Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman Outline: 1. Concepts of trace element and isotope geochemistry for the Earth’s mantle 2. Derivation of upper mantle’s composition 3. Some updates 4. Composition of uppermost 100km

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

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

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

Olivine (Mg,Fe) 2 SiO 4 Orthopyroxene (Mg,Fe)SiO 3 Melt Partial Melting Leads to Trace Element Partitioning Wark et al. (2003) U, Th and K all prefer the melt phase With Melt/Residue ~ 1000

Bulk Silicate Earth (Mantle before any crust was formed) Increasing Compatibility in Solid Residue Partial Melting Leads to Trace Element Partitioning Element Concentrations Normalized to Bulk Silicate Earth

Mantle melt (Ocean Crust) Partial Melting Leads to Trace Element Partitioning Element Concentrations Normalized to Bulk Silicate Earth Increasing Compatibility in Solid Residue

Mantle melt (Ocean Crust) Mantle residue after melt removal Partial Melting Leads to Trace Element Partitioning Element Concentrations Normalized to Bulk Silicate Earth Increasing Compatibility in Solid Residue

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

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

Elemental Abundances in Modern Ocean Crust Element Concentrations Normalized to Bulk Silicate Earth

Elemental Abundances in Modern Ocean Crust Element Concentrations Normalized to Bulk Silicate Earth

**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

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)

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

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) 1. Abyssal Peridotites - samples of mantle with melt removed Element Concentrations Normalized to Bulk Silicate Earth

Bulk Silicate Earth (BSE) McDonough & Sun (1995) Increasing Amount of Melt Removal 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!

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

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

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

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

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

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

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

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

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

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.

146 Sm --> 142 Nd t 1/2 = 103 My 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. Boyet and Carlson (2006)

Using a similar approach as I showed, they get: U = 5.4 ppb Th = 16 ppb K = 68.4 ppm (About x 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)??

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) 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) Assume DMM composition (average of W&H, 2005 and B&C, 2006) Water content is set 120 ppm Water content is set 120 ppm 1. Range of water = 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% H 2 O at 8 wt% MgO Find the potential temperature needed to make oceanic crust Find the potential temperature needed to make oceanic crust

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

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

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

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

CONCLUSIONS

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

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

87 Rb 87Sr t 1/2 = 49 Byr

Mineralogy and Buoyancy of the Lithosphere 0 Ma 50 Ma Solidus