SMARA


Eucrite
Polymict
standby for smara photo
Found April 2000
26° 41' N., 11° 44' W.

A single 12.87 kg stone was recovered from the Western Sahara. Smara is a fragmental breccia containing a variety of clast types including basalts, gabbros, microgabbros, and impact-melts, the latter measuring up to 1 cm in diameter. Continued research on this new find should reveal many more details about its formation history.

The probable source asteroid, 4 Vesta, has a diameter of ~525 km. It has an outer basaltic crust thin enough (~10–25 km) to have been completely excavated down to diogenitic material by the huge impact which left the 460-km-wide, ~13-km-deep crater near the south pole. Exposure age distributions of a statistical sampling of HED meteorites show that at least two major impact events occurred during its history—one at 22 m.y. ago and another at 39 m.y. ago. Many shallower impacts into eucritic strata also occurred. Some of the eucrite, diogenite, and howardite material was spalled into space by these impacts and entrained into the 3:1 and v6 resonances. A recent search has identified twenty small Vesta-like asteroids, eight of which bridge the gap between Vesta and the 3:1 resonance, which are composed of both eucritic and diogenitic fragments. Consequently, HED fragments were perturbed from these dynamical escape hatches into Earth-crossing orbits on time scales of tens-to-hundreds of m.y., where they were eventually swept up by Earth's gravity to land in our collections.

The general composition of eucrites consists of roughly equal amounts of anorthite, a plagioclase feldspar, and the clinopyroxene pigeonite. The parental magma was probably derived from the mafic mineral peridotite, a mixture of olivine, pigeonite, and plagioclase, which is the same mineral that forms the bulk of the Earth's upper mantle. Eucrite differentiation and crystallization occurred very early in Solar System history, ~4.565 b.y. ago, while cooling and metamorphism within individual ejecta blankets lasted an additional ~600 m.y. The photo above shows a 1.9 g partial slice of Smara exhibiting its brecciated texture.