The Earth II: The Core; Mantle Reservoirs

The Earth II:The Core; Mantle

ReservoirsLecture 46

Composition of the Core• In the case of the Earth’s core, we have only two

types of constraints:• Geophysical:

o density and seismic velocity derived from seismology and moment of inertia. Also must generate a geomagnetic field.

o Density suggests a material 5-10% less dense than Fe.• Cosmochemical:

o What materials of appropriate density are available in sufficient abundance to constitute 1/3 the mass of the Earth?

o Iron meteorites provide a compositional model of the core.o Again we turn to a chondritic model: we infer that siderophile elements

missing from the silicate Earth are in the core.o For refractory siderophile elements, they should be in chondritic relative

proportions.o For non-refractory siderophiles, the volatility trend provides a means of

estimating composition.

Volatility Trend

Composition of the Core

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).

Mantle Geochemical Reservoirs & Evolution

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.o 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.

Sr-Nd Mantle Array

Nd-Pb Isotope Systematics

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 εNd and εHf ratios 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.

How Much DUM?

• Suppose we consider the Earth as consisting of three reservoirs:o Primitive mantleo Continental crusto 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 εNd and Nd concentrations of the other 2 reservoirs and their masses.o 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.

OIB Reservoirs• The OIB are

more diverse.• They can be

divided into 4 main groups:o St. Helena (HIMU)o Kerguelen (EM I)o Society (EM II)o Hawaii (PREMA)

• This suggests several distinct (chemical) evolutionary pathways.

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.

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).

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.

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).