Zircons Hint at the Internal Structure and Geodynamics of Mars

The NWA 7533 zircon-rich meteorite containing fragments of the ancient crust of Mars. Credit: University of Copenhagen.

The uranium-bearing mineral zircon is an abundant constituent of Earth’s continental crust, providing information about the age and origin of the continents, while Mars’ crust is compositionally more similar to Earth’s oceanic crust, where zircon is rare. Therefore, zircon is not expected to be a common mineral on Mars. However, a recent analysis of an ancient martian meteorite reveals zircons with ages spanning 4.2 billion years, nearly the entire geologic history of Mars, suggesting that zircon may instead be abundant on the surface of the red planet.

Martin Bizzarro and Maria Costa from the Globe Institute at the University of Copenhagen and colleagues investigated the ancient martian meteorite NWA 7533, which was discovered in the desert of Morocco in 2011. The meteorite is a regolith breccia originating from the southern highlands of Mars interpreted to have components of mixed igneous, sedimentary, and impact origin. After crushing 15 grams of this rock, they extracted 57 zircon grains. By uranium-lead age-dating the zircons, they found that the majority of the grains crystallized nearly 4.5 billion years ago. They also made an unexpected discovery: some zircons had much younger ages, ranging from about 1.5 billion years down to 300 million. The plausible sources for these grains are the large volcanoes located in the northern hemisphere of Mars, but this requires significant dispersal in order for them to be incorporated into NWA 7533 in the southern highlands. The researchers also analyzed the hafnium isotopic composition in the same zircons to understand the internal structure of Mars. They found hafnium isotope signals in the young zircon population suggesting the existence of a primitive, deep-seated, isotopically homogenous, and presumably convecting, mantle reservoir that provides important evidence regarding the properties of the deep interior of Mars. The discovery that zircon may be abundant on the martian surface may also guide the future robotic exploration of the planet, especially for missions seeking to return samples to Earth. READ MORE