The Thickness and Heat Production of Archean Lithosphere: Constraints from Xenolith Thermobarometry and Surface Heat Flow

Home , Kalahari Craton

Mantle Petrology: Field Observations and High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd © The Geochemical Society. Special Publication No.6. 1999 Editors; Yingwei Fei, Constance M. Bertka, and Bjorn O. Mysen

The thickness and heat production of Archean lithosphere: constraints from xenolith thermobarometry and surface heat flow

ROBERTAL. RUDNICK'and ANDREWA. NYBLADE2 'Department of Earth and Planetary Sciences. Harvard University, Cambridge, MA 02138, U.S.A. 2Department of Geosciences, Pennsylvania State University, University Park, PA 16802, U.S.A

Abstract-The pressure-temperature arrays derived from thermobarometry of peridotite xenoliths are compared for four Archean cratons: Kalahari, Slave, Superior and Siberia. Except for Siberia, which shows significant scatter in the pressure-temperature data, the arrays for these cratons are relatively coherent and do not show large differences in temperature «200°C at 5 GPa), or temperature gradient, between cratons. This observation suggests that the heat flowing through the mantle root is similar for each region. If we assume that the P-T arrays reflect the present-day thermal regime of the cratons, inferences can be made regarding the heat production and thickness of cratonic lithosphere in regions where sufficient P-T data exist to define the conductive geotherm well. The Kalahari craton is one such region, where over 100 P-T data points exist for mantle xenoliths. We employ a grid search technique to generate some 18,000 geotherms by varying the thermal, structural and chemical properties of the lithosphere within reasonable limits. Successful models, defined as those producing geotherms falling within the 95% confidence limits of a linear least-squares fit to the P-T array, yield estimates of mantle 3 heat production between 0 and 0.07 µW/m (with most models :s 0.03 µW/m3) and lithospheric thickness of 200-250 km. Using the average surface heat flow for the Kalahari craton (47 ::t: 2 mw/nr'), the modeling results also limit crustal heat production to between 0.5 and 0.8 µW/m3 and require a significant heat flux through the Moho (17 to 25 mw/m''), and the base of the lithosphere (8 to 22 mw/m"),

INTRODUCTION ate for thermobarometry are now available from STUDYOFthe structure, composition and thermal evo- mantle xenoliths from several Archean cratons [e.g., lution of Archean-aged continental crust and under- Kalahari (Kaapvaal and Zimbabwe), Siberia, Slave, lying mantle lithosphere is fundamental to under- Superior and Tanzania - see below] and, although standing the processes of continental growth and differences exist between xenoliths from different evaluating the long-term stability of continents. Ar- cratons, there are several important similarities: chean cratons are stable regions where surface heat flow is observed to be relatively low and uniform 1) Both coarse-granular and deformed xenoliths (MORGAN, 1984; NYBLADEand POLLACK, 1993). occur in each craton, with the deformed or por- These conditions imply a steady state heat flux, phyroclastic xenoliths generally recording higher which is controlled by conduction of heat from two equilibration temperatures than the coarse-granular sources: 1) heat produced by radioactive decay of K, xenoliths. Th and U in the lithosphere and 2) heat conducted 2) The P-T array defined by coarse, low tempera- from the underlying convective mantle (POLLACK, ture xenoliths from each craton are similar, falling 1980). Depending upon the concentration and distri- close to a 40 mW/m2 reference geotherm of POLLACK bution of heat producing elements in the lithosphere and CHAPMAN(1977). and the thermal conductivity, a large range of con- 3) The maximum equilibration depth of cratonic ductive geotherms can be produced for any given peridotite xenoliths is :s7 GPa (230 km depth). surface heat flow, spanning temperature differences [Some rare garnet lherzolites from the Jagersfontein of up to several hundreds of degrees Celsius at depths kimberlite have inclusions in garnets that are inter- of 100-200 km (DAVIESand STREBECK,1982; RUD- preted as exsolution of pyroxene from an original NICKet al., 1998). Thus surface heat flow alone does majorite garnet (SAUTTERet al., 1991). Likewise, not tightly constrain the thermal conditions within ultra high pressure phases such as majorite (MOORE mantle lithosphere. and GURNEY,1985) and possible lower mantle min- Xenoliths, foreign rock fragments picked up and erals (HARRISet al., 1997; HARTEand HARRIS,1994), carried rapidly to the Earth's surface in magmas such have been reported as inclusions in diamonds. These as kimberlites and alkali basalts, provide some of the rare minerals and rocks may indicate formation of the few petrological and geochemical constraints on the kimberlite very deep within the earth (e.g., HAG- thermal properties and state of the deep lithosphere GERTY,1994; RINGWOODet al., 1992), but cannot be (e.g., BoYD, 1973). Mineral chemical data appropri- taken as evidence for the thickness of the lithosphere 3 4 R. L. Rudnick and A. A. Nyblade

unless one is prepared to argue that the lithosphere (SMITH,1999). This conclusion is consistent with the extends into the lower mantle.] evidence for near isotopic equilibration at the time of eruption for the Sm-Nd and Rb-Sr systems in rocks In this paper we assume that the poT arrays defined that are clearly much older (RICHARDSONet al., 1985). by cratonic xenoliths represent equilibration to a con- For Siberian samples, however, some evidence for ductive, steady-state geotherm at the time of kimber- major element disequilibria has been documented lite eruption (i.e., that the kimberlite magmatism has (BoYD et al., 1997). Thus, one must bear in mind that not thermally affected the xenoliths or the litho- any use of xenolith P-T arrays to infer thermal struc- sphere). Furthermore, because of the long time con- ture of cratonic lithosphere is subject to the underly- stant for thermal diffusion [IOO's of Ma for a heat ing assumption that equilibrium has been attained. If 2 flow anomaly on the order of 5 mW/m to propagate this is shown in the future to be incorrect, then the through a 200 km thick lithosphere (NYBLADE, conclusions drawn here must be revised accordingly. 1999)], we assume that these poT arrays are repre- The most recent and thorough experimental cali- sentative of present-day conditions beneath Phanero- brations of the Fe-Mg exchange thermometer be- zoic kimberlite pipes; the kimberlites included in this tween clinopyroxene and orthopyroxene and the Al study range from Mesozoic (Kalahari, Slave, Supe- in orthopyroxene barometer of Brey and colleagues rior) to Permian (Siberia). Given this, a "geotherrn (BREY and KOHLER, 1990 and BREY et al., 1990) window" can be defined for cratons with sufficient (hereafter termed the "BKN" thermobarometers) are data coverage (i.e., Kalahari craton) by using the PoT used to compare equilibration conditions of cratonic estimates to place bounds on the range of permissible xenoliths from four Archean cratons: Kalahari (BOYD temperatures at specified depths in the mantle. By and MERTZMAN,1987; SHEE, 1978; STIEFENHOFERet varying lithospheric thermal and structural parame- al. , 1997), Slave (BOYDand CANIL,1997; KOPYLOVA ters, families of geotherms that satisfy this geotherm et al., 1998; PEARSONet al., 1998), Siberia (BOYDet window can be calculated. The range of acceptable al., 1997; GRIFFINet al., 1996; POKHILENKOet al., geotherms provides limits on the amount of heat 1993; ZHURAVLEVet al., 1991) and Superior (MEYER production in the lithospheric mantle. Given surface et al., 1994; SCHULZE,1996) [Data for peridotite heat flow observations, this approach can also con- xenoliths from the Tanzanian craton have not been strain the amount of heat production in the crust. considered here since evidence exists for incipient Finally, the intersection of the conductive geotherms heating by rift-related magmas (DAWSONet al., 1997; with the mantle adiabat is used to define the thickness LEE and RUDNICK1999)]. Pressure and temperature of the Archean thermal boundary layer, and this have been calculated directly from mineral chemical thickness, when combined with the heat production data from the literature or supplied from the authors; of the crust and lithospheric mantle, yields estimates in all cases Fe3+ is assumed to be zero. of the proportion of surface heat flow coming from Figure 1 shows the P-T arrays for peridotite xenoliths the lithospheric mantle and convecting mantle. from the four cratons; coarse-granular peridotites are shown as solid symbols, porphyroclastic or p. T ARRAYS OF CRATONIC XENOLITHS deformed peridotites are shown as open symbols. The Thanks to the pioneering work of F. R. BOYD(e.g., data are plotted relative to the best fit conductive geo- BoYD, 1973; BOYD,1984; FINNERTYand BOYD, 1987) therm for the Kalahari craton (described below). Also the past several decades have witnessed large ad- shown in the Kalahari plot is the 40 mW/m2 geotherm vances in our understanding of the thermal structure of POLLACKand CHAPMAN(1977). The thermobarom- of Archean cratons through the application of ther- etry data for xenoliths from each craton generally form mobarometry to cratonic mantle xenoliths. An excel- a coherent trend. The exception is Siberia, for which lent summary of thermobarometry as applied to cra- evidence of mineralogical disequilibria has been found tonic xenoliths is found in SMITH(1999), in which he (BoYD et al., 1997; GRIFFINet al., 1996). More limited reviews the evidence for equilibration in these sam- scatter exists in the P-T arrays for the other cratons, ples. Zoning is common in the deformed, high tem- which is probably due to 1) analytical errors in mineral perature samples, leading to uncertainty regarding the compositions, propagated through the thermobarom- veracity of their PoT arrays (e.g., LEE, 1998; SMITH, etry, 2) uncertainties in the thermobarometry calibration 1999). Evidence for trace element and isotopic dis- (estimated at :t20°C and :to.3 GPa for the BKN cali- equilibria has been documented in some coarse gran- bration, BREYand KOHLER,1990), and possibly 3) sub- ular peridotites (GUNTHER and JAGOUTZ, 1994; tle variations in temperature (beyond the resolution of SHIMIZUet al., 1997), but the general lack of zoning thermobarometry) within a given craton. of major elements in these minerals is cited as evi- Deformed, high temperature peridotites were orig- dence for their equilibration to ambient conditions inally interpreted as deriving from the asthenosphere The Archean lithosphere 5

FIG. l. Pressure and temperature of equilibration for peridotite xenoliths in kimberlites from different Archean cratons, calculated using the BREY and KOHLER (1990) formulations of the 2 pyroxene thermometer and Al in orthopyroxene barometer. Solid symbols are coarse-granular peridotites, open symbols are deformed peridotites. The best fit geotherm for the Kalahari data is plotted for reference in each diagram. (a) Kalahari craton. Data sources: Lesotho (FINNERTYand BOYD, 1984; NIXONand BOYD, 1973) and Kimberly (unpublished data from F. R. BOYD), Kaapvaal craton; Letlhakane kimberlites, Zimbabwe craton (SHEE, 1978; STIEFENHOFERet al., 1997). Two geotherms are plotted: the Pollack and Chapman 40 mw/rrr' conductive geotherm and the best fit geotherm for the Ps'T data (see Table I for parameters). (b) Slave craton. Data sources: Jericho (KOPYLOVAet aI., 1998), Lac de Gras (PEARSONet al., 1998), Torrie (CANIL, unpublished data), Grizzly (BoYD and CANIL, 1997). (c) Siberian craton. Data sources: Udachnaya (BOYD et al., 1997; GRIFFIN et al., 1996; SPETSIUSand SERENKO, 1990), Mir (ZHURAVLEVet al., 1991) (d) Superior craton. Data sources: (MEYERet al., 1994; SCHULZE,1996; VICKER, 1997).

(NIXONand BoYD, 1973) because (1) they have por- with the zoning seen in garnets in many of these phyroclastic textures, indicating flow, (2) they were peridotites (GRIFFINet al., 1989; SMITHand BoYD, thought to be fertile, i.e., close in composition to a 1987), is interpreted to reflect metasomatic enrich- model primitive mantle composition, such as the ment of Fe, Ti and Zr shortly before the xenolith's famous sample PHN1611, used in several experi- entrainment in the kimberlites. The "kink" in the mental investigations of mantle melting (e.g., SCARFE geotherm appears to be an artifact of choice of ther- and TAKAHASHI,1986) and (3) they appeared to define mobarometer and geotherm. The kink is present a "kink" in the xenolith PoT array, which might have when using the FINNERTYand BOYD (1987) plus marked a thermal boundary layer at the base of the MACGREGOR(1974) combinations of the two-pyrox- lithosphere. However, this interpretation has not held ene thermometer and Al in orthopyroxene barometer, up in light of more recent investigations. A compila- but is not apparent when using the BKN thermoba- tion of major element chemistry for Kaapvaal peri- rometers. In addition, the POLLACKand CHAPMAN dotites shows that the average A1203 and CaO con- (1977) geotherm curves strongly away from the tent of the deformed peridotites is similar to that of xenolith PoT array below depths of 150 km, giving coarse-granular peridotites (McDONOUGHand RUD- rise to an apparent discrepency between the P-T array NICK, 1998), indicating that they are equally refrac- and the conductive geotherm (this is discussed more tory. However, Fe contents are higher (and Mg# fully below). Finally, Os isotopic measurements in- lower) than coarse-granular peridotites, and together dicate that many of these high temperature peridotites 6 R. L. Rudnick and A. A. Nyblade

experienced ancient melt depletion, and must there- tween depths of about 70 and 200 km (Fig. 1). We fore be pieces of the ancient lithosphere that were therefore use these data and the approach outlined in recently overprinted (PEARSONet al., 1995). For these the introduction to obtain estimates of crust and man- reasons, and the fact that they form a continuum with tle heat production and lithosphere thickness. the coarse granular peridotites in a temperature vs. To find the range of geotherms that satisfy the pressure plot (e.g., Fig. 1), we include them in our Kalahari P- T array, we first determine the 95% thermal modeling. confidence limits (2 standard deviations) of the P- T Although still widely used within the petrological array by fitting the data using linear and second- community today, the POLLACKand CHAPMAN(1977) order least squares regressions. Both regressions family of geotherms is not particularly applicable to yield similar fits to the data (r values of 0.969 and Archean cratons. They were modeling the thermal 0.970 for the linear and second-order fit, respec- structure of relatively thin continental lithosphere tively), and for simplicity we use the confidence (120 km). Because of this, the heat production in limits for the linear regression in our modeling, their mantle jumps from 0.01 µW/m3 between 40 and which are shown in Fig. 2. 120 km depth (their model lithosphere), to 0.084 Next we perform a grid search to obtain a family of µW/m3 below these depths - the pyrolite model of geotherms that fall within the 95% confidence limits the day. This high heat production at depths greater of the poT array. In this grid search, geotherms are than 120 km gives rise to the pronounced curvature calculated for a range of plausible lithospheric pa- of their geotherm, causing it to fall below the high rameters using the method outlined by CHAPMAN temperature end of the xenolith PoT array in every (1986). The parameter range is shown in Table 1. The craton. Although the heat production of lithospheric surface heat flow from the Kaapvaal and Zimbabwe mantle cannot be well constrained from the K, Th and Cratons is similar (JONES, 1988; JONES, 1992; Ny- U of cratonic xenoliths (see RUDNICKet al., 1998, for BLADEet al., 1990), giving an average heat flow for a more detailed discussion), the P- T array itself sug- the Kalahari Craton of 47 ::'::2 (st. error) mW/m2 gests that the heat production of the lithosphere is (NYBLADEet al., 1990). The heat flow range in Table 3 2 relatively low, :s0.03 µW/m , as discussed below. 1 is based on the :t2 mW/m uncertainty. Values of The well-defined Kalahari geotherm serves as a mean crustal heat production were chosen to span a marker by which to compare the absolute tempera- range wider than the range reported in the literature ture differences between different cratons. Variations in equilibration conditions of the coarse granular peridotites are minimal within individual cratons (ex- Temperature (oC) cept the Siberian craton where significant scatter ex- o ists even in one pipe), but significant differences exist between cratons. Peridotites from the Kalahari and 50 Superior cratons plot at higher temperatures for a 2 given pressure than peridotites from the Siberian and 100 ,,-... Slave cratons. However, the maximum temperature difference between different cratons «200°C at 5 g GPa) is smaller than the difference between xenoliths 150 ...c: from Archean cratons and those from adjacent Pro- ~ terozoic crust (e.g., BOYD, 1979; FRANZet al., 1996; 200 0 MITCHELL,1984). The overall uniformity of the xenolith P-T array between Archean cratons suggests a 8 range of geotherms similar amount of heat flowing through the mantle that fit 95% lithosphere at each locality. We will now use these confidence limits observations to make inferences about crustal and 10 mantle heat production and overall craton thickness. We focus our efforts on the Kalahari craton, for FIG. 2. poT data array for Kalahari craton with 95% confidence limits for the linear regression of the data. Also which a large data density exists. shown are the upper and lower bounds (gray field) for the family of geotherms (n = 535) that fall within the 95% confidence limits and the best fit geotherm to the regressed THE KALAHARI INFERENCES FROM data (Table 1). Adiabats correspond to potential tempera- P·T ARRAY tures of 1300 and 1400°C. Note that the high temperature xenoliths fall below the highest adiabat and there is no need The xenolith thermobarometry data from the Kala- to appeal to a special origin via diapiric upwelling to explain hari Craton reasonably constrain temperatures be- these data (cf. JORDAN, 1988). The Archean lithosphere 7

Table 1. Input and output values of parameters used in geotherm equation in the grid search test. The best fit geotherm is the one having the lowest RMS error with the regressed poT data

Kalahari model parameters Permissible Models

Input Range Minimum Maximum Best Estimate Surface Q (mW 1m2) 45-49 45 49 45 3 Crustal HP (µW1m ) 0.3-0.9 0.5 0.8 0.7 K Crust (WIm.K) 2.4-3.0 2.4 3 2.4 Mantle HP (µW1m3) 0-0.1 0 0.07 0 Crustal thickness (km) 35-45 35 45 38 Q from lithospheric 0 15 0 mantle (mW 1m2) Q from crust (mW1m2) 20 32 27 QMoho (mW/m2) 17 25 18 Q Base of Lithosphere 8 21 18 (mW/m2) Lithosphere Thickness 203 253 224 (km)

for Archean crust (see RUDNICKet al., 1998, and ble of successful models. In the case of crust and references therein). Distribution of heat producing mantle heat production, the range of values in the elements in the crust follows the three layer model of successful models is a subset of the range of input RUDNICKand FOUNTAIN(1995), with 60% in the upper values, demonstrating that the range of input values crust, 34% in the middle crust and 6% in the lower had little effect on the outcome of the grid search. In crust. The range of mean thermal conductivity for the addition, results from sensitivity analyses indicate crust was chosen to span a range of values typical of that the results of the grid search are not influenced crustal rocks (CERMAKand RYBACH,1982; Roy et al., by the choice of surface temperature used in calcu- 1981), including the effects of pressure and temper- lating the geotherms, nor by the details of how heat ature on conductivity (CHAPMAN,1986; SASS et al., production is distributed in the crust (in agreement 1992; ZOTH and HAENEL,1988). In the lithospheric with DAVIESand STREBECK,1982), as long as a large mantle, we used the thermal conductivity model of percentage of the heat production resides in the upper SCHATZand SIMMONS(1972), which accounts for the crust. Table 1 also shows the parameters of the "best effects of pressure and temperature. Since heat pro- fit" geotherm - the geotherm giving the lowest RMS duction in the lithospheric mantle is poorly con- error when compared to the regressed P- T data. This strained, a very large range of values was chosen so geotherm is plotted for reference against all the P-T as not to influence the results of the modeling. Lastly, data arrays in Fig. 1. the range of crustal thickness was chosen to span the Figure 3 shows histograms of the values of the range of average crustal thicknesses for Archean different parameters obtained from the successful crust (40 :t ,5 km, CHRISTENSENand MOONEY,1995; models. In most cases, the histograms approximate RUDNICKand FOUNTAIN,1995). The few published normal distributions (e.g., lithospheric thickness, heat estimates of crustal thickness for the Kalahari Craton flow into lithosphere, crustal heat production), indi- fall within the 40 :t 5 km range (DURRHEIMet al., cating that most successful models yield values in the 1992; DURRHEIMand GREEN,1992; GANEet al., 1956; middle of the output range. However, this is not the STUARTand ZENGENI,1987), suggesting that the Kala- case for mantle heat production, which shows a hari crust is not significantly different from Archean strongly skewed distribution, indicating that most of crust elsewhere. the successful models have low lithospheric mantle Some 18,000 geotherms were generated in the grid heat production (Fig. 3). search by incrementally changing each model param- Prom the above analysis, the bounds on average eter individually. Five hundred and thirty-five geo- crustal heat production in the Kalahari craton is 0.5 to therms fell within the range of acceptable tempera- 0.8 µ.W/m3 and lithospheric mantle heat production 3 tures defined by the 95% confidence limits (Fig. 2). ranges from 0 to 0.07 µ.W/m . However, only a very Table 1 gives the range (minimum and maximum) of small number of models have mantle heat production 3 values for each parameter represented in the ensem- as high as 0.07 µ.W/m , and the vast majority of the 8 R. L. Rudnick and A. A. Nyblade

160 200 QMoho (mW 1m2) QBaseoflithopshere (mW 1m2) 140 a b 120 100 80 I11III ..t100 60 40 L J L J 50 20 I o I •• 0 16 18 20 22 24 26 8 10 12 14 16 18 20 22 250 C Crustal Conductivity (WIm-K) d Lithospheric Thickness, km 200

150 100 100

50 50 r o 180 200 220 240 260 280 r------, 400 I: Crustalr....,c • .,..l heath",,,,,," production.,...... rlnr"f.1...... I 1 350 120 (µW/m3) 300 100 250 80 200 60 150 40 100 20 50 o o o om 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.5 0.6 0.7 0.8

FIG. 3. Histograms of the parameters derived from the grid search of geotherms that fall within the 95% confidence limits of the P-T data for the Kalahari craton. Arrows mark range of values for the lO geotherms that yield the closest fit (lowest rms) to the regressed PoTdata. (a) heat flux across the Moho, (b) heat flux through base of lithosphere, (c) crustal conductivity, (d) lithospheric thickness, (e) lithospheric mantle heat production, (f) crustal heat production. models have heat production < 0.03 µW/m3 (Fig. 3). tures between 1300 and 1400°C and thermal gra- Thus, although we cannot exclude a relatively high dients between 0.3 and 0.5°C/km (FE!, 1995; heat production in the lithospheric mantle on the NAVROTSKY, 1995). This yields a range of thicknesses basis of this modeling, we consider it unlikely. The between 200 and 250 km (Table 1), with the best fit best fitting geotherm is for a surface heat flow of 45 geotherm giving a maximum thickness of 224 km mw/rrr', a mean crustal heat production of 0.70 µW/ (using the hotter adiabat). m", a crustal thickness of 38 km, and no heat production in the lithospheric mantle. OTHER CRATONS The thickness of the conductive mantle root beneath the Kalahari craton can also be constrained Although the data density from thermo barometry from the analysis of the xenolith poT array. The of xenoliths in other cratons is not high enough to thickness is determined from the intersection of the allow a statistical treatment like the one provided family of conductive geotherms obtained from the above for the Kalahari craton, several qualitative grid search with the mantle adiabat. We adopt a range interpretations can nonetheless be made. The thermo- of possible mantle adiabats with potential tempera- barometry data from the Superior Province (Fig. 1) The Archean lithosphere 9

plot on top of the Kalahari data, suggesting that the TAYLORand McLENNAN (1985) and RUDNICKand conclusions regarding lithospheric mantle heat pro- FOUNTAIN(1995). The upper bound (0.8 µ,W/m3) is duction and thickness for the Kalahari craton can also higher than most estimates of Archean crustal heat be applied to the Superior craton. These interpreta- production (which are generally :::;0.7 µ,W/m3, tions are consistent with a recent analysis of the McLENNANand TAYLOR, 1996; WEAVERand TAR- thermal structure of that craton (JAUPARTet al., 1998). NEY, 1984) but is lower than that estimated for the In the Superior craton, however, surface heat flow is Archean crust of eastern China (0.9 µ,W/m3, GAO lower than in the Kalahari and the crust appears to be et al., 1998). As discussed previously (RUDNICKet thicker. Assuming that the P-T array observed for the al., 1998), this very high heat production inferred Kalahri craton is also representative of the Superior for the Chinese crust is anomalous and may be a craton, we re-ran our models changing the surface contributing factor in the high surface heat flow heat flow bounds to 35-39 mW/m2 and crustal thick- (-60 mw/rn'') observed there. ness bounds to 40-45 km. As expected, this yielded Heat production in the mantle lithosphere is a lower crustal heat production than for the Kalahari, poorly constrained quantity. Difficulties associated but similar mantle heat production and litihospheric with inferring heat production in the mantle litho- thickness. Our modeling suggests that, in contrast to the conclusions of JAUPARTet al. (1998), the amount sphere from xenolith compositions have been re- of heat flowing across the Moho in the Superior viewed by McDONOUGH(1990) and RUDNICKet al. Province is relatively large (18-25 mW/m2, cf their (1998) and include the enhancement of heat produc- estimate of 12-13 mw/m? ). This discrepancy im- ing element contents due to host infiltration, alter- plies that either the estimated heat production of ation and weathering, as well as the persistent ques- cratonic crust of JAUPARTet al. (1998) is too high, or tion regarding the representativeness of the xenoliths that the xenolith P-T array is hotter than the present for highly incompatible elements such as these. The geotherm beneath the craton. simple analysis of P-T data presented here may pro- In comparison to the Kalahari and Superior cra- vide a more robust approach to estimating mantle tons, both the Siberian and Slave cratons appear to heat production than xenolith compositions, since the have somewhat cooler lithospheres, suggesting a amount of curvature in the mantle geotherm directly lower amount of heat flowing from the underlying reflects the amount of heat production. mantle. Interestingly, surface heat flow data for these Is the 200-250 km thick Archean lithosphere es- two cratons lie at the extremes of observed surface timated here in accord with the results of numerous heat flow in Archean cratons. Average surface heat seismic studies of Archean cratons over the past two flow in the Siberian craton is very low - less than 35 to three decades? Estimates of Archean lithosphere 2 mW/m (DUCHKOV,1991), whereas the limited data thickness from many seismological studies are in the for the Slave (one measurement near Yellowknife) depth range of 300-400 km (e.g., GRAND, 1994; 2 suggest that heat flow is high -50-53 mW/m (LEWIS JORDAN,1975; JORDAN,1988; Su et al., 1994), sig- and WANG, 1992). If these values are confirmed by nificantly deeper than our estimate based on thermal additional measurements, then they would imply structure. However, more recent interpretations of large differences in the crustal composition between seismological data are consistent with a :s250 km these cratons. Moreover, the slightly cooler geo- thick lithosphere (RICARDet al., 1996; VAN DER LEE therms could allow a thicker lithospheric root, but and NOLET, 1997) and preliminary results from probably not much deeper than 250 km, based on broadband studies seeking to image the lithosphere extrapolation of the xenolith P-T arrays and the mantle adiabat in Fig. 1 (b and c). beneath the Australian (VAN DER HILSTet al., 1998), Tanzanian (RITSEMA et al., 1998), and Kalahari (JAMESet al., 1998) cratons estimate minimum thick- COMPARISON WITH PREVIOUS ESTIMATES nesses of 200-250 km, overlapping with our obser- In this section we compare our results with previ- vations. It remains to be seen whether the apparent ous estimates of crustal and mantle heat production discrepancies between the earlier seismological mod- and lithospheric thickness in Archean cratons. The els and the thermal modeling persist in light of newer mean crustal heat production determined from the seismological data. If they do, it implies either that Kalahari data of 0.5-0.8 µ,W/m3 falls within the the xenolith PoT arrays are too hot and are not rep- range of most other estimates based on a number of resentative of present-day conditions beneath the cra- techniques and assumptions (see RUDNICKet al., tons or that cratons may be sites of cold convective 3 1998). The lower bound (0.5 µ,W/m ) matches esti- downwelling in the mantle (e.g., KINGand ANDERSON, mates of average Archean crustal heat production of 1998). 10 R. L. Rudnick and A. A. Nyblade

CONCLUSIONS four-phase lherzolites II. New thermo barometers, and practical assessment of existing thermobarometers. J. Assuming that cratonic peridotite xenoliths record Petrol. 31, 1353-1378. the poT conditions of present-day Archean mantle BREY G. P., KOHLERT., and NICKELK. G. (1990) Geother- mobarometry in four-phase Iherzolites I. Experimental roots, the following conclusions can be drawn: results from 10 to 60 kb. J. Petrol. 31, 1313-1352. CERMAKA. and RYBACHL. (1982) Thermal conductivity 1) The average crustal heat production in the Kala- and specific heat of minerals and rocks. In Landolt- 3 hari craton lies between 0.5 and 0.8 µ,W/m , with a Bornstein Numerical Data and Functional Relationships 3 best estimate of 0.70 µ,W/m . in Science and Technology, Vol. 16 (ed. G. ANGEN- 2) The average lithospheric mantle heat production HEISTER).Springer-Verlag. 3 CHAPMAND. S. (1986) Thermal gradients in the continental is <0.07 µ,W/m , and, for most models less than 0.03 3 3 crust. In The nature of the lower continental crust, Vol. µ,W/m , with a best estimate of 0 µ,W/m . 24 (eds. J. B. DAWSON,D. A. CARSWELL,J. HALL, and 3) The intersection of the family of geotherms that K. H. WEDEPOHL),pp. 63-70. Geol. Soc. Spec. Publ. fit the xenolith data with a 1400°C mantle adiabat CHRISTENSENN. I. and MOONEY W. D. (1995) Seismic suggests maximum lithospheric thicknesses of 250 velocity structure and composition of the continental crust: a global view. J. Geophys. Res. 100(B7), 9761- km with a best estimate of 220 km for the Kalahari 9788, craton. DAVIES G. F. and STREBECKJ. W. (1982) Old continental 4) The heat flux across the Moho derived from the geotherms: constraints on heat production and thickness xenolith P-T data is 17-25 mW/m2, which is higher of continental plates. Geophys. J. R. astr. Soc. 69, 623- than recent estimates from the Superior Province 634. DAWSONJ. B., JAMESD., PASLICKC., and HALLIDAYA. M. (12-13 rnw/rrr'). The differences are not easily rec- (1997) Ultrabasic potassic low-volume magmatism and oncilable and imply either that the estimates of continental rifting in north-central Tanzania: association crustal heat production in the Superior Province are with enhanced heat flow. Proc. Sixth Int. Kimberlite too high or that the xenolith P- T arrays are too hot. Conf, Russian Geology and Geophysics 38, 69-81. DUCHKOVA. D. (1991) Review of Siberian heat flow data. In Terrestrial heat flow and the lithosphere struc- Acknowledgments-We extend a special thanks to Joe ture (eds. V. CERMAKand L. RYBACH), pp. 426-443. Boyd, Dante Canil, Bill Griffin, Dan Schulze and Johann Springer-Verlag. Stiefenhofer for sharing mineral chemical data. Bill Me- DURRHEIMR. J., BARKERW. H., and GREENR. W. E. (1992) Donough and Cin-Ty Lee are thanked for discussions and Seismic studies in the Limpopo Belt. Precam. Res. 55, Steve Haggerty for access to mineral chemical data in 187-200. unpublished theses. Suzan van der Lee and Bjorn Mysen DURRHEIMR. J. and GREEN R. W. E. (1992) A seismic provided constructive criticism of earlier drafts. Alayne refraction investigation of the Archaean Kaapvaal Cra- Moody is thanked for assistance with manuscript formating. ton, South Africa, using mine tremors as the energy Our work on cratonic lithosphere has been supported by source. Geophys. J. Int. 108,812-832. NSF grants EAR-9304555 to AAN and EAR-950651O to PEl Y. (1995) Thermal expansion. In Mineral Physics & RLR. Crystallography: A handbook of physical constants (ed. T. J. AHRENS),pp. 29-44. AGU. REFERENCES FINNERTYA. A. and BOYD F. R. (1984) Evaluation of thermobarometers for garnet peridotites. Geochim. Cos- BOYD F. R. (1973) A pyroxene geotherm. Geochim. Cos- mochim. Acta 48, 15-27. mochim. Acta 37, 2533-2546. FINNERTYA. A. and BOYD F. R. (1987) Thermobarometry BOYD F. R. (1979) Garnet lherzolite xenoliths from the for garnet peridotites: basis for the determination of ther- kimberlites of East Griqualand, South Africa. Carnegie mal and compositional structure of the upper mantle. In Inst. Washington Yearbook 79,296-302. Mantle Xenoliths (ed. P. H. NIXON), pp. 381-402. John BOYD F. R. (1984) Siberian geotherm based on lherzolite Wiley & Sons. xenoliths from the Udachnaya kimberlite, U.S.S.R. Ge- FRANZL., BREYG. P., and OKRUSCHM. (1996) Steady state ology 12, 528-530. geotherm, thermal disturbances, and tectonic develop- BOYD F. R. and CANILD. (1997) Peridotite xenoliths from ment of the lower lithosphere underneath the Gibeon the Slave craton, Northwest territories. In Seventh Annual kimberlite province, Namibia. Contrib. Mineral. Petrol. V.M. Goldschmidt Conference, LPI Contribution No. 126, 181-198. 921, pp. 34. Lunar and Planetary Institute. GANE P. G., ATKINSA. R., SELLSCHOPJ. P. F., and SELIG- BOYD F. R. and MERTZMANS. A. (1987) Composition and MAN P. (1956) Crustal structure in the Transvaal. Bull. structure of the Kaapvaallithosphere, southern Africa. In Seis. Soc. Am. 46,293-316. Magmatic Processes: Physicochemical principles, Spe- GAO S., Luo T.-C., ZHANGB.-R., ZHANGH.-F., HAN Y.-W., cial Pub. No. 1 (ed. B. O. MYSEN), pp. 13-24. The Hu Y.-K., and ZHAOZ.-D. (1998) Chemical composition Geochemical Society. of the continental crust as revealed by studies in East BoYD F. R., POKHILENKON. P., PEARSOND. G., MERTZMAN China. Geochim. Cosmochim. Acta, 62, 1959-1975. S. A., SOBOLEVN. V., and FINGERL. W. (1997) Compo- GRAND S. P. (1994) Mantle shear structure beneath the sition of the Siberian cratonic mantle: evidence from Americas and surrounding oceans. J. Geophys. Res. 99, Udachnaya peridotite xenoliths. Contrib. Mineral. Petrol. 11,591-11,621. 128,228-246. GRIFFIN W. L., KAMINSKYF. V., RYAN C. G., O'REILLY BREY G. P. and KOHLERT. (1990) Geothermobarometry in S. Y., WIN T. T., and ILUPINI. P. (1996) Thermal state The Archean lithosphere 11

and composition of the lithospheric mantle beneath the Solubility of Al203 in enstatite for spinel and garnet Daldyn kimberlite field, Yakutia. Tectonophys. 262, 19- peridotite. Am. Mineral. 59, 110-119. 33. McDONOUGHW. F. (1990) Constraints on the composition GRIFFINW. L., SMITHD., BOYDF. R., COUSENSD. R., RYAN of the continental lithospheric mantle. Earth Planet. Sci. C. G., SIE S. H., and SUTERG. F. (1989) Trace-element Lett. 101, 1-18. zoning in garnets from sheared mantle xenoliths. McDONOUGHW. F. and RUDNICKR. L. (1998) Mineralogy Geochim. Cosmochim. Acta 53, 561-567. and composition of the upper mantle. In Ultra High GUNTHERM. and JAGOUTZE. (1994) Isotopic disequilibria Pressure Mineralogy, Reviews in Mineralogy, Vol. 37 (SmlNd, Rb/Sr) between mineral phases of coarse (ed. R. J. HEMLEY),pp. 139-164. Mineral. Soc. Am. grained, low temperature garnet peridotites from Kimber- McLENNAN S. M. and TAYLORS. R. (1996) Heat flow and ley Floors, southern Africa. In Kimberlites, Related the chemical composition of continental crust. 1. Geol. Rocks and Mantle Xenoliths, (Proceedings Fifth Int. 104, 396-377. Kimb. Conf), Vol. 1 (eds. H. 0. A. MEYER and 0. H. MEYER H. 0. A., WALDMANM. A., and GARWOODB. L. LEONARDOS),pp. 354-364. C.P.R.M. (1994) Mantle xenoliths from kimberlite near Kirkland HAGGERTYS. E. (1994) Superkimberlites: a geodynamic Lake, Ontario. Can. Mineral. 32, 295-306. diamond window to the Earth's core. Earth Planet. Sci. MITCHELL R. H. (1984) Garnet lherzolites from the Lett. 122, 57- 69. Hanaus-I and Louwrensia kimberlites of Namibia. Con- HARRISJ., HUTCHINSONM. T., HURSTHOUSEM., LIGHTM., trib. Mineral. Petrol. 86, 178-188. and HARTE B. (1997) A new tetragonal silicate mineral MOORE R. 0. and GURNEY J. J. (1985) Pyroxene solid occurring as inclusions in lower-mantle diamonds. Na- solution in garnets included in diamond. Nature 335, ture 387, 486-488. 784-789. HARTE B. and HARRISJ. W. (1994) Lower mantle mineral MORGANP. (1984) The thermal structure and thermal evo- associations preserved in diamonds. Mineralogical Mag- lution of the continental lithosphere. In Structure and azine 58, 384-385. Evolution of the Continental Lithosphere, Vol. 15 (eds. JAMESD. E., VAN DER LEE S., GAO S., SILVERP., VANDE- H. N. POLLACKand V. R. MURTHY),pp. 107-193. Phys. CARJ., KUEHNELR., JORDANT. H., SALTZERR., GAHERTY and Chem. Earth. J., GOREJ., ZENGENIT., NGUURIT., WRIGHTc., WEBB S., NAVROTSKYA. (1995) Thermodynamic properties of min- BURFORDD., DOUCOUREM., MOLISANAM., GREEN R., erals. In Mineral Physics & Crystallography: A hand- ROBEY J., HARVEYJ., KOSTLINE., and REICHHARDTF. book of physical constants (ed. T. J. AHRENS),pp. 18-28. (1998) Southern Africa seismic experiment. EOS, Spring AGU. Meeting Abstracts, S228. NIXON P. H. and BOYD F. R. (1973) Petrogenesis of the JAUPARTC., MARESCHALJ. C., GUILLOu-FROTTIERL., and granular and sheared ultrabasic nodule suite in kimber- DAVAILLE A. (1998) Heat flow and thickness of the lites. In Lesotho Kimberlites (ed. P. H. NIXON), pp. 48- lithosphere in the Canadian shield. 1. Geophys. Res. 103, 56. Lesotho National Development Corporation. 15,269-15,286. NYBLADE A. A. (1999) Heat flow and the structure of JONES M. Q. W. (1988) Heat flow in the Witwatersrand Precambrian lithosphere. Lithos, (in press). Basin and environs and its significance for the South NYBLADEA. A. and POLLACKH. N. (1993) A global analysis African shield geotherm and lithosphere thickness. J. of heat flow from Precambrian terrains: implications for Geophys. Res. 93, 3243-3260. the thermal structure of Archean and Proterozoic litho- JONES M. Q. W. (1992) Heat flow anomaly in Lesotho: sphere. J. Geophys. Res. 98, 12207-12218. implications for the southern boundary of the Kaapvaal NYBLADEA. A., POLLACKH. N., JONESD. L., PODMOREF., craton. Geophys. Res. Lett. 19,2031-2034. and MUSHAYANDEBVUM. (1990) Terrestrial heat flow in JORDAN T. H. (1975) The continental tectosphere. Rev. East and Southern Africa .. J. Geophys. Res. 95, 17,371- Geophys. Space Phys. 13(3), 1-12. 17,384. JORDANT. H. (1988) Structure and formation of the conti- PEARSOND. G., CARLSONR. W., SHIREYS. B., BOYDF. R., nental lithosphere. In Oceanic and Continental Litho- and NIXON P. H. (1995) The stabilisation of Archaean sphere; similarities and differences (eds. M. A. MENZIES lithospheric mantle: A Re-Os isotope study of peridotite and K. Cox), pp. 11-37. 1. Petrol., Special Lithosphere xenoliths from the Kaapvaal craton. Earth Planet. Sci. Issue. Lett. 134,341-357. KiNG S. D. and ANDERSOND. L. (1998) Edge-driven con- PEARSONN. J., GRIFFINW. L., DOYLEB. J., O'REILLY S. Y., vection. Earth Planet. Sci. Lett., 160, 289-296. VAN ACHTERBERGHE., and KiVI K. (1998) Xenoliths KOPYLOVAM. G., RUSSELLJ. K., and COOKENBOOH. (1998) from kimberlite pipes of the Lac de Gras area, Slave Upper-mantle stratigraphy of the Slave craton, Canada: craton, Canada. In 7th Internat. Kimberlite Conf Ex- insights into a new kimberlite province. Geology 26, tended Abst., pp. 670-672. Univ. Cape Town. 315-319. POKHILENKON. P., SOBOLEVN. V., BOYD F. R., PEARSON LEE C.-T. (1998) Are inflected geotherms real? In 7th In- D. G., and SHIMIZUN. (1993) Megacrystalline pyrope ternat. Kimberlite Conf Abstracts, pp. 489-491. Univ. peridotites in the lithosphere of the Siberian platform: Cape Town. mineralogy, geochemical peculiarities and the problem of LEE, C.-T. AND RUDNICK, R. L. (1999) Compositionally their origin. Russ. Geol. Geophys. 34,56-67. stratified cratonic lithosphere: petrology and geochemis- POLLACKH. N. (1980) The heat flow from the earth: a try of peridotite xenoliths from the Labait Volcano, Tan- review. In Mechanisms of Continental Drift and Plate zania. In Proceedings of the Seventh Kimberlite Confer- Tectonics (eds. P. A. DAVIES and S. K. RUNCORN),pp. ence (eds. J. J. GURNEYand S. H. RICHARDSON)in press. 183-192. LEWIS T. J. and WANG K. (1992) Influence of terrain on POLLACKH. N. and CHAPMAND. S. (1977) On the regional bedrock temperatures. Global Planetary Change 98, 87- variation of heat flow, geotherms, and the thickness of the 100. lithosphere. Tectonophys. 38, 279-296. MACGREGORI. D. (1974) The system MgO-AI203-Si02: RICARDY., NATAFH.-C., and MONTAGNERJ.-P. (1996) The 12 R. L. Rudnick and A. A. Nyblade

three-dimensional seismological model a priori con- Kimberlite Conf., Russian Geology and Geophysics 38, strained: Confrontation with seismic data. J. Geophys. 205-217. Res. 101, 8457-8472. SMITH D. (1999) Temperatures and pressures of mineral RICHARDSONS. H., ERLANKA. J., and HART S. R. (1985) equilibration in peridotite xenoliths: review, discussion, Kimberlite-borne garnet peridotite xenoliths from old en- and implications. In Mantle Petrology: Field Observa- riched subcontinental lithosphere. Earth Planet. Sci. Lett. tions and High-Pressure Experimentation: A Tribute to 75, 116-128. Francis R. (Joe) Boyd, Vol. (this volume) (eds. Y. FE!, RiNGWOODA. E., KESSONS. E., HIBBERSONW., and WARE C. M. BERTKA, and B. O. MYSEN). The Geochemical N. (1992) Origin of kimberlites and related magmas. Society. Earth Planet. Sci. Lett. 113,521-538. SMITHD. AND BOYDF. R. (1987) Compositional heteroge- RITSEMAJ., NYBLADEA. A., OWENST. J., LANGSTONC. A., neities in a high-temperature lherzolite nodule and impli- and VANDECARJ. C. (1998) Upper mantle seismic veloc- cations for mantle processes. In Mantle Xenoliths (ed. ity structure beneath Tanzania, East Africa: implications P. H. NIXON), pp: 551-56l. John Wyllie & Sons. for the stability of cratonic lithosphere. J. Geophys. Res. SPETSIUSZ. V. and SERENKOV. P. (1990) Composition of 103,21,201-21,214. the continental upper mantle and lower crust beneath the Roy R. F., BECK A. W., and TOULOUKIANY. S. (1981) Siberian platform. Moscow "Nauka". (in Russian) Thermo-physical properties of rocks. In Physical Prop- STIEFENHOFERJ., VILJOENK. S., and MARSH J. S. (1997) erties of Rocks and Minerals (eds. Y. S. TOULOUKIAN, Petrology and geochemistry of peridotite xenoliths from W. R. JUDD,and R. F. Roy), pp. 409-502. McGraw-Hill. the Letlhakane kimberlites, Botswana. Contrib. Mineral. RUDNICKR. L. and FOUNTAIND. M. (1995) Nature and Petrol. 127, 147-158. composition of the continental crust: a lower crustal STUARTG. W. and ZENGENIT. G. (1987) Seismic crustal perspective. Rev. Geophys. 33(3), 267-309. structure of the Limpopo mobile belt, Zimbabwe. Tec- RUDNICKR. L., MCDONOUGHW. F., and O'CONNELL R. J. tonophysics 144, 323-335. (1998) Thermal structure, thickness and composition of Su W.-J., WOODWARDR. L., and DZIEWONSKIA. M. (1994) continental lithosphere. Chem. Geol. 145,399-415. Degree 12 model of shear velocity heterogeneity in the SASSJ. H., LACHENBRUCHA. H., MOSEST. H., and MORGAN mantle. J. Geophys. Res. 99, 6945-6980. P. (1992) Heat flow from a scientific research well at TAYLORS. R. and McLENNANS. M. (1985) The Continental Cajon Pass, California. J. Geophys. Res. 97,5017-5030. Crust: its Composition and Evolution. Blackwell. SAUTTERV., HAGGERTYS. E., and FIELD S. (1991) Ultra- VAN DER HILSTR. D., SIMMONSF. J., and KENNETTB. L. N. deep (300 kilometers) ultramafic xenoliths: petrological (1998) Constraints on the structure of the upper mantle evidence from the transition zone. Science 252,827-830. beneath Australia from waveform tomography. EOS, SCARFEC. M. and TAKAHASHIE. (1986) Melting of garnet Spring Meeting Abstracts, S228. peridotite to 13 GPa and the early history of the upper VAN DER LEE S. and NOLET G. (1997) Upper mantle S mantle. Nature 322, 354-356. velocity structure of North America. J. Geophys. Res. SCHATZJ. F. and SIMMONSG. (1972) Thermal conductivity 102,22815-22838. of earth materials at high temperatures. J. Geophys. Res. VICKERP. A. (1997) Garnet peridotite xenoliths from kim- 77, 6966-6983. berlite near Kirkland Lake, Canada. M.S. Thesis, Univ. SCHULZED. J. (1996) Kimberlites in the vicinity of Kirkland Toronto. lake and Lake Timiskaming, Ontario and Quebec. In WEAVERB. L. and TARNEYJ. (1984) Empirical approach to Searching for Diamonds in Canada, Vol. 3228 (eds. estimating the composition of the continental crust. Na- A. N. LECHEMINANT,D. G. RICHARDSON,R. N. W. DILA- ture 310, 575-577. BIO, and K. A. RICHARDSON),pp. 73-78. Geological Sur- ZHURAVLEVA. Z., LAZ'KO Y. Y., and PONOMARENKOA. I. vey Canada, Open File Rept. (1991) Radiogenic isotopes and REE in garnet peridotite SHEE S. R. (1978) The mineral chemsitry of xenoliths from xenoliths from the Mir kimberlite pipe, Yakutia. the Orapa kimberlite pipe, Botswana. PhD Thesis, Univ. Geokhimiya 7, 982-994. Cape Town. ZOTH G. and HAENELR. (1988) Thermal conductivity. In SHIMIZUN., POKHILENKON. P., BOYD F. R., and PEARSON Handbook of Terrestrial Heat Flow Density Determina- D. G. (1997) Geochemical characteristics of mantle xeno- tion (eds. R. HAENEL,L. RYBACH,and L. STEGENA),pp. liths from the Udachnaya kimberlite pipe. Proc. Sixth Int. 449-463. Kluwer Academic.