Geoscience Reference
In-Depth Information
Table 17.3
Parameters of end-members
Depleted
Enriched
Enrichment
Mantle
Mantle
Ratio
Parameter
(1)
(2)
(2)/(1)
87
Sr/
86
Sr
0.701-0.702
0.722
Sr
2
/Sr
1
13.8
Nd
24.6
−
16
.
4
Nd
2
/Nd
1
73.8
206
Pb/
204
Pb
16.5-17.0
26.5
Pb
2
/Pb
1
62.5
3
He/
4
He (
R
a
)
6.5
150
He
2
/He
1
2.0
18
O (permil)
5.4
17
O
2
/O
1
1.0
δ
La/Ce
0.265
0.50
Ce
2
/Ce
1
165.0
Sm/Nd
1.50-0.375 0.09-0.39
Partition Coefficients
Rb
0.02
Sr
0.04
La
0.012
Ce
0.03
Sm
0.02
Pb
0, 0.002
He
0.50
Nd
0.09
garnet--pyroxenite cumulate, delaminated lower
continental crust or an eclogite slab.
There are two situations that have particular
relevance to mantle-derived magmas. Consider a
'normal' depleted mantle magma. If it can rise
unimpeded from its source to the surface, such
as at a rapidly spreading ridge, it yields a rela-
tively unfractionated, uncontaminated melt, but
can nevertheless be a blend of melts. Suppose
now that the magma rises in a midplate envi-
ronment, and its ascent is impeded by thick
lithosphere. The magma will cool and crystal-
lize -- evolve -- simultaneously partially melting
or reacting with the surrounding mantle. Thus,
crystal fractionation and mixing occur together,
and the composition of the hybrid melt changes
with time and with the extent of fractionation.
Fractionation of garnet and clinopyroxene from
a tholeiitic or picritic magma at sublithospheric
depths (
and the partition coefficients,
D
, assumed in the
modeling are given in Table 17.3. The figures that
follow show various ratios for mixes of a fraction-
ating depleted melt and an enriched component.
We assume equilibrium crystal fractionation, as
appropriate for a turbulent or permeable magma
body, and constant
D
. La/Ce and Sm/Nd are used
in the following examples but the discussion
is general and the results are typical of other
LIL-pairs.
La/Ce versus
87
Sr/
86
Sr
La/Ce is high in melts relative to crystalline
residues containing garnet and clinopyroxene.
It is low in melt-depleted reservoirs and high
in enriched reservoirs. Low and high values of
87
Sr/
86
Sr are characteristics of time-integrated
depleted and enriched magmas, respectively.
Since high
87
Sr/
86
Sr implies a time-integrated
enrichment of Rb/Sr, there is generally a posi-
tive correlation of Rb/Sr and La/Ce with
87
Sr/
86
Sr.
Some magmas, however, exhibit high La/Ce and
low
87
Sr/
86
Sr. This cannot be explained by binary
mixing of two homogenous magmas, but can be
explained by mixing of magmas from a cooling
partially molten mantle.
On a theoretical La/Ce vs.
87
Sr/
86
Sr plot
(Figure 17.5), the mixing lines between the
crystallizing MORB and enriched mantle compo-
nents reverse slope when MORB has experienced
slightly more than 99% crystal fractionation.
The relationships for equilibrium partial melting
>
50 km) can generate alkalic magmas
with enriched and fractionated LIL patterns.
For purposes of illustration, let us investigate
the effects of combined eclogite fractionation
(equal parts of garnet and clinopyroxene) and
'contamination' on melts from depleted mantle.
'Contamination' is modeled by mixing an enri-
ched component with the fractionating depleted
magma. This component is viewed as a par-
tial melt generated by the latent heat associ-
ated with the crystal fractionation. The assumed
geochemical properties of the end-members, the
enrichment factors of the elements in question
