¹L. Miché Aaron-Hennig, ²Kim Seelos
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2025.116835]
¹Johns Hopkins University, 3400 N Charles St, Baltimore, MD 21218, USA
²Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, USA
Copyright Elsevier

Understanding the distribution and provenance of hydrated minerals within Noachian terrains is essential to deciphering Mars’ crustal formation and alteration history. The phyllosilicate and carbonate minerals typically found within Noachian geologic units, for instance, have been attributed to a warmer, wetter climate that preceded a transition to the sulfate-dominated, colder, drier, and more acidic conditions in the Hesperian and Amazonian. However, these broad associations may not hold true locally. In Tyrrhena Terra, the heart of the Noachian-aged cratered highlands, three isolated craters host an unusual occurrence of hydrated sulfates alongside a variety of other alteration minerals more typically associated with the Noachian era. This paper investigates the presence of these outcrops in order to understand their origin, relationship to these co-located minerals, and implications for aqueous history and crustal evolution of Mars. Using Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data along with other contextual remote sensing data, we present a mineralogical mapping and spectral analysis of primary and secondary minerals at each location where sulfates are observed. Based on our characterization, we have constrained the formation of the sulfates to be associated with epithermal alteration or sulfide oxidation rather than impact or mechanically induced alteration. This suggests a complex sequence of aqueous alteration, potentially involving one or more steps, which we intend to explore further in future studies. The discovery of these sulfate minerals within predominantly phyllosilicate and carbonate territories challenges the conventional timeline of Mars’ climate evolution, hinting that transitions between climatic epochs may have overlapped or been more regionally varied than previously thought.

^1,2Varsha M. Nair, ¹Amit Basu Sarbadhikari, ³G.N.S. Sree Bhuvan, ⁴T. Vijaya Kumar, ^5,6Nilanjana Sorcar, ⁶Sneha Mukherjee, ⁴E.V.S.S.K. Babu, ¹Jyotiranjan S. Ray
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.09.043]
¹Physical Research Laboratory, Ahmedabad 380009, India
²Indian Institute of Technology Gandhinagar, Gujarat 382355, India
³Department of Earth Sciences, Pondicherry University, Puducherry 605014, India
⁴CSIR-National Geophysical Research Institute, Hyderabad 500007 India
⁵National Centre for Earth Science Studies, Akkulam, Thiruvananthapuram 695011, India
⁶Korea Polar Research Institute, Incheon 21990, the Republic of Korea
Copyright Elsevier

Martian meteorite Northwest Africa (NWA) 1950 is a poikilitic shergottite whose whole-rock chemical and isotopic composition indicates its origin from an intermediate mantle source. In this study, we report the results of a detailed petrographic, in-situ trace element, and whole-rock Sr-Nd isotopic investigation carried out on NWA 1950 to understand the cause of the apparent absence of the depleted poikilitic shergottites among the Martian meteorites. The sample exhibits distinct poikilitic and non-poikilitic textural domains. The large pyroxene oikocrysts (up to ∼2 mm) enclose early-formed olivine and chromite inclusions (chadacrysts) in the poikilitic domain. The non-poikilitic domain comprises olivine, pyroxene, maskelynite, merrillite, and other late-stage minerals. Pyroxenes exhibit a highly LREE-depleted pattern, and maskelynite shows no evidence of LREE enrichment. Merrillite is characterized by low REE abundance and a slight negative Eu anomaly, resembling those of depleted olivine-phyric shergottites like Tissint. Olivine grains contain abundant melt inclusions in the poikilitic and non-poikilitic domains. Oxygen fugacity values in the poikilitic and non-poikilitic domains are QFM-2.8 and QFM-2.5, respectively. The measured 87Sr/86Sr and 143Nd/144Nd ratios for NWA 1950 whole-rock are 0.710432 ± 0.000027 and 0.513201 ± 0.000011, respectively. The REE pattern of melt inclusions is depleted, resembling the depleted shergottites, while the whole-rock REE and Sr-Nd isotopic compositions are of intermediate class. This characteristic of the NWA 1950 melt inclusions enables us to establish a genetic linkage between the intermediate and depleted shergottites and to find out the missing link for the depleted poikilitic shergottites.
The parent magma of NWA 1950 is more magnesian and less aluminous than that of the enriched poikilitic shergottites in the same manner that depleted olivine-phyric shergottites have more Mg and less Al than their enriched counterparts. Additionally, the modal proportion of pyroxene to plagioclase and REE abundance in merrillite of NWA 1950 closely matches the depleted olivine-phyric shergottites. The observed enrichment in trace elements and the Sr-Nd isotopic compositions cannot be explained by a depleted mantle source, as inferred from the REE patterns of the constituent minerals in NWA 1950. We propose such enrichment to have originated due to the heterogeneity in the shergottite mantle, aided by the entrapped melt pockets within the upper mantle. A mixture of 0.3–1.0 % trapped liquid in the Martian upper mantle with the parent melt can produce the high REE abundance and Sr-Nd isotopic composition of NWA 1950. Longer residence time in the magma chamber and slower cooling rate of the poikilitic shergottites than the extrusive shergottites could have aided the enrichment in the source region. Additionally, this study suggests that care must be taken in classifying the chemical and isotopic characteristics of the poikilitic shergottites in future studies.