RECOGNITION OF GREATER-THAN-OR-EQUAL-TO-3850 MA WATER-LAIN SEDIMENTS IN WEST GREENLAND AND THEIR SIGNIFICANCE FOR THE EARLY ARCHEAN EARTH

A layered body of amphibolite, banded iron formation (BIF), and ultram afic rocks from the island of Akilia, southern West Greenland, is cut by a quartz-dioritic sheet from which SHRIMP zircon (206)pb/(207)pb we ighted mean ages of 3865 +/- 11 Ma and 3840 +/- 8 Ma (2 sigma) can be calculated by different approaches. Three other methods of assessing t he zircon data yield ages of >3830 Ma. The BIFs are interpreted as wat er-lain sediments, which with a minimum age of similar to 3850 Ma, are the oldest sediments yet documented. These rocks provide proof that b y similar to 3850 Ma(1) there was a hydrosphere, supporting the chemic al sedimentation of BIF, and that not all water was stored in hydrous minerals, and (2) that conditions satisfying the stability of liquid w ater imply surface temperatures were similar to present. Carbon isotop e data of graphitic microdomains in apatite from the Akilia island BIF are consistent with a bio-organic origin (Mojzsis et al. 1996), exten ding the record of life on Earth to >3850 Ma. Life and surface water b y similar to 3850 Ma provide constraints on either the energetics or t ermination of the late meteoritic bombardment event (suggested from th e lunar cratering record) on Earth. Copyright (C) 1997 Elsevier Scienc e Ltd.