Hello Valuepro
I had a similar problem with that link of Peter's. The animation would start loading but then appear to grind to a halt. I have a pretty powerful PC and graphics chip so I don't think it was my computer. Had the same problem with the site when I originally linked it too. Oh well, the rest of that site is very informative.
That GPR exploration reference was interesting. Let us know if that company has any success won't you?
Teevee, I bow to your expertise on the geophones. Though, if one has to drill a hole to drop the unit in, the cost savings over drilling would seem problematic.
On the matter of more tech papers/research, here are a few more of interest.
A Steady-State Conductive Geotherm for the North-central Slave: Inversion of Petrological Data from the Jericho Kimberlite Pipe
perseus.geology.ubc.ca
Journal Geophysical Research : 104, B4, 1999, 7089-7101
James K. Russell and Maya G. Kopylova
Abstract.
Mantle xenoliths carried by kimberlite provide direct evidence for the rock types that constitute the roots to ancient cratons and provide critical constraints on the thermal state of the lithosphere. We derive a steady-state conductive geotherm for the north-central Slave craton based on published petrological data for mantle-derived xenoliths from the Middle Jurassic Jericho kimberlite. The preferred model geotherm assumes a two-layer lithosphere. The upper layer is D km thick, has constant thermal conductivity, and an exponential decrease in heat production with depth (Ao e -z/D); Ao is a fixed value based on the bedrock geology. Below D, the lithosphere has a higher, constant thermal conductivity and a constant value of radiogenic heat production (Am). Inverting the thermobarometric data produces model estimates of surface heat flow (qo) and the depth-scale parameter (D) of 54.1 mWm-2 and 25.8 km, respectively. The value of D corresponds to the thickness of upper to middle crust as defined by seismological for the southern Slave craton. On the basis of our analysis the Slave craton is characterized by high surface heat flow, relative to other Archean cratons, which is consistent with a single field measurement from the southern Slave craton . The high surface heat flow results from the high concentrations of radiogenic heat producing elements found in the Slave crustal rocks. Despite the relatively high surface flow, the underlying lithosphere is cool relative to mantle beneath the rest of North American and other Archean cratons. The model geotherm predicts a thermal gradient in the mantle (100-200 km) of between 5 and 3.7 (o C/km) and a mantle heat flow of 15.9-11.9 mWm-2 that is consistent with other estimates of mantle heat flow beneath Precambrian continental crust.
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perseus.geology.ubc.ca
Mapping the Lithosphere Beneath the North Central Slave Craton
M. G. Kopylova, J. K. Russell, H. Cookenboo
Proc. 7th Intern. Kimb. Conf., 1999, Rad Roof Design, Cape Town, pp..
ABSTRACT:
The architecture of the Slave upper mantle and stratigraphy of the lithosphere and asthenosphere is inferred from thermobarometry of mantle xenoliths in the Jericho kimberlite pipe (north central Slave Craton, North America). We combine a P-T array for the Slave peridotitic upper mantle with thermobarometry results for large suites of clinopyroxene-garnet xenoliths. The latter xenoliths are classified using petrography and mineral chemistry into pyroxenites (8% of the Jericho mantle xenoliths) and eclogites with massive or anisotropic fabric (25%). Anisotropic eclogite has higher content of jadeite and hedenbergite in clinopyroxene, and higher content of grossular in garnet. Mineral chemistry of the Jericho eclogite suggests two distinct protoliths of mantle and oceanic crustal origin for these rocks. Clinopyroxene-garnet thermometry shows a bimodal temperature distribution of clinopyroxene-garnet xenoliths: lower temperatures of origin for eclogite and higher temperatures of formation for pyroxenite. Eclogite derives from between 90 and 195 km, whereas pyroxenite has apparent source regions below 190 km. The petrological stratigraphy is compared to available geophysical cross-sections to constrain the interpretation of geophysical discontinuities below the Slave. There is evidence for an important boundary between 160 and 190 km and at 1100ûC in the north central Slave. This boundary is manifested by the juxtaposition of porphyroclastic peridotite with magmatic rocks, and a coincident and pronounced disturbance in the calculated P-T array. The boundary could be interpreted as the transition between the long-stabilized petrological lithosphere and asthenosphere that has been chemically and texturally modified by pre-kimberlitic magmatism. The lithosphere comprises recrystallized and texturally equilibrated rocks: coarse peridotite with eclogite lenses and layers. The asthenosphere at the time of Jurassic kimberlite magmatism was composed of porphyroclastic peridotite and magmatic pyroxenites. The 160 - 190 km boundary could easily account for the major discontinuity observed by various geophysical methods at this depth.
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library.iem.ac.ru
Petrology of Peridotite and Pyroxenite Xenoliths from the Jericho Kimberlite: Implications for the Thermal State of the Mantle beneath the Slave Craton, Northern Canada
M. G. KOPYLOVA1*, J. K. RUSSELL1 AND H. COOKENBOO2
1DEPARTMENT OF EARTH AND OCEAN SCIENCE, THE UNIVERSITY OF BRITISH COLUMBIA, VANCOUVER, B.C., CANADA, V6T 1Z4
2CANAMERA GEOLOGICAL LTD, 399 MOUNTAIN HIGHWAY, NORTH VANCOUVER, B.C., CANADA, V6B 2M9
RECEIVED NOVEMBER 21, 1997; REVISED TYPESCRIPT ACCEPTED MAY 19, 1998
We report comprehensive petrological and thermobarometric data for the following types of mantle xenoliths from the Jericho kimberlite in the Slave craton: (1) coarse peridotite; (2) porphyroclastic peridotite; (3) megacrystalline pyroxenite; (4) ilmenite-garnet wehrlite and clinopyroxenite. The varieties of the upper-mantle xenoliths, and their proportions, petrography and mineralogy, mainly resemble those from kimberlites of other cratons. Unique characteristics of the north-central Slave mantle include the presence of high-temperature megacrystalline pyroxenite and an unusually high proportion of pyroxenitic magmatic rocks related in origin to megacrysts. Other unique aspects of Jericho peridotitic mantle are: (1) a pronounced Cr enrichment in mineral chemistry within high-temperature peridotite; (2) an anomalously high proportion of chemically unequilibrated samples; (3) an uncommon pattern in silicate mineral chemistry in spinel-bearing peridotite that reflects equilibration with spinel. The central Slave mantle is colder than the mantle beneath the rest of the North American craton and the mantle beneath Kaapvaal and Siberian cratons. Thermobarometric data are fitted to a model steady-state geotherm with an exponential decrease in heat production with depth using a geologically constrained value of surface heat production at Jericho. The estimated model values of surface heat flow (Q0 = 52-53 mW/m2) show an excellent agreement with two heat flow measurements available for the Slave craton (53-55 mW/m2). Thus, the cool upper mantle here coexists with a highly radiogenic crust.
Keywords: mantle xenolith;peridotite;pyroxenite;Slave craton;thermal state
INTRODUCTION
Kimberlite-derived xenoliths provide constraints on the composition, structure and thermal state of the upper mantle underlying many of the world's major cratons. Before the 1990s the lack of kimberlite occurrences from the Slave craton (Northwest Canada) made the lithosphere underlying this craton terra incognita. The first kimberlite was found in the Lac de Gras area of the Slave craton in 1991, and the following `diamond rush' has discovered more than 150 kimberlite pipes (Pell, 1997). Xenoliths hosted by these kimberlite bodies offer the first opportunity to study the petrology of the upper mantle for this part of the Earth.
To date, there have been only preliminary petrological studies of mantle xenoliths of the Slave craton (Boyd & Canil, 1997; Kopylova et al., 1997; MacKenzie & Canil, 1998; Pearson et al., 1998). The purpose of this paper, therefore, is to provide the first detailed description of the mantle rocks underlying the Slave craton, to constrain their conditions of origin and to compare our results with parallel results from other cratons.
GEOLOGICAL SETTING
The Slave Structural Province of the Northwest Territories is one of several Archaean nuclei of the North American Craton. These nuclei, including the Superior Province, the Slave Province, and the Nain Province, were welded together in Palaeoproterozoic time (2·5-1·6 Ga) (Percival, 1996). The Slave craton comprises dominantly late Archaean (2·7-2·6 Ga) supracrustal and plutonic rocks (Padgham & Fyson, 1992), within which are blocks of older (4·0-2·8 Ga) gneiss and younger sedimentary rocks (Percival, 1996). The Earth's oldest known rocks, the Acasta gneisses (4·02 Ga), occur in the western part of the Slave craton (Bowring & Housh, 1995).
Kimberlite has episodically intruded the Slave lithosphere from the Cambrian to Tertiary (Pell, 1997). The distribution of kimberlite pipes in the central Slave Province (Fig. 1) is similar to that observed in other kimberlite fields; most pipes are distributed along a main trend (NNW), whereas subordinate clusters of pipes are arranged in a direction orthogonal to the main trend (NNE and ENE) (Kjarsgaard, 1996). Most of the kimberlite bodies in the Slave Province do not crop out at surface, but are concealed by lakes or glacial till and are fairly small (Pell, 1997); they are interpreted as variably eroded, carrot-shaped diatremes fashioned after `miniature' versions of the classic South African pipes (Kjarsgaard, 1996). The majority of the Slave kimberlite occurrences are in the Lac de Gras area, where crater, diatreme, and hypabyssal facies kimberlite have all been identified. Diamonds have been found in 35 pipes located within the Slave craton, and at least seven are known to have economic quantities of diamonds (Pell, 1997).
Figure 1. Schematic map of Slave craton (Padgham & Fyson, 1992), Northwest Territories, Canada (see inset), showing location of Jericho pipe against distribution of other kimberlite bodies within Slave craton (Pell, 1997). Faults are shown by bold lines.
This study focuses on xenoliths derived from the Jericho kimberlite pipe (65°59'55"N, 111°28'45"W). The Jericho pipe is located 400 km NE of Yellowknife near the northern end of Contwoyto Lake, intrudes Archaean granitoids (Bowie, 1994) and supracrustal rocks of the Slave craton, is dated at 172 ± 2 Ma (Rb-Sr and U-Pb geochronology; Heaman et al., 1997), and is significantly diamondiferous (1·18 ct/t, Lytton Minerals Ltd Press Release, 1997). The Jericho kimberlite is a multiphase intrusion consisting of a precursor dyke and at least two pipes (Cookenboo, 1998). Mineralogically, the Jericho kimberlite is a typical non-micaceous kimberlite lacking groundmass tetraferriphlogopite (Mitchell, 1995). Chemically, on the basis of concentrations of TiO2, K2O, SiO2 and Pb (Smith et al., 1985), the Jericho kimberlite is classified as Group Ia (Kopylova et al., 1998b), and is similar to most of the other Slave kimberlites (Pell, 1997).
Garnet and spinel
Garnet in Jericho peridotites contains high MgO, low CaO and moderate Cr2O3, and is classified as pyrope. All pyrope compositions plot within the `G9' field of lherzolitic, Ca-saturated garnet compositions established by Dawson & Stephens, (1975). However, some of the garnet-bearing samples are, in fact, harzburgite. Garnet in porphyroclastic peridotite is more Ti rich, Mg rich, and Cr rich (Fig. 7) on average than garnet in coarse peridotite: 7·70 wt % Cr2O3 ± 2·02 wt % vs 4·17 ± 1·26 wt %, respectively. Plotted as Cr2O3 vs CaO, pyrope compositions define two trends: a common `lherzolitic' trend (Sobolev et al., 1973) parallel to the G9-G10 boundary, and a more exotic trend showing less Cr2O3 enrichment with increasing Ca. The latter trend extends well into the compositional field of wehrlitic garnet (Sobolev et al., 1973) and comprises pyrope compositions from spinel-bearing peridotite. Ilmenite-garnet wehrlite-clinopyroxenite rocks contain titaniferous pyrope (1-3 wt % TiO2), which is more enriched in Ti than all previously described garnet in kimberlite and kimberlite-hosted xenoliths (Dawson & Stephens, 1975; Mitchell, 1986; Solovjeva et al., 1994).
Figure 7. A Cr2O3 vs CaO plot for garnet in Jericho peridotites and pyroxenites. Fields shown for garnets (e.g. G9, G10, and all non-peridotitic garnets) are from Dawson & Stephens, (1975). Symbols: 1, spinel-garnet coarse peridotite; 2, garnet coarse peridotite; 3, porphyroclastic non-disrupted peridotite; 4, porphyroclastic disrupted peridotite; 5, megacrystalline pyroxenites; 6, megacrysts; 7, ilmenite-garnet wehrlite-pyroxenite.
Trace element concentrations were measured in garnet from coarse, porphyroclastic, and ilmenite-bearing peridotite (Table 2). Coarse peridotite contains 11-48 ppm Ni, 0-12 ppm Ga, 5-26 ppm Y, and 2-57 ppm Zr. Porphyroclastic peridotite demonstrates similar levels of Ga (7-10 ppm) and Y (11-17 ppm), but elevated levels of Ni (39-92 ppm) and Zr (22-72 ppm). Garnet in ilmenite-garnet wehrlite is considerably richer in minor elements containing 25-88 ppm Ni, 7-36 ppm Ga, 8-22 ppm Y, and 18-145 ppm Zr.
More than half of the Jericho peridotitic garnets show Y concentrations exceeding 10 ppm and one-third of peridotitic garnets demonstrate high Zr (>30 ppm); such elevated levels of Y and Zr are usually taken as evidence of relatively `high-temperature' and `low-temperature' metasomatic enrichment, respectively (Griffin & Ryan, 1995; Griffin et al., 1996). The common linkage between enrichment in Y and Zr and temperature, though, is not observed at Jericho. Here, the cryptic metasomatism was not associated with any particular range of temperature, as no correlation is noted between Y and Zr concentration and Ni content. The metasomatic introduction of incompatible elements resulted in high variance of Ni, Y, and Zr from grain to grain far exceeding the normal variance seen in homogeneous equilibrated rocks.
Garnet from some samples also shows major element heterogeneity between grains and within grains. The within-grain zoning is commonly patchy in peridotitic garnet (Fig. 3g) and areas in rims are usually depleted in CaO (<1·2 wt %) and Cr2O3 (<2·5 wt %). Ti-pyrope in ilmenite-garnet wehrlite and clinopyroxenite exhibits extreme variation in TiO2 (from 0·34 to 3 wt % in one sample) and corresponding variations in CaO, Al2O3, and SiO2 apparently linked to garnet habits and degree of melting. Rims of euhedral garnet show oscillatory zoning in Ti and Cr (Fig. 3h).
Spinel (chromite with 0-0·4 wt % TiO2, 33-60% Cr2O3, and 9-16% MgO) occurs only in coarse peridotites and demonstrates common negative correlation of Mg and Cr. The low-Cr character places the Jericho xenolithic chromite far below the diamond inclusion field (Gurney & Zweistra, 1995) in the Cr-Mg space.
estimated using O'Neill & Wood, (1979).
DISCUSSION
Commonality of the Slave peridotitic mantle
The mantle beneath the northern part of the central Slave craton shows many features common to cratonic mantle elsewhere. Peridotite is the dominant rock type within the mantle underlying the Slave craton. Similar to mantle sampled by most Type I kimberlites, there are two distinct suites of low-temperature and high-temperature peridotite.
Jericho low-temperature garnet peridotite has Mg-enriched mineral compositions, typical of peridotite underlying other cratons. The similarity in olivine composition between the Slave and other cratons indicates their comparable degree of depletion. The modal mg-number of olivine in low-temperature Jericho peridotite (92-93) is identical to that reported from other Slave peridotite (Boyd & Canil, 1997; MacKenzie & Canil, 1998) and parallels the average range for Kaapvaal and Udachnaya (Boyd et al., 1997). Low-temperature peridotite is generally coarse textured, and only rarely displays disrupted textures.
At Jericho, xenoliths of high-temperature peridotite show Fe-, Ca- and Ti-enriched mineral compositions, and always have porphyroclastic textures. Both these features are shared by most other world-wide cratonic peridotite suites (Boyd, 1987; Harte & Hawksworth, 1989). In addition, garnet within high-temperature peridotite is always non-disrupted. Disrupted garnet is present only within the low-temperature suite, where it is attributed to higher stress regimes associated with cooler mantle. This parallels what has been described from the Kaapvaal craton (Boyd & Nixon, 1978).
The compositional range found in peridotitic garnet at Jericho suggests that the peridotitic mantle beneath the central Slave shares one more feature with portions of the Kaapvaal cratonic mantle. Specifically, there is a pronounced lack of G10 (harzburgitic) garnet compositions found in Jericho harzburgite (Fig. 7), in Jericho kimberlite concentrate (Kopylova et al., 1998a), and in the Lac de Gras harzburgite (Pearson et al., 1998). In the Kaapvaal craton, the absence of G10 garnet compositions was interpreted to result from equilibration of harzburgitic assemblages with a lherzolite-dominated mantle (Boyd & Nixon, 1978; Skinner, 1989). In such harzburgite, garnet is saturated in Ca despite the local absence of clinopyroxene in rocks.
One last feature that is shared by peridotite of the Slave and Siberian cratons is the presence of a late-stage Na-, Al- and Cr-depleted clinopyroxene. The clinopyroxene occurs as a fine-grained coating along orthopyroxene grain boundaries in peridotite xenoliths recovered from the Jericho pipe as well as from the Grizzly pipe (Boyd & Canil, 1997). Similar clinopyroxene has been described in Udachnaya peridotite and was ascribed to secondary crystallization during eruption (Boyd et al., 1997).
Unique features of the Slave peridotitic mantle
Some substantive differences between peridotitic xenoliths from Jericho and peridotite described from other cratons suggest corresponding differences in the character of the underlying peridotitic upper mantles. Jericho peridotitic xenoliths show three unique chemical features, including: (1) an anomalously high proportion of chemically unequilibrated samples; (2) a distinct Cr enrichment in mineral chemistry of high-temperature peridotite, relative to low-temperature samples; (3) unique trends in garnet and clinopyroxene compositions within spinel-bearing peridotite that derive from equilibration with spinel.
Almost half of the analysed samples of high- and low-temperature peridotite are chemically unequilibrated; minerals show abundant between- and within-grain chemical variation. The chemical variation is irregular, in that individual grains show patchy zoning, generally restricted to the rims, and mineral chemistry does not always correlate with grain shape and origin (e.g. porphyroclast vs neoblast). The most heterogeneous minerals are clinopyroxene and garnet; the least heterogeneous is olivine.
Olivine, orthopyroxene and garnet in high-temperature (spinel-free) peridotite are Cr rich compared with low-temperature peridotite, whereas clinopyroxene is not. However, it is mainly within high-temperature peridotite that clinopyroxene shows compositional zoning involving Cr enrichment. These aspects of Cr enrichment of mineral assemblages are not described for other high-temperature peridotite xenoliths. There is, however, a distant similarity to patterns seen in sheared low-temperature nodules from the Kimberley pipes. In two of the Kimberley nodules, orthopyroxene porphyroclasts were found to be lower in Cr, Ti, Al and Ca than orthopyroxene neoblasts (Boyd, 1975). Similar effects have been sought, but not found, in sheared lherzolite from Thaba Putsoa, Lesotho, and Frank Smith. Garnet from the Kimberley deformed rocks tends to be more Cr rich than garnet from coarse peridotite, but this may reflect the presence of chromite (Boyd & Nixon, 1978). Cr enrichment in minerals from Jericho high-temperature peridotite may result from one of two processes. First, the high Cr content could be a primary feature reflecting the absence of spinel in these deep-seated rocks. Alternatively, the high Cr content may derive from secondary processes linked to mantle deformational events. The latter explanation is suggested by rims of clinopyroxene porphyroclasts and neoblasts that show Cr enrichment, and by coexisting (albeit rare) porphyroblasts of olivine that have retained their early Cr- and Ca-depleted core compositions.
Equilibration with spinel affected the composition of clinopyroxene and garnet in Jericho spinel-garnet peridotite. Clinopyroxene compositions from spinel-garnet peridotite show an unusual negative correlation in Mg and Cr (Fig. 6). The trend in clinopyroxene compositions is reminiscent of the compositional variations commonly found in mantle, Ti-poor spinel and also characteristic of the Jericho chromite. Typically, the Cr contents of these spinel compositions are positively correlated with Fe (Haggerty, 1991), implying a negative correlation with Mg. If the partitioning of Cr and Mg into silicates is controlled by the presence of spinel with a constant Cr/Mg ratio, we might expect similar negative Mg-Cr correlations in contemporaneous silicate phases.
Pyrope garnet that has also equilibrated with spinel defines a strong and unique compositional trend that is less enriched in Cr relative to the common `lherzolitic' trend (Fig. 7). This trend in garnet composition is found in garnet from spinel-garnet peridotite and from heavy mineral concentrates at Jericho, and appears to be controlled by the substitution of uvarovite for pyrope (Ca2Cr3 Mg2Al3). Smith & Boyd, (1992) have described a similar trend for garnet compositions from spinel-garnet peridotite from South Africa. Furthermore, similar ranges in garnet composition have been reproduced experimentally (Fig. 10) for natural compositions of lherzolite (Brey et al., 1990; Brey, 1991). The experiments were performed at mantle conditions (i.e. 900-1200°C and 3-6 GPa) and used spinel-bearing starting materials. Based on these observations we suggest that this trend in garnet compositions results from equilibration with spinel. An outstanding problem is that this `spinel-garnet equilibrium' trend is apparently rarely found in mantle peridotite and kimberlite concentrates, despite the common occurrence of spinel in coarse low-temperature peridotite (Boyd, 1987). This suggests that coexisting spinel and garnet in mantle peridotite are not always at equilibrium.
Figure 10. A Ca-Cr plot for garnet exhibiting the spinel-garnet equilibration: garnet in Jericho spinel-garnet coarse peridotite (grey field), garnet in xenolith PHN 1569 from Thaba Putsoa kimberlite (Smith & Boyd, 1992), and garnet in experiments with natural lherzolite compositions at 900-1200°C, 33-60 kbar (Brey et al., 1990). Arrow denotes common lherzolitic trend.
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