BSE image and 40Ar/39Ar age
of basaltic regolith basalt 15639,6, brought by the Apollo 15 mission
This figure illustrates my approach to the project using Apollo sample 15639,6. I combine techniques to develop as full an understanding as possible of a sample and what it tells us about the history of the Moon.
The upper frame shows a back scattered electron image. An electron beam is scanned over the surface and electrons that are directed backwards are measured. Spots with heavier elements scatter more electrons back to the detector. Combined with other analyses it is possible to understand the detailed composition of the sample. Different mineral phases are labelled: olv=olivine, 2 types of pyroxene (aug=augite, pig=pigeonite), chr=chromite, spn=spinel, plag=plagioclase. The samples also contained minor minerals such as apatite and K-rich glass.
An aliquot of the sample was irradiated with neutrons. Irradiation converts a small proportion of potassium (39K) to a form of argon called "argon-39" (39Ar). This is useful because potassium also naturally produces a different form of argon called "argon-40"(40Ar). (The technical word is "isotope"). The older a sample is, the more radiogenic argon-40 will have accumulated for each milligram of potassium it contains.
We use an infra-red laser to heat the irradiated sample stepwise, and at each step the gas it has released is sent into a mass spectrometer to measure small amounts of argon-39 and argon-40. From each step we can work out an age based on the amount of potassium corresponding to the argon-39 measured, and the amount of accumulated argon-40. The ration between these 2 isotopes is multiplied by the inverse of 40K-decay constant to calculate and ages.
The lower frame shows how the apparent age varies with temperature, which is represented here by the proportion of 39Ar released per step (represented by one of the small segments, solid rectangles. From the consistent ages, over the 39Ar release, and usually at the intermediate and high temperature steps, we can work out how long ago the sample started to trap radiogenic argon-40 as it was produced from potassium.
References: Assis Fernandes et al. EPSC-Berlin abstract