Recently, scientists were able to analyze the composition of Mars through the delivery of meteorites from Mars to Earth. These include shergottites that formed 165 to 2400 million years ago and 1.345 billion-year-old nakhlites and chassignites. These are all igneous rocks that come from the Martian mantle. They contain crystals older than the rock that surrounds them, but no one knows why. However, there are also key differences reported between these three types of meteorites. Shergottites are geochemically similar to the shield and rejuvenating volcanic formations of Hawaii. Nakhlites and chassignites have a similar composition to melting rocks from that mantle that have reached the lithosphere, while shergottites have a similar composition to deep mantle sources. The Martian magma that formed these rocks is generated through melting episodes of the lithosphere.
Martian meteorites are identified by trapped chemicals within the rock that have identical signatures to the Martian atmosphere. They represent near-surface igneous rocks, including shergottites, nakhlites, chassignites, and the ancient crustal rocks. Shergottites are the most diverse of all of these rocks and define three groups, each one with different amounts of rubidium and neodymium. They come from reservoirs that were formed as early as 4.045 billion years ago. Nakhlites and chassignites are much younger and less varied. They are formed through cooler partial melts at a depleted mantle source. The relationship between these rocks is poorly understood. Both have enrichments from the same source, but nakhlites have a more complex origin. Shergottites came from deep mantle plumes and nakhlites from a partially melted lithosphere, that melted at a lower temperature.
The scientists collected 40 martian meteorites and analyzed them through the following methods. For the meteorites found, a diamond saw was used to cut away the fusion crust, which is a crust formed from rock melted by the atmosphere. Alternatively, the crust was crushed or sanded. 50 milligrams of the sample was digested in a distilled acid solution for at least seventy-two hours on a hot plate. This left the scientists with a clear solution. After several more procedures, trace element abundance was measured with a mass spectrometer. They were also able to measure the abundance of other elements using the mass spectrometer and a more diluted sample.
This study presents a dataset composed of groups of shergottites and nakhlites. The 24 shergottites spanned the known range of compositions, from incompatible trace-element depleted to trace-element enriched. Depleted shergottites have lower amounts of trace elements than intermediate or enriched shergottites. Enriched shergottites had more alkalis and were less mafic than intermediate or deplete shergottites, and collectively they spanned a range of compositions from picrobasalts to basalts.
The 16 nakhlites and chassignites span a range of compositions, from picrobasalts low in silicon dioxide to basaltic andesite. They have a wider range of La/Ybn, which is a rare earth element. They also have lower abundances of certain rare earth elements, like niobium and yttrium than depleted and intermediate shergottites, but higher abundances than enriched shergottites. This is expected because they formed from depleted mantle sources, which contained little to no amounts of these rare earth elements. They also have lower amounts of trace elements.
This study also contains some notable anomalies. Some of these meteorites were found in the desert, which gave them terrestrial contamination, altering certain elements, like barium. For example, NWA 2986 has a lead anomaly that might have come from terrestrial alteration. One particular meteorite, NWA 6963, had the composition of a shergottite, and the trace-element abundance of a nakhlite.
In a nutshell, several scientists have analyzed a group of meteorites and found key differences among them. They were extracted from the fusion crust using sawing or other methods and analyzed using a mass spectrometer. They discovered that shergottites had a variety of differences between the three groups, depleted, intermediate, and enriched. This was due to the different ways that they formed. They are also different from nakhlites and chassignites, a younger group of igneous rocks that formed near Mars’s lithosphere. They have lower amounts of rare earth elements and trace elements. This is because they were formed in a depleted mantle source. This study provided a lot of important information that will help scientists classify and differentiate between these groups of martian meteorites.