Difference between "mafic floor" and "olivine-bearing floor" in Jezero crater? (Perseverance landing site)

Question

Figure 3: Main geologic units within Jezero crater shown on a basemap constructed from images from the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter (MRO). Blue circle shows the location of the Perseverance rover landing ellipse.

From Destination Jezero Crater: Landing Site of the Mars 2020 Perseverance Rover

The image above shows the labels "mafic floor" and "olivine-bearing floor" in Jezero crater along with the Western delta and Perseverance's landing ellipse.

It looks like the Octavia E. Butler¹ landing site is somewhere between the two labels, and I'm not sure if there is a distinct border between the two or not.

Question: What is the difference between the "mafic floor" and "olivine-bearing floor" in Jezero crater? Are they exclusive or overlapping designations? Is there a borderline? In which did Perseverance land?

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¹Octavia E. Butler; landing site on Mars

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This image shows two possible routes (blue and purple) to the delta.

From CNN's Perseverance rover takes its first drive on Mars, sends back image

Answer 1

TL, DR: Olivine is a specific type of mafic or ultra-mafic mineral that is specifically identified in some parts of the overall mafic floor of the crater. The olivine is seen in specific areas of which the indicated "olivine-bearing floor" is most prominent.

Mafic: More than just olivine

In mineralogical terms, "mafic" is a more general term than "olivine", the latter being a specific type of mafic mineral. As shown in the diagram below, mafic minerals include predominantly olivine and also pyroxene, the latter having slightly more silica content and thus being slightly less "mafic".

In chemical terms olivines have the formula $\text{(Mg, Fe)}_2\text{SiO}_4$ which represents the lowest possible molar ratio of silica to metal oxide in a silicate because each silicon atom is "saturated" with extra oxygen atoms from the metal oxide component, thus breaking up the silicate network into discrete $\text{SiO}_4^{4-}$ tetrahedra. Pyroxene more closely resembles other silicate minerals in having less metal oxides relative to the silica content, and thus preserving the polymeric silicate networks.

Jezero Crater

The mineral classification of the various regions of Jezero crater is based data from visible and near-IR spectroscopy of the surface by Horgan et al. [1]. The floor of the crater contains the bulk of observed mafic matter similar to the Gale Crater discussed in this answer. The mafic matter exposed on the surface consists of a mosaic of olivine and pyroxenes (compare with the chart above), with a prominent olivine (red) region near the western edge where the olivine-bearing floor is identified.

Image from Ref. 1, Figure 6.

Reference

1. Briony H.N. Horgan, Ryan B. Anderson, Gilles-Dromart, Elena S. Amadord, Melissa S. Ricee, "The mineral diversity of Jezero crater: Evidence for possible lacustrine carbonates on Mars", Icarus 339, 15 March 2020, 113526.

Answer 2

There has been a series of publications on this topic last August, among which:

• Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars (Farley et al., 2022)
• Compositionally and density stratified igneous terrain in Jezero crater, Mars (Wiens et al., 2022)

The "olivine-bearing floor" of the figure in the question has been informally called the Séítah unit/formation (formal designation Cf-f-1) while the "mafic floor" has been informally called the Máaz unit/formation (formal designation Cf-fr).

Are they exclusive or overlapping designations?

As they are distinct geological units, their designations are exclusive.

Is there a borderline?

Yes, there is a contact between the two units, see the dashed white line of Fig. 1C in Farley et al. (2022) (not included here because of copyright). Note that it is an "interpreted distribution of the Séítah and Máaz formations", as it is hard to define the precise contact without carrying out fieldwork...

In which did Perseverance land?

According to the same figure, the landing site (marked "OEB") is clearly within the boundaries of the Máaz formation (i.e., the "mafic floor").

The Séítah formation is interpreted as a cumulate, formed by sedimentation of denser olivine crystals at the bottom of a magma body, while the origin of the Máaz formation is more debated. But the formation processes of these units is a whole other question, better suited for Earth Science SE maybe! :)