geochem exam 1

U-Pb (longer)

238U

206Pb

4.47

204Pb

Rb-Sr

87Rb

87Sr

48.8

86Sr

Sm-Nd

147Sm

143Nd

106

144Nd

K-Ar

40K

40Ar

1.25

36Ar

U-Pb (shorter)

235U

207Pb

0.7

204Pb

Th-Pb

232Th

208Pb

14

204Pb

Lu-Hf

176Lu

176Hf

37.1

177Hf

What are the four methods of geochronology?

Isochrons: Plots normalized daughter isotope vs. parent for multiple samples; do not need to assume the initial concentration of daughter isotope, can reveal the initial isotopic concentration and reservoir from which the magma originated from.

Inverse isochron: Inverse of isochron; used when concentration of the parent isotope is low.

Concordia-discordia: Plots two radiometric ratios for U-Pb systems; can show discordance caused by loss of Pb, loss of U, or gain of U in a non-closed system and crystallization/disturbance age.

Model age: Used when only one sample is available, shows time of extraction from mantle and not true crystallization age.

Why are phase diagrams used?

Show what chemical compositions are stable at certain conditions (temperature, pressure); phase equilibria, determines distribution of elements in the crust

What are partition coefficients?

~compatibility/incompatibility; relate the amount of a trace element that enters a growing crystal to the amount of that element in the coexisting fluid, used to see which trace elements are compatible, characteristic of each mineral species

Where do elements come from?

The Big Bang, stars/supernovas

7.50g of Fe3O4; how many moles of iron? Fe = 55.85 g/mol, O = 15.99 g/mol

0.0972 mol

Find the decay constant of 235U.

9.9e-10

What mass of magnetite (Fe₃O₄) contains 0.250 mol of Fe?

19.3g

How many moles of sulfur are in 43 grams of pyrite?

How many moles of copper are in 10 tons of rock at 43% Cu5FeS4?

How many more atoms of Cu are in 10g of bornite (Cu5FeS4) than 12g of chalcopyrite CuFeS2?

2.1e22