For the example to the right, offset values for Centering the secondary ion image in the center of the field aperture (FA) is necessary for each new analysis spot because primary beam paths and conditions of secondary ion extraction vary over the area of the sample surface,in particular when spot-to-spot movements over large distances (few mm) are required.
The sequence is as follows: In order to stabilize the secondary ion signal and to remove surface contamination, the primary beam is focused on the sample surface for a certain time period prior to data acquisition.
EPMA and CL services will be billed directly by Dr. GSS will charge a handling fee to cover costs associated with providing grain mounts and x-y grain-relocation files to Dr.
ion bombardement is significantly enhanced if the sample surface is saturated with regard to oxygen.
In this example (right), the duration of the presputtering was 3 min.
Subsequently, intensities were measured in 15 magnet cycles.
Zircon is especially useful because it frequently contains uranium in substitution for zirconium, but does not incorporate lead (as shown by the absence of Lead-204).
Concordia ages of 2,051 ± 9 Ma (2σ, MSWD = 0.1) and 2,056 ± 5 Ma (2σ, MSWD = 0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef.
Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity.
Thus all the lead in the zircon can be assumed to be radiogenic.
The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks.