GEOL 135 Lab – REE partitioning in melts and granite weathering
The Rare Earth Elements (REE)
The lanthanide series is formed by filling of 4f orbitals, and its constituents are commonly termed the
rare earth elements (REE). The common valence state is +3 for all REE over a wide range of oxygen
fugacity; Ce4+ can occur in highly oxidized environments at the earth's surface, and Eu2+ in reducing
environments in the crust and mantle. The rare earth elements are lithophile elements (low
electronegativities lead to the formation of highly ionic bonds), which substitute into many silicates and
phosphates. Ionic radii of the lanthanides decreases with increasing atomic number from La to Lu, called
the lanthanide contraction…
Because of their high charge and large radii, the rare earth elements are usually
relatively incompatible in silicate minerals. However, due to the lanthanide contraction,
the heavier rare earths are smaller and thus can "fit" within some lattice sites (although
there must be charge compensation for the 3+ valence), so incompatibility decreases
with increasing Z.
1. Producing REE diagrams
REE diagrams are plotted as concentration on the y-axis and each REE represented on the x-axis. Using
the chondrite REE data (in ppm by weight), plot the diagram for REE (La though Lu) corresponding to
the chondrite composition on Table 1 using excel (try making a LINE plot).
Why does the element concentration vary significantly in this fashion?
16.3
37.51
4.66
17.28
4.14
0.35
3.09
0.49
2.45
0.37
0.82
0.12
0.69
0.09
1.03
0.59
109.1
0.14
0.08
16.8
0.64
0.33
71.36
0.1
0.05
9.37
0.23
0.11
16.93
0.03
0.01
2.33
0.2
0.09
11.87
Table 1 – selected REE
0.03in these
0.01exercises
1.45
use
0.66
0.11
0.56
0.09
0.18
0.03
data0.15
for
0.02
Biotite
Muscovite
Apatite
OIB
37
80
9.7
38.5
10
3
7.62
1.05
5.6
1.06
2.62
0.35
2.16
0.3
Orthoclase
E-type
MORB
6.3
15
2.05
9
2.6
0.91
2.97
0.53
3.55
0.79
2.31
0.356
2.37
0.354
Plagioclase
N-type
chondrite MORB
0.2347
2.5
0.6032
7.5
0.0891
1.32
0.4524
7.3
0.1471
2.63
0.056
1.02
0.1966
3.68
0.0363
0.67
0.2427
4.55
0.0556
1.01
0.1589
2.97
0.0242
0.456
0.1625
3.05
0.0243
0.455
Granite
ppm
(w/w)
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Since the effect plotted for chondrite REE
affects all rocks, we typically normalize all REE
analyses based on the chondrite values so we can
see effects associated with other phenomena.
Plot the N- and E-type MORBs (N-MORB is a
7.74
3.97
103.28
3.15 E-Morb
60.72 is an
‘normal
mid-ocean
ridge basalt,
17.19
8.94
298.05
7.93
147.08
‘enriched’ MORB), OIB (Ocean Island Basalt, a
2.03
1.01
51.21
0.95
17.73
mantle
plume
a granite
as
7.4
3.71 material),
276.8 and
3.49
66.22
CHONDRITE-NORMALIZED
on
1.58
0.83
132.83
0.89 compositions
14.05
0.33
0.48 discuss
9.99the major
0.04 differences.
0.24
an
REE plot,
9.8
1.36
5.9
0.86
1.86
0.26
1.37
selected
0.19
rocks to
2. Rayleigh Fractionation of elements as melts crystallize
As melts crystallize, elements enter certain minerals according to a partitioning coefficient, KD (for a
comprehensive list of KD values, see: http://earthref.org/cgi-bin/er.cgi?s=kdd-s0-main.cgi). Using the
chondrite-normalized data above as a starting point and the KD data in Table 2, plot how Sm, Nd, Eu,
and Yb would change as a function of melt crystallization as Plagioclase and Clinopyroxene crystallize,
and then discuss how an OIB mantle material could be produced…
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Use the Rayleigh Fractionation equation for this:
KD
KD
PlagioclaseClinopyroxene
Cimelt = Ci0rockF(KD-1)
0.135
0.0536
0.062
0.0858
0.088
0.15
0.028
0.1873 Where the initial melt concentration for any element is Cimelt ,the
0.036
0.291 concentration of any element in the rock or mineral under consideration
0.73
0.458 is Ci0rock. , KD is the partition coefficient, and F is the fraction of melt
0.04
0.82 remaining. Here the REE concentrations for the chondrite is the initial
0.026
0.53
0.055
0.442
0.048
0.44
0.041
0.387 Table 2 – selected KD values for Plagioclase and Clinopyroxenes
0.036
1.09
0.031
0.43
0.025
0.433
3. REE changes as granite weathers
REE are often conservative, in other words REE distributions of minerals forming sediments are
representative of the parent rocks they come from, with some caveats. We will investigate how REE
changes as a function of granite weathering and the minerals REEs are coming from as granite dissolves.
Table 3 lists REE for Granite and the major minerals in granite (Data from Aubert et al., 2001 GCA) in
addition to soil, river sediment, soil porewater, and river water analyses.
Plot the REE patterns of each sample and calculate apparent KD for each mineral.
Investigate this dataset and address the following:
REE (ppm)
HPT Granite
Plagioclase
Orthoclase
Apatite
Muscovite
Biotite
top Soil
river sediment
soil porewater
river water
If river sediment is deposited and eventually forms a fluvial sedimentary rock, how well is the REE
preserved, and what elements see the most difference. Address why these specific REE distributions
may be different. Discuss this with supporting graphs and calculations.
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
16.3
37.51
4.66
17.28
4.14
0.35
3.09
0.49
2.45
0.37
0.82
0.12
0.69
0.09
7.74
17.19
2.03
7.4
1.58
0.33
1.03
0.14
0.64
0.1
0.23
0.03
0.2
0.03
3.97
8.94
1.01
3.71
0.83
0.48
0.59
0.08
0.33
0.05
0.11
0.01
0.09
0.01
103.28
298.05
51.21
276.8
132.83
9.99
109.1
16.8
71.36
9.37
16.93
2.33
11.87
1.45
3.15
7.93
0.95
3.49
0.89
0.04
0.66
0.11
0.56
0.09
0.18
0.03
0.15
0.02
60.72
147.08
17.73
66.22
14.05
0.24
9.8
1.36
5.9
0.86
1.86
0.26
1.37
0.19
17.63
38.27
4.35
15.4
2.72
0.25
1.54
0.18
0.76
0.13
0.31
0.05
0.27
0.04
16.4
38.9
4.8
20.5
5.5
0.9
6.2
1
5.2
0.88
2.3
0.3
1.86
0.27
188.4
114.5
12.8
58
17.6
3.9
19.8
3.6
19
3.2
8.7
1.7
10.9
1.3
21
34
7.2
41
17
3.1
20
3.8
22
4.1
12
2
13
2