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
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. Because of their high charge different international ocean drilling and features and are used to supple- and large radii, the REE are incompat- programs (Fig. 1). ment solar values in the estimation of ible elements. Incompatible elements cosmic composition (Fig. 2). are in geochemistry elements that dur- 3. The chemistry of the solar sys- ing melting process will be strongly tem and the cosmic composition 4. Geochemical information on enriched in the melt compared to the mantle evolution solid residues. The crystals are not The early geochemical evolution able to retain these elements during of material from the Earth has to be The MORB geochemistry is a key partial melting processes. However, seen in the context of cosmochemistry, to understand the geochemical evolu- the heavy REE have sufficiently small as the Earth is only a part of our so- tion of the Earth. The major element radii that they can be accommodated lar system. Geologically it is the most MORB chemistry is relatively homo- to some degree in many common studied planet. However, the chemis- geneous, so that nearly every oceanic minerals. The heaviest REE readily try of meteorites are crucial for under- crustal segment around the world has substitute in garnet, and hence can be standing and modelling the bulk Earth a very similar composition. Trace el- concentrated by it. Eu, in its 2+ state, composition. A fundamental constrain ements like the lanthanides, or rare substitutes for Ca2+ in plagioclase feld- on any understanding of the chemical earth elements (REE), have important spar more readily than the other REE. evolution of the Earth is the abundance applications in igneous and metamor- Thus plagioclase is often anomalous- of elements in the solar system. Due to phic petrology. The REE comprise the ly rich in Eu compared to the other the fact, that the planets are generally series of elements with atomic num- REE, and other phases in equilibrium thought to have originated in a slowly bers 57 to 71 (La to Lu). The REE with plagioclase become relatively rotating disk-shaped “solar nebula” of are the upper line of the two rows of depleted in Eu as a consequence. gas and dust with solar composition. elements commonly shown at the bot- REE patterns for average mid-ocean Some meteorites, the Chondrites, tom of the periodic table. The REE ridge basalt (N-MORB) are shown are chemically primitive, having com- all have very similar chemical and in Figure 3. MORB exhibits a light
Fig. 3: REE pattern illustrating the Earth differentiation into core, man- tle and crust. The REE concentra- tion had been more than 3 times enriched in the primitive mantle compared to the cosmic composi- tion after the separation of the Earth into core and mantle. This primi- tive mantle has been depleted in the light REE during the formation of the crust, the resulting depleted mantle is today the source of the N-MORB. In contrast the opposite reservoir, the continental crust is enriched in light REE compared to the primitive mantle. The average N- MORB composition and the average continental crust content, as well as the normalizing C1 Chondrite data are from the GERM Reservoir Data- base (http://Earthref.org).
Revue Technique Luxembourgeoise 1/2007 3 Fig. 4: 143Nd/144Nd vs. 87Sr/86Sr diagram for igneous rocks from the North Atlantic Igneous Province (NAIP) LIP. Isotopic differences are due to the contrast in Nd and Sr isotopic composition between old continental crust (e.g. gneiss, amphibolites) and mantle magma sources. Variations of 143Nd/144 Nd and 87Sr/86Sr in crustal rocks are related to age and the contrasting geochemical behaviour of Sm and Nd relative to Rb and Sr. The mantle plume model suggests that the correlation of initial Nd and Sr isotopes represent mixing lines, between “mantle zoo” reservoirs of distinct chemistry and age. DMM = Depleted MORB Mantle; DMM = Depleted MORB Mantle; HIMU = High μ (subducted oceanic crust); EM1 and EM2 = Enriched Mantle 1 (recycled pelagic ocean floor sediments / sub-continental lithosphere) and 2 (subducted continental material). Alternatively, the trend may result from mixing by assimilation / contamination of high-Sm/Nd, low-Rb/Sr mantle melts with old low-Sm/Nd, high-Rb/Sr crustal material, as has been shown in some sub-NAIP areas (Scotland). (Figure from Meyer et al., in press)
REE depleted pattern reflecting the The most prominent igneous rocks rounding pressure/temperature condi- incompatible element-depleted nature on the Earth surface – the MORB tions, and starts to rise again due to its of the upper mantle from which these – have a common mantle source, the smaller density compared to the lower magmas are derived. This incompat- geochemically depleted upper mantle mantle. Such a rising plume is restrict- ible element depletion of the mantle is (DM) (Fig. 2). It can be concluded that ed to a relatively small area on the generally thought to have resulted from during extensional processes resulting surface, and is believed to be a nearly extraction of partial melts, in which the in MOR this depleted reservoir is the fixed system compared to the plate incompatible elements were concen- major fused mantle involved in this movements. The rising mantle plume trated. In a theoretical model where the geodynamic process. material starts to melt due to its ex- continental crust and the mantle are the Most of the magmatic activity on cess temperature at the asthenosphere only two REE reservoirs, and decom- Earth is limited to the plate bounda- - lithosphere boundary. The arrival of pression melts from the upper mantle ries, however magmatism in the mid- mantle plumes is often related to the are light REE depleted, then the con- dle of plates is also known (e.g. Eifel, formation of Large Igneous Provinces tinental crust complementary reservoir Hawaii). After the introduction of the (LIP). The formation of such LIPs has has to be light REE enriched (Fig. 2). mantle plume hypothesis (Morgan, been recently discussed as potential This model is supported by data in Fig- 1971) this so-called intraplate volcan- source for biological mass extinctions ure 3: the upper continental crust has ism has mainly been explained by hot through the geological history. a light REE enriched pattern with a spots. In the mantle plume hypothesis During the rise through the man- so-called negative ‘Eu anomaly’. The geochemically enriched material from tle, plume material has multiple pos- available geochemical dataset allows subducted slabs is stored for geological sibilities for deep mantle interactions. to conclude, that the complementary times at the D’’ (core - mantle bound- Even geochemical core signatures are incompatible-element enriched reser- ary). During this period the enriched believed to be identified in proposed voir is the continental crust. material equilibrates with the sur- mantle plume derived magmas. Mainly
4 Revue Technique Luxembourgeoise 1/2007 Fig. 5: Pb isotope ratios in major terrestrial mantle reservoirs MORB and oceanic islands represent the isotopic variability of upper mantle and deep mantle respectively. As an example for intracontinental magmatism are plot- ted data from the CEVP, the Rhoen Mts. in Germany. (Modified after Meyer et al., 20002). isotopic investigations (cf. Fig. 4) de- This mantle plume model was for anic crust with different (continental) fined different geochemical reservoirs more than 25 years the ultimate ex- crustal segments. This package has in the mantle. Isotopes do not fraction- planation for intracontinental mag- been recycled in the deeper mantle. ate during partial melting of fractional matism. However, this hypothesis However, these enriched signatures melting processes, so they will reflect has recently been challenged due to can also be produced by mantle melt the characteristics of the source. the fact, that several primary classical / continental crust interactions (EM1 plume features required adaptations can be an assimilation signature of The detected geochemical mantle (cf. www.mantleplumes.org). Predic- lower continental crust, and EM2 zoo reservoirs (cf. Fig. 4, 5) are e.g.: tions of the mantle plume hypothesis correlates with a contamination of that are not observed for the Eifel upper continental crustal rocks). Al- (1) DM depleted mantle (MORB plume are for example, the plume ternative models for intracontinental source), track is nonexistent, and the seismic magmatic activities have been sug- tomographic image of the mantle gested, including delamination, me- (2) BSE Bulk Silicate Earth anomaly (proposed mantle plume) is teorite impact, small-scale rift-related restricted to the upper mantle. As for convection, and chemical mantle het- (3) PREMA prevalent mantle several plumes the mantle anomaly erogeneities. has been observed extending into the (4) HIMU (read: high μ = high 238U/204Pb deep mantle (Montelli et al., 2004), 5. Conclusions (Fig. 5)) former subducted and the lack of continuity below the Eifel recycled oceanic crust seems to be real. Furthermore, the Hard rock geochemistry of igneous Eifel is an integral area of the Central rocks provides clear evidence for the (5) EM1 enriched mantle type 1 European Volcanic Province (CEVP), chemical evolution of the Earth. The 87 (has near primordial Sr/86Sr and the CEVP has more signatures to differentiation of the Earth into core, (Fig. 4)) former subducted and be rift, instead of mantle plume, re- mantle and crust left behind geochemi- recycled oceanic crust plus sedi- lated. cal “fingerprints” (Fig. 2). These signa- ments Moreover, several isotopically en- tures can be traced back today in trace riched reservoirs (EMI, EMII, and element and isotopic characteristics of (6) EM2 enriched mantle type 2 (has HIMU) (Fig 4) are too enriched for primary mantle melts. The geochem- > 0.720, well above any reaso- any known mantle process, and they istry community has good reasons to nable mantle sources high 87Sr/ correspond to crustal rocks and/or assume, that the average solar nebula 86Sr (Fig. 4)) former subducted sediments. In the plume concept this composition is represented by the C1 and recycled oceanic crust with reservoirs had been produced in the Chondrite meteorites. Every process sediments mantle through subduction of oce- of the Earth evolution fractionated this
Revue Technique Luxembourgeoise 1/2007 5 composition. Geochemically the core models and availability of increasing- References IRUPDWLRQ LV UHÀHFWHG LQ WKH HQULFK- ly more consistent geochemical data ment of the incompatible elements and from projects investigating the role 0RUJDQ :- &RQYHFWLYH the depletion of “siderophile” elements of mantle-crust interaction (e.g. ESF plumes in the lower mantle. – Na- (e.g. Au, Ir, Os) in every primary man- EUROMARGINS project CRP-01- ture, 230. 42-43. tle melt source. The light REE deple- /(&(0$) VKRXOGKHOSWRUHVROYH tion of the MORB source can only be the problem of the challenged mantle - Meyer, R., Abratis, M., Viereck- explained as the conjugate reservoir of plume concept in the near future. *|WWH / 0lGOHU - +HUWRJHQ the enriched continental crust (Fig. 3). -DQG5RPHU5/ 0DQ- Magmatic activity mainly takes Acknowledgment telquellen des Vulkanismus in der place at divergent and convergent thüringischen Rhön. – Beitr. Geol. plate margins. Volcanism in the mid- Prof. P. Seck is thanked for the in- 7KULQJHQ dle of plates is less voluminous and vitation and the initiative to present the cause of it is more enigmatic. This UHVHDUFK IURP /X[HPERXUJLDQ VFLHQ- 50H\HU-YDQ:LMN/*HUQLJRQ intraplate activity is today a matter WLVWV DEURDG LQ /X[HPERXUJ ³0HUFL´ (in press.): North Atlantic Igneous of sharp debate. No obvious mecha- to the sponsors of the conference and Province: A Review of Models for QLVP¿WVUHDGLO\LQWRWKHSODWHWHFWRQLF VSHFLDOO\WKH$VVRFLDWLRQ/X[HPERXU- its Formation. c In: G. R. Foulger, SDUDGLJP )ROORZLQJ 0RUJDQ geoise des Ingénieurs and its president D. M. Jurdy (eds.) Plates, Plumes, this activity is commonly related to François Jaeger for sponsoring the ses- and Planetary Processes – Geolo- hot spots, deep astenospheric mantle sion of R.M.. Further the European Sci- gical Society of America Special upwellings. However, this scenario is ence Foundation, the Ministère de la Paper. presently strongly debated and alter- Culture, de l’Enseignement supérieur native models including delamination, HWGHOD5HFKHUFKH /X[HPERXUJ DQG meteorite impact, small-scale rift-re- Rotary International are acknowledged Contact lated convection, and chemical mantle for their support of R.M.’s Ph.D. re- heterogeneities have been proposed. search. R.M. is actually founded by Romain Meyer Ongoing development of the different %)5 /X[HPERXUJ [email protected]
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