran, Antarctica, by applying the aforementioned ¹⁰Be pyroxene decontamination procedure and radiocarbon derived production rates. A chronology for ice surface lowering was obtained for the adjacent Mackay Glacier, indicating the ice surface lowered approximately 60 m during a relatively rapid episode of thinning which occurred between ...