This has generally been interpreted as indicating that chondrules formed approximately 2 million years after CAIs.
However, it could also have meant that chondrules formed at the same time as CAIs but were endowed with less Al was uniformly distributed.
The uranium clock thus measures a time when the temperature became low enough that the uranium and lead isotopes did not equilibrate any more with their surroundings.
Compared to a uranium age of 4567.2±0.6 million years for CAIs, the time differences given by these ages are such that we could expect to find evidence for initial Models of the temperature profiles experienced by different metamorphic grades of the H ordinary chondrites.
In this instrument a finely focused ion beam (in our case oxygen) bombards the surface of the sample to be analyzed (in our case polished thin sections of the meteorites).
Al (the only stable isotope of aluminum) in several crystals from the two meteorites.
On a large crystal from Forest Vale we could make these measurements in many different areas.
Measurements are made by changing the magnetic field of the mass spectrometer to different values so that only ions of a given isotope are transmitted and counted. Because of the very low magnesium concentrations, measurements take up to 10 hours for a single spot.
This picture shows the recently installed Nano SIMS at Washington University.
The Nano SIMS is a new type of ion microprobe that allows elemental and isotopic analysis with very high spatial resolution and with high sensitivity.