Hello Valuepro
Here is the second half.
Unique aspects of the Jericho pyroxenitic mantle
The mantle underlying the north-central Slave craton comprises, in part, an unusually high proportion of magmatic-textured, non-peridotitic rocks that appear to be related in origin to megacrysts. The magmatic suite includes unique high-temperature megacrystalline pyroxenite and ilmenite-bearing rocks.
Pyroxenite rocks in cratonic mantle are found in the Kaapvaal craton in Matsoku (Harte et al., 1975) and Monastery kimberlites (Gurney et al., 1991), and in North America in Colorado-Wyoming kimberlite (Eggler & MacCallum, 1974), but are most widespread in Siberia. There, they have been described in the Mir, Obnazhennaya and Udachnaya pipes (Solovjeva et al., 1987; Spetsius & Serenko, 1990), and in the last they represent up to 6% of the mantle xenoliths (Solovjeva et al., 1994). In all of these occurrences, pyroxenite xenoliths have metamorphic granoblastic textures ranging from megacrystalline to fine grained (Solovjeva et al., 1987), and relatively low equilibrium temperatures (600-1100°C). Past workers have suggested that mantle-derived pyroxenite originated as relatively young, magmatic intrusions (Harte & Hawksworth, 1989; Harte et al., 1987) or perhaps as igneous cumulates (Harte et al., 1975). Subsequently, these igneous mantle rocks have been recrystallized, as evidenced by widespread exsolution of all pyroxenes, and sometimes deformed. In general, these rocks become increasingly granoblastic and fine grained as the extent of recrystallization increases (Solovjeva et al., 1987, , 1994).
Pyroxenite from Jericho differs from other occurrences of cratonic pyroxenite in two aspects. First, the suite is dominated by magmatic textures rather than textures of metamorphic recrystallization or subsolidus re-equilibration. Second, the suite records significantly higher equilibrium temperatures, indicating a formation within the deep thermally disturbed mantle. This combination of characteristics, quenching of high-temperature formation conditions and original magmatic textures, places substantial constraints on the thermal history of the pyroxenite. It implies that the samples were removed from ambient mantle conditions shortly after their formation or the surrounding mantle conditions changed drastically.
At Jericho, the suite of megacrystalline pyroxenite seems to be related genetically to the megacryst (low-Cr) suite. The mineral chemistry of the megacrysts is identical to that of megacrystalline pyroxenites (e.g. Fig. 6). Furthermore, there is a complete textural transition from megacrystalline pyroxenite to megacrystic intergrowths of garnet-pyroxene to isolated megacrysts. Pyroxenite and megacrysts occupy the same depth interval in the Slave mantle, as demonstrated by clinopyroxene-garnet thermometry (Kopylova et al., 1998c) and two-pyroxene-garnet thermobarometry (Table 3). The transitional nature of megacrystalline pyroxenites is a common feature found also in kimberlite-hosted xenoliths from other cratons. In part, this association has been masked by classification problems involving small megacrystalline pyroxenite xenoliths (Solovjeva et al., 1994). The problem arises in knowing whether to treat each sample as a representative of a megacryst suite or as a xenolith. For example, small pyroxenitic nodules comprising several grains of pyroxene are sometimes ascribed to a megacryst suite (Mitchell, 1986; Schulze, 1987) whereas megacrysts with extensive exsolution features have been described as polymineral xenoliths (Kirkley et al., 1984).
The Jericho megacrystalline pyroxenite could represent pegmatites derived from precursor mafic magmas. Crystallization within these earlier magmas, which are intrinsically related to kimberlite magmas, has been suggested as a source for Cr-poor megacrysts (Schulze, 1987).
The ilmenite-garnet wehrlite and clinopyroxenite suite of xenoliths described at Jericho belong to a rare variety of cratonic rocks containing ilmenite. Other descriptions of their counterparts in kimberlite-derived xenolith suites include ilmenite peridotite and pyroxenite xenoliths from Siberia (Ponomarenko, 1977; Rodionov et al., 1988; Solovjeva et al., 1994), Fe-Ti pyroxenite veins in peridotite xenoliths from South Africa (Harte et al., 1987), and other South African occurrences of exotic ilmenite-garnet-bearing pyroxenite xenoliths (Boyd et al., 1984; Clarke & McKay, 1990).
The Jericho ilmenite-bearing xenoliths share a number of traits with these occurrences. For example, these rocks tend to have: (1) highly variable mineral modes (Tomkins & Haggerty, 1984; Clarke & MacKay, 1990; Solovjeva et al., 1994), (2) sideronitic texture (Boyd et al., 1984; Rodionov et al., 1988; Clarke & MacKay, 1990; Solovjeva et al., 1994), (3) mosaic-textured silicate minerals in contact with ilmenite (Harte et al., 1987; Rodionov et al., 1988; Solovjeva et al., 1994), (4) megacrystalline grains of ilmenite and garnet (Rodionov et al., 1988; Solovjeva et al., 1994), and (5) abundant compositional zoning in minerals (Boyd et al., 1984; Rodionov et al., 1988).
In general, past workers have argued that these rocks represent crystallization products from a magma that is related both to the megacryst suite and to later kimberlite magmas (Boyd et al., 1984; Rodionov et al., 1988; Clarke & MacKay, 1990; Solovjeva et al., 1994). We suggest that the Jericho ilmenite-garnet wehrlites and clinopyroxenites originated by partial melting and magmatic and/or metasomatic recrystallization of pre-existing garnet-clinopyroxene segregations. The parental magmas were also a source for the ubiquitous ilmenite (± garnet and clinopyroxene) megacrysts found in the kimberlite, and were notably enriched in incompatible elements as evidenced by extremely high Zr and Ga contents in garnet.
On the basis of our observations at Jericho, we suggest that the two pyroxenitic suites (megacrystalline and ilmenite bearing) may be linked in origin to megacrystal magmas. These pyroxenitic xenoliths volumetrically form up to 13% of our xenolith collection. Even considering the over-representation of exotic rock types in the collection, such a proportion of pyroxenitic magmatic rocks is unusually high for a cratonic mantle. We think that this phenomenon might not be a characteristic of the north-central Slave mantle, but is rather related to temporal and spatial vagaries of the processes that created the Jericho kimberlite magma.
`Cool' upper mantle and `hot' crust
P-T arrays from kimberlite-hosted mantle xenoliths offer insight into the thermal structure of the mantle and commonly are used to constrain cratonic geotherms. Common practice is to match the observed P-T array against model steady-state, conductive geotherms established for average models for continental lithosphere and various values of model surface heat flow (Q0) (Pollack & Chapman, 1977). Alternatively, other workers have used forward modelling methods to simulate a geotherm for an assumed crust-mantle configuration (e.g. Bussod & Williams, 1991; Cull et al., 1991). The geometries of these conductive geotherms, whether based on average models of the Earth or specific models for a region, are highly sensitive to the architecture and composition of the crust and mantle (e.g. Chapman, 1986; Rudnick et al., 1998), and may be completely invalid if the assumed structure is inappropriate and can lead to serious misinterpretations (Rudnick et al., 1998). Therefore, we have taken a slightly different approach in our interpretation of the P-T array.
Our approach is to adopt and fit a relatively simple, one-dimensional, analytical model for steady-state conductive heat transfer to the P-T data array to constrain values of surface heat flow at the time of kimberlite emplacement (Russell & Kopylova, 1998). This method is relatively independent of assumptions about crustal structure and, therefore, is especially well suited for regions with sparse geophysical data or for deducing palaeo-geotherms. The model steady-state conductive geotherm assumes an exponential decrease in heat-producing elements over a critical depth D and has the form (Lachenbruch, 1968; Crowley, 1987)
At this point we invert the P-T array data (Fig. 8) to obtain model estimates of surface heat flow (Q0) and D given geologically defined values of surface heat production (A0) and thermal conductivity (K). We assigned a constant value of 2·5 to K, based on reported values of K for upper (2·37 W/m per K) and lower (2·64 W/m per K) crust from cool cratonic regimes (Chapman, 1986). The bedrock geology for an area of 14 300 km2 surrounding the Jericho kimberlite was used to calculate an average value of surface radiogenic heat production (A0) of 2·16 µW/m3 using the surface area proportions of rock types and the bulk chemical compositions reported by Davis, (1991) (Table 4).
Parameters used to calculate a weighted average value for surface heat generation based on the bedrock geology surrounding the Jericho kimberlite pipe
Table 4. Parameters used to calculate a weighted average value for surface heat generation based on the bedrock geology surrounding the Jericho kimberlite pipe
Our analysis utilizes only P-T data that are representative of the stable, low-temperature portion of the geotherm; we have not included P-T points from high-temperature peridotite in our inversion (Fig. 11). The results for high-temperature peridotite fall off the ambient geotherm and define a thermal disturbance that is seen in many Type I kimberlite geotherms and that is commonly ascribed to convection processes associated with the magma-bearing zone at the lithosphere to asthenosphere transition (e.g. Boyd & Gurney, 1986; Boyd, 1987; Harte & Hawkesworth, 1989).
Figure 11. Best-fit model conductive geotherms satisfying the FB-MG solution ([closed circle]) and BK solution ([open circle]) for the undisturbed parts of the P-T arrays. (See text for explanation.)
We fitted the model equation to P-T data sets resulting from both the FB-MG and the BK solutions. Optimal values of the model parameters Q0 and D have been derived by minimization of the [chi]2 merit function (Press et al., 1986):
where Tobs is the observed temperature value, Sj is the uncertainty in Tobs because of analytical variance and T* is the model temperature.
The model geotherm derived from the FB-MG data array has parameters D = 19·6 km and Q0 = 52·3 mW/m2. The optimal solution reproduces 95% of the P-T data to within ±40°C. Also shown (Fig. 11) is the model geotherm fitted to the BK array of P-T estimates. It should be noted that although the latter P-T estimates are offset to lower values of P and T, the corresponding model fit parameters are nearly identical in value (D = 19·5 km; Q0 = 52·9). Thus, regardless of which of these thermobarometric methods is preferred, the deduced thermal state of the Slave lithosphere is the same. We tested the sensitivity of our results to variations in K; a 10% change in the value of K (e.g. 2·75 W/m per K) causes a 5% variation in the fit parameters (e.g. <1 km in D and <3 mW/m2 in Q0).
The estimated model values of surface heat flow Q0 = 52·3-52·9 mW/m2 show an excellent agreement with two heat flow measurements available for the Slave craton: 54·4 ± 0·4 mW/m2 (Beck & Sass, 1966) and 53·3-54·4 mW/m2 (Lewis & Wang, 1992). These estimates suggest that the Slave craton has unusually high surface heat flux for a Precambrian terrane. However, this distinction does not extend deeper into the upper mantle. Our model predicts a thermal gradient in the mantle of 4·3°C/km, which implies a mantle heat flow of 12·9 mW/m2 for an assumed K for the mantle of 3·0 W/m per K. This assumption is based on latest estimates of thermal diffusivity of the upper mantle [(7-8) * 10-7 m2/s; Katsura, 1995); heat capacity (1·24 J/kg per K; Stacey, 1992) and density (Turcotte & Schubert, 1982). The mantle heat flow of the north-central Slave craton is, therefore, identical to other estimates of mantle heat flow for the Canadian shield (12 mW/m2, Mareschal et al., 1997; and 14 mW/m2, R. Hyndman & T. Lewis, in preparation) and for other Precambian terranes (10-15 mW/m2, Rudnick et al., 1998). The relatively cool upper mantle below the Slave craton is, however, not at odds with the observed high surface heat flow. Heat production within the radiogenically enriched Slave crust more than adequately compensates the surface heat flux for the slightly cooler deep mantle underlying the craton.
CONCLUSIONS
(1) The following types of mantle xenoliths from the Middle Jurassic Jericho kimberlite in the Slave craton of Northern Canada are described: (a) coarse peridotite (48%; spinel-garnet and spinel free); (b) porphyroclastic peridotite (31%; non-fluidal, fluidal non-disrupted and fluidal disrupted); (c) megacrystalline pyroxenite (16%); (d) ilmenite-garnet wehrlite and clinopyroxenite (5%).
(2) The varieties of the upper-mantle xenoliths, and their proportions, petrography and mineralogy, mainly resemble those from kimberlites of other cratons. Based on major and minor element thermobaromety, the peridotite samples are assigned to a low-temperature and a high-temperature suite. The low-temperature depleted peridotite is coarse or disrupted porphyroclastic; high-temperature fertile peridotite samples always show non-disrupted porphyroclastic textures.
(3) 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: (a) an anomalously high proportion of chemically unequilibrated samples; (b) a distinct Cr enrichment in mineral chemistry of high-temperature peridotite, relative to low-temperature samples; (c) unique trends in garnet and clinopyroxene compositions within spinel-bearing peridotite that reflect equilibration with spinel.
(4) The mantle below Jericho comprises, in part, an unusually high proportion of magmatic-textured, non-peridotitic rocks that appear to be related in origin to megacrysts. The magmatic suite includes unique high-temperature megacrystalline pyroxenite and ilmenite-bearing rocks. In contrast to other occurrences of cratonic pyroxenites, the Jericho pyroxenites have magmatic rather than metamorphic granoblastic textures, and higher equilibrium temperatures, indicating formation within the deep thermally disturbed mantle.
(5) The cool upper mantle below the central Slave coexists with a highly radiogenic, hot crust. 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. A new approach was used to invert the Jericho thermobarometric data to obtain model estimates of surface heat flow given geologically defined values of surface heat production and thermal conductivity. The estimated model values of surface heat flow of the north-central Slave craton are Q0 = 52-53 mW/m2; whereas the mantle heat flow is 13 mW/m2. These values show excellent agreement with both measured values of surface heat flow for the Slave craton and theoretically predicted values of mantle heat flow beneath cratons.
Dawson, J. B. & Stephens, W. E. (1975). Statistical classification of garnets from kimberlites and associated xenoliths. Journal of Geology 83, 589-607.
Sobolev, N. V., Lavrent'ev, Yu. G., Pokhilenko, N. P. & Usova, N. P. (1973). Chrome-rich garnets from the kimberlites of Yakutia and their parageneses. Contributions to Mineralogy and Petrology 40, 39-52.
Mitchell, R. H. (1986). Kimberlites: Mineralogy, Geochemistry, and Petrology. New York: Plenum, 442 pp.
Gurney, J. J. & Zweistra, P. (1995). The interpretation of the major element compositions of mantle minerals in diamond exploration. Journal of Geochemical Exploration 53, 293-309.
Pell, J. A. (1997). Kimberlites in the Slave Craton, Northwest Territories, Canada. Geoscience Canada 24, 77-91.
Percival, J. A. (1996). Archean cratons. In: Richardson, D. G., DiLabio, R. N. W. & Richardson, K. A. (ed.) Searching for Diamonds in Canada. Geological Survey of Canada, Open File 3228, 161-169.
geotech.org
This paper suggests an emplacement age during the middle Jurassic which according to the Age Matrix link above, puts Jericho at about 136my+/-, however the third paper also suggests an age of around 172my +/-2my which puts Jericho emplacement in the early Jurassic or late Triassic which would have sandblasted as Russett ribbed, the under belly of something big.
Regards |
