| Meteorite RBT04262 is one of only two Martian meteorites made of composite lithologies. Lithology 1 is composed of coarse-grained olivines enclosed in poikilitic pigeonites, resembling lherzolitic shergottites. Lithology 2 is finer-grained and composed of olivine phenocrysts set within a groundmass of augite, olivine, plagioclase (shocked to maskelynite) and accessory phases such as Ca-phosphates, representing an olivine-phyric shergottite. Lithology 1 may be an early-formed cumulate while Lithology 2 may represent a cooled liquid laden with accumulated olivine crystals. Geochemical and textural observations suggest that all components of RBT04262 formed as intrusions. As the majority of Martian meteorites also have a cumulate origin, it is speculated here that crust formation on Mars may be dominated by magmatic underplating or intrusion rather than by subaerial flows. An intrusive-origin for Martian crust has important implications as intrusions can reset the radiometric ages or magnetic signatures of the Martian crust without erasing the cratering history. |
