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

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thesis
posted on 15.11.2021, 12:39 authored by Collins, Julia Anne

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 ~13.5 ka and 11 ka.  This thesis presents a successful test of decontamination procedures, new production rates, and an example application, showing the promise of ¹⁰Be in pyroxene as a chronometer. The development of ¹⁰Be in pyroxene allows environments without quartz-bearing rocks to be dated using this widely used nuclide. The pairing of ¹⁰Be with ³He in pyroxene would allow complex exposure histories to be determined, expanding the application.

History

Copyright Date

01/01/2015

Date of Award

01/01/2015

Publisher

Te Herenga Waka—Victoria University of Wellington

Rights License

Author Retains Copyright

Degree Discipline

Geology

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level

Masters

Degree Name

Master of Science

ANZSRC Type Of Activity code

970104 Expanding Knowledge in the Earth Sciences

Victoria University of Wellington Item Type

Awarded Research Masters Thesis

Language

en_NZ

Victoria University of Wellington School

School of Geography, Environment and Earth Sciences

Advisors

Norton, Kevin; Mackintosh, Andrew