Minor and Trace Elements

Question 1

What is Goldschmidt’s rules?

Answer 1

Ions with the same size, charge and valence fit as easily into a mineral lattice
Ions with the same charge but different size, smaller size favoured
Ions with same size but different charge, greater charge favoured

Question 2

How do elements fractionate?

Answer 2

between two competing phases i.e. mineral and melt or two minerals in a melt

Question 3

What is Henry’s Law?

Answer 3

If concentration of an element in a magma increases the same concentration increase will be seen in the mineral

Question 4

What is the equation for distribution coefficient and what does it mean?

Answer 4

Kd=Cs/Cl
Distribution coefficient = concentration of element in the solid/concentration of element in the liquid
If Kd>1 element is compatible in the mineral , Kd<1 element is incompatible in mineral, stays in melt

Question 5

What two types of element are incompatible?

Answer 5

High field strength and high charge are incompatible

Low charge and large ion size are incompatible

Question 6

Give examples of incompatible trace elements

Answer 6

K/Rb - show role of amphibole, if amphibole present all K/Rb will be in the mineral
Sr - excluded from all but plag
Ba - excluded from all but K-feldspar

Question 7

Explain the relationship between Ni and Sc+Cr

Answer 7

Ni partitions into olivine then pyroxene
Sc and Cr partition into pyroxene
By Ni/Sc and Ni/Cr can work out how olivine and pyroxene were produced in a melt

Question 8

Explain batch melting

Answer 8

Cl/Co=1/Kd(1-F)+F
Cl=concentration in melt Co=concentration in rock initially Kd=distribution coefficient F=weight fraction of melt - melt/melt+rock
If Kd 1 compatible
As F>1 all trace elements leave melt

Question 9

What is Raleigh fractionation?

Answer 9

Cl/Co=F(Kd-1)

Shows separation in a fractional crystallisation

Question 10

What are HREE and Eu2+ compatible with?

Answer 10

HREE more compatible especially in garnet

Eu2+ more compatible in plagioclase