Presentation on theme: "The Chemical Composition of the Solar System and the Earth"— Presentation transcript:
1The Chemical Composition of the Solar System and the Earth
2Two suggestionsExpertise comes from making all possible mistakes (Niels Bohr)Nothing can be obtained in geochemistry without careful analytical work (C.J. Allegre)
3Further readings W.M. White, Geochemistry. A on-line text book. S.R. Taylor and S.M., The Continental Crust: Its Composition and Evolution. Blackwell, Oxford.W.F. McDonough and S.Sun, The Composition of the Earth, Chemical Geology, 120:Geochemical Earth Reference Model (GERM)
4Why do we study element abundances? Fundamental for any geochemical studies
5Confusing termsAbundance: for a large system, e.g., Cosmos, Sun, Moon, Earth, crust, regional crustContent/concentration: for a smaller system, e.g., rocks, minerals, natural water.
15Classification of elements (McDonough and Sun, 1995)
16Classification of elements according to volatility
17Ca-Al inclusionThere is much interest in high T component, i.e., the so-called Refractory Inclusions (RI) or Ca-Al inclusions (CAI), because their composition represents that of the first condensates from a high T gas.
18Carton illustrating the process involved in formation of chondrites and their components
19Abundances of elements in sun’s photosphere vs their abundances in CI chondrites (White, 2001)
20Comparison of element abundances in solar photosphere and CI carbonaceous chondrites (Taylor and McLennan, 1995)
24Characteristic of element abundances of the solar system H and He accounts for 98% in mass.Exponential decrease in abundance for elements with atomic number<45.Elements with even mass show significantly higher abundances than the neighboring elements with odd mass.He exhibit an abnomously high abundance compared to the neighboring Li, Be and B.O and Fe show a peak.Isotopes with atomic weight being factor of 4 have high abundance. 4He (Z=2, N=2), 16O (Z=8, N=8),40Ca (Z=20, N=20).
25Even-odd mass effect5860666470576862596367657169
26Sequence of decreasing element abundances in the solar system HHeOCN, NeMg, SiFeS1010 to 109107106105
28Steller structure at the onset of supernova stage (White, 2001) The E-process (Si burning)The S-process (neutron capture)The r-process (Rapid neutron capture)Principle mechanism for forming heavier isotopesThe p-process (Proton capture)Responsible for the lightest isotopes ofa given element
36The Earth’s Interior Mantle: Peridotite (ultramafic) Upper to 410 km (olivine ® spinel)Low Velocity Layer kmTransition Zone as velocity increases ~ rapidly660 spinel ® perovskite-typeSiIV ® SiVILower Mantle has more gradual velocity increaseFigure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
37The Earth’s Interior Core: Fe-Ni metallic alloy Outer Core is liquid No S-wavesInner Core is solidFigure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
42Rock NamesPeridotite: ultramafic rock composed of olivine, 2 pyroxenes (opx-cpx) and Al-phase (i.e., plagioclase, spinel, garnet, with the specific phase being a function of pressure, 0-10, 10-25, >25 Kb respectively), includes: lherzolite, harzburgite, dunite
43Eclogite: mafic (i.e., basaltic) rock composed of Na-rich clinopyroxene and garnet
44Pyroxenite: mafic to ultramafic rock, dominantly composed of pyroxene, often containing an Al-phase (e.g., plagioclase, spinel, garnet)
45Non-Rock NamesPrimitive Mantle/Silicate Earth: model composition for the crust + mantle.Pyrolite: model composition for the primitive mantle, name derived from pyroxene-olivine-ite.Piclogite: model composition for the mantle, name derived from picritic-eclogite (picrite = olivine-rich basalt).
46Lherzolite: A type of peridotite with Olivine > Opx + Cpx Dunite90PeridotitesWehrliteHarzburgiteLherzolite40Olivine WebsteritePyroxenitesOrthopyroxenite10Websterite10ClinopyroxeniteOrthopyroxeneClinopyroxeneFigure 2-2 C After IUGS
47Mantle rock mineral assemblage Simple: 4 or 5 phasesOlivine (Ol)Orthopyroxene (OPX)Clinopyroxene (CPX)Plagioclase (Pl)Spinel (Sp)Garnet (Gt)
51Phase diagram for aluminous 4-phase lherzolite: Al-phase =Plagioclaseshallow (< 50 km)Spinel50-80 kmGarnetkmSi ® VI coord.> 400 kmNote: the mantle will not melt under normal ocean geotherm!Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,
54Melting of MantleMelt: BasaltResidue: Peridotite
55Lherzolite is probably fertile unaltered mantle Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting15Tholeiitic basalt10Partial MeltingWt.% Al2O35Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.LherzoliteHarzburgiteResiduumDunite0.00.20.40.60.8Wt.% TiO2
56How does the mantle melt?? 1) Increase the temperatureNo realistic mechanism for the general caseLocal hot spots OKvery limited areaFigure Melting by raising the temperature.
572) Lower the pressureAdiabatic rise of mantle with no conductive heat lossDecompression melting could melt at least 30%Adiabatic rise of mantle with no conductive heat lossSteeper than solidusIntersects solidusD slope = heat of fusion as mantle meltsDecompression melting could melt at least 30%Figure Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.
583) Add volatiles (especially H2O) Remember solid + water = liq(aq) and LeChatelierDramatic lowering of melting point of peridotiteFigure Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.
59Experiments on melting enriched vs. depleted mantle samples: Tholeiite easily createdby 10-30% PMMore silica saturatedat lower PGrades toward alkalicat higher PFigure 10-17a. Results of partial melting experiments on depleted lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73,
60Experiments on melting enriched vs. depleted mantle samples: 2. Enriched MantleTholeiites extend tohigher P than for DMAlkaline basalt fieldat higher P yetAnd lower % PMFigure 10-17b. Results of partial melting experiments on fertile lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic magmas. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73,
68Common lherzolite xenoliths come from a depth of 50-80 km: lithosphere
69Phase diagram for aluminous 4-phase lherzolite: Al-phase =Plagioclaseshallow (< 50 km)Spinel50-80 kmGarnetkmSi ® VI coord.> 400 kmNote: the mantle will not melt under normal ocean geotherm!Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,
73Mantle model circa 1975Homogeneous mantleLarge-scale convection (drives plate tectonics?)Figure 10-16a After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.
74Newer mantle model Upper depleted mantle = MORB+ crust sources Lower undepleted & enriched OIB sourceLayered mantleUpper depleted mantle = MORB sourcedepleted by MORB extraction > 1 GaLower = undepleted & enriched OIB sourceBoundary = 670 km phase transitionSufficient D density to impede convection so they convect independentlyIt is interesting to note that this concept of a layered mantle was initiated by the REE concentrations of oceanic basaltsLater support came from isotopes and geophysicsFigure 10-16b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.
88The Earth’s Interior Core: Fe-Ni metallic alloy Outer Core is liquid No S-wavesInner Core is solidFigure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
89Composition of the Core Poorly constrained beyond its major constituents (i.e., an Fe-Ni alloy).Presence of 5-15% of light element(s) (S, O, Si).The dominant depository of siderophile elements in the Earth.
90Limits on the compositions of the core and bulk Earth (McDonough & Sun, 1995)
91Liquid silicate-liquid metal partition coefficients
92Comparison of element distributions in the Earth and carbonaceous chondrites The Earth is more strongly depleted in volatile elements
93Figure 1-5. Relative atomic abundances of the seven most common elements that comprise 97% of the Earth's mass. An Introduction to Igneous and Metamorphic Petrology, by John Winter , Prentice Hall.
98Plate Tectonics – Igneous Genesis 1. Mid-ocean Ridges2. Intracontinental Rifts3. Island Arcs4. Active ContinentalMargins5. Back-arc Basins6. Ocean Island Basalts7. Miscellaneous Intra- Continental Activitykimberlites, carbonatites, anorthosites...
99Composition of the Oceanic Crust (Taylor and McLennan, 1995)
100Part 3-4 The Continental crust The continental crust accounts for 41% of the Earth surface.Approximately 31% of continental area is submerged beneath the oceans.
101Importance of Determining Crustal Composition Basic constraints on evolution of the Earth.Most accessible part of the Earth and the best known.Place for formation of most of ore deposits.Important depository for highly incompatible elements (U, K, Cs).Essential for environmental studies and geochemical exploration.
102Study of the composition of the continental crust can be traced back to earliest stage of geochemical studiesF.M.Clarke, 1889F.M.Clarke and H.S.Washington, 1924V.M.Goldschmidt,1933, who is regarded as the father of modern geochemistry.S.R.Taylor, 1994D.M.Shaw, 1967S.R.Taylor and S.M. McLennan, 1985K.H.Wedepohl, 1992
103What is the Continental Crust ? Extends vertically from the surface to the Mohorovicic discontinuity, a jump in compressional wave Vp speeds from ~7 km/s to ~8 km/s that is interpreted to mark the crust-mantle boundary.Stratification in seismic velocity and thus rock type and chemical composition.Lateral and vertically heterogeneous and great diversity in rock type.
104Structure and compositional model of the continental crust (Wedepohl, 1995)
105Metamorphic Facies Lower Crust Middle Crust Upper crust Fig Temperature-pressure diagram showing the generally accepted limits of the various facies used in this text. Boundaries are approximate and gradational. The “typical” or average continental geotherm is from Brown and Mussett (1993). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Middle CrustThe boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observedThey are thus separated by mineral reaction isogradsThe limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phaseThe 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.Upper crust
106Part 3-4-1 The Upper Continental Crust : the most accessible part of the Earth
107Methods of StudiesLarge-scale regional sampling (e.g., the Canadian Shield)Using fine-grained clastic sediments
108Large-scale regional sampling Examples: The Canadian Shield and Eastern China.The most reliable method for upper crustal composition estimation.The only method for major element composition studies.Expensive and time-consuming.Not pertain to the study of upper crustal composition in the geological history.
111Fine-Grained Clastic Sediments as Natural Sampling of the Exposed Upper Continental Crust Shale, mudstone, siltstone, graywack, tillite, and loess.Simple and much less expensive.The only way for studying upper crustal composition in geological history.Unsuitable to providing the major element composition.Limited to REE, Y, Th, Sc, Co.
112Geological influences on sedimentary rock composition-Solubility Water-upper crust partition coefficient:Ky=Natural water/Upper crustSeawater residence time y: time for replacement of seawater element concentration.y=My/Fywhere My is the mass of element y in the oceans and Fy is the annual mean flux of element y through the ocean reservoir.
113Residence time vs seawater partition coefficients
114Weathering Ca, Na and Sr are lost K, Rb, Cs and Ba are retained. Al, Ga, HSFE (Ti, Zr, Hf, Ta, Th) and REE, Y, Sc are immobile.
115CIA: Chemical Index of Alteration CIA=Al2O3/(Al2O3+CaO CIA: Chemical Index of Alteration CIA=Al2O3/(Al2O3+CaO*+Na2O+K2O) in molecular proportionPlagioclase is the dominant phase in the continental crust subjected to weathering.
117Erosion and transportation The sand-size effect.Quartz and heavy minerals (zircon, rutile, magnetite, chromite) are enriched in sandstone.
118Diagenesis Sensitive to redox conditions Fe and Mn are soluble in anoxic conditions.Fe, Cu, Mo, Pb, Zn, V, Ni, S, C are clearly enriched in anoxic sediments due to incorporation in sulphides and/or absorption on organic compounds.U is enriched also in anoxic sediments due to reduction of soluble 6+U to insoluble 4+U.
119Metamorphism Poorly understood. Li and Pb may increase. Most elements and particularly REE, Y, Th, HFSE, Cr and Sc are immobile.
120Sedimentary rocks as crustal samples Insoluble elements (log 4; Ksw -4) are likely to be transferred almost quantitatively into clastic sediments and give the best information regarding the source-exposed upper crust.
121Quantitatively transferred into fine-grained clastic sediments REE in fine-grained sediments provide quantitative info on the upper crust compositionQuantitatively transferred intofine-grained clastic sediments
127Estimation of upper crustal composition Major elements:large-scale samplingTrace elements: large-scale sampling fine-grained clastic sedimentsusing REE and their ratios toother elements
128Various upper crustal major element estimates Taylor & McLennanShaw et al.WedepohlCondieGao et al.Rudnick & Gao198519671995199319982002SiO26664.9366.2165.4665.84TiO20.50.520.550.650.60Al2O315.214.6314.9613.6514.31FeOT4.503.974.705.134.92MnO0.070.0680.10.10MgO188.8.131.522.522.47CaO184.108.40.206.313.46Na2O3.93.512.753.13K2O220.127.116.112.582.66H2O0.792.11P2O18.104.22.168.14Total100.1797.9898.8098.4199.71
129Upper crustal compositional estimates (Taylor and McLennan, 1985)
130Comparison of loess and upper crustal compositions (Taylor and McLennan, 1985)
132Methods of StudiesAmphibolite- and granulite-facies xenoliths entrained mostly in basalts.Exposed deep crustal sectionsCorrelation of seismic velocities of rocks with lithologiesHeat flow constraints
133Crustal structure based on deep crusal xenoliths (Mengel et al., 1992)
142Calculation of velocities in depth V(z)=V(0) + [(dV/dP)T P + (dV/dT)PT]dzWhere V(0) and V(z) are velcities at a reference state and at depth z.For common rocks, (dV/dP)T =210-4 to 710-4 km s-1 MPa-1; (dV/dT)P = -210-4 to -610-4 km s-1 C-1
143Effect of heat flow on Vp (Rudnick and Fountain, 1995)
144150 MPa 下侵入岩Vp随成分的变化 (Fountain and Christensen, 1989)
145Relation between SiO2 and Vp of granulites (Rudnick and Fountain, 1995)
154Comparison of various REE and trace element estimates of continental crust (Rudnick and Fountain, 1995)
155Comparison of continental crust and various basalts (Hoffmann, 1994)
156Compositional characteristics of continental crust The upper crust is granitic with 66% SiO2 and with a significant negative Eu anomaly.The middle crust is tonalitic with 61% SiO2.The lower crust is mafic in many regions with ~52% SiO2 and may be more evolved for some cratons (e.g., North China Craton) and collision belts.Relative depletion in Nb and enrichment in Pb characterize the continental crust and continental crustal rocks- “the arc signature”.The total continental crust has an andesitic/granodioritic bulk composition with 59-62%. It contains a significant proportion of the bulk silicate Earth’s incompatible element budget (33-35% of Rb, Ba, K, Pb, Th and U).
157Schematic model for growth and evolution of the continental crust (Taylor and McLennan, 1995)
159Relative Vp change with depth under varying surface heat flows
160Contrasting lower crustal velocities for Archean and Proterozoic provinces (Durrheim and Mooney, 1991)
161The following slides are not used in the lectures
162Generation of tholeiitic and alkaline basalts from a chemically uniform mantle Variables (other than X)TemperaturePressureVariables (other than X)Temperature= % partial meltingPressureFig indicates that, although the chemistry may be the same, the mineralogy variesPressure effects on eutectic shiftFigure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,
163Liquids and residuum of melted pyrolite Tholeiite produced at < 30 km depth by 25% PM60 kmAlkalis are incompatible so tend to concentrate in first low % partial melts20% PM -> alkaline basalt30% PM -> tholeiite (only 25% or less at 30 km so looks like tholeiitic nature suppressed with depth)Note that residuum is Ol + Opx (harzburgite)Note also the thermal divide between thol and alk at low pressure for FXFigure 10-9 After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2,
164Initial Conclusions: Tholeiites favored by shallower melting 25% melting at <30 km ® tholeiite25% melting at 60 km ® olivine basaltTholeiites favored by greater % partial melting20 % melting at 60 km ® alkaline basaltincompatibles (alkalis) ® initial melts30 % melting at 60 km ® tholeiite
165Crystal Fractionation of magmas as they rise Tholeiite ® alkalineby FX at med to high PNot at low PThermal divideAl in pyroxenes at Hi PLow-P FX ® hi-Alshallow magmas(“hi-Al” basalt)Figure Schematic representation of the fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). After Wyllie (1971). The Dynamic Earth: Textbook in Geosciences. John Wiley & Sons.
166Primary magmasFormed at depth and not subsequently modified by FX or AssimilationCriteriaHighest Mg# (100Mg/(Mg+Fe)) really ® parental magmaExperimental results of lherzolite meltsMg# = 66-75Cr > 1000 ppmNi > ppmMultiply saturated
167SummaryA chemically homogeneous mantle can yield a variety of basalt typesAlkaline basalts are favored over tholeiites by deeper melting and by low % PMFractionation at moderate to high depths can also create alkaline basalts from tholeiitesAt low P there is a thermal divide that separates the two seriesIn spite of this initial success, there is evidence to suggest that such a simple approach is not realistic, and that the mantle is chemically heterogeneous
168REE data for oceanic basalts Ocean Island Basalt (Hawaiian alkaline basalt)Looks like partial melt of ~ typical mantleMid Ocean Ridge Basalt (tholeiite)How get (+) slope??increasing incompatibilityFigure 10-13a. REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).
169Spider diagram for oceanic basalts Same approach for larger variety of elementsStill OIB looks like partial melt of ~ typical mantleMORB still has (+) slopeLooks like two mantle reservoirsMORB source is depleted by melt extractionOIB source is not depletedis it enriched?increasing incompatibilityFigure 10-13b. Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).
170REE data for UM xenoliths LREE enrichedLREE depletedor unfractionatedREE data for UM xenolithsLREE depletedor unfractionatedLREE enrichedDepleted types (+) slopeFertile types (-) slopeEnriched?Figure Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.
173The Geothermal Gradient Continental Gradient higher than Oceanic GradientRange for bothHighest at Surfacewater and cold surfaceIn the future we will often use average values rather than the rangesFigure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18,
174Fig Temperature- pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.The high P/T series, for example, typically occurs in subduction zones where “normal” isotherms are depressed by the subduction of cool lithosphere faster than it can equilibrate thermallyThe facies sequence here is (zeolite facies) - (prehnite- pumpellyite facies) - blueschist facies - eclogite facies.The medium P/T series is characteristic of common orogenic belts (Barrovian type)The sequence is (zeolite facies) - (prehnite-pumpellyite facies) - greenschist facies -amphibolite facies - (granulite facies)Crustal melting under water-saturated conditions occurs in the upper amphibolite facies (the solidus is indicated in Fig. 25-2)The granulite facies, therefore, occurs only in water-deficient rocks, either dehydrated lower crust, or areas with high XCO2 in the fluid