What is an angrite and why is it important?
Angrites are rare volcanic meteorites, among the oldest igneous rocks known, crystallising within a few million years of the Solar System's birth. They are used as precise time markers.
Angrites are a small group of basaltic achondrites, named after Angra dos Reis, which fell in Brazil in 1869. Only a few dozen are known. They are made mostly of calcium- and aluminium-rich pyroxene, calcium-rich olivine and calcium-rich plagioclase, an unusual mineral mix not seen in other meteorites.
Their chemistry shows they come from a parent body strongly depleted in volatile elements such as sodium and potassium, so it was probably heated early, losing these elements to space. Their oxygen isotopes are close to, but distinct from, those of the HED meteorites.
Angrites are important for two reasons. First, they are some of the oldest volcanic rocks known. Some quickly cooled angrites, such as D'Orbigny and Sahara 99555, crystallised about 4,563 to 4,564 million years ago, only around 4 million years after the first solids formed. This shows that some planetesimals melted and erupted lavas very early.
Second, because they cooled quickly and stayed undisturbed, angrites record several radiometric clocks that agree well. They are dated precisely by both the long-lived uranium–lead method and short-lived methods such as manganese–chromium and hafnium–tungsten. This makes them anchors: scientists use angrite ages to convert relative ages from short-lived isotopes into absolute ages for other meteorites. The parent body of angrites has not been identified among known asteroids.