Earth is the only body in our solar system with an extensive silica-rich (andesitic or granitic) crust, which is attributed to multi-stage igneous processing linked to plate tectonics. In contrast, most differentiated meteorites are basaltic, with significantly lower silica contents, and are thought to have formed through single-stage, high-degree partial melting of their parent bodies. Recently, however, a few unique achondrites with much higher silica contents have been discovered. The origin of these andesitic meteorites has been attributed to very low-degree partial melting and rapid, near-fractional extraction of such melts from chondritic bodies.
To better understand these rare achondrites, a team led by Robert W. Nicklas of the University of California San Diego made a detailed petrologic and geochemical study of Erg Chech (EC) 002, a meteorite found in the Algerian Sahara in May 2020. EC 002 is an igneous-textured rock consisting predominantly of elongated pyroxene grains in a groundmass of Na-rich plagioclase feldspar. Its bulk composition is equivalent to andesite, similar to Earth’s bulk continental crust. EC 002 is also strongly depleted in highly siderophile elements (HSE), which tend to be strongly concentrated in metal.
The depletion of HSE in EC 002 makes it unlike other known andesitic achondrites. Nicklas and coauthors propose a two-stage formation model in which a chondritic body first differentiated, forming an iron-rich core and depleting the rest of the body in HSEs, followed by a second heating event in which the parent melt of EC 002 was formed. Radiometric age dating of EC 002, conducted previously by another team, places the formation of EC 002 within the first 2.25 million years of solar system history. If the preferred interpretation of Nicklas and coauthors is correct, this would mean that multi-stage igneous processing occurred on some planetesimals shortly after accretion. However, other scenarios to explain the HSE depletion are possible and remain to be tested. READ MORE