Cratonic mantle

Neither of these proposals, however, can explain the subchondritic Ca/Al of most cratonic perido-tites. If the latter are residues at higher P, more aluminum remains in the residue relative to calcium, the opposite trend of lower P residues. The anomalously low Ca/Al of cratonic perido-tites may be unique to the formation of sub-cratonic mantle, or is a secondary feature with unexplained origin. 

If continental peridotite xenoliths are divided into cratonic and noncratonic compositions, the vast majority of highly enriched neodymium isotope compositions originate in cratonic mantle and very few are evident in noncratonic mantle. Enriched neodymium isotope compositions can also be found in mantle sampled by orogenic peridotites (Reisberg and Zindler, 1986 Pearson et ai, 1993 Chapter 2.04) but not the extreme values evident in cratonic CLM. This is expected, given the great antiquity of cratonic CLM. Further subdivision of cratonic samples (Figure 37) shows... 

Cratonic mantle peridotites. Over 230 whole-rock cratonic xenoliths have now been analyzed for Re-Os isotope compositions. Given that many peridotite xenoliths have experienced relatively recent rhenium introduction, it is generally best to use rhenium-depletion model ages (Trd) that do not rely on the measured rhenium content of the rock for model age calculation. For cratonic peridotite xenoliths, the frequency distribution of rhenium-depletion ages shows a wide range, with a pronounced mode at 2.5-2.75 Gyr and some samples that have ages of >3.5 Gyr... 

Calculated bulk rock trace-element systematics of eclogites have wider implications for mantle recycling models and bulk silicate earth mass balance. The subchondritic Nb/Ta, Nb/La, and Ti/Zr of both continental cmst and depleted mantle require the existence of an additional reservoir with superchondritic ratios to complete the terrestrial mass balance. Rudnick et al. (2000) have shown that rutile-bearing eclogites from cratonic mantle have suitably superchondritic Nb/Ta, Nb/La, and Ti/Zr such that if this component formed 1 -6% by weight of the bulk silicate earth, this would resolve the mass imbalance. This mass fraction far exceeds the likely mass of eclogite in the continental lithosphere and so the material is proposed to reside in the lower mantle, possibly at the core-mantle boundary. 

Boyd F. R., Pokhilenko N. P., Pearson D. G., Mertzman S. A., Sobolev N. V., and Finger L. W. (1997) Composition of the Siberian cratonic mantle evidence from Udachnaya peridotite xenoliths. Contrib. Mineral. Petrol. 128, 228-246. 

Hanghoj K., Kelemen P. B., Bernstein S., Blustztajn J., and Frei R. (2(X)1) Osmium isotopes in the Wiedemann Fjord mantle xenoliths a unique record of cratonic mantle formation by melt depletion in the Archaean. Geochem. Geophys. Geosys. 2 (20010109) 2000GC000085. 

Herzberg C. (1999) Phase equihbrium constraints on the formation of cratonic mantle. In Mantle Petrology Field Observations and High Pressure Experimentation, Spec. Publ. Geochem. Soc. (eds. Y. Fei, C. Bertka, and B. O. My sen) Geochemical Society, Houston, vol. 6, pp. 241-250. 

MacKenzie J. M. and Canil D. (1999) Composition and thermal evolution of cratonic mantle beneath the central Archean Slave Province, NWT, Canada. Contrib. Mineral. Petrol. 134, 313 -324. 

Schmidberger S. S. and Francis D. (1999) Nature of the mantle roots beneath the North American craton mantle xenohth evidence from Somerset Island kimberlites. Lithos 48, 195-216. 

Direct evidence for the compositional effects of partial melt extraction is preserved in samples of upper-mantle lithosphere with a range of ages, including Archean cratonic mantle, Proterozoic subcontinental mantle, and modern oceanic mantle. Samples of upper mantle are collected as xenoliths, peridotites dredged from oceanic fracture zones, and slices of upper mantle tectonically exposed at the surface, and extensive samples exist from both oceanic and continental settings (see Chapters 2.04 and 2.05). Here, data sets are assembled for oceanic and subcontinental mantle lithosphere, and compositional trends are compared to those predicted for partial melt extraction from fertile peridotite in order to deduce the role that melt extraction has played in producing compositional variability in upper-mantle lithosphere, and to place constraints on the thermal evolution of the mantle. 

Modern oceanic mantle is defined solely by abyssal peridotites, which are samples of harzbur-gite and Iherzolite collected from fracture zones at oceanic spreading centers, and these samples are representative of shallow oceanic lithosphere that has been processed at mid-ocean ridges (see Chapter 2.04). Two types of continental mantle lithosphere are considered (i) cratonic mantle, which refers to xenoliths collected from kimberlites that sample portions of mantle beneath stable, Archean cratons and (ii) off-craton mantle, which refers to xenoliths collected from alkalic basalts that have sampled portions of the subcontinental mantle adjacent to ancient cratonic mantle (see Chapter 2.05). Also included with off-craton lithosphere are orogenic Iherzolites and ophiolites, which are slices of mantle tectonically emplaced typically at convergent margins. 

In this section the normative modal and major-element compositions of a large number of samples collected from both oceanic and continental mantle lithosphere are presented. Compositional trends exhibited by oceanic and off-craton mantle are used to develop a model fertile upper-mantle composition. The reader is referred to Chapters 2.04 and 2.05 in this volume, and to reviews by McDonough (1990), McDonough and Rudnick (1998), and Griffin et al. (1999b) for alternative and detailed perspectives of the mantle sample. 

Circum-cratonic mantle beneath continental crust is sampled as xenoliths in alkalic basalt... 

In order to focus more closely on the melt extraction component in olf-craton mantle, a subset of data has been selected. A single criterion... 

Figure 9 Normative spinel Iherzolite mineral abundances (wt.%) versus rock Mg for a subset of 292 off-craton mantle compositions (shaded circles). Data from a given locality were included if compositions had a correlation between Mg and normative olivine with a correlation coefficient (i ) of 0.6 or better. Data sources for xenoliths are Beccaluva et al. (2001a,h), Reid and Woods (1978), Rivalenti et al. (2000), Stem et al. (1999), Vaselli et al. (1995), Wiechert et al. (1997), Xu et al. (1988), and Zangana et al. (1999). Data sources for orogenic Iherzolites are Bodinier et al. (1988), Frey et al. (1985, 1991), Hartmann and Wedepohl (1993), and Lugovic et al. (1991). Open circles are the reconstmcted abyssal peridotite compositions from Figure 7. Also shown are estimates of primitive mantle from Table 2 white square = 1, circle = 2, triangle = 3, diamond = 5, inverted triangle = 6, ex = 7, and shaded star = 8.

Figure 10 Major-element oxides versus Mg for the off-craton mantle subset (shaded circles) and reconstructed abyssal peridotite compositions (open circles). Primitive mantle compositions from Table 2 are also shown with symbols as in Figure 9.

Figure 12 Normative garnet Iherzolite mineral abundances (wt.%) versus rock Mg for low-temperature cratonic mantle. Data are open circles, Kaapvaal craton (Boyd and Mertzman, 1987 and references therein) filled diamonds, Tanzanian craton (Lee and Rudnick, 1999) open triangle, Siberian craton (Boyd et al., 1997) cross. Slave craton, northwest Canada (Kopylova and Russell, 2000) filled triangle, northern Canadian craton (Schmidberger and Francis, 1999) and inverted triangle, central Greenland craton (Bernstein et al., 1998).

Here, the fertile upper-mantle composition derived in the previous section is assumed for oceanic and olf-craton mantle, and the model of Kinzler and Grove (1992a, 1993) is used to model melt extraction at pressures of < 2.5 GPa. 

Melt extraction at higher pressures as applied to cratonic mantle is modeled using the experimental data of Walter (1998) for a composition that is very similar to the primitive mantle of McDonough and Sun (1995). 

The off-craton mantle subset is shown relative to isobaric batch melt extraction curves on plots of normative olivine and major-element oxides versus Mg in Figure 17. Generally speaking, off-craton mantle compositions are consistent with effectively 0-30% melt extraction from fertile upper mantle in the range of 1-5 GPa. The chemical signature recorded in off-craton mantle mimics closely the maximum degree of melt extraction recorded in oceanic mantle, but in contrast there are many samples that show little or... 

Figure 18 Normative olivine and opx (garnet Uierzo-Ute norm) versus rock Mg showing low-T cratonic mantle relative to batch melt extraction trends from 3 GPa to 7 GPa (see Figure 6). Open circles are compositions from the Kaapvaal, Siberian, and Slave cratons, and the filled circles are compositions from the Tanzanian, Canadian, and Greenland cratons.

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