The development of lithospheric keels beneath the earliest continents: time constraints using PGE and Re–Os isotope systematics
D. G. Pearson, G. J. Irvine, R. W. Carlson, M. G. Kopylova, D. A. Ionov, 2002. "The development of lithospheric keels beneath the earliest continents: time constraints using PGE and Re–Os isotope systematics", The Early Earth: Physical, Chemical and Biological Development, C. M. R. Fowler, C. J. Ebinger, C. J. Hawkesworth
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Continued studies of xenolith suites found in kimberlites on and around the Kaapvaal Craton, together with those from newly discovered localities on other cratons, are providing new insights into the generation and evolution of the Earth’s oldest continents. Comparison of modal abundance data with melt depletion models, together with trace element and isotope systematics in Kaapvaal low-temperature peridotites, suggest that much or all of the diopside and garnet in these rocks may have formed significantly after initial melt depletion. The Re-Os isotope system has been instrumental in providing an improved understanding of the timing of the formation of cratonic lithospheric keels. New studies that focus on carefully selected whole-rock peridotites and use combined platinum group element (PGE) and Re-Os isotope analysis provide better constraints on the significance of Re-Os model ages. The large database of Re-Os isotope analyses for peridotites for the Kaapvaal Craton indicate formation of significant amounts of lithospheric mantle in Neoarchaean time, associated with voluminous mafic magmatism. Formation of lithospheric mantle in Neoarchean time (3.0–2.5 Ga) follows the cessation of major crustal differentiation events at c. 3.1 Ga and marks the onset of craton stabilization. Some lithospheric mantle was produced in Palaeo- to Mesoarchaean time (3.8–3.0 Ga) in southern Africa, which preserved ancient crustal fragments. Large-scale preservation of Archaean continental masses was effective only after the formation of substantial, buoyant, rigid, deep lithospheric keels and their stabilization in Neoarchean time. Formation of lithospheric mantle beneath the surrounding Proterozoic crustal regions occurred in Mesoproterozoic time, with lower degrees of mantle melting than associated with the cratonic peridotites. This circum-cratonic mantle is of similar age to the oldest overlying crust and has been coupled to the margins of the craton since its formation. Major magmatic events, some coincident with the formation of circumcratonic mantle, added new lithosphere to the Kaapvaal mantle root but failed to destroy it. The mechanically strong, buoyant lithospheric keels beneath cratons protect their crust from subduction and recycling over 3 Ga time periods.
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This volume contains a series of papers that cover a wide range of aspects, including geophysics, structure and tectonics, atmosphere, origin of life, biosphere, deep mantle geochemistry, early oceans, microbial ecology, on the development of the Earth in the first 2000 Ma of its history.The aim of this publication is to facilitate future discussions and understanding of this area of research.
This book is divided into three parts:
Geophysical and petrological constraints on Archaean lithosphere
Models of cratonic evolution and modification
Constraints on the Archaean environment
Subjects covered include the chemical and biological controls on the atmosphere and oceans, early controls on the carbon cycle and photosynthesis, petrologic, isotopic, tectonic and seismic evidence for the composition and structure of Archaean lithosphere.
This volume should be of interest to geologists and geophysicists who work on the Archaean, and students at all levels.