New insights into the origin of the Eoarchean Narssaq ultramafic complex (southern West Greenland) from trace element mineral chemistry

The Archean mantle is an invaluable source of information about the geodynamic processes active during the early Earth. However, our knowledge on Archean mantle compositions is still fragmentary due to its poor exposure on the present-day Earth. The Eoarchean Narssaq ultramafic complex within the Itsaq Gneiss Complex of southern West Greenland is one of the most extensively studied Eoarchean ultramafic sections. According to van de Locht et al. (2020), the ultramafic complex represents a remnant of the Archean mantle. Conversely, Zhang and Zilas (2024) proposed a cumulate origin for the same lithologies. Such contrasting interpretations have significant implications for the inferred geodynamic scenario active during the Eoarchean.

We carried out a new field survey on the Narssaq ultramafic complex collecting representative samples of dunites and hornblende peridotites. All samples include olivine, amphibole and phlogopite, as major rock-forming minerals. Accessory orthopyroxene, chromite and magnetite were in places also found. Mineral phases were characterized for major and trace elements concentration by electron microprobe and laser ablation ICP-MS, respectively. On selected amphiboles, images on trace element distribution were also carried out by LA-ICP-TOF-MS.

Preliminary data reveal that olivine is Fo_90-91 and is characterised by exceptionally low contents of incompatible trace elements and high contents of Ni (>3500 ppm) and fluid mobile elements, such as B and Li (up to 4 and 6 ppm, respectively). Amphibole is mostly tremolitic in composition, although, amphibole grains in equilibrium with olivine and phlogopite locally preserve core ghost domains consisting of Mg-hornblende. These domains are characterized by extremely high Cr contents (>8500 ppm), nearly flat chondrite-normalized REE pattern at about 10 times CI chondrite with a marked negative Eu anomaly, and relatively high contents of both crustal (e.g., Pb, Th, U) and fluid mobile elements (e.g., Li, Be and B). Phlogopite is exceptionally Cr- and Ni-rich and displays low contents in incompatible trace elements, including those for which the affinity is higher, such as Cs, Rb and Ba; Li and B are up to 6 ppm.

The typically null concentrations of fluid immobile incompatible trace elements in olivine and the constantly high and similar contents of fluid mobile elements in all mineral phases indicates that the entire ultramafic system underwent re-equilibration and re-crystallisation during the metamorphic evolution of the Narssaq ultramafic complex. The mantle or cumulate origin of the complex is thus difficult to be assessed. Notwithstanding, we propose that amphibole crystallisation was related to a melt with a significant crustal component that interacted with the pre-existing ultramafic system. Further trace element and stable isotope micro-analyses are in progress, to verify if amphibole crystallisation was related to the emplacement or partial melting of the host Itsaq gneisses.

van De Locht J., Hoffmann J.E., Rosing M.T., Sprung P., Munker C. Preservation of Eoarchean mantle processes in 3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta 280 (2020) 1–25.

Zhang L. & Zilas K. Eoarchean ultramafic rocks represent crustal cumulates: A case study of the Narssaq ultramafic body, southern West Greenland. Earth Planet. Sci. Lett. 625 (2024) 118508