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Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman

Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman. Outline: Concepts of trace element and isotope geochemistry for the Earth’s mantle Derivation of upper mantle’s composition Some updates Composition of uppermost 100km. Mid-Ocean Ridge Spreading Center :.

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Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman

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  1. Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman Outline: Concepts of trace element and isotope geochemistry for the Earth’s mantle Derivation of upper mantle’s composition Some updates Composition of uppermost 100km

  2. Mid-Ocean Ridge Spreading Center : Mantle Melting and Production of Crust Removes U and Th from the Mantle ~100 km deep

  3. Underwater Basaltic Eruption, Hawaii “Pele Meets the sea” by Pyle et al. (1990), Lava video productions

  4. U, Th and K also removed by continental crust formation Depleted Mantle upwelling beneath ridges

  5. Partial Melting Leads to Trace Element Partitioning Olivine (Mg,Fe)2SiO4 Melt U, Th and K all prefer the melt phase With Melt/Residue ~ 1000 Wark et al. (2003) Orthopyroxene (Mg,Fe)SiO3

  6. Partial Melting Leads to Trace Element Partitioning 100 10 1 0.1 0.01 Element Concentrations Normalized to Bulk Silicate Earth Bulk Silicate Earth (Mantle before any crust was formed) Increasing Compatibility in Solid Residue

  7. Partial Melting Leads to Trace Element Partitioning 100 10 1 0.1 0.01 Mantle melt (Ocean Crust) Element Concentrations Normalized to Bulk Silicate Earth Increasing Compatibility in Solid Residue

  8. Partial Melting Leads to Trace Element Partitioning 100 10 1 0.1 0.01 Mantle melt (Ocean Crust) Element Concentrations Normalized to Bulk Silicate Earth Mantle residue after melt removal Increasing Compatibility in Solid Residue

  9. If trace element fractionation happened a long time ago… 87Rb 87Sr 86Sr is not radiogenic

  10. Isotopic Compositions of Mid-Ocean-Ridge Basalts Ancient depletion of upper mantle

  11. Elemental Abundances in Modern Ocean Crust 100 10 1 0.1 0.01 Element Concentrations Normalized to Bulk Silicate Earth

  12. Elemental Abundances in Modern Ocean Crust 100 10 1 0.1 0.01 Element Concentrations Normalized to Bulk Silicate Earth

  13. **Upper mantle has ~3% mafic melt removal - a big effect for incompatible trace elements (like Th, U and K). **Seismic properties, based on major element chemistry, don’t change much from small degrees of melt extraction. Calculated by L. Stixrude

  14. Constraints on the Trace Element Composition of DMM 1. Abyssal Peridotites = Define trends of melt depletion for the upper mantle (same assumptions as McDonough and Sun (1995) 2. Isotopic composition of Mid-Ocean Ridge Basalts = Parent/daughter ratios in DMM (Rb/Sr, Sm/Nd, U/Pb, Th/Pb, Lu/Hf) Requires 1 more assumption than BSE calculation 3. Canonical Ratios = Some trace element ratios are nearly constant in MORBs and assumed to be the same in the MORB source (Ce/Pb, Nb/Ta, Nb/U, Ba/Rb) Workman and Hart (2005)

  15. 1. Abyssal Peridotites - samples of mantle with melt removed From Henry Dick

  16. 1. Abyssal Peridotites - samples of mantle with melt removed Element Concentrations Normalized to Bulk Silicate Earth Data from: Dick (1984), Dick (1989), Johnson et al. (1990), Johnson & Dick (1992), Dick & Natland (1996), Salters & Dick (2002), Hellebrand et al. (2002), Tartorotti et al. (2002)

  17. 1. Abyssal Peridotites - samples of mantle with melt removed ** Slope is a function of relative partitioning of the two elements. ** Where is modern upper mantle on this trend? ** Use Sm-Nd isotope system to plot position of upper mantle…BUT need to know information about the AGE of mantle depletion! Bulk Silicate Earth (BSE) McDonough & Sun (1995) Increasing Amount of Melt Removal

  18. The only solid material we know has definitely been extracted from the mantle and STAYED extracted from the mantle is the continental crust. Depleted Mantle upwelling beneath ridges

  19. Continental Growth Models ---> identify age (i.e. history) of mantle depletion **A consensus is merging toward the middle

  20. 2. Isotopic composition of Oceanic Crust Melt is continually removed from the upper mantle through time, starting at 3 Ga Sm/Nd = 0.411 (Calculated) Present day Nd Isotopic value (Observed)

  21. Defining a unique position on the mantle depletion trends BSE Sm/Nd = 0.411

  22. 3. “Canonical” ratios Some trace elements don’t fractionate from each other! So ratio in melt equals ratio in residue Spreading Center Lavas PETDB Database

  23. Composing Trace Element Composition of Upper Mantle Abyssal Peridotite Constraints Element Concentrations Normalized to Bulk Silicate Earth

  24. Composing Trace Element Composition of Upper Mantle Parent/Daughter Constraints Element Concentrations Normalized to Bulk Silicate Earth

  25. Composing Trace Element Composition of Upper Mantle Cannonical Ratios Constraints Element Concentrations Normalized to Bulk Silicate Earth

  26. Composing Trace Element Composition of Upper Mantle Connecting the Dots… Element Concentrations Normalized to Bulk Silicate Earth -- Internally consistent model (error for many elements is 1-5%) -- Is it accurate?

  27. So how much U, Th and K is that? U = 3.2 ± 0.5 ppb (16% of the BSE value) Th = 7.9 ± 1.0 ppb (10% of the BSE value) K = 50 ppm (20% of the BSE value) Workman and Hart likely gives minimum values.... New information is coming out to suggest this.

  28. New evidence from a short lived isotope informs us about the Early Earth (>4 billion years ago)… Shows that a crust was formed early in earth history, creating a very old depleted mantle. 146Sm --> 142Nd t1/2 = 103 My Boyet and Carlson (2006)

  29. Using a similar approach as I showed, they get: U = 5.4 ppb Th = 16 ppb K = 68.4 ppm (About 1.4 - 2x higher than our previous estimate) These numbers are only valid for the modern UNMELTED upper mantle What about the upper ~100km that has melt removed (and hence much to all of the U and Th removed)??

  30. Estimating the Compositional Structure of Oceanic Lithosphere • Use the pHMELTS model: most recent iteration of a thermodynamic model for phase equilibria (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998; Ghiorso et al., 2002, Asimow and Langmuir, 2003; Asimow et al., 2004) • Assume DMM composition (average of W&H, 2005 and B&C, 2006) • Water content is set 120 ppm 1. Range of water = 70-200 ppm (Michael, 1988; Michael et al., 1995; Danyushevsky et al., 2000; Saal et al., 2002; Workman and Hart, 2005) 2. Water content that generates a MORB with 0.2 wt% H2O at 8 wt% MgO • Find the potential temperature needed to make oceanic crust

  31. What is the Potential Temperature of the Mantle? pHMELTS model runs + Error is ±50° Roughly 1km for every 25 degrees

  32. Effect of water on the mantle’s melting temperature A B Hirth and Kohlstedt (1996) Recent iteration by Asimow and Langmuir (2003)

  33. Melt Extraction from Upper Mantle U and Th (ppb), K (ppm) crust crust F = 0.5% Dry solidus 120 ppm H2O

  34. crust U and Th (ppb) K (ppm) Sediments Altered Ocean Crust Depth (km) Unaltered Ocean Crust, 56 ppb ? How deep? At least ~500 km. Maybe higher U, Th, K at depth…PM values?

  35. CONCLUSIONS

  36. Peridotites = Residues of DMM Melting Fractional Melting: (Sobolev & Shimizu, 1993; Johnson et al., 1990; Johnson and Dick, 1992) Reconstituted peridotites: (No plag peridotites)

  37. Peridotites = Residues of DMM Melting Primitive Upper Mantle (PUM) McDonough & Sun (1995) Linearized relationship between two elements, A & B, in a residue of fractional melting: Where slope, R Depletion By Fractional Melting

  38. 87 Rb 87Sr t1/2 = 49 Byr

  39. Mineralogy and Buoyancy of the Lithosphere 50 Ma 0 Ma Solidus

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