Trace Element Geochemistry

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• Solid solutions
• Example Olivine (Mg,Fe)2SiO4
• two endmembers of similar crystal form and
  structure
• Forsterite Mg2SiO4 and Fayalite Fe2SiO4
• isomorphism
• complete mixing range exists between forsterite
  and fayalite (isomorphous mixture)
• Mg/Fe ratio is variable, but (MgFe)/Si ratio
  is constant!
• Olivine is a solid solution between these two
  endmembers!
• (like a solution, olivine is homogeneous over
  the entire range of Mg/Fe ratios)
• Mg2 and Fe2 substitute for each other in the
  olivine crystal structure

2
Solid solutions
pure albite melts at 1118 oC, pure anorthite at
1557 oC, mixtures are intermediate Melt with 60
anorthite (and 40 albite) cools to 1470 oC (X)
before the first crystals with composition Y
appear. These crystals are higher in anorthite
than the melt and hence deplete the melt in
anorthite (the melt follows X -gt K). The older
crystals continuously re-equilibrate with the
melt until crystallization stops at K. At K, all
melt is crystallized and these crystals have
composition L (which is the starting composition
of the melt).
3
Solid solutions
pure albite melts at 1118 oC, pure anorthite at
1557 oC, mixtures are intermediate Melting
plagioclase with 40 anorthite (and 60 albite)
starts at ca 1245 oC (A) and produces melt of
composition B. This melt is higher in albite than
the solid and hence depletes the solid in albite
as melting proceeds (the solid and melt follow A
-gt C and B -gt D, resp.). At D all solid is melted
and turned into a liquid with 40 anorthite and
60 albite (which is the starting composition of
the plagioclase).
4
Eutectic melts
D -gt E and A -gt E are the melting points of the
possible diopside anorthite mixtures
For example Melt with 20 anorthite and 80
diopside cools (M -gt N) the first diopside
crystallizes at 1350 oC composition of melt
changes (N -gt E) at E the melt has 58 diopside
and 42 anorthite, anorthite begins to
crystallize temperature remains constant until
crystallization is complete E eutectic (temp.)
lowest temperature at which melt exists in the
system E eutectic composition of a diopside
anorthite melt
5
Partial melting a solid (in the mantle a
peridotite or in the continental crust a granite,
for example) moves above the solidus and starts
to (partially) melt. batch melting (equilibrium
fusion, equilibrium partial melting) melt and
residual solid continually react with each other
and re-equilibrate at the site of melting until
the melt escapes as a single batch of magma
(probably applicable to production of felsic
melts in the upper and lower continental
crust). fractional melting (Rayleigh
melting) small amount of melt forms and is
instantly removed from the source (probably
applicable to production of basaltic melts in the
mantle).
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• Geochemistry-Quiz 2, 11/04/04
• 1. What is the difference between LILEs and
  HFSEs? (give an example of each)
• 2. How do KD values depend on temperature,
  pressure, and SiO2 content?
• 3. What is the difference between batch melting
  and Rayleigh melting?
• 4. Why is Dr important for magma genesis?

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Homework Assignment 2

• Partial Batch Melting
• CL/C0 1 / DRS F(1-DRS)
• CL C0 / DRS F(1-DRS)
• CL concentration in liquid (partial melt)
• C0 concentration in unmelted solid (mantle
  peridotite)
• F fraction of melt produced during partial
  melting
• DRS bulk distribution coefficient for the
  residual solid (peridotite after melt extraction)
• Calculate the Sr and Yb concentrations in a melt
  after 3 and 30 partial melting, respectively,
  of a mantle with 50 ppm Sr and 3 ppm Yb leaving a
  residuum with
• a) 40 ol 20 opx 20 cpx 20 plag
• b) 40 ol 20 opx 20 cpx 20 gt
• Display the results in a Sr vs. Sr/Yb plot
• and discuss them !

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Partial Melting