Geo388H/376E Sample exam questions 

Note: these are not meant to be a comprehensive overview of the subjects covered; rather they are meant to provide examples of the type of question that may be asked.

1. a) Discuss in words the relationship between vibrational frequency and atomic mass, and between atomic mass and H2O vapor pressure.

b) Write an isotope exchange reaction, the associated expression for the equilibrium constant, and a general equation that describes the temperature dependence of fractionation factors. In general, how do the values for equilibrium constants for isotope exchange reactions differ from equilibrium constant values for mineral-solution reactions, and how do these differences bear on the ability of isotope disequilibrium to drive fluid-rock interaction?

2. Listed below are some data for two groundwater samples, A and B, from a Tertiary limestone aquifer. Answer the following questions regarding these samples:

a) Mos Def asks you to determine the O and Sr isotopic compositions of a groundwater formed by mixing equal amounts of each of these two waters together. Write two equations that will help him do this.

b) Illustrate a fluid mixing curve between groundwaters A and B in dD vs. d13C space. Mark the positions of 25:75, 50:50, and 75:25 mixtures.

 

Sample     Cl     d18O         dD               d13C                    87Sr/86Sr         Sr              HCO3

                    (g/g)     (SMOW)     (SMOW)        (PDB)                                                           (mmol/kg)          (mg/l)

__________________________________________________________________________

A        10-1       -14.6     -108           -13                            0.7277                 55         123

B        10-2         -3           -13            1.0                          0.7077                 0.11         123

 

*****************************************************************************

3. For the Devil’s Hole calcite record of oxygen isotope variations (Winograd et al., 1992)

             a. discuss the different paths that water molecules may take from the ocean to precipitation of calcite in Devil’s Hole

            b. how the different paths could affect the O-isotope signal recorded in the calcite,

            c. for each path note the steps at which isotope fractionation may occur

            d. for one path, note the direction of isotopic shift caused by the fractionation at each step, and how changes in an environmental variable can affect the magnitude of the shift.