1 Composition of the Earth GLY 4200 Fall, 2014

2 Interior of the Earth Earth’s interior is divided into zones, with differing properties and compositions Since we live on the crust, it is the most studied The core and mantle are very important in understanding the behavior of the earth

3 Composition of the Crust – Major Elements Earth’s crust is composed predominantly of eight elements Figure for Si here is correct – figure 5.2 in text has a misprint Numbers are in weight percent

4 Abundances Measurements We can specify abundances using differ methods The most common are:  Weight per cent  Atom per cent  Volume percent

5 Comparison of Methods ElementWeight %Atom % O Si Al Fe Ca Na K Mg

6 Minor and Trace Element Definition Minor elements have abundances between 0.1 to 1.0 weight percent Elements with abundances less than 0.1% are called trace elements Their abundance is usually given in parts per million (ppm) or parts per billion (ppb)

7 Minor and Trace Elements in Crust Only 17 elements occur with abundances of at least 200 parts per million (ppm) – in addition to those on the major element slide, these include: ElementWeight %ElementWeight ppm Ti0.44%F625 H0.14%Sr375 P0.10%S260 Mn0.09%C200 Ba0.04%

8 Ores Many valuable elements are in the trace element range, including the gold group (Au, Ag, and Cu) and the platinum group (Pt, Pd, Ir, Os), mercury, lead, and others Useage does not always reflect abundance – copper (55 ppm) is used more than zirconium (165 ppm) or cerium (60 ppm)

9 Effect of Pressure As pressure increases, minerals transform to denser structures, with atoms packed more closely together This is seen in the mantle The upper mantle is dominated by the mineral olivine, Mg 2 SiO 4 Magnesium is in VI, and Si in IV

10 Transition Zone In the transition zone, from about 410 to 660 kilometers below the surface, olivine transforms to denser structures  olivine (ρ = 3.22 gm/cm 3 ) → wadsleyite (ρ = 3.47 gm/cm 3 ) → ringwoodite (ρ = 3.55 gm/cm 3 ) Wadsleyite is β- Mg 2 SiO 4 and Ringwoodite is γ-Mg 2 SiO 4, polymorphs of olivine

Hydrous Transition Zone The transition zone has been investigated as a hydrous zone within the earth’s mantle Experiments by Ye et al. (2012) on synthetic ringwoodite indicate it can hold about 2.5% water as hydroyxl groups Wadsleyite is also reported to hold up to 3% water This has significant implications for the physical properties and rheology of the transition zone Pearson et al. (2014) found ringwoodite in diamonds from the Juína district of Mato Grosso, Brazil and concluded that this provided direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent water This was the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine 11

12 Lower Mantle Pressures are so great that silicon becomes six coordinated (CN = VI), and some magnesium becomes eight-coordinated (perovskite structure)  Ringwoodite (ρ = 3.55 gm/cm 3 ) → MgSiO 3 (perovskite structure) and (Mg, Fe)O (magnesiowűstite - halite structure)

13 Core The core is divided into two regions, the liquid outer core and the solid inner core There is a definite chemical discontinuity between the lower mantle and the outer core The main elements in the core are an iron and nickel alloy Increasing temperature first melts the alloy to make the outer core Increasing pressure freezes the alloy to produce the inner core

14 Outer Core Ranges from 2900 to 5100 kilometers below the earth Composition is iron with about 2% nickel Density of 9.9 gm/cm 3 is too low to be pure metal Best estimates are that silica makes up 9-12% of the outer core

15 Inner Core From 5100 to 6371 kilometers below surface 80% iron, 20% nickel alloy Pressures reach about 3 megabars, or 300,000 megapascals Temperature at the center is about 7600ºC