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

According to [1, 2], mica schists of the Korva Tundra Group (~1.91 Ga [3, 4]) of the Tana Belt were buried at the middle crustal level and later exhumed simultaneously with granulites of the Lapland Complex (~1.91 Ga [5]). The Kandalaksha Group (KG) is composed of higher-temperature rocks [1, 6], but their geodynamic evolution is similar to that of the Korva Tundra Group [2]. Since the KG rocks have not been previously dated in this area, their synchronous metamorphism with Lapland granulites has not been proved. To solve this problem, metamorphic PT conditions and U‐ Pb zircon ages were determined for two KG samples taken in the Pados‐Yavr interfluve. The results are presented in this work. The hypersthene‐biotite plagiogneiss (sample Pal-9) is a fine- to medium-grained light gray blastomylonitic rock with biotite schistosity and vague quartz‐plagioclase gneissosity. This appearance is related to uneven distribution of melanocratic and leucocratic minerals. The schistosity defined by the preferred orientation of biotite coincides with banding. The texture is lepidogranoblastic. Major rock-forming minerals (Qtz and Pl) in equal proportions compose from 50 to 80% of rock. The Bt content varies within 30‐40% in the melanocratic portions and within 5‐10% in the leucocratic portions. Orthopyroxene (10‐15%) is mainly accumulated in the melanocratic bands and lenses. Opx occurs as an accessory mineral in other parts of the rock. However, both forms of Opx lack mineral inclusions. Some large Opx grains are rimmed by Bt flakes. Secondary minerals are nearly absent. The garnet‐clinopyroxene crystalline schist (sample Lap-34) is a medium-grained dark green rock with a lenticular-banded structure and porphyritic heterogranoblastic texture. The rock is similar in appearance to eclogite. However, the Cpx composition does not correspond to omphacite. Its structure is defined by the uneven distribution of Grt and leucocratic minerals. Cpx is the major rock-forming mineral (up to 90%). Garnet accounts for ~10%. Qtz and Pl account for no more than 1‐2%. Locally, Cpx decreases and gives way to amphibole (up to 10‐15%). Ilmenite, rutile, and zircon are accessory minerals. The Grt porphyroblasts are unevenly distributed as intermittent chains, which are aligned with Pl-rich lenses and bands. Scarce inclusions in the Grt are typically small (up to 1 mm) Cpx, which occasionally form chains marking the surface of intergrown garnet grains in a porphyroblast. The representative analyses of equilibrium Cpx, Grt, and Pl are shown in Table 1. Occasionally, Grt contains Pl inclusions (~0.5 mm). In the Hbl-rich domains, Grt contains Hbl inclusions that are compositionally similar to the groundmass hornblende. Hornblende is typically absent or developed as thin (10‐15 µ m) interrupted rims around clinopyroxene. Only locally, Hbl occurs as small xenomorphic grains up to 0.2 mm across. Elongated grains with convoluted boundaries are less common. Xenomorphic Pl grains (up to 0.2‐0.3 mm across) more often occur as a rim in the Cpx-rich granoblastic matrix. The larger (up to 0.5 mm) equant Pl grains form lenticular aggregates from 1 × 10 to 1 × 15 mm in size. The garnet has a constant grossular mole fraction ( ) = 0.28), while the Mg index, X Mg = Mg/(Mg + Fe) decreases by 10 mol % from the core to the rims. Cpx lacks inclusions and occurs as two morphological types. The predominant type is represented by xenomorphic subequant or slightly elongated polygonal grains up to 0.5‐0.7 µ m across. The second type is represented by large (up to 5 × 4 mm across) weakly XCa

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Metamafic Rocks of the Tana Belt, Kola Peninsula: Isotopic Age and PT Conditions

Multiple isotope studies relating to cave development and landscape evolution in the Cradle of Humankind, South Africa

Author response: Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa

Nitrogen, Neon and Argon Isotopes in Hypatia, a Diamond Bearing Pebble from the Lybien Desert Glass Strewnfield


 An integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton.
 In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.

Jan Kramers

Papers

Metamafic Rocks of the Tana Belt, Kola Peninsula: Isotopic Age and PT Conditions

Multiple isotope studies relating to cave development and landscape evolution in the Cradle of Humankind, South Africa

Author response: Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa

Nitrogen, Neon and Argon Isotopes in Hypatia, a Diamond Bearing Pebble from the Lybien Desert Glass Strewnfield

Petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton