The ureilite meteorites are a major group of ultramafic meteorites (consisting of the minerals olivine and pyroxene) thought to represent the mantle of a partially differentiated, carbon-rich asteroid. Some fundamental aspects of the formation and evolution of this body are poorly understood, including the nature and fate of the plagioclase-bearing crustal rocks that should have formed as complements to the mantle. However, the recent discovery of a main group of ureilite containing primary plagioclase may prove to be a critical missing link in deciphering the igneous history of the ureilite parent body.

MS-MU-012 is a 15.5-gram clast from the polymict ureilite Almahata Sitta, which fell in Sudan in 2008. Cyrena Goodrich from the Lunar and Planetary Institute, along with a team of international collaborators, analyzed this unusual ureilitic sample with a host of high-precision techniques to better understand what this rock means for the formation of the ureilites.

Based on its mineralogy, petrology, and chemistry, MS-MU-012 belongs to a group called the “Hughes-type ureilites” named after the meteorite Hughes 009. What marks this sample as unusual, however, is that it contains the mineral plagioclase. The authors interpreted this as indicating that MS-MU-012 and other Hughes-type ureilites contained a melt component. They proposed a scenario for the differentiation of the ureilite parent body in which melting began approximately 1 million years after the formation of the first solids in the solar system and proceeded with rapid extraction of melts to form a crust. Late melts, formed several million years later, are represented by the Hughes-type ureilites, based on the compositions of trapped melt inclusions. These later melts were not as efficiently extracted and were emplaced as intrusions in the crust. This work shows that discovering new types of ureilites can improve our understanding of their origin and evolution. READ MORE