2004 Annual Science Report
University of California, Los Angeles Reporting | JUL 2003 – JUN 2004
Dating Earliest Life From Akilia Island, Greenland
New mapping, geochronology, and geochemistry of supracrustal rocks on Akilia Island, Greenland, support a >3800 Ma age and chemical sedimentary origin for controversial Fe-rich quartz-pyroxene rocks that have been reported to contain evidence for early life. Supracrustal lithologies comprise laterally continuous, mappable units of mafic amphibolite, ultramafics, and two Fe-rich quartz-pyroxene units. Map relations support the interpretation that quartz-pyroxene rocks were part of a predeformational volcano-sedimentary stratigraphy, but are inconsistent with alternative hypothesis of an origin as metasomatic veins. U-Pb ion-microprobe measurements of zircons indicate a minimum formation age of 3600 Ma for the quartz-pyroxene rock and a possible metasedimentary block in amphibolite gneisses. A >3800 Ma depositional age of Akilia supracrustals has previously been inferred based on controversial crosscutting relations involving orthogneiss in the high-strain portion of the supracrustals. We identified two new crosscutting metatonalites. Ion-microprobe measurements of zircon cores yielded 3746±11 Ma and 3835±31 Ma ages and Th/U zircon consistent with igneous growth in the bulk composition from which they were extracted. Low bulk Zr minimizes the possibility of zircon inheritance. Ion-microprobe depth-profiling study of a single zircon from the older orthogneiss yields an igneous core age of 3825±8 Ma, and shows an episode of zircon growth at 3731±10 Ma, which may have occurred during contact metamorphism at the time of emplacement of the second tonalitic sheet. These results support a >3825 Ma depositional age of the Akilia supracrustals. As an alternative to a chemical sedimentary origin for Fe-rich quartz-pyroxene rocks, it has been proposed that the pyroxene-rich bands are metasomatized ultramafic rocks and that quartz-rich bands are metamorphosed quartz veins. However, we find a large contrast in whole-rock d 18 O between the quartz-pyroxene rock (12.6 ‰) and adjacent metaigneous units (7-8 ‰) that argues against significant mass transfer. Moreover, immobile trace elements, Rare Earth Element (REE) patterns, S isotopes and PGE data are consistent only with an origin as hydrothermal deposits chemically sedimented in a volcanic/hydrothermal setting with minor, locally derived ultramafic and mafic detrital components. These results validate the interpretation that the quartz-pyroxene rock of the Akilia supracrustals is >3800 Ma metasediment.