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

Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa

On the Fern Pass rockslide (Eastern Alps, Austria), projecting boulders collected surface runoff and delayed percolation of water into the rockslide mass, leading to decimetre-scale, fluctuating, phreatic/vadose diagenetic systems along their contact. In these systems, aragonite and calcite precipitation were nourished mainly by dissolution of carbonate-rock flour. Cement precipitation was limited to southern- and eastern-exposed “runoff haloes” of boulders and mainly resulted in cemented breccias. Aragonite precipitation was related to dissolved Mg2+ and/or to high CaCO3 supersaturation in evaporative-concentrated pore waters. Early aragonite cement yielded a 234U/230Th age of 4,150 ± 100 years. Relative to other radiometric ages (36Cl, 14C; by other authors) for the rockslide event, the U–Th age of the aragonite is the most precise proxy of depositional age. Carbonate cements are present in other rockslide and rockfall deposits also. U–Th dating of such cements is thus a comparatively rapid and inexpensive method of minimum-age dating catastrophic mass movements.

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Aragonite and calcite cementation in “boulder-controlled” meteoric environments on the Fern Pass rockslide (Austria): implications for radiometric age dating of catastrophic mass movements

Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes;Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes

An open-system fractional crystallization model for very early continental crust formation

The Rhenosterkoppies Greenstone Belt, situated in the northern Kaapvaal Craton close to the Southern Marginal Zone of the Limpopo Belt, was studied by structural mapping, petrography, and geochronology. The belt consists mainly of a 300-500-m-thick alternating sequence of amphibolite and banded iron formation horizons, with intercalated talc-tremolite schist, rare metapelite, and strongly foliated sheets of mainly granodioritic gneiss. This mainly supracrustal sequence broadly defines an antiform with axis plunging shallowly to the east. Granodioritic gneisses are exposed in the centre of the antiform. Their contact with the supracrustal sequence is sheared. The gneisses have T DM model ages between approximately 2950 and approximately 3100 Ma, in agreement with T DM ages of gneisses from the northernmost Kaapvaal Craton and the Limpopo Belt Southern Marginal Zone in general. Four deformational phases were recognized, the most prominent of which produced a pervasive layer-parallel foliation and a consistent northeast-trending stretching lineation, and probably represents the regional north-vergent thrusting episode recognized elsewhere in the northern Kaapvaal Craton. The shearing occurred under upper greenschist to lower amphibolite-facies conditions. Upper amphibolite-facies relics are found throughout the greenstone belt. The shearing was dated directly at 2729+ or -19 Ma by stepwise leaching Pb/Pb analysis of synkinematic titanite. Dating of a retrograde post-kinematic titanite-epidote paragenesis within the granitoid gneisses in the core of the anticline yielded 2743+ or -21 Ma, placing a lower age limit of approximately 2720 Ma on this tectonism. It was followed by the updoming of the anticline, recognizeable as a separate event as the main lineation is rotated by it. In the northernmost part of the greenstone belt, close to the Hout River Shear Zone, a local medium-to high-pressure, lower amphibolite-facies metamorphic episode associated with localized metasomatism was dated at 2634+ or -49 Ma by Pb stepwise leaching of kyanite. This is consistent with other age constraints on the tectonics of the Southern Marginal Zone of the Limpopo Belt. North-vergent tectonism in the northernmost Kaapvaal Craton thus predates tectono-metamorphism in the Southern Marginal Zone by more than 50 Ma.

Jan Kramers

Papers

Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa

Aragonite and calcite cementation in “boulder-controlled” meteoric environments on the Fern Pass rockslide (Austria): implications for radiometric age dating of catastrophic mass movements

Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes;Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes

An open-system fractional crystallization model for very early continental crust formation

Structure, metamorphism, and geochronology of the Rhenosterkoppies greenstone belt, South Africa