Composition Of Subcontinental Lithospheric Mantle Research Articles

Lithospheric magnetic structure of cratonic regions in Central-Eastern China inferred from aeromagnetic anomalies: Insights into magnetization in the uppermost mantle

Most of the subcontinental lithospheric mantle (SCLM) beneath Proterozoic cratons consists of refertilized Archaean SCLM. Variations in SCLM composition and its physical properties significantly affect the stabilization and preservation of the ancient continents. In this paper, aeromagnetic data are analyzed to reveal the magnetic structure of the lithospheric mantle beneath two major Precambrian blocks in central-eastern China, i.e., the Upper Yangtze Block (UYB) and Ordos Block (OB). After being reduced to the pole, the Fourier power spectrum of the aeromagnetic anomalies is calculated to determine the depth to magnetic sources. Considering the lower spatial resolution of the power spectral analysis in dealing with the long-wavelength aeromagnetic anomalies, we applied the scale-normalized continuous wavelet transform (CWT) on the magnetic data to trace the magnetic sources, with special focus on deeper ones. Synthetical model of a magnetic layer and application to the profile data validate the effectiveness of this scale normalization scheme in improving the wavenumber/spatial resolution in the CWT scalogram.In order to present a detailed magnetic structure, we carried out 2.5D forward modeling work on the magnetic data of a 2280 km-long nearly N-S profile across the UYB and OB. Due to the inherent ambiguity in the modeling results, the CWT-based spectral analysis is successfully adopted to provide source depth constraints for the initial model. The constrained forward modeling results indicate strong inhomogeneities among main tectonic blocks of studied area, like humans have different fingerprints. The magnetization of OB is larger than that of UYB since its Archean to Paleoproterozoic metamorphic basement are widely exposed at the surface, while the Precambrian basement of UYB is mostly overlain by unmetamorphosed Sinian cover and weakly metamorphosed Neoproterozoic strata. The most interesting aspect is that deep-seated magnetic sources might reside in the uppermost mantle of UYB and OB, suggesting vertical layering in the SCLM in cold cratonic regions.

Sr-Nd isotopic study of dolerite dykes in the Western Dharwar craton, southern India: Implications for the evolution of the subcontinental lithospheric mantle in late Archean

Mafic dyke swarms are one of the major geologic features that represent crustal extension and rifting episodes which result in the transfer of large volumes of mantle materials to the crust. The rapid emplacement of dykes within a short span of time helps to preserve the composition of subcontinental lithospheric mantle (SCLM) and understand its evolution over time. Here we report major and trace element geochemistry and Sr-Nd isotopic characteristics of dykes from the Tiptur area in the late Archean Western Dharwar craton of southern India. The studied mafic dykes are un-metamorphosed and preserve pristine igneous textures and mineralogy. Three different groups of dykes were identified and categorized as olivine dolerites, dolerites and REE enriched dolerites, based on the distinct petrographical and geochemical characteristics. Olivine dolerites are having a high Mg# suggesting a lower degree of fractionation and is indicative of a less evolved magma. Dolerites have comparatively lower Mg# than the olivine dolerites indicating that the former have undergone significant fractional crystallization as confirmed by the higher Ni contents. The low initial Sr isotopic ratio for olivine dolerites (87Sr/86Sr = 0.70088) with negative epsilon Nd values are suggesting a late Archean enriched subcontinental lithospheric mantle (SCLM) in the source region. The dolerites (87Sr/86Sr = 0.70132 to 0.70327) and REE enriched dolerites (87Sr/86Sr = 0.70405 to 0.70653) show signatures of a more enriched source. The evolution of SCLM from a DMM source enriched by the subduction of the mafic oceanic crust in the late Archean to a mantle enriched by continent-derived sediments towards the early Proterozoic are reflected in the dolerite and REE enriched dykes.

Assessing the presence of volatile-bearing mineral phases in the cratonic mantle as a possible cause of mid-lithospheric discontinuities

A number of possible hypotheses have been proposed to explain the origin of mid-lithospheric discontinuities (MLDs), typically characterized by ∼2-6% reductions in seismic shear wave velocity (VS) at depths of 60 km to ∼150 km in the cratonic sub-continental lithospheric mantle (SCLM). One such hypothesis is the presence of low-shear wave velocity, hydrous and carbonate mineral phases. Although, the presence of hydrous silicates and carbonates can cause a reduction in the shear wave velocity of mantle domains, the contribution of volatile metasomatism to the origins of MLDs has remained incompletely evaluated. To assess the metasomatic origin of MLDs, we compiled experimental phase assemblages, phase proportions, and phase compositions from the literature in peridotite + H2O, peridotite + CO2, and peridotite + H2O + CO2 systems at P-T conditions where hydrous silicate and/or carbonate minerals are stable. By comparing the experimental assemblages with the compiled bulk peridotite compositions for cratons, we bracket plausible proportions and compositions of hydrous silicate and carbonate mineral phases that can be expected in cratonic SCLMs. Based on the CaO and K2O contents of cratonic peridotite xenoliths and the estimated upper limit of CO2 content in SCLM, ≤∼10 vol.% pargasitic amphibole, ≤∼2.1 vol.% phlogopite and ≤∼0.2 vol.% magnesite solid solution can be stable in the SCLM. We also present new elasticity data for the pargasite end member of amphibole based on first principles simulations for more accurate estimates of aggregate VS for metasomatized domains in cratonic mantle. Using the bracketed phase compositions, phase proportions, and updated values of elastic constants for relevant mineral end members, we further calculate aggregate VS at MLD depths for three seismic stations in the northern continental U.S. Depending on the choice of background wave speeds of unmetasomatized peridotite and the cratonic geotherm, the composition and abundance of volatile-bearing mineral phases bracketed here can explain as much as 2.01 to 3.01% reduction in VS. While various craton formation scenarios allow formation of the amphibole and phlogopite abundances bracketed here, presence of volatile-bearing phases in an average cratonic SCLM composition cannot explain the entire range of velocity reductions observed at MLDs. Other possible velocity reduction mechanisms thus must be considered to explain the full estimated range of shear wave speed reduction at MLD depths globally.

Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr–Nd–Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U+Th+K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84–6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085–0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100−6800+9400 and 54000−9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03±0.15 to 13.66±0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31–9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe=1.0833−0.0053+0.0216 and 136Xe/132Xe=0.3761−0.0034+0.0246).The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17–7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065–0.079). We suggest that this MORB-like metasomatism was capable of overprinting the noble gas composition of Gobernador Gregores due to recent metasomatism of the SCLM because of asthenospheric mantle upwelling in response to opening of the Patagonian slab window. The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 380 up to 6560, with mantle source 40Ar/36Ar between 8100−700+1400 and 17700−3100+4400. The lower 40Ar/36Ar ratio of the Gobernador Gregores mantle source, compared with that of Pali-Aike, attests that the Patagonia SCLM was affected significantly by atmospheric contamination associated with the recycled oceanic lithosphere.

Continental-root control on the genesis of magmatic ore deposits

Some giant ore deposits are formed from magma, but the precise controls on their formation are unclear. A Perspective article analyses the distribution of some diamond, platinum-group element and gold deposits worldwide, and suggests that the structure and composition of sub-continental lithospheric mantle could play a role in ore genesis.

Thermal and compositional structure of the subcontinental lithospheric mantle: Derivation from shear wave seismic tomography

Seismic tomography can provide unique information on the structure of the subcontinental lithospheric mantle (SCLM), but seismic velocity reflects both temperature and composition. We present a methodology for evaluating and isolating the relative contributions of these effects, which produces maps of regional geotherm and broad compositional constraints on the SCLM from the inversion of shear wave (Vs) seismic tomography. This approach uses model geotherms quantized in steps of 2.5 mW/m2 and three mantle compositions corresponding to typical Archean, Proterozoic, and Phanerozoic SCLM. Starting from an assumed composition for a volume of SCLM, lithospheric density at surface pressure and temperature is calculated for each geotherm at each point; the optimum geotherm is taken as the one yielding a density closest to the mean value derived from mantle xenoliths (3.31 g/cm3), since density varies with composition. Results requiring densities or geotherms outside the known natural range of these parameters worldwide require the choice of a different mantle composition. This technique, applied iteratively to a 275 km × 275 km Vs model developed by S. Grand (University of Texas, Austin), results in maps of the geotherm and regional density, which allow interpretation of SCLM composition within broad limits. These results can then be compared with local (paleo)geotherms and data for mantle composition, derived from xenolith suites. Application of this technique to the SCLM beneath Africa, Siberia, and North America shows good correlation with regional geological features, xenolith data, and other geophysical data.

Lithosphere mapping beneath the North American plate

Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections. In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140–160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to <100 km depth toward the northern and southern edges of the craton. Strongly layered SCLM suggests that plume subcretion may also have played a role in the construction of the lithosphere beneath Michigan and Saskatchewan. Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts. SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado–Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain buoyant, modified Archean SCLM. Possible juvenile Proterozoic SCLM beneath the Colorado Plateau is significantly less refractory. The SCLM beneath the Kansas kimberlite field is highly melt-metasomatised, reflecting its proximity to the Mid-Continent Rift System. A traverse across the continent shows that the upper part of the cratonic SCLM is highly magnesian; the decrease in mg# with depth is interpreted as the cumulative effect of metasomatic modification through time. The relatively small variations in seismic velocity within the continental core largely reflect the thickness of this depleted layer. The larger drop in seismic velocity in the surrounding Proton and Tecton belts reflects the closely coupled changes in SCLM composition and geotherm.

The evolution of lithospheric mantle beneath the Kalahari Craton and its margins

The compositional structure and thermal state of the subcontinental lithospheric mantle (SCLM) beneath the Kalahari Craton and the surrounding mobile belts have been mapped in space and time using >3400 garnet xenocrysts from >50 kimberlites intruded over the period 520–80 Ma. The trace-element patterns of many garnets reflect the metasomatic refertilisation of originally highly depleted harzburgites and lherzolites, and much of the lateral and vertical heterogeneity observed in the SCLM within the craton is the product of such metasomatism. The most depleted, and possibly least modified, SCLM was sampled beneath the Limpopo Belt by early Paleozoic kimberlites; the SCLM beneath other parts of the craton may represent similar material modified by metasomatism during Phanerozoic time. In the SW part of the craton, the SCLM sampled by “Group 2” kimberlites (>110 Ma) is thicker, cooler and less metasomatised than that sampled by “Group 1” kimberlites (mostly ≤95 Ma) in the same area. Therefore, the extensively studied xenolith suite from the Group 1 kimberlites probably is not representative of primary Archean SCLM compositions. The relatively fertile SCLM beneath the mobile belts surrounding the craton is interpreted as largely Archean SCLM, metasomatised and mixed with younger material during Paleoproterozoic to Mesoproterozoic rifting and compression. This implies that at least some of the observed secular evolution in SCLM composition worldwide may reflect the reworking of Archean SCLM. There are strong correlations between mantle composition and the lateral variations in seismic velocity shown by detailed tomographic studies. Areas of relatively low Vp within the craton largely reflect the progressive refertilisation of the Archean root during episodes of intraplate magmatism, including the Bushveld (2 Ga) and Karroo (ca. 180 Ma) events; areas of high Vp map out the distribution of relatively less metasomatised Archean SCLM. The relatively low Vp of the SCLM beneath the mobile belts around the craton is consistent with its fertile composition. The seismic data may be used to map the lateral extent of different types of SCLM, taking into account the small lateral variations in the geotherm identified using the techniques described here.

The origin and evolution of Archean lithospheric mantle

Abstract The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model” of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere.

Cr‐pyrope garnets in the lithospheric mantle 2. Compositional populations and their distribution in time and space

Three novel statistical approaches (Cluster Analysis by Regressive Partitioning [CARP], Patient Rule Induction Method [PRIM], and ModeMap) have been used to define compositional populations within a large database (n &gt; 13,000) of Cr‐pyrope garnets from the subcontinental lithospheric mantle (SCLM). The variables used are the major oxides and proton‐microprobe data for Zn, Ga, Sr, Y, and Zr. Because the rules defining these populations (classes) are expressed in simple compositional variables, they are easily applied to new samples and other databases. The classes defined by the three methods show strong similarities and correlations, suggesting that they are statistically meaningful. The geological significance of the classes has been tested by classifying garnets from 184 mantle‐derived peridotite xenoliths and from a smaller database (n &gt; 5400) of garnets analyzed for &gt;20 trace elements by laser ablation microprobe–inductively coupled plasma‐mass spectrometry (LAM–ICPMS). The relative abundances of these classes in the lithospheric mantle vary widely across different tectonic settings, and some classes are absent or very rare in either Archean or Phanerozoic SCLM. Their distribution with depth also varies widely within individual lithospheric sections and between different sections of similar tectonothermal age. These garnet classes therefore are a useful tool for mapping the geology of the SCLM. Archean SCLM sections show high degrees of depletion and varying degrees of metasomatism, and they are commonly strongly layered. Several Proterozoic SCLM sections show a concentration of more depleted material near their base, grading upward into more fertile lherzolites. The distribution of garnet classes reflecting low‐T phlogopite‐related metasomatism and high‐T melt‐related metasomatism suggests that many of these Proterozoic SCLM sections consist of strongly metasomatized Archean SCLM. The garnet‐facies SCLM beneath Phanerozoic terrains is only mildly depleted relative to Primitive Upper Mantle (PUM) compositions. These data emphasize the secular evolution of SCLM composition defined earlier [Griffin et al., 1998, 1999a] and suggest that at least part of this evolutionary trend reflects reworking and refertilization of SCLM formed in the Archean time.

The density structure of subcontinental lithosphere through time

This study uses information on composition, thermal state and petrological thickness to calculate the densities of different types of subcontinental lithospheric mantle (SCLM). Data from mantle-derived peridotite xenoliths and garnet–xenocryst suites document a secular evolution in the composition of SCLM: the mean composition of newly formed SCLM has become progressively less depleted, in terms of Al, Ca, mg# and Fe/Al, from Archean, through Proterozoic to Phanerozoic time. Thermobarometric analyses of xenolith and xenocryst suites worldwide show that the mean lithospheric palaeogeotherms rise from low values (corresponding to surface heat flows of 35–40 mW/m2) beneath Archean terranes, to higher values (>50 mW/m2) beneath regions with Phanerozoic crust. The typical thickness of the lithosphere (defined as a chemical boundary layer), ranges from about 250 to 180 km, 180–150 km and 140–60 km for Archean, Proterozoic and Phanerozoic terranes respectively. The depth of this lithosphere–asthenosphere boundary corresponds to a temperature of 1250–1300°C. Using the estimated compositions, average mineral compositions and experimental data on the densities of mineral end-members (tables 1 and 2), we calculate mean densities at 20°C for Primitive Mantle (3.39 Mg m−3) and for SCLM of Archean (3.31±.016 Mg m−3), Proterozoic (3.35±0.02 Mg m−3) and Phanerozoic (3.36±0.02 Mg m−3) age. Curves of density and cumulative density versus depth, which take into account variations in geotherm with tectonothermal age, have been constructed for each age type of lithospheric section to assess the buoyancy of these columns relative to the asthenosphere, modelled as a Primitive Mantle composition. The density curves show that Archean SCLM is significantly buoyant relative to the asthenosphere at depths greater than about 60 km. Proterozoic sections deeper than about 100 km thick also are significantly buoyant. The buoyancy of Archean and Proterozoic SCLM sections, combined with their refractory composition, leads to high viscosities and explains the longevity and stability of old SCLM. Replacement of Archean lithosphere, as beneath the present-day eastern Sino–Korean craton, probably involves mechanical dispersal by rifting, accompanied by the rise of hot, fertile asthenospheric material. Fertile Phanerozoic lithosphere is buoyant when the geotherm is sufficiently high, as in many Cenozoic volcanic provinces. However, as the geothermal gradient relaxes toward a stable conductive profile, Phanerozoic SCLM sections thinner than about 100 km become denser than the asthenosphere, and hence gravitationally unstable. This could help to induce delamination of the SCLM and upwelling of asthenospheric material, beginning a new cycle. The tectonic consequences of such lithosphere replacement would include uplift and magmatism, and basin formation during subsequent thermal relaxation.

Secular evolution of subcontinental lithospheric mantle

Chemical and petrographic observations on mantle-derived xenoliths and xenocrysts define a fundamental distinction between Archean cratonic mantle and Phanerozoic circumcratonic mantle. Archean xenoliths are more depleted on average, and those from South Africa and Siberia, the best-studied areas, have higher Si/Mg than Phanerozoic xenoliths of similar Mg#; subcalcic harzburgites are wellrepresented in Archean xenolith and xenocryst suites, but rare in younger ones. Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks. Typically, garnet suites from volcanics penetrating Archean crust have higher Cr#, lower Y, Y/Ga and HREE/Sc, and higher Zr/Y than those from volcanic rocks in Proterozoic and Phanerozoic terranes. These differences, and the rarity of subcalcic garnets in mantle beneath Proterozoic and Phanerozoic crust, are consistent with a decrease in mean modal cpx/gnt and (cpx+gnt) from Archean to Proterozoic to Phanerozoic mantle. Temperatures calculated from the Ni content of peridotitic garnets (TNi) can be referred to a local paleogeotherm to map the distribution of garnet types with depth. These maps show that Archean mantle sections are typically strongly stratified, with concentrations of harzburgitic rocks and depleted lherzolites in the middle to lower parts of the sections, and more fertile lherzolites at depth. Proterozoic mantle sections are less depleted and typically less strongly stratified, while the garnet-facies parts of Phanerozoic sections are still less depleted and essentially homogeneous. Inversion of TNi and composition data on garnets allows mapping of the vertical distribution of the XFo of coexisting olivine in mantle sections. Archean sections typically show decreasing XFo with depth; Proterozoic sections are less regular, and many show the lowest XFo in the upper parts of the sections. These differences imply changes in the processes that have produced subcontinental lithospheric mantle (SCLM) at different times in Earth history. In xenolith suites, the Cr content of peridotitic garnet correlates well with the Al content of the host peridotite, and Al is closely related to the contents of other major and minor elements. These correlations allow calculation of the mean composition of a mantle section, given the Cr content of garnet xenocrysts from that section. The calculated mean composition of SCLM beneath Archean, Proterozoic and Phanerozoic terrains shows a secular evolution in all measures of depletion, such as Al, Ca, Mg#, and Fe/Al; Proterozoic SCLM is intermediate in composition between Archean and Phanerozoic SCLM. Cenozoic SCLM, exemplified by garnet peridotites from young extensional areas (China, Siberia, Australia) is only mildly depleted (²10% melt extraction) relative to Primitive Mantle. Similar compositions are found in some exposed spinel lherzolite massifs in extensional settings, such as Zabargad Island. Shallow spinel-facies SCLM beneath some Phanerozoic terrains, especially in Europe (both xenolith suites and exposed massifs), is more depleted and may represent reworked Proterozoic SCLM; this is consistent with some Re- Os depletion ages. Archean xenolith suites show positive correlations between Al, Fe and Cr that are not present in younger mantle samples; these correlations indicate that no Cr-Al phase was present on the liquidus during the melting event that produced Archean mantle. We suggest that most Archean SCLM was derived by high-degree melting at depths ³150 km, and that subduction processes have not been significant in its genesis. Phanerozoic circumcratonic xenolith suites are typically much less depleted than ophiolitic and abyssal peridotites; this suggests that subducted oceanic mantle is not a major component of Phanerozoic SCLM. Instead, the fertile composition of circumcratonic xenolith suites, and comparison with the Zabargad situation, suggests that most existing Phanerozoic SCLM probably has been generated in extensional post-orogenic environments. Density modelling shows that Phanerozoic SCLM, once it has cooled to typical conductive geotherms, will be gravitationally unstable with respect to the asthenosphere. This instability provides a mechanism for removing such lithosphere and replacing it with upwelling fertile material on a short time scale. In contrast, typical sections of Archean and Proterozoic SCLM are gravitationally stable, allowing their long-term preservation. The broad correlation of SCLM composition with crustal age implies quasi-contemporaneous formation of crustal volumes and their underlying SCLM, and crust-mantle coupling over periods of aeons beneath Archean and Proterozoic terranes; it also requires an evolution in fundamental processes involved in the formation of continents and their roots. This evolution may be related to the secular cooling of Earth, and a consequent decrease in the melting temperature and depth of melting in mantle plumes.

The Composition of Sub-Continental Lithospheric Mantle: Garnet-Based Estimates

Data from xenoliths, garnet concentrates and peridotite massifs demonstrate secular evolution in the composition of subcontinental lithospheric mantle (SCLM), related to the last major tectonothermal event in the overlying crust. The garnet data show that subcalcic (cpx-free) harzburgites are restricted to Archaean mantle, and that the dominant lherzolites become progressively less depleted (in terms of major-element composition) from Archaean through Proterozoic to Phanerozoic time. This broad correlation of SCLM composition with crustal age implies quasi-contemporaneous formation of crustal volumes and their underlying SCLM, and crustmantle coupling over periods measured in aeons. In most xenolith suites, concentrations of majorand minor elements are well-correlated with A1203 contents, while in garnet peridotites there is a good correlation between the Cr203 content of the garnet and the A1203 content of the host rock. Algorithms relating garnet composition to bulk-rock composition allow calculation of mean SCLM compositions from garnet concentrates; this procedure gives good agreement with aveages or medians of large xenolith suites of both Archaean and Phanerozoic age (Table 1). Application of this approach to garnet concentrates (>13,000 analyses) from 28 regions of different crustal age yields estimates of mean composition ('Gnt-SCLM') for SCLM of Archaean, Proterozoic and Phanerozoic age (Table 2). Proterozoic GntSCLM is similar to averages of orogenic peridotite massifs and xenolith suites of known Proterozoic age. Phanerozoic Gnt-SCLM and garnet peridotite xenoliths are similar to Zabargad Island peridotites, but less depleted than the average of spinel peridotite xenolith suites from extensional regions with Phanerozoic crust; these suites may include relict older SCLM. Even if the spinel peridotite data are used as an estimate of mean Phanerozoic SCLM,