Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

The composition of the Earth

The composition of the continental crust

We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.

Three natural zircon standards for u‐th‐pb, lu‐hf, trace element and ree analyses

Arguments against syn-magmatic sills in the Bushveld Complex, South Africa

The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

The Natal Group and Msikaba Formation remain relatively poorly understood with regards to their provenance and relative age of deposition; a much-needed geochronological study of the detrital zircons from these two units was therefore undertaken. Five samples of the Durban and Mariannhill Formations (Natal Group) and the Msikaba Formation (Cape Supergroup) were obtained. A total of 882 concordant U–Pb ages of detrital zircon populations from these units were determined by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Major Neoproterozoic and secondary Mesoproterozoic detrital zircon age populations are present in the detrital zircon content of all the samples. Smaller contributions from Archean-, Palaeoproterozoic-, Cambrian- and Ordovician-aged grains are also present. Due to the presence of a prominent major population of 800–1000 Ma zircons in all the samples, late Stenian – Tonian ancient volcanic arc complexes overprinted by Pan-African metamorphism of Mozambique, Malawi and Zambia, along with areas of similar age within Antarctica, India and Sri Lanka, are suggested as major sources of detritus. The Namaqua–Natal Metamorphic Complex is suggested as a possible source of minor late Mesoproterozoic-aged detritus. Minor populations of Archean and Palaeoproterozoic zircons were likely sourced from the Kaapvaal and Grunehogna Cratons. Post-orogenic Cambrian – Lower Ordovician granitoids of the Mozambique Belt (Mozambique) and the Maud Belt (Antarctica) made lesser contributions. In view of the apparent broad similarity of source areas for the Natal Group and Msikaba Formation, their sedimentation occurred in parts of the same large and evolving basin rather than localized in small continental basins, and the current exposures merely represent small erosional relicts.

Detrital zircon U–Pb ages of the Palaeozoic Natal Group and Msikaba Formation, Kwazulu-Natal, South Africa: provenance areas in context of Gondwana

In the Northern Calcareous Alps (NCA), meteoric cementation of Quaternary talus slopes was mainly sourced by dissolution of matrix and lithoclasts, by leaching of glacial till, and by groundwaters entered from underneath. Cement precipitation can take place within a few hundreds to a few thousands of years after talus deposition, but later diagenetic changes locally are indicated. Downslope along well-preserved talus successions, a change in prevalent diagenetic pathways is related to prolonged availability of pore waters from the apex to the toe of the slope. Talus slopes contain a significant proportion of carbonate mud probably produced by a combination of physical, chemical, and biological processes. 234U/230Th cementation ages of talus successions are scattered over a total range of 5–480 ka. The talus relicts of the NCA thus became cemented at highly different times during the late Quaternary. With the available data, we could not identify a specific palaeoclimatic significance of talus cementation.

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Meteoric diagenesis of Quaternary carbonate-rocky talus slope successions (Northern Calcareous Alps, Austria)

The conflict between independently published ages for the Mushandike Granite, Zimbabwe (2.92 ± 0.17 Ga and 3.45 ± 0.13 Ga) has been resolved in favour of the older age by SHRIMP U–Pb analyses of zircon. Two samples yield indistinguishable estimates of 3374 ± 7 and 3368 ± 11 Ma for the crystallization age of the magma. Together with published data from elsewhere in southern Zimbabwe, the results imply a widespread magmatic event at about 3.35 Ga. A single zircon core giving 3.46 Ga, together with the granite's previously measured Nd model age, suggests that the Mushandike magma could have incorporated remobilized basement similar to the c . 3.5 Ga Tokwe gneisses which crop out 30 km to the west. The published Rb–Sr and Pb–Pb datasets show evidence of late Archaean disturbance of Sr and Pb isotope systematics. In the absence of exposed contacts between the Mushandike granite and the neighbouring Mushandike stromatolitic limestone, the new U–Pb emplacement age suggests that the limestone is unconformable on the granite.

Jan Kramers

Papers

Arguments against syn-magmatic sills in the Bushveld Complex, South Africa

The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

Detrital zircon U–Pb ages of the Palaeozoic Natal Group and Msikaba Formation, Kwazulu-Natal, South Africa: provenance areas in context of Gondwana

Meteoric diagenesis of Quaternary carbonate-rocky talus slope successions (Northern Calcareous Alps, Austria)

The Mushandike granite: further evidence for 3.4 Ga magmatism in the Zimbabwe craton