Lecture 18: Chemical Geodynamics, or Mantle Blobology

1. Lecture 18: Chemical Geodynamics, or Mantle Blobology • Questions • What can geochemistry tell us about the deep interior of the Earth? • Is the mantle homogeneous and if not how many reservoirs are there? How long have they maintained their separate identities? • How do we use radiogenic isotope ratios and trace element ratios in basalts to make such inferences about the mantle? • Reading • Albarède, Chapter 8

2. Summary of Earth Differentiation (nucleosynthesis, mixing) Solar Nebula (volatiles) (gas-solid equilibria) (refractories) Condensation and Accretion (late veneer) (continuing cometary flux?) (siderophile & chalcophile) (melting; gravity and geochemical affinity) (atmophile) (lithophile) (lost due to impacts) Core Silicate Earth Primitive Atmosphere (freezing) (catastrophic impact) Moon Primitive Mantle Inner Core Outer Core (partial melting; liquid-crystal partitioning) (?) degassing Upper Mantle Lower Mantle Continental Crust (plate tectonics: partial melting, recycling) (hotspot plumes) degassing Modern Ocean & Atmosphere Oceanic Crust

3. Geochemistry and Geodynamics • A range of models have been proposed… “Whole mantle convection model” “Standard model” “Strongly Layered model” “Lava lamp model”

4. Geochemistry and Geodynamics • Our only data about the history of the Earth’s structure is derived from geochemical inference, because geophysics only samples the present (exception: paleomag) • However, geochemistry only samples the surface, so inferences about depths within the Earth are indirect, and must be supplemented by geological or geophysical constraints. • In some cases, mantle samples are directly available as xenoliths or peridotite massifs, but mostly the mantle delivers its chemical signals to us in basaltic magmas.

5. Geochemistry and Geodynamics • What information in a basalt can be taken as direct information about the source region? • Not major element composition…partial melting and shallow differentiation both separate major elements from one another in complicated ways • Not trace element concentration…even knowing all the partition coefficients, these are functions of extent and style of melting as well as source composition • Stable isotopes, maybe, if high temperature fractionation is negligible • Ratios of incompatible trace elements…yes. If both elements are sufficiently incompatible that they are quantitatively extracted, then liquid ratio equals source ratio. • Ratios of heavy long-lived isotopes…yes. Arguments based on diffusion strongly suggest that basalts are produced in isotopic equilibrium with their source.

6. Heterogeneity of Oceanic Basalts • Observation: while less diverse than continental rocks, oceanic basalts do display a significant diversity of isotopic compositions in 87Sr/86Sr. • Focus on oceanic basalts because they are uncontaminated by continents. MORB = mid-ocean ridge basalt OIB = ocean island basalt

7. Isotopic Equilibrium and Disequilibrium • So heterogeneous isotopic compositions come out of the mantle. What does this mean about the heterogeneity of the mantle itself? • The essential argument for isotopic equilibrium between source and melt was presented by Hofmann and Hart (1978). Consider two cases: • (1) The mantle is uniform on a regional scale (10-1000 km3) due to efficient mechanical stirring, but not in chemical or isotopic equilibrium on a local (cm) scale due to inefficient diffusion. • In case (1), isotope heterogeneity in erupted basalts might reflect, for example, different degrees of melting if radiogenic Sr accumulates in phlogopite and is contributed to the melt only as phlogopite melts. • (2) The mantle contains regional inhomogeneities that have survived the stirring process for long times, but is isotopically equilibrated by diffusion on a local (100 m?) scale at least during melting. • In case (2), isotope heterogeneity in erupted basalts reflects regional-scale difference in their source compositions only

8. Isotopic Equilibrium and Disequilibrium • Case (1): Regional homogeneity, local disequilibrium • Case (2): Regional heterogeneity, local equilibrium

9. Isotopic Equilibrium and Disequilibrium • Isotope heterogeneity on the meter scale can certainly persist for long times in the solid state • Trace element diffusion coefficients in minerals are ~10–12 cm2/s at 1200 °C • Hence timescale for diffusion across 1 m distances is t ~ L2/D ~ 3 x 108 years • Stirring of viscous fluids stretches and thins heterogeneities but it also takes many millions of years to thin them to diffusive lengths. • Isotope heterogeneity on cm scale probably cannot survive a melting episode • Typical diffusion coefficients in silicate liquids are ~10-7 cm2/s at 1200 °C • Hence typical transport distance by diffusion is 2 cm per year or 200 m in 10 ka • As soon as partial melt fills all the grain boundaries, the distance over which solid-state diffusion must act drops from the scale of heterogeneity to the size of a crystal! • It follows that basalt liquids are expected to have isotope ratios that are faithful copies of their sources averaged over at least several meters

10. Isotopic Equilibrium and Disequilibrium • We can see evidence of this in the comparison of isotopic composition between basalts and associated residual peridotites: • Basalts are more homogeneous and more radiogenic than peridotite suites. Taken to imply that Nd (and Os) from a recycled component was in the source but is not sampled in the residual assemblage. • Consistent with regional heterogeneity, local homogenization

11. Isotopes in Oceanic Basalts • What then is the interpretation of the pattern of Sr isotope heterogeneity among MORB and OIB? • Sr by itself is very hard to interpret…we don’t know bulk earth value because Rb is volatile on accretion • Sr and Pb isotope variations do not correlate in any simple way, which caused much gnashing of teeth 30-40 years ago • It took the introduction of Nd isotope data to begin a real debate between meaningful models • Sm and Nd are refractory, so we know CHUR composition and by inference BSE (this has been challenged lately by 142Nd data) • Sr and Nd isotopes in oceanic rocks do correlate, inversely • MORB and crust are seen to be complementary (recall trace element story from lecture 2), but the meaning of OIB is ambiguous the “mantle array”

12. The Sm-Nd mantle array • The distribution of OIB data between MORB and Bulk Silicate Earth is consistent with at least three models: • The “standard model” -- MORB samples the upper mantle which is complementary to continental crust extraction; OIB samples the lower mantle which is primitive; the mantle array is the result of mixing between depleted and primitive. • Or, different parts of the mantle may have been depleted to various degrees and never homogenized…this would also generate an array of data from depleted to primitive, but with a very different spatial distribution of mantle reservoirs! • Or, there may be no primitive reservoir involved at all, and OIB may be mixtures between depleted MORB mantle and various enriched components like recycled oceanic crust or subducted sediment • There might still be a primitive mantle somewhere, but it might not ever be sampled by volcanism Hofmann and White model

13. Isotopic Mass Balance • Assuming eNd for bulk silicate earth = 0; eNd, [Nd] and mean age of continents; and eNd for upper mantle, can we distinguish standard and whole-mantle models by mass balance? Let’s calculate what volume fraction of the whole mantle must be depleted to balance the continents.

14. Isotopic Mass Balance • For times short compared to the half-life of 147Sm, • Or, in epsilon notation, with initial eNd = 0, • IF: • There are only three reservoirs: c, d, and p (and p is primitive) • We know the Sm/Nd and [Nd] of the continental crust, and the eNd of depleted mantle • THEN we get a relationship between the age T of crust formation and the ratio of the masses of continental crust and depleted mantle: • For T ~ 2.5 Ga (which we get independently from ƒSm/Ndc and eNdc) and the known mass of continental crust, depleted mantle is 0.3 times the mass of the whole mantle. • This fits beautifully with the standard model, since the upper mantle is 1/3 of the mantle. • BUT if there is another large reservoir (stored subducted materials or magma ocean dregs), this messes up the whole calculation. With enough enriched material with eNd > 0 in this reservoir, the entire remainder of the mantle could be depleted. So this is equally consistent with the Hofmann and White model!

15. The Sm-Nd mantle array • How do we choose between these models? For starters, get more data and more isotope systems! • Problem 1 with standard model: with more data, we find that OIB extend beyond primitive mantle (PRIMA) composition, both to higher 87Sr/86Sr and lower eNd. Hence they must contain some enriched material. • Problem 2 with standard model: the array is not consistent with two-component mixing…the width of the trend is way outside analytical error and requires at least two enriched components. • Problem 3 with standard model: the MORB data are spatially organized by ocean, so the upper mantle is not homogenous either • Problem 4 with standard model: add other isotopes and the binary-ish mantle array breaks down altogether

16. The mantle isotope zoo So how many components do you need? For Sr-Nd-Pb-Pb-Pb space, at least four: • DMM = depleted MORB mantle • HIMU = High U/Pb component • EMI = Enriched Mantle I (low Nd) • EMII = Enriched mantle II (high Sr) If 206Pb, 207Pb, 208Pb are not really independent, then four end members to span data in 3-space (Sr-Nd-Pb) is trivial, but the same components also bound data in Hf and Os space.

17. The Worm-o-gram How do the four bounding components mix with one another? Is there evidence of an “internal” component, that everything mixes towards? If so, what is it? Some authors see mixing towards particular locations, and argue that these represent common components with well-defined compositions: FOZO, C, PREMA More on this when we talk about noble gas systems.

18. An oddity • The DUPAL (Dupré and Allègre) anomaly: nearly all the isotopically unusual hotspots are in a well-defined latitude band between 0° and 50°S. • If this has any geodynamic significance, nobody has figured out what it is! • So what are DMM, HIMU, EMI, and EMII? Are they well-defined reservoirs with sensible histories and physical locations in the mantle, or merely arbitrary points in multi-isotope space? • Before we can answer that we need to think more about trace elements, since parent-daughter ratios over time determine the isotope characteristics of the end members. Getting back to geodynamics...

19. Trace Element Ratios • Another kind of tracer of mantle sources should be ratios of incompatible elements in basalts, but one has to be careful to avoid effects of recent fractionation • Two cases that do not work: Sm/Nd and Lu/Hf • Nearly all MORB samples plot above Bulk Earth in Hf and Nd isotopes, meaning their long term Lu/Hf and Sm/Nd ratios have been higher than chondritic. But nearly all MORB samples have subchondritic measured Lu/Hf and Sm/Nd ratios. • It follows that Lu/Hf and Sm/Nd were fractionated recently (by the melting process itself), which turns out to requires garnet in the source (P > 2.5 GPa).

20. Trace Element Ratios • Two that do work, for MORB & OIB melting: Nb/U & Ce/Pb Nb/U and Ce/Pb in oceanic basalts do not correlate with [Nb] and [Ce]. This implies (1) that the ratio in basalt does not depend on extent of melting, and (2) that depleted and enriched sources are equal also, so the ratio in the residue does not get fractionated. Hence either the elements have equal partition coefficients or are both incompatible enough to be totally extracted. But the ratio in MORB and OIB is not chondritic! The continent-forming process did fractionate these element pairs (probably because arc processes involve oxidizing fluids), and crust and mantle are complementary reservoirs. But…OIB do not mix towards primitive value, so there is no evidence here of a primitive reservoir sampled by any basaltic magma!

21. Trace Element Mass Balance • If we know the Nb/U ratio of the primitive mantle, depleted mantle, and continental crust, we should be able to calculate the masses of each of these reservoirs. • UCCXCC + UDMMXDMM = UBSE • NbCCXCC + NbDMMXDMM = NbBSE • XCC + XDMM = 1 -> • (Nb/U)DMM ~ 47, XCC (relative to whole silicate earth) ~ 0.6%, UCC = 0.9-1.3 ppm. • Conclusion: it does not work…something must be missing, because the continental crust appears to be 0.7 to 1.15% of the crust+depleted mantle system. Either there is a hidden reservoir of Nb or U somewhere, or some fraction of the mantle remains primitive and is not sampled by either MORB or OIB. • Possible hidden reservoir is again subducted oceanic crust, perhaps eclogite with rutile to hold a lot of Nb

22. Anomalous fractionations involving continents • Why are some trace element ratios different in continents than in mantle, even though basalt genesis does not fractionate them? Let’s look at Ce/Pb again: • Which is the anomalous element, Ce or Pb? In the spidergram, Pb clearly stands out as high in CC, low everywhere else. • Where do the continents get this signature? Where are continents made? In island arcs.

23. Anomalous fractionation • In this case study of the Aleutians, we know Ce/Pb ratio and Pb isotope composition of the North Pacific sediment and ocean crust being subducted. • In Pb-isotope space, the arc lavas appear to get all their Pb from mixtures of these two components. • But the lavas are not a simple mix of MORB and sediment. The low 207Pb/204Pb component has a lowered Ce/Pb ratio…Pb must be preferentially extracted (relative to Ce) from the subducting basalt (but not from the sediment). • Implication: Pb is mobile in aqueous fluid, leading to low- Ce/Pb arc source and high-Ce/Pb residual slab.

24. Origin of the four mantle components • DMM is easy…it is ambient upper mantle, depleted ~2 Ga ago by extraction of the continents. • However, MORB can be polluted by influence of nearby plumes (Schilling’s effect), so not all MORB plot right at DMM: Isotopic composition of mid-Atlantic ridge samples near the Azores hotspot… Begs questions: How well mixed is DMM reservoir? Is even “pure DMM” recharged with a flux from somewhere? How does the upper mantle stay fertile over time?

25. Origin of the four mantle components • EMII is almost certainly recycled continental material, presumably subducted terrigenous sediment. • Isotopic composition of young pelagic sediment is a pretty good match for EMII isotopes, but not perfect…sediments must be aged for a while. • As we saw, continents (and hence also continent-derived sediments) have very high Pb concentrations. Hence U/Pb is not very high and EMII does not evolve to especially enriched 206Pb/204Pb. But Th/U is high (due to scavenging of Th from seawater), so 208Pb/204Pb increases faster. • Because Sr/Pb and Nd/Pb ratios are lower than in other components, mixing arrays towards EMII should be strongly curved in isotope ratio-ratio space, as observed. • Even though sediment signature is transferred to arc basalts at subduction zones, some sediment or some sediment-derived trace elements must be subducted, to arise elsewhere in OIBs.

26. Origin of the four mantle components • HIMU is usually attributed to subducted, altered ancient oceanic crust. • The preferential extraction of Pb from the basaltic part of slab at subduction zones leaves a high U/Pb residual component, which will evolve to high 206Pb/204Pb with time. • But it is necessary that Rb also be removed relative to Sr during subduction, or HIMU would have wrong 87Sr/86Sr. • Note that HIMU-DMM mixing arrays are linear, which implies Sr/ Pb and Nd/Pb ratios are similar in these end members…a problem? • Other authors think HIMU is a component of metasomatically altered continental lithospheric mantle…no agreement on this. • Some even think HIMU has high U/Pb because of late segregation of Pb into the core...

27. Origin of the four mantle components • EMI is problematic. All kinds of ideas are in play... • It is close to Bulk Earth, except in eNd (but wait…perhaps BSE does not have eNd = 0…). Perhaps it is slightly modified BSE (modified how? Nobody says). • It also resembles lower continental crust, from xenoliths and granulite terranes. Perhaps EMI and EMII are distinguished by intracontinental differentiation, and EMI is recycled by delamination whereas EMII is recycled by erosion and subduction. • H2O-rich and CO2-rich fluids mobilize trace elements differently. It is possible that HIMU and EMI could be complementary products of migration of CO2-rich fluids from continental lithospheric mantle into lower continental crust. • Another issue to revisit after we talk noble gases.

28. The Upper Mantle as an Open System • For Pb, we can prove that there is continuing input to the upper mantle-ocean crust system from some other long-lived reservoir, probably the lower mantle. If this input balances Pb flux to continents at arcs, the upper mantle might be in steady-state for incompatible elements. • This argument is based on Th/U ratios: the continental crust has a chondritic Th/U ratio (3.9), but the MORB source has a much lower Th/U ratio (2.5). • If input to upper mantle is chondritic in Th/U, and output to continents is chondritic, upper mantle could be in steady state, even with a different Th/U ratio, but this requires a short residence time of Pb in upper mantle.

29. Th/U ratios, Th isotopes and Pb isotopes • Trying to match up Th/U ratio and 208Pb/206Pb composition of MORBs is a different exercise from the Sm/Nd and Lu/Hf problem presented above, because we can correct accurately for effects of melting and there is still a discordance. • For a source in secular equilibrium, the activity of 230Th is equal to that of 238U. Hence the (232Th/230Th) activity ratio is a measure of the Th/U ratio of the source: • Since MORB has a Th excess due to melting processes, the measured value is an upper limit for the Th/U of the source. Data: • Mid-Atlantic Ridge kTh= 2.5 • East Pacific Rise kTh = 2.5±0.2 • Hawaii and Iceland kTh = 3.0 • Tristan da Cunha kTh = 3.7 • Here is a trace-element ratio indicator in which hotspots are closer to primitive!

30. Th/U ratios, Th isotopes and Pb isotopes • The long-term history of Th/U in a source, on the other hand, is determined from the Pb isotopes (where T is the age of the Earth): • Data: • Mid-Atlantic Ridge kPb= 3.78±0.07 • East Pacific Rise kPb = 3.73±0.06 • Indian Ridges kPb = 3.89±0.11 • Hawaii and Iceland kPb = 3.83±0.04 • Tristan da Cunha kPb = 4.17 • Some hotspots have long-term Th/U higher than chondritic! • SO…the maximum present day Th/U of the MORB source (from Th isotopes) is much less than the long-term Th/U average reflected in the Pb isotopes of the same source, and this is not a recent melting effect. • The Pb in MORB cannot have been in a low Th/U reservoir for more than 600 Ma…this is a maximum for residence time of Pb in the upper mantle.

31. Th/U ratios, Th isotopes and Pb isotopes • Where is the reservoir from which Pb is input to the upper mantle? • Upper continental crust has chondritic Th/U and can be recycled by erosion (hence EMII flavored hotspots), but it has the wrong 207Pb/204Pb ratio, since its U was fractionated from Pb more than 1 Ga ago when 235U was more abundant. • Continental lithospheric mantle might be the reservoir, but this would require its entire mass to exchange with the upper mantle every few hundred Ma, which is inconsistent with the long-term stability of cratonic lithosphere. • That leaves only the lower mantle, which is so big and Pb-rich that over geologic time only half the mass of the upper mantle would have to be replaced by lower mantle to give the necessary flux (~10% per Ga). • Bottom line: the more incompatible the element, the shorter its residence time in the upper mantle-oceanic crust system (~200 Ma for the perfectly incompatible element) • Hence DMM is roughly equal to BSE in Pb isotopes (which are replaced much faster than 238U decay), but quite different in Nd and Sr isotopes, since these elements are more compatible (especially in arcs). • For the most incompatible elements the global system has evolved to a steady-state where output to the continents is balanced by input from lower mantle. • Convective isolation (layering?) is necessary to explain long-term evolution of components, but it cannot be perfect…it must be leaky.