Journal ArticleDOI
Genesis of Archean Peridotitic Magmas and Constraints on Archean Geothermal Gradients and Tectonics
TLDR
In this article, a model of peridotitic komatiite magma was developed in which selective removal of garnet during ascent produces CaO/Al 2 O 3 > 1 and heavy rare-earth element depletion in the resultant peridoteitic or mafic extrusions.Abstract:
The experimentally determined extrusion temperature (1650° ± 20°C) for Archean peridotitic komatiite magma implies diapirism of upper mantle peridotite from a depth of at least 200 km. Models of magma genesis are developed in which selective removal of garnet during ascent produces CaO/Al 2 O 3 > 1 and heavy rare-earth element depletion in the resultant peridotitic or mafic extrusions. Magma genesis can be interpreted with a model of the Archean geotherm, lithosphere, and plate tectonics processes resembling the modern Earth, but this does not account for the distinctive preservation and character of Archean greenstone belts. A preferred model of a steeper Archean geotherm, thin lithosphere (≃50 km), and asthenosphere with approximately 5 percent melting does not allow eclogitization and subduction of basaltic oceanic crust, but it postulates scraping off of such crust against and between primitive sialic nucleii. This model may account for the distinctive characteristics of the Archean greenstone and “granite” terranes.read more
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Journal ArticleDOI
Speculations on evolution of the terrestrial lithosphere–asthenosphere system—Plumes and plates
TL;DR: In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe-Ni core resulted in extensive melting of the silicate mantle and in the formation of a near-surface magma mush ocean as mentioned in this paper.
Journal ArticleDOI
Crustal evolution of the early earth: The role of major impacts
Herbert Frey,Herbert Frey +1 more
TL;DR: In this article, it was shown that at least 50% of the global sialic crust would have been converted into impact basins by 4 Ga ago, and the minimum number of such basin-forming impacts can be calculated by scaling from the observed (minimum) number preserved on the Moon.
Journal ArticleDOI
High pressure metamorphism in the Scourian of NW Scotland: Evidence from garnet granulites
David Savage,Jane D. Sills +1 more
TL;DR: In this paper, a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12-15 kb and 1,000° C was reported.
Journal ArticleDOI
Speculations on the evolution of the lithosphere
TL;DR: In this article, the authors focus on constraints that must be respected by models for Precambrian evolution of the crust, using geologic data (presence of continental margins and rift zones, paleomagnetic polar-wander curve, age and distribution of andesites) as a guide to extrapolate back in time.
Journal ArticleDOI
Geotherms, evolution of the lithosphere and plate tectonics
TL;DR: For the purpose of calculating Precambrian geotherms, modern “average” geotherm need to be defined as discussed by the authors, and this method only indicates that in all probability subduction did not begin before 1000 Ma ago.
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