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Composition of Earth’s Interior: A New View

Composition of Earth’s Interior: A New View. Richard W. Carlson DTM, Carnegie Institution of Washington and Maud Boyet DTM and ENS, Lyon. AGU, May 2006. How to Estimate the Composition of a Planet: Look to the Sun.

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Composition of Earth’s Interior: A New View

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  1. Composition of Earth’s Interior: A New View Richard W. Carlson DTM, Carnegie Institution of Washington and Maud Boyet DTM and ENS, Lyon. AGU, May 2006

  2. How to Estimate the Composition of a Planet:Look to the Sun Solar and CI compositions from Palme, and Palme and O’Neill, Treatise on Geochemistry, 2003

  3. The Earth is not a CI Chondrite Earth’s Mantle is Siderophile-Depleted Earth is Volatile-Depleted Bulk Earth from McDonough and CI from Palme and O’Neill, Treatise on Geochemistry, 2003

  4. Refractory Lithophile Elements should be Present in Chondritic Relative Abundances, but are not in Earth’s Crust A Schematic Compositional Cross-Section of the Silicate Earth Continental Crust Mass Fraction 0.45% Mantle Source of Oceanic Crust 30-70% 70-30% Primitive, Undifferentiated, “Chondritic” Mantle Source of Oceanic Crust

  5. Timing of Earth Differentiation: 146,147Sm-142,143Nd Systematics Long-lived chronometer: 147Sm 143Nd (T1/2 = 106 Ga) 147Sm abundance decreased by only 3% in 4.56 Ga Short-lived chronometer: 146Sm 142Nd (T1/2= 103 Ma) 146Sm exists only in the first 500 Myr of solar system history MORB: Sm/Nd > Chondritic, 143Nd/144Nd = 0.51315 (higher than chondritic) Continental Crust: Sm/Nd < Chondritic, 143Nd/144Nd = 0.5130 to 0.5108, Up to 0.3% lower than chondritic)

  6. New 142Nd Chondritic Reference: Its not Terrestrial! Assumption of 142Nd equality between Nd standard and chondrites used for more than 20 years Weighted mean ± 2s Basaltic eucrites -0.22 ± 0.04 -0.40 ± 0.17 Enstatite -0.17 ± 0.04 Ordinary -0.31 ± 0.03 Carbonaceous Nyquist et al., 1995 Prinzhofer et al., 1992 Literature values Jacobsen and Wasserburg 1984 Jacobsen and Wasserburg 1980 Relative to Terrestrial Nd standard

  7. Chondrites vs. Terrestrial samples • All terrestrial samples have 142Nd excesses relative to chondrites. • A few Isua samples have excess 142Nd compared to all other terrestrial samples(cf. Boyet et al., 2003 and Caro et al, 2003; 2006). Two different explanations : 1. The Earth does not have chondritic relative abundances of refractory lithophile elements. 2. All terrestrial samples derive from a mantle reservoir characterized by a high Sm/Nd ratio formed by an early differentiation event. Boyet and Carlson, submitted

  8. Option 1: Earth ≠ Chondritic? • Sm and Nd refractory lithophile elements • Very small range of Sm/Nd among chondrites (~3%) and basaltic eucrites 4.567 Ga 143Nd/144Nd 0.209 147Sm/144Nd

  9. Implications of the non-chondritic 142Nd/144Nd in terrestrial rocks Prior to continent extraction, the mantle did not have chondritic Sm/Nd. To estimate mass of mantle involved in continent formation, recombine continental crust with an estimate of the composition of the continent-depleted mantle. Increase the mass of depleted mantle until a target Sm/Nd is reached. New estimate suggests that most of the mantle has been depleted by continent formation Previous estimates suggest ~30-40% of the mantle has been depleted by continent formation

  10. Size and Composition of the Reservoirs Composition in trace elements: • Mass balance calculation: • EER = Early Enriched Reservoir • EDR = Early Depleted Reservoir • EDR+EER = Bulk Silicate Earth • The smaller the enriched reservoir, the more enriched in lithophile element it must be. • Note the high concentrations of U, Th (and K) in the D” sized EER

  11. Size and Composition of the Reservoirs Reservoir Mass(1025g) Th(ppb)U(ppb) K(ppm) TW Cont. Crust 2.26 5600 1300 15000 7.3 Enriched=D” 17 920 230 2650 9.3 Enriched>1600km 111 150 40 440 10.4 Primitive (60%) 242 79 20 240 11.7 Early Depleted 290-390 43-53 11-13 ~150 9.5-10.3 Depleted (>D”) 387 15.7 5.4 85 4.7 MORB Mantle 161 7.9 3.2 50 1.1

  12. Schematic Earth’s Mantle Evolution time Formation of different silicate reservoirs Magma ocean crystallization ~4.5+ Ga Two separated reservoirs “Whole mantle convection” Today

  13. Conclusions: • 1. All terrestrial samples come from a depleted reservoir: • +e142Nd, High Sm/Nd compared to chondrites • 2. Different silicate reservoirs form in the first 30 Myr of Earth history. Most of the mantle more depleted in U, Th, K than assumed previously. • 3. No trace of the complementary enriched reservoir • Geochemical modeling better matched for a small reservoir D”? If so, strongly enriched in U, Th, K • 4. Details of reservoir masses have significant consequences for concentrations and distributions of radioactive elements and, hence, heat production • -

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