According to a team headed by Dr Violaine Sautter from the National Museum of Natural History in Paris, France, Martian crustal components bear a strong resemblance to tonalite-trondhjemite-granodiorites, rocks that predominated in Earth’s continental crust more than 2.5 billion years ago.
Igneous clast named Harrison embedded in a conglomerate rock in Gale crater shows elongated light-toned feldspar crystals. The mosaic merges an image from Curiosity’s Mastcam with higher-resolution images from ChemCam’s Remote Micro-Imager. Image credit: NASA / JPL-Caltech / LANL / IRAP / U. Nantes / IAS / MSSS.
“Mars has been viewed as an almost entirely basaltic planet, with igneous rocks that are dark and relatively dense, similar to those forming the Earth’s oceanic crust,” said team member Dr Roger Wiens of Los Alamos National Laboratory.”
“However, Gale crater, where NASA’s Curiosity rover landed, contains fragments of very ancient igneous rocks (3.6 billion years old) that are distinctly light in color, which were analyzed by the rover’s ChemCam instrument.”
Dr Sautter, Dr Wiens and their colleagues from the United States, France and the UK observed images and chemical results of 22 of these rock fragments.
The team determined that these pale rocks are rich in feldspar, possibly with some quartz, and they are unexpectedly similar to Earth’s granitic continental crust.
“We present geochemical data and images of 22 specimens analyzed by Curiosity that demonstrate that these light-toned materials are feldspar-rich magmatic rocks,” the scientists wrote in a paper published in the journal Nature Geoscience.
These rocks belong to two distinct types: alkaline compositions containing up to 67 percent SiO2 and 14 percent total alkalis (Na2O + K2O) with fine-grained to porphyritic textures on the one hand, and coarser-grained textures consistent with quartz diorite and granodiorite on the other hand.
“Combined with the identification of feldspar-rich rocks elsewhere and the low average density of the crust in the Martian southern hemisphere, we conclude that silica-rich magmatic rocks may constitute a significant fraction of ancient Martian crust and may be analogous to the earliest continental crust on Earth,” the scientists wrote.
_____
V. Sautter et al. In situ evidence for continental crust on early Mars. Nature Geoscience, published online July 13, 2015; doi: 10.1038/ngeo2474
