Biogenic Carbon in Pink Corundum from Southern West Greenland
Some of the oldest ruby is found in the southern part of West Greenland. Although the most famous locality is the Aappaluttoq deposit—where corundum is hosted by igneous rocks (N. Keulen et al., “Formation, origin and geographic typing of corundum (ruby and pink sapphire) from the Fiskenæsset complex, Greenland,” Lithos, Vol. 366–367, 2020, article no. 105536)—several smaller occurrences are found throughout the region. A number of these smaller occurrences are hosted by metamorphosed sedimentary rocks that were tectonically interleaved with ultramafic rocks (figure 1). Fluid-assisted movement of silica from the metasedimentary rocks to the ultramafic rocks and reverse movement of minor amounts of chromium during metamorphism have been proposed to cause the chemical conditions necessary for ruby growth.
Pink corundum from these smaller occurrences contains various types of mineral inclusions, most notably rutile and graphite (figure 2). Two recent studies investigated these rocks with the goals of dating the absolute timing of corundum growth, understanding the corundum formation mechanisms, and determining the origin of graphite (V. van Hinsberg et al., “The corundum conundrum: Constraining the compositions of fluids involved in ruby formation in metamorphic melanges of ultramafic and aluminous rocks,” Chemical Geology, Vol. 571, 2021, article no. 120180; C. Yakymchuk et al., “Corundum (ruby) growth during the final assembly of the Archean North Atlantic Craton, southern West Greenland,” Ore Geology Reviews, Vol. 138, 2021, article no. 104417). Samples were collected from an occurrence near the town of Maniitsoq and from a second occurrence on Storø Island to the northeast of Greenland’s capital, Nuuk.
Rutile inclusions are about 2.5 billion years old at both localities, based on uranium–lead geochronology. However, the pink corundum may be slightly older than this, given that the isotopic clock in rutile may only start sometime on the post-growth cooling history. Nevertheless, this constrains the timing of corundum growth during the final stabilization of the continental crust in the region.
This research also analyzed the carbon isotope composition of graphite to determine its origin. In general, biogenic activity modifies carbon isotopes; living organisms preferentially incorporate the lighter isotope (12C) compared with the heavier isotope (13C). Graphite from the Storø Island location was very enriched in the lighter isotope and was interpreted as a bona fide organic signature, which suggests that the graphitic carbon was originally part of a living organism. Graphite from the Maniitsoq pink corundum locality was less enriched in the lighter carbon isotope, but the isotope signature was still most consistent with a biogenic origin. Although the presence of carbon of biogenic origin is not surprising given the sedimentary nature of the host rocks, the research team modeled the chemical conditions necessary for corundum growth and suggested that the presence of graphite created the conditions in the rocks that facilitated corundum formation at the imposed depths and temperatures of metamorphism.
Although the recognition of biogenic carbon in the form of graphite in pink corundum occurrences is based on a limited number of samples, future work should be able to determine how widespread this association is in southern West Greenland. In addition, in situ analysis of carbon in graphite inclusions using secondary ion mass spectrometry would help further test the hypothesis of a biogenic origin of graphite in 2.5 billion-year-old pink corundum.
