Hello Eyewatch
For what its worth, here are a few of the other papers and sites I came across in my technical research last month many of which are particularly relevant to SUF's claims and exploration activities here in the NWT and around the world.
I strongly recommend you use the URL's to read the originals, as the graphics/schematics bring everything into focus.
In particular, read and look at the schematics of the second paper: Accelerated Errosion Of Mantle Terranes In The Slave Craton.
***
7TH INTERNATIONAL KIMBERLITE CONFERENCE, CAPE TOWN, 13-17 APRIL, 1998
DIAMONDS FROM THE DEEP: PIPE DO-27, SLAVE CRATON, CANADA
es.mq.edu.au
R.M. Davies1, W.L. Griffin1,2, N.J. Pearson1, A. Andrew3, B.J. Doyle4, S.Y. O'Reilly1
1 GEMOC, Macquarie,
2 CSIRO EM,
3 CSIRO Petr. Expl.,
4 Kennecott Canada Inc.
This is the first report of an ongoing investigation of diamonds (mineral inclusions, diamonds’ physical and chemical characteristics) from kimberlite pipe DO-27, near Lac de Gras in the Slave Craton, Canada. This study is a component of our Lithosphere Mapping project on the Slave Craton, which integrates petrological and geophysical data to understand the composition, structure and origin of the lithospheric mantle; this information is critical to diamond exploration models for the Slave Craton, which has a unique lithospheric structure (Griffin et al., this volume).
Physical Characteristics
Diamonds examined weigh between 0.01 and 0.42 carats; 75% were <0.10 carat. 75% of the stones are coloured, from shades of brown (55%) to yellow/brown (5%), yellow (9%) and grey (6%). Morphology ranges from planar octahedra and composite octahedra with minor resorption (30% of stones) to heavily resorbed dodecahedra. All resorption categories (Robinson et al., 1989) are represented, and more than half of the diamonds have lost 25% to 65% of their original mass. Resorbed forms consist of equal proportions of dodecahedra, flattened dodecahedra, aggregates and fragments with resorbed external faces. 12% of stones are cubes and cubo-octahedra, many of which are fibrous and/or have hopper faces. Octahedral diamonds have smooth finely stepped planar surfaces and ribbed edges. Negative etch trigons and hexagons are common on primary faces. Dodecahedral forms preserve dissolution laminae and large drop-shaped hillocks. Ruts are common and resorbed surfaces are often frosted. Slip plane dislocations resulting from plastic deformation are evident on about half the stones, as glide planes and shagreen texture on resorbed dodecahedral faces.
Diamond Inclusions
Mineral inclusions were extracted by breaking diamonds in an enclosed cell, then placed in epoxy on glass slides and polished for electron microprobe analysis. Small inclusions exposed on cleavage surfaces were analysed in situ. Representative analyses are given in Table 1.
Eclogitic paragenesis: =50% of the inclusions are eclogitic. Eclogitic garnets have 9-16% CaO and variable Na2O contents; no majorite component is present. Their composition suggests they are derived from kyanite-bearing eclogites, similar to observed xenoliths (Pearson et al., this vol.). One "omphacite" has a high level of opx solid solution, implying a high-T origin; another contains Jd=25%. Diopside inclusions also occur but may be epigenetic (Table 1). Low-Ni (<2.9% Ni) iron sulfides of the eclogite paragenesis were recovered from 5 diamonds .
Peridotitic paragenesis: This paragenesis includes olivine, Cr-pyrope and pentlandite. Pyrope inclusion 27G has very high Cr2O3, but is only mildly subcalcic. It is extremely depleted in Y and Zr, but contains significant Sr, as is typical of diamond-inclusion Cr-pyrope garnets. Two lherzolitic garnets intergrown with diamonds give TNi = 1130 °C and 920 °C. All of the olivines have high mg# (92.8-94.0), suggesting a harzburgitic paragenesis. One pentlandite inclusion has been recovered.
Super-deep paragenesis : At least 25% of the inclusion-bearing stones contain inclusions of ferropericlase ((Fe,Mg)O) or Mg-perovskite. The ferropericlase inclusions have mg#, Cr and Ni contents similar to inclusions reported from Orroroo, Koffiefontein and Sloan (Scott-Smith et al., 1984; Otter and Gurney, 1989). In diamond 14A the ferropericlase is accompanied by an inclusion with MgSiO3 stoichiometry, interpreted as the corresponding Mg-perovskite phase, and by a tiny inclusion of essentially pure Ni.
Phlogopite and other possible epigenetic phases. Diamond 26D contained a granular mass of phlogopite with irregular intergrowths of heterogeneous almandine garnet and diopside. This assemblage is believed to be due to infiltration of fluids along a crack between two parts of the stone. Phlogopite, calcite, sphene and phlogopite + diopside + calcite intergrowths have been found in other stones.
FIGURE 1. Carbon-Isotope compositions of DO27 diamonds
Carbon Isotopes
Carbon isotopes were measured by mass spectrometry on 0.1 mg fragments. 2/3 of the *13C values lie between -3.5‰ and -5.5‰; the remainder scatter to very low values (Fig. 1). Peridotitic diamonds cluster in the main peak, while eclogitic diamonds range from -4.1‰ to -21.1‰. Eclogitic garnets have the lightest carbon values <-15‰, while eclogitic sulfides and omphacite range between -4.1 and -14.4‰. The isotopically light carbon of the eclogitic diamonds is taken as evidence of crustal derivation of the carbon.
FTIR Characteristics
Nitrogen contents and aggregation states were calculated by deconvolution of FTIR absorbance spectra of whole stones, using the guidelines of Mendelssohn and Milledge (1995). The eclogitic and peridotitic paragenesis diamonds have similar N contents and N-aggregation characteristics. 2/3 of the diamonds are of the IaA-IaB type; nitrogen contents are 200-900 ppm (mean 500). Nitrogen aggregation states show a bimodal distribution: one mode, with <20% IaB, is more common to the eclogitic diamonds which also have higher average N contents; the other mode scatters between 40-80 %IaB, and %IaB does not correlate with higher N contents. The diamonds with high aggregation states all show plastic deformation, which is inferred to enhance nitrogen aggregation (Evans et al., 1995) 35% of the diamonds contain no detectable nitrogen (Type II). All diamonds with the superdeep inclusion phases are Type II, as in the Sao Luiz diamond suite (Wilding et al., 1991). Other Type II diamonds are peridotitic (olivine and Cr-pyrope inclusions), and ca 10% of the eclogitic diamonds are Type II. CO2 has only been found in eclogitic diamonds (cf Chinn et al., 1995).
Conclusions
The correlation of nitrogen contents and carbon isotope compositions with inclusion paragenesis suggests that the non-inclusion bearing diamonds in this study are derived largely from the eclogitic paragenesis and the "superdeep" ferropericlase-bearing paragenesis. Based on this limited sample, we estimate that *50% of the diamonds are eclogitic, and =25% of the superdeep paragenesis.
The "superdeep" mineral assemblage represented by the ferropericlase and Mg-perovskite inclusions is stable only at lower-mantle depths (>650 km), and its occurrence in diamond-inclusion suites has been interpreted as evidence for the ascent of plumes from the lower mantle or from the core-mantle boundary (Scott-Smith et al., 1984; Kesson and Fitz Gerald, 1991). Its presence at Lac de Gras may have major genetic implications for the other diamonds as well. The association of abundant "superdeep" inclusions with a high proportion of eclogitic diamonds, many of which have very low *13C, suggests to us that a significant proportion of the diamonds from DO27 originated in the deep mantle, in a volume that contained a high proportion of subducted crustal material. The ascent of a megalithic diapir (Ringwood, 1982, Haggerty, 1994) containing this subducted material may have played an important role in the construction of the lithosphere beneath the Slave Craton. In particular, it may have produced the unique two-layered lithospheric mantle found beneath the Lac de Gras region (Griffin et al., this volume).
References
Chinn, I.L., Gurney, J.J., Milledge, H.J., Taylor, W.R. and McCallum, M.E., 1995, CO2-bearing diamonds from the George Creek K1 kimberlite dyke of the Colorado-Wyoming State Line district, Abst. 6th Int. Kimb. Conf., 113-115.
Evans, T., Kiflaw, I., Luyten, W., van Tendeloo, G. and Woods, G.S., 1995, Conversion of platelets into dislocation loops and voidite formation in Type IaB diamonds, Proc. R. Soc. Lond.A, 449, 295-313.
Haggerty, S.E., 1994, Superkimberlites: A geodynamic diamond window to the Earth’s core, Earth. Planet. Sci. Lett., 122, 57-69.
Kesson, S.E. and Fitz Gerald, J.D., 1991, Partitioning of MgO, FeO, NiO, MnO and Cr2O3 between magnesian silicate perovskite and magnesiowustite: implications for the origin of inclusions in diamond and the compositin of the lower mantle, Earth Planet. Sci. Lett., 111,229-240.
Mendelssohn, M.J. and Milledge, H.J., 1995, Geologically significant information from routine analysis of the mid-Infrared spectra of diamonds, Int. Geol. Rev., 37, 95-110.
Otter, M.L. and Gurney, J.J., 1989. Mineral inclusions in diamond from the Sloan diatremes, Colorado-Wyoming State Line kimberlti district, N. America, Geol. Soc. Australia Spec. Publ., 14, 2, 1042-1053.
Ringwood, A.E., 1982, Phase transformations and differentiation in subducted lithosphere: Implications for mantle dynamics, basalt petrogenesis and crustal evolution, Jour. Geol., 90, 611-643.
Robinson, D. N., Scott, J. A., van Niekerk, A. and Anderson, V. G., 1989, The sequence of events reflected in the diamonds of some southern African kimberlites: In Kimberlites and Related Rocks Geol. Soc. Australia Spec. Publ., 14, 990-1000.
Scott-Smith, B.H., Danchin, R.V., Harris, J.W. and Stracke, K.J., 1984, Kimberlites near Orroroo, South Australia, In J. Kornprobst (ed.), Kimberlites and related rocks, Elsevier, Amsterdam, pp 121-142.
Wilding, M.C., Harte, B. and Harris, J.W., 1991, Evidence for a deep origin for the Sao Luiz diamonds, Abst. 5th Int. Kimb. Conf., 456-458.
Table 1. Representative analyses of inclusions in DO-27 diamonds
Paragen. Superdeep Peridotitic
Sample 15B(1) 14A(8) 14A(8) 14A(8) 15B 22B(2)-1 27G Diam. 1
Phase Fe-Pericl. Fe-pericl. Mg-Perov. Ni metal Olivine Olivine Cr-Pyr. Cr-Pyr.
SiO2 0.03 0.04 57.22 0.00 41.22 41.44 40.26 41.04
TiO2 0.02 0.01 0.03 0.02 0.00 0.01 0.17 0.2
Al2O3 0.17 0.08 2.03 0.01 0.02 0.02 11.91 15.72
Cr2O3 0.45 0.67 0.36 0.00 0.06 0.05 14.76 10.31
FeO 29.69 20.97 4.22 0.79 (Fe) 7.06 6.90 7.11 7
MnO 0.31 0.23 0.07 0.03 0.09 0.06 0.15 0.39
MgO 68.44 78.25 36.11 0.00 51.66 51.51 20.01 18.22
CaO 0.02 0.09 0.13 0.00 0.04 0.03 5.00 6.91
Na2O 0.07 0.27 0.04 0.00 0.00 0.00 0.02 0
K2O 0.01 0.04 0.00 0.02 0.00 0.01 0.00 0
NiO 1.55 1.28 0.03 96.2 (Ni) 0.34 0.30 0.00 0
Total 100.76 101.92 100.25 97.1 100.51 100.33 99.39 99.79
mg# 80.4 86.9 93.8 92.9 93.0 83.4 82.3
Paragen. Eclogitic Epigenetic?
Sample D27-28 22I 23j 27F 26d 26d 26d 12E(1c)
Phase Garnet Garnet Garnet Cpx Phlogopite Garnet Cpx Cpx
SiO2 42.73 39.27 39.48 54.30 41.02 41.10 53.88 53.67
TiO2 0.79 0.97 0.82 0.41 3.83 0.01 0.31 0.47
Al2O3 19.31 21.07 21.21 5.48 12.21 23.04 0.7 0.65
Cr2O3 0.07 0.06 0.06 0.07 0.18 0.02 0.45 0.05
FeO 14.16 16.80 16.00 7.93 5.43 9.12 3.17 5.17
MnO 0.29 0.40 0.38 0.10 0.06 0.20 0.08 0.09
MgO 9.87 8.58 8.81 12.55 21.64 18.29 17.62 15.58
CaO 13.05 12.46 13.03 14.56 0.04 6.87 22.52 23.04
Na2O 0.11 0.20 0.20 3.53 0.24 0.04 0.52 0.57
K2O 0.00 0.00 0.00 0.16 10.08 0.02 0.02 0.02
NiO 0.00 0.00 0.00 0.05 0.19 0.00 0.06 0.00
Total 100.38 99.81 99.99 99.14 94.92 98.71 99.33 99.32
mg# 55.39 47.7 49.5 73.8 87.7 78.10 90.8 84.31
KIMBERLITES, ACCELERATED EROSION AND EVOLUTION OF THE LITHOSPHERE STRUCTURE AND MANTLE TERRANES: SLAVE CRATON, CANADA
es.mq.edu.au
Griffin, W.L.1,2, Doyle, B.J.3, Ryan, C.G.2,1, Pearson, N.J.1, O’Reilly, S.Y.1,
Natapov,L.1, Kivi, K.4, Kretschmar, U.5 and Ward, J.6
1. GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia
2. CSIRO Exploration and Mining, P.O. Box 126, North Ryde, NSW 2113, Australia
3. Kennecott Canada Exploration Inc., 200 Granville St., Vancouver, B.C. V6C 1S4, Canada
4. Kennecott Canada Exploration Inc., 1300 Walsh St., Thunder Bay, Ontario P7E 4X4, Canada
5. Kretschmar Geoscience, 408 Bay St., Orillia, Ontario L3V 3X4, Canada
6. Ashton Mining of Canada Inc., Unit 123-930, West 1st St., North Vancouver, B.C. V7P 3N4, Canada
Xenoliths and heavy mineral concentrates (>1500 garnets and chromites from 21 kimberlite intrusions) were used to map the composition, structure and thermal state of the lithospheric mantle beneath the Lac de Gras area. P-T estimates for xenoliths from the A154 pipe, combined with those of Boyd and Canil (1997) for the Grizzly pipe, define a geotherm that is close to or below a 35 mW/m2 conductive model geotherm for temperatures <900 °C, and close to a 38 mW/m2 model at T *900 °C (Pearson et al., this volume). P-T estimates based on garnet and chromite concentrates (Ryan et al., 1996) define a similar stepped geotherm (Fig. 1), which may separate two layers with different conductivity. High-T sheared xenoliths are present, but do not define a "kink" in the geotherm.
Fig. 1. P-T plot for garnet xenocrysts
A boundary between two layers of the lithospheric mantle is well defined by plots of trace- and major-element data vs Nickel Temperature (TNi) for concentrate garnets (Fig. 2). Projection of the temperature of this boundary to the geotherm derived from xenoliths and concentrates places it at a depth of 150±10 km beneath the Lac de Gras area. Both harzburgitic and lherzolitic garnets from the shallower layer have extremely low levels of Y, Zr, Ti and Ga. The median contents of these elements (Ti, 380 ppm; Y, 1.5 ppm; Ga, 4 ppm, Zr, 5 ppm) are lower than 90% of the values found in garnets from Archean cratons worldwide (Griffin et al., 1998), indicating that this layer is ultradepleted. The garnets of the deeper layer are more typical of Archean garnets worldwide (median Ti, 1800 ppm; Y, 8 ppm; Ga, 8 ppm, Zr, 33 ppm), and depleted compositions like those of the shallower ultradepleted layer are rare in the deeper layer. A small population of garnets with Ti-Zr-Y rich signatures characteristic of high-T sheared xenoliths suggests that the base of the (chemically defined) lithosphere has TNi = 1200-1250 °C, corresponding to 200-220 km beneath the central part of the Lac de Gras area.
Garnet and chromite data and available xenoliths indicate that the shallower layer consists of approximately 60% harzburgite and 40% lherzolite, both with similar olivine compositions (mean Fo=92.7). At depths shallower than 100 km, garnet disappears and the Al-Cr phase is spinel. The deeper layer of the lithospheric mantle contains <17% harzburgite, and most of this is concentrated in the upper part of this layer (Fig. 2); mean olivine composition is Fo=91.5. T estimates on xenoliths indicate that all eclogites are derived from the deeper layer, but are bimodally distributed with Al-rich compositions near the bottom of the layer, and Al-poor compositions near the top (Pearson et al., this vol.).
The two-layered lithospheric structure has been mapped in detail over an area ca 60x30 km around Lac de Gras, and is consistent from pipe to pipe. The wider lateral extent of this structure (Fig. 3) has been mapped using concentrates from outlying pipes, including the Ranch Lake pipe to the north, the Cross Lake cluster to the SW, the Drybones pipe near Yellowknife, and isolated exploration samples. Limited data from pipes A44 and PL01 indicate that the two-layer structure extends at least 50 km E and NE from Lac de Gras. This structure also is well-defined beneath the Ranch Lake pipe, ca 80 km NW of Lac de Gras, where the boundary is at ca 140 km depth. However, at Ranch Lake the base of the (chemically defined) lithosphere has shallowed to ca 170 km, and the proportion of harzburgite in both the shallower and deeper layer has decreased.
Fig. 2. Y and Zr vs TNi for garnets, showing the two lithosphere layers, and the LAB boundary. Symbols as in Fig. 1
Concentrate garnets from the Camsell Lake area, 100 km S of Lac de Gras (Pokhilenko et al., 1997) do not include the population of low-Ca, moderate-Cr harzburgitic garnets that characterises the shallow layer beneath Lac de Gras. The range of FeO contents in the Camsell Lake garnets also requires relatively fertile lherzolitic material at shallow depth, and the proportion of G10 garnets in the concentrate is 15-17%, as in the deeper layer beneath Lac de Gras. The distribution of CaO contents in low-Cr garnets also is bimodal at Camsell Lake, as in the deeper layer at Lac de Gras. These data suggest that the shallow ultradepleted layer is absent, or is < 100 km thick, in the southern part of the craton, and that most of the lithospheric mantle consists of material similar to the deeper layer beneath Lac de Gras.
Ultramafic xenoliths from the Jericho pipe, ca. 80 km N of Ranch Lake, are lherzolitic, with relatively low-Cr garnets (Kopylova et al.,this vol.). The TNi distribution of Cr, Zr and Y contents of garnet shows that the shallow ultra- depleted layer is absent, and the stratigraphy and garnet composition are characteristic of Proterozoic, rather than Archean, mantle (Griffin et al., 1998). This suggests a major boundary in the lithospheric mantle, which may correspond to the position of the Proterozoic Kilohigok Basin.
Concentrate garnets from the pipes of the Cross Lake cluster define a 38 mW/m2 geotherm similar to that in the deeper layer of the Lac de Gras section, but without any step; the base of the lithosphere lies at ca 180 km. The ultradepleted upper layer is absent, or <100 km thick; the depth range 100-140 km is occupied almost entirely by moderately depleted lherzolites. The analysed section contains ca 16% harzburgite (as defined by "G10" garnets), concentrated in the depth range 140-160 km, and the median garnet Cr, Zr and Y contents are similar to those of the deeper layer beneath Lac de Gras. High-Ca eclogitic garnets are present, but the bimodal distribution seen in the Lac de Gras sections is not evident.
Peridotitic garnets from the Drybones pipe on the SW margin of the craton are derived entirely from <140 km depth, and are are lherzolitic, with a median Cr2O3 content of ca 5%. Many garnets from depths <130 km are depleted in Zr and Y, but the lack of harzburgitic garnets in this layer distinguishes it from the mantle at similar depths beneath Lac de Gras. This section is most typical of Proterozoic, rather than Archean, lithospheric mantle.
The shallow, ultradepleted layer of the lithospheric mantle has been mapped over an area of at least 18,000 km2 centred on Lac de Gras (Fig. 3). It appears to be absent, or <100 km thick, beneath the SE part of the craton, and apparently does not cross the Sleepy Dragon Belt to the Cross Lake area. The deeper layer appears to extend further, rising to depths ?120 km beneath the Camsell Lake and Cross Lake areas. The shallow layer is interpreted as lithosphere formed in a convergent-margin setting, analogous to modern depleted sub-arc mantle, during the 2.75-2.6 Ga accretion of the eastern part of the Slave craton to the existing continental nucleus of the western part. The deeper layer of the lithospheric mantle contains many diamonds (=25% of the population) with the ultradeep ferropericlase-Mg-perovskite assemblage (Davies et al., this volume), many eclogitic diamonds with very low *13C, and kyanite eclogite xenoliths with anorthositic-troctolitic compositions (Pearson et al., this volume), suggesting a component of recycled crustal material. This deeper layer is interpreted as the head of a plume or diapir, incorporating both moderately depleted mantle and subducted crustal material, that has risen from >650 km depth to accrete onto the base of the craton. The pre-existing lithosphere was ca 150 km thick and ultradepleted beneath the centre of the craton (Fig. 3), but thinner and/or less refractory to the S and SW. This mantle-underplating event may be related to the widespread post-orogenic (2.5-2.6 Ga) granitoid magmatism in the Slave Province, and heat from this event may have caused further depletion of the shallow layer where it was present.
The apparently Proterozoic lithospheric mantle beneath the Jericho pipe may represent older lithosphere reworked during the Mackenzie plume event (1.27 Ga); similar mantle is reflected in garnet concentrates from kimberlites on Victoria Island, closer to the plume focus (this work). Alternatively, it may have been generated during rifting processes that led to the formation of the Proterozoic Kilohigok Basin. The mantle sampled by the Drybones pipe on the SW corner of the craton is not typically Archean; it may have been reworked or replaced during Proterozoic subduction from the west, or during transcurrent movement along the major MacDonald Fault.
Fig. 3. Terrane map of Slave Craton, showing minimum extent of the shallow ultradepleted layer (dark shading) and of the deeper less depleted layer (light shading). I, Anton Terrane (ancient continental core); IA, Sleepy Dragon domain; II, Hackett River Terrane (island arcs); IIA, Contwoyto Terrane (accretionary prism); IIB, Beechy Lake domain; III, passive margins, Kilohigok Basin (Proterozoic cover); IV, Proterozoic mobile belt.
References
Boyd, F.R. and Canil, D., 1997. Goldschmidt Conf. Abstracts, 34-35.
Griffin, W.L., Doyle, B.J., Ryan, C.G., Pearson, N.J., O’Reilly, S.Y, Davies, R., Kivi, K. and van Achterbergh, E., 1998. Jour. Petrol., subm.
Pokhilenko, N.P., McDonald, J.A., Melnik, U., McCorquodale, J., Reimers, L.F. and Sobolev, N.V., 1997. Russian Geology and Geophys., 38, 550-558.
Ryan, C.G., Griffin, W.L. and Pearson, N.J., 1996. Jour Geophys. Res., 101, 5611-5625.
I'll send the rest in another post.
Regards |
