B4
Basaltic shergottite
Martian volcanism ~ 200 million years ago

In 1976, two NASA landers, Vikings I and II, landed on the surface of Mars and measured the atmospheric composition. This compositional information could then be compared from rocks ejected from the Martian surface through relatively large impact events which eventually land on Earth. Due to the relatively high temperatures and pressures during Martian impact events, a small portion of the rocks become molten, capturing Martian atmosphere upon solidification, which can be sampled and compared with the Viking data once collected from the surface of Earth. Dr. Donald Bogard, a NASA scientist, measured trapped gas in the Martian shergottite EET 79001 and found that the isotopic composition of the trapped gas is consistent with that of the Martian atmosphere measured by the Viking landers, indicating that the rock really had come from Mars. Martian meteorites are presently classified into mainly four types: shergottite, nakhlite, chassignite, and ALH 84001. These rocks are considered to be products of igneous processes on Mars. Based on petrological signatures, shergottites are further classified into three types: basaltic, lherzolitic, and olivine-phyric. Shergottites are also classified based on their chemical characteristics into three types: enriched, depleted, and intermediate, having high, poor, and moderate abundances of incompatible elements, respectively. Based on whole-rock radio isotope dating, ALH 84001 formed about 4.1 billion years ago. The formation ages of nakhlite and chassignite meteorites are determined to be about 1.3 billion years ago. Shergottites have a wide range of formation ages from about 180 million to 500 million years ago. The displayed item is a cut slab of an enriched basaltic shergottite, Zagami. The chemical zoning structures of pyroxene grains in Zagami suggest a two-stage crystallization process: (stage 1) crystallization inside a magma chamber indicated by homogeneous core composition due to slow cooling, and (stage 2) rapid cooling near the surface which results in a gradient of compositional zoning. Based on such information recorded in Martian meteorites, we are able to obtain a picture about the past igneous activity and crustal evolution on Mars. (Takafumi Niihara)