The oldest rocks on Earth:: time constraints and geological controversies
Balz S. Kamber, Stephen Moorbath, Martin J. Whitehouse, 2001. "The oldest rocks on Earth:: time constraints and geological controversies", The Age of the Earth: From 4004 BC to AD 2002, C. L. E. Lewis, S. J. Knell
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Ages in the range 3.6–4.0 Ga (billion years) have been reported for the oldest, continental, granitoid orthogneisses, whose magmatic precursors were probably formed by partial melting or differentiation from a mafic, mantle-derived source. The geological interpretation of some of the oldest ages in this range is still strongly disputed.
The oldest known supracrustal (i.e. volcanic and sedimentary) rocks, with an age of 3.7–3.8 Ga, occur in West Greenland. They were deposited in water, and several of the sediments contain 13C-depleted graphite microparticles, which have been claimed to be biogenic.
Ancient sediments (c. 3 Ga) in western Australia contain much older detrital zircons with dates ranging up to 4.4 Ga. The nature and origin of their source is highly debatable. Some ancient (magmatic) orthogneisses (c. 3.65–3.75 Ga) contain inherited zircons with dates up to c. 4.0 Ga. To clarify whether zircons in orthogneisses are inherited from an older source region or cogenetic with their host rock, it is desirable to combine imaging studies and U-Pb dating of single zircon grains with independent dating of the host rock by other methods, including Sm-Nd, Lu-Hf and Pb/Pb.
Initial Nd, Hf and Pb isotopic ratios of ancient orthogneisses are essential parameters for investigating the degree of heterogeneity of early Archaean mantle. The simplest interpretation of existing isotopic data is for a slightly depleted, close-to-chondritic, essentially homogeneous early Archaean mantle; this does not favour the existence of a sizeable, permanent continental crust in the early Archaean.
By analogy with the moon, massive bolide impacts probably terminated on Earth by c. 3.8–3.9 Ga, although no evidence for them has yet been found. By c. 3.65 Ga production of continental crust was well underway, and global tectonic and petrogenetic regimes increasingly resembled those of later epochs.
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The Age of the Earth: From 4004 BC to AD 2002
The age of the Earth has long been a subject of great interest to scientists from many disciplines, particularly geologists, biologists, physicists and astronomers. This volume, The Age of the Earth: from 4004 BC to AD 2002, brings together contributors from these different subjects, along with historians, to produce a comprehensive review of how the Earth’s age has been perceived since ancient times. Touching on the works of eminent scholars from the seventeenth to nineteenth centuries, it describes how concepts of the Earth’s history changed as geology slowly separated itself from religious orthodoxy to emerge as a rigorous and self-contained science. Fossils soon became established as useful markers of relative age, while deductions made from geomorphological processes enabled the discussion of time in terms of years. By the end of the nineteenth century biologists and geologists were fiercely debating the issue with physicists who were unwilling to give them the time needed for evolution or uniformitarianism.
With the discovery of radioactivity, attempts to calculate the Earth’s age entered a new era, although these early pioneers in radiometric dating encountered many difficulties, both technical and intellectual, before the enormity of geological time was fully recognized. This effort affected both the theory and practice of geology. Geochronology was largely responsible for it maturing into a professional scientific discipline, as increasingly refined techniques measured not only the age of the rocks, but the rate of processes which now elucidate many aspects of the Earth’s evolution.
Even today the Earth’s chronology remains a contentious topic — particularly for those dating the oldest rocks — and it is implicated in debates surrounding our hominid ancestors, the origins and development of life, and the age of the universe.
The Age of the Earth: from 4004 bc to AD 2002 will be of particular interest to geologists, geochemists, and historians of science, as well as astronomers, archaeologists, biologists and the general reader with an interest in science.