Application of Spallogenic Noble Gases Induced by Energetic Proton Irradiation to Problems in Geochemistry and Thermochronometry

diffusion kinetics;helium;neon;noble gases;thermochronometry

Shuster, David Lawrence ; Farley, Kenneth A.

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: diffusion kinetics;    helium;    neon;    noble gases;    thermochronometry;   

Others  :  https://thesis.library.caltech.edu/2357/1/Shuster_Thesis_Final.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Synthetic components of 3He, 4He, and 21Ne were generated within natural minerals via irradiation with 150 and 220 MeV proton beams.Fluences of ~10^14 and ~10^16 protons/cm2 induced 3He concentrations of ~10^8 and ~10^9 atoms/mg, respectively.Controlled degassing experiments on irradiated samples of terrestrial apatite (Ca5(PO4)3F), titanite (Ca(TiO)(SiO4)), olivine ((Mg,Fe)2SiO4), goethite (FeOOH), and quartz (SiO2) demonstrate that the proton-induced nuclides are spatially uniform across samples <= 1 mm in diameter and sequential degassing quantifies solid state diffusion kinetics of helium and neon.Diffusion kinetics of proton-induced 3He in Durango apatite (Ea = 147.9 ± 1.3 kJ/mol; ln(Do/a2) = 16.0 ± 0.3 ln(s-1)) and Fish Canyon tuff titanite (Ea = 183.7 ± 2.7 kJ/mol; ln(Do/a2) = 13.3 ± 0.5 ln(s-1)), are indistinguishable from those determined for natural radiogenic 4He in the same samples.Experiments indicate that lattice damage potentially introduced via proton irradiation did not significantly modify the natural 4He diffusion kinetics in the two samples, and that the proton-induced 4He component is relatively negligible in abundance.Therefore, sequentially measured 4He/3He ratios reflect the natural spatial distribution of radiogenic 4He.Combined with a (U-Th)/He age and helium diffusion kinetics, the distribution limits the time-temperature (t-T) path a mineral experienced through geologic time.This is the basis for a new methodology called 4He/3He thermochronometry, which for apatite constrains continuous t-T paths between 80 oC and 20 oC.Proton-induced 3He diffusion parameters in olivine are: Ea = 153.8 ± 1.1 kJ/mol and ln(Do/a2) = 3.0 ± 0.2 ln(s-1).At 25 oC, the helium diffusion kinetics in a goethite sample (Ea = 163 ± 2.4 kJ/mol; ln(Do/a2) = 26.0 ± 0.6 ln(s-1)) predict 90% 4He retention over 11.8 Ma, consistent with the observed deficit gas fraction and (U-Th)/He age of 10.7 Ma.This indicates that goethite (U-Th)/He dating is a viable weathering geochronometer.In quartz, the diffusion kinetics for proton-induced 21Ne and 3He (Ea = 153.7 ± 1.5 (kJ/mol); ln(Do/a2) = 15.9 ± 0.3 (ln(s-1)) and Ea = 84.5 ± 1.2 (kJ/mol); ln(Do/a2) = 11.1 ± 0.3 (ln(s-1), respectively), indicate that cosmogenic neon will be quantitatively retained in inclusion-free quartz at Earth surface temperatures whereas cosmogenic helium will not.