39Ar-40Ar dating method - This is a variant of the K-Ar radiometric dating technique. In this decay system, 40K decays to 40Ar with a half-life of ~1.3 Ga. For the 39Ar-40Ar method, samples are irradiated in a nuclear reactor which converts 39K to 39Ar, by the following reaction:
39K(n,p)39Ar
39Ar is then measured as a proxy for K, along with 40Ar, to obtain a K-Ar date. Usually, an incremental heating technique is used to measure the isotopic composition of Ar gas that is evolved as temperature is increased. The evolved gas is passed through a mass spectrometer which is tuned to measure the abundances of various Ar isotopes. K/Ca ratios can be inferred for the gas released, because 37Ar is created from 40Ca in the nuclear reactor; thus 37Ar can be used as a proxy for Ca, just as 39Ar can be used as a proxy for K.
Arrhenius diagram - This is a type of diagram often used in diffusion studies in which the logarithm of diffusion coefficient (abbreviated D, not to be confused with partition coefficient) is plotted against 1/T, as in Fig.1 of the paper. Straight lines appear on this type of diagram because diffusion rates increase exponentially with an increase in temperature, as given by:
D = Do exp(-Q/RT)
(Eq. 1 in the paper). Here T = temperature in Kelvin, Do = a constant, R = gas constant, and Q = activation energy for diffusion. On an Arrhenius diagram, the y-intercept of a line at 1/T ~ 0 is given by log Do, and the slope of the line is given by -Q/R. Usually, diffusion coefficients are measured in the laboratory by measurements of how rapidly a species diffuses through a substance.
diffusivity parameter D/a2 - This parameter is a measure of diffusion rate, where D = diffusion coefficient, and a = length scale for diffusion. The 39Ar-40Ar dating method can be used to estimate the diffusivity parameter for Ar diffusion in minerals, by measuring how rapidly Ar gas is evolved from a substance as temperature is maintained at some constant value. This diffusivity can be converted to a diffusion coefficient by assuming a diffusion length scale.
lunar highlands - This is the light colored, heavily cratered terrane on Earth's Moon. The highlands are composed of feldspar-rich plutonic rocks and are older (>3.8 Ga) than the dark-colored maria, which are basaltic plains.
lunar cataclysm (late heavy impact bombardment) -
This is the hypothesis that the Moon experienced a distinct episode of
heavy impact bombardment ~3.9 Ga ago. Some workers subscribe instead
to the hypothesis that there was no distinct late bombardment, but rather
an exponential decline in the impact bombardment rate at that time. These
workers argue that the apparent clustering of ages of highland rocks at
~3.9 Ga (Fig. 6 in the paper) is an artifact, caused mainly by impact-induced
isotopic resetting. Evidence in favor of this argument is that
cratering data imply that terranes significantly older than 3.9 Ga exist
on the Moon, and that a few large, late-forming impacts appear to have
widely redistributed material on the Moon. This argument has been
going on ever since the 1970s.