The circumstances of the recovery, as related by Robert Haag, begins with a man chasing crows out of his cornfield near Zagami Rock, Nigeria. All of a sudden the man heard a loud explosion and was buffetted by a shock wave. Seconds later there was a puff of smoke and a thud about ten feet away. Afraid that an artillery shell had landed, he waited a few minutes before approaching the two-foot hole.
A 40 pound, calcium-rich, basaltic achondrite had just completed a 3 million-year journey from Mars. Evidence supporting this origin includes a crystallization age of only 180 m.y. ago, residual magnetic properties, a mineral composition that contains water (kaersutite), trapped gases in percentages the same as those found by the Viking and Pathfinder missions, and a weak gravity field on the crystallizing minerals. Along with Zagami, twelve other meteorites comprising three separate groups, shergottites, nakhlites, and a chassignite, fit the criteria for Martian origin.
The shergottite group has been divided into two distinct subgroups; those with a volcanic origin consisting primarily of the clinopyroxenes pigeonite and augite, to which Zagami belongs, and those with a plutonic origin consisting primarily of olivine, chromite, and orthopyroxene known as lherzolites or harzburgites. Shergottites as a group have a large percent of their composition as plagioclase feldspar which was shocked after crystallization to 31 GPa, creating maskelynite intergrowths. The pyroxene crystals of pigeonite and augite have Mg-rich cores indicating crystallization in a slowly cooling magma chamber at a depth of ~7-15 km. This is consistent with a typical Tharsis-type volcanic magma chamber such as Olympus Mons. Studies also suggest a possible second stage, rapid-cooling period in a thick lava flow also consistent with that of a Tharsis-type terrain.
Crystallization of Zagami from the molten state took place a recent 180 m.y. ago. A subsequent shock event occurring ~3 m.y. ago resulted in the maskelynization of plagioclase and was probably the impact event that launched the rock into space. This cosmic ray exposure (CRE) age for Zagami matches that of two of the other four basaltic shergottites suggesting a simultaneous ejection event on Mars. The three lherzolites likewise have CRE ages that coincide at ~4 m.y. ago, being ejected from Mars just 1 m.y. before the three basaltic shergottites. The other two basaltic shergottites, EET79001 and Dar al Gani 476, have very similar CRE ages of 0.82 (+/-0.2) m.y. and 1.1 m.y. respectively, as well as similar mineralogy, representing at least one additional magma source on Mars than the other members of the two subgroups.
Zagami is very similar to terrestrial basalts in even minor and trace element content. However, factors such as a high CRE age, Fe- and Mn-rich composition, lack of water, and oxygen isotopic differences, point to an origin from Mars rather than from Earth.
