The Earth II: The Core; Mantle Reservoirs

1. The Earth II:The Core; Mantle Reservoirs Lecture 46

3. Composition of the Core • In the case of the Earth’s core, we have only two types of constraints: • Geophysical: • density and seismic velocity derived from seismology and moment of inertia. Also must generate a geomagnetic field. • Density suggests a material 5-10% less dense than Fe. • Cosmochemical: • What materials of appropriate density are available in sufficient abundance to constitute 1/3 the mass of the Earth? • Iron meteorites provide a compositional model of the core. • Again we turn to a chondritic model: we infer that siderophile elements missing from the silicate Earth are in the core. • For refractory siderophile elements, they should be in chondritic relative proportions. • For non-refractory siderophiles, the volatility trend provides a means of estimating composition.

4. Volatility Trend

5. Composition of the Core

6. Understanding Core Formation • Metal/silicate partition coefficients depend on pressure and oxygen fugacity. • Today, the core–mantle boundary, is at 135 GPa and 3000–4000 K. • Experiments suggest metal silicate equilibration took place at lower pressure (as in planetesimals, there were the building blocks of Earth).

7. Mantle Geochemical Reservoirs & Evolution

8. Mantle Reservoirs • We previously looked at the composition of the silicate Earth (BSE). This composition is also known as ‘primitive mantle’ (mantle after core segregation, but before crust formation). • In reality, the mantle is processed, heterogeneous, and no known sample of mantle matches exactly the ‘primitive mantle’ composition. • Isotope ratios of basalts (particularly oceanic ones) provide views of the time-integrated composition of their sources. • Basalts are useful because they are common and because their production involved larger regions (>100 km3) of mantle. Elemental compositions are changed during melting, but isotope ratios are not. • Isotope ratios shows a fundamental two-fold division of basalts: MORB and OIB.

9. Sr-Nd Mantle Array

10. Nd-Pb Isotope Systematics

11. MORB & the Depleted Mantle • Seafloor spreading creates 3 km2 new area of ocean floor each year (an equal area is subducted) and ~20 km3 of mid-ocean ridge basalt (MORB) forms to fill the gap. They are the most voluminous magmas on the planet. • Compare to others, they have uniform tholeiitic (richer in Si, poorer in alkalis than alkali basalt) and are relatively poor in compatible elements. • They have low 87Sr/86Sr and 206Pb/204Pb and high εNdand εHfratios implying low time-integrated Rb/Sr, U/Pb, Nd/Sm, and Hf/Lu ratios - that is low values of ratios of more-to-less incompatible element ratios. • They provide evidence of a voluminous (incompatible element-) depleted upper mantle (DUM) or DMM (depleted MORB mantle). • The origin of the this DUM is most readily explained by removal of an (incompatible element-rich) melt that has formed the continental crust.

12. How Much DUM? • Suppose we consider the Earth as consisting of three reservoirs: • Primitive mantle • Continental crust • Depleted Mantle • We write a series of mass balance equations that allow us to solve for the fractional mass of depleted mantle, assuming we know the εNdand Nd concentrations of the other 2 reservoirs and their masses. • We don’t necessarily know the εNd of continental crust, but we do know its Sm/Nd ratio and can guess at its age. • We can solve for the relative mass of depleted mantle. • Likely answer is ~30% if BSE εNd = 0 (chondritic) but 40-100% if the Earth has εNd = 3-7, consistent with collisional erosion. • Bottom line: at a minimum, melt has been extracted from a lot (~30%) or perhaps most of the mantle to form the continental crust. • If substantial volumes of primitive mantle remain, we see little direct evidence of it.

13. OIB Reservoirs • The OIB are more diverse. • They can be divided into 4 main groups: • St. Helena (HIMU) • Kerguelen (EM I) • Society (EM II) • Hawaii (PREMA) • This suggests several distinct (chemical) evolutionary pathways.

14. Primitive Mantle • Convergence of OIB arrays at Zindler & Hart’s PREMA (prevalent mantle) together with the observations that the highest 3He/4He ratios occur in basalts with εNd of 3-7 suggests primitive mantle might be a background component of many OIB sources, provided primitive mantle has εNd of 3-7 as collisional erosion (or the EER hypothesis) predicts.

15. Mantle Plumes • Oceanic island volcanoes (e.g., Hawaii, Iceland, Azores) are widely (but not universally) thought to be products of mantle plumes - columns of hot (but solid) rock rising convectively from the deep mantle (perhaps from the core-mantle boundary driven by heat from the core). • Although still a bit controversial, seismic evidence is increasingly consistent with this. • Thus OIB sample deep mantle reservoirs (reservoir(s) could be small: D’’ a candidate).

16. Evolution of OIB reservoirs • Many OIB have 87Sr/86Sr greater and εNd lower than BSE. This requires something other than melt extraction. • Incompatible element pattern consistent with melt enrichment. • Although plumes come from the deep mantle, incompatible element patterns suggest upper mantle processes (deep mantle melts have very different incompatible element patterns). • Thus although they come from the deep mantle, their chemistry bears the signature of upper mantle processing. • Slope on 207Pb/204Pb-206Pb/204Pb plots suggest heterogeneity is ancient, but not as old as the Earth itself. Extended rare earth or “spider” diagram, in which the BSE-normalized abundances are plots and elements are ordered by incompatiblility.

17. Mantle Plumes from Ancient Oceanic Crust • Hofmann and White (1982) proposed that the distinct composition of OIB sources (mantle plumes) comes from oceanic crust (+continent derived sediment) subducted into the deep mantle. • This material is heated (by the core) and eventually rises to the surface as mantle plumes (finally melting in the upper most 100-200 km).