SIO 224

Models for bulk Earth, crust, mantle, and core composition

Composition of Earth cannot be understood in isolation• Earth formation closely linked to sun and meteorite formation• Nucleosynthesis in stars, from mainly H + He to other elements

Earth composition continued…..

Post-accretional chemical planetary processes• shift from low-P to high-P processes on planets• element segregation - grouping of elements, from

cosmochemical to geochemical

Earth composition continued….. lithophile elements (oxygen, oxides, silicate minerals,

Greek lithos - stone) chacophile (sulphides, Greek khalkos=copper) siderophile (metallic, Greek sideros=iron)

Earth composition composition…..Goldschmidt’s classification is based on distribution inmeteorites and Earth’s major geochemical reservoirs, but elements can still be further grouped based on their relative behavior in the Earth’s silicate portion, mantle and crust

Relative abundances:• of the >100 known elements, only 90 occur naturally on Earth• only 14 elements make up > 99% of the naturally occurring

inorganic chemical compounds (minerals) H, C, O, Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn, and Fe O, Mg, Si, Fe, Al, and Ca make up > 99% of the BSE

Normal igneous rock composition:Major element > 1.0 wt. % of the rock or mineralMinor element 0.1 - 1.0 wt. %Trace element <0.1 wt. % (<1,000 ppm)

Core composition:

The Earth’s mantle:

How do we know the composition & mineralogy of the mantle?

• Cosmochemical constraints• Geophysical constraints• Experimental & theoretical constraints• Direct samples of the mantle - basalts - crystalline samples * alpine/orogenic

peridotite* abyssal peridotite* ophiolite* nodules/xenoliths* xenoliths in/& kimberlite/lamproite

COSMOCHEMICAL

PREVIOUSLY…..

The six most abundant, nonvolatile rock-forming elements in the Sun are Si (100), Mg (104), Fe (86), S (43), Al (8.4), and Ca (6.2). As the mantle of the Earth contains neither metal nor significant amounts of Fe3+, the sum of all oxides (by weight) must add up to 100%:

MgO + SiO2 + Al2O3 + CaO + FeO = 100% (1)

By inserting into Equation (1) average solar system abundance ratios, e.g., Si/Mg, Ca/Mg, and Al/Mg, one obtains

2:62 x MgO + FeO = 100% (2)

Considering that iron is distributed between mantle and core, the mass balance for iron can be written as

Fecore x 0.325 + Femantle x 0.675 = Fetotal (3)

and similarly for magnesium, assuming a magnesium-free core,Mgmantle x 0.675 = Mgtotal (4)

By assuming that sulfur is quantitatively contained in the core and accounting for nickel in the core (Fetotal/Ni = 17), the amount of iron in the core, Fecore, is calculated to be 75%. From Equations (2) to (4) and by using the solar abundance ratio for Fetotal/Mgtotal, the hypothetical composition of the Earth’s mantle is obtained as:

Earth’s mantle solar model

MgO 35.8 SiO2 51.2 FeO 6.3 Al2O3 3.7 CaO 3.0

Common mantle minerals:• Olivine• Orthoppyroxene• Clinopyroxene• Spinel• Garnet

DIRECT SAMPLES (Peridotite)

Mineralogy of the source

Pyrolite: hypothetical mixture of of residualmantle material(xenolith) + primitivebasaltic magma

Note:

All peridotites are metamorphic rocks that have had complex subsolidus history after melt extraction ceased - strain, crystal segregation, deformation, metasomatism, etc. Thus peridotites show compositional variations, particularly in their trace element contents. Nevertheless, they show definite and coherent trends - the least-depleted peridotites (lowest MgO, but highest CaO, Al2O3 and other incompatible trace elements that partitioninto the liquid phase during partial melting (i.e., fertile) plot closest to the composition of the primitive mantle (PM).

Trace element content of the PM has also been estimated basically following similar assumptions and arguments used for the majors.

HSE (Os, Ir, Pt, Ru, Rh, Pd, Re, Au) are low in the Earth’s mantle,but not low enough as expected - hence the “late veneer” hypothesis..

Mantle samples

Composition of the mantle of the Earth assuming average solar system element ratios for the whole Earth versus PM mantle compositions

Ref. solar model (1) (2) (3) (4) (5) (6) (7) (8)MgO 35.8 36.77 38.1 38.3 36.8 35.5 37.8 37.8 37.77Al2O3 3.7 4.49 3.3 4.0 4.1 4.8 4.06 4.4 4.09SiO2 51.2 45.40 45.1 45.1 45.6 46.2 46.0 45.0 46.12CaO 3.0 3.65 3.1 3.5 3.5 4.4 3.27 3.5 3.23FeOt 6.3 8.10 8.0 7.8 7.5 7.7 8.1 7.49Total 100 98.41 97.6 98.7 97.5 98.6 98.8 98.7

Mg#, molar Mg/(Mg+Fe); FeOt, all Fe as FeO; (RLE/Mg)N, refractory lithophile elements normalized to Mg- and CI-chondrites. References: (1) Palme & ONeil’04 (2) Ringwood’79 = “pyrolite” model (3) Jagoutz et al.’79 (4) Wa¨nke et al.’84 (5) Palme & Nickel’85 (6) Hart & Zindler’86 (7) McDonough & Sun’95 (8) Alle`gre et al.’95

Pressure increases with depth:P = gh; for the upper few hundred km, = 3.3 g/cc = 0.33 kbar h, where h is in km.

GEOPHYSICAL & EXPERIMENTAL CONSTRAINTS

Is the mantle compositionally layered or not………

Oceanic basalts as probes of the upper mantle

Adiabatic decompression partial melting of the mantle:

Intraplate magmatism: linear island chains & LIPs

Partial melting of the mantle (intraplate setting):

Typical normalized elementpatterns of terrestrialigneous rocks

Convergent margin magmatism

Partial Melting of the mantle (subduction zone setting):

Major elements of island arc volcanic rocks & magma series

IAB : MORB:MgO < (Mg#)<K2O >Al2O3 >(although variable &some C-A basaltshave 17-20% (high-Albasalts), some believethat these are parentalto C-A series rocks)

Trace elements of island arc volcanic rocks & magma series(continued)

Note enrichment in LIL and depletion in HFS

Several potential source components for island arc magmas

Continental crust

From Rudnick & Gao, 2005

Continents (early studies):•an average intermediate or andesitic composition•only 0.6% by mass of BSE, but 20-70% of incomp. elements•contains the oldest rocks (4.0 Ga Acasta gneiss) & minerals

(4.4 Ga detrital zircon) = rich geological history•seismically divided into

•upper- [granodiorite]•deep-

•middle- •lower-crust [high-grade metamorphic rocks & granulite xenoliths; increasing metamorphic grade &mafic rx = more mafic?]

•vertically stratified and laterally heterogeneous

Upper crustcompo-sition:

- weighted averages of exposedrocks

- averages of fine-grained sedimentsor glacialdeposits

Deep crust composition:(1) samples from deep crust(2) seismic velocities(3)heat flow measurements

Comparison of (a) REE & (b) additional trace element compositions of the upper, middle and lower crust recommended by Rudnick & Gao ‘05.

Middle crust -metasedimentary rocks,but dominated by DTTG

Deep crust -mafic

Comparison of (a) REE & (b) additional trace element compositions of the bulk crust - this study = Rudnick& Gao, ‘05.

Bulk cc composition:•intermediate•high Mg#•up to 50% of BSE’s inc. el.•depleted in Nb relative to La•enriched in Pb•subchondritic Nb/Ta

If the crust grows ultimately by igneous processes,then the disparity between crust and primary mantle melt compositions requires additional process(es) such as:1) Recycling of mafic/ultramafic lower crust and upper

mantle (density foundering or delamination)2) Mixing silicic melts from subducted slab and mafic melt

from mantle peridotite (e.g., Archean DTTG)3) Weathering of the crust, with preferential recycling of Mg

& Ca into the mantle via subduction (not supported by observation)

4) Ultramafic cumulates complementary to andesitic crust are present in the upper mantle

But Nb depletion suggests that ~80% of the crust was generated in a convergent margin

Major elements of island arc volcanic rocks & magma series

IAB : MORB:MgO < (Mg#)<K2O >Al2O3 >(although variable &some C-A basaltshave 17-20% (high-Albasalts), some believethat these are parentalto C-A series rocks)

Selected references:

Ringwood, A.E., 1979, Origin of the Earth and Moon, Springer-Verlag, N.Y., 295 p.

Holland, H.D. and Turekian, K.K., 2003, Treatise on Geochemistry, vol. 1, Meteorites, Comets and Planets.

Stevenson, D.J., 1981, Models of the Earth’s Core,Science 214, #4521, 611-619.