Geoscience Reference
In-Depth Information
Table 14.6
Representative values of large-ion lithophile and
high-field-strength elements (HFSE) in basalts (ppm)
Midocean-
Back-Arc-Basin
Island-Arc
Ocean-Island
Element
Ridge Basalt
Basalt
Basalt
Basalt
Rb
0.2
4.5--7.1
5--32
5
Sr
50
146--195
200
350
Ba
4
40--174
75
100
Pb
0.08
1.6--3
9.3
4
Zr
35
121
74
125
Hf
1.2
2.8
1.7
3
Nb
1
25
2.7
8
Ce
3
32
6.7--32.1
35
Th
0.03
1--1.9
0.79
0.67
U
0.02
0.4
0.19
1.18
component. The existence of similar basalts at
midocean ridges and back-arc basins and the
subtle differences in trace-element and isotopic
ratios provide clues as to the composition and
depth of the MORB reservoir.
the residual solid, respectively;
D
is the bulk dis-
tribution coefficient for the minerals left in the
residue, and
F
is the fraction of melting. Each
element has its own
D
that depends not on the
initial mineralogy but on the residual minerals,
and, in some cases, the bulk composition of the
melt. For the very incompatible elements (
D
l);
essentially all of the element goes into the first
melt that forms. The so-called compatible ele-
ments (
D
Trace-element modeling
An alternative to mixing known components to
infer
the composition of the depleted
MORB mantle
is to calculate the partitioning of
elements with a particular model in mind. For
example, instead of using MORB and Q, one can
suppose that the MORB source was the result of a
certain kind of melting process, and calculate the
properties of the melt and the residue. This usu-
ally gives a more depleted result than the mixing
method.
The trace-element contents of basalts contain
information about the composition, mineralogy
and depth of their source regions. When a solid
partially melts, the various elements composing
the solid are distributed between the melt and
the remaining crystalline phases. The equation
for equilibrium partial melting is simply a state-
ment of mass balance:
1) stay in the crystalline residue, and
the solid residual is similar in composition to
the original unmelted material. The above equa-
tion is for equilibrium partial melting, also called
batch melting. The reverse is equilibrium crystal
fractionation, in which a melt crystallizes and
the crystals remain in equilibrium with the melt.
The same equations apply to both these situati-
ons. The effective partition coefficient is a weigh-
ted average of the mineral partition coefficients.
The Rayleigh fractionation law
C
m
/
C
o
=
F
(
D
−
1)
applies to the case of instantaneous equilibrium
precipitation of an infinitesimally small amount
of crystal, which immediately settles out of the
melt and is removed from further equilibration
with the evolving melt. The reverse situation is
called fractional fusion.
Nickel and cobalt are affected by olivine and
orthopyroxene
C
r
D
where
C
m
,
C
o
and
C
r
are the concentrations of
the element in the melt, the original solid and
C
o
C
m
=
F
=
D
(1
−
F
)
+
fractionation
since
D
is
much
greater
than
1
for
these
minerals.
These
are
