Petrology and Geochemistry of Poikilitic Shergottite Northwest Africa 1950: bridging the gap between depleted and intermediate sources in Martian Mantle

· by · in ,

Varsha M. Nair, Amit Basu Sarbadhikari, G.N.S. Sree Bhuvan, T. Vijaya Kumar, Nilanjana Sorcar, Sneha Mukherjee, E.V.S.S.K. Babu, Jyotiranjan S. Ray

Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 9 October 2025

LINK

“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.”

Related posts (automatically generated):

  1. Heavy halogen geochemistry of martian shergottite meteorites and implications for the halogen composition of the depleted shergottite mantle source
  2. Petrology and shock history of the first depleted-like poikilitic shergottite Asuka 12325OPEN ACCESS 
  3. Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars
  4. Heterogeneous martian mantle: Evidence from petrology, mineral chemistry, and in situ U-Pb chronology of the basaltic shergottite Northwest Africa 8653
  5. The oxygen fugacity of intermediate shergottite NWA 11043: implications for Martian mantle evolution
  6. Petrology and Mineralogy of the Martian Olivine Gabbroic Shergottite Northwest Africa 13581: Insights into the Enriched Martian Mantle Source
  7. Uniform oxygen fugacity of shergottite mantle sources and an oxidized martian lithosphere
  8. Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior
  9. Detailed mineralogy and petrology of highly shocked poikilitic shergottite Northwest Africa 6342
  10. Heterogeneous distribution of H2O in the Martian interior: Implications for the abundance of H2O in depleted and enriched mantle sources.
Tags: , , , ,