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Signatures of Early Earth Differentiation in the Deep Mantle?

Signatures of Early Earth Differentiation in the Deep Mantle?. Richard W. Carlson Department of Terrestrial Magnetism Carnegie Institution of Washington. COMPRES, June 15, 2011. Continental Crust Formation has Caused Chemical Differentiation of the Mantle. Mass Fraction. 0.45%.

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Signatures of Early Earth Differentiation in the Deep Mantle?

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  1. Signatures of Early Earth Differentiation in the Deep Mantle? Richard W. Carlson Department of Terrestrial Magnetism Carnegie Institution of Washington COMPRES, June 15, 2011

  2. Continental Crust Formation has Caused Chemical Differentiation of the Mantle Mass Fraction 0.45% Sample/Bulk-Silicate-Earth 30-70% 70-30% WHEN DID THIS SEPARATION OCCUR? Sm-Nd model ages for MORB = 200 - 2000 Ma Pb-Pb model age for oceanic basalts ~1800 Ma “Average” continental crust Sm-Nd model age ~2000 Ma

  3. LLSVPs: A Remnant of Early Differentiation or Modern Subduction? Garnero and McNamara, 2008

  4. Some Meteorites are Compositionally Similar to the Sun.These Serve as a Starting Point for Estimating Bulk Earth Composition, but how well is the Chondrite Model Matched by Real Earth Rocks? N C In? For most elements, CI chondrites provide a good approximation of solar composition Li Solar and CI compositions from Palme and O’Neill, Treatise on Geochemistry, 2003

  5. The Bulk Earth is NOT CI Chondritic: Volatile Depletion is a Characteristic of Many Solar System Objects, Including Earth From McDonough TOG, 2003 CI-normalized terrestrial volatile element abundances decrease with decreasing condensation temperature. Same pattern, though less extreme, is seen in “primitive” meteorites. Volatile depletion of Earth may be a “pre-accretion” phenomena

  6. Dating Early Earth Differentiation Actively-used short-lived radioactive isotopes Condensation – Volatile Loss: Al-Mg, Mn-Cr, Pd-Ag, I-Xe Metal – Silicate Separation: Fe-Ni, Pd-Ag, Hf-W Silicate Differentiation: Al-Mg, Fe-Ni, Mn-Cr, Hf-W, Sm-Nd

  7. The Bulk Earth is NOT CI Chondritic: Volatile Depletion is a Characteristic of Many Solar System Objects, Including Earth From McDonough TOG, 2003 CI-normalized terrestrial volatile element abundances decrease with decreasing condensation temperature. Same pattern, though less extreme, is seen in “primitive” meteorites. Volatile depletion of Earth may be a “pre-accretion” phenomena

  8. Earth Formed Volatile DepletedChondrite Mn/Cr variation correlates with 53Cr/52Cr. Earth has a lower 53Cr/52Cr than almost all chondrites. Mn more volatile than Cr. Earth’s volatile depletion occurred while 53Mn was alive (t1/2 = 3.7 Ma) Earth From Qin et al., GCA 2010

  9. Earth’s Mantle is Depleted in Siderophile Elements Palme and O’Neil, TOG, 2003

  10. Reconciling Mn-Cr, Pd-Ag, and Hf-W Constraints on the Timescale of Earth Volatile-Depletion and Core Formation 26 Myr accretion of volatile-poor material (86% of Earth mass) 4% CI added at 26 Myr (Adds another 9% of Earth Mass) Schonbachler et al., Science 2010

  11. Refractory Lithophile Elements SHOULD be Present in the BSE in Chondritic Relative Abundances, but Often They are Not “Fertile” mantle xenoliths (from Palme and O’Neill, TOG, 2004, after Jagoutz et al., 1979)

  12. 146,147Sm-142,143Nd Systematics Short-lived chronometer: 146Sm 142Nd (T1/2= 103 Ma) 146Sm exists only in the first 500 Ma of Solar System history Coupled to the long-lived chronometer: 147Sm 143Nd (T1/2 = 106 Ga) 147Sm abundance decreased by only 3% in 4.56 Ga Zircon 4.4 Ga Isua 3.8 Ga

  13. 142Nd Variation in Earth Materials Limited and Restricted Only to Rocks Older than 3.5 Ga 142Nd excesses measured in 3.8 Ga samples from SW Greenland and Anshan, China (up to 0.15e). 142Nd deficiencies in Nuvvuagittuq, Quebec, Canada • Evidence for early • differentiation, but not all old rocks show this • No heterogeneities preserved after 3.5 Ga in the convecting Earth’s mantle External Precision

  14. Is “Terrestrial” 142Nd/144Nd Chondritic? – No! • 142Nd/144Nd ratios measured in carbonaceous, ordinary and some enstatite chondrites, and eucrites, are lower than laboratory standard and terrestrial samples • Excess 142Nd in Earth rocks indicative of higher than chondritic Sm/Nd ratio while 146Sm was still extant. Open symbols show data from Nyquist et al., 1995; Andreasen and Sharma, 2006; Rankenburg et al., 2006. Closed symbols are data from Boyet and Carlson, 2005; Carlson et al., 2007.

  15. chondritic evolution Constraints on the Timing of Earth Differentiation 5 Ma, 147Sm/144Nd=0.209 30 Ma, 147Sm/144Nd=0.212 60 Ma, 147Sm/144Nd=0.216 100 Ma, 147Sm/144Nd=0.222 Mid-ocean ridge basalts Archean samples Differentiation event occurred during the first <30 Ma of Earth history

  16. Predicted Parental Mantle Reservoir from 142Nd Overlaps with high 3He/4He Reservoir (Ra) Reservoir parental to terrestrial mantle “primordial” chondrite reservoir

  17. The Broader Trace Element Characteristics of this Ancient Depleted Source Jackson et al., Nature 2010

  18. Similar Normalized Incompatible Element Patterns Found for Other Major Flood Basalts, in this case, Ontong-Java Basalts The flat primitive-mantle-normalized patterns defined by alteration-resistant incompatible elements in the Kwaimbaita- and Kroenke-type basalts (see Fitton & Godard, 2004) point to a mantle source not too different from estimated primitive mantle in most of its inter-element ratios. However, the observed isotopic values (e.g., eNd(t) ~ +6) are clearly far-removed from those estimated for primitive mantle (eNd = 0). (Tejada and Mahoney, MantlePlumes.org) 10 1 10 1 All these samples have e143Nd between +4 and +7

  19. Size and Composition of the Reservoirs. So What? 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 MORB Mantle 161 7.9 3.2 50 1.1

  20. Two Ways to Create an EDR – EER Pair Magma Ocean Overturn Shallow Differentiation Basal Magma Ocean (Labrosse et al., Nature 2007)

  21. How Did the Non-Chondritic Mantle Form? Melting is the easiest way to fractionate the lithophile elements, but what were the conditions of melting? Corgne et al., 2005 – 25 GPa

  22. Signatures of Early Earth Differentiation in the Deep Mantle? • Earth accreted first, and mostly, from volatile-depleted material • Core formation occurred while the accreting material shifted from volatile-poor (reduced?) to volatile-rich (oxidized) • First ~85% of Earth’s mass mostly volatile-poor • What has been called “primitive” mantle is in fact incompatible element depleted • Earth is non-chondritic in refractory lithophile element abundances? • Signature of an early differentiation event? • Deep fractionation of perovskite or subduction of a shallow “KREEP” crust? • Only the depleted reservoir is sampled at Earth’s surface – the complementary enriched reservoir must be buried in the deep mantle – LLSVPs?

  23. If core formation were simple 33 ± 2 Ma after Solar System formation or 4.534 Ga When Did Earth’s Core Form? If Earth grew slowly and involved many “accumulation events”, then the answer depends on the details of Earth accumulation Parts in 10,000 Parts in 10,000 182Hf  182W (t1/2 = 9 Ma) Chondrite Hf/W = 1 Metal Hf/W = 0 Mantle Hf/W = 10

  24. Pd-Ag Core Formation Timescale Too Fast for Hf-W! Accrete volatile-rich material first, but this violates Mn-Cr 107Pd  107Ag (t1/2 = 6.5 Myr) Pd/Ag CI = 3 Pd/Ag Earth = 13 Pd/Ag Core > 400 Pd/Ag Mantle = 0.5 Dashed curves are for accumulation of material as volatile-depleted as Earth today (Pd/Ag = 12.9). Solid curves are for accumulation of CV3 chondrites (Pd/Ag = 8.5). Numbers along the curves give the mantle Pd/Ag ratio after core formation. If Earth accumulated from volatile-rich material, then Pd-Ag offers no constraints on the timing of core formation. (From Schonbachler et al., Science 2010)

  25. The Importance of that Last 1% Earth = 6 x 1024 kg Ocean = 1.4 x 1021 kg CI Chondrite = 18 wt% H2O 1% Earth Mass of CI Chondrite contains 1021 kg water

  26. Early Earth 142Nd/144Nd and 143Nd/144Nd Evolution The +15 ppm 142Nd/144Nd of the SW Greenland Archean rocks require a 147Sm/144Nd > 0.225. The reduction in 142Nd/144Nd between 3.9 and 3.5 Ga requires mixing between high- and low-Sm/Nd reservoirs formed within tens of Ma of Earth formation. 142Nd/144Nd in Archean Mantle-Derived Rocks Initial 143Nd in Mantle-Derived Rocks

  27. r-, s-process Variability Explains at Least some of the 142Nd/144Nd Range Between C- and O-, E-Chondrites, but not the Earth-Chondrite Offset

  28. Crust Formation (with its characteristic LREE enrichment) Started Early Example – the 4.3 Ga Nuvvuagittuq Terrane, Quebec, Canada 3.8 Ga 4.0 Ga O’Neil et al., Science, 2008

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