JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 6
PAGES 981–1001
2002
Lamproites from Gaussberg, Antarctica:
Possible Transition Zone Melts of Archaean
Subducted Sediments
D. T. MURPHY∗, K. D. COLLERSON AND B. S. KAMBER
ADVANCED CENTRE FOR QUEENSLAND UNIVERSITY ISOTOPE RESEARCH EXCELLENCE (ACQUIRE),
DEPARTMENT OF EARTH SCIENCES, STEELE BUILDING, ST. LUCIA CAMPUS, UNIVERSITY OF QUEENSLAND,
ST. LUCIA, BRISBANE, QLD. 4072, AUSTRALIA
RECEIVED MAY 22, 2001; REVISED TYPESCRIPT ACCEPTED DECEMBER 17, 2001
Petrogenetic models for the origin of lamproites are evaluated using
new major element, trace element, and Sr, Nd, and Pb isotope data
for Holocene lamproites from the Gaussberg volcano in the East
Antarctic Shield. Gaussberg lamproites exhibit very unusual Pb
isotope compositions ( 206Pb/204Pb = 17·44–17·55 and 207Pb/
204
Pb = 15·56–15·63), which in common Pb isotope space plot
above mantle evolution lines and to the left of the meteorite isochron.
Combined with very unradiogenic Nd, such compositions are shown
to be inconsistent with an origin by melting of sub-continental
lithospheric mantle. Instead, a model is proposed in which late
Archaean continent-derived sediment is subducted as K-hollandite
and other ultra-high-pressure phases and sequestered in the Transition
Zone (or lower mantle) where it is effectively isolated for 2–3 Gyr.
The high 207Pb/204Pb ratio is thus inherited from ancient continentderived sediment, and the relatively low 206Pb/204Pb ratio is the
result of a single stage of U/Pb fractionation by subduction-related
U loss during slab dehydration. Sr and Nd isotope ratios, and trace
element characteristics (e.g. Nb/Ta ratios) are consistent with
sediment subduction and dehydration-related fractionation. Similar
models that use variable time of isolation of subducted sediment
can be derived for all lamproites. Our interpretation of lamproite
sources has important implications for ocean island basalt petrogenesis as well as the preservation of geochemically anomalous
reservoirs in the mantle.
INTRODUCTION
KEY WORDS: lamproites; Pb isotopes; mantle Transition Zone; subducted
sediment; anomalous mantle reservoirs
Lamproites are ultrapotassic mantle-derived volcanic
rocks with low CaO, Al2O3 and Na2O contents, high
K2O/Al2O3, relatively high Mg content, and extreme
enrichment in incompatible elements (Foley et al., 1987).
They are geographically widespread, yet volumetrically
insignificant. Unlike kimberlites, which are found mainly
in Archaean cratons, lamproites intrude continental crust
of varying age (e.g. Graham et al., 1999; Turner et al.,
1999), ranging from Archaean (Western Australia) to
Palaeozoic (Southern Spain). The oldest known (diamond-bearing) lamproites are of Proterozoic age and
occur in Western Australia and India (Pidgeon et al.,
1988; Rao et al., 1999). Lamproite magmatism appears
to be more common in the Phanerozoic than in the
Precambrian, even when the lower preservation potential
of ancient examples is taken into account (Mitchell &
Bergman, 1991). Gaussberg in Antarctica (56 ± 5 ka) is
the youngest lamproite occurrence yet recognized on the
Earth (Tingey et al., 1983).
Lamproites, kimberlites, and carbonatites are interpreted to have been derived from geochemically anomalous mantle whose mineralogy, chemistry, and
temporal evolution is a matter of continuing debate
(e.g. Mitchell & Bergman, 1991; Ringwood et al., 1992;
Collerson et al., 2000). The level of incompatible element
enrichment in lamproites precludes significant contamination by continental crust (Collerson & McCulloch,
1983), thus they are excellent isotopic proxies for their
mantle source. It is generally assumed from Sr, Nd, and
∗Corresponding author. Telephone: +61 7 33659776. Fax: +61 7
33651277. E-mail: [email protected]
 Oxford University Press 2002
JOURNAL OF PETROLOGY
VOLUME 43
Pb isotope compositions that lamproites, kimberlites, and
carbonatites form by melting of portions of the subcontinental lithospheric mantle that were enriched by
metasomatism and contain phlogopite, clinopyroxene, or
K-richterite (Mitchell & Bergman, 1991; Mitchell, 1995;
Edgar & Mitchell, 1997). However, the occurrence of
ultra-high-pressure syngenetic mineral inclusions in diamonds from kimberlites in China, Brazil and Africa
(Wang & Sueno, 1996; McCammon et al., 1997; Stachel
et al., 2000), and ultradeep majorite-bearing xenoliths in
‘alnöites’ from Malaita, Solomon Islands (Collerson et
al., 2000) suggest that some highly enriched melts may
originate in the deep mantle (Ringwood et al., 1992).
Furthermore, the presence of primordial noble gas ratios
in carbonatites (Sasada et al., 1997; Marty et al., 1998;
Dauphas & Marty, 1999) is not consistent with a subcontinental lithospheric mantle origin for these melts.
Continental-derived sediment, which may be subducted along with oceanic lithosphere, is inherently Krich and contains high-pressure phases such as K-hollandite, which could provide a source component for
lamproitic melts. Although most subducted sediment is
probably mixed into the convecting mid-ocean ridge
basalt (MORB) source mantle, tomographic images (e.g.
Van der Hilst et al., 1991; Christensen, 1996; Simons
et al., 2000) show that oceanic lithosphere commonly
descends to the Transition Zone, and in some subduction
zones even penetrates the lower mantle (Van der Hilst
et al., 1991; Christensen, 1996). Subduction of oceanic
lithosphere has been an integral process of the Earth’s
convecting system since the Archaean and it has been
estimated that approximately 40 times the volume of the
present-day oceanic crust may have been recycled into
the mantle over time (Kramers & Tolstikhin, 1997;
Kamber & Collerson, 2000). As a result, it is plausible that
a considerable amount of subducted material, including
continent-derived sediment, could be stored in the Transition Zone and the lower mantle.
This paper, expanding on earlier research on Gaussberg by Collerson & McCulloch (1983) and Williams et
al. (1992), uses new major element, trace element, and
Sr–Nd–Pb isotope data for an extensive suite of lamproite
samples from Gaussberg (collected by K.D.C. during the
1997 Austral summer) to develop a new model for the
formation of lamproite magmas.
GEOLOGICAL BACKGROUND,
SAMPLES, AND PETROGRAPHY
Gaussberg is a 370 m high volcanic feature, situated at
67°S, 89°E on the East Antarctic Shield (Fig. 1). The
physical, volcanological, and lithological variations in the
volcano were mapped by K.D.C. during a 1997 Austral
NUMBER 6
JUNE 2002
Fig. 1. Gaussberg location map; modified from Tingey et al. (1983).
summer expedition to Gaussberg. Several phases of lamproitic activity were recognized. In particular, it was
shown that the present >300 m high volcanic construct
was built on an earlier eroded volcanic feature. Field
observation showed that the volcano erupted sub-glacially, producing pillow lava flows as well as hyaloclastite
deposits. Constant pillow flow directions on all points of
the volcano indicate that the current cone shape is an
artefact of glacial erosion. Thus the original volcano must
have been much larger. This conclusion is also supported
by the observation that moraine within the adjacent
Philippi Glacier is dominated by lamproite fragments.
Samples were collected from all representative localities
ranging from the base of the volcanic feature to its
summit. Details of sample locations are shown in Fig. 2.
The lamproite lavas from Gaussberg range in texture
from almost aphyric to hypohaline and are commonly
highly vesicular. Microphenocrysts are dominated by
subhedral leucite, olivine, and clinopyroxene ranging in
size up to 1 mm, with phlogopite in some samples. These
sit in a yellow brown glassy matrix containing variable
proportions of quench crystals of very fine-grained leucite,
phlogopite, apatite, and ilmenite.
Exposed rocks at Gaussberg are generally fresh and
particular care was taken to obtain unaltered sample
material. All studied samples were examined microscopically and found to be remarkably fresh. They preserve fine petrographic structural and textural detail.
Phenocrysts are virtually unaltered. Leucite occurs as
0·5–2 mm hexagonal crystals with characteristic complex
twinning. They often contain rings of melt inclusions,
which is a common feature of magmatic leucite (Mitchell
& Bergman, 1991). Olivines are clear and colourless, and
lack signs of significant alteration. They often contain
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MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
Fig. 2. A topographic map of the Gaussberg volcanic feature. Samples
were collected at the base of Gaussberg near Drygalski’s camp (location
1; Table 2), along the beach below Nord Kap (location 2; Table 2),
from the prominent pillow cliff above Nord Kap (location 3; Table 2),
on the NE slope of the cone below the KDC terrace camp (KDC)
(location 4; Table 2), from the summit region (location 5; Table 2),
and from the first terrace to the SW of the summit (location 6; Table
2).
very large melt inclusions and have very well developed
crystal faces, implying rapid formation during magma
ascent. Clinopyroxene crystals are relatively large (often
>2 mm) and show no significant alteration. Devitrification
textures in the glass matrix are rare.
Research Excellence (ACQUIRE). Sample preparation
and analytical procedures used were identical to those
of Eggins et al. (1997) except that Tm was not used as
an internal standard and W-2, a US Geological Survey
diabase standard, was used as the calibration standard.
Our preferred concentrations for W-2 and the measured
concentrations and relative standard deviations for AGV1 (an average of 26 analyses of nine digestions analysed
over 4 years) are shown in Table 1. Concentrations for
W-2 were derived partly by analysing it relative to
synthetic standards (Li, Cr, Ni, Ga, Rb, Sr, Y, Zr, Nb,
Cs, Ba, Hf, Ta, Pb, Th, and U) or are based on an
assessment of published standard data (A. Greig, personal
communication, 2001).
All isotope measurements were carried out on a VG
54-30 Sector multicollector mass spectrometer in static
mode in the ACQUIRE laboratory. Procedures were
identical to those of Wendt et al. (1999). The long-term
(5 years) reproducibility of the NBS SRM 987 Sr and
La Jolla Nd standards at ACQUIRE is 87Sr/86Sr =
0·710251 ± 17 and 143Nd/144Nd = 0·511861 ± 11,
respectively. During the course of this study, the NBS
SRM 981 Pb standard yielded an average fractionation
per mass unit of 0·0007049 using pyrometer control and
reproducibilities of 206Pb/204Pb = 0·0038, 207Pb/204Pb =
0·006 and 208Pb/204Pb = 0·0236.
MAJOR ELEMENT CHEMISTRY
ANALYTICAL PROCEDURES
Whole-rock major element concentrations were determined at the Department of Earth Sciences, University
of Queensland by inductively coupled plasma optical
emission spectrometry (ICP OES) on a Perkin Elmer
Optima 3300DV system. Approximately 50 mg of
crushed sample were dissolved in 5 ml of 32% HCl, 3 ml
of 70% HNO3, and 2 ml of 50% HF using a microwave
oven. After digestion, 39·5 ml of 3·5% boric acid and
0·5 ml of a 1000 ppm Lu internal standard were added.
Samples were then placed in the microwave for a further
10 min before analysis. An andesite standard, JA-3 (Imai
et al., 1995), was used as the calibration standard and
results for other standards analysed as unknowns are
shown in Table 1. Major element totals in Table 2
include BaO, SrO and ZrO2 and assume Fe2+/(Fe2+ +
Fe3+) to be 0·63, the average ratio determined by Sheraton & Cundari (1980) for Gaussberg lamproites. Loss
on ignition was not determined but major element concentrations are very similar to previously published results
(Sheraton & Cundari, 1980).
Trace elements were analysed by ICP mass spectrometry (ICP-MS) on a Fisons PQ2+ system at the
Advanced Centre for Queensland University Isotopic
Lamproites have unusually high K contents, a characteristic more typical of highly evolved melts. However,
they also have high MgO contents, which are typical of
primitive magmas. The characteristically high K/Al ratio
is one of the main criteria for the recognition of lamproites
(Foley et al., 1987). Although this peculiar chemistry
partly reflects source composition, an important question
remains regarding the nature of the primary melt composition.
Complete chemical analyses for 24 Gaussberg lamproite samples are reported in Table 2. Despite representing all major phases of exposed volcanism at
Gaussberg, the studied lamproites show little variation
in major element chemistry, with very narrow ranges in
K2O (11·5–13·5), Al2O3 (9·5–10·6), and MgO (7·5–9·5).
Because of the lack of variation in the geochemistry of
the Gaussberg lamproites (Table 2) it is difficult to assess
the role of fractional crystallization on the basis of major
(and trace) element content.
To assess the role of fractional crystallization, we
compare chemistries of worldwide lamproites, which
collectively show stronger trends on K2O vs K2O/Al2O3
(Fig. 3a) and K2O vs K2O/CaO plots (Fig. 3b). Compared with Harker diagrams, these plots discriminate
more clearly between fractionation trends caused by
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JUNE 2002
two plausible K-rich phases. The trends shown in Fig.
3 are, however, consistent with fractionation and/
or accumulation of K-poor mineral phases such as
clinopyroxene (vector a in both Fig. 3a and b) or
olivine (vector c in both Fig. 3a and b). Thus, although
the high-MgO and low-Al2O3 contents may be inherited
primary features of lamproite melt, part of the compositional range defined by these elements may be due
to the accumulation of mafic minerals. By analogy
with other lamproites, fractional crystallization cannot
be discounted as a process that has affected Gaussberg
lamproites. For this reason we will deal mainly with
aspects of lamproite chemistry that are insensitive to
fractional crystallization.
ISOTOPE GEOCHEMISTRY
Fig. 3. (a) K2O vs K2O/Al2O3 and (b) K2O vs K2O/CaO systematics
of the Gaussberg lamproites (Φ), Yakokut lamproites, Siberia (Α)
(Mues-Schumacher et al., 1995), Western Australian olivine (Β) and
phlogopite lamproites (Χ), Leucite Hills lamproites (Μ) (Mitchell &
Bergman, 1991), and Meta do Lenco lamproites, Brazil (Ο) (Gibson et
al., 1995). The fractionation vectors shown represent the accumulation
of (a) clinopyroxene, (b) olivine and (c) leucite from the average
Gaussberg composition. Although these vectors are not representative
of all lamproites, they give an indication of the nature of possible trends
related to fractional crystallization in lamproites.
leucite, clinopyroxene, olivine, and phlogopite (not
shown in figures). This is because these minerals have
distinctly different K2O/Al2O3 and K2O/CaO ratios
and, therefore, distinct trends are evident when either
fractionation or accumulation of any of these phases
occurred. For example, olivine has a K2O/Al2O3 ratio
of zero and a K2O/CaO of close to zero, which
means that when olivine is accumulated in a melt
these ratios in the melt do not change whereas the
K2O content is systematically reduced (vector b in
both Fig. 3a and b). It is significant that the compositional trends predicted for accumulation of leucite
and phlogopite are not compatible with the trends
defined by lamproite data. Therefore, major element
variations in lamproites and specifically their K enrichment are not due to the accumulation of the only
A number of detailed studies on the isotopic chemistry
of lamproites have concluded that such melts originate
from a source that had experienced a prolonged history
of K, Rb and light rare earth element (LREE) enrichment
(Collerson & McCulloch, 1983; Vollmer et al., 1984;
Fraser et al., 1985; Nelson, 1992; Lambert et al., 1995;
O’Brien et al., 1995). Foley et al. (1987) concluded that
this source was metasomatized, enriched sub-continental
lithospheric mantle. However, none of these models
provide explanations for the nature of source enrichment
and lack an obvious geological link between enrichment
of the source and the melting process. Furthermore,
ancient metasomatic source enrichment is inherently
difficult to test. In this paper we combine the temporal
constraints on the lamproite source imposed by the
Rb–Sr, Sm–Nd, and U–Pb decay schemes to evaluate
the possibility that the Gaussberg lamproites were derived
from metasomatized lithospheric mantle and to test alternative explanations.
The isotopic compositions of the Gaussberg lamproites
show almost no variation in Pb, Nd, and Sr isotope ratios
(Table 3), with no discernible trends in 207Pb/204Pb vs
206
Pb/204Pb space (Fig. 4a), 208Pb/204Pb vs 206Pb/204Pb
space (Fig. 4b), or Nd vs 87Sr/86Sr space (Fig. 4c). This
indicates that it is very unlikely that the isotope data
represent a mixture of more than one source. In isotope
plots, lamproites occupy a region that is more commonly
associated with crustal rocks, rather than mantle-derived
melts. As mentioned above and discussed in detail below,
crustal contamination could only have had subordinate
effects on the Pb, Nd, and Sr contents of Gaussberg
lamproites (Collerson & McCulloch, 1983). Therefore,
their unusual (for mantle-derived melts) isotopic composition reflects that of the mantle source.
984
MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
Table 1: Standard data for major element and trace element analysis
Element
JB-3
BIR-1
Element
W-2
SiO2
51·16
47·43
Li
9·2
TiO2
1·49
1·04
Be
0·62
Al2O3
17·50
15·66
Sc
36
12
T-Fe2O3
262
117·7
12·06
11·70
V
MnO
0·18
0·18
Cr
92·8
MgO
5·29
9·89
Co
45
CaO
9·28
13·21
Ni
AGV-1
RSD (%)
10·5
2·09
Element
W-2
AGV-1
RSD (%)
1·1
Ce
23·2
69·7
0·7
2·0
Pr
3·03
2·0
Nd
12·91
8·54
31·7
1·0
0·9
1·7
Sm
3·27
5·79
1·2
18·5
Eu
1·094
1·583
1·4
15
1·6
Tb
0·615
0·656
0·9
70·0
15·29
3·2
Gd
3·71
4·75
1·1
8·92
Na2O
2·80
1·82
Cu
103·0
61·6
6·5
Dy
3·81
3·57
1·0
K2O
0·79
0·02
Zn
77·0
85·5
3·7
Ho
0·803
0·694
1·1
P2O5
0·26
0·00
Ga
17·42
20·3
1·4
Er
2·222
1·832
1·0
Rb
19·80
67·1
1·0
Tm
0·327
0·265
1·3
0·6
Yb
2·058
1·664
1·2
1·1
Lu
0·301
0·245
1·8
1·2
Hf
2·4
5·0
1·3
Sr
194·8
Y
20·1
Zr
87·9
659
18·09
229
Nb
7·28
14·29
1·0
Ta
0·454
Sn
1·95
5·72
12·1
Pb
7·53
1·267
2·4
Th
2·10
6·13
1·1
1·3
U
0·505
1·915
1·4
Cs
Ba
La
0·888
169·7
10·52
1217
38·5
0·834
36·2
1·2
5·0
1·3
RSD (relative standard deviation) refers to repeat analyses of AGV-1.
Testing the sub-continental lithospheric
mantle hypothesis
Representative Nd vs 87Sr/86Sr data for a selection of
lamproites for which high-quality data are available are
shown in Fig. 4c. Lamproites exhibit a very extreme
range in Nd from −4 (Wannamaker et al., 2000) to
−26 (Fraser et al., 1985). Such compositions require an
extended period of source evolution in isolation from the
convecting upper mantle. This can be demonstrated
when the Nd-isotope evolution of Gaussberg lamproites
is compared with that of the MORB source mantle (e.g.
Nägler & Kramers, 1998). Using the 147Sm/144Nd decay
equation we calculate the Sm/Nd ratio necessary to
produce the Gaussberg source from the MORB source
mantle for any given time. From a plot of 147Sm/144Nd
ratios against time (Fig. 5a) it is clear that the Gaussberg
source must be at least 0·6 Gyr old. Taking a very
conservative 147Sm/144Nd ratio of 0·078 for the source,
isolation from convecting mantle must have occurred 1
Gyr ago. In this example, the 147Sm/144Nd ratio of MORB
source was assumed to be 0·238 (Nägler & Kramers,
1998) and the implied enrichment that would have accompanied isolation thus would have lowered the ratio
more than three-fold. Using a more realistic enriched
source 147Sm/144Nd ratio of 0·15, which represents a
much less dramatic LREE enrichment, the time required
for isolation is >3 Gyr.
The 147Sm/144Nd ratios that appear to characterize
the Gaussberg source are significantly lower than those
reported for sub-continental lithospheric mantle (McDonough, 1990). Xenoliths interpreted as being samples from
the sub-continental lithospheric mantle have variable but
substantially higher 147Sm/144Nd and 143Nd/144Nd ratios
than are required in the Gaussberg source. Thus, if
Gaussberg lamproites originated in the lithospheric
mantle, their source would have to represent a portion
that is not normally sampled in xenolith populations.
Foley et al. (1987) have suggested that the source of
lamproites could be enriched lithospheric veins. If such
veins formed in the garnet stability field, where heavy
REE (HREE) would be retained in the residue, they
could, over time, evolve to 143Nd/144Nd ratios similar to
those of Gaussberg. A separate thermal event would then
be required to explain why these veins melted after >1
Gyr of stability in the lithospheric mantle.
Constraints imposed by the Rb–Sr isotope system
indicate that the Gaussberg source must have undergone
a prolonged period of Rb enrichment to explain the
radiogenic Sr isotope characteristics of the lamproites
(Collerson & McCulloch, 1983). An estimate of the Sr
evolution of the upper mantle was calculated using an
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Table 2: Major (wt %) and trace (ppm) element concentrations of Gaussberg lamproites
Sample:
KC97-1/1
KC97-1/2
KC97-20A
KC97-20H
KC97-19A
K-97-19B
KC97-19C
KC97-4A
Location:∗
1
1
2
2
3
3
3
4
SiO2
50·8
49·1
51·3
49·2
48·7
49·2
48·9
TiO2
3·3
3·4
3·3
3·1
3·3
3·2
3·3
3·6
Al2O3
10·3
9·6
9·8
9·8
9·4
9·6
9·4
9·8
Fe2O3
6·5
6·7
6·6
6·3
6·9
6·6
6·6
7·0
MnO
0·1
0·1
0·1
0·1
0·1
0·1
0·1
0·1
MgO
7·8
8·1
8·3
8·3
8·7
8·2
7·8
8·1
CaO
4·4
4·3
4·7
4·1
4·4
4·4
4·6
4·4
Na2O
1·8
1·9
1·3
1·1
0·7
1·2
1·5
0·8
11·6
11·7
11·4
11·1
11·0
11·2
11·3
11·8
K2O
P2O5
Total†
1·5
1·6
1·6
1·4
1·6
1·5
1·5
1·5
98·5
96·7
98·9
95·0
95·3
95·6
95·5
99·8
Li
6·89
6·02
6·82
6·88
7·25
6·73
7·21
Be
6·10
4·19
5·55
5·78
5·84
5·43
5·79
Sc
8·8
V
Cr
52·2
94·1
365
12·7
61·8
218
13·7
84·9
270
14·0
86·3
274
14·8
109·0
313
14·2
78·7
327
14·5
91·2
304
6·61
7·19
16·3
76·1
337
Co
32
19
28
27
30
29
29
29
Ni
264
154
230
206
268
266
247
246
Cu
59
24
32
33
35
32
46
31
Zn
109
52
74
74
83
77
80
80
Ga
21·9
14·0
20·0
20·1
19·7
19·8
20·2
21·5
Rb
183
257
302
304
285
310
310
306
Sr
1816
1355
1599
1631
1722
1566
1700
1430
Y
Zr
Nb
20·74
1018
107·3
17·29
715
80·2
20·82
902
91·7
20·96
954
20·54
938
96·8
98·9
20·20
861
90·5
Sn
4·64
5·77
5·64
6·52
6·24
5·38
Cs
1·906
1·555
1·778
1·775
1·914
2·016
Ba
5431
4697
5022
5217
5581
4790
21·76
954
99·9
10·45
1·974
5234
22·73
1420
111·0
8·71
2·073
5085
La
163·0
153·7
178·4
185·0
186·7
177·6
190·4
187·6
Ce
316
272
324
335
337
323
344
346
Pr
34·7
28·3
34·2
35·2
35·8
34·2
36·3
36·4
Nd
116·3
93·5
113·7
117·2
118·1
113·6
120·0
121·5
Sm
15·23
11·85
14·54
14·92
15·16
14·65
15·31
15·53
Eu
3·79
2·93
3·63
3·79
3·79
3·67
3·87
3·93
Tb
1·172
0·859
1·072
1·093
1·093
1·058
1·120
1·164
Gd
9·53
7·28
9·13
9·29
9·26
8·99
9·60
9·76
Dy
5·34
3·87
4·96
5·05
5·00
4·85
5·24
5·44
Ho
0·898
0·661
0·839
0·837
0·823
0·814
0·869
0·922
Er
2·047
1·523
1·921
1·952
1·869
1·879
2·005
2·099
Yb
1·456
1·106
1·405
1·414
1·343
1·342
1·501
1·570
Tm
0·256
0·185
0·229
0·235
0·226
0·223
0·240
0·263
Lu
0·204
0·153
0·193
0·197
0·185
0·189
0·202
Hf
Ta
25·0
5·42
17·5
4·66
22·0
4·79
23·2
4·99
22·9
5·07
21·1
4·63
23·1
5·11
0·216
35·3
5·33
Pb
38·8
29·6
34·9
36·6
36·1
33·3
36·5
39·9
Th
15·9
20·3
22·3
23·2
23·3
22·0
24·4
27·5
U
3·32
2·23
2·88
2·91
986
2·45
2·87
3·24
3·61
MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
Sample:
KC97-4B
KC97-4C
KC97-4D
KC97-16F
KC97-17A
KC97-17B
KC97-23F
KC97-5C
Location:∗
4
4
4
4
4
4
4
5
SiO2
51·1
51·4
50·9
52·4
50·2
50·5
49·6
TiO2
3·4
3·4
3·3
3·2
3·0
3·1
3·2
3·5
Al2O3
9·6
9·5
9·6
10·3
10·4
10·3
9·9
10·0
Fe2O3
6·8
6·7
6·9
6·7
6·2
6·4
6·6
6·7
MnO
0·1
0·1
0·1
0·1
0·1
0·1
0·1
0·1
MgO
7·7
7·8
8·5
8·8
8·4
8·2
8·4
7·9
CaO
4·4
4·3
4·3
4·7
4·5
4·7
4·7
4·3
Na2O
0·8
1·0
0·9
1·7
1·2
1·5
1·3
2·0
11·5
11·0
11·7
12·1
11·3
12·0
11·5
12·3
K2O
P2O5
Total†
1·5
1·4
1·6
1·5
1·4
1·5
1·5
1·6
97·3
96·9
98·3
102·0
97·2
98·5
97·3
100·3
Li
6·61
5·99
6·32
6·79
6·91
6·79
6·92
Be
7·30
7·26
6·93
5·58
5·65
5·61
5·70
Sc
V
Cr
51·5
16·4
78·2
348
9·3
68·1
335
15·6
74·0
347
14·4
86·1
360
14·6
86·9
319
14·6
85·8
316
14·8
88·4
343
7·10
5·77
14·1
85·4
295
Co
30
30
29
30
28
29
29
27
Ni
258
274
252
280
225
243
231
230
Cu
33
52
30
35
32
32
32
32
Zn
82
103
76
78
76
78
77
76
Ga
21·2
21·2
20·7
19·8
20·4
19·8
20·3
19·5
Rb
305
160
323
307
313
305
310
297
Sr
1505
1465
1539
1698
1738
1721
1737
1601
Y
Zr
Nb
23·10
1423
111·0
19·46
1335
102·2
20·74
1314
103·8
20·48
897
20·48
887
96·6
95·9
21·02
899
96·8
21·61
911
98·1
Sn
8·93
5·93
8·51
5·95
6·35
6·33
6·08
Cs
2·324
1·184
2·278
2·131
2·158
2·121
2·178
Ba
5092
4302
5049
5011
5119
5023
5108
21·92
955
96·8
6·37
1·712
5192
La
195·2
154·0
182·8
181·8
184·7
187·9
188·3
187·2
Ce
360
300
333
332
336
344
343
339
Pr
37·8
32·7
35·1
35·2
35·6
36·4
36·3
35·7
Nd
125·6
110·0
116·2
116·8
118·5
121·4
120·9
118·3
Sm
15·94
14·17
14·53
14·91
15·07
15·44
15·45
15·12
Eu
3·96
3·38
3·66
3·80
3·83
3·94
3·95
3·77
Tb
1·183
1·074
1·065
1·063
1·077
1·107
1·129
1·111
Gd
9·90
8·81
8·96
9·14
9·25
9·48
9·60
9·36
Dy
5·46
4·95
4·93
4·92
4·96
5·05
5·11
5·08
Ho
0·926
0·845
0·832
0·805
0·814
0·834
0·864
0·866
Er
2·156
1·953
1·940
1·898
1·891
1·928
1·990
2·022
Yb
1·602
1·439
1·432
1·338
1·324
1·367
1·424
1·465
Tm
0·261
0·252
0·239
0·223
0·223
0·224
0·234
0·245
Lu
0·223
0·196
0·197
0·183
0·184
0·191
0·198
Hf
Ta
35·4
5·40
34·3
5·06
32·5
4·99
21·7
4·90
21·6
4·89
22·0
4·98
22·0
4·99
0·206
23·2
5·00
Pb
40·1
50·5
39·2
34·8
34·7
35·6
35·4
35·9
Th
28·3
17·0
26·6
23·3
23·2
23·5
23·9
23·9
U
3·55
3·71
3·79
3·14
987
3·12
3·15
3·20
3·13
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 6
JUNE 2002
Table 2: continued
Sample:
KC97-11
KC97-12A
KC97-12B
KC97-13A
KC97-14
KC97-27
KC97-28A
KC97-28B
Location:∗
5
5
5
5
5
5
6
6
SiO2
50·8
50·4
50·4
51·0
51·6
50·3
50·3
TiO2
3·4
3·5
3·5
3·4
3·2
3·4
3·2
3·3
Al2O3
10·0
9·9
9·8
10·1
9·5
9·9
10·4
10·0
Fe2O3
6·5
6·7
6·6
6·6
6·6
6·6
6·3
6·6
MnO
0·1
0·1
0·1
0·1
0·1
0·1
0·1
0·1
MgO
7·5
8·0
7·5
7·6
7·4
7·5
7·1
8·0
CaO
4·2
4·3
4·3
4·3
4·2
4·5
4·2
4·3
Na2O
1·9
2·0
2·0
2·3
2·0
1·8
1·8
1·9
12·5
12·2
12·0
11·6
11·7
12·2
12·2
12·2
K2O
P2O5
Total∗
1·5
1·6
1·6
1·5
1·5
1·5
1·4
1·5
98·8
99·1
98·4
99·0
98·1
98·2
97·4
99·9
Li
6·79
7·44
6·77
6·94
6·24
6·92
6·49
Be
5·72
6·07
5·76
5·61
5·91
5·80
5·33
Sc
V
Cr
51·6
13·9
86·5
288
14·9
94·8
287
12·4
85·6
257
14·3
90·9
325
13·8
69·0
249
14·0
88·3
327
14·5
82·3
289
6·80
5·58
13·6
85·7
265
Co
27
30
25
31
23
28
26
27
Ni
221
244
186
268
185
339
212
219
Cu
33
40
47
35
30
33
30
38
Zn
74
84
97
93
71
76
67
74
Ga
19·1
21·3
20·3
20·4
18·0
19·5
18·6
19·6
Rb
283
319
264
310
286
298
303
299
Sr
1603
1754
1593
1723
1371
1651
1487
1642
Y
Zr
Nb
20·94
962
97·1
Sn
6·27
Cs
1·640
Ba
5216
22·86
1040
105·7
10·25
1·812
5686
20·68
993
97·9
21·45
932
22·82
1106
99·2
88·7
20·90
950
98·4
20·08
867
89·0
4·46
6·84
6·90
6·09
5·76
1·626
2·108
1·952
1·890
1·823
5007
5205
4609
5316
4800
21·01
922
94·3
8·03
1·802
5090
La
187·2
204·1
171·1
187·8
173·7
185·5
174·3
183·1
Ce
339
369
320
342
316
337
314
331
Pr
35·5
39·0
33·9
36·2
33·5
35·3
33·0
34·8
Nd
118·1
128·6
112·9
119·4
111·0
116·8
109·6
115·2
Sm
14·98
16·37
14·41
15·32
14·32
14·82
13·85
14·69
Eu
3·74
4·09
3·50
3·85
3·50
3·74
3·47
3·68
Tb
1·085
1·179
1·085
1·109
1·083
1·079
1·017
1·074
Gd
9·19
10·09
8·94
9·48
8·98
9·17
8·55
9·04
Dy
4·98
5·46
4·98
5·15
5·12
4·96
4·70
4·99
Ho
0·844
0·922
0·837
0·860
0·883
0·834
0·795
0·826
Er
1·934
2·092
1·919
1·974
2·148
1·941
1·875
1·937
Yb
1·407
1·537
1·389
1·441
1·611
1·392
1·352
1·423
Tm
0·237
0·256
0·246
0·235
0·265
0·229
0·229
0·235
Lu
0·196
0·211
0·193
0·200
0·225
0·195
0·188
Hf
Ta
23·5
5·03
25·2
5·40
24·6
5·08
22·5
5·07
27·4
4·41
23·0
5·06
21·1
4·64
0·200
22·5
4·80
Pb
37·5
42·7
36·0
35·5
36·3
36·3
33·0
35·5
Th
23·5
25·5
21·0
24·0
25·5
24·0
22·5
23·3
U
3·17
3·42
3·00
3·25
∗For sample locations, see Fig. 2.
†See text for details.
988
3·26
3·19
2·93
3·12
MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
Fig. 4. Radiogenic isotope plots of Gaussberg lamproites (Φ) compared
with representative lamproites and OIB. The lamproites include the
5·7–10·8 Ma Spanish lamproites (Κ) (Turner et al., 1999), the 17–24 Ma
Western Australian lamproites (Β) (Mitchell & Bergman, 1991), the
27 Ma Smoky Butte lamproite, Montana, USA (Η) (Fraser et al., 1985),
the 97–106 Ma Prairie Creek lamproite, USA (Α) (Mitchell & Bergman,
1991), the 1·1–1·2 Ma Leucite Hills lamproites (Μ) (Mitchell &
Bergman, 1991), the >85 Ma Meta do Lenco lamproites, Brazil (Ο)
(Gibson et al., 1995) and the 22 Ma Robbers Roast lamproite, Utah,
USA (+) (Wannamaker et al., 2000). OIB (small Ο) compilation from
Kamber & Collerson (2000). (a) Plot of 207Pb/204Pb vs 206Pb/204Pb;
meteorite isochron and upper-mantle evolution line (MORB) from
Kramers & Tolstikhin (1997). The lamproites, with one exception, plot
to the left of the meteorite isochron, and above the Kramers &
Tolstikhin (1997) depleted mantle evolution line. This contrasts with
OIB, which all plot to the right of the meteorite isochron. (b) Plot of
208
Pb/204Pb vs 206Pb/204Pb; lamproites consistently evolve with a higher
Th/U ratio than depleted mantle (as evidenced by the high 208Pb/
206
Pb ratios). Although most OIB also have high 208Pb/206Pb ratios, they
evolved to consistently higher 206Pb/204Pb ratios than lamproites. (c)
Plot of Nd vs 87Sr/86Sr; there is a very strong distinction between OIB
and lamproites, with no overlap. Lamproites plot in a field of much
less radiogenic Nd with very variable Sr isotope composition (but still
higher than typical OIB).
initial 87Sr/86Sr at 4·5 Ga of 0·69897 (BABI; Papanastassiou & Wasserburg, 1969) and 87Rb/86Sr ratio of
0·08923 (Allègre et al., 1983) and a linear decrease in the
87
Rb/86Sr ratio with time to the present-day depleted
upper-mantle 87Sr/86Sr value of 0·702706 (Allègre et al.,
1983). If the Gaussberg source separated from the upper
mantle 1 Gyr ago it would have had a 87Rb/86Sr ratio
of 0·53 (Fig. 5b, analogous to Fig. 5a). Isolation at 3 Ga
would have required a ratio of 0·22 and would be
consistent with derivation of the source by partial melting
in the garnet stability field, followed by isolation in the
sub-continental lithospheric mantle, as indicated by the
Sm/Nd systematics.
Isotopic evolution of Sr and Nd depends on the respective parent/daughter ratios, and time. In the case of
the Gaussberg source, the parent/daughter ratio is not
known nor is the time over which the source evolved.
Although difficult to constrain, modelling of the Sr and
Nd isotope evolution using estimated parent/daughter
ratios shows that the Gaussberg source could have been
directly derived from the upper mantle, but only if
it remained isolated in the sub-continental lithospheric
mantle for a period in excess of 1 Gyr.
This scenario is not compatible with Pb isotopes. The
Pb isotopic composition of Gaussberg is very unusual for
contemporary mantle-derived rocks. In 207Pb/204Pb vs
206
Pb/204Pb space (Fig. 4a), Gaussberg lamproites plot
above terrestrial evolution lines, but to the left of the
meteorite isochron. This apparently contradicts the first
Pb paradox (i.e. future paradox), which states that average
marine sediment and MORB have evolved to the right
of the meteorite isochron (Allègre, 1969; Kramers &
Tolstikhin, 1997). Importantly, all ocean island basalts
(OIB) plot to the right of the meteorite isochron (Kamber
& Collerson, 1999). Only certain crustal rocks plot close
to Gaussberg Pb. These are generally Archaean granitoids
that have undergone U loss, associated with later highgrade metamorphism (e.g. Montgomery & Hurley, 1978).
The very unusual Pb isotope composition of the Gaussberg lamproites places strong constraints on any model
of lamproite source evolution, because 207Pb/206Pb evolution is non-linear as a result of the difference in half-life
between 238U and 235U. A temporal model can therefore
be developed to calculate whether, and at what time, the
Gaussberg source separated from a specific reservoir such
as the MORB source mantle. Such a temporal model
989
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 6
JUNE 2002
Table 3: Pb, Nd and Sr isotope ratios of the Gaussberg lavas
±
207
±
143
±
87
17·415
0·017
15·594
0·016
17·550
0·052
15·594
0·047
38·155
0·040
38·259
0·114
0·511894
0·000004
0·709248
0·000009
0·511911
0·000004
0·709742
KC-97-4a
17·526
0·035
15·618
0·032
38·396
0·000006
0·077
0·511875
0·000004
0·709816
KC-97-4b
17·557
0·009
15·614
0·009
0·000006
38·321
0·023
0·511884
0·000004
0·709759
KC-97-4c
17·509
0·021
15·579
0·000007
0·019
38·253
0·048
0·511877
0·000004
0·709708
KC-97-4d
17·499
0·028
0·000007
15·577
0·025
38·244
0·062
0·511893
0·000005
0·709741
KC-97-5c
17·527
0·000007
0·024
15·583
0·023
38·240
0·055
0·511881
0·000006
0·709796
KC-97-11
0·000007
17·502
0·014
15·580
0·013
38·171
0·033
0·511913
0·000004
0·709726
0·000006
KC-97-12a
17·500
0·038
15·581
0·035
38·182
0·085
0·511907
0·000004
0·709720
0·000006
KC-97-12b
17·544
0·014
15·630
0·014
38·434
0·034
0·511915
0·000004
0·709724
0·000006
KC-97-13a
17·444
0·029
15·566
0·026
38·145
0·063
0·512013
0·000007
0·709345
0·000010
KC-97-14a
17·500
0·038
15·629
0·035
38·428
0·085
0·511870
0·000004
0·709891
0·000006
KC-97-16f
17·440
0·048
15·579
0·044
38·175
0·106
0·511999
0·000006
0·709185
0·000009
KC-97-17a
17·443
0·020
15·582
0·015
38·162
0·042
0·512015
0·000007
0·709192
0·000010
KC-97-17b
17·449
0·040
15·618
0·036
38·255
0·088
0·511915
0·000004
0·709231
0·000006
KC-97-19a
17·498
0·057
15·590
0·051
38·244
0·125
0·511927
0·000005
0·709532
0·000010
KC-97-19b
17·496
0·021
15·585
0·019
38·221
0·045
0·511917
0·000005
0·709544
0·000009
KC-97-19c
17·483
0·061
15·579
0·056
38·211
0·134
0·511909
0·000005
0·709541
0·000009
KC-97-20a
17·574
0·240
15·626
0·214
38·348
0·524
0·511942
0·000005
0·709735
0·000010
KC-97-20h
17·559
0·022
15·614
0·020
38·318
0·048
0·511922
0·000005
0·709742
0·000011
KC-97-23f
17·457
0·038
15·593
0·035
38·220
0·084
0·511922
0·000007
0·709233
0·000010
KC-97-27
17·501
0·031
15·598
0·021
38·241
0·097
0·511892
0·000004
0·709598
0·000007
KC-97-28a
17·609
0·010
15·611
0·008
38·331
0·021
0·511922
0·000005
0·709747
0·000007
KC-97-28b
17·526
0·046
15·588
0·042
38·251
0·101
0·511899
0·000004
0·709742
0·000009
Sample
206
KC-97-1/1
KC-97-1/2
Pb/204Pb
Pb//204Pb ±
208
Pb/204Pb
Nd/144Nd
Sr/86Sr
±
All uncertainties are absolute 2-sigma standard deviations (Pb) and standard errors (Nd and Sr).
can then be evaluated together with the Nd and Sr
isotopic constraints.
The temporal evolution of Pb isotopes in the MORB
source mantle and in various crustal reservoirs has been
modelled by Kramers & Tolstikhin (1997). Pb isotope
evolution of a source that separated from the MORB
source mantle at a specific time can be calculated using
the Kramers & Tolstikhin (1997) 207Pb/204Pb and 206Pb/
204
Pb values as the initial values in the decay equations
with the parent/daughter ratio as the only remaining
unknown variable. The 238U decay equation can be solved
for a given time to yield the present-day Gaussberg 206Pb/
204
Pb ratio. From this analysis the 238U/204Pb () for the
source can be derived (Fig. 5c). Invariably, we find that
when this is applied to the 235U decay equation, the
corresponding 207Pb/204Pb ratio is lower than that of
Gaussberg and plots to the left of the meteorite isochron
but below terrestrial models. Solving the 235U decay
equation produces isotope compositions that plot above
terrestrial mantle models but to the right of the meteorite
isochron. As an example, we show the results of such
calculations using a separation age of 2·5 Ga in Fig. 5d.
As stated above, 207Pb/204Pb ratios are much lower than
the observed values if the 238U decay scheme is used for
calculation of . Conversely, the 235U decay equation
yields 206Pb/204Pb ratios that are too high. This finding
reflects the fact that it is impossible to obtain evolution
paths that converge close to the Gaussberg value. Thus
the Gaussberg source could not have originated from
2·5-Gyr-old MORB mantle if it evolved with a singlestage . This is true irrespective of the proposed time of
isolation, because the U/Pb ratio calculated via the two
independent decay schemes do not converge over the
age of the Earth (Fig. 5c).
Hence, if Gaussberg Pb evolved from the upper mantle,
a multi-stage history must be invoked, such as that
previously proposed by Williams et al. (1992). A similar
multi-stage history was also proposed by Nelson (1992)
for the Ellendale lamproite of Western Australia. For
example, a source starting with upper-mantle Pb isotope
ratios at 2·5 Ga would have had to evolve with a of
26 for 0·6 Gyr before experiencing a strong decrease
990
MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
Fig. 5. Temporal evolution models for radiogenic isotope systematics of the Gaussberg lamproites. (a) Plot of the 147Sm/144Nd ratio required to
produce the present-day Gaussberg 143Nd/144Nd ratio from the upper mantle, calculated from the 143Nd/144Nd ratios of Nägler & Kramers (1998)
at 100 Myr intervals from 4·3 Ga to the present (continuous line). Also shown is the Sm/Nd evolution line for the MORB source mantle (dashed
line). This line is not smooth because of the dynamic nature of the Nd isotope evolution of MORB source mantle [as modelled by Nägler &
Kramers (1998)]. The older the separation age, the smaller the required decrease in 147Sm/144Nd ratio, as indicated by the convergence of the
curves with increasing age. A minimum age of 0·6 Ga is obtained for a 147Sm/144Nd ratio of zero. (b) Plot of the 87Rb/86Sr ratio required to
produce the present-day Gaussberg 87Sr/86Sr ratio from MORB source mantle (continuous line), analogous to (a). An approximation of the
87
Rb/86Sr evolution of the MORB source mantle was calculated with an initial 87Sr/86Sr ratio of 0·69897 and 87Rb/86Sr ratio of 0·085 and a
linear decrease in the 87Rb/86Sr ratio with time (dashed line). This line is artificially smooth because of the linear nature of the calculations used
in its derivation. Again, the older the separation age, the smaller the required increase in 87Rb/86Sr ratio. (c) Plot of the 238U/204Pb ratio required
to produce the present-day Gaussberg Pb isotope composition from the upper mantle, analogous to (a) and (b), calculated independently via the
238
U/206Pb parent–daughter system (continuous line) and via the 235U/207Pb parent–daughter system (dashed line). As with the Nd isotope
evolution of the mantle (a) these curves are not smooth because of the dynamic nature of the MORB mantle [as modelled by Kramers &
Tolstikhin (1997)]. The two curves do not converge over the entire history of the Earth, indicating that there is no U/Pb ratio that can produce
the Gaussberg lamproites from the upper mantle in a single stage at any time in the Earth’s history. (d) Position of average Gaussberg Pb in
common Pb isotope space (Ε), relative to MORB source mantle (dashed line) of Kramers & Tolstikhin (1997). Also shown are two evolution
paths (a) and (b), starting from the MORB source mantle at 2·5 Ga, yielding the Gaussberg 207Pb/204Pb ratio (a) and 206Pb/204Pb ratio (b),
respectively. This illustrates that the high 207Pb/204Pb ratio of Gaussberg could not be produced in a single stage from MORB source mantle
without driving the 206Pb/204Pb ratio far to the right of the meteorite isochron. (e) Plot of a two-stage evolution path yielding the correct Gaussberg
Pb isotope composition both (Ε) in 206Pb/204Pb and 207Pb/204Pb. It starts from the MORB source mantle at 2·5 Ga with a of 26 for the first
0·6 Gyr (dashed line represents its continuation to the present day) and a of 1·55 from 1·9 Ga to the present day [analogous to fig. 6 of
Nelson (1992)]. This scenario (and similar solutions for older mantle separation ages) requires first an approximately two-fold increase of ,
followed by an unprecedented drop in by a factor of 17. (f ) Plot of the 238U/204Pb ratio required to produce the present-day Gaussberg Pb
isotope composition from continental sediment approximated by the ‘erosion mix’ model of Kramers & Tolstikhin (1997), analogous to (c). The
two curves converge between 2·6 and 2·5 Ga, indicating that it is possible to derive the Gaussberg Pb isotope composition from Archaean
continental-derived sediment with one episode of U/Pb fractionation. This is due to the inherently high 207Pb/204Pb ratio of old upper continental
crust.
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in to 1·55 for the remaining 1·9 Gyr. This is graphically
shown in Fig. 5e. Similar two-stage evolution histories
can be calculated for different separation times, provided
that the separation occurred before 2·2 Ga.
The 2·5 Ga model requires the initial fractionation
event to increase the from the upper mantle value of
6·68 to 26. Although extreme, a melting event could
cause such a change in the U/Pb ratio and could also
have increased the Rb/Sr ratio and reduced the Sm/Nd
ratio. However, the Pb isotope data require a subsequent
event of massive U depletion at >1·9 Ga. It is unlikely
that melt extraction could have removed more than 90%
of the U from the source without complementary Pb
depletion. Furthermore, melt extraction would also reduce the Rb/Sr ratio and increase the Sm/Nd ratio, a
situation that is not consistent with a source that has
experienced K and LREE enrichment. Therefore, if the
Gaussberg source separated from the MORB mantle
in the late Archean, a third event of relatively recent
metasomatism would be required. This would have
strongly enriched K and LREE but also flooded the
source with Sr, Nd and Pb, which would probably
exhibit MORB source mantle isotope compositions. The
evolution of such a source would become increasingly
difficult to model, relying on increasingly older ages of
isolation from the MORB source mantle and increasingly
dramatic enrichment and depletion events, as was suggested by Williams et al. (1992). In the following, we
propose a new and more elegant solution to explain
the isotope evolution of the source of the Gaussberg
lamproites.
A single-stage mantle evolution model for
the lamproite source
The complexity of lithospheric mantle models is largely
caused by the high 207Pb/206Pb ratio of the Gaussberg
lamproites. It therefore appears more likely that Gaussberg Pb evolved for a significant length of time early
in the Earth’s history in a high- environment before
reduction of in a U/Pb fractionation event. Isotopically
similar evolutions are typically found in high- Archaean
gneisses that have experienced substantial U loss induced
by subsequent metamorphism (e.g. Montgomery &
Hurley, 1978).
Continental crustal Pb has had a major influence
on the isotope evolution of Pb in all terrestrial silicate
reservoirs, including the MORB source mantle (Kramers
& Tolstikhin, 1997). For example, recycling of U and Pb
from the upper crust into the MORB source mantle as
subducted sediment explains the second terrestrial Pb
paradox (Kramers & Tolstikhin, 1997; Collerson &
Kamber 1999) and has strongly influenced the 206Pb/
204
Pb isotope evolution of the mantle (Kramers & Tolstikhin, 1997). According to Ringwood et al. (1992) it
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JUNE 2002
may be possible for some subducted sediment to remain
isolated as high-pressure mineral phases in the Transition
Zone or lower mantle. This provides a mechanism for
Pb that evolved for a considerable time in continental
crust to be isolated and stored for a considerable length
of time in the mantle. Thus, continental-derived material
can evolve in a closed system until a subsequent melting
event. Slab dehydration driven by metamorphism at
the time of subduction is a plausible mechanism for
fractionation of parent/daughter ratios (e.g. Becker et al.,
2000).
We now test whether this hypothesis could provide a
model for the Gaussberg magma source. Kramers &
Tolstikhin (1997) modelled the Pb isotope evolution of
terrestrial sediment that has been subducted into the
mantle throughout Earth’s history. We use this modelled
sediment reservoir, termed ‘erosion mix’, as an approximation to the Pb isotope composition of terrestrial
sediment to test whether the Pb isotope evolution of the
Gaussberg source can be reconstructed from it. We
calculate from ‘erosion mix’ the required to produce
Gaussberg Pb isotope ratios via the 238U and the 235U
decay equations, analogous to the calculation for Fig. 5c.
The solutions converge at >2·6 Ga (Fig. 5f ). This
indicates that the Gaussberg Pb isotopes can indeed be
derived from ‘erosion mix’ with initial 206Pb/204Pb, 207Pb/
204
Pb and 208Pb/204Pb values of 14·32, 15·04 and 33·74
with a of 6·4 and a 232Th/238U () of 5·2 from 2·6 Ga.
If the Gaussberg source was derived from subducted
sediment, only relatively small changes in and are
necessary. Relative to the original ‘erosion mix’, a lowering of the original from 13·8 to 6·4 and an increase
of the original from 3·9 to 5·2 is required. This could
have occurred in a number of ways:
(1) if the subducted material represented a mixture
between silicate sediment and an appreciable amount of
carbonate (carbonate is enriched in Pb, is low in U and
has a high Th/U ratio);
(2) if accessory phases such as monazite, which concentrate Th, were stable during subduction (hence increasing the Th/U ratio);
(3) if high O2 fugacity prevailed during subduction;
oxidation of U4+ to the more soluble U6+ could have
caused U to be lost relative to both Th and Pb, thereby
reducing and increasing .
Trace element data reported for eclogites that are
interpreted to have undergone subduction zone dehydration-induced metamorphism (Becker et al., 2000)
show a large range in (1–30) and (0·2–6·2). The
Gaussberg source model and lie within this range.
More importantly, the model-derived of 5·2 for the
Gaussberg source is in good agreement with the calculated from 230Th/232Th isotope results (Williams et al.,
1992).
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LAMPROITES FROM GAUSSBERG, ANTARCTICA
Above, we demonstrated that an extreme 147Sm/144Nd
ratio would be required if the Gaussberg source had
evolved directly from depleted mantle. However, the Nd
of subducted sediment (approximated again by ‘erosion
mix’) at 2·6 Ga was significantly negative (–5), because
it contained unradiogenic Nd that evolved in the continental crust (Nägler & Kramers, 1998). Thus a much
less extreme 147Sm/144Nd ratio of 0·155 is required to
produce Gaussberg Nd isotope ratios from the ‘erosion
mix’ at 2·6 Ga with an initial 143Nd/144Nd ratio of
0·50923. To achieve the calculated 147Sm/144Nd ratio of
the Gaussberg source from the ‘erosion mix’ ( 147Sm/
144
Nd ratio of 1·05 at 2·6 Ga; Nägler & Kramers, 1998)
the Sm/Nd ratio must have increased. During subduction, slabs undergo LREE depletion, which increases
the 147Sm/144Nd ratio. This is seen in ultra-high-pressure
rocks such as jadeite quartzites (ultra-high-pressure metasediments) from the Dabie Mountains, China, that have
a range of 147Sm/144Nd ratios from 0·11 to 0·16 (Liou et
al., 1997). This does not conflict with the LREE-enriched
nature of Gaussberg lamproites because their REE characteristics are likely to predominantly reflect melting
processes.
It is not difficult to produce the observed 87Sr/86Sr
ratios in a mildly enriched isolated mantle reservoir over
a period of 2–3 Gyr. Therefore, even less enrichment is
required if the source started with a more radiogenic Sr
isotope composition (such as that of subducted sediment).
Because the Sr isotope composition of weathered and
eroded continental material is likely to have been heterogeneous (as it is today) no quantitative treatment of the
source evolution in Rb–Sr is warranted. In summary,
producing the Gaussberg source Nd and Sr isotope
signature from continental-derived sediment in the Transition Zone is a simpler solution than enrichment of subcontinental lithospheric mantle and is consistent with
207
Pb/206Pb evolution of the lamproites.
TRACE ELEMENT CHEMISTRY
In this section we evaluate whether our model is consistent
with the trace element chemistry of Gaussberg lamproites.
Gaussberg lavas have very unusual trace element chemistry as they show extreme, but irregular enrichment in
incompatible elements (Table 2; Fig. 6a). However, they
show very limited variation in both major and trace
element concentrations (Table 2). It is probable that
fractional crystallization has affected the trace element
chemistry to some degree, but this mechanism cannot
be used to explain the extraordinary enrichment in
incompatible elements, the shape of REE patterns (Fig.
7), or the general trace element systematics discussed
below. Nevertheless, where possible we use combinations
of elements that have very similar partition coefficients
Fig. 6. (a) N-MORB normalized (Sun & McDonough, 1989) trace
element concentrations of average Gaussberg lamproite (Ε) compared
with average continental crust (Β) after Hofmann (1988). (b) GLOSS
normalized (Plank & Langmuir, 1998) trace element concentrations of
average Gaussberg lamproite (Ε) compared with average continental
crust (Β) after Hofmann (1988) and N-MORB (Φ) after Sun &
McDonough (1989).
(in MORB melting) to examine features that have been
least affected by fractional crystallization. This allows us
to study the effect of crustal contamination (if any) on
Gaussberg lamproites, and to evaluate features of the
source chemistry.
Continental contamination
993
Gaussberg lamproites are considerably more enriched in
incompatible elements (Fig. 6a) than average continental
crust (Hofmann, 1988). This does not, per se, preclude the
possibility of crustal contamination. There are, however, a
number of direct and indirect lines of evidence against
significant crustal contamination:
(1) comparison of MORB-normalized REE plots of
Gaussberg lamproites (Fig. 7) and average continental crust
(Hofmann, 1988) shows that continental crust is enriched
in LREE but not nearly to the extent of the lamproites. If
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Fig. 7. N-MORB normalized (Sun & McDonough, 1989) REE concentrations of individual Gaussberg lamproite samples (Χ) compared
with average continental crust (Β) after Hofmann (1988). The important
feature is the crossover between lamproites and the continental crust
patterns.
contamination had occurred, the LREE concentration of
the lamproites would be reduced! At the same time,
the lamproites are depleted in HREE relative to both
continental crust and MORB. Contamination would therefore readily increase their HREE content. In other words,
the slope of the normalized REE pattern would flatten. In
addition, it is unlikely that crustal contamination would
have affected all the sampled flows equally. The slopes of
individual lamproite samples, however, do not deviate
from the average to any significant degree (Fig. 7), which
indicates insignificant contamination.
(2) A worst-case scenario is calculated where we assume
that the uncontaminated lamproitic melt had a Lu content
of zero and that the Lu content of Gaussberg lamproites
is directly proportional to crustal contamination. Even
in this hypothetical scenario, contamination could only
account for 30% of Gaussberg Pb.
(3) If the Gaussberg Pb isotopic signature were to be
explained by 30% crustal contamination, the composition
of that crust would have to plot well outside the known
terrestrial Pb isotope space, at very high 207Pb/206Pb ratios
seen only in lunar samples.
(4) Most of the other known lamproites also plot above
the depleted mantle and to the left of the meteorite
isochron in 207Pb/204Pb vs 206Pb/204Pb space (Fig. 4a).
This implies that their sources evolved in a very similar
way to Gaussberg. It is very unlikely that they experienced
crustal contamination to a similar degree and by crust
with a similarly unusual isotope composition.
We therefore conclude, in agreement with Collerson
& McCulloch (1983) that the chemistry of Gaussberg
lamproites is a reflection of source chemistry and source
mineralogy with no significant crustal contamination.
Inferred source chemistry
Gaussberg lamproites have very unusual trace element
characteristics, with extreme but irregular enrichments
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in incompatible trace elements (Fig. 6a). These unusual
geochemical features are likely to reflect particular source
features. In this section we review aspects of the chemistry
of the lamproites, using relative element abundances
that have not been strongly influenced by fractional
crystallization, to elaborate the nature of the Gaussberg
source. We first discuss the abundance of the moderately
to highly incompatible elements Sr, Pb, Nb, and Ta, as
these show significant anomalies in Gaussberg lamproites
(Fig. 6a). Second, we will discuss their superchondritic
Nb/Ta ratio, a feature that sets them apart from most
terrestrial volcanic rocks. Third, we discuss the contents
of U, Th and Cs relative to K and Rb. Finally, we discuss
the high concentrations of the compatible elements Mg,
Ni, and Cr of lamproites in the context of a subducted
sediment source for lamproites.
The concentration of Pb in Gaussberg lamproites is
high. This manifests as a prominent positive spike on a
MORB-normalized trace element variation diagram (Fig.
6a). Sr, on the other hand, is significantly under-abundant. These anomalies could not have originated during
melting, because Sr and Pb have similar compatibility
during dry mantle melting (Hofmann, 1988). If the source
of Gaussberg lamproites was in the sub-continental lithospheric mantle (which is depleted as a result of the
production of continental crust), a very specific enrichment event is required where Pb is preferentially
enriched and Sr depleted relative to elements with similar
compatibility. It is possible to fractionate elements with
similar partition coefficients, for example during unmixing of two immiscible melts or fluids (Veksler et al.,
1998). Gaussberg source lithospheric mantle would have
required a very specific metasomatism by a fluid or melt
with relatively high Pb but low Sr content. This complex
and specific history, although not impossible, is very
difficult to test.
Our model, on the other hand, assumes that the source
of Gaussberg lamproites originated from subducted, continental-derived sediments. Therefore we compare the
lamproites with an estimate of present-day Global Subducting Sediment (GLOSS; of Plank & Langmuir, 1998).
When normalized to GLOSS, the lamproites no longer
have a negative Sr anomaly (Fig. 6b), because sediments
inherit a negative Sr anomaly from continental crust.
Moreover, in this normalization, Pb has a negative anomaly (Fig. 6b). This is because normalization to GLOSS
removes the effect of inheritance of relatively high Pb
concentrations from continental crust in sediment. This
feature points to the influence of subduction-dehydration
on the chemistry of the Gaussberg source before its
storage in the deep mantle. In this regard, Pb is a highly
mobile element (Brenan et al., 1995) and is readily lost
from slabs via dehydrating fluids.
Prominent negative anomalies are present for Nb and
Ta in a MORB-normalized trace element diagram (Fig.
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MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
6a). Conventionally, the negative Nb and Ta anomalies
(often associated with negative Ti anomalies) of lamproites
are explained by residual rutile in the lithospheric mantle
(Mitchell & Bergman, 1991). If this explanation is applied
to Gaussberg lamproites it implies that the residue is
strongly enriched in rutile. If we assume that the Th/
Nb ratio of the Gaussberg source was similar to that of
MORB (0·052), and that Nb was retained in rutile to
produce the Th/Nb ratio of the lamproites (0·245), the
source would have retained >4·5 times the Nb content
of the lamproites. Because rutile is the main Ti carrier,
a linear correlation between Nb and Ti can be expected.
However, in Gaussberg lamproites Ti does not show a
negative anomaly in a MORB-normalized pattern (Fig.
6a) and is not correlated with Nb. Titanium in the
Gaussberg source was either decoupled from Nb or the
negative Nb and Ta anomalies are primary features
of a source other than the sub-continental lithospheric
mantle. We favour the latter explanation and propose
that the low Nb and Ta contents were inherited from
continental crust. When normalized to GLOSS, Nb and
Ta show positive anomalies (Fig. 6b). This again reflects
the process of dehydration during subduction where Nb
and Ta are compatible and therefore not significantly
lost relative to U and Th (Kamber & Collerson, 2000;
Rudnick et al., 2000).
The Gaussberg lamproites have superchondritic Nb/
Ta ratios (17·2–20·8), which are extremely rare in terrestrial volcanic rocks. This is not likely to be an analytical
artefact for several reasons. First, the analytical protocol
in the ACQUIRE laboratory consistently reproduces the
Nb and Ta concentrations of standard rocks (Table 1).
For example, the propagated 1-sigma standard deviation
on the Nb/Ta ratio of AGV-1 from the concentration
data (Table 1) is 17·14 ± 0·28. Second, reproducibility
of the Nb/Ta ratio is even slightly better than those of
the Nb and Ta contents. For example, the average Nb/
Ta ratio of AGV-1 by ACQUIRE ICP-MS analysis is
17·14 ± 0·27. Third, the only potential deviation from
true values is towards lower Nb/Ta ratios because the
less abundant element Ta is more sensitive to sample
carry-over and occasional high aberrant blanks (or beaker
memory). Thus, we regard the superchondritic Nb/Ta
ratio of the lamproites to be a real feature.
Virtually all OIB and MORB have chondritic to slightly
subchondritic Nb/Ta ratios (Kamber & Collerson, 2000).
Because Nb and Ta are not significantly decoupled
during melting, the Gaussberg source must have had a
superchondritic Nb/Ta ratio. Nb/Ta ratios in the subcontinental lithospheric mantle range from chondritic for
depleted samples ( Jochum et al., 1989) to crustal (i.e.
significantly subchondritic) for enriched samples (McDonough, 1990). Samples of the sub-continental lithospheric
mantle that are interpreted to have undergone metasomatism contain spinel with reaction rims enriched in
Nb and Ta (Bodinier et al., 1996). These rims dominate
the Nb and Ta budget, but they consistently have low Nb/
Ta ratios. Therefore, melts of enriched sub-continental
lithospheric mantle should have low Nb/Ta ratios.
Nb/Ta ratios of sediments are also low (e.g. GLOSS =
14·2; Plank & Langmuir, 1998) and melting of unmodified
sediment should lead to melts with low Nb/Ta ratios.
However, Nb and Ta are fractionated in subduction
zones, either during dehydration (Kamber & Collerson,
2000) or during partial melting with residual rutile (Rudnick et al., 2000). This is inferred from the fact that
eclogitic metabasalts have substantially higher Nb/Ta
ratios (33·7 ± 3·5; Kamber & Collerson, 2000) than
their protoliths. High-P, medium-T metasediments from
the Swiss Alps (Henry et al., 1996) also show strong
unidirectional Nb/Ta fractionation, with some dehydrated samples recording a Nb/Ta increase from 11
in the original sediment to 18 in their metamorphosed
equivalent. Therefore, the high Nb/Ta of Gaussberg
lamproites may well be a source feature that records Ta
loss during subduction-zone processes. On a global scale,
lamproites exhibit a large range in Nb/Ta from 14 to
20 (Gibson et al., 1995; Lambert et al., 1995). In our
model this reflects variable extent of dehydration of
heterogeneous sediment.
Further features of Gaussberg lamproites are relative
depletions in Cs, Th, and U (and Pb) compared with
GLOSS (Fig. 6b). These can also be explained by subduction metamorphism, because Cs, Th and U are among
the most fluid-mobile during dehydration (Brenan et al.,
1994). They are strongly enriched in arc magmas (Pearce
& Peate, 1995) whereas complementary depletions are
seen in high-pressure metabasalts (Becker et al., 2000)
and metasediments (Liou et al., 1997). In view of interelement differences in distribution behaviour (i.e. lacking
the chemical similarity of Nb and Ta) and redox sensitivity
(U), no quantitative treatment is warranted.
Lamproites are not only strongly enriched in incompatible elements but also have substantial concentrations of compatible elements. The high
concentrations of MgO, Ni, and Cr are difficult to
explain if the lamproite source consisted entirely of deeply
subducted sediment. The isotopic and incompatible trace
element evidence does not, however, require a pure
metasediment source. Pb, Nd, and Sr isotope compositions can be reconciled with a substantial contribution
of basaltic melt from melting of surrounding peridotite.
There is the added possibility that eclogitic oceanic crust
with which the metasediment was associated also melted
and contributed towards the compatible element budget
of the final lamproite melt. Further deliberations on the
issue of compatible element (including MgO) contents of
lamproites are at present not warranted because it is
unclear what compositions can be expected for melting
of peridotite and eclogite at Transition Zone pressure.
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However, Os-isotope systematics could probably discriminate between a peridotite and eclogite origin of the
compatible elements in lamproites.
MELTING ENVIRONMENT
Knowledge of potential melting environments and the
properties of melting assemblages are important components of any model for lamproite formation. Gaussberg
lamproites require isolation of their source from the
convecting mantle for at least 2 Gyr. Plausible physical
environments are restricted to the sub-continental lithospheric mantle, the Transition Zone, and the lower
mantle because only they can withstand entrainment into
convecting upper mantle on a time scale >0·5 Gyr.
In this section we discuss physical (i.e. pressure and
temperature) and mineralogical controls on potential
lamproite sources provided by high-pressure and -temperature experiments on relevant compositions.
Results of experiments at lithospheric pressures and
temperatures (Mitchell, 1995; Sato et al., 1997; Edgar &
Mitchell, 1997; Konzett et al., 1997) suggest that an
assemblage rich in phlogopite, K–Ti-amphiboles, clinopyroxene, apatite, and a mixture of exotic K–Ba–Zr–Nb
titanates could be the source of lamproites. These mineral
phases are potentially stable throughout the sub-continental lithospheric mantle possibly extending into the
diamond stability field (Sato et al., 1997; Konzett et al.,
1997), and could be represented by MARID (mica,
amphibole, rutile, ilmenite, and diopside) xenoliths found
in kimberlites (Dawson & Smith, 1977). Experimental
evidence for the upper pressure and temperature stability
of such paragenesis is incomplete. If such a source is
envisaged for Gaussberg, melting could have occurred
by decompression or thermal perturbation (by a plume).
If we assume a geothermal gradient equivalent to a
surface heat flow of 44 mW/m2 it seems possible that an
enriched sub-continental lithospheric mantle source could
melt as a result of decompression (Sato et al., 1997).
However, decompression melting proceeds to varying
degrees, leading to fractionation trends in major and
trace elements. Such trends are not seen in Gaussberg
lamproites. This is supported by the lack of evidence for
the surface expression of sub-continental lithospheric
mantle thinning or erosion (e.g. rifting, basin formation)
or other likely features of ascent of hot asthenosphere
beneath the sub-continental lithospheric mantle. Rather,
the East Antarctic shield in the vicinity of Gaussberg
supports the burden of an ice sheet.
Alternatively, melting could have been caused by thermal perturbation from a plume head. This, too, seems
an unlikely scenario for Gaussberg lamproites. Because
plume heads incorporate, upon ascent through the asthenosphere, material from surrounding mantle their
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melting products (i.e. OIB) invariably represent mixtures
of deep and shallow mantle material. This is clearly
expressed in the Pb-isotope systematics of OIB, yet no
mixing trends are seen in the chemical and isotopic
composition of Gaussberg lamproites. Furthermore, the
Pb isotope composition of all plume-related OIB is so
distinctive (Fig. 4a) that a plume source for Gaussberg
can be excluded. In addition, Gaussberg is situated
500 km from the closest expression of plume volcanism
at Heard Island (Collerson & McCulloch, 1983). As
volcanism occurred contemporaneously at Heard Island
and Gaussberg it is very unlikely that the two are related.
Therefore in agreement with Collerson & McCulloch
(1983) and Tingey et al. (1983), we envisage a separate
isolated magmatic event.
In the deep mantle, only the highly viscous lower
mantle or the Transition Zone could contain lamproite
source material. We have argued that the source of
lamproites is deeply subducted sediment. Mantle tomographic studies show that oceanic lithospheric slabs containing a small amount of subducted sediment can
accumulate in the Transition Zone (e.g. Simons et al.,
1999). The important factor that governs the behaviour
of slabs at Transition Zone depths is buoyancy, which is
controlled by the relative densities of mineral phases.
Experimental studies on crustal compositions (Irifune et
al., 1994) have been carried out for Transition Zone
pressures and temperatures, but not yet for lower-mantle
conditions. Under Transition Zone conditions, the following minerals derived from continental sedimentary
protoliths have been observed: majorite, K-hollandite,
stishovite, CAS (calcium aluminium silicate), and Caperovskite (Irifune et al., 1994), which are consistent with
natural Transition Zone samples found as xenoliths from
Malaita, Solomon Islands (Collerson et al., 2000). The
presence of K-hollandite in this assemblage provides a
potential source for highly potassic melts. The total
density of such material is dependent predominantly on
the proportion of stishovite and K-hollandite (specific
gravity of 4·3 and 3·9 g/cm3, respectively), but it appears
plausible that certain assemblages could achieve neutral
buoyancy at the Transition Zone and thus could reside
in the Transition Zone for a long period of time.
Melting of this K-rich assemblage could be triggered
by decompression to >350 km, where K-hollandite is
no longer stable and converts to wadeite (Irifune et al.,
1994). Wadeite-bearing assemblages are not thermally
stable and melt to produce a high K/Na liquid (Irifune
et al., 1994). Entrainment of the Transition Zone source
into asthenospheric convection cells or perturbation of
the Transition Zone by a plume would explain decompression responsible for the adiabatic melting of
wadeite.
Our proposed mode of lamproite formation (sediment
subduction, storage and subsequent melting) invokes a
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MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
logical sequence of integral processes of plate tectonics.
Because lamproites are very rare it would appear that
the vast majority of subducted sediment is not stored in
the Transition Zone but is mixed into the asthenosphere,
possibly because of buoyancy once thermal equilibrium
is reached. Mixing of continental sediment is required by
other geochemical observations; in particular, solutions to
the second terrestrial Pb paradox (Kramer & Tolstikhin,
1997; Collerson & Kamber, 1999; Kamber & Collerson,
2000).
IMPLICATIONS FOR OTHER
LAMPROITES
Our proposed alternative model for the petrogenesis
of lamproites was developed specifically to match the
characteristics of lamproites exposed at Gaussberg, Antarctica. We next discuss its relevance for other lamproite
occurrences. We first review the highly variable Pb isotope
chemistry of lamproites with reference to three examples,
which encompass the wide array of lamproite Pb isotope
compositions. We then discuss the Sr and Nd isotope
characteristics of lamproites, specifically the two trends
observed in 87Sr/86Sr vs 143Nd/144Nd space (Fig. 4c).
Our Pb isotope evolution model for the Gaussberg
lamproite source requires continental-derived sediment
to have been isolated within the mantle since the late
Archaean. Global lamproites, although mostly plotting
to the left of the meteorite isochron, display a large range
in both 207Pb/204Pb and particularly 206Pb/204Pb isotope
ratios (Fig. 4a). This isotope variation can be explained
with our model if the time of source isolation is adjusted
and if small allowances are made for the wide range in
the 207Pb/206Pb ratio of Archaean continental crust. The
following three examples illustrate these effects.
Lamproites from Western Australia (17–24 Ma; Mitchell & Bergman, 1991) have very high 207Pb/204Pb isotope
ratios (Fig. 4a). They plot above ‘erosion mix’, which
indicates that they cannot be derived from this modelled
evolution of subducted continent-derived sediment. The
source of Western Australian lamproites must have
evolved from a source that had a higher 207Pb/206Pb ratio
than ‘erosion mix’ (i.e. a record of very high early in
the Earth’s history). However, the temporal Pb isotope
evolution of the ‘erosion mix’ represents a global average
of subducted sediment. It is well known that the isotope
(and chemical) composition of sediment varies in modern
subduction zones (e.g. Plank & Langmuir, 1998). By
analogy, the Pb-isotope composition of sediment in Archaean subduction zones is also likely to have varied.
The degree of isotopic variability in Pb was probably
even larger than it is today, because the late Archaean
was the time when the largest variability existed in
terrestrial 207Pb/206Pb ratios. This is exemplified by the
characteristic ‘banana shape’ of the terrestrial common
Pb-isotope array (e.g. Kramers & Tolstikhin, 1997). It is
further interesting to note that the Yilgarn craton of
Western Australia is a member of the group of very high
207
Pb/206Pb cratons (Oversby, 1975) that also includes
the Zimbabwe, Slave, and Wyoming cratons. Thus, the
Pb isotopes of sediments from such cratons subducted in
the Archaean would provide possible material from which
the source of Western Australian lamproites could subsequently have evolved with a low .
By contrast, the Smoky Butte lamproites (27 Ma; Mitchell & Bergman 1991) plot significantly to the left of
the meteorite isochron below ‘erosion mix’ and close to
the MORB source evolution line (Fig. 4a). Calculations
analogous to those in Fig. 5f indicate that the source of
these lamproites could be subducted continental-derived
sediment that was isolated for 3 Gyr. This demonstrates
that relatively minor adjustments of the isolation age (i.e.
3·0 instead of 2·6 Ga) have substantial effects on the
present-day isotope composition of possible lamproite
sources because of the strong age-sensitivity of the 235U–
207
Pb isotope system.
The only lamproites that plot to the right of the
meteorite isochron (Fig. 4a) in a position very close to
that of modern-day continental crust are from Spain
(5·7–10·8 Ma: Mitchell & Bergman 1991). In our model,
they are explained as melting products of continentalderived sediment that was very recently subducted into
the mantle and has not yet had time to evolve to the left
of the meteorite isochron. This appears to be compatible
with their occurrence close to the Alpine front, along
which sediment was subducted throughout the relatively
recent closure of the Tethys ocean.
The Sr and Nd isotope characteristics of lamproites
are also highly variable and show two distinct trends in
87
Sr/86Sr vs 143Nd/144Nd space. Lamproites from Gaussberg and specifically Western Australia plot on a trend
of increasing 87Sr/86Sr whereas lamproites from Smoky
Butte and Leucite Hills plot on a trend of decreasing Nd
with little change in 87Sr/86Sr ratio (Fig. 4c). The variations probably represent heterogeneities in the sedimentary material being subducted into the mantle and
more importantly in subduction-zone processes (i.e. the
extent to which Rb/Sr and Sm/Nd were fractionated
during subduction dehydration). The Western Australian
lamproites may have evolved with high Rb/Sr ratios
because K-bearing phases dominated over Ca-bearing
phases in the continent-derived sediment that made up
their source during its subduction into the deep mantle.
Similarly, the Smoky Butte and Leucite Hills lamproites
may have evolved with low Rb/Sr ratios as a result of
the stability of carbonate and low Sm/Nd as a result
of the stability of monazite during subduction of their
sources.
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In conclusion, the highly variable Pb isotope composition of lamproites is an expected feature of our model
because subduction has been operational for a long time
and because there is local variability in continental Pb
isotope composition that was probably even more pronounced during the late Archaean. Variations in Nd and
Sr isotope ratios can be explained by differences in slab
mineralogy during dehydration and the proportion of
carbonate and phosphate in the sediment.
IMPLICATIONS FOR OIB
PETROGENESIS
The isotopic variability of OIB has led geochemists
to propose the existence of chemically distinct mantle
components, which have remained relatively isolated for
up to billions of years in order to evolve the observed
range in isotope composition (Hofmann, 1997). Although
there is general consensus that the mantle is not chemically homogeneous, there is little agreement about the
nature and origin of its heterogeneities. Potential endmember mantle reservoirs that are widely accepted include recycled oceanic lithosphere, sub-continental lithospheric mantle, a lower-mantle component, and recycled
sediment (Zindler & Hart, 1986). According to Zindler
& Hart (1986), isotopic variability observed in OIB is
explained by mixing of these reservoirs in different proportions.
If, as we propose, lamproites are melts of deeply
subducted sediment in the Transition Zone, there should
be significant overlap of their isotopic characteristics with
those of OIB, which are believed to have a sedimentary
source component (e.g. EMII; enriched mantle II). However, in a comparison of the two melt types the following
observations can be made:
(1) in 207Pb/204Pb vs 206Pb/204Pb space, only one lamproite occurrence plots significantly to the right of the
meteorite isochron (Southern Spain; Turner et al., 1999)
whereas all OIB plot to the right. Thus, except for the
anomalous Spanish lamproites there is no significant
overlap between the two and importantly no clear mixing
trends exist (Fig. 4a).
(2) 208Pb/204Pb vs 206Pb/204Pb space also shows very
little overlap between lamproites and OIB, with the
majority of lamproites plotting to the left of the OIB
field (Fig. 4b).
(3) There is no overlap between the Nd isotopic compositions of OIB and lamproites. It should be noted in
particular that the Spanish lamproites have significantly
negative Nd values (Fig. 4c). The Nd isotope ratios of
lamproites are far less radiogenic than those of even the
most enriched OIB.
(4) Sr isotope ratios of lamproites slightly overlap with
those of the most radiogenic OIB (Fig. 4c).
NUMBER 6
JUNE 2002
Thus a comparison of lamproite and OIB isotope
systematics indicates that subducted sediment is unlikely
to be a significant component of the OIB source, a
conclusion that contrasts with some interpretations of Hf
isotopes in OIB (e.g. Blichert-Toft et al., 1999).
On the basis of Pb isotope ratios, Chauvel et al. (1992)
postulated that certain OIB (EMI; enriched mantle I)
may represent mixtures of subducted pelagic sediment
and recycled oceanic crust. In that scenario, subducted
sediment is represented by Pb isotope ratios very similar
to those of the most radiogenic lamproites, plotting above
the depleted mantle but slightly to the left of the meteorite
isochron, whereas HIMU (high ) OIB represent ancient
subducted oceanic crust. In 207Pb/204Pb vs 206Pb/204Pb
space, EMI OIB plot on a potential mixing trend between
HIMU and subducted sediment. Although Chauvel et
al.’s (1992) model is mathematically viable for Pb isotope
ratios, it fails to explain three features of OIB isotope
characteristics. First, because the Pb content of sediment
is over 60 times higher than that of MORB whereas its
Nd content is only 3·5 times higher than in MORB, the
postulated subducted sediment component in the OIB
source would have no substantial effect on Nd isotope
ratios. Second, and more important, in 207Pb/204Pb vs
206
Pb/204Pb space, lamproites plot in a broad field similar
in size to that of OIB (Fig. 4a). This isotope variability
was not taken into account by Chauvel et al. (1992), who
modelled the sediment with Pb compositions comparable
with relatively radiogenic lamproites. However, because
a very small contribution of unradiogenic sediment would
strongly affect the Pb isotope composition of OIB, we
would expect to find at least some OIB (with postulated
sediment source component) to plot significantly to the
left of the meteorite isochron. This is not seen. Third,
the putative Pb-isotope mixing space between lamproites
and HIMU OIB is occupied by the EMI OIB, the
majority of which have very high 3He/4He (Moreira et
al., 2001), which implies a primitive undegassed component in their source. This is very difficult to reconcile
with a source consisting of oceanic crust and subducted
sediment, both of which will have undergone degassing
at the Earth’s surface.
Trace element systematics have been used to argue for
(Sims & DePaolo, 1997) and against (Hofmann, 1997)
the presence of subducted sediment in the OIB source.
In view of the complex and heterogeneous nature of the
fractionation processes that have affected lamproites (i.e.
sedimentary subduction, and melting processes with residual Transition Zone mineralogy), we suggest that the
use of trace element ratios as fingerprints for involvement
of highly enriched mantle source components, such as
subducted sediment, should be treated with caution.
Element pairs that have near identical partition coefficients during MORB melting (e.g. Nb/Ta, Th/Nb,
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MURPHY et al.
LAMPROITES FROM GAUSSBERG, ANTARCTICA
Y/Ho, and Ce/Pb) are apparently fractionated in subduction-related dehydration. As a result of the inherent
heterogeneities in both the subducted sediment and in
the degree of subduction dehydration, a wide range of
ratios can be produced (e.g. the Nb/Ta ratios of lamproites range from 14 to 20). Such variability is not seen
in OIB (Kamber & Collerson, 1999). In addition, the
presence of unusual mineral phases such as K-hollandite
in the melt source will also have a significant effect on the
fractionation of trace element ratios. The mineral–melt
partition coefficients for elements such as the REE, Pb,
U, and Th are notably different in K-hollandite compared
with more typical mantle minerals (Irifune et al., 1994).
These observations mean that the trace element systematics of melts from subducted sediment are expected
to be highly variable. Such variability could result in the
atypical trace element systematics of lamproites [e.g.
negative Cs, Sr, Nb, and Ta anomalies, positive Pb and
Ba anomalies (Fig. 6a)], which are not seen in OIB.
A further implication of our model is that although
lamproites and many OIB originate in the deep mantle,
their sources could occupy separate regions that do not
significantly mix. If the sedimentary source of lamproites
is neutrally buoyant and thermodynamically stable for
billions of years in the Transition Zone it would indicate
that OIB sources are unlikely to originate in the Transition Zone. Rather, in agreement with interpretation of
Os and He isotopic data for OIB (Walker et al., 1995;
Moreira et al., 2001) as well as mantle tomographic studies
(Shen et al., 1998), OIB are more likely to be sourced in
the lower mantle.
SUMMARY AND CONCLUSIONS
We argue that new comprehensive major and trace
element and Pb–Sr–Nd isotope data for lamproite
samples from Gaussberg, Antarctica, are difficult to reconcile with existing models of lamproite formation,
including melting of enriched lithospheric veins ultimately
derived from asthenospheric mantle (Foley et al., 1987)
and melting of subduction-related metasomatized lithosphere (Nelson, 1992). Both of these models interpret
the source of lamproites to be in the sub-continental
lithospheric mantle. In each case a complex and specific
series of fractionation events is required to explain the
combined Pb, Nd and Sr isotope systematics. A source
in the sub-continental lithospheric mantle is also difficult
to reconcile with the highly unusual lamproite trace
element characteristics. Our main concern with these
models is not the proposed nature of processes, but rather
the complex sequence of events that would have had to
be repeated for each lamproite occurrence.
We propose an alternative, more straightforward model
for lamproite petrogenesis that operates with processes
that are integral components of the Earth’s convection
system. We show that lamproites could be melts derived
from sediment that was subducted to the Transition Zone
or lower mantle where it remained isolated. Variability
in Pb isotope ratios among different lamproite occurrences in our model reflects the time of source isolation, which in some cases can be up to 2–3 Gyr. This
new model explains the Pb, Nd, and Sr isotope evolution
of the Gaussberg lamproites, specifically their high 207Pb/
206
Pb ratio, with a single fractionation event related to
dehydration during subduction. In addition, the model
provides plausible explanations for the unusual trace
element systematics of lamproites, which could inherit
the trace element signature of continental-derived sediment [e.g. negative Sr, Nb, and Ta anomalies (Fig. 6a)],
modified during subduction-related dehydration [e.g. the
high Nb/Ta ratio and the negative Cs, U, Th and Pb
anomalies when compared with GLOSS (Fig. 6b)], and
during melting (e.g. the enrichment in LREE and Ba).
Our model also has implications for OIB petrogenesis
and mantle dynamics. It is widely believed that subducted
sediment is an important component of the source of
certain OIB (Chauvel et al., 1992). This is, however, not
compatible with both the Pb and Nd isotope ratios of
subducted sediment if they are indeed represented by
lamproites. Furthermore, no mixing trends exist between
OIB and lamproites in 207Pb/204Pb vs 206Pb/204Pb space
(Fig. 4a) or in Sr vs Nd space (Fig. 4c). Rather our model
implies that the sources of OIB and lamproites could be
physically separated in the mantle and do not mix.
The existence of lamproites reflects the presence of
chemically distinct reservoirs in the Transition Zone or
lower mantle. Irrespective of the nature of these, a
minimum isolation time of 2 Gyr is unavoidable for those
lamproite sources, whose Pb isotopes plot farthest to the
left of the meteorite isochron. The sparse occurrence of
lamproites suggests either that such mantle domains are
very rare or that they are highly stable in the deep
mantle. Our model is feasible from a geodynamic point
of view, but further experimental work is required to
explore in detail the nature of the source and define the
melting mechanism.
ACKNOWLEDGEMENTS
Reviews by Catherine Chauvel, Marge Wilson, and an
anonymous reviewer greatly improved the final manuscript. Field support for this project was provided by
ANARE via an ASAC grant to K.D.C. Laboratory
investigation costs were partially defrayed by ARC grant
A39701577 to K.D.C. & J.X. Zhao. We would like
to acknowledge the Australian Antarctic Division for
logistical support, and Robyn Frankland, Andy Ciacci,
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JOURNAL OF PETROLOGY
VOLUME 43
and Darryn Schneider for field assistance. Alan Greig is
thanked for sharing his ICP-MS expertise.
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