Shergottites are the most common type of martian meteorite. Their composition is basaltic, similar to igneous rocks from Iceland and Hawaii, but with some important differences.
When the shergottites are dated using common isotope-decay techniques, they generally are found to be only a few hundred-million years old. On Mars, the only sources of igneous rocks of that age are the large volcanoes, such as Olympus Mons, which are also made of basalt.
But, when J. Brian Balta and Harry Y McSween Jr. compare the shergottites to measurements of the composition of those volcanoes by orbiting spacecraft, they find that they do not match, particularly in their silica contents. Despite the shergottites matching the volcanoes in age, the volcanoes appear to be made of a different type of basalt from most of the meteorites in our collection.
Magmas can dissolve small amounts of water in them, and that water can change the silica contents of magmas in a way that could explain both the volcanoes and the shergottites. Balta and McSween propose that the shergottites represent magmas generated with water and the volcanoes represent drier magmas.
Magmas similar to the shergottites could therefore have been a major source of the water present on the martian surface early in its history.
Water and the composition of Martian magmas by J. Brian Balta and Harry Y. McSween, Jr., Dept. of Earth and Planetary Sciences, University of Tennessee,DOI: 10.1130/G34714.1.