GEOCHEMISTRY OF MULTIPLE CARBONATE UNITS FROM THE ~3.7 GA ISUA GREENSTONE BELT, SW GREENLAND

Carbonate bodies (ferroan dolomite, siderite, calcite) occur in multiple locations across the ~3.7 Ga Isua Greenstone Belt (IGB), SW Greenland. Early research suggested a sedimentary origin, akin to shallow-water quartz-carbonate shelf environment, potentially inhabited by microbial communities that left a record of isotopically light carbon in some samples. Subsequent work identified the connection of carbonate bodies forming as the product of metasomatism. We have collected and analyzed samples from a variety of geologic positions (concordant, discordant), and in contact with different rock types (inferred protoliths in parentheses), including, quartz-magnetite-amphibole schist (BIF), quartzite, (chert), anythophyllite schist (ultramafic igneous), actinolite-tremolite schist (mafic igneous), and dark carbonated schist (tonalite). There are three likely origins for the carbonate bodies: (1) chemical sediments precipitated from sea water, or (2) metasomatic carbonates derived from hydrothermal interactions with host rocks, or (3) remobilization of primary, or early formed carbonate into secondary metasomatic carbonates. The dark carbonated schist, informally called the “boulder bed,” presents like a cobble-boulder conglomerate with “clasts” of tonalite set in a dark, carbonate-rich matrix. Both REE and trace-element patterns of “clasts” (actually severed fold hinges) and carbonated matrix are similar to tonalite, suggesting that carbonate addition occurred metasomatically. Anthophyllite-hosted carbonate occurs as disconnected cm-scale lenses and disseminations within the schist and typify metasomatic carbonate. In shale-normalized REE+Y plots, these samples have distinct positive Eu and Y anomalies, and small but positive La anomalies, like chemical sediments derived from sea water. Carbonates within all other hosts also have shale-normalized, positive, La, Eu, and Y anomalies. However, most trace elements show considerable deviation relative to Isua BIF. Rather than supporting a sedimentary precipitate origin, these results suggest REE+Y patterns may be mobilized from rocks with pre-existing sea-water patterns (e.g., BIF) or derived from interactions with sea-water during alteration as has been documented elsewhere.