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Journal of Petrology | Volume 43 | Number 6 | Pages 981-1001 | 2002
© Oxford University Press 2002
Lamproites from Gaussberg, Antarctica: Possible Transition Zone Melts of Archaean Subducted Sediments
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
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONPetrogenetic 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/204Pb = 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 continent-derived 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.
KEY WORDS: lamproites; Pb isotopes; mantle Transition Zone; subducted sediment; anomalous mantle reservoirs
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INTRODUCTION |
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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 Pb isotope compositions that lamproites, kimberlites, and carbonatites form by melting of portions of the sub-continental 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 sub-continental lithospheric mantle origin for these melts.
Continental-derived sediment, which may be subducted along with oceanic lithosphere, is inherently K-rich 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.
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GEOLOGICAL BACKGROUND, SAMPLES, AND PETROGRAPHY |
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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 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.
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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 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.
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ANALYTICAL PROCEDURES |
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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).
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Trace elements were analysed by ICP mass spectrometry (ICP-MS) on a Fisons PQ2+ system at the Advanced Centre for Queensland University Isotopic 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 AGV-1 (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.
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MAJOR ELEMENT CHEMISTRY |
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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 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 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.
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), Yakokut lamproites, Siberia (
) (Mues-Schumacher et al., 1995
) and phlogopite lamproites (•), Leucite Hills lamproites (
) (Mitchell & Bergman, 1991
) (Gibson et al., 1995 |
ISOTOPE GEOCHEMISTRY |
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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 206Pb/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.
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) (Turner et al., 1999
) (Fraser et al., 1985
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.
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), relative to MORB source mantle (dashed line) of Kramers & Tolstikhin (1997)
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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 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 87Rb/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 206Pb/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 high-grade 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 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/204Pb 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/204Pb ratio. From this analysis the 238U/204Pb (µ) for the source can be derived (Fig. 5c). Invariably, we find that when this µ 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 µ. 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 single-stage µ. 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 µ of 26 for 0·6 Gyr before experiencing a strong decrease in µ 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 µ 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-µ environment before reduction of µ in a U/Pb fractionation event. Isotopically similar evolutions are typically found in high-µ 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/204Pb isotope evolution of the mantle (Kramers & Tolstikhin, 1997). According to Ringwood et al. (1992) it 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 µ 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/204Pb and 208Pb/204Pb values of 14·32, 15·04 and 33·74 with a µ 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 µ and are necessary. Relative to the original ‘erosion mix’, a lowering of the original µ 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:
- 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);
- if accessory phases such as monazite, which concentrate Th, were stable during subduction (hence increasing the Th/U ratio);
- 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 µ 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 µ (1–30) and (0·2–6·2). The Gaussberg source model µ 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).
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/144Nd 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 sub-continental lithospheric mantle and is consistent with 207Pb/206Pb evolution of the lamproites.
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TRACE ELEMENT CHEMISTRY |
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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 (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.
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Continental contamination
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:
- 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 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.
- 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.
- 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.
- 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 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. 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 sub-continental 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 inter-element 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. However, Os-isotope systematics could probably discriminate between a peridotite and eclogite origin of the compatible elements in lamproites.
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MELTING ENVIRONMENT |
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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 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 Ca-perovskite (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 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).
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IMPLICATIONS FOR OTHER LAMPROITES |
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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 µ 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 207Pb/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 µ.
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–207Pb 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 continental-derived 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 87Sr/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.
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.
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IMPLICATIONS FOR OIB PETROGENESIS |
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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 end-member 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:
- 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).
- 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).
- 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.
- Sr isotope ratios of lamproites slightly overlap with those of the most radiogenic OIB (Fig. 4c).
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 µ) 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 206Pb/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, 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.
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SUMMARY AND CONCLUSIONS |
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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/206Pb 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.
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ACKNOWLEDGEMENTS |
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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, and Darryn Schneider for field assistance. Alan Greig is thanked for sharing his ICP-MS expertise.
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FOOTNOTES |
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*Corresponding author. Telephone: +61 7 33659776. Fax: +61 7 33651277. E-mail: d.murphy{at}earth.uq.edu.au
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REFERENCES |
|---|
Allègre, C. J. (1969). Comportement des systèmes U–Th–Pb dans le manteau supérieur et modèle d’évolution de ce dernier au cours des temps géologiques. Earth and Planetary Science Letters 5, 261–269.
Allègre, C. J., Hart, S. R. & Minster, J. F. (1983). Chemical structure and evolution of the mantle and continents determined by inversion of Nd and Sr isotopic data, II, numerical experiments and discussion. Earth and Planetary Science Letters 66, 191–213.[Web of Science]
Becker, H., Jochum, K. P. & Carlson, R. W. (2000). Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones. Chemical Geology 163, 65–99.
Blichert-Toft, I., Frey, F. A. & Albarède, F. (1999). Hf isotope evidence for pelagic sediments in the source of Hawaiian basalts. Science 285, 879–882.
Bodinier, J. L., Merlet, C., Bedini, R. M., Simien, F., Remaidi, M. & Garrido, C. J. (1996). Distribution of niobium, tantalum, and other highly incompatible trace elements in the lithospheric mantle: the spinel paradox. Geochimica et Cosmochimica Acta 60, 545–550.
Brenan, J. M., Shaw, H. F., Phinney, D. L. & Ryerson, F. J. (1994). Rutile–aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th—implications for high-field strength element depletions in island-arc basalts. Earth and Planetary Science Letters 128, 327–339.
Brenan, J. M., Shaw, H. F. & Ryerson, F. J. (1995). Experimental-evidence for the origin of lead enrichment in convergent-margin magmas. Nature 378, 54–56.
Chauvel, C., Hofmann, A. W. & Vidal, P. (1992) HIMU-EM: the French-Polynesian connection. Earth and Planetary Science Letters 110, 99–119.[Web of Science]
Christensen, U. R. (1996). The influence of trench migration on slab penetration into the lower mantle. Earth and Planetary Science Letters 140, 27–39.
Collerson, K. D. & Kamber, B. S. (1999). Evolution of the continents and the atmosphere inferred from Th–U–Nb systematics of the depleted mantle. Science 283, 1519–1522.
Collerson, K. D. & McCulloch, M. T. (1983). Nd and Sr isotope geochemistry of leucite-bearing lavas from Gaussberg, East Antarctica. In: Oliver, R. L., James, P. R. & Jago, J. B. (eds) Antarctic Earth Science; Fourth International Symposium. Cambridge: Cambridge University Press, pp. 676–680.
Collerson, K. D., Hapugoda, S., Kamber, B. S. & Williams, Q. (2000). Rocks from the mantle transition zone: majorite-bearing xenoliths from Malaita, southwest Pacific. Science 288, 1215–1223.
Dauphas, N. & Marty, B. (1999). Heavy nitrogen in carbonatites of the Kola Peninsula: a possible signature of the deep mantle. Science 286, 2488–2490.
Dawson, J. B. & Smith, J. V. (1977). The MARID (mica–amphibole–rutile–ilmenite–diopside) suite of xenoliths in kimberlite. Geochimica et Cosmochimica Acta 41, 309–323.[Web of Science]
Edgar, A. D. & Mitchell, R. H. (1997). Ultra high pressure–temperature melting experiments on a SiO2-rich lamproite from Smoky Butte, Montana: derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle. Journal of Petrology 38, 457–477.
Eggins, S. M., Woodhead, J. D., Kinsley, L. P. J., Mortimer, G. E., Sylvester, P., McCulloch, M. T., Hergt, J. M. & Handler, M. R. (1997). A simple method for the precise determination of 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation. Chemical Geology 134, 311–326.[Web of Science]
Foley, S. F., Venturelli, G., Green, D. H. & Toscani, L. (1987). The ultrapotassic rocks—characteristics, classification, and constraints for petrogenetic models. Earth-Science Reviews 24, 81–134.
Fraser, K. J., Hawkesworth, C. J., Erlank, A. J., Mitchell, R. H. & Scottsmith, B. H. (1985). Sr, Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites. Earth and Planetary Science Letters 76, 57–70.
Gibson, S. A., Thompson, R. N., Leonardos, O. H., Dickin, A. P. & Mitchell, J. G. (1995). The Late Cretaceous impact of the Trindade Mantle Plume—evidence from large-volume, mafic, potassic magmatism in SE Brazil. Journal of Petrology 36, 189–229.
Graham, S., Lambert, D. D., Shee, S. R., Smith, C. B. & Reeves, S. (1999). Re–Os isotopic evidence for Archean lithospheric mantle beneath the Kimberley block, Western Australia. Geology 27, 431–434.
Henry, C., Burkhard, M. & Goffe, B. (1996). Evolution of synmetamorphic veins and their wallrocks through a western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major- and trace-element systematics. Chemical Geology 127, 81–109.
Hofmann, A. W. (1988). Chemical differentiation of the Earth—the relationship between mantle, continental-crust, and oceanic-crust. Earth and Planetary Science Letters 90, 297–314.[Web of Science]
Hofmann, A. W. (1997). Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219–229.
Imai, N., Terashima, S., Itoh, S. & Ando, A. (1995). 1994 compilation of analytical data for minor and trace elements in seventeen GSJ geochemical reference samples. Igneous Rock Series. Geostandards Newsletter 19, 135–213.
Irifune, T., Ringwood, A. E. & Hibberson, W. O. (1994). Subduction of continental-crust and terrigenous and pelagic sediments—an experimental-study. Earth and Planetary Science Letters 126, 351–368.
Jochum, K. P., McDonough, W. F., Palme, H. & Spettel, B. (1989). Compositional constraints on the continental lithospheric mantle from trace-elements in spinel peridotite xenoliths. Nature 340, 548–550.
Kamber, B. S. & Collerson, K. D. (1999). Origin of ocean island basalts: a new model based on lead and helium isotope systematics. Journal of Geophysical Research—Solid Earth 104, 25479–25491.
Kamber, B. S. & Collerson, K. D. (2000). Role of ‘hidden’ deeply subducted slabs in mantle depletion. Chemical Geology 166, 241–254.[Web of Science]
Konzett, J., Sweeney, R. J., Thompson, A. B. & Ulmer, P. (1997). Potassium amphibole stability in the upper mantle: an experimental study in a peralkaline KNCMASH system to 8·5 GPa. Journal of Petrology 38, 537–568.
Kramers, J. D. & Tolstikhin, I. N. (1997). Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust. Chemical Geology 139, 75–110.
Lambert, D. D., Shirey, S. B. & Bergman, S. C. (1995). Proterozoic lithospheric mantle source for the Prairie-Creek lamproites—Re–Os and Sm–Nd isotopic evidence. Geology 23, 273–276.
Liou, J. G., Zhang, R. Y. & Jahn, B. (1997). Petrology, geochemistry and isotope data on a ultrahigh-pressure jadeite quartzite from Shuanghe, Dabie mountains, east–central China. Lithos 41, 59–78.
Marty, B., Tolstikhin, I., Kamensky, I. L., Nivin, V., Balaganskaya, E. & Zimmermann, J. L. (1998). Plume-derived rare gases in 380 Ma carbonatites from the Kola region (Russia) and the argon isotopic composition in the deep mantle. Earth and Planetary Science Letters 164, 179–192.
McCammon, C., Hutchison, M. & Harris, J. (1997). Ferric iron content of mineral inclusions in diamonds from São Luiz: a view into the lower mantle. Science 278, 434–436.
McDonough, W. F. (1990). Constraints on the composition of the continental lithospheric mantle. Earth and Planetary Science Letters 101, 1–18.
Mitchell, R. H. (1995). Melting experiments on a sanidine phlogopite lamproite at 4–7 GPa and their bearing on the sources of lamproitic magmas. Journal of Petrology 36, 1455–1474.
Mitchell, R. H. & Bergman, S. C. (1991). Petrology of Lamproites. New York: Plenum.
Montgomery, C. W. & Hurley, P. M. (1978). Total-rock U–Pb and Rb–Sr systematics in the Imataca Series, Guayana Shield, Venezuela. Earth and Planetary Science Letters 39, 281–290.
Moreira, M., Breddam, K., Curtice, J. & Kurz, M. D. (2001). Solar neon in the Icelandic mantle: new evidence for an undegassed lower mantle. Earth and Planetary Science Letters 185, 15–23.[Web of Science]
Mues-Schumacher, U., Keller, J., Konova, V. & Suddaby, P. (1995). Petrology and age-determinations of the ultramafic (lamproitic) rocks from the Yakokut Complex, Aldan Shield, Eastern Siberia. Mineralogical Magazine 59, 409–428.[Abstract]
Nägler, T. F. & Kramers, J. D. (1998). Nd isotopic evolution of the upper mantle during the Precambrian: models, data and the uncertainty of both. Precambrian Research 91, 233–252.
Nelson, D. R. (1992). Isotopic characteristics of potassic rocks—evidence for the involvement of subducted sediments in magma genesis. Lithos 28, 403–420.[Web of Science]
O’Brien, H. E., Irving, A. J., McCallum, S. & Thirlwall, M. F. (1995). Strontium, neodymium, and lead isotopic evidence for the interaction of postsubduction asthenospheric potassic mafic magmas of the Highwood Mountains, Montana, USA, with ancient Wyoming Craton lithospheric mantle. Geochimica et Cosmochimica Acta 59, 4539–4556.
Oversby, V. M. (1975). Lead isotopic systematics and ages of Archaean acid intrusives in the Kalgoorlie–Norseman area, Western Australia. Geochimica et Cosmochimica Acta 39, 1107–1125.
Papanastassiou, D. A. & Wasserburg, G. J. (1969). Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects. Earth and Planetary Science Letters 5, 361–376.
Pearce, J. A. & Peate, D. W. (1995). Tectonic implications of the composition of volcanic arc magmas. Annual Review of Earth and Planetary Sciences 23, 251–285.[Web of Science]
Pidgeon, R. T., Smith, C. B. & Fanning, C. M. (1988). Kimberlite and lamproite emplacement ages in Western Australia. In: Ross, J., Jaques, A. L., Ferguson, J., Green, D. H., O’Reilly, S. Y., Danchin, R. V. & Janse, A. J. A. (eds) Kimberlites and Related Rocks. Special Publication, Geological Society of Australia 14, 369–381.
Plank, T. & Langmuir, C. H. (1998). The chemical composition of subducting sediment and its consequences for the crust and mantle. Chemical Geology 145, 325–394.[Web of Science]
Rao, N. V. C., Miller, J. A., Gibson, S. A., Pyle, D. M. & Madhavan, V. (1999). Precise Ar-40/Ar-39 age determinations of the Kotakonda kimberlite and Chelima lamproite, India: implication to the timing of mafic dyke swarm emplacement in the Eastern Dharwar craton. Journal of the Geological Society of India 53, 425–432.[Web of Science]
Ringwood, A. E., Kesson, S. E., Hibberson, W. & Ware, N. (1992). Origin of kimberlites and related magmas. Earth and Planetary Science Letters 113, 521–538.
Rudnick, R. L., Barth, M., Horn, I. & McDonough, W. F. (2000). Rutile-bearing refractory eclogites: missing link between continents and depleted mantle. Science 287, 278–281.
Sasada, T., Hiyagon, H., Bell, K. & Ebihara, M. (1997). Mantle-derived noble gases in carbonatites. Geochimica et Cosmochimica Acta 61, 4219–4228.
Sato, K., Katsura, T. & Ito, E. (1997). Phase relations of natural phlogopite with and without enstatite up to 8 GPa: implication for mantle metasomatism. Earth and Planetary Science Letters 146, 511–526.
Shen, Y., Solomon, S. C., Bjarnason, I. T. & Wolfe, C. J. (1998). Seismic evidence for a lower-mantle origin of the Iceland plume. Nature 395, 62–65.
Sheraton, J. W. & Cundari, A. (1980). Leucitites from Gaussberg, Antarctica. Contributions to Mineralogy and Petrology 71, 417–427.
Simons, F. J., Zielhuis, A. & Van der Hilst, R. D. (1999). The deep structure of the Australian continent from surface wave tomography. Lithos 48, 17–43.[Web of Science]
Sims, K. W. W. & DePaolo, D. J. (1997). Inferences about mantle magma sources from incompatible element concentration ratios in oceanic basalts. Geochimica et Cosmochimica Acta 61, 765–784.[Web of Science]
Stachel, T., Harris, J. W., Brey, G. P. & Joswig, W. (2000). Kankan diamonds (Guinea) II: lower mantle inclusion parageneses. Contributions to Mineralogy and Petrology 140, 16–27.
Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic systamatics of oceanic basalts: implications for mantle composition and processes. In: Norry, M. J. & Saunders, A. D. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Tingey, R. J., McDougall, I. & Gleadow, A. J. W. (1983). The age and mode of formation of Gaussberg, Antarctica. Journal of the Geological Society of Australia 30, 241–246.
Turner, S. P., Platt, J. P., George, R. M. M., Kelley, S. P., Pearson, D. G. & Nowell, G. M. (1999). Magmatism associated with orogenic collapse of the Betic–Alboran Domain, SE Spain. Journal of Petrology 40, 1011–1036.
Van der Hilst, R., Engdahl, R., Spakman, W. & Nolet, G. (1991). Tomographic imaging of subducted lithosphere below Northwest Pacific island arcs. Nature 353, 37–43.
Veksler, I. V., Petibon, C., Jenner, G. A., Dorfman, A. M. & Dingwell, D. B. (1998). Trace element partitioning in immiscible silicate–carbonate liquid systems: an initial experimental study using a centrifuge autoclave. Journal of Petrology 39, 2095–2104.
Vollmer, R., Ogden, P., Schilling, J. G., Kingsley, R. H. & Waggoner, D. G. (1984). Nd and Sr isotopes in ultrapotassic volcanic rocks from the Leucite Hills, Wyoming. Contributions to Mineralogy and Petrology 87, 359–368.
Walker, R. J., Morgan, J. W. & Horan, M. F. (1995). Os187 enrichment in some plumes—evidence for core–mantle interaction. Science 269, 819–822.
Wang, W. & Sueno, S. (1996). Discovery of a NaPx–En inclusion in diamond: possible transition zone origin. Mineralogical Journal 18, 9–16.
Wannamaker, P. E., Hulen, J. B. & Heizler, M. T. (2000). Early Miocene lamproite from the Colorado Plateau tectonic province, Southeastern Utah, USA. Journal of Volcanology and Geothermal Research 96, 175–190.
Wendt, J. I., Regelous, M., Niu, Y. L., Hekinian, R. & Collerson, K. D. (1999). Geochemistry of lavas from the Garrett Transform Fault: insights into mantle heterogeneity beneath the eastern Pacific. Earth and Planetary Science Letters 173, 271–284.[Web of Science]
Williams, R. W., Collerson, K. D., Gill, J. B. & Deniel, C. (1992). High Th/U ratios in subcontinental lithospheric mantle—mass-spectrometric measurement of Th isotopes in Gaussberg lamproites. Earth and Planetary Science Letters 111, 257–268.
Zindler, A. & Hart, S. (1986). Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493–571.[Web of Science]
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