MINERAL VESTIGES OF THE EARLIEST HABITABLE ENVIRONMENTS ON EARTH

The oldest vestiges of habitable environments on Earth are preserved as highly metamorphosed sedimentary rocks. These rocks of Hadean-Eoarchean age (~4.5 to 3.6 billion years old) represent the only available archive of mineral environments in which life originated and possibly contain the altered remains of the earliest microbial ecosystems. To trace possible mineralogical-geochemical evidence of early life, minerals that contain the most basic elements of life (C, H, N, O, P, and S – known hereafter as CHNOPS) may be targeted in Hadean-Eoarchean metasedimentary rocks for crystallographic, molecular, isotopic, and trace element analyses. For instance, the CHNOPS elements can originate from decomposed biological organic matter or metabolic by-products in the initial depositional environment and subsequently become incorporated and preserved in authigenic minerals in metasedimentary rocks. However the CHNOPS elements are also found in a variety of igneous rock-forming minerals and/or they can be younger contaminants transported by crustal fluids during metamorphism. These facts have led to opposing views on datasets from Eoarchean metasedimentary rocks and have created debates that are still ongoing. In this contribution, I will explore the inventory of minerals that contain some of the CHNOPS elements in Eoarchean metasedimentary rocks formed in marine environments (such as banded iron formations and metapelites) in order to outline the various geochemical lines of evidence that have been use to discuss the possibility of life at that time. Examples of new correlated micro-analytical approaches that can illuminate the debate will be presented for some of the most ancient mineral environments on Earth.