Part 12- Geothermometry Flashcards
How is a temperature calculated from stable isotope fractionations?
The difference in isotope composition between two phases in dynamic equilibrium.
Why are stable isotope fractionations very suitable for thermometry?
They are independent of pressure.
Why are MORBS and fresh xenoliths suitable for O isotope compositions?
Partial melting and melt extraction does not alter the O composition.
Why are fresh unaltered mantle samples required for O isotope compositions?
Composition is readily changed by reaction with sea water.
What kind of range of O isotope compositions do MORBs show?
What does this confirm?
Have a narrow range of compositions.
Confirms that the mantle is fairly homogeneous and well mixed with regards to delta18O values.
Igneous, metamorphic and sedimentary rocks from the CC show a very large range of O isotope compositions. Why?
Fractionations occur through interactions with meteoric fluids.
How are O isotopes used as tracers?
E.g. If an igneous rock has a delta18O value of +5.5% it hasn’t long left the mantle/hasn’t experienced any alteration.
What is the principal cause of C isotope variations?
Biological processes especially photosynthesis.
What are the 2 ‘layers’ of C isotope fractionation in marine plants?
- Photosynthesis
* CO2 dissolving in sea water
What DIC stand for?
Dissolved Inorganic Carbon
Why is surface sea water depleted in DIC?
Biological uptake
Why is deep sea water enriched in DIC?
Dead biological material ‘rains down’.
Why is the DIC of surface water enriched in heavy 13C isotope (high delta13C values)?
Preferential uptake of 12C by primary producers.
Why are evaporation-condensation processes of particular interest in geochemistry?
Generate very large isotope effects.
For a condensation process what does the maths model assume?
Equilibrium extends between the vapour and an infinitesimally small batch of liquid
