Petrogenesis of the olivine cumulate outcrop Issole – The missing link between the Séítah and Máaz formations in Jezero crater, Mars

Two lithologic units have been mapped and studied on the floor of Jezero crater, Mars: Séítah, which consists of layered olivine-rich cumulates, and Máaz, a series of basaltic to trachyandesitic lava flows. While Séítah and Máaz are close in proximity and stratigraphy, their potential geologic and petrological relationship remains unclear. Here, we present observations from the Planetary Instrument for X-ray Lithochemistry (PIXL) of an olivine cumulate outcrop – Issole – within the Séítah formation. The rock analyzed at Issole is a wehrlite dominated by olivine (Fo_46±1) and interstitial phases, including augite, late olivine, spinel, and feldspathic material. Compared to other outcrops from Séítah, Issole is more iron-rich and records substantial alteration processes. We combine mineral chemistry, textural analysis, and thermodynamic modeling to show that Séítah olivines crystallized from a basaltic parent magma, compositionally similar to the most primitive basalts in the Máaz formation. Crystallization of this parent magma produces residual melts that follow the magmatic differentiation trend defined by Máaz basalts. Moreover, the mineral assemblages predicted by our model during crystallization are consistent with observations from Séítah rocks but show some differences in composition. These differences can be reconciled by considering post-cumulus processes that modified the mineral assemblage in Séítah, including FeMg exchange between olivine and pyroxene. Our results indicate that Séítah and Máaz are likely genetically related, and their formation involved both accumulation of crystals at depth and eruption of lavas. The emplacement of Séítah likely occurred as a near-surface, sill-like igneous intrusion into previously erupted Máaz lava flows. The relationship between Séítah and Máaz demonstrates that magmatic differentiation processes, similar to those responsible for the formation of some Martian meteorites, can produce highly diverse lithologies and mineral textures in the Martian crust.