Geology Reference
In-Depth Information
Sr, P, Nd, Zr, Sn, Ti and Y is shown (hatched area) in a chondrite-normalized spider
diagram, which demonstrates extremely high concentration of Sr, Pb, and REE.
There is also extreme degree of LREE enrichment. In a plot of
ε
Nd
versus
87
Sr/
86
Sr ratio, it is observed that the rocks of Leucite hills and Presley Peak has Sr
isotopic ratio > 0.705, and lamproites of Western Australia show extreme enrich-
ment with respect to the same ratio.
The Gaussberg lamproites have initial values ranging between 0.7105 and
0.7120. All these rocks are characterized by
ε
Nd
values lower than
8. The MORB
−
basalts have positive
ε
Nd
values between 5 and 13 and lower Sr isotropic ratios.
Initial
ε
Nd
values for Manning Massif tristanite, Mt. Bayliss alkali mela-syenite and
Bunger Hills trachybasalt dykes are
9.3,
12.3 and <
16.3, respectively.
−
−
−
Pb isotope diagram showing composition of lamproites from Western
Australia, Gaussberg (Nelson et al. 1986), Leucite Hills and Antarctica dykes are
shown in Fig.
5.8
a. The Pb isotopic signature of the Western Australian and
Gaussberg lamproites has been interpreted by Nelson et al. (1986) in terms of
involvement of an extremely ancient source component, which had high U/Pb ratio,
early in its history, followed by a mixing with more recent rock materials, which
lowered the U/Pb ratio. They estimate a minimum of 2.1 Ga for this differentiation
event. The high
207
Pb/
204
Pb and low
206
Pb/
204
Pb ratios of these rocks contrast with
The Pb
-
41.0
40.0
W A lamproites
Bunger Hills
Mt Bayliss
Gaussberg
39.0
Priestley
Peak
MORB
Manning
Massif
38.0
37.0
Leucite Hills
15.9
W A lamproites
Mt Bayliss
Priestley
Peak
15.8
Manning Massif
15.7
Bunger Hills
Gaussberg
15.6
MORB
Leucite Hills
15.5
17.0
17.5
18.0
18.5
19.0
19.5
206
/ 204
Pb
Pb
Fig. 5.8 a and b
208
Pb/
204
Pb versus
206
Pb/
204
Pb and
207
Pb/
204
Pb versus
206
Pb/
204
Pb isotope data
for K-rich rocks from various localities. The data on Mid-Oceanic Ridge basalts are also plotted for
comparison (after Nelson et al. 1986)
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