Composition of the Earth and its reservoirs: Geochemical observables

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1 Composition of the Earth and its reservoirs: Geochemical observables Cin-Ty A. Lee Rice University MYRES-I 2004

2 The Earth is dynamic and heterogeneous Atmosphere Midocean Ridge Plume Ocean Crust Oceanic Continental Crust Base of lithosphere Mantle Core? Not to scale

3 The Earth is differentiating: But are we mixing or unmixing reservoirs? Atmosphere Temp ( o C) Midocean Ridge Plume Ocean Crust Oceanic Continental Crust Base of lithosphere Mantle Core? P (GPa) 5 Saturated H2 O 200 Depth (km) 10 B 300 Not to scale

4 Gastronomy Tackley, 2000

5 Definition of heterogeneity Heterogeneity: a qualitative term describing how well-mixed a system is Reservoirs: chemically distinct regions in a system that have physically defined boundaries

6 At what point do we consider a heterogeneity a reservoir? Sampling Lengthscale L 1 ~ l o L 2 > l o L 3 >> l o

7 Tell me what lengthscale you re interested in 3000 km OPX SP OL CPX 1 cm Tackley, 2000

8 Forms of compositional heterogeneities Major - wt. % Minor - ~0.1 wt. % Trace - <100 ppm Isotopic What types of compositional heterogeneities lead to variations in physical parameters?

9 Three Steps to Bliss 1. Bulk Earth Composition 2. Composition of reservoirs 3. Quantifying the size and distribution of reservoirs

10 STEP 1 Towards a Bulk Earth Composition Bulk Silicate Earth (Primitive Mantle) Bulk Earth Bulk Silicate Earth Primitive Mantle Core Continental & Oceanic Crusts + Depleted Mantle + Primitive Mantle + Core + In the Beginning First 30 Ma Today Core

11 What types of samples can we work with? Mantle rocks (xenoliths, massifs, ophiolites) Lavas/Magmas Sediments Meteorites?

12 The search for the holy grail Do samples of Primitive Mantle exist? Na Al 2 O 3 wt. % CaO wt. % 2 O wt. % MANTLE PERIDOTITES Melting trend MgO (wt. %)

13 Can we use primitive meteorites, e.g. undifferentiated meteorites as a proxy for the undifferentiated Earth? Yes and No

14 Al/Si Terrestrial array CV CK CM CO L HCR CI BSE Cosmochemical Fractionation trend EH EL LL Mg/Si

15 Drake and Righter, 2002; after: Clayton, R. N. & Mayeda, T. K. Oxygen isotope studies in carbonaceous chondrites. Geochim. Cosmochim. Acta 63, (1999). Clayton, R. N. Oxygen isotopes in meteorites. Annu. Rev. Earth Planet. Sci. 21, (1993). Clayton, R. N., Mayeda, T. K., Goswami, J. N. & Olsen, E. J. Oxygen isotope studies in ordinary chondrites. Geochim. Cosmochim. Acta 55, (1991).

16 It is safe to say that Earth is derived from Earth-like materials

... but all is not lost CI chondrites Solar Elemental abundance (per 10 6 Si atoms) 10 8 10 6 10 4 10 2 1 10-2 He H C O N Fe Mg S Al Ni Co Ti Cu Ge Rb Sr B Pb Be Li Th 10-2 1 10 2 10 4 10 6 10 8 CI

17 ... but all is not lost CI chondrites Solar Elemental abundance (per 10 6 Si atoms) He H C O N Fe Mg S Al Ni Co Ti Cu Ge Rb Sr B Pb Be Li Th CI carbonaceous chondrite (per 10 6 Si atoms) The Early Evolution of the Inner Solar System: A Meteoritic Perspective C. M. O'D. Alexander, A. P. Boss, R. W. Carlson SCIENCE, Volume 293, Number 5527, pp

18 50% condensation temperatures (pressure 1E-4 bars) H He Li 1225 Na 970 K 1000 Rb 1080 Be B 964 C N O F 736 Mg 1340 Ca 1520 Sc 1644 Ti 1590 Sr Y Zr 1750 V Nb Cr 1300 Mo 1600 Cs Ba La-Lu Hf Ta W 1800 Fr Ra Ac-Lr Mn 1190 Tc Re 1800 Fe 1336 Ru 1600 Os 1800 Co 1351 Rh Ir 1600 Ni 1354 Pd 1334 Pt 1411 Cu 1037 Ag 952 Au 1225 Zn 660 Al 1680 Ga 997 Si 1311 Ge Cd In 470 Sn 720 Hg Tl 428 Pb 496 P 1267 As 1157 Sb 912 Bi 451 Ne S 648 Cl Ar Se 684 Te 680 Br I Kr Xe Po At Rn La 1500 Ac Ce Pr Nd Pm Sm Eu 1290 Th 1590 Pa U 1540 Gd Tb Dy Ho Er Tm Yb 1420 Lu 1590 Np Pu Am Cm Bk Cf Es Fm Md No Lr Refractory, >1400 K Transitional ~ K Moderately volatile ~ K Highly volatile <800 K

19 Nebular differentiation Planetary differentiation Refractory Moderately volatile Volatile Lithophile silicate loving Siderophile Fe loving Atmophile - atmosphere

20 PM Establish concentrations in Primitive Mantle using refractory lithophile element ratios for chondrites Fig. 6 from McDonough & Sun (1995)

21 STEP 2. RESERVOIR DOGS Atmosphere Midocean Ridge Plume Ocean Crust Oceanic Continental Crust Base of lithosphere x x WMDs Mantle Core? Not to scale

22 Partial melting as a major differentiation process Mid-ocean Ridge Temp ( o C) A Dry melting regime P (GPa) B Dry solidus Liquidus Depth (km) 7

23 Major Element Effects Bulk Silicate Earth MgO (wt. %) basalt Lower Crust 5 0 Continental Crust Andesite Dacite Rhyolite SiO 2 (wt. %)

24 Partition coefficient melt D = C solid C melt D > 1 compatible in solid D < 1 incompatible in solid Solid residue

25 Using magmas as windows to the mantle Cont. Crust Melts and crust normalized to primitive mantle MORB source is depleted in highly incompatible elements (DMM = depleted MORB mantle) 10 1 Plume MORB Primitive Mantle Continental Crust is enriched in highly incompatible elements Cs Rb Ba Th U Nb La Ce Pr Sr Nd Zr Hf SmGd Tb Dy Ho Y Er Yb Lu Plume is enriched or more primitive in character incompatible Increasing D More compatible

26 Trace-elements may suffer from fractionation during magmatic processes Isotopes in general are NOT fractionated

27 Sm/Nd < 1 melt Rb/Sr > 1 Sm/Nd > 1 Rb/Sr < 1 Solid residue 147 Sm 143 Nd 87 Rb 87 Sr 143 Nd/ 144 Nd Depleted Mantle Cont. Crust Bulk Earth 87 Sr/ 86 Sr Cont. Crust Depleted Mantle Bulk Earth Gy 0 time Gy 0 time

28 Depleted Mantle radiogenic Nd unradiogenic Sr Continental Crust unradiogenic Nd radiogenic Sr Hofmann, 1997 Nature 385:

29 At least the upper part of the Earth s mantle is depleted in highly incompatible elements This depleted portion appears to be complementary to the continental crust How much of the mantle is depleted?

30 Mass Balance Magic Concentration (ppm) Cont. Relative Size MORB PM DMM DMM/PM Crust of DMM Cs Rb Ba U K Sr Relative Size of DMM Cs Rb Ba U K Sr Possibly 30 to 70% of mantle has been processed to make Cont. Crust Mass Balance Says NOTHING about geometry

31 What do hotspots tell us? (Ocean Island Basalts OIB) Hofmann, 1997 Nature 385:

32 Hotspot magmas appear to be variably enriched in incompatible elements Source feature Or Sampling problem?

33 Sampling problem is a possibility BUT average isotopic composition of some OIBs differ from that of MORBs OIB source must differ slightly in composition from that of MORB source IF OIBs are associated with plumes + IF plumes derive from the lower mantle The lower mantle must be slightly enriched. 87 Sr/ 86 Sr 143 Nd/ 144 Nd Do plumes exist? Meibom & Anderson 2003

34 Other Reservoirs in the Silicate Earth Subducted oceanic crust SCLM subcontinental lithospheric mantle Subducted sediment Primordial mantle???

35 Tackley, 2000

36 Where do we go from here? More of the same data? New data? NEW QUESTIONS?

37 YOU can be an armchair geochemist Use compiled datasets GEOROC RidgePetDB GERM But don t abuse the data sets

38 New directions New data

39 DIRECTION 1 Is a homogeneous Bulk Silicate Earth a valid assumption? Was the primordial mantle stratified? If so, how does this affect geochemical mass balance conditions? Is the lower part of the mantle Fe-rich? Can we ignore mass fluxes across the core-mantle boundary?

40 What data may be able to answer these questions? Short-lived radionuclides 182 Hf- 182 W, 146 Sm- 142Nd, 107 Pd- 107 Ag More precise data on first series transition elements Fe, Mn, Ni, Co, Cr, V, Sc

41 Direction 2 Constrain the size and distribution of reservoirs Geochemistry, Geophysics, Geodynamics, Petrology G 3 P Do geochemical heterogeneities reflect variations in physical properties that are detectable by geophysical methods?

42 Clearly, major-element variations affect physical properties of the mantle. The physical effects of trace-element and isotopic heterogeneities are unclear. However, trace-element and isotopic heterogeneities may be correlated with variations in oxygen fugacity and water content, both of which may have profound effects on elasticity, conductivity, and viscosity. 2FeO + O 2 = Fe 2 O 3

43 Future? Effects of fo 2 and H 2 O on physical properties of peridotite Characterizing the variation of fo 2 and H 2 O in the mantle and what processes control these parameters Geochemical research on partitioning behavior of redox sensitive elements valence state of redox-sensitive elements quantification of H 2 O content in the mantle

44 Conclusions Geochemistry has provided a considerable knowledge base for our understanding of how the Earth works. Further progress will require interdisciplinary collaboration and the generation of new questions and areas of focus. More geochemical data will obviously be helpful, but existing databases should be mined to reveal areas that need more refinement.

The Periodic Table. Periodic Properties. Can you explain this graph? Valence Electrons. Valence Electrons. Paramagnetism

The Periodic Table. Periodic Properties. Can you explain this graph? Valence Electrons. Valence Electrons. Paramagnetism Periodic Properties Atomic & Ionic Radius Energy Electron Affinity We want to understand the variations in these properties in terms of electron configurations. The Periodic Table Elements in a column