Origin of life from apatite dating
Such organisms could have survived thermal assaults from giant impacts, especially if sequestered deep in the oceans or in rocks away from a destructive surface zone bathed both in the intense ultraviolet radiation of the early Sun and a rain of extraterrestrial debris ~4 b.y. To better understand this early era, we have to unravel the timing of events from a heavily modified early Archean rock record. M., 1999, Geochronology and thermochronology by the Ar method, 2nd ed.: New York, Oxford University Press, 269 p. R., 2000, Age significance of U-Th-Pb zircon data from early Archaean rocks of West Greenland A reassessment based on combined ion-microprobe and imaging studies Comment: Chemical Geology (in press). R., and Mason, B., 1977, Petrogenesis and geochemistry of metabasaltic and metasedimentary enclaves in the Amîtsoq gneisses, West Greenland: American Mineralogist, v. EARLY ARCHEAN (3500 Ma) HISTORY OF WEST GREENLAND The diverse rock types present in the Isua district of West Greenland are all contained within extensive early Archean (36003900 Ma) gneisses dominantly of tonalitic-granodioritic composition (Black et al., 1971; Nutman et al., 1996) (Fig. This multiply metamorphosed terrane, termed the Itsaq Gneiss Complex (Nutman et al., 1996), contains 3850 Ma on Akilia Island The oldest known sediment (Nutman et al., 1997), and the oldest known rock with evidence of biological processes active during time of deposition (Mojzsis et al., 1996), is a layer ~3 m thick of BIF within a body of amphibolite on the southern tip of Akilia island, West Greenland (Fig. This BIF on Akilia was chosen as the type locality for the Akilia association, a term used for volcano-sedimentary enclaves found throughout the Itsaq Gneiss Complex that are not part of the larger and better preserved Isua supracrustal belt (Mc Gregor and Mason, 1977). Kamber and Moorbath (1998) argued that the lack of 3850 Ma Pb-Pb ages in feldspars from gneisses collected throughout southern West Greenland specifically preclude a 3850 Ma protolith age for any rocks in West Greenland. W., 1995, Carbon isotope thermometry in marbles of the Adirondack Mountains, New York: Journal of Metamorphic Geology, v.
On the basis of the age of these rocks, the emergence of the biosphere appears to overlap with a period of intense global bombardment.
More detailed field studies through the 1980s and 1990s, combined with geochronological work on the intruding gneisses in the coastal regions of southern West Greenland, demonstrated that the rocks around Godthåbsfjord (in the southwestern coastal area), including those of the Akilia association, contain older components (Kinny, 1986) than most rocks in the Isua district, 150 km distant.
On Akilia island itself, gneissic sheets crosscut the amphibolite enclave containing BIF and yield U-Pb zircon ages as old as ca. S., 1999, Rapid equilibration of CO and formate under hydrothermal conditions (with a comment on the abiotic synthesis of hydrocarbons during serpentinization): Geological Society of America Abstracts with Programs, v.
This age for the Akilia island sediments is also significant because it would place their deposition simultaneous with the Late Heavy Bombardment of the Moon at 38003900 Ma, and coeval with the presence of abundant liquid water on the surface of Mars. J., 1988, Impact frustration and the origin of life: Nature, v.
Several of the Akilia gneisses that cut the BIF generally contain three zircon age populations: ca. 5) (Mojzsis and Harrison, 1999; Whitehouse et al., 1999).
The granitoid protolith of the orthogneisses is characterized by relatively low Zr contents (~120 ppm) and high crystallization temperatures (900 °C).