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Hard Rock Geochemistry and Earth Mantle Evolution by Romain Meyer and Jan Hertogen

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Hard Rock Geochemistry and Earth Mantle Evolution by Romain Meyer and Jan Hertogen Editée par l’Association Lux. des Ingénieurs, Architectes et Industriels L-1330 Luxembourg - 4 Bd G.-D. Charlotte REVUE Tel. 451354 Fax. 450932 http://www.aliai.lu Publication de l’Association Lux. des Ingénieurs, Architectes et Industriels TECHNIQUE l’Association Lux. des Ingénieurs l’Association Lux. des Ingénieurs Industriels l’Ordre des Architectes et des Ingénieurs-conseils LUXEMBOURGEOISE janvier-mars 2007 1 Hard Rock Geochemistry and Earth Mantle Evolution by Romain Meyer and Jan Hertogen This paper is based on the talk of R. Meyer (16. October 2006) in the Cycle de Conférences: Les Chercheurs Luxem- bourgeois à L’Étranger: “Reise zum Erdmantel: Petrologische und Geochemische Aspekte zum Vulkanismus (Voyage vers le manteau terrestre: Aspects pétrologiques et géochimiques sur le volcanisme)” 1. Introduction The physical background of this its rise to the surface. Other approach- model is based on recordings of seis- es are the composition of primary The crust, upper mantle, lower man- mic waves from such crustal earth- mantle magmas reflecting the mantle tle, outer core, and inner core are the quakes. Seismic waves bend and re- source geochemistry. However, the re- textbook subdivisions of the Earth’s flect at the interfaces between different ally deep layers, can only be reached interior. The core is composed mostly materials. In addition, the two types in Hollywood science fiction produc- of iron and is so hot that the outer core of seismic wave behave differently, tions, e.g. to save the Earth from catas- is molten. In contrast, the inner core is depending on the material. Compres- trophe by a drill down to the core (The under such extreme pressure that it re- sional waves travel and refract through Core). Even with the most optimistic mains solid. Most of the Earth’s mass, both fluid and solid materials. Shear news of technical evolution, this re- is in the mantle and composed of sili- waves, however, cannot travel through mains wishful thinking and a borehole cates of iron, magnesium, calcium and fluids like melts or water. will not be able to reach the core. As a aluminium. At > 1000°C, the mantle is These geophysical observations had result, analogues had been of primary solid but can deform slowly in a plastic to be explained in a consistent geo- interest, and observations from astro- manner. The crust is much thinner than chemical Earth evolution model. In physics as well as from meteorites any of the other layers, and its miner- this challenge petrologists have been have been able to fill the observational alogy is dominated by the less dense mainly restricted to secondary infor- gaps. Iron meteorites are today consid- calcium and sodium aluminium-sili- mation of the layers. The mantle can ered as equivalents to the Earth core. cates. Being relatively cold, the crust only be directly investigated on the This situation leads modern petrology is brittle, which is the precondition to base of mantle fragments (xenoliths) to consistent model assumptions and cause earthquakes. that are transported by magma during such a model will be discussed in this 1 Fig. 1: (A) Photograph of an ODP (Leg104 642E) MORB core sample from the N Atlantic, and (B) Cross polarized light microphotograph of a typical tholeiitic MORB from the N Atlantic (C) Photo of the unique IODP scientific research vessel Joi- des Resolution, specially designed to drill deep into the sea floor (Pho- to C from www-odp.tamu.edu). Transform boundaries have only limited magmatic activity and due to this observation they can not be used for the geochemical evolution of the paper. In the next section an approach parts of the mantle. The key principle mantle. In contrast both other bound- for the mantle composition and evolu- of plate tectonics is that the lithosphere ary types are strongly associated with tion will be discussed, on the base of exists as separate and distinct tectonic enormously igneous activity. primary mantle magmas. plates, riding on the visco-elastic solid A convergent plate boundary forms asthenosphere. The plate boundaries at a subduction zone when one or both 2. Volcanoes and primary mantle are commonly associated with geolog- of the tectonic plates is composed of melts ical events such as earthquakes and the oceanic lithosphere. The less dense creation of mountains, volcanoes and plate, or lithosphere usually rides over A volcano is an opening in the oceanic trenches. The majority of the the denser plate, which is subducted. Earth’s surface that allows hot, molten world’s active volcanoes occur along This type of plate convergence is asso- rock, and gases to escape from deep such plate boundaries, with the Ring ciated with (volcanic) island arcs such below the surface. The word ‘volcano’ of Fire around the Pacific being the as e.g. Japan or, the Andes. In subduc- comes from the little island Vulcano most active and famous. tion zones, the loss of volatiles from (38°24′00″N, 14°58′00″E) in the Tyr- Plates are able to move because of the subducted slab into the mantle in- rhenian Sea, about 25 km north of Sic- the relative density of oceanic litho- duces partial melting of the overriding ily and the southernmost of the Aeolian sphere and the relative weakness of metasomatized mantle and generates Islands. Centuries ago, people believed the asthenosphere. Dissipation of heat low-density, calc-alkaline magma that that Vulcano was the chimney of the from the mantle is recognized to be the buoyantly rise through the lithosphere forge of Vulcan - the blacksmith of the original source of energy driving plate of the overriding plate. As a result Roman gods. Volcan was named in tectonics, but it is no longer thought the extruded magmas are the product the Greek mythology Hephaistos - the that the plates ride passively on asthe- of a combination of volatiles from god of fire and craftsmanship. Romans nospheric convection currents. Instead, the subducted slab and mantle mate- thought that the hot lava fragments and it is accepted that the excess density of rial. Hence, the erupted magmas will clouds of dust erupting form Vulcano the oceanic lithosphere sinking in sub- not have a primary primitive mantle came from Vulcan’s forge as he ham- duction zones drives plate motions. signature. The divergent boundaries mered thunderbolts for Jupiter, king of When it forms at mid-ocean ridges within continents initially produce rift the gods, and weapons for Mars, the (MOR), the young hot oceanic litho- valleys (e.g. the Rhinegraben). Not all god of war. Today geoscientists attri- sphere is initially less dense than the continental rifts eventually evolve to bute the volcanic activities at Vulcano underlying asthenosphere, but it be- the stage of continental breakup. The and other volcanoes mainly to release comes more dense with age, as it con- so-called “failed” rifts were once loci of excess heat by geodynamic plate ductively cools. The greater density of of strain localization and crustal exten- tectonic processes. The plate tectonic old lithosphere relative to the underly- sion, but are now no longer considered theory was developed to explain the ing asthenosphere allows it to sink into active. Other rifts proceed more suc- observed evidence for large scale mo- the deep mantle at subduction zones, cessfully to their final stage and result tions of the Earth’s crust. providing most of the driving force for in continental separation and seafloor Next to the division of the Earth plate motions. spreading with oceanic crust forma- in crust, mantle and core, geodyna- Three types of plate boundaries are tion. Therefore, most active divergent mists use another classification of the distinguished by the way the plates plate boundaries exist between oce- crust and the mantle. This division of move relative to each other. anic plates and are called mid-oceanic the outer parts of the Earth into litho- ridges (MOR). At MOR crystallized sphere and asthenosphere is based 1. Transform boundaries (where two magma (MOR basalts, MORB) (Fig. on their mechanical properties. The plates slide against each other); 1a, 1b) continuously forms new oce- lithosphere is cooler and more rigid, anic crust along the ridge axes. This whilst the asthenosphere is hotter 2. Convergent boundaries (where new magma emerges at and near the and mechanically weaker. Also, the two plates collide head-on with axis because of decompression melt- lithosphere loses heat by conduction each other); and ing in the underlying Earth’s mantle. whereas asthenosphere transfers heat Due to this constant fusion process, by convection and has a nearly adia- 3. Divergent boundaries (where two with homogeneous tholeiitic mag- batic temperature gradient. The litho- plates move apart from each oth- mas (named after the rock Tholeyit of sphere comprises both crust and upper er). Tholey, Saarland), these rocks have 2 Revue Technique Luxembourgeoise 1/2007 Fig. 2: Schematic sketch of the che- mical differentiation of the Earth from a homogeneous cosmic com- position into core, mantle and crust. physical properties. This is based on the fact, that they all form stable 3+ ions of similar size. The only differ- ences in chemical behaviour are a consequence of the small but steady decrease in ionic size with increasing atomic number. A small number of the REE also exist in oxidation states the highest geochemical potential to positions similar to that of the sun. other than 3+ but the only geologi- start a geochemical investigation of Chondrites are named after the round- cal important ions are Ce4+ and Eu2+. the mantle evolution. During the last ed fragments, a.k.a. chondrules that These form a smaller and a larger ion years MORB have been thoroughly they contain. The C1 or CI Chondrites relative to the common 3+ oxidation petrologically investigated during the have the most primordial composition state.
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