Re-Os systematics of mantle xenoliths from the East African Rift: age, structure, and history of the Tanzanian craton

Chesley, John T.; Rudnick, Roberta L.; Lee, Cin-Ty A.

doi:10.1016/s0016-7037(99)00004-6

Cited by 203 publications

(150 citation statements)

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“…and comparable to averages from Tanzania as well as the Slave and Siberian cratons (Lee and Rudnick, 1999;McKenzie and Canil, 1999;Kopylova et al, 1999;, but much lower than low temperature peridotites from the Kaapvaal .…”

Section: Garnet Fades Peridotitessupporting

confidence: 65%

Major element and isotope geochemistry (Sr Nd and Hf) of mantle derived peridotites carbonatites and kimberlites from Canada and Greeland insights into mantle dynamics

Bizzarro¹

2003

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Illhérité de la differentiation initiale de la Terre. Notre étude confirme donc qu'une telle hétérogénéité était toujours présente aux environ de 3 Ga, mais plus elusive dans les roches juvéniles d'âge <2.7 Ga. Les nouvelles données isotopiques de l'Hf ici présentées favorisent une source et origine communes pour les carbonatites, kimberlites et certain basaltes océaniques. IV ABSTRACTWe report on the major element composition of a suite of kimberlite-hosted peridotite xenoliths from the northern margin of the western Archean craton of Greenland, as well as the isotopic composition (Nd, Sr and Hf) of carbonatites and kimberlites from eastern Canada and southwestern Greenland.Our results indicate that the lithospheric mantle underneath the Sarfartoq area of southwestern Greenland extends to approximately 225 km, and consists of refractory (olivine Mg # >0.9) harzburgites and lherzolites. Moreover, the Sarfartoq peridotites are not characterized by the OPX enrichment observed in peridotites from the Kaapvaàl craton of South Africa, and show more affinity with peridotites from east Greenland (Wiedemann Fjord), Somerset Island (Canadian arctic) and Tanzania. Detailed analyses of constituent minerals reveal that the lithospheric mantle underneath the Sarfartoq area is compositionally layered as follows: (1) an internally stratified upper layer (70 to 180 km) consisting of coarse, un-deformed, refractory garnet-bearing and garnet-free peridotites and, (2) a lower layer (180 to 225 km) characterized by fertile, CPX-bearing, porphyroclastic garnet lherzolites. The stratification observed in the upper refractory harzburgite layer (70-180 km) is reflected by an increase in fertility (e.g. lower olivine abundance and fosterite content) with depth. The sharp nature of the stratification may indicate a two-step process for the formation of this lithosphère, i.e. multistage growth of the lithospheric root.In Sr values suggests that apatite and carbonate precipitated predominantly under non-equilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger-(regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources, or from a common isotopically heterogeneous mantle. This analytical technique provides a rapid and effective approach to determine the extent of isotopic heterogeneity within a single sample or individual grains, with analytical precision approaching that obtained by conventional TIMS.High-precision Hf isotopic analyses and U-Pb ages of carbonatites and kimberlites from Greenland and eastern North America, including Earth's oldest known carbonatite (3 Ga), indicate derivation from an enriched mantle source. This previously unidentified mantle reservoir of unradiogenic Hf isotopic composition, preserved in the deep mantle for at least 3 billion years, may account for the mass imbalance in Earth's Hf-Nd budget. Our results show that the Early to mid-Archean terrestrial mantle was isotopically much more...

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Section: Garnet Fades Peridotitessupporting

confidence: 65%

Major element and isotope geochemistry (Sr Nd and Hf) of mantle derived peridotites carbonatites and kimberlites from Canada and Greeland insights into mantle dynamics

Bizzarro¹

2003

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“…This indicates that episodic events, as might be associated with underplating of a craton by plume material rather than gradual convective replacement of the base of the lithosphere, occurred. The laterally narrow Tanzania craton above 140 km depth has remained stable [Chesley et al, 1999]. Somerset Island at the edge of the Canadian shield has remained stable down to 150 km where the rocks are now 1100°C [Schmidberger et al, 2002].…”

Section: Evidence From Xenoliths and Magmasmentioning

confidence: 99%

Survival of Archean cratonal lithosphere

2003

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[1] I use thermal convection models to search for combinations of physical parameters that are compatible with the results of xenolith studies on the history and present thermal structure of cratonal lithosphere. The cratonal lithosphere above $180 km depth formed in the Archean and remained stable until recently sampled. The mantle adiabat cooled $150 K over this time. The temperature change across the rheologically active boundary layer at the lithospheric base is <300 K over a depth range of several tens of kilometers. Modern cratonal lithospheric thicknesses are relatively uniform, $225 km, much thicker than old oceanic lithosphere. Cratonal lithosphere is now in quasi-steady state with conductive heat flow to the surface in balance with heat supplied to the base of the lithosphere by convection driven by local temperature contrasts within the rheologically active boundary layer. This heat flow and the lithosphere thickness changed little after the cratonal lithosphere stabilized. One possibility is that chemically buoyant lithosphere forms a conductive lid above the convecting normal mantle. To survive, the chemical lithosphere also needs to be more viscous than normal mantle. A reasonable situation has chemical lithosphere a factor of 20 more viscous than normal mantle with weakly temperature-dependent viscosity (a factor of e over 100 K) from 0.2 Â 10 20 Pa s along the modern mantle adiabat. However, a chemical lid is unnecessary for lithospheric thickness to change slowly over time. For example, the base of the lithosphere may be stable if the viscosity at its base (along an adiabat) decreases rapidly with depth.

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“…The Sr and Nd isotope systematics of lavas from the rift valley provide evidence that the compositional heterogeneity is at least partly related to one or more chemical fractionations (depletions) in the Proterozoic, and a series of metasomatic events, one of which undoubtedly occurred shortly before and during the Cenozoic activity, others of which may be the result of subduction, accretion and magmatic events related to Archaean, Proterozoic and Panafrican orogenies (Norry et al 1980;Cohen et al 1984;Davies & Macdonald 1987;RehkaÈ mper et al 1997;Dawson & Smith 1988;Rudnick et al 1993;Chesley et al 1999;Burton et al 2000;SpaÈ th et al 2000).…”

Section: Comparisons With Ocean Island Basaltsmentioning

confidence: 99%

Magmatism of the Kenya Rift Valley: a review

MacDonald

2002

Transactions of the Royal Society of Edinburgh: Earth Sciences

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Tertiary±Recent magmatism in the Kenya Rift Valley was initiated c. 35 Ma, in the northern part of Kenya. Initiation of magmatism then migrated southwards, reaching northern Tanzania by 5±8 Ma. This progression was accompanied by a change in the nature of the lithosphere, from rocks of the Panafrican Mozambique mobile belt through reworked craton margin to rigid, Archaean craton. Magma volumes and the geochemistry of ma®c volcanic rocks indicate that magmatism has resulted from the interaction with the lithosphere of melts and/or¯uids from one or more mantle plumes. Whilst the plume(s) may have been characterised by an ocean island basalttype component, the chemical signature of this component has everywhere been heavily overprinted by heterogeneous lithospheric mantle. Primary ma®c melts have fractionated over a wide range of crustal pressures to generate suites resulting in trachytic (silica-saturated and -undersaturated) and phonolitic residua. Various Neogene trachytic and phonolitic¯ood sequences may alternatively have resulted from volatile-induced partial melting of underplated ma®c rocks. High-level partial melting has generated peralkaline rhyolites in the south±central rift. Kenyan magmatism may, at some future stage, show an increasing plume signature, perhaps associated ultimately with continental break-up. KEY WORDS: Plume±lithosphere interactionsThe Red Sea and the Gulf of Aden are two arms of a postulated triple junction centred on the Afar region (Fig. 1). Oceanic crust began to form under these arms c. 10 Ma. The third arm, the East African Rift System (EARS), extends southwards through Ethiopia, Kenya and Tanzania into Mozambique. Whilst lithospheric extension is occurring beneath the EARS, the rift has not developed to the stage of ocean crust formation. The EARS has come to be seen, therefore, as a prime location in which to investigate the early stages of continental break-up.This review focuses on the magmatism of the Kenya Rift section, used here to indicate that section of the EARS extending from northern Kenya to northern Tanzania. The primary aim is to demonstrate the complex polybaric evolutionary history of the magmatism, starting with the postulated plume-induced generation of primary magmas by variable degrees of partial melting of heterogeneous, metasomatised lithosphere, through to generation of salic magmas by assimilation±crystal fractionation processes over a range of crustal pressures and by partial melting of crustal protoliths, also over a range of pressures.

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Re-Os systematics of mantle xenoliths from the East African Rift: age, structure, and history of the Tanzanian craton

Cited by 203 publications

References 40 publications

Major element and isotope geochemistry (Sr Nd and Hf) of mantle derived peridotites carbonatites and kimberlites from Canada and Greeland insights into mantle dynamics

Major element and isotope geochemistry (Sr Nd and Hf) of mantle derived peridotites carbonatites and kimberlites from Canada and Greeland insights into mantle dynamics

Survival of Archean cratonal lithosphere

Magmatism of the Kenya Rift Valley: a review

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