Mineralogy, Morphology, and Emplacement History of the Maaz Formation on the Jezero Crater Floor From Orbital and Rover Observations

Funder: CNESFunder: CNRSFunder: IRIS OCAVFunder: NASA Return Sample Science Participating Scientist ProgramAbstractThe first samples collected by the Perseverance rover on the Mars 2020 mission were from the Maaz formation, a lava plain that covers most of the floor of Jezero crater. Laboratory analysis of these samples back on Earth would provide important constraints on the petrologic history, aqueous processes, and timing of key events in Jezero crater. However, interpreting these samples requires a detailed understanding of the emplacement and modification history of the Maaz formation. Here we synthesize rover and orbital remote sensing data to link outcrop‐scale interpretations to the broader history of the crater, including Mastcam‐Z mosaics and multispectral images, SuperCam chemistry and reflectance point spectra, Radar Imager for Mars' subsurface eXperiment ground penetrating radar, and orbital hyperspectral reflectance and high‐resolution images. We show that the Maaz formation is composed of a series of distinct members corresponding to basaltic to basaltic‐andesite lava flows. The members exhibit variable spectral signatures dominated by high‐Ca pyroxene, Fe‐bearing feldspar, and hematite, which can be tied directly to igneous grains and altered matrix in abrasion patches. Spectral variations correlate with morphological variations, from recessive layers that produce a regolith lag in lower Maaz, to weathered polygonally fractured paleosurfaces and crater‐retaining massive blocky hummocks in upper Maaz. The Maaz members were likely separated by one or more extended periods of time, and were subjected to variable erosion, burial, exhumation, weathering, and tectonic modification. The two unique samples from the Maaz formation are representative of this diversity, and together will provide an important geochronological framework for the history of Jezero crater.</jats:p

Samples collected from the floor of Jezero crater with the Mars 2020 Perseverance rover

The first samples collected by the Mars 2020 mission represent units exposed on the Jezero Crater floor, from the potentially oldest Séítah formation outcrops to the potentially youngest rocks of the heavily cratered Máaz formation. Surface investigations reveal landscape-to-microscopic textural, mineralogical, and geochemical evidence for igneous lithologies, some possibly emplaced as lava flows. The samples contain major rock-forming minerals such as pyroxene, olivine, and feldspar, accessory minerals including oxides and phosphates, and evidence for various degrees of aqueous activity in the form of water-soluble salt, carbonate, sulfate, iron oxide, and iron silicate minerals. Following sample return, the compositions and ages of these variably altered igneous rocks are expected to reveal the geophysical and geochemical nature of the planet’s interior at the time of emplacement, characterize martian magmatism, and place timing constraints on geologic processes, both in Jezero Crater and more widely on Mars. Petrographic observations and geochemical analyses, coupled with geochronology of secondary minerals, can also reveal the timing of aqueous activity as well as constrain the chemical and physical conditions of the environments in which these minerals precipitated, and the nature and composition of organic compounds preserved in association with these phases. Returned samples from these units will help constrain the crater chronology of Mars and the global evolution of the planet’s interior, for understanding the processes that formed Jezero Crater floor units, and for constraining the style and duration of aqueous activity in Jezero Crater, past habitability, and cycling of organic elements in Jezero Crater

Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars.

The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater's sedimentary delta, finding that the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Séítah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Magnesium-iron carbonates along grain boundaries indicate reactions with carbon dioxide-rich water under water-poor conditions. Overlying Séítah is a unit informally named Máaz, which we interpret as lava flows or the chemical complement to Séítah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks have been stored aboard Perseverance for potential return to Earth

Mantle metasomatism

Mantle metasomatism is a relatively recent concept introduced in the early 1970s when detailed studies of lithospheric mantle rock fragments (xenoliths), brought to the surface of in basaltic to kimberlitic magmas, became widespread. Two main types of metasomatism were defined: modal (or patent) metasomatism describes the introduction of new minerals; cryptic metasomatism describes changes in composition of pre-existing minerals without formation of new phases. A new type of metasomatism is introduced here, stealth metasomatism; this process involves the addition of new phases (e.g. garnet and/or clinopyroxene), but is a “deceptive” metasomatic process that adds phases indistinguishable mineralogically from common mantle peridotite phases. The recognition of stealth metasomatism reflects the increasing awareness of the importance of refertilisation by metasomatic fluid fronts in determining the composition of mantle domains. Tectonically exposed peridotite massifs provide an opportunity to study spatial relationships of metasomatic processes on a metre to kilometre scale. The nature of mantle fluids can be determined from the nature of fluid inclusions in mantle minerals and indirectly from changes in the chemical (especially trace-element) compositions of mantle minerals. Metasomatic fluids in off-craton regions cover a vast spectrum from silicate to carbonate magmas containing varying types and abundances of dissolved fluids and solutes including brines, C-O-H species and sulfur-bearing components. Fluid inclusions in diamond and deep xenoliths reveal the presence of high-density fluids with carbonatitic and hydro-silicic and/or saline-brine end-members. The deep cratonic xenolith data also reinforce the importance of highly mobile melts spanning the kimberlite-carbonatite spectrum and that may become immiscible with changing conditions. A critical conceptual advance in understanding Earth’s geodynamic behaviour is emerging from understanding the linkage between mantle metasomatism and the physical properties of mantle domains recorded by geophysical data. For example, metasomatic refertilisation of cratonic lithospheric mantle increases its density, lowers its seismic velocity and strongly affects its rheology. Introduction of heat-producing elements (U, Th, K) increases heat production, and the key to understanding electromagnetic signals from mantle domains may be closely related to fluid distribution and type (e.g. carbonatitic) and its residence in or between grains. The lithospheric mantle is a palimpsest recording the multiple fluid events that have affected each domain since it formed. These events, involving different fluids and compositions, have repeatedly overprinted variably depleted original mantle wall-rocks. This produces a complex, essentially ubiquitously metasomatised lithospheric mantle, heterogeneous on scales of microns to terranes and perhaps leaving little or no “primary” mantle wall-rock. Decoding this complex record by identifying significant episodes and processes is a key to reconstructing lithosphere evolution and the nature and origin of the volatile flux from the deep Earth through time.63 page(s