Composition of the Earth GLY 4200 Fall 2012

Composition of the Earth GLY 4200 Fall, 2012 1

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 2

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 3

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

Comparison of Methods Element O Si Al Fe Ca Na K Mg Weight % 46. 60 27. 72 8. 13 5. 00 3. 63 2. 83 2. 59 2. 09 Atom % 62. 55 21. 22 6. 47 1. 92 1. 94 2. 64 1. 42 1. 82 5

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) 6

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: Element Weight % Element Weight ppm Ti 0. 44% F 625 H 0. 14% Sr 375 P 0. 10% S 260 Mn 0. 09% C 200 Ba 0. 04% 7

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) 8

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 Si. O 4 • Magnesium is in VI, and Si in IV 9

Transition Zone • In the transition zone, from about 400 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) 10

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) → Mg. Si. O 3 (perovskite structure) and (Mg, Fe)O (magnesiowűstite - halite structure) 11

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 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 12

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 13

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 14