Geoscience Reference
In-Depth Information
Table 2.6
Concentration C
i
0
(ppm) of Ni, La, and Yb in the peridotitic
source and mineral/liquid partition coefficients K
i
min/liq
Element
i
Ni
La
Yb
C
i
0
ppm
2500
0.5
0.4
K
i
ol
/
liq
10
0
0
K
i
opx
/
liq
1
0
0.05
K
i
cpx
/
liq
1
0.01
0.3
87
Sr/
86
Sr vs. [Mg]
a.
87
Sr/
86
Sr vs. [Sr]
b.
87
Sr/
86
Sr vs. [Sr]/[Mg]
c.
87
Sr/
86
Sr vs. [Mg]/[Sr]
d.
87
Sr/
86
Sr vs. 1/[Sr]
e.
87
Sr/
86
Sr vs. [Mg]/([Mg]
[Sr])
10. Plot the Sr concentrations and
87
Sr/
86
Sr ratios vs. the Mg concentrations in mixtures of
hydrothermal solutions and seawater (0-100 wt% mixing ratios) using the following
data:
87
Sr/
86
Sr
f.
+
1260 ppm in seawater;
87
Sr/
86
Sr
=
0.709, [Sr]
=
8 ppm, [Mg]
=
=
=
=
0 ppm in the hydrothermal end-member. The atomic
weight of Sr is 87.62 and the atomic abundance of isotope 86 is 9.86%.
11. Partial melting of a peridotite produces basaltic liquids. We assume that the min-
eral composition of the residue is invariable and made of 70 wt% olivine, 20 wt%
orthopyroxene and 10 wt% clinopyroxene. Calculate the concentrations of the follow-
ing elements: nickel (Ni), lanthanum (La), ytterbium (Yb), and the La/Yb ratio in the
melt for melt fractions
F
equal to 0.002, 0.01, 0.02, and 0.1. Concentrations in the
12. The melt calculated in the previous exercise for
F
0.703, [Sr]
16 ppm, [Mg]
0.1 rises and fractionates min-
erals in conduits before the residual liquids are erupted as differentiated lavas. The
cumulates contain 70 wt% olivine and 30 wt% clinopyroxene. Calculate the Ni, La,
and Yb concentrations and the La/Yb ratios for a fraction crystallized
X
equal to 0.01,
0.1, and 0.2. Compare the relative evolution of these parameters resulting from partial
melting and fractional crystallization.
13. Very low partition coefficients are particularly difficult to assess experimentally
because their mineral concentrations are very low and prone to contamination by co-
existing phases. This is the case for rare-earth elements between olivine and melt. Let
us call
r
the ionic radius of the rare-earth elements in octahedral coordination in this
mineral. Use the partition coefficients of Sm (
r
=
0.96 Å,
K
Sm
ol
=
liq
=
0.000424), Gd
/
0.94 Å,
K
Gd
ol
0.87 Å,
K
Yb
ol
=
liq
=
=
liq
=
(
r
0.000973), and Yb (
r
0.0196) to evaluate
/
/
the olivine/liquid partition coefficients of La (
r
=
1.03 Å), Ce (
r
=
1.01 Å), and Nd
0.98 Å). (Hint: calculate the three coefficients of the parabola relating ln
K
ol
/
liq
with
r
.)
(
r
=
