 Meteorites are asteroid or comet material that have fallen to Earth. There are over 40,000 meteorites that we know about. Some have no uranium. They can be used to measure the solar system's initial lead ratios. The Kenyon-Diablo meteorite fits in this category. Some contain intact material from the circumstellar disk during the planetesimal building process. These are the ones that never went through a melt and re-hardening process, like all the rocks on the Earth and the Moon. The meteorite Allande fits into this category. We'll start with Kenyon-Diablo, the meteorite that was responsible for meteor crater in Arizona. It is estimated to have fallen to Earth around 50,000 years ago. Fragments of the meteorite have been actively collected since the mid-1800s. The largest fragment is the Hollinger meteorite with a mass of 639 kilograms or 1,400 pounds. This iron meteorite contains troryolite, an iron sulfide mineral that has almost no uranium. Since the mineral contains no uranium, all the lead present in the troryolite is the lead originally present when the meteorite formed. This includes the radiogenic isotope lead 206 and 207 that decayed from uranium before the meteorite formed, as well as the natural, non-radiogenic lead 204. Thus, using mass spectrometry as always, this Kenyon-Diablo troryolite gives us the primordial ratios for lead 206 over 204 and 207 over 204. In fact, these two numbers are generally used as the standard for our solar system's original lead concentrations. In 1969, Allande created a fireball over the northern Mexico sky. As the meteorite burst, numerous fragments rained down around the small village of Pueblito de Allande. Over 2,000 kilos of debris have been found, and new pieces are still being discovered every now and then. It's a type of meteorite called carbonaceous chondrite that's a stony meteorite with lots of carbon and containing small mineral granules called chondrules. A lot of meteorites have experienced significant heat that melted and reorganized their minerals. But Allande was not one of them. Its pieces remained as they were when they formed. Here's a slice of it. We're particularly interested in the little pale whitish-gray bits called calcium-aluminum inclusions, CAIs for short. These are thought to be the very first solids condensed in the circumstellar disk. Meaning these would give us the starting date for the solar system. Other interesting pieces of the meteorite are these round darker gray bits. These were the first liquid droplets to condense out of the disk gas. They are also some of the oldest minerals that formed in the solar system, but not as old as CAIs. We are interested in two findings associated with Allande. One is the ratio of Uranium-238 to Uranium-235 isotopes. In Allande, it was around 137.88 Uranium-238s for each Uranium-235. This ratio has held up across Earth, Moon, and meteorite rocks. The other is the ratio of radiogenic-led 207 and 206 to non-radiogenic-led 204. A large number of these ratios were determined from the various CAIs and condrules, but we'll use just one, with lead 207 over 204 at 22.76 and 206 over 204 at 30.06.