Abstract
Despite an apparent north/south topographic dichotomy that formed >4.0 Ga, the young Martian meteorites (<2.4 Ga) and first-order remote sensing observations revealed a surface of Mars that is uniformly basaltic. This simplistic vision has been challenged by the discovery of a brecciated meteorite and additional spacecraft data that all point to the presence of alkaline igneous rocks, thereby demonstrating an unexpected igneous diversity on Mars. In the present paper, we review a variety of effusive alkaline rocks (basalts to trachytes) recognized so far in the southern hemisphere of Mars as observed from a unique 4.47 Ga Martian meteorite, as well as ground, and orbital data. The complementary of effusive alkaline rocks and plutonic orthopyroxene-rich rocks in early Mars is discussed. We propose that mantle-derived magmas at high extent of melting at rather low pressure either erupted forming orthopyroxene-rich lavas, or crystallized at shallow crustal depths, fractionating orthopyroxene which sank to the bottom of the chamber and residual alkaline magmas which erupted at the surface of Mars. Widespread low pressure fractionation processes could also be related to heavy bombardment on the early Martian crust generating melt sheets that ultimately differentiated. The Noachian crust is more diverse than being merely basaltic.
Highlights
The chemistry and mineralogy of the Martian surface have increasingly been better constrained these past 40 years thanks to Martian meteorites and remote analyses obtained by rovers, landers, and orbiting spacecraft
The Hesperian or Noachian alkaline rocks from Columbia Hills and the Amazonian evolved melts included within olivine cumulus of chassignites were likely produced by fractional crystallization at crustal depths, the Noachian alkaline clasts within the Martian breccia potentially formed by fractionation at the top of an alkaline-rich impact melt sheet of a preexisting alkaline crust that was produced 4.43 Ga ago by low degree of partial melt of a fertile mantle, and the formation of Noachian alkaline rocks from Gale crater remain debated
While terrestrial plate tectonics and significant volcanism have erased the first continental crust on Earth, numerous impact bombardments and extensive basaltic volcanism have likely buried a differentiated crust on Mars
Summary
The chemistry and mineralogy of the Martian surface have increasingly been better constrained these past 40 years thanks to Martian meteorites and remote analyses obtained by rovers, landers, and orbiting spacecraft. The simple view of Mars being a basaltcovered world had been challenged these past ten years by recent observations of differentiated rocks that combine new Martian meteorite finds [Agee et al, 2013, Humayun et al, 2013, Santos et al, 2015, Hewins et al, 2017], remote analyses from the Mars Science Laboratory (MSL) Curiosity rover [Cousin et al, 2017, Payré et al, 2020, Sautter et al, 2016, 2015, 2014, Stolper et al, 2013], and spectroscopic. The geoid to topographic ratios calculated from orbital data question the basaltic nature of the crust, suggesting buried felsic crustal components beneath the mafic surface in the southern hemisphere, [Baratoux et al, 2014, Sautter et al, 2016, Wieczorek and Zuber, 2004]. Such differentiated magmatism in a terrestrial planet such as Mars without plate tectonics (i.e. in a stagnant lid system) will be discussed
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