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Early Earth Systems: A Geochemical Approach…
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Early Earth Systems: A Geochemical Approach (edição: 2007)

de Hugh R. Rollinson (Autor)

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Early Earth Systems provides a complete history of the Earth from its beginnings to the end of the Archaean. This journey through the Earth's early history begins with the Earth's origin, then examines the evolution of the mantle, the origin of the continental crust, the origin and evolution of the Earth's atmosphere and oceans, and ends with the origin of life. Looks at the evidence for the Earth's very early differentiation into core, mantle, crust, atmosphere and oceans and how this differentiation saw extreme interactions within the Earth system. Discusses Archaean Earth processes within the framework of the Earth System Science paradigm, providing a qualitative assessment of the principal reservoirs and fluxes in the early Earth. "The book would be perfect for a graduate-level or upper level undergraduate course on the early Earth. It will also serve as a great starting point for researchers in solid-Earth geochemistry who want to know more about the Earth's early atmosphere and biosphere, and vice versa for low temperature geochemists who want to get a modern overview of the Earth's interior."       Geological Magazine, 2008… (mais)
Membro:klasse
Título:Early Earth Systems: A Geochemical Approach
Autores:Hugh R. Rollinson (Autor)
Informação:Wiley-Blackwell (2007), Edition: 1, 296 pages
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Early Earth Systems: A Geochemical Approach de Hugh R. Rollinson

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A textbook on the Hadean and Archean periods of the Earth's formation, from 4.6 billion years ago to 2.5 billion years ago. Recent enough (2007) that it is probably not grossly out of date, but pricey even for a textbook at over $100 for a modest-sized paperback.

The most striking thing about the book is that it is more a catalog of the issues needing resolution than a story giving the answers. This is perhaps unsurprising considering how much geological evidence from this early time period has been destroyed by subsequent tectonics.

Archean rocks form the cores of the continents, the continental shields. These are mostly granite-greenstone belts surrounded by high-grade gneiss, but the author prefers to divide these into greenstone belts, granite-gneiss terranes, and sedimentary basins, the latter the youngest of the Archean rocks. The greenstone belts contain the oldest examples of magma generated from the mantle and so are our best window into the early mantle. The granite-gneiss terranes show us early continental crust formation.

Some of the things we do seem to know surprised me. There seems to be pretty good agreement that the Earth formed from the collision of smaller planetisimals in which iron cores had already largely separated from silica mantles; there's no other good way to explain the radioisotope and geochemical data. The radioisotopes tell us that separation had to take place very early, and the geochemistry tells us that separation likely took place in a magma ocean at considerable pressure. Well, so another thing agreed on is that there was a primordial magma ocean that cooled very rapidly; it was largely solidified within a few tens of millions of years. That I also didn't know.

The original crust was something like oceanic crust today -- low-silica basalt. There is a lot of debate over the origin of continental crust, but it seems to have been generated in pulses, with one big pulse towards the end of the Archean. There is evidence, but controversy, about crust formation; Archean crust is particularly stable, and this is apparently because it has particularly cold and stable mantle lithosphere beneath it. The Archean lithosphere is apparently unusually depleted (has been a source for an unusually great volume of magma) but this is disputed; Archean crust itself is apparently more silicic than younger crust, though this is also disputed; it may have formed directly from melting of subducting slabs, rather than melting of mantle wedges over subducting slabs (the mechanism for crust formation today), producing characteristic tonalite-trondjemite-granodiorite batholiths, though this is also disputed.

Everyone seems on board now with the idea that subducting slabs sink clear into the lower mantle; beyond that everything is disputed, including the degree of chemical stratification of the mantle and where plumes arise, if there even are true plumes, which is now also disputed. One view is that subduction cools the mantle and hot spot plumes cool the core; this is disputed. There is a view that the lower mantle has a different chemistry from the upper mantle, and a competing view that the mantle is a kind of plum-pudding of blobs of different composition; that the mantle is heterogeneous seems not to be disputed. There is fairly good reason to think that there is a significant water reservoir, of all places, at a phase transition zone about 400-600 km down, where a mineral phase appears that has room in its lattice for water molecules; there may be something like 2-3 oceans' worth of water locked up here. It is suggested that this acts as a chemical filter for rising magma, explaining some geochemistry that is otherwise hard to reconcile with the geophysics.

The ocean formed very early on; this seems not to be much disputed any more. Where it came from is disputed. The chemical environment was highly reducing until 2.3 billion years ago, when the oxygen levels rose really quite abruptly; why is disputed, though life was certainly involved. Sulfur was mostly reduced early on, except that Archean sedimentary basins characteristically contain oxidized barium sulfate, whose origin is disputed. Eukaryotes appeared early, though how early is disputed. Chloroplasts were originally cyanobacteria and mitochondria were purple sulfur bacteria; this seems to not be much disputed.

The book lays down the various arguments for all sides in the disputes fairly evenhandedly. Great fun.

Thumbs up, though I recommend borrowing rather than purchasing at this price. ( )
  K.G.Budge | Aug 8, 2016 |
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Early Earth Systems provides a complete history of the Earth from its beginnings to the end of the Archaean. This journey through the Earth's early history begins with the Earth's origin, then examines the evolution of the mantle, the origin of the continental crust, the origin and evolution of the Earth's atmosphere and oceans, and ends with the origin of life. Looks at the evidence for the Earth's very early differentiation into core, mantle, crust, atmosphere and oceans and how this differentiation saw extreme interactions within the Earth system. Discusses Archaean Earth processes within the framework of the Earth System Science paradigm, providing a qualitative assessment of the principal reservoirs and fluxes in the early Earth. "The book would be perfect for a graduate-level or upper level undergraduate course on the early Earth. It will also serve as a great starting point for researchers in solid-Earth geochemistry who want to know more about the Earth's early atmosphere and biosphere, and vice versa for low temperature geochemists who want to get a modern overview of the Earth's interior."       Geological Magazine, 2008

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