Subcratonic Lithospheric Mantle Research Articles

Phase equilibria and trace element modeling of Archean sanukitoid melts

Sanukitoids are Mg-rich, Late Archean granitoids, which are interpreted to have formed by the interaction of crustal and mantle melts possibly in a subduction zone setting. Geochemically and chronologically they seem to represent the transition between the tonalite-trondhjemite-granodiorite series rocks predominant in the Archean and the modern-style, subduction related magmatic suites. Studying their formation conditions can therefore provide important insights into the geodynamic processes shaping the cratonic crust in the Late Archean.We present a combination of phase equilibria and trace element modeling to study the melting conditions and protolith sources, which best reproduce the characteristic major and trace element concentrations of sanukitoid melts (high Mg#, V, Cr and Ni but enriched in LILE). After comparing our model to results from experiments for validation, we explore the formation conditions and melt sources for sanukitoids of two localities: the Amazonian craton and the Superior Province. Mixtures of 70% crustal melt (TTG) and 30% of relatively undepleted peridotite are most favorable to reproduce major and trace element characteristics of granodioritic sanukitoids (62–67wt.% SiO2). The P-T conditions of melting vary significantly with the composition of the crust–mantle mixture but range within 1100–1250°C and 2.0–3.2GPa generating relatively high melt fractions between 50% and 65%. Less differentiated sanukitoid melts (49–53wt.% SiO2) can result from either 50/50 or 70/30 crust–mantle mixtures, depending strongly on the composition of the TTG source. The addition of small proportions of carbonatite (∼5%) can account for the elevated concentrations of Ba, Sr, P, Eu and LREE in most sanukitoids of the Superior Province. In contrast to mere trace element models where the degree of melting and the P-T conditions are poorly constrained, the combination with phase equilibria calculations allows for a better-defined model that is consistent with formation conditions in the subcratonic lithospheric mantle or possibly a mantle wedge.

Relationships between oxygen fugacity and metasomatism in the Kaapvaal subcratonic mantle, represented by garnet peridotite xenoliths in the Wesselton kimberlite, South Africa

A suite of 12 peridotite xenoliths from the Wesselton kimberlite was studied and found to sample the subcratonic lithospheric mantle over a pressure range from 3.6 to 4.7 GPa and a temperature range of 880 to 1120 °C. Major, minor and trace element compositions indicate that both metasomatised and un-metasomatised samples are present over this pressure range. Fe3 +/∑ Fe in garnet from four xenoliths was determined using Fe K-edge XANES spectroscopy, enabling the redox state of the sampled subcratonic mantle to be determined for three garnet bearing samples. ΔlogfO2[FMQ] varied from 0 to − 3.3 over the sampled pressure interval, with the un-metasomatised samples falling within the global trend of decreasing ΔlogfO2[FMQ] with increasing depth. Superimposed on this was an oxidation trend, at higher pressures (≥ 4.5 GPa), with ΔlogfO2 increasing by 1.5 to 2 units in the metasomatically enriched samples, indicating a clear link between metasomatism and oxidation. One potential source of this oxidation is a carbonated silicate melt, which will increase in carbonate content as ΔlogfO2 increases. Mantle minerals affected by such a melt have the potential to shift from the field of diamond stability into that of carbonate, threatening the stability of diamond.

Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa

To constrain the relations between deformation and metasomatism in the subcratonic lithospheric mantle, we have analyzed the microstructures and crystal preferred orientations in 50 mantle xenoliths from the Kaapvaal craton. Water contents in olivine and pyroxenes were measured in 14 samples equilibrated at different depths. Coarse-granular microstructures recording deformation by dislocation creep followed by annealing predominate. Mylonitic (sheared) peridotites with partially or totally recrystallized microstructures are however common below 140km. Refractory compositions predominate, but multiple metasomatic events resulted in orthopyroxene enrichment or secondary crystallization of clinopyroxene and phlogopite. Coherent orthopyroxene and olivine CPO in most coarse-grained peridotites implies in pre- to syn-kinematic orthopyroxene enrichment or epitaxial growth on primary orthopyroxene. Undeformed, interstitial orthopyroxene, clinopyroxene, and phlogopite with random orientations in coarse-grained peridotites record post-kinematic modal metasomatic events. Deformation of these phases in the sheared peridotites implies that mylonitization results from a later event, which affected locally the deep cratonic lithosphere. Olivine CPO recording dominant [100] glide predominate at all depths. Only two samples, equilibrated at ~3.3GPa show olivine [001] and orthopyroxene [001] axes subparallel, suggesting dominant [001] glide. Water contents in olivine are maximum (150wt.ppmH2O) in peridotites equilibrated at ~160km depth. Peridotites equilibrated below 180km depth are, in contrast, almost dry. Lack of correlation between olivine mg# and water content indicates that the high water contents in olivine record re-hydration after the extensive partial melting, which produced the cratonic root. The vertical variation in water contents in olivine observed in the Kaapvaal peridotites may result from hydrogen addition or loss during extraction by the kimberlites. Comparison with magnetotelluric electrical conductivity data suggests, however, that the observed vertical variation of water contents in olivine may be representative of the present-day state of the Kaapvaal mantle, implying that extensive metasomatism resulted in hydration of the cratonic mantle at intermediate depths. The annealed microstructures of Kaapvaal peridotites indicate however that this metasomatism was not followed by remobilization of the cratonic root.

Origin of the stratification of cratonic mantle lithosphere

Compiled PTX diagram and trans-sects for sub-cratonic mantle lithosphere (SCLM) >of 150 kimberlitic pipes including those from Yakutia. Major regularities were determined for by the comparison of the PTX diagram for the subcratonic lithospheric mantle (SCLM) according to mantle xenoliths from large amount of the kimberlite pipes from N America Yakutia, Baltica , South and Central Africa. Proterozoic pipes from Africa like Premier Roberts Victor show in general higher heating degree which is visible especially by the by the high temperature PT subadiabatic arrays for SCLM beneath Premier from the deep level at 70 kbar (Fig. 1). The diamond inclusions and diamond eclogites from Roberts Victor support this thesis. (Fig 2). But common pipes beneath Kaapvaal craton refer to 40mw/m2 geotherm in the middle part and heating at 60 kbar and different PT paths for the mantle eclogites. Diamond inclusions reveal colder and deeper conditions and several PT paths.

Continent Formation in the Archean and Chemical Evolution of the Cratonic Lithosphere: Melt-Rock Reaction Experiments at 3-4 GPa and Petrogenesis of Archean Mg-Diorites (Sanukitoids)

To better understand the genetic relationship between granitoid rocks of the Archean cratons and the underlying ‘keel’ of subcratonic lithospheric mantle (SCLM), we have conducted two types of experiments in a multi-anvil apparatus at 3–4 GPa: (1) peridotite assimilation experiments, in which natural, hydrous TTG (trondhjemite–tonalite–granodiorite) melts are reacted with mantle peridotite at relatively high melt–rock ratios; (2) liquidus saturation experiments on hydrous Mg-rich diorite (sanukitoid) melts. Our results demonstrate that liquids similar to typically late- to post-tectonic sanukitoid intrusions can form by hybridization of initial TTG melts by assimilation of olivine-bearing peridotite, and that these primitive granitoid melts are in equilibrium with reaction residues consisting of olivine-free garnet websterite or garnet pyroxenite. The experimental melts retain the distinctive trace element signature of TTGs, overprinted by a ‘primitive’ mantle signature (i.e. high Mg-number, elevated Cr and Ni abundances), whereas the various phases of the crystalline residues acquire trace element signatures reflecting equilibration with Mg-rich granitoid melts. At low melt:rock ratios, metasomatism by TTG melts may be responsible for the silica enrichment and high modal orthopyroxene content of some cratonic peridotites and cryptic trace element overprints in garnet, clinopyroxene, and orthopyroxene. Our results demonstrate that the lithospheric keel of Archean cratons represents the product of reaction between TTG melts and previously depleted mantle peridotite at relatively low melt:rock ratios, as evidenced by the trace element signature in garnet pyroxenite and orthopyroxene-enriched garnet peridotite xenoliths, whereas Late Archean sanukitoids represent the products of these same reactions at relatively high melt:rock ratios.

Formation of diamond in the Earth’s mantle

The principal sources of natural diamonds are peridotitic (about2/3 of diamonds)and eclogitic (1/3) domains located at 140–200 km depth in the subcratonic lithospheric mantle. There,diamonds probably form during redox reactions in the presence of melt (likely for eclogiticand lherzolitic diamonds) or under subsolidus conditions in the presence of CHO fluids(likely for harzburgitic diamonds).Co-variations of δ13C and the nitrogen content of diamonds suggest that two modes of formation mayhave been operational in peridotitic sources: (1) reduction of carbonates, thatduring closed system fractionation drives diamond compositions to higherδ13C values and lower nitrogen concentrations and (2) oxidation ofmethane, that in a closed system leads to a trend of decreasingδ13C with decreasing nitrogen. The present day redox state of subcratonic lithospheric mantle isgenerally too reduced to allow for methane oxidation to be a widespread process.Therefore, reduction of carbonate dissolved in melts and fluids is likely the dominantmode of diamond formation for the Phanerozoic (545 Ma–present) and Proterozoic(2.5 Ga–545 Ma). Model calculations indicate, however, that for predominantly Paleoarchean(3.6–3.2 Ga) to Mesoarchean (3.2–2.8 Ga) harzburgitic diamonds, methane reductionis the principal mode of precipitation. This suggests that the reduced presentday character (oxygen fugacity below carbonate stability) of peridotitic diamondsources may be a secondary feature, possibly acquired during reducing Archean (>2.5 Ga) metasomatism. Recycling of biogenic carbonates back into the mantle through subductiononly became an important process in the Paleoproterozoic (2.5–1.6 Ga) and diamondsforming during carbonate reduction, therefore, may predominantly be post-Archean in age.For eclogitic diamonds, open system fractionation processes involving separation of aCO2 fluid appear to dominate, but in principal the same two modes of formation(methane oxidation, carbonate reduction) may have operated. Direct conversionof graphitized subducted organic matter is not considered to be an importantprocess for the formation of eclogitic diamonds. The possible derivation of 12C enriched carbon in eclogitic diamonds from remobilized former organic matter is, however,feasible in some cases and seems likely involved, for example, in the formation ofsublithospheric eclogitic diamonds from the former Jagersfontein Mine (SouthAfrica).

An alternative model for silica enrichment in the Kaapvaal subcontinental lithospheric mantle

The Mg- and Si-rich nature of the sub-cratonic lithospheric mantle (SCLM) beneath the Kaapvaal Craton indicates extensive melt depletion, followed by a Si-enrichment process. Six highly silica enriched peridotites from Kimberley containing high amounts of orthopyroxene (Opx) or garnet (Grt) that are locally concentrated in clots, were investigated to constrain the timing and nature of the Si-enrichment process. A clinopyroxene-bearing lherzolite containing an Opx-clot was studied to quantify the effects of recent metasomatism on the Si-enriched samples. Minerals from the lherzolite, together with Opx from harzburgites and Opx- and Grt-clots have Hf–Nd isotope ratios at the time of kimberlite eruption, 90 Ma, comparable to group I kimberlites and are close to trace element equilibrium with kimberlitic melts. This implies the xenoliths underwent major interaction with kimberlitic melts close to the time of kimberlite eruption. Harzburgites and mineral clots record equilibration pressures and temperatures of, respectively, between 3.5–4.3 GPa and 930–1060 °C. The garnets in Opx-clots have low Lu/Hf and ε Hf(t) −15, whereas garnets from Grt-clots have high Lu/Hf and ε Hf(t) +10. In contrast, Grt from both Grt- and Opx-clots have low Sm/Nd and ε Nd −10. The whole rock platinum group element (PGE) concentrations are an order of magnitude higher in the Grt-clot than the Opx-clot. Measured 187Os/ 188Os range from 0.1085 to 0.1222. The Grt-clot bearing sample yields Nd–Hf–Os isotope model ages that suggest formation in the Neoproterozoic (∼650 Ma). In contrast, an Opx-clot yields T RD ages of 2.8 Ga, which is interpreted as the time of formation of the host harzburgite. The Opx-clots and host harzburgites have comparable Lu–Hf isotope systematics that imply Opx growth at ∼1.3 Ga and hence their formation is not related to the Grt-clots. Garnets from Opx- and Grt-clots have elevated high-field strength element (HFSE) concentrations, and lack HFSE depletion relative to other trace elements with comparable degrees of incompatibility in the mantle (La/Nb < 0.5). In addition, calculated melts in equilibrium with Grt have strongly fractionated REE (Nd/Yb > 300) and HREE depletion (Yb N < 0.1) suggesting equilibration with a hydrous melt that is more HREE depleted than a kimberlitic melt. Previous models that related Si-enrichment to subduction are inconsistent with the lack of HFSE depletion (La/Nb < 0.5). Therefore the favoured model for Opx- and Grt-clot formation is infiltration of a hydrous melt in a within plate geodynamical environment associated with volcanism in the Mid-proterozoic and Neoproterozoic, respectively. This implies that Si-enrichment of the Kaapvaal SCLM may be a consequence of numerous localised magmatic events rather than a single craton-wide process.

Formation and temporal evolution of the Kalahari sub-cratonic lithospheric mantle: Constraints from Venetia xenoliths, South Africa

The ~533 Ma Venetia Diamond Mine is located between the Kaapvaal and Zimbabwe Cratons and the study of selected xenoliths provides the opportunity to investigate the temporal evolution of the sub-continental lithospheric mantle (SCLM) underneath southern Africa, as well as the extent and potentially the timing and nature of the Si-enrichment that characterizes the Kaapvaal SCLM. Most peridotite xenoliths contain 15–25% orthopyroxene, confirming Si-enrichment of the Venetian SCLM. Mineral major element compositions for 36 peridotitic mantle xenoliths record less melt depletion than inclusions in Venetian diamonds (e.g., olivine Mg# 88.7–93.4; mode 92.5). Olivine Mg# suggest on average ~40% melt extraction and reconstructed whole rock HREE concentrations can be modelled by 20% fractional melting in the garnet stability field followed by ~10 to 13% in the spinel stability field. Calculated Nb/Sr and Ce/Yb ratios for melts in equilibrium with garnet and clinopyroxene suggest that the xenolith suite underwent metasomatism by both hydrous fluids and kimberlite/carbonatite-type melts. Garnet Nd ( T CHUR(Nd) = 2.1 Ga) and Hf ( T CHUR(Hf) = 1.8 Ga) model ages for one sample with an exclusively hydrous metasomatic character are indistinguishable from previously reported Re–Os ages of Venetian peridotitic and eclogitic diamond inclusions. Based on a geochemical and isotopic approach, we propose that the Venetian SCLM formed by shallow melting in the Archaean followed by lateral accretion. Hydrous fluids, either associated with remobilisation due to regional heating or subduction, led to metasomatism and possibly to Si-enrichment at ~2.0 Ga before a final stage of metasomatism associated with kimberlite magmatism.

Simultaneous generation of Archean crust and subcratonic roots by vertical tectonics

Archean cratons are characterized by granite-greenstone belts, by abundant tonalites, trondhjemites, and granodiorites (TTGs), and by the presence of strong and buoyant subcratonic lithospheric mantle (SCLM). Individually, mechanisms for their formation remain controversial, and together they provide no clear picture of the tectonic style prevalent during the late Hadean and Early Archean. Following earlier observation that the dome-and-keel structure of granite-greenstone terranes resembles salt diapirs, we present numerical calculations that show that in pre-Archean to Middle Archean thermal regimes, mafic to ultramafic volcanics overlying a felsic basement will overturn diapirically in as few as 10 Ma, displacing as much as 60% of the volcanics to the lower crust. This suggests that diapirism may have dominated Hadean and Early Archean crustal tectonics. Furthermore, repeated cycles of bimodal volcanism and crustal recycling by diapirism provide a mechanism for making TTGs, leaving a restite that contributes to the SCLM from above. This diapiric tectonism later declined as the Earth cooled and plate tectonics became the dominant paradigm.

Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism

The oxidation state, reflected in the oxygen fugacity (fO2), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO2 FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO2 trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO2 = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred.

The origin of cratonic diamonds — Constraints from mineral inclusions

The origin of cratonic diamonds is reviewed on the basis of nearly 5000 analyses of silicate, oxide and sulphide inclusions in diamonds. Compositional fields are defined for common minerals of the peridotitic, eclogitic and websteritic inclusion suites and used to establish the characteristics of diamond source rocks in the subcratonic lithospheric mantle. Peridotitic inclusion compositions overlap with the record established from cratonic garnet peridotite xenoliths and xenocrysts but reflect overall higher levels of depletion in basaltic components. The interior of the Kaapvaal Block (Kalahari Craton) is by far the best studied diamond source region in the world but appears to be unique rather than representative because of extreme levels of chemical depletion preserved in the peridotitic inclusion suite. Major and trace element characteristics of peridotitic diamond sources indicate polybaric melt extraction proceeding from the garnet into the spinel stability fields, most readily explained by protolith depletion in Archean mid-oceanic ridge environments. Eclogitic mineral inclusions broadly reflect basaltic source compositions and show chemical trends that are indicative of igneous fractionation and cumulate enrichment in magmatic precursors. In agreement with mounting evidence from xenolith studies, eclogitic diamond sources are linked to subducted oceanic protoliths. A more mafic character relative to present-day MORB may relate to (i) higher degrees of partial melting in Archean or early Proterozoic spreading centres and (ii) secondary melt depletion during subduction, or after emplacement in the subcratonic lithosphere. In line with a subduction origin of eclogitic diamond source rocks, mines with predominantly eclogitic diamond populations are generally (but not invariably) associated with craton margin settings or lithosphere with a post-Archean tectonothermal history. The websteritic suite is poorly defined and reflects a range of broadly pyroxenitic source rocks intermediate between peridotite and eclogite. Geothermometry, based on inclusions and nitrogen aggregation in diamonds, indicates that crystallization and mantle storage of peridotitic, eclogitic and websteritic diamonds occurred under the same thermal conditions. Geobarometry for peridotitic inclusions shows that the majority of diamonds formed at depths of less than 200 km along model geotherms corresponding to 38 to 42 mW/m 2 surface heat flow. Lower geothermal gradients observed for diamonds from the Kalahari and Slave cratons likely represent re-equilibration of touching inclusion pairs to cooling ambient conditions, suggesting that diamond formation was accompanied by transient heating events. Diamond precipitation is interpreted to have occurred during metasomatic events under super-solidus (melt dominated) and sub-solidus (CHO-fluid dominated) conditions. Increasing evidence for a reduced character of the subcratonic lithospheric mantle implies that diamond precipitation through redox reactions requires upward migration of carbonate-bearing melts/fluids. In such a redox scenario high solubility of sulphate relative to sulphide in melts/fluids may provide an explanation for a high abundance of sulphide inclusions as a consequence of co-precipitation with diamond in response to decreasing oxygen fugacity. Such comparatively oxidized metasomatic agents cannot derive from the reduced deep upper mantle and, therefore, likely relate to recycling of oceanic lithosphere.

Accretion of arc-oceanic lithospheric mantle in the Mediterranean: Evidence from extremely high-Mg olivines and Cr-rich spinel inclusions in lamproites

Si-rich Mediterranean type lamproites (48–56 wt.% SiO 2) are olivine-phyric, mantle-derived volcanics, in which both phenocrystic and xenocrystic olivine are present. Here we demonstrate the phenocrystic origin of the most extremely NiO–MgO enriched olivine in lamproites with Mg# up to 0.95, that host Cr-rich (Cr# around 0.95) spinels. Our comprehensive study of olivine–spinel pairs from Mediterranean lamproites enables us to constrain the extent of depletion of their mantle source. Olivine–spinel pairs from primitive Mediterranean lamproites plot in the most refractory part of the olivine–spinel mantle array diagram, showing even more refractory character than mineral pairs from boninites and cratonic mantle xenoliths. This indicates involvement of an ultra-depleted mantle component which had previously lost up to 40% basaltic components. These characteristics would fit the involvement of depleted subcratonic lithospheric mantle which underwent komatiite extraction in the Archaean, but mantle of such composition and age is known neither from ultramafic xenolith suites nor in tectonically emplaced ultramafic massifs in western and central Europe. Here, we explore the possibility that the extremely depleted component of the mantle source of Mediterranean lamproites is derived from an island-arc oceanic lithosphere accreted during Alpine collisional processes. This may provide compelling evidence for recent accretion of oceanic lithospheric blocks during Mesozoic subduction–collision processes in the Mediterranean.

Diamonds from Jagersfontein (South Africa): messengers from the sublithospheric mantle

The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ13C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions.

Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1

The enriched mantle 1 (EM-1) component in ocean–island basalts (OIB, e.g., Kerguelen, Pitcairn and Walvis Ridge) has been attributed to melting in upwelling mantle plumes of either: (i) shallow-recycled delaminated subcontinental-lithospheric-mantle or crust; or (ii) deep-recycled metasomatised lithosphere, oceanic plateau or oceanic crust plus a few percent of pelagic sediment. We present new geochemical data for OIB samples from the central South Atlantic; these include 100 to 30 Ma alkali and tholeiitic basalts from the Walvis Ridge and Rio Grande Rise and < 3 Ma basanites and basalts from Tristan da Cunha, Inaccessible and Gough. Additionally, we have analysed Cretaceous mafic–potassic magmas from south-west Africa and eastern South America in order to establish the compositional variation of metasomatised lithospheric mantle that may have been delaminated during Gondwana break-up. The results of our rare-earth-element inversion and Sr-, Nd- and Pb-isotopic mixing models suggest that the ‘depleted’ mantle plume component resembles FOZO and that the composition of the ‘enriched’ mantle component in central South Atlantic OIB has varied both spatially and temporally. At least three different enriched mantle end-members are required to explain the compositional range of 100 to 30 Ma OIB magmas. These resemble the source regions of mafic–potassic magmas from: (i) the Congo craton and Damara belt of south-west Africa; (ii) the São Francisco craton and Brasilia belt of south-east Brazil; and (iii) the Rio Apa–Luis Alves craton of southern Brazil and Paraguay. The most isotopically enriched EM-1 basalts ( ɛNd = − 0.8 to − 4.5), generated on the Walvis Ridge and Rio Grande Rise between 89 and 78 Ma, appear to contain a 10% to 15% contribution from a melt source region with low ɛNd, 206Pb / 204Pb and high [La / Nb] n , similar in composition to metasomatised subcratonic lithospheric mantle beneath southern Brazil and Paraguay. Reconstructions of plate motions indicate that, at the time of continental break-up, this would have been located above the sites of subsequent EM-1 melt generation. Shallow-mantle recycling of metasomatised lithosphere delaminated from south-west Africa and south-east Brazil may explain the further variations in trace-element and isotopic ratios of 100 and 80 to 30 Ma basalts from the Walvis Ridge, respectively. Recent magmas from Tristan, Inaccessible and Gough contain a melt contribution from an enriched mantle source with high 206Pb / 204Pb, 208Pb / 204Pb and 87Sr / 86Sr ratios and low ɛNd, relative to bulk-Earth. This does not resemble the isotopic composition of mafic–potassic magmas from continents adjacent to the South Atlantic and we therefore propose that it is a deep-recycled mantle component. The high Pb-isotopic ratios of recent Tristan plume magmas are inconsistent with a melt contribution from recycled oceanic crust plus a few percent pelagic sediments. We suggest that the deep mantle component is recycled metasomatised lithospheric mantle. Recycled metasomatised lithospheric mantle, of variable composition, only appears to have been an intrinsic part of the sub-oceanic Tristan mantle plume and the bulk-rock composition of present-day Tristan plume related melts are therefore inappropriate for modelling the contribution of plume-derived melts to the Paraná–Etendeka continental flood-basalts. This relationship may also apply to other large igneous provinces and ocean islands along their associated hot spot tracks.

EVIDENCE FOR FORMATION AND MODIFICATION OF THE KAAPVAAL CRATON (SOUTHERN AFRICA) IN ARCHAEAN SUBDUCTION ZONES

Considerable progress has been made in the understanding of craton formation and evolution in southern Africa in recent years, thanks to newly available geophysical and geochemical data. Recent tomographic studies and Re-Os analyses of on- and off-craton mantle xenoliths have clearly shown that the Kaapvaal craton is underlain by a, cold, depleted lithosphere at least 200 km thick that was stabilized in the Archaean. Several important questions regarding the consolidation and modification of the lithosphere, however, remain unanswered or highly debated. Firstly, the sub-cratonic lithospheric mantle (SCLM) is distinct from oceanic and most continental mantle in that it is more depleted in magmaphile elements (Fe, Al, Ca, HREE), but at the same time strongly enriched in incompatible trace elements like LILE and LREE. The Kaapvaal SCLM also has a distinctly higher Si/Mg ratio and therefore higher modal orthopyroxene content (at a given Mg/Fe) than oceanic and most continental upper mantle. Resolution of this issue is critical to understanding the mechanism of lithosphere formation. We carried out a detailed petrological and geochemical study on low-temperature peridotite xenoliths from Kimberley (located at the craton core) and northern Lesotho (at the south-eastern craton edge) in order to obtain information about the processes that led to the depletion and re-enrichment of the Kaapvaal SCLM. Samples have been characterized for Re-Os isotope systematics, major and trace element concentrations in whole rocks and minerals, and garnet and clinopyroxene Lu-Hf, Sm-Nd and Rb-Sr isotopes. The combined results require a multistage history of the Kaapvaal mantle. They are most consistent with a model that involves introduction of orthopyroxene and trace element enrichment of the garnets by aqueous fluids, possibly derived from a subducting oceanic slab. This is consistent with the observed enrichment in Si (orthopyroxene) and the Re-Os systematics. Infiltration of hydrous fluids would also increase the degree of mantle melting and could therefore explain the strong major and HREE depletion of the SCLM. The Nd-Hf isotope characteristics of the garnets require the trace element enrichment to be ancient. We therefore suggest that melting and metasomatism of the Kaapvaal SCLM took place in subduction zone settings, probably during amalgamation of smaller pre-existing terranes in the Late Archaean (~2.9 Ga). Trace element and Nd and Hf isotope disequilibrium between garnet and clinopyroxene is preserved in many samples and indicates that garnet and clinopyroxene are not cogenetic. Calculated equilibrium liquids and Hf-Nd isotopic compositions for clinopyroxene suggest that most diopside in the xenoliths studied crystallized from an infiltrating kimberlite- like melt, either during group II kimberlite magmatism at ~200-110 Ma ago (Kimberley) or shortly prior to eruption of the host kimberlite around 90 Ma (N. Lesotho).

Modification of cratonic lithosphere by major tectono-magmatic events: implications for the survival of Archaean cratons

The majority of the Sub-Cratonic Lithospheric Mantle (SCLM) has been shown to be of Archaean age (Pearson, 1999). This implies that the SCLM plays a fundamental control in stabilizing and preserving overlying continental crust and isolating significant volumes of mantle from the convecting asthenosphere (Jordan, 1988). It is clear from the consideration of accretion models that, over time, Archaean crust has been destroyed or modified by erosion and/or major tectono-magmatic events. Determining the processes that have modified the CLM is therefore fundamental to our understanding of the evolution of continental crust and the mantle. The aim of this project is to understand the factors/ processes that have led to the local preservation/destruction of Archaean continents. For this reason CLM xenoliths suites will be examined from two geodynamic settings that have been subjected to major tectono-magmatic events. One craton survives (Bushveld Province) while the other becomes rifted (Tanzania). In Kaapvaal craton, recent geophysical and petrological studies show a large anomaly beneath Premier, a region affected by 2.05 Ga Bushveld magmatism. The 1.2 Ga Premier pipe provides a natural probe of the mantle in this area. Later, Northern Tanzania xenoliths will be studied as they are from a region where an Archaean craton (Chesley et al., 1999) is actively rifting. These xenoliths will provide information on how SCLM is modified by magmatic and metasomatic events before a craton no longer acts as a stable continental core. To assess if differences between these two tectonic settings are the result of SCLM modification, we need to understand what percentage of the SCLM has been modified by what processes and the changes with time. Our approach consists of petrology, major and trace element, and multi-isotope studies related to detailed spatial sampling of mineral domains. Whole rock and Re-Os analyses will provide strong mass balance constraints on compositions and ages of the CLM at different depths. Petrology studies will allow calculation of pressure corrected xenoliths densities. Because P estimates can be used to constrain the depth of origin of xenoliths, we can therefore assess the extent to which the CLM has been modified at different depths and then model the predicted thermal and mechanical response of the craton. Whole rock and mineral compositional data will be presented from Premier, allowing an estimate of composition with depth. Preliminary Re-Os, PGE and trace elements in minerals from the first part of the suite will be discussed. Isotopic analysis will be initiated later this year.

The Geochemistry of the Platinum-Group Elements (PGE) in Kimberlites and the Nature of the PGE in the Subcratonic Lithospheric Mantle

The Geochemistry of the Platinum-Group Elements (PGE) in Kimberlites and the Nature of the PGE in the Subcratonic Lithospheric Mantle