Journal of Petrology Advance Access originally published online on January 4, 2006
Journal of Petrology 2006 47(4):801-820; doi:10.1093/petrology/egi096
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Cr-Saturation Arrays in Concentrate Garnet Compositions from Kimberlite and their Use in Mantle Barometry
1 DE BEERS CONSOLIDATED MINES LTD., P.O. BOX 82232, SOUTHDALE, 2135 SOUTH AFRICA
2 MINERAL SERVICES SOUTH AFRICA, P.O. BOX 38668, PINELANDS, 7430 SOUTH AFRICA
RECEIVED DECEMBER 6, 2004; ACCEPTED NOVEMBER 30, 2005
 |
ABSTRACT |
|---|
TOP
ABSTRACT
INTRODUCTIONThe spinel–garnet transition in Cr/Al-enriched peridotitic bulk compositions is known from experimental investigations to occur at 20–70 kbar, within the pressure range sampled by kimberlites. We show that the Cr2O3–CaO compositions of concentrate garnets from kimberlite have maximum Cr/Ca arrays characterized by Cr2O3/CaO
0·96–0·81, and interpret the arrays as primary evidence of chromite–garnet coexistence in Cr-rich harzburgitic or lherzolitic bulk compositions derived from depth within the lithosphere. Under Cr-saturated conditions on a known geotherm, each Cr/Ca array implicitly delineates an isobar inside a garnet Cr2O3–CaO diagram. This simplification invites a graphical approach to calibrate an empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromite–garnet harzburgite xenoliths from the Roberts Victor kimberlite tightly bracket a graphite–diamond constraint (GDC) located at Cr2O3 = 0·94CaO + 5·0 (wt %), representing a pivotal calibration corresponding to 43 kbar on a 38 mW/m2 conductive geotherm. Additional calibration points are established at 14, 17·4 and 59·1 kbar by judiciously projecting garnet compositions from simple-system experiments onto the same geotherm. The garnet Cr/Ca barometer is then simply formulated as follows (in wt %):
- if Cr2O3
0·94CaO + 5, then P38 (kbar) = 26·9 + 3·22Cr2O3 – 3·03CaO, or
- if Cr2O3 < 0·94CaO + 5, then P38 (kbar) = 9·2 + 36[(Cr2O3 + 1·6)/(CaO + 7·02)].
- if Cr2O3 < 0·94CaO + 5, then P38 (kbar) = 9·2 + 36[(Cr2O3 + 1·6)/(CaO + 7·02)].
A small correction to P38 values, applicable for 35–48 mW/m2 conductive geotherms, is derived empirically by requiring conventional thermobarometry results and garnet concentrate compositions to be consistent with the presence of diamonds in the Kyle Lake kimberlite and their absence in the Zero kimberlite. We discuss application of the P38 barometer to estimate (1) real pressures in the special case where chromite–garnet coexistence is known, (2) minimum pressures in the general case where Cr saturation is unknown, and (3) the maximum depth of depleted lithospheres, particularly those underlying Archaean cratons. A comparison with the PCr barometer of Ryan et al. (1996, Journal of Geophysical Research 101, 5611–5625) shows agreement with P38 at 55 ± 2 kbar, and 6–12% higher PCr values at lower P38. Because the PCr formulation systematically overestimates the 43 kbar value of the GDC by 2–6 kbar, we conclude that the empirical Cr/Ca-in-garnet barometer is preferred for all situations where conductive geotherms intersect the graphite–diamond equilibrium.
KEY WORDS: Cr-pyrope; chromite; P38 barometer; mantle petrology; lithosphere thickness
|
INTRODUCTION |
|---|
The aluminous phases in peridotitic rock types in the lithospheric upper mantle are well known from experimental evidence and xenolith studies to transform from plagioclase-facies to spinel-facies to garnet-facies with increasing pressure (Green & Ringwood, 1970
; Harte & Hawkesworth, 1989). The subsolidus transition of spinel peridotite to garnet peridotite occurs via the generalized reaction pyroxene(s) + spinel = garnet + olivine and is located at pressures of 16–20 kbar in Cr-free, Al-bearing peridotitic bulk compositions (MacGregor, 1965; O'Hara et al., 1971; Robinson & Wood, 1998). At increasing peridotite Cr/(Cr + Al) ratios the reaction involves a multivariant pyroxene(s) + Cr-spinel + Cr-pyrope + olivine assemblage that progressively shifts the spinel–garnet transition to pressures as high as 70 kbar, primarily as a result of strong partition of Cr into spinel relative to garnet (MacGregor, 1970; O'Neill, 1981; Klemme, 2004). The implied coexistence of Cr-spinel with Cr-pyrope garnet in depleted Cr/Al-enriched peridotitic bulk compositions has been confirmed by observation in kimberlite-borne xenoliths (e.g. Dawson et al., 1978; Field & Haggerty, 1994; Schulze, 1996; Menzies, 2001) and in multiphase touching inclusions inside diamond (e.g. Sobolev et al., 1976, 1997; Kopylova et al., 1997). Early experimental results in the model MAS–Cr system (Malinovsky & Doroshev, 1975) showed that Cr-in-garnet isopleths have a weak negative dP/dT under Cr-saturated conditions, initiating the development of a promising Cr-in-garnet geobarometer by additional experimentation and related thermodynamic modelling (O'Neill, 1981; Irifune et al., 1982; Chatterjee & Terhart, 1985; Irifune, 1985; Nickel, 1986; Webb & Wood, 1986; Doroshev et al., 1997; Brey et al., 1999; Girnis & Brey, 1999; Girnis et al., 1999, 2003; Klemme & O'Neill, 2000; Klemme, 2004). Despite this effort, the few available thermodynamic calibrations of the Cr-in-garnet barometer have found little use in everyday mantle barometry, in part because they are mathematically complicated, but also because Cr–Al disequilibrium is often demonstrable in experimental charges and key uncertainties consequently remain regarding the stability of, and Cr–Al mixing in, high-temperature, high-pressure pyroxenes and garnet.
Semi-empirical Cr-in-pyrope thermobarometer models have nevertheless been calibrated for application to garnet compositions in heavy mineral concentrates derived from alkaline magmas such as kimberlite (Griffin & Ryan, 1995; Ryan et al., 1996). The models are typically applied to a wide range of Cr-pyrope compositions and are commonly used in conjunction with Ni-in-garnet thermometry to constrain lithospheric geotherms at the time of kimberlite eruption (Ryan et al., 1996; Tainton et al., 1999; Griffin et al., 2004). In these applications the coexistence of garnet with chromite is unknown and garnet compositions provide only an estimate of minimum pressure (Nickel, 1989; Griffin & Ryan, 1995). Similar semi-empirical Cr-based thermobarometers could, in principle, also be developed for orthopyroxene compositions (Nickel, 1989; Klemme & O'Neill, 2000), but the near-absence of orthopyroxene in kimberlitic heavy mineral concentrates would effectively curtail actual application. The Cr/(Cr + Al) ratio of clinopyroxene is a key ingredient of a recently developed thermobarometer applicable to garnet-facies lherzolitic clinopyroxene grains that commonly occur in kimberlitic heavy mineral concentrates (Nimis, 1998; Nimis & Taylor, 2000).
The objective of this study is documentation of chromite-saturation arrays defined by the Cr2O3–CaO compositions of xenocrystic Cr-pyrope garnets that occur in heavy mineral concentrates derived from kimberlite. A consequence of the straightforward empirical approach adopted here is the formulation of a simple, graphically intuitive minimum-pressure geobarometer that is valid for common harzburgitic and lherzolitic Cr-pyrope compositions. The barometer is calibrated using robust constraints provided by simple-system experimental results and the graphite–diamond equilibrium in natural chromite + garnet peridotite xenoliths, and requires only the garnet Cr2O3 and CaO content as input variables, plus an assumed geotherm. The work was inspired partly by 30–60 kbar isobars projected on a garnet Cr2O3–CaO diagram by Malinovsky & Doroshev (1977) and was initially developed to an extent similar to that presented here for Anglo American Research Laboratories (Pty.) Ltd. (AARL) by the two senior authors [unpublished work of Grütter and Smit (1994), but see Grütter (1994) and Grütter & Sweeney (2000)]. The simplicity of our empirical barometer suggests that it will be applied to a number of lithosphere-scale problems, and we discuss its use in situations involving real-pressure barometry, minimum-pressure barometry and base-of-lithosphere estimates. A brief comparison with the PCr barometer of Ryan et al. (1996) is also provided.
|
ANALYTICAL METHODS AND DATA SOURCES |
|---|
A large proportion of the mineral compositions utilized in this work represent electron microprobe analyses for concentrate garnets prepared from samples of kimberlite. Most analyses were performed at AARL in Johannesburg, South Africa, in the mid-1980 s to early-1990s. An ARL electron microprobe equipped with nine wavelength-dispersive spectrometers was used and operated at an acceleration potential of 20 kV and a beam current of 30 nA. Counts were collected for 10 s on the K-
peak of all elements and background count levels estimated from long-term empirical trends. Standards comprised pure oxides MgO (Mg), Cr2O3 (Cr), Al2O3 (Al), TiO2 (Ti) and Fe2O3 (Fe), as well as natural minerals rhodonite (Mn) and wollastonite (Ca,Si). Apparent concentrations were corrected for matrix effects with an online computer program (Bence & Albee, 1968). Lower limits of detection are calculated to be of the order of 0·06 wt % (at 2
) for all elements. The calculated precision of CaO and Cr2O3 determinations is 2·5% relative (at 2), and was confirmed to be of that order by reanalysing the same spot 10, 25 and 100 times on an appropriate variety of garnet grains. The AARL concentrate garnet compositions are augmented in certain instances with data provided by the Kimberlite Research Group (KRG) at the University of Cape Town. These, and ancillary compositional data for non-garnet species in peridotite xenoliths, were compiled from published literature and selected unpublished theses. Additional garnet Cr2O3–CaO values were also digitized from published diagrams using the freeware utility DataThief II. Appropriate reference is made in figure captions to identify and acknowledge various data sources.
|
USEFUL PRINCIPLES |
|---|
P–T–XCr relations
The effect of pressure on Cr/(Cr + Al) in coexisting spinel and garnet in model lherzolite and Ca-free harzburgite lithologies is illustrated for fixed temperature in Fig. 1. Cr-spinel and garnet can coexist over a wide pressure interval for any given bulk peridotite Cr/(Cr + Al) ratio, and the mineral compositions show progressive Cr/Al enrichment to higher pressure. Garnet compositions in Ca-free harzburgites have
20% lower Cr/(Cr + Al) than in lherzolites at any given pressure, implying that an equal-pressure tieline (an isobar) in a garnet Cr2O3 vs CaO diagram must show clear positive dependence on CaO. The barometer developed in this work takes advantage of this relationship. The P–XCr relations in Fig. 1 similarly predict that chromites in Ca-free chromite–garnet harzburgites have markedly lower Cr/(Cr + Al) than do chromites in chromite–garnet lherzolites, at any given pressure.
|
Garnet compositions in lherzolite xenoliths from alkali basalts are constrained by P, T and bulk Cr/(Cr + Al) ratios to have Cr/(Cr + Al) <0·2 (Fig. 1), which calculates to Cr2O3 <7·0 wt % based on the stoichiometry of pyropic garnet. That garnets in these xenoliths always contain significantly lower Cr2O3 (typically <3·0 wt % Cr2O3, Stern et al., 1986
; Ionov et al., 1993) should be ascribed to low-pressure sampling of garnet-facies mantle by alkali basalts, or to the absence of garnet-saturated bulk compositions at depth. Higher bulk Cr/(Cr + Al) ratios in xenoliths from kimberlites permit garnet compositions to extend to Cr/(Cr + Al) 0·35 (12 wt % Cr2O3) in chromite-saturated lherzolite, and to Cr/(Cr + Al) 0·45 (16 wt % Cr2O3) in chromite-saturated Ca-free harzburgite (Fig. 1). Such Cr-rich pyropic garnet compositions are typically observed in concentrates from cratonic, diamond-bearing kimberlites (e.g. Gurney & Switzer, 1973; Sobolev et al, 1973), implying that chromite-saturated garnet peridotites may commonly occur at pressures inside the diamond stability field. This P–T–XCr relationship suggests that high-Cr concentrate garnets from diamondiferous kimberlites could display distinctive compositional evidence of chromite saturation. In what follows below, we identify such Cr-saturated compositional arrays as having garnet Cr2O3/CaO 0·8–0·96 and use them as a cornerstone in the empirical calibration of our Cr/Ca-in-garnet barometer.
The compositions of coexisting Cr-pyrope garnet and chromite in model harzburgite systems (MAS–Cr ± Fe) have been examined experimentally at 900–1500°C at T/P conditions that fall above, or well above, the geothermal gradients characteristic of cratonic lithosphere (Fig. 2 and references therein). Attainment of Cr–Al equilibrium in garnet and pyroxene is an admitted problem, even at these high temperatures, and the experimental results currently permit considerable latitude in the P–T location of garnet Cr/(Cr + Al) isopleths (e.g. Girnis & Brey, 1999; Girnis et al., 2003). Agreement does exist, however, that the Cr/(Cr + Al)Grt isopleths show weak to moderately negative dP/dT, such that the isopleths intersect common conductive lithospheric geotherms at a high angle and are predicted to show a regular increase of 0·01Cr/(Cr + Al)Grt per kbar along (i.e. down) any given geotherm (Fig. 2). We capitalize on this fortuitous situation by explicitly choosing a model 38 mW/m2 conductive geotherm (after Pollack & Chapman, 1977) as a mixed P–T standard state (Wood & Fraser, 1976) for our barometer. This ‘unconventional’ approach summarily simplifies the relationship of pressure with Cr/(Cr + Al)Grt in Cr-saturated systems and permits the use of a modest linear correction to reduce calculated pressures for hotter geotherms. As an illustration of the calibration procedure followed below, we note from Fig. 2 that the graphite–diamond transition intersects a 38 mW/m2 geotherm at 43 kbar (Point I), corresponding to Cr/(Cr + Al)Grt 0·12 based on the isopleths of Malinosky & Doroshev (1975). Conversely, the point labelled J occurs at Cr/(Cr + Al)Grt 0·28 on a 38 mW/m2 geotherm, which, for the isopleths illustrated, corresponds to P 61 kbar. The same Cr/(Cr + Al)Grt would yield P 58 kbar along a hotter 44 mW/m2 geotherm.
|
The garnet Cr2O3–CaO diagram, Cr-saturated version
The garnet Cr2O3 vs CaO diagram (Fig. 3) is used globally by mantle petrologists and the diamond exploration community to represent the mineral compositions and assemblage of garnet-facies upper mantle peridotites, pyroxenites, eclogites and megacrysts (e.g. Gurney & Switzer, 1973
; Sobolev et al., 1973; Griffin et al., 1999; Grütter et al., 2004). Although denominated by only Cr2O3 and CaO, the diagram serves as a fairly comprehensive description of Cr-pyrope compositions because linear stoichiometric relations within the garnet formula (Ca,Mg,Fe)3(Al,Cr)2Si3O12 dictate that trivalent cation contents are completely specified by garnet Cr2O3 content, whereas divalent cation occupancy is specified by garnet CaO content and Mg/Fe ratio. Under the chromite-saturated conditions along a conductive geotherm that are of interest in this work, the diagram assumes special significance: if P is known from Cr/(Cr + Al)Grt, then T is specified by the geotherm, and garnet Ca and Mg/Fe may be inverted to calculate the Mg/(Mg + Fe) of coexisting olivine and pyroxene using conventional Fe–Mg exchange thermobarometers (e.g. O'Neill & Wood, 1979; Harley, 1984). Cr/(Cr + Al)Grt, P, T and calculated Fe–Mg site occupancy of pyroxene may then be used to estimate the tetrahedral Cr and Al contents of the coexisting orthopyroxene, although the parameters of the pyroxene solution model to be used are still being refined (Gasparik, 1987; Nickel, 1989; Klemme & O'Neill, 2000). The garnet PCr barometer of Ryan et al. (1996) uses an iterative inversion of this nature to construct a hypothetical orthopyroxene composition and hence to solve the pressure of Cr saturation as a function of T and garnet composition. Our approach instead follows that of Malinovsky & Doroshev (1977) by developing the barometer empirically within the P–T–X projection plane represented by a garnet Cr2O3–CaO diagram and a geotherm implicitly specified by the Cr-saturated condition (see Fig. 3). Because small changes in bulk Mg/(Mg + Fe) do not materially affect Cr/Al partition between chromite, garnet and pyroxene (Brey et al., 1999), we formulate our barometer without consideration of the small variations in Mg/Fe that occur in natural Cr-pyrope garnets.
|
Kopylova et al. (2000)
described a ‘chromite–clinopyroxene–garnet equilibrium’ (CCGE) trend in comparatively rare orthopyroxene-absent spinel–garnet wehrlite assemblages derived from the low-temperature, low-pressure (650–850°C, 25–36 kbar) mantle underlying the Jericho kimberlite in northern Canada. Garnet compositions in the CCGE trend are moderately calcic and fall along a linear array with Cr2O3/CaO 1·8, distinct from Cr2O3/CaO 4·0 for a lherzolite assemblage (Kopylova et al., 2000; Fig. 3). Similar linear CCGE-type compositional arrays have now been recognized in moderately calcic garnets from low-temperature (typically 700°C < T < 900°C) spinel–garnet wehrlite xenoliths in a number of kimberlite localities (Kopylova et al., 2000, and references therein; Carbno & Canil, 2002; Lehtohnen et al., 2004). These recent discoveries indicate that Cr-saturated garnet compositions are likely to define linear arrays within a garnet Cr2O3–CaO diagram and that Cr saturation at low garnet Cr2O3 content occurs at low pressure and low temperature on cool, cratonic geotherms. For instance, a garnet with 1·48 wt % Cr2O3 and 5·63 wt % CaO in a Jericho spinel–garnet lherzolite xenolith is calculated to derive from P–T (kbar–°C) of 14·4–520, 23·3–633, 26·7–690 or 31·8–781, based on conventional Al-in-opx thermobarometry [sample 22-1 of Kopylova et al. (1999)].
The graphite–diamond constraint (GDC)
Figure 4a portrays the Cr2O3–CaO compositions of garnets in graphite- or diamond-bearing peridotite xenoliths that also contain primary Cr-spinel. Additional data are available for diamond-bearing xenoliths without Cr-spinel (Fig. 4b) and graphite-bearing xenoliths without Cr-spinel (Fig. 4c). The mineral assemblage and compositional data for these xenoliths have been compiled (and often re-verified) from a large number of published and a few unpublished sources and are included as a supplementary dataset (Electronic Appendix 1, available at http://www.petrology.oxfordjournals.org). With a few notable exceptions, the mineral compositions are described as unzoned and similar data are reported for the same xenolith analysed at different facilities. Assemblages are internally consistent, except for sample A-90a, which is intensely serpentinized and has a low Cr/Ca garnet in a garnet + spinel + diamond assemblage (Sobolev et al., 1984). As may be expected from the presence of diamond, the xenoliths are derived predominantly from economic or near-economic kimberlites located on cratons with cool, conductive geotherms (ranging from 36 to 41 mW/m2; Finnerty & Boyd, 1987; Bell et al., 2003; Griffin et al., 2003). The localities represented are Finsch, Jagersfontein, the mines at Kimberley, Newlands, Premier, Roberts Victor, Star (all in South Africa), Letlhakane (in Botswana), Kao, Liqhobong, Mothae, Thaba Putsoa (in Lesotho), Aikhal, Dalnaya, Mir, Udachnaya (in Yakutia) and Schaffer-03 (in the USA).
|
With one exception in 148 available analyses the chromite-saturated garnet compositions show a remarkably consistent separation of graphite-bearing from diamond-bearing samples, indicating that minerals in these mantle samples attained Cr–Al and Ca–Fe–Mg equilibrium at the P–T conditions prevailing along common cratonic geotherms (Fig. 4a; Grütter, 1994
). The division of graphite- from diamond-bearing samples is well constrained compositionally and, serendipitously, is defined by four xenoliths from the Roberts Victor locality—a Group-2 kimberlite situated in the central Kaapvaal craton. Acid-digestion residues of the four samples contain Cr-pyrope garnet, coarse euhedral Cr-spinel, graphite in samples RV160 and RV171, and diamond in samples RV175 and RV180 (Viljoen et al. 1994; K. S. Viljoen, personal communication, 1997). The garnet compositions hence define a unique chromite-saturated trend that transects the harzburgite compositional field in Cr2O3–CaO space (Fig. 4). We denote this relationship as the graphite–diamond constraint (henceforth the GDC) and describe it with the linear expression (in wt % oxides) Cr2O3 = 0·94CaO + 5·0.
Significant variations in the position of the GDC as a function of Fe content are not expected because the Mg/(Mg + Fe) ratios of olivine in natural chromite–garnet harzburgite xenoliths is typically restricted to the range 0·93 ± 0·01 (e.g. Electronic Appendix 1). Other minor components, such as Fe2O3, TiO2 and possibly MnO, could influence spinel–garnet relations in peridotites (MacGregor 1970; Fursenko, 1981; Woodland & O'Neill, 1995), but their relative variation in natural harzburgitic garnet compositions is evidently too small to cause noticeable displacement of the GDC in a Cr2O3–CaO diagram (Fig. 4a).
The P–T conditions of the GDC are, by mantle standards, reasonably well known. Garnets in graphite-bearing sample RV160 have the same Ni content as those in diamond-bearing sample RV175 (37 ± 3 ppm Ni, Viljoen et al., 1994), implying that the two samples bracket the graphite–diamond transition at the same mantle temperature, within error. This observation independently verifies the tight Cr–Ca constraints on the GDC (Fig. 4a) and pins the graphite–diamond transition at Roberts Victor at T = 941 ± 50°C [TNi calibration of Ryan et al. (1996)] and P = 42·9 ± 1·3 kbar [on the graphite–diamond equilibrium of Kennedy & Kennedy (1976)]. Conventional thermobarometers place xenoliths from Group-2 kimberlites in the central Kaapvaal craton on a 38 mW/m2 model conductive geotherm (Skinner, 1989; Menzies et al., 1999; Menzies, 2001; Bell et al., 2003), thereby corroborating an intersection with graphite–diamond at P 43 kbar and T 940°C (Point I in Fig. 2). The unique P–T–X constraints provided by the GDC at Roberts Victor dictate calibration of our barometer at P–T conditions along a 38 mW/m2 conductive geotherm, and a value of P = 43 kbar for graphite–diamond.
|
Cr-SATURATION ARRAYS IN CONCENTRATE GARNET COMPOSITIONS |
|---|
Cr-pyrope garnets in heavy mineral concentrates derived from kimberlite occur as single, xenocrystic mineral grains disaggregated from their host rock and their paragenesis is formally unknown. Although derivation from wehrlitic, lherzolitic or harzburgitic assemblages can be inferred from garnet Ca-saturation characteristics (e.g. Schulze, 1995
; Fig. 3), the Cr-saturation status of a xenocryst garnet cannot be determined. We consequently apply a limiting approach to identify concentrate garnets that may have coexisted with chromite—for a large number of grains analysed from a single kimberlite intrusion, those most likely to be Cr saturated have the highest Cr content at any given Ca content. Application of this concept to concentrate garnet compositions from the Venetia K1 kimberlite highlights the presence of six high-Cr garnets with variable Ca that occur on a linear array with Cr2O3/CaO slope near unity (Fig. 5). Although the compositional array is poorly constrained at Cr2O3 >13·5 wt % by the absence of high-Cr garnets with moderate Ca, it nevertheless defines a limiting Cr-content envelope for low-Ca garnet compositions. Garnets with higher Cr do not occur in a database of over 5000 analyses representing concentrate from all of the 11 known Venetia kimberlites (De Beers, unpublished data, 2004). The six grains highlighted in Fig. 5 are thus the most Cr-rich of all peridotitic garnets contained within the Venetia kimberlite province; their alignment along a linear array falling essentially parallel to the GDC is considered exceptionally strong evidence in support of chromite-saturated conditions at high garnet Cr content, and thus at pressures well in excess of the GDC.
|
The upper Cr limit for peridotitic garnets from the Udachnaya kimberlite is essentially coincident with that at Venetia. A significant range of Ca is recorded in high-Cr garnets from Udachnaya and the upper Cr limit is constrained to be subparallel to the GDC and linear, or approximately so, over the full range of Ca possible in harzburgitic garnet compositions (Fig. 6). Concentrate garnets account for two of three high-Cr data points from Udachnaya—the third represents harzburgite xenolith Uv-379/86 in which high-Cr primary chromite coexists with high-Cr garnet (Griffin et al., 1993
). This relationship affirms chromite saturation as the cause of arrays with Cr2O3/CaO 0·96 that delimit concentrate garnet compositions from Venetia and Udachnaya. The Cr-saturated arrays for both kimberlites intercept the Ca-free axis at 10·0 wt % Cr2O3 (Figs 5 and 6), which recalculates to Cr/(Cr + Al)Grt 0·28 and represents an extreme high-pressure calibration point for our barometer (labelled point J in Fig. 2).
|
Concentrate from the Finsch kimberlite contains many low-Ca garnets with elevated Cr content. The compositions of six high-Cr grains fall within analytical error of a linear array formulated as Cr2O3 = 0·96CaO + 7·79 (in wt %) which parallels the GDC, but at 2·79 wt % higher Cr2O3 content (Fig. 7). We interpret the array as demarcating Cr-saturated conditions at a pressure higher than the GDC, but less than at Udachnaya. High-Cr lherzolitic garnets are exceptionally rare at Finsch when compared with Venetia and Udachnaya (see Figs 5, 6 and 7), implying that chromite-saturated garnet lherzolite xenoliths may be a rarity at Finsch. Low- to moderate-Cr garnet lherzolite xenoliths are common at Finsch and, although derived from high pressure on a 38 mW/m2 geotherm (53–62 kbar at 1100–1250°C), none are reported to contain primary Cr-spinel (Gurney & Switzer, 1973
; Shee et al., 1982; Skinner, 1989; Bell et al., 2003). In contrast to high-Cr garnet harzburgites, the Finsch garnet lherzolites apparently have bulk Cr/(Cr + Al) ratios too low to stabilize chromite at high pressure and temperatures of 1200°C (see Fig. 1).
|
Peridotitic garnets derived from concentrate or xenoliths from the Jagersfontein kimberlite commonly have moderate to low Cr contents, and only a small proportion have Cr contents higher than the GDC (
1·7%; see Fig. 8). Three such grains span the Ca content range of the entire harzburgite compositional field and align perfectly in an array parallel to the GDC that we interpret as resulting from chromite saturation. That the interpreted Cr-saturation array represents a limiting upper envelope is supported by three additional analyses, also spanning a large Ca range, which fall outside analytical error at marginally lower Cr content (Fig. 8).
|
Lherzolitic compositions dominate the concentrate garnets derived from the Koffiefontein kimberlite, and all of these fall to lower Cr than a Cr-saturation array defined by three of the few harzburgitic garnets recovered at this locality (Fig. 9). The Koffiefontein Cr-saturation array is coincident with the GDC within analytical error. Peridotitic garnets included in Koffiefontein diamonds overlap to lower Ca the compositions of harzburgitic concentrate garnets, at Cr/Ca contents similar to or lower than the GDC (Fig. 9). This apparent contradiction of the GDC is explicable by the absence of chromite coexisting with garnet in the described diamond-inclusions (Rickard et al., 1986
; Griffin et al., 1992). Making the reasonable assumption that a typically cratonic geotherm pertains, the concentrate data locate chromite–garnet harzburgites at the GDC (i.e. at the graphite–diamond transition) in the Koffiefontein lithosphere, whereas the diamond-inclusions relate to chromite-free garnet–diamond harzburgites derived from within the diamond stability field. Apart from the occurrence of diamond itself, the presence of diamond-facies harzburgite at Koffiefontein is forecast by a single very low-Ca concentrate garnet with Cr2O3 0·9 wt % higher than the GDC (see Fig. 9). We discuss the significance of such compositionally isolated high-Cr/Ca analyses further below.
|
Concentrate garnets from the Cleve-01 kimberlite show clear truncation of Cr-rich lherzolitic compositions at a Cr2O3/CaO slope similar to the GDC (Fig. 10). However, a line fitted to all the highest Cr data at given Ca produces a Cr-saturation array with a slope subtly shallower than the GDC (Fig. 10). The difference in slope is considered robust, as omission of any one, two or three randomly selected high-Cr data points fails to produce fitted lines with a Cr2O3/CaO slope higher than or equal to the GDC. Wyatt et al. (1994)
postulated the geotherm at Cleve-01 to be close to cratonic and related its lack of diamonds to a gravitational settling event that occurred during transit of the kimberlite magma through the uppermost mantle. This explanation is considered implausible given the presence of common 0·3–2·0 mm sized mantle-derived ilmenite, chromite, garnet and chromian diopside in the kimberlite, raising the prospects that diamond-hosting lithologies may have been absent from the lithosphere sampled by Cleve-01, or that the kimberlite magma entrained mantle materials from within the graphite stability field only. The latter interpretation is supported by the presence of a Cr-saturated array at Cr contents appreciably below the GDC and by the absence of xenocryst garnets with Cr content higher than the GDC.
|
The compositions of garnets overlying the Nzega kimberlite in Tanzania are shown in Fig. 11. Roughly 75% of the garnets record 750°C < TNi < 800°C, placing them in a narrow temperature interval some 165°C lower than the GDC and well inside the graphite stability field on the
37 mW/m2 model conductive geotherm established for the locality (see Tainton et al., 1999). The coexistence of chromite with garnet over such a narrow and low-temperature interval is expected to produce a well-constrained Cr-saturation array displaced to appreciably lower Cr content than the GDC. An appropriately situated Cr-saturation array is defined for Nzega by six garnets with Ca contents covering low-Ca harzburgitic to Ca-rich lherzolitic compositions (Fig. 11). The array is noted to have a slope similar to that at Cleve-01, and subtly different from the GDC.
|
Garnet lherzolite and spinel–garnet lherzolite xenoliths are infrequently observed together with abundant spinel peridotite xenoliths in alkali basalt vents. The compositions of garnets from alkali basalts should logically also show Cr-saturation arrays, and two examples are given here (Figs 12 and 13). The Cr-saturation arrays in both instances slope at Cr2O3/CaO
0·27, whether interpreted for numerous concentrate garnet compositions from Tieling, China (Fig. 12), or for lesser amounts of data representing spinel–garnet lherzolite xenoliths from Pali-Aike, Chile (Fig. 13). The marked change in slope relative to the GDC is a consequence of the decreased separation between ‘garnet-in’ curves for lherzolite and harzburgite at lower pressures (see Fig. 1). The effect is particularly noticeable in fertile, Cr/Al-poor bulk compositions and consequently affects formulation of our empirical Cr/Ca-in-garnet barometer for low-Cr garnet compositions.
|
|
|
CALIBRATION OF A SIMPLE BAROMETER |
|---|
When collated as a family of lines, the Cr-saturation arrays outlined above form a simple graphical template that may be used to formulate an empirical barometer, valid for common harzburgitic and lherzolitic garnet compositions derived from kimberlite (Fig. 14). Arrays richer in Cr than the GDC may be visualized as falling parallel to the GDC and differing from one another only by the value of their ordinate within line segment I–J. Arrays poorer in Cr than the GDC are related to one another by differences in slope and in ordinate values. A rotation about a point located near –1·5 wt % Cr2O3 and –7·0 wt % CaO would serve to describe them. Calibration of the barometer requires a mathematical description of these highly intuitive graphical attributes, as well as established pressure values for garnet compositions near the extremities of the area of interest (points labelled L, H, I and J in Fig. 14). We hence proceed by finding pressures referenced to our chosen standard-state 38 mW/m2 model conductive geotherm for spinel-saturated garnet compositions corresponding to points L, H, I and J.
|
Point L in Fig. 14 represents a garnet in a fertile, Cr-free lherzolite that would be in equilibrium with Fo90 olivine, two pyroxenes and Al-spinel at P = 14·0 kbar and T
390°C on a 38 mW/m2 geotherm [based on the CMAS model of Gasparik (2000) with an Fe correction from O'Neill (1981)]. Garnet composition H occurs on the same geotherm at P = 17·4 kbar and T 470°C in a Cr-free harzburgite assemblage with Fo90 olivine, enstatite and Al-spinel (based on the same model as above). Point I occurs at Cr/(Cr + Al)Grt = 0·14 in a depleted Ca-free, Cr-enriched harzburgite–dunite assemblage with Fo93 olivine and high-Cr spinel. Point I assumes P–T values of the GDC (P = 43 kbar at T 940°C) on a 38 mW/m2 geotherm, as outlined above.
Point J occurs at Cr/(Cr + Al)Grt = 0·28, corresponding to the highest Cr concentrate garnet from kimberlite that would coexist with chromite in a Ca-free chromite–garnet harzburgite–dunite assemblage with Fo93 or Fo94 olivine. Current experimental results at 1200°C yield pressures of 51 to 61 kbar for J (Table 1, Fig. 2), representing a range too large to use in our calibration. However, we note the isothermal pressure difference between J and I averages 19·0 ± 2·9 kbar across six separate experimental investigations (or 19·0 ± 0·4 kbar omitting two outliers; Table 1), implying that J occurs at roughly 19 kbar higher pressure than the GDC (from Fig. 14). By applying a small pressure reduction to account for the temperature difference between J and I along a 38 mW/m2 geotherm (Fig. 2), we estimate that point J occurs at P 59·1 kbar and T 1150°C.
|
The barometer is then formulated (in wt % oxides and kbar) as follows:
- if Cr2O3 0·94CaO + 5, then P38 = 26·9 + 3·22Cr2O3 – 3·03CaO, or
- if Cr2O3 < 0·94CaO + 5, then P38 = 9·2 + 36[(Cr2O3 + 1·6)/(CaO + 7·02)]
- if Cr2O3 < 0·94CaO + 5, then P38 = 9·2 + 36[(Cr2O3 + 1·6)/(CaO + 7·02)]
where the subscript explicitly specifies the reference 38 mW/m2 model conductive geotherm (as shown in Fig. 2). Figure 15 shows a graphical representation of isobars defined by the formulation. The calibration is not valid for wehrlitic garnet compositions because their chromite-saturation arrays appear to fall along trends at high angles to those in harzburgites and lherzolites (see CCGE trend in Fig. 3).
|
Pressures calculated with the P38 formulation have to be reduced for geotherms hotter than the 38 mW/m2 reference state because garnet Cr-content isopleths in chromite-saturated model harzburgite and lherzolite compositions have negative dP/dT (e.g. Fig. 2). Because the magnitude of the correction is poorly known (–24 to –55 bar/°C, Table 1), we construct an empirical correction based on mantle materials from the Zero and Kyle Lake kimberlites, which have elevated geotherms. Ca-poor Cr-pyrope xenocrysts from the Zero kimberlite, South Africa, provide P38 pressures up to 49·8 kbar, but thermobarometry on peridotite xenoliths (Shee et al., 1989
) and clinopyroxene xenocrysts (using Nimis & Taylor, 2000) limits actual pressures to 45·6 kbar on a 48 mW/m2 conductive geotherm. Such a geotherm barely intersects the diamond stability field (Fig. 16), consistent with the absence of diamond at Zero. The Kyle Lake kimberlites in Ontario, Canada, contain Ca-poor, high-Cr garnets (Sage, 2000) with P38 values up to 59·1 kbar. Clinopyroxene thermobarometry constrains actual pressures up to 56·0 kbar, at high temperatures in the diamond stability field on a 46 mW/m2 geotherm (Fig. 16). The Kyle Lake kimberlites contain numerous brown-tinted and graphite-coated diamonds, features that are considered consistent with deformation and/or residence of the diamonds at high mantle temperatures (Harris, 1992). The 48 mW/m2 and 46 mW/m2 conductive geotherms for Zero and Kyle Lake adequately account for the occurrence and state of diamonds in the kimberlites, and for the two localities combined indicate dP/dT of garnet Cr-content isopleths to be approximately –20 bar/°C (dashed arrows in Fig. 16). Our empirical barometer thus includes a small pressure correction for variable geothermal gradients, which is expressed (in kbar) in simplified linear format as
 |
]. Substituting for Zero, we compute P38 = 49·8 kbar to be equivalent to P48 = 45·9 kbar. It is worth noting that a typical convective mantle adiabat intersects the graphite–diamond equilibrium at high temperature on a 48 mW/m2 geotherm (Fig. 16). As our barometer is calibrated to be consistent with conductive geotherms that intersect the graphite–diamond equilibrium, we caution against its application where this is not the case (Fig. 16).
|
|
APPLICATIONS AND DISCUSSION |
|---|
Because of its inherent mathematical simplicity, formulation in terms of only two oxide-denominated variables and comparatively low temperature dependence, the barometer presented in this work may be applied in several situations where Cr-pyrope garnets occur. Here we discuss application to garnet compositions for (1) the special case where garnet–chromite coexistence can be demonstrated, (2) the general case where chromite coexistence with garnet is indeterminate, and (3) base-of-lithosphere depth estimates. A brief comparison is also made with the PCr barometer of Ryan et al. (1996)
.
Chromite-present (real-pressure) barometry
The empirical approach taken to construct our barometer required special selections to be made of kimberlite-derived garnet concentrate data, which emphasize the presence of chromite-saturated compositional arrays (Figs 5–13 and summary in Fig. 14). The barometer yields a real-pressure estimate under such Cr-saturated conditions, and appropriate calculated values for selected garnet compositions are listed in Table 2.
|
Given that the highest Cr/Ca garnets at Venetia and Udachnaya coexist with chromite (Figs 5 and 6), we estimate P38
60 kbar for them, and P35 61 kbar for the 35 mW/m2 conductive geotherms established by conventional thermobarometry at these localities (Griffin et al., 1996; Stiefenhofer et al., 1999). Identical real-pressures of P38 = 52·5 kbar are calculated for three of six chromite-saturated high-Cr/Ca harzburgitic garnets from Finsch (Fig. 7), even though their Cr2O3 and CaO contents differ considerably (Table 2). The pressures calculated for garnets from Venetia, Udachnaya and Finsch are well in excess of that required for diamond to be present at these localities, implying that these kimberlites have entrained depleted, Cr/Al-enriched garnet peridotite over a considerable depth range from within the diamond stability field.
The geotherm applicable to Group-1 kimberlites on the Kaapvaal craton is well known to follow a 40 mW/m2 conductive model, marginally hotter than the 38 mW/m2 geotherm of Group-2 kimberlites (Finnerty & Boyd, 1987; Bell et al., 2003). We accordingly calculate P40 pressures of 47·7 and 41·9 kbar for chromite-saturated harzburgitic garnets from the Jagersfontein and Koffiefontein kimberlites, respectively. Table 2 shows the difference between P40 and P38 estimates to be rather small, indicating that either model could be applied to garnet compositions from Kaapvaal kimberlites without incurring perceptible error. A 40 mW/m2 geotherm is also considered appropriate for concentrate from the Cleve-01 kimberlite (Wyatt et al., 1994), implying that chromite-saturated garnet lherzolite occurs at P40 = 36·4 kbar (Table 2, Fig. 10). In the absence of concentrate garnets with materially higher Cr content, this real-pressure represents the deepest mantle material from Cleve-01, consistent with the absence of diamond at this locality.
Chromite-absent (minimum-pressure) barometry
The vast majority of garnet concentrates derived from kimberlite (or exploration samples) show little or no direct compositional evidence for Cr saturation. Our barometer can still be applied in this generalized circumstance because Cr-pyrope compositions provide minimum-pressure estimates when chromite coexistence cannot be demonstrated (Nickel, 1989). Thus chromite-absent diamond-associated garnets with 5·0 wt % Cr2O3 plot at Cr contents below the GDC (open symbols in Fig. 4b) and return P38 values in the range 33–35 kbar, which can be reconciled with the presence of diamond only if the calculated pressures represent minima.
Our barometer is likely to find extensive application to garnet concentrate compositions where the highest Cr/Ca grain occurs in compositional isolation, and not associated with a chromite-saturated compositional array. We assign a chromite-absent status to such grains from the Koffiefontein and Nzega kimberlites (see Figs 9 and 11) and, assuming appropriate geothermal models, respectively calculate minimum pressures of P40 45·1 kbar and P37 34·7 kbar for them (Table 2). In this application the highest Cr/Ca grains provide estimates of the minimum thickness of depleted lithospheric material entrained by the kimberlite. Applying this concept to Cr-pyrope xenocrysts from kimberlites in central Quebec, Canada, we calculate the depleted lithosphere underlying the Portage area to extend across the range P38 52·4 to 61·9 kbar, i.e. well into the diamond stability field on an assumed 38 mW/m2 geotherm (Fig. 17a, Table 2). The Beaver Lake kimberlite occurs some 90 km to the south of the Portage area and provides evidence of depleted peridotite extending beyond P38 42·0 kbar, based on the composition of a marginally lherzolitic Cr-pyrope xenocryst (Fig. 17b, Table 2); the actual lithosphere must extend deeper because trace diamond occurs at this locality (Girard, 2001).
|
Maximum-value barometry to establish lithosphere depth
The examples outlined above demonstrate that the maximum value obtained during general application of the minimum-pressure barometer depends critically on the prevalence of high-Cr/Ca garnets in a concentrate, particularly those related to harzburgite (Figs 9, 11 and 17). The incidence of such garnet compositions in a dataset is in turn related to the colour representation and quantity of concentrate garnets analysed, the distribution of harzburgitic or Cr-rich lherzolitic bulk compositions at depth within the lithosphere, and the vagaries of how such mantle rock types are entrained by, and disaggregate within, kimberlite magmas. Real-pressure barometry is not possible in the absence of evidence of Cr saturation (e.g. Figs 4–11), and estimates of the depth extent of depleted lithosphere should rely on procurement of several hundred to thousands of garnet analyses that represent a cluster of spatially related kimberlites. Examples of maximum-value depth constraints provided by such densely populated ‘mature’ datasets are listed in Table 3 and are briefly discussed below.
|
Our barometer shows Cr-rich peridotite to occur at depths of at least 60–65 kbar in the central portions of the East China, Siberian, Slave and the West African Man cratons (Table 3). Although minimum pressures for extreme high-Cr/Ca garnets from Daldyn (Russia) and Gahcho Kue (southern Slave) approach 66 kbar, there is no evidence of Cr-rich, depleted lithosphere at depths of 70 kbar (Table 3), implying that it does not exist at such pressures, or that kimberlite magmas cannot entrain depleted peridotite from such depths. However, conventional thermobarometry results document derivation of garnet peridotite xenoliths from 65–70 kbar in the Daldyn, Northern Slave and Southern Slave regions (Griffin et al., 1996
; Kopylova et al., 1999; Koplova & Caro, 2004), providing reassurance that application of maximum-value Cr barometry to ‘mature’ datasets yields robust estimates of the depth extent of Cr-rich, depleted lithosphere.
The western portion of the Kaapvaal craton is richly endowed with numerous and diverse kimberlite magma types that occur in an 450 km long traverse stretching from Kimberley across the craton margin to the off-craton, Proterozoic Namaqualand Metamorphic Complex (Skinner et al., 1994; Grütter et al., 1999). Cr-pyrope concentrates from these kimberlites show depleted peridotite to occur at 53 kbar in the Finsch area (Fig. 7, Table 2), 55 kbar in the Kimberley area (Table 3) and 53 kbar in the vicinity of Roberts Victor, located some 80 km ENE of Kimberley (Table 2). These maximum-value barometry results suggest that the depleted lithosphere underlying the diamond mines in the western Kaapvaal craton is significantly thinner than the mantle roots of many other Archaean cratons. The available data support thinning of the Kaapvaal lithosphere to depths equivalent to pressures of 49 kbar at the inboard edge of the craton, and only 46 kbar underlying the Marydale domain along the outboard edge of the craton (Table 3; Grütter et al., 1999, fig. 4b). Peridotitic mantle material beneath the off-craton Namaqua and Gibeon kimberlite provinces is dominated by moderately depleted lherzolitic bulk compositions, which evidently do not extend beyond depths equivalent to pressures of 45 and 38 kbar, respectively (Table 3). The off-craton lithospheric sections thus fall within the graphite stability field assuming a 44 mW/m2 geotherm applies, consistent with the diamond-absent status of kimberlites in the Namaqua and Gibeon provinces.
Comparison with the PCr barometer of Ryan et al. (1996)
A comparison of typical results for the P38 (this work) and PCr (Ryan et al., 1996) barometer formulations is presented in Figs 18 and 19. Garnet compositions falling at nominal P38 values of 27, 35, 43, 51 and 59 kbar were selected from (1) open-file data for diamond exploration till samples from the central Slave craton (n = 86, Armstrong, 2001); (2) concentrate from Slave craton kimberlites (n = 20, mostly from Griffin et al., 2004); (3) inclusions in diamonds (n = 11, several sources) and (4) chromite-saturated garnet peridotites with graphite or diamond (n = 16, a subset of data in Electronic Appendix 1). P38 and PCr values were calculated assuming equilibration of garnet with chromite, orthopyroxene and olivine at temperatures along a 38 mW/m2 conductive geotherm. The comparison is therefore made at nominal P–T (kbar–°C) of 27–650, 35–815, 43–940, 51–1050 and 59–1140 for a range of garnet Cr2O3–CaO compositions (Fig. 18a), thereby replicating the P–T–X conditions inherent in the P38 calibration of this work. All compositional data, data sources and thermobarometry results are provided as supplementary data (Electronic Appendix 2, available at http://www.petrology.oxfordjournals.org).
|
Our thermobarometry results show PCr > P38 in general, but PCr
P38 at very high Cr content (Fig. 18b). The PCr and P38 formulations generally agree to within 2 kbar for a limited pressure range at 55 ± 2 kbar, but PCr yields 5 to 7 kbar higher pressures than P38 for all very low-Ca compositions (those with CaO <1·2 wt %; Fig. 18a and b, and Electronic Appendix 2). Excluding these low-Ca data, we find that PCr typically yields 7–10% higher pressures than P38 at 27 kbar < P38 < 53 kbar, this being a mantle interval commonly sampled by diamondiferous kimberlites. The combination of PCr with TNi for pyrope garnet (Ryan et al., 1996) would therefore typically underestimate mantle geotherms compared with a P38–TNi combination. Figure 18c shows the underestimate to amount to 2 mW/m2 on average for cool, cratonic geotherms, though earlier modelling indicates a larger underestimate occurs for hot geotherms (see Grütter & Sweeney, 2000).
The GDC provides an additional opportunity to compare results for the P38 and PCr barometers. On assuming Cr-saturated conditions in an orthopyroxene–olivine assemblage, both barometers should reproduce a pressure of 43 kbar at a temperature of 940°C when applied to GDC-like garnet compositions. Our calculations (Electronic Appendix 2) show that PCr systematically overestimates pressure by 6–12% at these conditions and illustrate an unresolved inverse dependence of PCr on garnet CaO content (Fig. 19b). We conclude that the barometer formulated in this work yields substantially different results from the PCr barometer of Ryan et al. (1996), for common cratonic geothermal gradients and the range of Cr-pyrope compositions that characteristically occur in heavy mineral concentrates from kimberlite.
|
|
SUMMARY AND CONCLUSION |
|---|
Our empirical analysis of Cr-pyrope compositions in heavy mineral concentrates from kimberlite shows chromite saturation in natural harzburgite and lherzolite to be expressed as linear arrays with Cr2O3/CaO near unity in garnet Cr2O3 vs CaO diagrams (Figs 4–13). When conceptualized as pressure increments along a standard-state conductive mantle geotherm, a 38 mW/m2 model being chosen in this work, the arrays may be used as a graphical template (Fig. 14) to calibrate a simple, effective Cr/Ca-in-garnet barometer (Fig. 15) with low, negative temperature dependence (Fig. 16). Our P38 barometer formulation is internally consistent with Cr-saturated garnet compositions in cratonic harzburgite and lherzolite assemblages and is closely constrained at the graphite–diamond transition (Fig. 4), which roughly bisects the calibrated compositional range (Fig. 15). We anticipate general application of the barometer to xenocrystic Cr-pyrope compositions derived from kimberlite, for which minimum entrainment depths can be estimated (e.g. Fig. 17). The maximum depth obtained approaches the base of the Cr/Al-enriched, depleted lithosphere with increased maturity in the number and diversity of xenocryst sources considered (Table 3). The PCr barometer of Ryan et al. (1996)
cannot reproduce the compositional and P–T constraints of our P38 formulation (Fig. 18) and we conclude that our empirical barometer is preferred for all situations where conductive geotherms intersect the graphite–diamond equilibrium (Figs 16 and 19). |
SUPPLEMENTARY DATA |
|---|
Supplementary data for this paper are available at Journal of Petrology online.
|
ACKNOWLEDGEMENTS |
|---|
We thank Gerhard Brey and Andrei Girnis for informative discussions on spinel–garnet relations in peridotites and gratefully acknowledge the contribution made by numerous researchers who painstakingly documented assemblages and mineral compositions in carbon-bearing peridotite xenoliths. The De Beers Group of Companies and Mineral Services is acknowledged for support during various stages of this project and for facilitating access to critical concentrate datasets. H.S.G. publishes with permission of De Beers Consolidated Mines Limited. Journal reviews by Gerhard Brey, Dante Canil and Bill Griffin markedly improved the presentation and overall clarity of the manuscript.
|
FOOTNOTES |
|---|
Present address: Rio Tinto Research and Technology Development, 1 Research Avenue, Bundoora 3083, Australia. 
* Corresponding author. Present address: BHP Billiton World Exploration Inc., Suite 800, Four Bentall Centre, 1055 Dunsmuir Street, Vancouver, B.C., V7X 1L2, Canada. Fax: +1 (604) 683-4125. E-mail: herman.grutter{at}bhpbilliton.com
|
REFERENCES |
|---|
Armstrong, J. P. (2001). Kimberlite Indicator Mineral Chemistry Database (KIMC): a preliminary digital compilation of Kimberlite Indicator Mineral Chemistry extracted from publicly available assessment filings; Slave craton and environs, Northwest Territories and Nunavut, Canada. DIAND NWT Geology Division, EGS Open Report 2001-02 (CD-ROM).
Bell, D. R., Schmitz, M. D. & Janney, P. E. (2003). Mesozoic thermal evolution of the Southern African mantle lithosphere. Lithos 71, 273–287.[CrossRef][Web of Science]
Bence, A. E. & Albee, A. L. (1968). Empirical correction factors for the electron microanalysis of silicates and oxides. Journal of Geology 76, 382–403.[Web of Science]
Brey, G. P., Doroshev, A. M. & Kogarko, L. (1991). The join pyrope–knorringite: experimental constraints for a new geothermobarometer for coexisting garnet and spinel. Extended Abstracts, 5th International Kimberlite Conference, 24–28 June 1991, pp. 26–28.
Brey, G. P., Doroshev, A. M., Girnis, A. V. & Turkin, A. I. (1999). Garnet–spinel–olivine–orthopyroxene equilibria in the FeO–MgO–Al2O3–SiO2–Cr2O3 system: I. Composition and molar volumes of minerals. European Journal of Mineralogy 11, 599–617.
Burgess, S. R. & Harte, B. (1999). Tracing lithosphere evolution through the analysis of heterogeneous G9/G10 garnets in peridotite xenoliths, I: major element chemistry. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the 7th International Kimberlite Conference. Cape Town: Red Roof Design, pp. 66–80.
Carbno, G. B. & Canil, D. (2002). Mantle structure beneath the SW Slave Craton, Canada: constraints from garnet geochemistry in the Drybones Bay kimberlite. Journal of Petrology 43, 129–142.
Chatterjee, N. D. & Terhart, L. (1985). Thermodynamic calculation of peridotite phase relations in the system MgO–Al2O3–SiO2–Cr2O3, with some geological applications. Contributions to Mineralogy and Petrology 89, 273–284.
Dawson, J. B., Smith, J. V. & Delaney, J. S. (1978). Multiple spinel–garnet peridotite transitions in upper mantle: evidence from a harzburgite xenolith. Nature 273, 741–743.
Doroshev, A. M., Brey, G. P., Girnis, A. V., Turkin, A. I. & Kogarko, L. N. (1997). Pyrope–knorringite garnets in the Earth's upper mantle: experiments in the MgO–Al2O3–SiO2–Cr2O3 system. Russian Geology and Geophysics 38, 559–586.
Field, S. W. & Haggerty, S. E. (1994). Symplectites in upper mantle peridotites: development and implications for the growth of subsolidus garnet, pyroxene and spinel. Contributions to Mineralogy and Petrology 118, 138–156.[CrossRef][Web of Science]
Finnerty, A. A. & Boyd, F. R. (1987). Thermobarometry for garnet peridotite xenoliths: a basis for the thermal and compositional structure of the upper mantle. In: Nixon, P. H. (ed.) Mantle Xenoliths. New York: John Wiley, pp. 381–402.
Fursenko, B. A. (1981). Synthesis of new high pressure garnets: Mn3Cr2Si3O12 and Fe3Cr2Si3O12. Bulletin de Minéralogie 104, 418–422.
Gasparik, T. (1987). Orthopyroxene thermobarometry in simple and complex systems. Contributions to Mineralogy and Petrology 96, 357–370.[CrossRef][Web of Science]
Gasparik, T. (2000). An internally consistent thermodynamic model for the system CaO–MgO–Al2O3–SiO2 derived primarily from phase equilibrium data. Journal of Geology 108, 103–119.[CrossRef][Web of Science][Medline]
Girard, R. (2001). Caractérisation de l'intrusion kimberlitique du lac Beaver, Monts Otish—pétrographie et minéralogie. Ressources Naturelles Quebec, Report MB2001-08, 23 pp. (in French).
Girnis, A. V. & Brey, G. P. (1999). Garnet–spinel–olivine–orthopyroxene equilibria in the FeO–MgO–Al2O3–SiO2–Cr2O3 system: II. Thermodynamic analysis. European Journal of Mineralogy 11, 619–636.
Girnis, A. V., Stachel, T., Brey, G. P., Harris, J. W. & Phillips, D. (1999). Internally consistent geothermobarometers for garnet harzburgites: model refinement and application. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the 7th International Kimberlite Conference. Cape Town: Red Roof Design, pp. 247–254.
Girnis, A. V., Brey, G. P., Doroshev, A. M., Turkin, A. I. & Simon, N. (2003). The system MgO–Al2O3–SiO2–Cr2O3 revisited: reanalysis of Doroshev et al.'s (1997) experiments and new experiments. European Journal of Mineralogy 15, 953–964.
Green, D. H. & Ringwood, A. E. (1970). Mineralogy of peridotitic compositions under upper mantle conditions. Physics of the Earth and Planetary Interiors 3, 359–371.[CrossRef]
Griffin, W. L. & Ryan, C. G. (1995). Trace elements in indicator minerals: area selection and target evaluation in diamond exploration. Journal of Geochemical Exploration 53, 311–337.[CrossRef][Web of Science]
Griffin, W. L., Gurney, J. J. & Ryan, C. G. (1992). Variations in trapping temperatures and trace elements in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites, and implications for lithosphere stratigraphy. Contributions to Mineralogy and Petrology 110, 1–15.[CrossRef][Web of Science]
Griffin, W. L., Sobolev, N. V., Ryan, C. G., Pokhilenko, N. P., Win, T. T. & Yefimova, E. S. (1993). Trace elements in garnets and chromites: diamond formation in the Siberian lithosphere. Lithos 29, 235–256.[CrossRef][Web of Science]
Griffin, W. L., Kaminsky, F. V., Ryan, C. G., O'Reilly, S. Y., Win, T. T. & Ilupin, I. P. (1996). Thermal state and composition of the lithospheric mantle beneath the Daldyn kimberlite field, Yakutia. Tectonophysics 262, 19–33.[CrossRef][Web of Science]
Griffin, W. L., Fisher, N. I., Friedman, J. H., Ryan, C. G. & O'Reilly, S. Y. (1999). Cr-pyrope garnets in the lithospheric mantle I. Compositional systematics and relations to tectonic setting. Journal of Petrology 40, 679–704.[CrossRef][Web of Science]
Griffin, W. L., O'Reilly, S. Y., Natapov, L. M. & Ryan, C. G. (2003). The evolution of lithospheric mantle beneath the Kalahari craton and its margins. Lithos 71, 215–241.[CrossRef][Web of Science]
Griffin, W. L., O'Reilly, S. Y., Doyle, B. J., Pearson, N. J., Coopersmith, H., Malkovets, V. & Pokhilenko, N. (2004). Lithosphere mapping beneath the North American plate. Lithos 77, 873–922.[CrossRef][Web of Science]
Grütter, H. S. (1994). Spinel–garnet–carbon phase relations in coarse Kaapvaal-type peridotites and implications for garnet–orthopyroxene equilibration. Extended Abstracts, International Symposium on the Physics and Chemistry of the Upper Mantle, São Paulo, June 1994, pp. 5–7.
Grütter, H. S. & Sweeney, R. J. (2000). Tests and constraints on single-grain Cr-pyrope barometer models: some initial results. Extended Abstracts, GeoCanada 2000: The Millenium Geoscience Summit, 29 May–2 June 2000, Calgary, Abstract 772, 4 pp. (on CD-ROM).
Grütter, H. S., Apter, D. B. & Kong, J. (1999). Crust–mantle coupling: evidence from mantle-derived xenocrystic garnets. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the 7th International Kimberlite Conference. Cape Town: Red Roof Design, pp. 307–313.
Grütter, H. S., Gurney, J. J., Menzies, A. H. & Winter, F. (2004). An updated classification scheme for mantle-derived garnet, for use by diamond explorers. Lithos 77, 841–857.[CrossRef][Web of Science]
Gurney, J. J. (1984). A correlation between garnets and diamonds. In: Glover, J. E. & Harris, P. G. (eds) Kimberlite Occurrence and Origin: a Basis for Conceptual Models in Exploration. Geology Department and University Extension, University of Western Australia Publication 8, 143–166.
Gurney, J. J. & Switzer, G. S. (1973). The discovery of garnets closely related to diamonds in the Finsch pipe, South Africa. Contributions to Mineralogy and Petrology 39, 103–116.[CrossRef][Web of Science]
Harley, S. L. (1984). An experimental study of the partitioning of Fe and Mg between garnet and orthopyroxene. Contributions to Mineralogy and Petrology 86, 359–373.[CrossRef][Web of Science]
Harris, J. (1992). Diamond geology. In: Field, J. E. (ed.) The Properties of Natural and Synthetic Diamond. London: Academic Press, pp. 345–393.
Harte, B. & Hawkesworth, C. J. (1989). Mantle domains and mantle xenoliths. In: Ross, J. (ed.) Kimberlites and Related Rocks. Geological Society of Australia Special Publication 14, 649–686.
Ionov, D. A., Ashchepkov, I. V., Stosch, H.-G., Witt-Eickschen, G. & Seck, H. A. (1993). Garnet peridotite xenoliths from the Vitim volcanic field, Baikal region: the nature of the garnet–spinel peridotite transition zone in the continental mantle. Journal of Petrology 34, 1141–1175.
Irifune, T. (1985). Experimental study of the system Mg3Al2Si3O12–Mg3Cr2Si3O12 at high pressure and high temperature. Journal of the Faculty of Science, Hokkaido University (Series IV) 21, 417–451.
Irifune, T., Ohtani, E. & Kumazawa, M. (1982). Stability field of knorringite Mg3Cr2Si3O12 at high pressure and its implication to the occurrence of Cr-rich pyrope in the upper mantle. Physics of the Earth and Planetary Interiors 27, 263–272.[CrossRef][Web of Science]
Kennedy, C. S. & Kennedy, G. C. (1976). The equilibrium boundary between graphite and diamond. Journal of Geophysical Research 81, 2467–2470.
Klemme, S. (2004). The influence of Cr on the garnet–spinel transition in the Earth's mantle: experiments in the system MgO–Cr2O3–SiO2 and thermodynamic modeling. Lithos 77, 639–646.[CrossRef][Web of Science]
Klemme, S. & O'Neill, H. S. C. (2000). The effect of Cr on the solubility of Al in orthopyroxene: experiments and thermodynamic modelling. Contributions to Mineralogy and Petrology 140, 84–98.[CrossRef][Web of Science]
Kopylova, M. G. & Caro, G. (2004). Mantle xenoliths from the southeastern Slave craton: evidence for chemical zonation in a thick, cold lithosphere. Journal of Petrology 45, 1045–1067.
Kopylova, M. G., Gurney, J. J. & Daniels, L. R. M. (1997). Mineral inclusions in diamonds from the River Ranch kimberlite, Zimbabwe. Contributions to Mineralogy and Petrology 129, 366–384.[CrossRef][Web of Science]
Kopylova, M. G., Russell, J. K. & Cookenboo, H. (1999). Petrology of peridotite and pyroxenite xenoliths from the Jericho kimberlite: implications for the thermal state of the mantle beneath the Slave craton, Northern Canada. Journal of Petrology 40, 79–104.[CrossRef][Web of Science]
Kopylova, M. G., Russell, J. K., Stanley, C. & Cookenboo, H. (2000). Garnet from Cr- and Ca-saturated mantle: implications for diamond exploration. Journal of Geochemical Exploration 68, 183–199.[CrossRef][Web of Science]
Lehtonen, M. L., O'Brien, H. E., Peltonen, P., Johanson, B. S. & Pakkanen, L. K. (2004). Layered mantle at the Karelian Craton margin: P–T of mantle xenocrysts and xenoliths from the Kaavi–Kuopio kimberlites, Finland. Lithos 77, 593–608.
MacGregor, I. D. (1965). Stability fields of spinel and garnet peridotites in the synthetic system MgO–CaO–Al2O3–SiO2. Carnegie Institute of Washington Yearbook 64, 126–134.
MacGregor, I. D. (1970). The effect of CaO, Cr2O3, Fe2O3 and Al2O3 on the stability of spinel and garnet peridotites. Physics of the Earth and Planetary Interiors 3, 372–377.[CrossRef]
Malinovsky, I. Y. & Doroshev, A. M. (1975). Effect of temperature and pressure on the composition of Cr-bearing phases of the garnet + enstatite + spinel + forsterite associations. In: Experimental Research on Mineralogy. Bulletin Institute Geology and Geophysics, Novosibirsk, pp. 121–125 (in Russian).
Malinovsky, I. Y. & Doroshev, A. M. (1977). Evaluation of P–T conditions of diamond formation with reference to chrome-bearing garnet stability. Extended Abstracts, 2nd International Kimberlite Conference, Santa Fe, N.M., unpaginated.
Masun, K. M., Doyle, B. J., Ball, S. & Walker, S. (2004). The geology and mineralogy of the Anuri kimberlite, Nunavut, Canada. Lithos 76, 75–97.[CrossRef][Web of Science]
Menzies, A. H. (2001). A detailed investigation into diamond-bearing xenoliths from Newlands kimberlite, South Africa. Ph.D. thesis, University of Cape Town, 253 pp.
Menzies A. H., Shirey, S. B., Carlson, R. W. & Gurney, J. J. (1999). Re–Os systematics of Newlands peridotite xenoliths: implications for diamond and lithosphere formation. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the 7th International Kimberlite Conference. Cape Town: Red Roof Design, pp. 566–583.
Nickel, K. G. (1986). Phase equilibria in the system SiO2–MgO–Al2O3–CaO–Cr2O3 (SMACCR) and their bearing on spinel/garnet lherzolite relationships. Neues Jahrbuch für Mineralogie, Abhandlungen 155, 259–287.[Web of Science]
Nickel, K.G. (1989). Garnet–pyroxene equilibria in the system SMACCR (SiO2–MgO–Al2O3–CaO–Cr2O3): the Cr-geobarometer. In: Ross, J. (ed.) Kimberlites and Related Rocks. Geological Society of Australia Special Publication 14, 901–912.
Nimis, P. (1998). Evaluation of diamond potential from the composition of peridotitic chromian diopside. European Journal of Mineralogy 10, 505–519.
Nimis, P. & Taylor, W. R. (2000). Single clinopyroxene thermobarometry for garnet peridotites. Part 1. Calibration and testing of a Cr-in-cpx barometer and enstatite-in-cpx thermometer. Contributions to Mineralogy and Petrology 139, 541–554.[CrossRef][Web of Science]
O'Hara, M. J., Richardson, S. W. & Wilson, G. (1971). Garnet peridotite stability and occurrence in crust and mantle. Contributions to Mineralogy and Petrology 32, 48–68.[CrossRef][Web of Science]
O'Neill, H. St. C. (1981). The transition between spinel lherzolite and garnet lherzolite, and its use as a geobarometer. Contributions to Mineralogy and Petrology 77, 185–194.[CrossRef][Web of Science]
O'Neill, H. S. C. & Wood, B. J. (1979). An experimental study of Fe–Mg partitioning between garnet and olivine and its calibration as a geothermometer. Contributions to Mineralogy and Petrology 70, 59–70.[CrossRef][Web of Science]
Pollack, H. N. & Chapman, D. S. (1977). On the regional variation of heat flow, geotherms, and lithospheric thickness. Tectonophysics 38, 279–296.[CrossRef][Web of Science]
Rickard, R. S., Harris, J. W., Gurney, J. J. & Cardoso, P. (1986). Mineral inclusions in diamonds from Koffiefontein mine. In: Ross, J. (ed.) Kimberlites and Related Rocks. Geological Society of Australia Special Publication 14, 1054–1062.
Robinson, J. A. C. & Wood, B. J. (1998). The depth of the garnet/spinel transition in fractionally melting peridotite. Earth and Planetary Science Letters 164, 277–284.[CrossRef][Web of Science]
Ryan, C. G., Griffin, W. L. & Pearson, N. J. (1996). Garnet geotherms: pressure–temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research 101, 5611–5625.[CrossRef]
Sage, R. P. (2000). Kimberlites of the Attawapiskat area, James Bay lowlands, northern Ontario. Ontario Geological Survey Open File Report 6019, 341 pp.
Schulze, D. J. (1995). Low-Ca garnet harzburgites from Kimberley, South Africa: abundance and bearing on the structure and evolution of the lithosphere. Journal of Geophysical Research 100, 12513–12526.[CrossRef]
Schulze, D. J. (1996). Chromite macrocrysts from southern African kimberlites: mantle xenolith sources and post-diamond re-equilibration. In: Kogbe, C. A. (ed.) Diamond Deposits in Africa. Special Issue Africa Geoscience Review 3, 203–216.
Shee, S. R., Gurney, J. J. & Robinson, D. N. (1982). Two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa. Contributions to Mineralogy and Petrology 81, 79–87.[CrossRef][Web of Science]
Shee, S. R., Bristow, J. W., Bell, D. R., Smith, C. B., Allsopp, H. L. & Shee, P. B. (1989). The petrology of kimberlites, related rocks and associated mantle xenoliths from the Kuruman province, South Africa. In: Ross, J. (ed.) Kimberlites and Related Rocks. Geological Society of Australia Special Publication 14, 60–82.
Skewes, M. A. & Stern, C. R. (1979). Petrology and geochemistry of alkali basalts and ultramafic inclusions from the Palei-Aike volcanic field in southern Chile and the origin of the Patagonian plateau lavas. Journal of Volcanology and Geothermal Research 6, 3–25.[CrossRef][Web of Science]
Skinner, C. P. (1989). The petrology of peridotite xenoliths from the Finsch kimberlite, South Africa. South African Journal of Geology 92, 197–206.[Abstract]
Skinner, E. M. W., Smith C. B., Viljoen, K. S. & Clark, T. C. (1994). The petrography, tectonic setting and emplacement ages of kimberlites in the south western border region of the Kaapvaal craton, Prieska area, South Africa. In: Meyer, H. O. & Leonardos, O. H. (eds) Kimberlites, Related Rocks and Mantle Xenoliths. CPRM Special Publication 1A, 80–95.
Skinner, E. M. W., Apter, D. B., Morelli, C. & Smithson, N. K. (2004). Kimberlites of the Man craton, West Africa. Lithos 76, 233–259.
Smith, D. & Wilson, C. R. (1985). Garnet–olivine equilibration during cooling in the mantle. American Mineralogist 70, 30–39.[Abstract]
Sobolev, N. V., Lavrent'ev, Y. G., Pokhilenko, N. P. & Usova, L. V. (1973). Chrome-rich garnets from the kimberlites of Yakutia and their paragenesis. Contributions to Mineralogy and Petrology 40, 39–52.[CrossRef][Web of Science]
Sobolev, N. V., Botkunov, A. I., Lavrent'ev, Y. G. & Usova, L. V. (1976). New data on mineral compositions as coexisting with diamonds of kimberlite pipe ‘Mir’ (Yakutia). Geologiya i Geofizika 12, 3–15 (in Russian).
Sobolev, N. V., Pokhilenko, N. P. & Yefimova, E. S. (1984). Diamond-bearing peridotite xenoliths in kimberlites and the problem of the origin of diamonds. Russian Geology and Geophysics 25, 63–80.
Sobolev, N. V., Pokhilenko, N. P. & Afanas'ev, V. P. (1993). Kimberlitic pyrope and chromite morphology and chemistry, as indicators of diamond grade in Yakutian and Arkhangelsk provinces. In: Dunne, K. P. E. & Grant, B. (eds) Mid-Continent Diamonds. GAC–MAC Symposium Abstracts, Edmonton, June 1993, pp. 63–69.
Sobolev, N. V., Yefimova, E. S., Reimers, L. F., Zakharchenko, O. D., Makhin, A. I. & Usova, L. V. (1997). Mineral inclusions in diamonds of the Arkhangelsk kimberlite province. Russian Geology and Geophysics 38, 379–393.
Stachel, T., Viljoen, K. S., Brey, G. & Harris, J. W. (1998). Metasomatic processes in lherzolitic and harzburgitic domains of diamondiferous lithospheric mantle: REE in garnets from xenoliths and inclusions in diamonds. Earth and Planetary Science Letters 159, 1–12.[CrossRef][Web of Science]
Stern, C. R., Saul, S., Skewes, M. A. & Futa, K. (1986). Garnet peridotite xenoliths from the Pali-Aike alkali basalts of southernmost South America. In: Ross, J. (ed.) Kimberlites and Related Rocks. Geological Society of Australia Special Publication 14, 736–744.
Stiefenhofer, J., Viljoen, K. S., Tainton, K. M., Dobbe, R. & Hannweg, G. W. (1999). The petrology of a mantle xenolith suite from Venetia, South Africa. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the 7th International Kimberlite Conference. Cape Town: Red Roof Design, pp. 836–845.
Tainton, K. M., Seggie, A. G., Bayly, B. A., Tomlinson, I. & Quadling, K. E. (1999). Garnet thermobarometry: implications for mantle heat flow within the Tanzanian craton. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the 7th International Kimberlite Conference. Cape Town: Red Roof Design, pp. 852–860.
Viljoen, K. S., Robinson, D. N., Swash, P. M., Griffin, W. L., Otter, M. L., Ryan, C. G. & Win, T. T. (1994). Diamond- and graphite-bearing peridotite xenoliths from the Roberts Victor kimberlite, South Africa. In: Meyer, H. O. & Leonardos, O. H. (eds) Kimberlites, Related Rocks and Mantle Xenoliths. CPRM Special Publication 1A, 285–303.
Webb, S. A. C. & Wood, B. J. (1986). Spinel–pyroxene–garnet relationships and their dependence on Cr/Al ratio. Contributions to Mineralogy and Petrology 92, 471–480.[CrossRef][Web of Science]
Wood, B. J. & Fraser, D. G. (1976). Elementary Thermodynamics for Geologists. Oxford: Oxford University Press, 303 pp.
Woodland, A. B. & O'Neill, H. St. C. (1995). Phase relations between Ca3Fe2Si3O12–Fe3Fe2Si3O12 garnet and CaFeSi2O6–Fe2Si2O6 pyroxene solid solutions. Contributions to Mineralogy and Petrology 121, 87–98.[CrossRef][Web of Science]
Wyatt, B. A., Shee, S. R., Griffin, W. L., Zweistra, P. & Robison, H. R. (1994). The petrology of the Cleve kimberlite, Eyre peninsula, South Australia. In: Meyer, H. O. & Leonardos, O. H. (eds) Kimberlites, Related Rocks and Mantle Xenoliths. CPRM Special Publication 1A, 62–79.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  A. Juhin, G. Morin, E. Elkaim, D. J. Frost, M. Fialin, F. Juillot, and G. Calas Structure refinement of a synthetic knorringite, Mg3(Cr0.8Mg0.1Si0.1)2(SiO4)3 American Mineralogist, January 1, 2010; 95(1): 59 - 63. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A.B. Woodland, M. Bauer, T. B. Ballaran, and M. Hanrahan Crystal chemistry of Fe32+Cr2Si3O12-Fe32+ Fe23+Si3O12 garnet solid solutions and related spinels American Mineralogist, February 1, 2009; 94(2-3): 359 - 366. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. A. Gibson, J. Malarkey, and J. A. Day Melt Depletion and Enrichment beneath the Western Kaapvaal Craton: Evidence from Finsch Peridotite Xenoliths J. Petrology, October 22, 2008; (2008) egn048v1. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Juhin, G. Calas, D. Cabaret, L. Galoisy, and J.-L. Hazemann Structural relaxation around substitutional Cr3+ in pyrope garnet American Mineralogist, May 1, 2008; 93(5-6): 800 - 805. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. McLean, A. Banas, S. Creighton, S. Whiteford, R. W. Luth, and T. Stachel GARNET XENOCRYSTS FROM THE DIAVIK MINE, NWT, CANADA: COMPOSITION, COLOR, AND PARAGENESIS Can Mineral, October 1, 2007; 45(5): 1131 - 1145. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||