Journal of Petrology Advance Access originally published online on July 29, 2004
Journal of Petrology 2004 45(9):1725-1745; doi:10.1093/petrology/egh031
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Journal of Petrology 45(9) © Oxford University Press 2004; all rights reserved
Intensive Variables in Kimberlite Magmas, Lac de Gras, Canada and Implications for Diamond Survival
SCHOOL OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF VICTORIA, PO BOX 3055, 3800 FINNERTY ROAD, VICTORIA, BC, V8W 3P6, CANADA
RECEIVED OCTOBER 1, 2003; ACCEPTED MARCH 19, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Crystallization temperatures (T) and oxygen fugacities (fO2) of kimberlite magma are estimated from oxides included in olivine phenocrysts from the Leslie, Aaron, Grizzly and Torrie kimberlite pipes in the central Slave Province, Canada. Crystallization temperatures recorded by olivine–chromite pairs at an assumed pressure of 1·0 GPa are 1030–1170°C ± 50°C, with a mean of
1080°C. At these temperatures, the fO2 of coexisting olivine and chromite is 2–3 log units less oxidized than the nickel–nickel oxide (NNO) buffer at a silica activity limited by the presence of monticellite. Mass balance of olivine, bulk-rock and liquid compositions in equilibrium with olivine phenocryst rims suggests that these kimberlites represent crystallization from a magma with 11–28 mol % of liquid, 10 mol % of earlier precipitated olivine phenocrysts and 62–79 mol % of mantle xenocryst olivine. The calculated T–fO2 values indicate that diamonds entrained in the Lac de Gras kimberlites were probably transported to the surface within the stability field of graphite but close to the graphite–CO2 boundary. KEY WORDS: chromite; crystallization temperature; kimberlite; olivine; oxygen fugacity
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Kimberlites are exotic and complex rocks that, for a number of reasons, are attractive for study. First, the composition and intensive variables (e.g. T, fO2) of these mantle-derived magmas shed light on the processes and conditions in their deep mantle source. Secondly, because kimberlites are the main, primary source of diamonds, knowing these intensive variables constrains the likely presence and quality of diamonds in a pipe.
Despite decades of study on kimberlites, there is still no comprehensive model for their origin and evolution (Mitchell, 1995
). One of the major unknowns is the primary composition of kimberlite melt formed in the mantle. Kimberlite magma sampled at the surface is a hybrid of minerals formed by crystallization and xenoliths derived from the mantle and crust. There are no unambiguous criteria to isolate material crystallized from the melt phase from that contributed by xenolithic material.
The best approximations of natural primary magmas are aphanitic kimberlites, such as at Wesselton, South Africa (Shee, 1986) or Jericho, Canada (Price et al., 2000). Even in these special cases, it remains unclear whether volatile (H2O, CO2) contents or their ratios have been preserved during emplacement. Experimental studies of natural and synthetic compositions can provide an estimate of liquidus temperatures for kimberlites (Edgar et al., 1988; Canil & Scarfe, 1990; Ringwood et al., 1992; Edgar & Charbonneau, 1993; Girnis et al., 1995; Dalton & Presnal, 1998), but these estimates can also be suspect because of the diversity in compositions chosen as starting material in the experiments, and the effect of unconstrained primary CO2/H2O on resulting determinations of liquidus temperatures and mineralogy.
Application of mineral geothermometers and oxygen barometers for the estimation of T and fO2 in kimberlite magma is usually not possible, because of the high degree of alteration that many pipes experience during their emplacement. One possible measure of redox conditions in kimberlite is the chemical composition of ilmenite. Pipes that contain ilmenites with low Fe3+/Fe2+ and high Mg are considered to have crystallized at low but undefined fO2 conditions (Fipke et al., 1995). This method has two shortcomings: it has only been empirically developed for South African kimberlites and may not apply to pipes from other provinces, such as those in North America (Orr & Luth, 2000); also, the paucity or absence of ilmenite in some kimberlite pipes limits its application (Fipke et al., 1995).
We examined some extremely fresh kimberlites from the Lac de Gras kimberlite field, in the Northwest Territories, Canada. Olivine phenocrysts in these kimberlites contain inclusions of magnesiochromite, allowing application of Mg–Fe exchange thermometry and oxygen barometry to examine T–fO2 evolution during emplacement of hypabyssal, diatreme and crater phases of pipe development. We then highlight the influence of all of these variables on the preservation of diamond in kimberlite melt and the prediction of economic potential of a kimberlite pipe.
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GEOLOGY, SAMPLES AND PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Geology
The Lac de Gras kimberlite field is located in the east-central part of the Archean Slave Province, Canada (Fig. 1). The Lac de Gras kimberlites have Eocene and Late Cretaceous emplacement ages (Davis & Kjarsgaard, 1997
; Creaser et al., 2003) and intrude metamorphosed Archean sedimentary, volcanic and plutonic rocks (Ward et al., 1995; Pell, 1997). Most of the Slave Province kimberlite pipes do not outcrop at the surface and are usually small, ranging from 2 to 12 hectares in surface area (Pell, 1997). The majority of these pipes are filled by crater facies (pyroclastic and epiclastic) kimberlite intruded by hypabyssal facies magma (Kirkley et al., 1998; Pell, 1997). Diatreme facies kimberlite is rare in comparison with South African examples. This study examines fresh kimberlite sampled in drill core from four kimberlite pipes: Grizzly, Torrie, Leslie and Aaron, with an emphasis on the latter two (Fig. 1). The composition, age and other data for the four kimberlites are summarized in Table 1.
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The Leslie pipe is 7 hectares in area (Fig. 2a). It was emplaced in porphyritic biotite granite and filled with very fresh hypabyssal facies kimberlite to the present land surface. Only a remnant of tuffitic kimberlite breccia is intersected at a depth of 400 m (Berg & Carlson, 1998
). The nearby pipes of the same age are filled with crater facies kimberlite to a depth of at least 300 m (Berg & Carlson, 1998, and references therein). Thus, the absence of crater facies at Leslie cannot be the result of erosion but rather reflects either an explosively formed open vent that was subsequently filled by magmatic kimberlite, or an earlier tuffitic kimberlite breccia phase displaced by a hypabyssal intrusion (Berg & Carlson, 1998). Bulk-rock geochemical studies of Leslie samples by Berg & Carlson (1998) indicate that geochemical variations represent one intrusive phase or multiple phases of chemically identical magma batches. The Leslie pipe incorporates both wall-rock granitoid and mantle xenoliths.
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The Aaron pipe is mainly filled with pelletal volcaniclastic kimberlite that incorporates fragments of shale and biotite- and garnet-bearing country rock. The south-east wall of the pipe is composed of hypabyssal kimberlite (Fig. 2c). The tuff and tuff breccia of the Torrie pipe intrude late Archean K-rich granitoids and have abundant clay and organic fragments (Orr & Luth, 2000
).
Samples
Samples of the Leslie (LS-1 to -5) pipe were selected from 81 pieces of core collected every 2–3 m from drill-hole LDC-09 (Fig. 2a) in the interval between 20 and 231 m. Sample LS-5 is located near the contact with wall rock and shows extensive serpentinization, whereas all four others are massive and extremely fresh hypabyssal kimberlite—well suited for study of their primary mineral compositions.
Aaron samples (AN-1 to -7) were chosen from 95 pieces of core, collected every 1–2 m from drill-hole 97-57 in the interval between 47 and 160 m (Fig. 2c) and represent different kimberlite facies. Samples AN-1, 2, 6 and 7 are grey, hypabyssal kimberlite, with little or no alteration; AN-3 is more altered and contains xenoliths of country rock; AN-4 and AN-5 are friable, green volcaniclastics, with serpentine alteration and abundant wall-rock xenoliths.
Two samples from the Grizzly pipe (GR95-19 and GR95-43) are fresh macrocrystal hypabyssal kimberlite (similar to the Leslie kimberlite). Four samples from the Torrie pipe (TQY 94-17-3, TQY 94-17-13, TQY 94-17-15, TQY 94-17-18) are organic-bearing kimberlite tuff and tuff breccia. Sample locations within the drill-core were not recorded during collection of samples from the two latter pipes.
Petrography
All Leslie samples and the hypabyssal facies of the Aaron pipe are macrocrystic hypabyssal monticellite kimberlite (Fig. 3a), according to the terminology of Scott-Smith (1992). They contain abundant macrocrysts and phenocrysts of olivine, macrocrysts of garnet surrounded by secondary reaction rims, pale to emerald green clinopyroxenes (± garnet inclusions, reaction rims), kink-banded altered biotite and mantle-derived micro-xenoliths, set in relatively fresh matrix of dominantly monticellite and late olivine. Anhedral olivine macrocrysts (up to several mm) constitute 20–45% of the rock and are oval, with minor serpentinization along margins and fractures.
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Olivine phenocrysts were distinguished using two criteria: euhedral shape and parallel extinction under crossed polars. They are small (0·2–1 mm), fresh and sometimes broken. Phenocrysts locally show distinctive zoning on back-scattered electron images (Fig. 3b), with the cores darker or brighter than the margins. The interiors are often rounded and anhedral, indicating partial dissolution before overgrowth by a later generation of olivine. The margins of phenocrysts often contain inclusions of euhedral (1–10 µm) aluminous magnesian chromite (Fig. 3f). The matrix includes fresh, euhedral microphenocrysts of olivine (up to 0·2 mm) and abundant monticellite (up to 80 µm) (Fig. 3c), surrounded by a fine-grained aggregate of serpentine, with zoned perovskite, altered chromite, magnetite, late-stage phlogopite (in Leslie samples), Mg-rich ilmenite, calcite, apatite and minor sulfides (pyrite).
Samples close to the contact with wall rocks are more extensively altered. LS-5 from the Leslie pipe has olivine macrocrysts and phenocrysts that show significant serpentinization along fractures and margins. Calcite and serpentine aggregates occur as pseudomorphs of olivine microphenocrysts and monticellite. The groundmass is altered to calcite and abundant clay minerals and contains laths of phlogopite. Mineralogically, this rock is a macrocrystic, hypabyssal, calcite, serpentine kimberlite. Sample AN-3 from the contact between hypabyssal and volcaniclastic kimberlite of the Aaron pipe is even more altered. The olivine macrocrysts and phenocrysts are often broken, highly fractured and serpentinized at the margins. Garnet macrocrysts are completely pseudomorphed by chlorite.
The volcaniclastic kimberlite of the Aaron pipe contains abundant xenoliths of biotite-rich metamorphic country rock. Megacrysts and phenocrysts are broken and fractured. Olivine phenocrysts with fresh cores have margins partially or completely replaced by a fine-grained, brownish-red aggregate (Fig. 3d), set in a carbonate-rich matrix. The matrix is not uniform, consisting of calcite–serpentine aggregates with abundant opaque minerals or dominated by calcite that forms rounded grains (Fig. 3e) surrounded by aggregates of serpentine and clay minerals.
Olivine phenocrysts in the Leslie, Aaron, Grizzly and Torrie kimberlites contain numerous oxide inclusions. The inclusions (1–10 µm) are mainly of magnesiochromite, but Torrie olivines also contain inclusions of rutile and ilmenite. All oxide inclusions are located within the rims of olivine phenocrysts and can often be found in the same olivine grain (Fig. 3f). The oxides do not show any particular distribution relative to one another and probably crystallized at the same stage. The euhedral shape of the oxide minerals indicates crystallization from a melt.
Petrographic study of the Leslie and Aaron samples shows that in both of these pipes, the degree of secondary alteration increases with depth in the borehole. LS-1, 2, 3 and AN-1 are extremely fresh kimberlites, LS-4 and AN-2 are slightly altered, LS-5 and AN-3 more altered, and AN-4 and -5 are volcaniclastic kimberlite and show extensive alteration and contamination by country rock.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Electron microprobe analyses of minerals were carried out with a JEOL JXA 8900R electron microprobe (EMP) at the University of Alberta (U of A) and a CAMECA SX50 electron microprobe at the University of British Columbia (UBC). Major and minor elements were determined at 15·0 kV acceleration voltage and a beam current of 20·1 nA (UBC) and 15 nA (U of A) with a 5µm beam for olivines and a 1µm beam for chromites, ilmenites and rutile. Analytical conditions were 20 s of counting time on peaks for all major elements, 60 s for Ni and 80 s for V and Zn. Data reduction was done with the ‘PAP’
(
Z) method (Pouchou & Pichoir, 1985). Natural and synthetic standards were used for calibration. Analyses obtained for the same chromite grain at both UBC and U of A differ by less than 5% for major elements, by up to 10% for Ti and by more than 10% for V, Mn, Ni and Zn. The Fe3+ and Fe2+ of the chromites and ilmenites were calculated on the basis of perfect stoichiometry (Droop, 1987).
Whole-rock analyses were performed at the Geochemical Laboratories of McGill University, Montreal, Quebec, using methods described by Price et al. (2000). The major and trace elements were measured by X-ray fluorescence (XRF) spectrometry, and CO2 using a LECO induction furnace and absorption bulb. H2O was calculated as difference between LOI and CO2.
Oxygen and carbon isotope analyses of whole-rock and carbonate fractions were performed at the University of Alberta, using a Finnigan MAT 252 mass spectrometer. Oxygen from silicate was extracted by the BrF5 technique (Clayton & Toshiko, 1983). C and O from carbonates were liberated using phosphoric acid (McCrea, 1950).
Calculation of Fe3+/
Fe in oxides
A common problem in the application of oxygen barometers is the uncertainty in the Fe3+/Fe of the phases determined by stoichiometry. To address this problem, Wood & Virgo (1989) proposed using secondary spinel standards, with Fe3+ determined by Mössbauer spectroscopy to correct stoichiometrically calculated Fe3+ in spinel. We analyzed three magnesiochromites with Fe3+ known from Mössbauer spectroscopy (Kopylova & McCammon, 2003). The composition of these chromites is very close to the composition of chromite inclusions in kimberlite olivine phenocrysts (Table 2). We observed no systematic relationship between Al/(Al + Cr) and (Fe3+/Fetot)true-probe for our standards and were thus unable to use the correction of Wood & Virgo (1989); however, the chromite standards allowed us to evaluate the possible effect of stoichiometric calculation of Fe3+ on the T–fO2 values. The maximum differences in fO2 and T calculated for the Fe3+ values obtained by both methods are +0·2/–0·7 log units and less than ±50°C, respectively (Fig. 4)—similar to those calculated with the oxygen barometer. We use these values as uncertainties in our calculations.
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Olivine
Olivine is the most abundant mineral in kimberlite, occurring as mantle xenocrysts, as large rounded anhedral macrocrysts, and as euhedral and subhedral phenocrysts. Being the most common mineral on the liquidus of kimberlite melt (Mitchell, 1986
), olivine phenocryst compositions can provide information about the melt from which they crystallized. Olivine phenocrysts and xenocrysts in kimberlite cannot be distinguished by composition—only by shape (Mitchell, 1986). We analyzed only euhedral grains (0·3–0·8 mm long) showing parallel extinction that we were confident were the product of melt crystallization. We address the compositional differences between olivine phenocrysts from different kimberlite pipes, the uniformity of olivine populations within one pipe, and examine zoning patterns in some olivines.
Olivine phenocrysts from kimberlites typically show rims with constant composition surrounding cores of diverse compositions. This diversity has been explained by crystallization from different magma batches of kimberlite (Mitchell, 1986). In our samples, the cores have the same average Mg-number as the rims, but show a much broader range of Mg-numbers (Fig. 5a; Electronic Appendix, available at http://www.petrology.oupjournals.org).
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The different composition of olivine cores can be easily recognized on back-scattered electron (BSE) images (Fig. 3b). Olivine cores often have irregular shapes, with rounded edges similar to dissolution forms. The dark phenocryst cores (Mg-number 0·92–0·93) have low Cr2O3 and CaO—common for mantle olivines (Larsen & Pedersen, 2000
, and references therein). These cores are similar in composition to olivine macrocrysts from the same kimberlite, and their irregular shape indicates that they are probably broken pieces of larger crystals. The largest range in MgO content and the most Fe-rich compositions were observed in phenocryst cores from the Leslie kimberlite (Fig. 5c). The origin of the olivine cores is unknown, but the rims, we believe, are the product of kimberlite melt crystallization. Sharp compositional changes on the rim–core boundary of zoned phenocrysts are inconsistent with diffusive processes and, therefore, show that the rims grew on cores of a previously existing olivine, and are not the result of re-equilibration with the groundmass (Fig. 6). The Mg-numbers of the olivine rims from the Leslie, Aaron and Grizzly kimberlites cluster at around 0·92. Rims in the Torrie kimberlite are more Fe-rich (Mg-number 0·91) (Fig. 5d) and may indicate different conditions or bulk composition of the melt.
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A very exotic composition of an olivine rim (Mg-number 0·97) was recorded in grain LS3-k (Fig. 6) from the Leslie kimberlite. The very low NiO content and extremely high CaO and MnO contents of this olivine indicate crystallization at a very late stage (Fig. 6; Electronic Appendix, http://www.petrology.oupjournals.org) or some significant change in the melt conditions (e.g. higher fO2).
Phenocryst cores and rims have different Ni, Ca and Mn contents (Fig. 7). Cores are higher in NiO than rims (Fig. 7a), as described by Mitchell (1973). In the Leslie kimberlite, core compositions form two trends, with a positive correlation between Mg-number and NiO that may indicate their different origin (Fig. 7c). The rims form two separate groups in terms of NiO content (Fig. 5b, Fig. 7a). The higher NiO group plots with the cores. The compositions that we used in thermobarometry are all from the low-NiO group. The cores have low CaO contents, probably reflecting crystallization at high pressures (Simkin & Smith, 1970) (Fig. 7b), whereas rims have a larger CaO range. This may suggest a significant decrease of P (change in magma depth) during crystallization of phenocryst rims. The cores from the Leslie kimberlite show a negative trend between CaO and Mg-number, and MnO behaves similarly to CaO.
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Thus, the composition of olivine phenocryst cores and rims is uniform in the four pipes. The only exceptions are the more Fe-rich composition of the rims from the Torrie kimberlite, and a larger compositional range and presence of some compositional trends in the cores from the Leslie kimberlite, which are absent in all the other pipes studied.
Oxides
Oxide minerals are found in kimberlites as macrocrysts of unknown origin, xenocrysts, inclusions in phenocryst olivine and as late-stage groundmass minerals (Mitchell, 1986). The chromite inclusions in olivine phenocrysts in all four kimberlites in this study are titanian aluminous magnesian chromites (according to the classification of Mitchell, 1986), with Cr/(Cr + Al) = 0·7–0·9, Mg/(Mg + Fe2+) = 0·59–0·64 and 1·1–4·2 wt % TiO2 (Table 2). Their composition is distinctively different from the magnesian ulvöspinel in the groundmass. Compared with chromite inclusions in the Wesselton kimberlite (Shee, 1984), Lac de Gras chromites have slightly higher Cr, Al and Mg, and lower Ti and Fe.
There are compositional variations in chromites between the different kimberlite pipes (Fig. 8). All chromites have a similar range of Mg-number, but plot in different fields on a Cr/(Cr + Al) vs Ti diagram. Chromites from the Torrie pipe have lower Cr contents than chromites from the three other pipes. The Ti content in chromite increases in the order: Aaron < Grizzly < Leslie < Torrie (Fig. 8b). None of the chromites shows any significant correlation between Ti and Fe3+/Fetot (Fig. 8c).
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Ilmenite inclusions in olivine phenocrysts from the Wesselton kimberlite (Shee, 1984
) are distinctively different in composition from other groups of ilmenites found in the same kimberlite. The Torrie ilmenite inclusions (Table 3) are very close in composition to those of Wesselton, except for two grains that have higher MgO contents similar to the groundmass ilmenites from Wesselton. These two inclusions probably came into contact with the latest magma during crystallization of the groundmass and their original compositions may have been altered. For geothermometry and oxygen barometry (see below), we used only ilmenites with unaltered compositions.
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Rutile forms tabular, euhedral inclusions in olivine (Fig. 3f) and is high in Cr2O3 (2·6–3·6 wt %) and Nb2O5 (3·3 wt %) (Table 3). The composition of the rutile corresponds to the microphenocrystal olivine paragenesis, according to the classification of Mitchell (1995)
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Major element chemistry
Major element compositions for the Leslie and Aaron samples from this study are compared with others from the literature, in Table 4 and Fig. 9. Compared with primitive magma compositions from the Jericho kimberlite, all samples from this study have higher MgO and SiO2 contents (Fig. 9a). All hypabyssal samples lie on an olivine-control line, whereas near-contact samples (LS-5 and AN-3) are displaced from this trend, towards a serpentine-control line.
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The covariation of Al2O3 and SiO2 is commonly used to distinguish contamination in kimberlites (Mitchell, 1986
). All Leslie hypabyssal samples lie within the contamination-free field (Fig. 9b), whereas the Aaron hypabyssal samples overlap this field. The Aaron volcaniclastic and Leslie tuffisitic kimberlite breccia samples lie along a trend consistent with contamination from granitoid country rocks (Ward et al., 1995). In all hypabyssal samples, the Si/Mg atomic ratio, which can be used as an indicator of crustal contamination (Ilupin & Lutz, 1971; Mitchell, 1986), is less than 0·88 (Table 4)—characteristic for uncontaminated rocks. Another measure of crustal contamination and/or weathering is the Contamination Index (C.I.) = (SiO2 + Al2O3 + Na2)/(MgO + 2K2O). A value of C.I. of less than 1 corresponds to uncontaminated fresh kimberlites (Clement, 1982). In volcaniclastic samples from Aaron, Si/Mg is greater than 0·88 and C.I. greater than 1 (Table 4), indicative of country-rock contamination or alteration, consistent with the Al2O3–SiO2 plot (Fig. 9b). The Leslie tuffitic kimberlite breccia (Berg & Carlson, 1998) has higher C.I. and probably reflects contamination. The Cr and Ni content of hypabyssal samples is higher than that in volcaniclastic samples.
Stable isotopes
Stable isotopes were used to estimate the influence of meteoric and magmatic fluids on the emplacement and crystallization of kimberlite magma in the Leslie and Aaron pipes. The carbonate fraction of the Leslie and Aaron kimberlites has a mean 
13CPDB of –4·9
, which is similar to the isotopic composition of mantle-derived material (e.g. Kirkley et al., 1989) and close to the Jericho (Price et al., 2000) and Wesselton kimberlite (Kirkley et al., 1989) carbon isotope ratios (Table 5). The 18OSMOW of the carbonate fraction for samples AN-2, 3 and LS-4 has a mean of 9·5, and falls within the field of the Wesselton kimberlite representing primary mantle ratios. LS-3 shows enrichment in 18O that is similar to the Jericho carbonates. The nature of this enrichment is not well understood, but it is not likely to be a result of interaction with meteoric waters (which have negative 18O) or of surface weathering processes (18O 26·5, Sheppard & Dawson, 1975). Other possible explanations for 18O enrichment in carbonates include oxygen isotope exchange with magmatic fluid. Extensive crystallization of groundmass serpentine (18O 2) prior to carbonate crystallization could also shift the isotopic ratios of residual liquids to higher 18O (Kirkley et al., 1989). The whole-rock 18O values for the Leslie and Aaron kimberlites are –1·5 to 3·7—lower than those of the Jericho kimberlite (Table 5). These values are typical of kimberlite groundmass minerals (serpentine and Fe–Ti oxides with 18O of 2 and 3, respectively) (Sheppard & Dawson, 1975). Sample AN-3, which is close to the contact between hypabyssal and volcaniclastic kimberlite, has a negative 18O value, possibly indicating interaction with meteoric fluids along the interface of these two facies.
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GEOTHERMOMETRY AND OXYGEN BAROMETRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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The lack of any significant alteration in olivine phenocrysts and oxide inclusions permits application of geothermometers and oxygen barometers to estimate the T and fO2 of crystallization of the kimberlite magma. Fresh euhedral olivines were identified with a petrographic microscope, and further examined with a scanning electron microscope (SEM) for the presence of oxide inclusions. Any oxide inclusions located on cracks or in slightly altered parts of olivine grains were ignored.
Olivine–spinel
For olivine–spinel assemblages, the reaction
 | (1) |
, requires electron microprobe data for olivine and spinel, and the T of equilibration. The composition of the chromite inclusions from the Lac de Gras kimberlites in terms of Mg-number, Cr-number and Fe3+/Fetot is within the range of spinel compositions used in the experimental calibration. The kimberlite chromites, however, contain much higher Ti. Temperatures were determined by the olivine–spinel FeMg–1 exchange thermometer (TOl–Sp) of O'Neill & Wall (1987), as simplified by Ballhaus et al. (1991). Results at four different equilibration pressures are given in Table 6.
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The oxygen barometer [reaction (1)] requires the presence of orthopyroxene. Ballhaus et al. (1991)
reported that, for orthopyroxene-undersaturated rocks, the oxygen barometer yields the maximum fO2 values. Because the silica activity (aSiO2) of kimberlites is well below that required to stabilize orthopyroxene, a correction has to be made to the calculated fO2 values. The log 
term in the Ballhaus et al. (1991) equation can be substituted with an expression for aSiO2, according to the reaction
 | (2) |
 | (3) |
simplified the expression for the oxygen barometer by canceling orthopyroxene against the ideal part of 
. This simplification is valid for rocks with 
close to 0·1 and silica activity close to that of the forsterite–enstatite (Fo–En) buffer. The difference in fO2 values calculated with the oxygen barometer between orthopyroxene-saturated and -undersaturated rocks is approximately 
. Thus, the corrected fO2 values can be rewritten as
 | (4) |
Silica activity is not well known for kimberlites. Mitchell (1973, 1986) proposed that the groundmass mineral assemblages limit aSiO2 at a given T and P. The upper limit of aSiO2 in the kimberlites from this study is constrained by the presence of monticellite, and so lies below the diopside–monticellite (Di–Mnt) buffer:
 | (5) |
. The difference in aSiO2 between the Fo–En and Di–Mnt buffers was used to adjust the fO2 values obtained from the Ol–Sp oxygen barometer [see equation (4) above] to give the maximum fO2 for the kimberlite magmas. The results recalculated in 
NNO units (= log fO2 sample – fO2 NNO buffer at P and T) are shown in Table 6 and Fig. 10.
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We assume that olivine–chromite co-crystallization probably occurred at or below 1·0 GPa, because monticellite, which limits aSiO2, is not stable at higher P in the melting interval of kimberlite (Edgar et al., 1988
).
Olivine–ilmenite
Olivine phenocrysts from the Torrie kimberlite also contain inclusions of chromite, Mg-rich ilmenite and rutile. The revised Fe–Mg exchange olivine–ilmenite (TOl–Ilm) thermometer in QUILF-95 (Andersen & Lindsley, 1981; Andersen et al., 1993) can be used to estimate the equilibration temperature of Ol–Ilm assemblages. The QUILF-95 method has been calibrated between 400 and 1500°C, to 3·2 GPa and Xgek = 0–1. The calculated TOl–Ilm values are similar to those recorded by TOl–Sp (Table 6, Fig. 10).
The ilmenites in the Torrie olivines have compositions that are very similar to bleb-like ilmenite inclusions in euhedral olivine phenocrysts from the Wesselton kimberlites (Shee, 1984), and are distinctively different from macrocryst and groundmass ilmenites. Some of the Torrie ilmenite inclusions contain >18 wt % MgO and correspond to groundmass ilmenites (Shee, 1984), probably resulting from re-equilibration with the groundmass. In the QUILF-95 calculations, these ilmenites show an absence of equilibrium and were not used in the T–fO2 calculations.
Ilmenite–rutile
Coexisting ilmenite and rutile inclusions in the Torrie olivine phenocrysts serve as an oxygen barometer, based on the reaction
 | (6) |
). We calculated activities 
and 
from Ghiorso & Sack (1995) for each ilmenite inclusion, using TOl–Ilm at 1·0 GPa. The 
is the mole fraction of TiO2 in rutile. The ilmenite compositions on which the rutile–ilmenite barometer is calibrated are more Fe-rich than the Torrie ilmenites. Nevertheless, the estimated fO2 values are similar to those calculated from the Ol–Sp oxygen barometer when uncorrected for the lower aSiO2 in kimberlites (Table 6, Fig. 10). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Crystallization temperatures and redox regime in kimberlite magma
The values of TOl–Sp calculated at 1·0 GPa for olivine rims in the Leslie, Aaron, Grizzly and Torrie kimberlites are between 1030 and 1170°C, with a mean value of 1080°C (Fig. 10). The whole range of calculated TOl–Sp for the four pipes is within the uncertainty of ±50°C. As expected, TOl–Sp correlates positively with the Mg content of the chromites, but such a correlation can also be an artefact of analyzing small chromite inclusions (5–10 µm) in an Mg-rich host phase such as olivine. The potential exists for excitation of the surrounding olivine, increasing the MgO and SiO2 content of the chromites and, as a result, raising the calculated T. To obviate this problem, we rejected from consideration all chromites with >0·2 wt % SiO2. The absence of any correlation between SiO2 in chromites and TOl–Sp suggests that our criteria for rejection are sound. We also believe that these results represent the real crystallization T of a kimberlite melt with olivine and chromite on its liquidus, for several reasons. First, if the chromite inclusions did not crystallize simultaneously with olivine, then they would more probably concentrate along a distinct boundary between the core and rim in olivine grains. On the contrary, chromites are randomly distributed throughout the olivine rims. Secondly, Fe–Mg exchange between spinel inclusions and olivine is very rapid in mafic magmas (Scowen et al., 1991
), implying equilibrium compositions of olivine–chromite pairs in our samples. Thirdly, the TOl–Sp values plot within the olivine + spinel + monticellite ± calcite + liquid fields in the melting interval of kimberlite, and so are consistent with the mineralogy of our samples, and olivine–spinel co-precipitation observed experimentally in kimberlites (Edgar et al., 1988).
We do not know the pressure (P) at which the olivine phenocrysts crystallized; however, pressure has only a minor influence on TOl–Sp (20°C/GPa) and fO2 (0·03 log units/GPa). The TOl–Sp values from olivine–chromite co-crystallization would be below the kimberlite solidus at pressures of >2·0 GPa (Foley, 1990; Canil & Fedortchouk, 1999), so we suggest 2·0 GPa as a maximum value.
TOl–Ilm records 1064, 1134 and 1332°C at 1·0 GPa (from QUILF-95, Andersen et al., 1993)—similar to TOl–Sp when calculated at 1·0 GPa, but differing by more than 150°C at 3·0 GPa (Table 6). The only olivine phenocryst (TQ15-n) where both TOl–Ilm and TOl–Sp can be applied shows only a 35°C difference when calculated at 1·0 GPa, signifying consistency between thermometers, and a suitable assumed P for the calculations.
The range of fO2 values calculated from coexisting Ol and Sp for the four pipes is from NNO = +0·4 to +0·9 (Fig. 10, Table 6) at the silica activity of the Fo–En buffer. Assuming Ol–Sp co-precipitation at a pressure of below 1·0 GPa, and a lower silica activity of the melt limited by the presence of monticellite, the corrected fO2 values are NNO = –3·0 to –2·2. Application of the Ilm–Ru oxygen barometer to the Torrie samples produces values of NNO = +0·7 and +0·9 (Table 7)—very close to the Ol–Sp data when not corrected for lower silica activity. As discussed above, kimberlite ilmenites are more Mg-rich than the compositional range used to calibrate the Ilm–Ru oxygen barometer (Zhao et al., 1999). The MgO content of ilmenites negatively correlates with the calculated log fO2 values, suggesting that the Torrie Ilm–Ru assemblage also yields the maximum fO2 values.
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Ballhaus et al. (1991)
used Ti-free spinels in their experimental calibration. The high TiO2 (1–4 wt %) in the kimberlite chromites from this study might have influenced the fO2 calculation. The absence of a correlation between the Ti content of our chromites and calculated fO2 suggests that this is not the case. Furthermore, a study by Andersen et al. (1991) shows that, for Mg-rich olivines, there is only a minor effect of Ti content in spinels on isopleths of XMgsp for coexisting Olss–Spss.
Figure 11a shows that the Ol–Sp assemblages in the margins of olivine phenocrysts record fO2 values similar to those estimated by Mitchell (1973, 1986) for olivine and magnetite in the groundmass of kimberlite. This suggests that during kimberlite crystallization from 1100 (Ol–Sp phenocrysts) to 600°C (groundmass), the fO2 of the melt evolved parallel to the NNO buffer.
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Estimation of kimberlite melt composition and amount of accumulated olivine
Olivine–liquid equilibria at known crystallization temperatures in our samples can provide constraints on kimberlite melt compositions along their liquid line of descent, and on the proportions of liquid and olivine from different sources (xenocrysts and phenocrysts) during crystallization. The partitioning of MgO and FeO between olivine and melt is a function of T, P and melt composition (e.g. Langmuir et al., 1992
). Using the composition of olivine phenocrysts in our kimberlites and their crystallization temperatures, we can determine the MgO and FeO content in the kimberlite liquid that was in equilibrium with these phenocrysts. Different expressions for this relationship have been proposed (Roeder & Emslie, 1970; Ford et al., 1983; Langmuir et al., 1992; Beattie, 1994; Sugawara, 2000). The effect of alkalis in the melt is incorporated in the expression of Langmuir et al. (1992):
 | (7) |
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Some constraints can be placed on the proportions of liquid and olivine (xenocrysts and phenocrysts) during crystallization of the olivine phenocryst rims. Mass-balance of bulk-rock MgO and FeO of the unaltered kimberlite samples (LS-1, 2, 3, 4 and AN-1, 2, 6, 7) with olivine and the melt composition calculated using KD ol–liqFeMg is shown in Fig. 12a. Melt compositions will plot along the line determined by the distribution coefficient of MgO and FeO between olivine and liquid, established by Roeder & Emslie (1970)
, which is independent of T (Fig. 12a). A tie-line between melt compositions along this line and the most Mg-rich olivine cores (Fo94·2 in the Electronic Appendix, http://www.petrology.oupjournals.org) recorded in our samples provides the upper constraints on the proportion of melt (28 mol %) and accumulated olivine (72 mol %). A lower limit of 11 mol % liquid and 89 mol % of accumulated olivine is constrained by the calculated MgO and FeO content of the kimberlite melt (Fig. 12a). For simplicity, all compositions in Fig. 12a were recalculated with all iron as FeO because of the reduced nature of the Lac de Gras kimberlites and low Fe3+/Fetot values estimated for the Leslie and Aaron kimberlites by the method of Kress & Carmichael (1988) at the T and fO2 determined from Ol–Sp thermobarometry.
Constraints on the amounts of mantle xenocrysts and earlier phenocrystic olivine accumulated in the melt require knowledge of the composition of the kimberlite melt when it first reached its liquidus. Some approximations for the primary kimberlite melt are aphanitic kimberlites from the Wesselton mine (South Africa) (Edgar et al., 1988) and from the Jericho pipe (Northwest Territories, Canada) (Price et al., 2000). The MgO/FeO ratios of the Jericho and Wesselton primary kimberlite liquids evolve during olivine fractional crystallization along the lines (indicated by arrows) shown in Fig. 12b and reach the MgO/FeO ratio of the kimberlite melts equilibrated with the Ol–Sp assemblage in this study after crystallization of 41 and 37 mol % (Jericho), and 26 mol % (Wesselton) of olivine (Table 7).
The tie-lines between the compositions of the Jericho and Wesselton aphanitic kimberlites and mantle olivine give the proportion of xenocrysts accumulated in these melts (Fig. 12b). The amount of earlier generations of phenocrysts present in the melt by the time of Ol–Sp co-precipitation in rims can be then calculated (Table 7). For all three inferred primary melt compositions, the mole proportion of olivine phenocrysts present in the kimberlite melt during Ol–Sp crystallization is 10 mol % (Table 7), regardless of the primary composition of the kimberlite melt, which is unknown. Kimberlites in this study contain between 11 and 28 mol % liquid, 10 mol % of the earlier generations of phenocrysts, and 62–79 mol % of accumulated mantle olivine.
Application to diamond dissolution
Diamonds in transit to the Earth's surface in kimberlite can undergo transformation into graphite, or oxidation into CO2 or carbonate. These processes are widely observed in natural diamonds as resorption, resulting in rounded crystals, etching or frosting, and can lead to the complete destruction of diamond, with significant decreases in the economic grade of a kimberlite pipe. It is not well understood whether the resorption of diamonds occurs in the mantle during kimberlite accumulation and ascent to shallower levels (Harris, 1987; Taylor et al., 1995), or during emplacement in kimberlite magma (Robinson et al., 1989). There is also the potential for reaction of diamonds with heated meteoric waters during surface eruptions (Sheppard & Dawson, 1975).
High-temperature kimberlite melt is a very reactive medium for diamonds and can possibly cause resorption. The degree and kinetics of diamond dissolution are controlled by T, fO2 and fCO2 (e.g. Arima, 1998). Assuming that Ol–Sp co-crystallization occurred below 1·0 GPa, the maximum fO2 values of kimberlite magmas (at a silica activity of the Di–Mnt buffer) are in the graphite stability field very close to the position of the D/GCO buffer (Fig. 11). With a decrease in P, fO2 values shift into the field of CO2 stability. Thus, any diamonds liberated from xenoliths at depth would have experienced dissolution in the graphite stability field, whereas those coming into contact with the magma at the latest stages of eruption might have been resorbed in the CO2 stability field. Diamond dissolution experiments in silicate melt at conditions within the graphite stability field (Sonin et al., 1997; Arima, 1998) produced resorption features that are similar to those recorded in natural diamonds, in agreement with our fO2 data.
This study allows us to evaluate the possible T–P–fO2 path of the Lac de Gras kimberlites at depth. According to Carmichael (1991), the redox state of erupted magmas is determined in their source region. Extrapolating the calculated fO2 conditions to higher T and P in the mantle (Fig. 13) shows that these kimberlites transported diamonds through the graphite stability field well below the D/GCO buffer without carbonatization.
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The Ol–Sp thermometry and oxygen barometry can potentially be applied to kimberlite pipes that show significant differences in the degree of resorption of their diamond populations. Correlation of any of the dissolution features in diamonds with the T and fO2 of kimberlite magmas may help to better understand the nature of the resorption process, or possibly to predict the degree of diamond preservation in a particular pipe.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Inclusions of chromite are common in the rims of olivine phenocrysts in hypabyssal and volcaniclastic kimberlites from the Leslie, Aaron, Grizzly and Torrie pipes in the Lac de Gras area and can be used for estimation of T and fO2 in kimberlite magmas.
TOl–Sp shows that the rims of olivine phenocrysts crystallized at 1030°C (Leslie) to 1170°C (Torrie) at a maximum fO2 of NNO –3·0 to –2·2 (calculated at 1·0 GPa and silica activity of the melt limited by the presence of monticellite).
Mass balance of the melt equilibrated with olivine phenocryst rims and the bulk-rock suggests crystallization of the olivine–chromite assemblage from a magma that contained between 11 and 28 mol % of liquid, 10 mol % of earlier-precipitated olivine phenocrysts and between 62 and 79 mol % of mantle xenocryst olivine.
The T–fO2 values obtained for four Lac de Gras kimberlites show that the diamonds entrained in these kimberlites ascended in the stability fields of graphite. At the latest stages of eruption (below 0·1 GPa), movement into the CO2 stability field is possible.
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Supplementary Data |
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TOPABSTRACTINTRODUCTIONGEOLOGY, SAMPLES AND PETROGRAPHYANALYTICAL METHODSMINERAL CHEMISTRYGEOCHEMISTRYGEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSSupplementary DataREFERENCES |
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Supplementary data for this paper are available on Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We sincerely thank Jon Carlson and BHP Billiton Diamonds Inc. for access to drill-core, for support and suggestions during sampling and permission to publish. L. Shi and M. Raudsepp provided assistance with EMP analyses at U of A and UBC, respectively. M. Raudsepp is thanked for valuable suggestions during microprobe work. We especially thank M. Kopylova and C. McCammon for providing us with the chromite grains with Fe3+ measured by Mössbauer spectroscopy. The manuscript has been improved by the review of N. Green, A. Woodland and J. K. Russell. This research was supported by a UVic graduate scholarship to Y.F. and NSERC of Canada Discovery grant to D.C.
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FOOTNOTES |
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* Corresponding author. Telephone: 001 250 472 4180. Fax: 001 250 721 7200. E-mail: dcanil{at}uvic.ca
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REFERENCES |
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 T. C. Birkett FIRST-ROW TRANSITION ELEMENTS, Y AND Ga IN KIMBERLITE AND LAMPROITE: APPLICATIONS TO DIAMOND PROSPECTIVITY AND PETROGENESIS Can Mineral, October 1, 2008; 46(5): 1269 - 1282. [Abstract] [Full Text] [PDF]
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T. F.D. Nielsen and K. K. Sand THE MAJUAGAA KIMBERLITE DIKE, MANIITSOQ REGION, WEST GREENLAND: CONSTRAINTS ON AN Mg-RICH SILICOCARBONATITIC MELT COMPOSITION FROM GROUNDMASS MINERALOGY AND BULK COMPOSITIONS Can Mineral, August 1, 2008; 46(4): 1043 - 1061. [Abstract] [Full Text] [PDF]
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 P. L. Roeder and D. J. Schulze Crystallization of Groundmass Spinel in Kimberlite J. Petrology, August 1, 2008; 49(8): 1473 - 1495. [Abstract] [Full Text] [PDF]
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V. S. Kamenetsky, M. B. Kamenetsky, A. V. Sobolev, A. V. Golovin, S. Demouchy, K. Faure, V. V. Sharygin, and D. V. Kuzmin Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins J. Petrology, April 1, 2008; 49(4): 823 - 839. [Abstract] [Full Text] [PDF]
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L. Melluso, M. Lustrino, E. Ruberti, P. Brotzu, C. de Barros Gomes, L. Morbidelli, V. Morra, D. P. Svisero, and F. d'Amelio MAJOR- AND TRACE-ELEMENT COMPOSITION OF OLIVINE, PEROVSKITE, CLINOPYROXENE, Cr-Fe-Ti OXIDES, PHLOGOPITE AND HOST KAMAFUGITES AND KIMBERLITES, ALTO PARANAIBA, BRAZIL Can Mineral, February 1, 2008; 46(1): 19 - 40. [Abstract] [Full Text] [PDF]
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A. Bellis and D. Canil Ferric Iron in CaTiO3 Perovskite as an Oxygen Barometer for Kimberlitic Magmas I: Experimental Calibration J. Petrology, February 1, 2007; 48(2): 219 - 230. [Abstract] [Full Text] [PDF]
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D. Canil and A. J. Bellis Ferric Iron in CaTiO3 Perovskite as an Oxygen Barometer for Kimberlite Magmas II: Applications J. Petrology, February 1, 2007; 48(2): 231 - 252. [Abstract] [Full Text] [PDF]
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P. J. DOWNES, J.-A. WARTHO, and B. J. GRIFFIN Magmatic Evolution and Ascent History of the Aries Micaceous Kimberlite, Central Kimberley Basin, Western Australia: Evidence from Zoned Phlogopite Phenocrysts, and UV Laser 40Ar/39Ar Analysis of Phlogopite-Biotite J. Petrology, September 1, 2006; 47(9): 1751 - 1783. [Abstract] [Full Text] [PDF]
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S. TAPPE, S. F. FOLEY, G. A. JENNER, L. M. HEAMAN, B. A. KJARSGAARD, R. L. ROMER, A. STRACKE, N. JOYCE, and J. HOEFS Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton J. Petrology, July 1, 2006; 47(7): 1261 - 1315. [Abstract] [Full Text] [PDF]
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 Y. Kozai and M. Arima Experimental study on diamond dissolution in kimberlitic and lamproitic melts at 1300-1420 {degrees}C and 1 GPa with controlled oxygen partial pressure American Mineralogist, November 1, 2005; 90(11-12): 1759 - 1766. [Abstract] [Full Text] [PDF]
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