title: Development of techniques for analysis of cosmogenic nuclides, application to exposure histories and controls on production rates
Creators: name: Carracedo Plumed, Ana
Publisher: name: University of Glasgow
Year: 2017
Subjects: subject: Cosmogenic nuclides, production rates, exposure dating.

description: Cosmogenic nuclides produced in terrestrial rocks provide a versatile way of quantifying Earth surface processes. Accurate calculation of the surface age and erosion rates determined from cosmogenic nuclide concentrations requires knowledge of their production rate. Cosmogenic 3He (3Hecos) and 21Ne (21Necos) are commonly used nuclides, and in contrast to 10Be and 26Al, they can be used in a wide array of minerals. The production rate of 3Hecos and 21Necos depends on mineral chemical composition and in mafic minerals varies with the Mg/Fe ratio. Theoretical models have estimated that the production of 3Hecos in olivine is ~10% higher than in pyroxene and ~55% higher in the case of 21Necos. The main aims of this work are to test empirically the 3Hecos and 21Necos production rate variations with chemical composition of the target mineral, and to apply stable (3Hecos) and radioactive (10Becos) cosmogenic nuclides in olivine to unravel the exposure history of the Mount Hampton nunatak in West Antarctica.

The ThermoScientific Helix SFT multi-collector mass spectrometer at SUERC was set up to perform simultaneous collection of 3He and 4He improving the measurement reproducibility by approximately 3 times compared to peak jumping collection using the MAP 215-50. The He extraction technique using a diode laser was refined achieving ±0.4% (2σ) and ±4.6% (2σ) precision for analysis of ~15 mg of pyroxene and olivine respectively. For samples of greater mass (50-250 mg) achieving complete degassing was challenging, especially in the case of olivine. However, reliable data was obtained when degassing pyroxene with exposure ages for the lava flows from Kula Volcanic Province, Western Turkey, ranging from &lt; 1.8 to 11.2 ka; which is in agreement with previously published data (Heineke et al., 2016). The laser technique was successfully used for extracting He isotopes from detrital gold grains from different localities around Scotland. Unravelling the source of 3He has proved challenging and requires further development. However, the extraction technique has proved reliable and has the potential for analysis of tens to hundreds of grains from the same deposit in relatively sort time generating large data sets required for provenance studies.

The homogeneous composition of mafic minerals from six lherzolite xenoliths from Mount Hampton yields low variation theoretical production rates. 3Hecos and 21Necos were measured in olivine, orthopyroxene, clinopyroxene and spinel from each xenolith. The results confirm empirically the compositional control of cosmogenic nuclide
production rates. Therefore, a correction factor to scale for composition is recommended. The high altitude (~3 km) and high latitude (~76 ̊S) of the Mount Hampton xenoliths has generated some of the highest concentrations measured on terrestrial samples, comparable to those of extra-terrestrial material. Relative production rate variation of cosmogenic isotopes with altitude was found from the analysis of 3Hecos/10Becos ratios in accessory minerals from the Himalayas (~27 ̊N) at elevations of 2.8 to 4.8 km (Gayer et al, 2004; Amidon et al., 2008) suggesting tertiary spallation reactions in rocks with sufficient energy to produce 3Hecos but not enough energy to produce 10Becos. This was not observed in the Mount Hampton xenoliths with 21Necos/3Hecos ratios varying from 0.25 ± 0.04 in the case of clinopyroxene to 0.40 ± 0.04 in the case of olivine. These are in agreement with previously published ratios from lower altitudes and simply reflect the chemical composition control of the 3Hecos and 21Necos production rates.

The combination of stable and radioactive cosmogenic nuclides is an established tool for unravelling complex exposure histories in quartz-bearing surfaces. Here I have measured 10Becos in olivine from six xenoliths from Mount Hampton and combined with the results of 3Hecos from olivine from the same samples to understand the rate of landscape change above the West Antarctic Ice Sheet (WAIS). Several scenarios of complex exposure history were considered with the data being consistent with two possible scenarios for the Mount Hampton nunatak: (1) intermittent cold-based ice cover during the Plio-Pleistocene glacial cycles with a long-term erosion rate of &lt; 7 m/Ma and (2) being ice-free since the time of formation (11.4 Ma) and experiencing a dramatic increase on erosion rates involving the removal of up to 3 m of material in the last 1.5 Ma at an average erosion rate of ~0.3 m/Ma since eruption. The results from this work demonstrate that dramatic changes in erosion rates over long periods of time can generate cosmogenic nuclide signatures typical of complex exposure without requiring burial and revealed the dynamic nature of the landscape in the high elevation mountaintops above the WAIS.