The Early Earth

P.G. Eriksson    Department of Geology, University of Pretoria, Pretoria 0002, Republic of South Africa

W. Altermann    Centre Biophysique Moléculaire (CBM), 45071 Orléans, Cedex 2, France
Centre National de la Recherche Scientifique (CNRS), 45071 Orléans, Cedex 2, France

D.R. Nelson    Department of Applied Physics, Curtin University of Technology Perth, W.A. 6845, Australia
Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, 6004, Australia

W.U. Mueller    Department Sciences de la Terre, Université du Québec à Chicoutimi, Chicoutimi, Québec G7H 2B1, Canada

O. Catuneanu    Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada

1.1 INTRODUCTION

D.R. NELSON

Inferences about the pre-4.0 Ga geological history of the Earth have been based traditionally either on the study of the oldest identified remnants on the Earth’s surface (e.g., Maas et al., 1992;Nutman et al., 1996; Amelin et al., 1999; Nelson et al., 2000; Ryder et al., 2000; Wilde et al., 2001; Mojzsis et al., 2001), or on modelling of the differentiation of global chemical reservoirs (e.g., Arndt and Chauvel, 1990; Bennett et al., 1993; Bowring and Housh, 1995; Kramers and Tolstikhin, 1997; Snow and Schmidt, 1998; Albarède et al., 2000; Canfield et al., 2000; Nutman et al., 2001). A major limitation of these approaches arises from the limited tangible evidence available for study of early Earth—the preserved rock record commences at 4030 Ma (Stern and Bleeker, 1998; Bowring and Williams, 1999), more than 500 My after the Earth’s formation. As a consequence, these approaches have so far provided only broad constraints on the mechanisms and time scales of accretion and early differentiation of the Earth, and of physicochemical conditions on the Earth’s surface during this time. In section 1.2 of this chapter, a new approach to the study of the early Earth, based on detailed chemical and isotopic studies of meteorites in combination with advances in our understanding of nucleosynthesis, has been investigated.

In 1960, the remarkable discovery by J.H. Reynolds of the isotope xenon-129 (129Xe) within the earliest-forming phase of a primitive meteorite (Reynolds, 1960; see also Jeffery and Reynolds, 1961) was eventually to lead to a breakthrough in our understanding of the timing of accretion and differentiation of the Earth. The 129Xe detected by Reynolds had accumulated in situ from the radiogenic decay of the long-extinct nuclide iodine-129 (129I), which has a half-life of only c. 16 My. The daughter products of a number of other extinct nuclides have since been identified within primitive meteorites, and it is now generally accepted that their short-lived radioactive parent nuclides were synthesised during supernova explosions in the vicinity and shortly before the formation of our solar system. These catastrophic nucleosynthesis events mark the time at which the radioactive isotopes that are widely used for geochronology were formed. As they are now long extinct, short-lived nuclides cannot be used directly to obtain absolute dates relative to the present-day, but their short half-lives have been used to constrain precisely the relative chronologies of planetary formation milestones for the early solar system (see Fig. 1.1-1).

The Earth and other terrestrial planets formed by the collision and amalgamation of smaller rocky planetesimals within the early solar system’s protoplanetary disk. During the later stages of this accretion process, progressively larger planetary embryos were formed and collided. These violent collisions resulted in the episodic reforming of the growing proto-Earth, along with the destruction of much of the evidence of the extent of earlier differentiation. The Earth’s Moon also probably formed as a result of such a catastrophic collision during the later stages of Earth accretion. As the planetary embryos grew, the impact rate decreased and the chances of survival of these early-formed fragments of the Earth’s surface increased. In section 1.3 of this chapter, Simonson et al. argue that terrestrial impact structures predating the Proterozoic era (> 2.5 Ga) are unlikely to have survived, due to the fragmentary state of preservation of the Earth’s rock record from this time. Fortunately, evidence of such early impact events may be preserved in the Earth’s stratigraphic archive, as thin layers rich in distinctive sand-sized spherules. In section 1.4, Abbott and Hagstrum estimate that in the time interval between 3.8 and 2.5 Ga, there were more than 350 impact events large enough to produce an impact layer of global extent. It has also been proposed (section 1.4) that major magmatic and (by implication) crust-formation events during the Archaean could have been related to major impact episodes. Although it is widely acknowledged that major impacts must have played an important role in the formation of the Earth’s early continental crust, this “extraterrestrial” influence has largely been overlooked in most previous studies of the Earth’s Archaean terranes. Recognition and detailed investigation of impact-related sedimentary rocks preserved in the Earth’s stratigraphic record currently is still in its infancy, but the way ahead is clearer from studies such as those documented in sections 1.3 and 1.4 of this chapter.

1.2 EARTH’S FORMATION AND FIRST BILLION YEARS

D.R. NELSON

Introduction

In this section, a new approach to the study of the early Earth, commencing before the time of formation of our solar system at 4571 Ma and working forward in time towards 3500 Ma, has been investigated. This approach explores recent insights into the processes active during formation of the early Earth arising from detailed chemical and isotopic studies of meteorites, combined with advances in our understanding of nucleosynthesis.

Many meteorites are fragments of asteroids formed early in the evolutionary history of the solar system, that were too small to have undergone much internal heating (see Hutchison et al., 2001). Some contain refractory calcium- and aluminium-rich inclusions that condensed from the nebula when temperatures were so high that other elements were volatile, shortly after formation of the Sun and during dissipation of the nebula. Others represent disrupted fragments of planetesimals and differentiated planetary bodies, including the Moon and Mars, formed later in the accretion history of the solar system. Some meteorite classes are samples of the interiors of disrupted planetary bodies, and have formed prior to, during and after active differentiation of these bodies. They may therefore provide unique information about the processes operating during the early differentiation of the Earth into silicate crust and mantle, and metallic core. The identification of short-lived (with half-lives less than 100 My) radioactive, now extinct, nuclides within some classes of meteorites has imposed important new constraints on the early evolution of the solar system and on accretion and differentiation rates for planetary bodies such as the Earth. Short-lived nuclides potentially offer the means to precisely constrain early solar system chronology, and of planetary accretion and differentiation processes, in relation to the time of nucleosynthesis.

To fully appreciate the insights offered by extinct nuclides into the chronology of the early solar system and formation and differentiation its planets including the Earth, an understanding of the processes involved in the synthesis of the elements prior to the formation of our solar system is required. Details of nucleosynthesis within stars were formulated by the pioneering work of E.M. Burbidge, G.R. Burbidge, Fowler, Hoyle and co-workers (Burbidge et al., 1957) and independently, by Cameron (1957). With the exception of the element hydrogen (H) and possibly some of the helium (He), lithium (Li), beryllium (Be) and boron (B) which may have been synthesised during the Big Bang or by spallation reactions, elements lighter than iron (Fe) now present in our solar system were created primarily by fusion reactions within the interiors of stars. Elements heavier than Fe were mostly synthesised by two major neutron-capture processes; the “slow” or s-process, which refers to the slow capture, relative to the rate of β-decay, of neutrons within stars, and by the “rapid” or r-process, mostly in catastrophic supernovae events during which unstable intermediate isotopes form by the capture of neutrons in a neutron-dense environment and so rapidly that they do not have time to decay. (Some less abundant neutron-deficient, proton-rich nuclei were synthesised by a third process, the “proton” or p-process.) In this section, the...