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Cosmogenic Nuclides (Which nuclides? (Most suitable nuclide depends on:,…
Cosmogenic Nuclides
Cosmogenic nuclides
= rare isotopes of chemical elements
form via cosmic rays reacting with atmos/surface minerals
Useful for discerning dates / rates geomorphic processes
Isotope
= set of nuclides w same number of protons
but different number of neutrons (thus different atomic mass)
e.g. Be
Be-7
4 protons & 3 neutrons
Be-10
4 protons & 3 neutrons
Radiocarbon vs In-Situ Cosmogenics
Meteoric
C-14
measures time since organism death
i.e. C-14 decay into N-14
In-situ cosmogenics
Build up of nuclides with time
Cosmogenics applications
Geomorphic processes produce fresh surfaces
∴ Cosmogenics allow determination dates & rates of earth surface processes
Which nuclides?
Most suitable nuclide depends on:
Timescale of interest
Rock types available
Means of extraction & isotope msmt
'Useful' cosmogenic nuclides are:
Produced in nr-surface material
Stable
keep accumulating as long as exposed i.e. do not decay
or atleast sufficiently long-lived for Earth sciece apps
Low geological background concentrations
Suitable methodology available
Typical
minerals
& CNS
Quartz
yields: Be, C-14, Ne
Limestone/basalts
Cl-36 & He-3
Calcite/olivine
Stable isotopes
Can be used to date Myr back
e.g. He, Ne, Ar
Key production pathways
Spallation
Muon capture
Neutron capture
CN Summary
Several nuclides may be measured
each w advantages & disadvant
Form by interaction of cosmic radiation w target minerals
Production rate varies w latitude & elevation
99% production occurs in upper 3 m Earth's surface