Journal of Petrology

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Journal of Petrology Volume 42 Number 6 Pages 1095-1117 2001
© Oxford University Press 2001

Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

S. S. SCHMIDBERGER^,* and D. FRANCIS

EARTH AND PLANETARY SCIENCES, McGILL UNIVERSITY, 3450 UNIVERSITY STREET, MONTRÉAL, QUÉBEC H3A 2A7, CANADA

Received April 11, 2000; Revised typescript accepted October 16, 2000

	ABSTRACT

TOP ABSTRACT INTRODUCTION SAMPLES AND PROCEDURES ANALYTICAL DATA DISCUSSION CONCLUSIONS REFERENCES

 
Peridotites that sample Archean mantle roots are frequently incompatible trace element enriched despite their refractory major element compositions. To constrain the trace element budget of the lithosphere beneath the Canadian craton, trace element and rare earth element (REE) abundances were determined for a suite of garnet peridotites and garnet pyroxenites from the Nikos kimberlite pipe on Somerset Island, Canadian Arctic, their constituent garnet and clinopyroxene, and the host kimberlite. These refractory mantle xenoliths are depleted in fusible major elements, but enriched in incompatible trace elements, such as large ion lithophile elements (LILE), Th, U and light rare earth elements (LREE). Mass balance calculations based on modal abundances of clinopyroxene and garnet and their respective REE contents yield discrepancies between calculated and analyzed REE contents for the Nikos bulk rocks that amount to LREE deficiencies of 70–99%, suggesting the presence of small amounts of interstitial kimberlite liquid (0·4–2 wt %) to account for the excess LREE abundances. These results indicate that the peridotites had in fact depleted or flat LREE patterns before contamination by their host kimberlite. LREE and Sr enrichment in clinopyroxene and low Zr and Sr abundances in garnet in low-temperature peridotites (800–1100°C) compared with high-temperature peridotites (1200–1400°C) suggest that the shallow lithosphere is geochemically distinct from the deep lithosphere beneath the northern margin of the Canadian craton. The Somerset mantle root appears to be characterized by a depth zonation that may date from the time of its stabilization in the Archean.

KEY WORDS: Canada; mantle; metasomatism; peridotite; trace elements

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SAMPLES AND PROCEDURES ANALYTICAL DATA DISCUSSION CONCLUSIONS REFERENCES

 
The mantle underlying many Archean cratons has anomalously high seismic velocities to depths of 350–400 km (Jordan, 1988; Grand, 1994), indicating the presence of cold refractory roots, depleted in the fusible major elements compared with fertile mantle (Boyd & Mertzman, 1987; McDonough, 1990). These deep residual peridotite roots probably contribute to the stability of Archean continental lithosphere because of their lower density and higher viscosity compared with that of the surrounding asthenospheric mantle (Boyd & McCallister, 1976; Jordan, 1979; Pollack, 1986). Mantle xenoliths that are hosted by kimberlites and alkaline basalts are our only window into the subcontinental lithosphere. They provide essential evidence on the chemical composition and evolution of the upper mantle to depths of >200 km. Studies of these mantle xenoliths enable us to characterize the abundance and distribution of major, minor and trace elements in peridotites and between their constituent minerals. Most subcratonic peridotite samples have undergone a complex history of melt extraction that has changed their chemical composition and resulted in depletion of the residual mantle in fusible major elements such as Fe, Al and Ca (e.g. Nixon, 1987; Herzberg, 1993; Boyd et al., 1997). In contrast to their depletion in incompatible major elements, however, many peridotite xenoliths have unexpectedly high abundances of incompatible trace elements, such as large ion lithophile elements (LILE) and light rare earth elements (LREE; Erlank et al., 1987; Menzies et al., 1987). Although these findings have been widely interpreted to indicate that the lithospheric mantle has been affected by interaction with incompatible element enriched percolating melts or fluids over time (Hawkesworth et al., 1983; Menzies & Hawkesworth, 1987), it is important to establish the chemical nature of these metasomatic agents and whether metasomatism is an ancient feature or associated with the kimberlite magmatic event itself.

Previous studies on Somerset Island peridotites have shown that Re–Os isotope systematics for these xenoliths yields Re depletion ages of up to 2·7 Ga (Irvine et al., 1999), indicating the existence of Archean lithospheric mantle underneath the northern margin of the Canadian craton. In this study, we present trace element and REE data for a suite of garnet peridotites and garnet pyroxenites, constituent garnet and clinopyroxene, and the host kimberlite from the Nikos kimberlite pipes on Somerset Island, in the southern Canadian Arctic (Fig. 1). The geochemical data provide information on mantle depletion and constraints on incompatible trace element enrichment during metasomatism following melt extraction. REE distribution patterns for constituent garnet and clinopyroxene give insights into trace element partitioning and are used to constrain the change in chemical composition of the refractory mantle root with depth. Models of trapped liquid compositions along grain boundaries indicate that the Nikos kimberlite probably acted as the metasomatic agent during sample entrainment.

View larger version (42K): [in this window] [in a new window]   Fig. 1. Geological map of Somerset Island (after Steward, 1987) showing kimberlite locations. Light gray, Paleozoic cover rocks; striped, Late Proterozoic cover rocks; dark gray, Precambrian basement; bold lines, normal faults.

 

	SAMPLES AND PROCEDURES

TOP ABSTRACT INTRODUCTION SAMPLES AND PROCEDURES ANALYTICAL DATA DISCUSSION CONCLUSIONS REFERENCES

 
Kimberlite
The recently discovered Cretaceous (100 Ma) Nikos kimberlite (Heaman, 1989; Smith et al., 1989; Pell, 1993; Fig. 1) consists of three individual pipes that have been described in more detail by Schmidberger & Francis (1999). The Nikos kimberlites were emplaced into late Archean crystalline basement overlain by Paleozoic cover rocks of the northern margin of the Canadian craton (Steward, 1987; Frisch & Hunt, 1993). Although it has been argued that Somerset Island is part of the Proterozoic Innuitian tectonic province (Trettin et al., 1972), late Archean Re depletion ages for other Somerset Island peridotite xenoliths (Irvine et al., 1999) indicate the presence of an Archean mantle root beneath the southern Canadian Arctic.

The southernmost Nikos kimberlite pipe (NK3) is characterized by a non-brecciated, magmatic texture, and appears to represent kimberlite liquid (Schmidberger & Francis, 1999). The magmatic kimberlite exhibits a porphyritic microcrystalline texture, consisting of phenocrysts of olivine, phlogopite, and spinel in a very fine-grained carbonate-rich matrix of calcite, serpentine, perovskite and apatite. Calcite occurs as aggregates of tabular sub-parallel crystals showing flow texture around larger olivine phenocrysts, which has been interpreted to be a primary magmatic feature (Schmidberger & Francis, 1999). Crushed whole-rock kimberlite samples were carefully handpicked under the binocular microscope before grinding to eliminate contamination from xenocrysts and country rock fragments.

Mantle xenoliths
The mantle xenolith suite for this study consists of large (10–30 cm), well-preserved garnet peridotites and lesser garnet pyroxenites, 30 of which were analyzed for their major and trace element and REE contents in this study (see Tables 2 and 3, below). The majority of the Nikos peridotites show coarse textures, typical of mantle xenoliths, with large crystals of olivine, orthopyroxene, clinopyroxene and garnet, although a few xenoliths with porphyroclastic textures are observed. Small differences in modal clinopyroxene contents do not justify dividing the suite according to the IUGS classification into harzburgitic (clinopyroxene <5 wt %) and lherzolitic (clinopyroxene >5 wt %) rock types, and the more general term peridotite is preferred for the xenoliths in the present study. A detailed description of mineralogy and petrology of the mantle xenoliths has been given by Schmidberger & Francis (1999). Whole-rock analyses of the peridotite xenoliths are strongly depleted in fusible major elements such as Fe, Al and Ca when compared with primitive mantle compositions (McDonough, 1990; Schmidberger & Francis, 1999; see Table 2). High magnesium numbers [mg-number = Mg/(Mg + Fe)] between 0·90 and 0·93 and their refractory olivine-rich mineralogy confirm the depleted nature of these peridotites (see Table 2). The Somerset whole-rock characteristics are similar to those for kimberlite-hosted peridotites from the Canadian Slave province (Kopylova et al., 1999; MacKenzie & Canil, 1999), and a detailed comparison of both xenolith suites has been given by Schmidberger & Francis (1999). Temperature and pressure estimates of last equilibration for the Nikos xenoliths (800–1400°C and 25–60 kbar) suggest that the peridotites were entrained from depths between 80 and 190 km (Schmidberger & Francis, 1999; see Table 2).

View this table: [in this window] [in a new window]   Table 2: Major element analyses and calculated mineral modes for bulk xenoliths

 

View this table: [in this window] [in a new window]   Table 3: Trace and rare earth element analyses for bulk xenoliths

 

View this table: [in this window] [in a new window]   Table 1: Major, trace and rare earth element analyses for Nikos kimberlites

 
The pyroxenites contain large crystals of clinopyroxene and orthopyroxene, and smaller crystals of garnet. These pyroxene-rich rocks have a range of mg-numbers (Schmidberger & Francis, 1999; see Table 2). The high-Mg pyroxenites contain olivine and abundant garnet (15–25 wt %) and have mg-numbers (0·88–0·90; NK2-7, NK3-14) that overlap those of the peridotites. The low-Mg pyroxenites (mg-number 0·85; NK3-1, NK3-17), on the other hand, are less garnet-rich (9–10 wt %) and have higher modal amounts of clinopyroxene (67–68 wt %) compared with the high-Mg pyroxenites (22–39 wt %).

Clinopyroxene and garnet
The emerald green clinopyroxene of the Somerset peridotites is a chromian diopside (Schmidberger & Francis, 1999), and the garnets are chromian pyrope, the majority of which are purple in color (see Table 5, below). A small number of samples contain blood red garnets (NK1-5, NK2-2, NK3-4). These garnets tend to have higher MgO, SiO₂, and Al₂O₃, but lower CaO, Cr₂O₃ and MnO contents, compared with the purple garnets (see Table 5). The high temperature estimates of 1200–1400°C (Schmidberger & Francis, 1999) for samples with red garnet possibly suggest that these xenoliths were derived from deep lithospheric mantle, whereas the temperatures calculated for xenoliths with purple garnet are much more variable (800–1400°C). Major element analyses of garnet and clinopyroxene indicate that these minerals are not chemically zoned and are compositionally homogeneous on the scale of a thin section. Inclusion-free crystals of clinopyroxene and garnet were handpicked under the binocular microscope from 10 peridotites and one low-Mg pyroxenite sample, and were analyzed for their REE and trace element (Ti, V, Cr, Sr, Y, Zr) contents with a Cameca IMS 3f ion microprobe at Woods Hole Oceanographic Institution. The major elements of these separates were determined using electron microprobe analysis at McGill University, as described by Schmidberger & Francis (1999).

View this table: [in this window] [in a new window]   Table 5: Major, trace and rare earth element analyses of garnets

 

View this table: [in this window] [in a new window]   Table 4: Major, trace and rare earth element analyses of clinopyroxenes

 

	ANALYTICAL DATA

TOP ABSTRACT INTRODUCTION SAMPLES AND PROCEDURES ANALYTICAL DATA DISCUSSION CONCLUSIONS REFERENCES

 
Kimberlite
The kimberlite whole-rock analyses are extremely high in CaCO₃ (27–39 wt %; Table 1), indicating liquid compositions intermediate between kimberlite and carbonatite (Woolley & Kempe, 1989; Ringwood et al., 1992). Primitive mantle normalized trace element patterns for the Nikos kimberlites (Table 1; Fig. 2a) show a strong enrichment in LILE (e.g. Ba, Sr) and Nb, Th, U and Pb, typical of small-degree partial melts such as kimberlites (Mitchell, 1986; Dalton & Presnall, 1998). The REE patterns for the Nikos kimberlites exhibit steep slopes with (La/Sm)_N of seven and relative depletion in heavy rare earth elements (HREE; Fig. 2b). Although LREE characteristics are similar to those for kimberlites from South Africa, Zaire, North America and India (Mitchell & Brunfelt, 1975; Paul et al., 1975; Wedepohl & Muramatsu, 1979; Cullers et al., 1982; Muramatsu, 1983; Fieremans et al., 1984; Mitchell, 1986; Fig. 2b), HREE levels are distinctly lower in the Nikos kimberlites, particularly from Er to Lu. With the exception of Ba and Pb, which are strongly enriched, the levels of most other trace elements in the Nikos kimberlites overlap the fields for kimberlites from South Africa, Zaire, North America and India (Fig. 2b).

View larger version (34K): [in this window] [in a new window]   Fig. 2. (a) Primitive mantle normalized trace element and (b) chondrite-normalized REE patterns for Nikos kimberlites compared with data for kimberlites from South Africa, Zaire, North America and India (Mitchell & Brunfelt, 1975; Paul et al., 1975; Wedepohl & Muramatsu, 1979; Cullers et al., 1982; Muramatsu, 1983; Fieremans et al., 1984; Mitchell, 1986). Primitive mantle and chondrite values after Sun & McDonough (1989).

 
Mantle xenoliths
Incompatible trace element patterns for the bulk peridotites are subparallel and indicate that LILE (e.g. Ba, Sr), Nb, Th and U concentrations are enriched when compared with primitive mantle abundances (Sun & McDonough, 1989; McDonough & Sun, 1995; Table 3; Fig. 3a). Their trace element patterns are, however, clearly distinct from those of the Nikos kimberlites, which have much higher LILE, Nb, Th, U and Pb contents (Fig. 3a). Chondrite-normalized HREE patterns for the peridotites are flat, whereas the LREE are fractionated with (La/Sm)_N of 4–6, resulting in concave-upward LREE patterns. The kimberlites, on the other hand, have much steeper slopes for LREE and, in particular, HREE compared with the peridotites (Fig. 3b).

View larger version (49K): [in this window] [in a new window]   Fig. 3. (a) Primitive mantle normalized trace element and (b) chondrite-normalized REE patterns for Nikos peridotites. Data for Nikos kimberlites are plotted for comparison.

 
The pyroxenites also contain high concentrations of incompatible trace elements such as LILE, Nb, Th and U, compared with values for primitive mantle (Table 3; Fig. 4a). The high-Mg pyroxenites (NK2-7, NK3-14), however, are considerably more enriched in incompatible trace elements than the low-Mg pyroxenites (NK3-1, NK3-17), and have trace element patterns that overlap those for peridotites showing the highest abundances of these elements (Fig. 4a). Incompatible trace element patterns for the low-Mg pyroxenites, on the other hand, are similar to those for peridotites with the lowest trace element levels. Furthermore, the high-Mg pyroxenites have strongly fractionated LREE patterns [(La/Sm)_N = 4–14], whereas low-Mg pyroxenites exhibit much less LREE fractionation [(La/Sm)_N = 2]. The HREE abundances for both the high- and low-Mg pyroxenites tend to be higher than those for the peridotites (Fig. 4b), probably reflecting their higher modal abundance of garnet (av. 15 wt %) compared with that of the peridotites (av. 7 wt %; Table 2).

View larger version (56K): [in this window] [in a new window]   Fig. 4. (a) Primitive mantle normalized trace element and (b) chondrite-normalized REE patterns for Nikos pyroxenites. Data for Nikos kimberlites and peridotite field are plotted for comparison.

 
Clinopyroxene and garnet
The clinopyroxenes show convex-upward chondrite-normalized REE patterns with enriched LREE compared with chondrites (1–100 times) and relatively depleted HREE, with approximately chondritic abundances (Table 4; Fig 5a and b). The clinopyroxenes from peridotites yielding high temperatures of equilibration (1200–1400°C) are only moderately enriched in LREE abundances (1–10 times chondrite; Fig. 5a). In comparison, clinopyroxenes from low-temperature peridotites (800–1100°C; NK1-3, NK1-4, NK1-14, NK2-3) exhibit significantly greater LREE enrichment (up to 100 times chondrite; Fig. 5b) and high Sr contents (>100 ppm; Table 4). In contrast, HREE patterns for both high-temperature and low-temperature peridotites are indistinguishable.

View larger version (35K): [in this window] [in a new window]   Fig. 5. Chondrite-normalized REE patterns for coexisting clinopyroxene and garnet in Nikos peridotites. (a) Clinopyroxene from high-temperature peridotites is mildly enriched in LREE but depleted in HREE compared with coexisting garnet. (b) Clinopyroxene–garnet pairs with highly LREE-enriched clinopyroxene from low-temperature xenoliths.

 
The clinopyroxenes in high-temperature Nikos peridotite xenoliths have LREE contents that overlap those of clinopyroxenes in high-temperature peridotites (>1100°C) from the Kaapvaal craton in South Africa, although the HREE concentrations of the Kaapvaal clinopyroxenes tend to be higher (Shimizu, 1975; Boyd, 1987; Fig. 6a). Unlike many of the high-temperature Kaapvaal xenoliths (Boyd, 1987), the high-temperature Somerset peridotites are not characterized by strong deformation textures. In comparison, clinopyroxenes from high-temperature Siberian peridotites (Shimizu et al., 1997) have LREE contents that are intermediate between those of the low- and high-temperature Nikos clinopyroxenes, and overlapping HREE contents (Fig. 6a).

View larger version (44K): [in this window] [in a new window]   Fig. 6. (a) Chondrite-normalized REE patterns for Nikos clinopyroxene and (b) garnet compared with fields based on data from high-temperature (high-T ) and low-temperature (low-T ) peridotites from the Kaapvaal craton and high-temperature peridotites from the Siberian craton (Shimizu, 1975; Shimizu et al., 1997).

 
The low-temperature Nikos peridotites have clinopyroxenes with LREE contents that overlap those of clinopyroxenes from low-temperature Kaapvaal peridotites (<1100°C), although the latter have considerably lower HREE contents (Fig. 6a). Clinopyroxenes from low-temperature Siberian peridotites appear to exhibit highly variable REE patterns.

The REE patterns for garnets are HREE enriched and LREE depleted compared with those of coexisting clinopyroxene, crossing the latter between Sm and Eu (Table 5; Fig. 5a and b). Garnet REE patterns for low-temperature xenoliths are indistinguishable from those for high-temperature peridotites. Strong depletion in LREE compared with HREE is consistent with garnet–liquid REE partition coefficients (Hauri et al., 1994; Halliday et al., 1995, and references therein) and has been interpreted to represent equilibrium REE distribution between garnet and a coexisting liquid (Shimizu, 1999). Sinusoidal patterns, characterized by an enrichment in the middle rare earth elements (MREE) compared with LREE and HREE (which have been interpreted to reflect recent, non-equilibrated melt–mineral reactions in other xenolith suites; Hoal et al., 1994; Shimizu, 1999), are not observed in the Nikos garnets. The LREE contents of garnet in the low-Mg pyroxenite NK3-1 (Fig. 5a) are extremely low, indicating the highly depleted nature of this sample.

The REE patterns for the Nikos garnets are subparallel to those for garnets from high-temperature Kaapvaal peridotites, although the latter contain higher overall REE abundances (Shimizu, 1975; Fig. 6b). Garnets from low-temperature Kaapvaal peridotites have sinusoidal, non-equilibrated REE patterns with higher LREE and considerably lower HREE contents than those in the Nikos garnets (Fig. 6b). The LREE concentrations in the Nikos garnets are lower than those of garnets from Siberian high-temperature peridotites, but their HREE contents overlap (Shimizu et al., 1997; Fig. 6b).

	DISCUSSION

TOP ABSTRACT INTRODUCTION SAMPLES AND PROCEDURES ANALYTICAL DATA DISCUSSION CONCLUSIONS REFERENCES

 
Mantle composition
Late Archean Re depletion ages for other Somerset Island peridotites (up to 2·7 Ga; Irvine et al., 1999) indicate that the northern margin of the Canadian craton is underlain by an Archean lithospheric mantle root, which may be only slightly younger than the Archean subcontinental lithosphere beneath the central Slave province, whose kimberlite xenoliths yield Re depletion ages of up to 3 Ga (Irvine et al., 1999). In comparison, peridotite xenoliths in kimberlites on the Kaapvaal and Siberian cratons yield Re depletion ages (3·3–3·5 Ga; Pearson et al., 1995a, 1995b) indicating that their mantle roots stabilized by the mid Archean.

Pressure and temperature estimates for the Nikos peridotites suggest the existence of a lithospheric mantle root to a depth of at least 190 km beneath Somerset Island, significantly deeper than the lithospheric mantle beneath Proterozoic mobile belts surrounding the Archean cratons (<160 km; Finnerty & Boyd, 1987; Nixon, 1987). High mg-numbers and depletion in fusible major elements, typical of Archean mantle xenolith suites (e.g. Boyd, 1987; Boyd et al., 1997), indicate the refractory nature of the Nikos peridotites, which have been interpreted to represent the residues of large-degree partial melting in the mantle (30% melt extraction; Schmidberger & Francis, 1999). These results are supported by experimental studies (Walter, 1998) on the melting of pyrolitic mantle at high pressures and temperatures, which indicate that the most refractory Nikos peridotites would require a minimum of 30% melt extraction from a primitive mantle source.

Melt extraction should also result in the depletion in incompatible trace elements of the residual mineral assemblages (Harte, 1983; Carlson & Irving, 1994). Trace element patterns for the Nikos peridotites, however, are enriched in LILE, Th, U and LREE compared with those estimated for primitive mantle compositions (McDonough & Sun, 1995). These signatures suggest that interaction with melts or fluids during metasomatism has resulted in incompatible trace element enrichment of the mantle beneath Somerset Island. Metasomatism and partial melt extraction may have been related to a single magmatic event (e.g. Shi et al., 1998) or incompatible trace element enrichment could reflect the infiltration of a metasomatic agent into lithospheric mantle that had previously been depleted in fusible major elements, as proposed for the mantle beneath the Kaapvaal craton in South Africa (e.g. Menzies & Hawkesworth, 1987; Hoal et al., 1994; Pearson et al., 1995a).

Depleted subcontinental lithosphere
It is important to establish the chemical composition of cratonic mantle roots because their refractory nature preserves a record of the large degree of partial melting involved in their formation during stabilization of the continents (Boyd & McCallister, 1976; Jordan, 1979; Walter, 1998). High-pressure melting experiments show that the incipient melting of a carbonate-bearing garnet peridotite produces alkalic liquids such as carbonatites and kimberlites and then picritic to komatiitic melts at larger degrees of partial melting, leaving a refractory residue that is highly depleted in fusible major elements such as Fe, Al and Ca (e.g. Takahashi & Scarfe, 1985; Baker & Stolper, 1994; Dalton & Presnall, 1998). Experimental data suggest that melting at pressures of 4·5–6 GPa (depths of up to 200 km) and temperatures of 1500–1700°C produces harzburgitic residues because of the preferential melting of clinopyroxene (Canil, 1992; Walter, 1998). Loss of clinopyroxene, the main LREE carrier in peridotites, to the liquid phase depletes the residue in these elements, whereas the stability of garnet in the restite favors the retention of HREE in the residual mineral assemblage (Frey, 1969; Nagasawa et al., 1969; Hauri et al., 1994). Progressive melting should result in chondrite-normalized bulk-rock REE patterns characterized by smooth LREE-depleted profiles (Navon & Stolper, 1987). The Somerset peridotites are, however, characterized by enrichment in LILE and LREE compared with the HREE, despite their refractory major element signatures, as are peridotite xenolith suites from cratonic areas in South Africa and Siberia (Erlank et al., 1987; Menzies et al., 1987). These presumed metasomatic effects make it difficult to establish the chemical composition of the mantle roots before their interaction with metasomatic agents.

Previous studies have shown that bulk REE contents in peridotites are controlled by clinopyroxene and garnet, whereas orthopyroxene and olivine contribute little to REE budgets (e.g. Shimizu, 1975; Eggins et al., 1998). The whole-rock REE compositions can, therefore, be modeled by quantitative mass balance calculations using the modal abundances of clinopyroxene and garnet and their respective REE contents. These mass balance calculations indicate that, in contrast to the analyzed whole-rock compositions, the calculated whole rocks have much lower LREE abundances, although having similar HREE contents (Table 6; Fig. 7). In particular, La and Ce are deficient in the calculated peridotite compositions by 99–90% for the high-temperature peridotites (e.g. NK1-7; Fig. 7a) and 80–70% for the low-temperature peridotites (e.g. NK1-4; Fig. 7b). Nd, Sm and Eu deficiencies in the calculated whole rocks of both low-temperature and high-temperature peridotites range from 70 to 40%, but decrease for Dy, Er and Yb to <20% (Fig. 7). The calculated whole-rock REE patterns for the low-temperature peridotites are approximately chondritic (e.g. NK1-4; Fig. 8b) or only slightly enriched (NK2-3). In contrast, the calculated LREE of high-temperature peridotites are depleted compared with chondrites (e.g. NK1-7; Fig. 8a) and the calculated whole-rock patterns correspond to those expected for refractory mantle, with a residual mineral assemblage of olivine + orthopyroxene + garnet ± clinopyroxene, that has been depleted by a large degree of partial melting (30–40%; Navon & Stolper, 1987; Walter, 1998).

View this table: [in this window] [in a new window]   Table 6: Calculated rare earth element contents for bulk xenoliths

 

View larger version (30K): [in this window] [in a new window]   Fig. 7. REE mass balance results for high-temperature peridotite NK1-7 and low-temperature peridotite NK1-4 showing deficiency or surplus (%) of each element calculated using the modal abundances of clinopyroxene and garnet and their respective REE contents compared with the analyzed whole-rock abundance. The REE contents of olivine and orthopyroxene were assumed to be zero.

 

View larger version (27K): [in this window] [in a new window]   Fig. 8. Calculated chondrite-normalized REE patterns (Calc. Whole Rock) for high-temperature peridotite NK1-7 and low-temperature peridotite NK1-4 using mass balance calculations based on REE contents of constituent garnet and clinopyroxene and their respective modal abundances. REE patterns for analyzed whole rock (Whole Rock), garnet and clinopyroxene are plotted for comparison.

 
The large discrepancy for the LREE between the calculated and analyzed whole-rock compositions that was obtained for all xenoliths suggests that an interstitial phase(s) is present, either a trapped melt or an accessory mineral phase, that contributes significantly to the LREE budget of the peridotites.

Interstitial kimberlite liquid
To account for the excess LREE contents of the whole-rock analyses, we modeled the possible presence of interstitial melt along grain boundaries of the peridotites. The calculations are based on bulk-rock trace element analyses and mineral modes, determined using a high-pressure norm calculation described by Schmidberger & Francis (1999), and a trapped melt in equilibrium with them (Bédard, 1994). The distribution of REE between minerals and trapped liquid in these calculations is determined using mineral–melt partition coefficients for the constituent peridotite phases (olivine, orthopyroxene, clinopyroxene, garnet) and a kimberlitic melt (Fujimaki et al., 1984). The mass balance calculations indicate that the discrepancies can be explained by the presence of 0·1–2·5 wt % trapped liquid with a composition overlapping that of the Nikos kimberlite (Table 7; Fig. 9). The similarity of the trapped liquid trace element composition to that of the kimberlite suggests the presence of interstitial kimberlitic melt or a kimberlite-derived phase(s) to account for the excess LREE in the analyzed whole rocks. The addition of 0·4–2 wt % kimberlite liquid to the calculated whole-rock compositions yields REE patterns that are remarkably similar to those of the analyzed bulk rocks (Table 6; Fig. 10).

View this table: [in this window] [in a new window]   Table 7: rare earth element contents for calculated interstitial liquid

 

View larger version (24K): [in this window] [in a new window]   Fig. 9. Calculated chondrite-normalized REE patterns for possible interstitial liquids in equilibrium with constituent mineral phases of the Nikos peridotites (after Bédard, 1994). Nikos kimberlites are plotted for comparison.

 

View larger version (22K): [in this window] [in a new window]   Fig. 10. Calculated chondrite-normalized REE patterns (Calc. WR and Kimb.) for (a) high-temperature peridotite NK1-7 and (b) low-temperature peridotite NK1-4 before and after the addition of 0·4 wt % (NK1-7) and 1·2 wt % (NK1-4) interstitial kimberlite liquid.

 

Accessory minerals
The mineral apatite is known to be a major carrier for LREE with elemental abundances up to several thousand ppm (e.g. Ce: 2600–3000 ppm; Kramers et al., 1983; Irving & Frey, 1984; O’Reilly et al., 1991) and accessory apatite has been described in peridotite xenoliths (Dawson, 1980; Kramers et al., 1983; O’Reilly et al., 1991). Even if present in only trace amounts, apatite can play a major role in the LREE budget of mantle rocks (O’Reilly et al., 1991).

We modeled the presence of apatite as a possible accessory mineral phase in the Nikos peridotites using an apatite REE composition determined by isotope dilution analysis (Kramers et al., 1983) from a metasomatized South African xenolith suite. Similar REE abundances have been reported for apatite from metasomatized Australian spinel lherzolites using proton microprobe analysis (O’Reilly et al., 1991). The inclusion of small modal amounts of apatite (0·01–0·15 wt %) in the mass balance calculations also yields whole-rock REE patterns that are similar to the bulk-rock analyses (Table 6). Minor amounts of apatite could, therefore, also account for the LREE surplus in the Nikos whole-rock analyses. Although apatite is as yet undetected in thin sections, the presence of 0·1 wt % apatite along grain boundaries would be hard to observe optically. The presence of apatite in the Nikos peridotites could have resulted from interaction of the xenoliths with their host kimberlite, precipitating intergranular apatite during xenolith transport at 100 Ma. Whole-rock phosphorus abundances (Table 2), however, although consistent with mass balance calculations assuming the presence of a trapped interstitial kimberlite liquid in the Somerset bulk rocks (Tables 1 and 6), are 2–5 times lower for most xenoliths than those required by the apatite contents necessary to match the trace elements (Table 6).

Mineral phases such as phlogopite were also considered as possible LREE carriers; however, mass balance calculations indicate that >25 wt % mica would be required to account for the excess LREE contents. Although traces of phlogopite (maximum 1 wt %) are observed in some Nikos peridotite xenoliths, its modal abundance is completely insufficient to explain the discrepancy between calculated and analyzed whole-rock peridotite compositions.

The results of these mass balance calculations indicate that the shallow low-temperature peridotites had in fact flat LREE patterns, whereas those of the deeper-seated high-temperature peridotites were LREE depleted compared with chondrites before contamination by their host kimberlite.

Pyroxenites
The dominant rock type (>90%) of the Nikos xenolith suite is peridotitic in composition, representing the Archean cratonic mantle root beneath Somerset Island, whereas pyroxenite appears to represent only a minor constituent (<10%; Schmidberger & Francis, 1999). The major element chemistry and mineralogy of the Nikos pyroxenites indicate that the division into high-Mg pyroxenites and low-Mg pyroxenites (Schmidberger & Francis, 1999) correlates with distinct incompatible trace element patterns. In contrast to their higher mg-numbers and a more refractory whole-rock composition, the high-Mg pyroxenites are more enriched in incompatible trace elements such as LILE and LREE than the low-Mg pyroxenites.

The major and trace element differences between high-Mg and low-Mg pyroxenites suggest that there is no direct genetic link between these two types of pyroxenites, an interpretation that is supported by their distinct Nd–Sr–Pb isotope systematics (Schmidberger & Francis, 1998). The low-Mg pyroxenites have REE profiles showing less fractionation between LREE and HREE compared with those of the peridotites and the high-Mg pyroxenites. The coarse textures, low mg-numbers and relatively depleted trace element signatures compared with the remaining samples suggest that they are cumulates, possibly representing veins in a peridotitic lithospheric mantle.

The similarity of the high-Mg pyroxenites to the peridotites in terms of their mg-numbers and high abundances of LILE and LREE is supported by their overlapping Nd–Sr–Pb isotope systematics (Schmidberger & Francis, 1998). These pyroxenites may constitute a pyroxene-rich component in the lithospheric mantle produced by small-scale segregation of peridotite compositions into olivine- and pyroxene-rich layers by metamorphic differentiation (Boyd et al., 1997).

Trace element partitioning
The significance of the distribution of incompatible elements between mineral pairs can be evaluated by comparing their trace element abundance ratios with experimentally determined trace element partition coefficients and with those reported in other natural systems (Irving & Frey, 1978, 1984; Harte & Kirkley, 1997; Shimizu et al., 1997). The clinopyroxene–garnet REE partition coefficients for the Nikos xenoliths decrease from La to Yb, which is consistent with mineral–melt distribution coefficients for garnet that increase from the LREE to the HREE as those for clinopyroxene decrease (e.g. Halliday et al., 1995, and references therein; Fig. 11). REE partitioning for clinopyroxene–garnet pairs from the high-temperature peridotites (e.g. La_Cpx/Garn: 20–40) is consistent with experimental data for high-temperature systems up to 1400°C (La_Cpx/Garn: 20–80; Hauri et al., 1994; Halliday et al., 1995, and references therein; Fig. 11). These findings suggest that the distribution of REE between clinopyroxene and garnet equilibrated at high temperatures in these xenoliths. In contrast, clinopyroxene–garnet pairs from the low-temperature peridotites and the low-Mg pyroxenite sample yield clinopyroxene–garnet partition coefficients that are significantly higher than those for the high-temperature peridotites (La_Cpx/Garn: 200–840; Fig. 11). These partition coefficients overlap those observed in natural systems at temperatures between 800 and 1100°C (La_Cpx/Garn: 60 to >1000; Griffin & Brueckner, 1985; Harte & Kirkley, 1997), suggesting that REE equilibration in the shallow lithosphere occurred at lower temperatures. These results confirm findings that clinopyroxene–garnet partition coefficients for the REE (e.g. Ce_Cpx/Garn; Fig. 12) vary significantly with temperature (Griffin & Brueckner, 1985).

View larger version (37K): [in this window] [in a new window]   Fig. 11. Clinopyroxene–garnet partition coefficients in Nikos peridotites. Fields for high-temperature experimental data (to 1400°C) after Hauri et al. (1994), Halliday et al. (1995) and references therein, and data for natural systems (800–1100°C) after Griffin & Brueckner (1985) and Harte & Kirkley (1997).

 

View larger version (92K): [in this window] [in a new window]   Fig. 12. Temperature dependence of clinopyroxene–garnet Ce partitioning (CeCpx/Garn) in the Nikos peridotites.

 

Mantle stratification
Pressure estimates for the low-temperature peridotites indicate entrainment from depths between 80 and 150 km in the lithospheric mantle. The clinopyroxenes in low-temperature peridotites have REE patterns (Fig. 5b) that show strong enrichment in the LREE with abundances of up to 100 times chondrite (e.g. La: 6–27 ppm). These clinopyroxenes are also characterized by high Sr abundances (100–400 ppm; Fig. 13a and b). In contrast, the clinopyroxenes in the high-temperature peridotites from depths estimated to be between 160 and 190 km have significantly lower LREE and Sr abundances than clinopyroxenes in the shallower low-temperature peridotites (e.g. La up to 10 times chondrite; Figs 5a and 13a and b).

Incompatible trace element abundances in the Nikos garnets range considerably and also correlate with temperature and pressure estimates for their xenoliths. Garnets in the low-temperature peridotites are characterized by low Zr and Sr contents (9–60 ppm and 0·2–0·7 ppm, respectively), whereas garnets in the high-temperature peridotites have Zr and Sr abundances (35–100 ppm and 0·4–1·1 ppm, respectively) that are significantly higher (Fig. 13c and d). Although this variation of Zr and Sr in garnet with depth could be interpreted as reflecting temperature-dependent partitioning of these elements between clinopyroxene and garnet, Zr and Sr contents in the calculated whole rocks using modal abundances of clinopyroxene and garnet and their respective Zr and Sr abundances also correlate with depth of derivation. The garnet in a peridotite from 150 km depth (NK1-3) has Zr and Sr contents overlapping those of garnets from the high-temperature peridotites, but its coexisting clinopyroxene has trace element abundances and clinopyroxene–garnet partition coefficients similar to those of the low-temperature peridotites (Fig. 13). The intermediate depth estimate (150 km) for this sample is consistent with it representing a transition between an upper and lower lithospheric mantle.

View larger version (53K): [in this window] [in a new window]   Fig. 13. Depth distribution of (a) La and (b) Sr abundances in clinopyroxenes and (c) Zr and (d) Sr abundances (ppm) in garnets of low- and high-temperature peridotites.

 

The correlation between incompatible trace element characteristics in clinopyroxene and garnet and estimated depth of entrainment for the Nikos peridotites is supported by calculated REE whole-rock patterns for the low-temperature peridotites that are distinct from those of the high-temperature peridotites, suggesting the existence of a vertically zoned lithospheric mantle root beneath Somerset Island. The strong LREE and Sr enrichment observed for clinopyroxenes in the low-temperature peridotites compared with those in the high-temperature peridotites suggests that the shallow lithospheric mantle beneath the Canadian craton is a significant reservoir for incompatible trace elements. The shallow lithospheric mantle could have been metasomatized by incompatible element rich fluids or LREE-enriched melts that intersected their solidus boundaries during passage through the lithosphere, originating from deeper levels of the subcontinental mantle (Wyllie, 1987). This, however, is difficult to reconcile with relatively low incompatible trace element abundances observed in the clinopyroxenes of underlying high-temperature peridotites that appear to have been less affected by metasomatic melts of fluids ascending from depths. It therefore appears likely that the subcontinental mantle root underneath Somerset Island is characterized by a vertical zonation in trace element distribution and that the shallow lithosphere is geochemically distinct from the deep lithosphere beneath the northern Canadian craton. The existence of this vertical layering may date from the time of its stabilization in the Archean.

Trace element systematics for clinopyroxene and garnet from Kaapvaal and Siberian peridotite xenoliths suggests that the continental lithosphere beneath these cratons is also chemically layered (e.g. Shimizu, 1975; Shimizu et al., 1997). The shallow subcontinental lithospheric mantle is characterized by mineral compositions enriched in incompatible trace elements, whereas the deep lithospheric mantle has relatively low mineral incompatible trace element abundances (Shimizu, 1975; Shimizu et al., 1997; Fig. 6). It is not likely that these trace element enriched signatures reflect interaction between mineral phases and the host kimberlite during sample transport, as this process should have equally affected both the low- and high-temperature peridotites. The trace element enriched clinopyroxene compositions of the shallow lithosphere beneath Somerset Island are similar to those observed for the Kaapvaal and the Siberian cratons, which may suggest that the upper portions of lithospheric mantle are generally enriched in incompatible trace elements compared with the deeper lithosphere.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION SAMPLES AND PROCEDURES ANALYTICAL DATA DISCUSSION CONCLUSIONS REFERENCES

 
Despite their high mg-numbers and refractory major element compositions, the Nikos peridotites exhibit whole-rock incompatible trace element patterns that are enriched in LILE, Th, U and LREE compared with primitive mantle compositions. The calculated whole-rock REE patterns of both the high- and low-temperature xenoliths have much lower LREE contents compared with the LREE-enriched analyzed whole-rock compositions, although having similar HREE contents. The REE deficiency amounts to 99–70% for La and Ce, but 20% or less for the HREE, indicating the presence of an interstitial phase containing the excess LREE abundances observed in the analyzed peridotites. Models of an interstitial melt in equilibrium with constituent peridotite minerals yield REE patterns that are comparable with those of the Nikos kimberlites. Mass balance calculations indicate that small amounts of kimberlitic liquid (0·4–2 wt %) may account for excess REE contents that are observed in the Nikos peridotites. Trace amounts of apatite (0·01–0·15 wt %), crystallized from the host kimberlite, might also solve mass balance problems for the incompatible REE abundances. Whole-rock phosphorus contents, however, appear to be lower than those required by the apatite mass balance calculations. Most significantly, however, the calculated whole-rock compositions indicate that the shallow low-temperature peridotites were characterized by flat LREE patterns, whereas the deeper-seated high-temperature peridotites had depleted LREE patterns before contamination by their host kimberlite.

REE partitioning between garnet and clinopyroxene for high-temperature xenoliths that sample the deep lithospheric mantle is consistent with high-temperature experimental data indicating that these clinopyroxene–garnet pairs equilibrated at high temperatures and pressures in the lower lithosphere. REE clinopyroxene–garnet partition coefficients for the low-temperature xenoliths are larger than those for the high-temperature xenoliths and overlap data for mantle-derived rocks from natural systems at lower temperatures (800–1100°C). These findings suggest that REE partitioning between constituent clinopyroxene and garnet in the subcontinental lithosphere is a function of temperature and thus depth.

The strong enrichment in REE and Sr in clinopyroxenes and low Zr and Sr contents in garnets observed for the low-temperature peridotites compared with abundances of these elements in the high-temperature peridotites suggest that the shallow subcontinental lithosphere beneath the northern Canadian craton is geochemically distinct from the underlying lower lithospheric mantle. The Somerset mantle root appears to be characterized by a depth zonation in incompatible elements that may date from the time of its stabilization in the Archean.

	ACKNOWLEDGEMENTS

 
We thank Tariq Ahmedali and Glenna Jackson for performing the whole-rock X-ray fluorescence analyses, and Glenn Poirier for assistance in obtaining the electron microprobe results. We are grateful to Nobu Shimizu from Woods Hole Oceanographic Institution and Tony Simonetti for their help with ion microprobe analyses. We also thank Fabien Rasselet for assistance in the field, hand picking separates of magmatic kimberlite for major and trace element analysis, and performing the CO₂ analyses with the assistance of Constance Guignard. We appreciate the comments of Roberta Rudnick, Bill Griffin and Graham Pearson, which have significantly improved the manuscript.

	FOOTNOTES

 
^*Corresponding author. Telephone: 1-514-398-4885. E-mail: stefanie{at}eps.mcgill.ca

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