42 research outputs found

    Pb isotope variations in Archaean time and possible links to the sources of certain Mesozoic-Recent basalts

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    THE GENERATION OF CONTINENTAL-CRUST - AN INTEGRATED STUDY OF CRUST-FORMING PROCESSES IN THE ARCHEAN OF ZIMBABWE

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    The Archaean craton of Zimbabwe includes two major episodes of crust generation at 3.5 and 2.9 Ga recorded in the emplacement of tonalite-gneiss granitoids. A total of 180 samples of representative gneisses and massive tonalites and sills has been collected from three areas in the southern part of the craton, at Mashaba, Chingezi, and Shabani. These rocks have been analysed for major, trace, and rare earth elements to evaluate the effects of the fractional crystallization and partial melting processes in the generation of this segment of Archaean crust.Three groups are distinguished on the basis of their major and trace element contents, and they follow two main trends of differentiation: the sodic and the calc-alkaline (sensu stricto) trends. Group I samples are tonalitic in composition and follow a sodic trend characterized by decreasing CaO/Na2O ratios. Y and Sr behave as compatible elements and are negatively correlated with Rb. REE patterns are moderately fractionated with La/Yb(n)=4-23.5. The characteristics of this group have been described only in the Archaean craton from Swaziland. Group II is an intermediate Group with a marked decrease in Na2O/K2O with increasing differentiation, similar to the Archaean tonalite-trondhjemite-granodiorite suites from Finland or the Pilbara Block, Australia. Samples display biotite tonalite and trondhjemite compositions, and Y, Sr, and Rb are all incompatible. The REE patterns are strongly fractionated, with La/Yb(n)=23-44, and with small positive or negative Eu anomalies, as observed in other Archaean tonalite-trondhjemites. Group III is composed mainly of trondhjemites and granites similar to many post-Archaean granitoids: they follow a calc-alkaline trend (sensu stricto) with decreasing CaO/Na2O and Na2O/K2O. Sr and Y are incompatible, whereas Rb increases with differentiation. REE patterns are variably fractionated, with La/Yb(n) = 6-36, with high REE contents, and marked negative Eu anomalies.The above geochemical features are explained in a three-stage petrogenetic model. The first stage consists of 6-20% melting of upper-mantle peridotite and the generation of tholeiitic basalts, as observed in the associated greenstone belts. The second stage involves 4-25% partial melting of metamorphosed basalts with a Gt amphibolite (15-45% Pl + 30-50% Hb + 2-35% Cpx + 3-15 % Gt) residue resulting in the Group I samples, under water-unsaturated conditions at intermediate pressure (approximately 16 kbar), or with an eclogite residue to generate the parental magmas for the Group II rocks. The third stage is low-pressure fractional crystallization (&lt; 8 kbar) of liquids generated during this second stage, leaving a 19-20% Qtz + 36-42% P1 +/- 0-2% Hb +/- Mt cumulate for the more evolved Group II samples, and 55% fractional crystallization of a 14% Qtz + 37.6% P1 (An26) +/- 3.3% Bt + 0.1 % Ilm +/- 0.8% Mt cumulate for Group III samples. The highly fractionated REE patterns of the Group II rocks are inherited from the second stage of partial melting of the metamorphosed basalt source rocks with an eclogite residue. Thus Group II and III initial liquids were generated through partial melting of eclogite and Gt amphibolite, respectively. The genetic relationships between Group I sodic and Group III calc-alkaline suites are evaluated, with the latter resulting from various stages of fractional crystallization processes of parental magmas within the sodic suite.</p

    The importance of the moderately siderophile and volatile germanium in chondrites and planetary reservoirs to reconstruct planet formation

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    International audienceThe moderately siderophile and volatile elements are strong tracers of condensation, accretion-collision and metal-silicate differentiation processes in the earliest history of the solar nebula, and of terrestrial planets, the Moon, and asteroids, respectively. Among them, germanium, with T 50% cond = 825 K and metal-silicate partition coefficient of 17 for the Earth, show significant isotopic fractionation (Δ 74 Ge metal/silicate) at two scales, namely (1) at a meteorite scale between metal and silicate phases in ordinary chondrites or pallasites, and (2) at reservoir scales, between iron meteorites taken as proxies for core and the silicate Earth. Establishing the isotopic budget of planetary mantles (e.g.Earth, Mars, Vesta) and the Moon implies to determine the isotopic composition of chondrites as proxy for undifferentiated parent bodies. Here we present new high precision germanium isotopic data obtained on (1) bulk carbonaceous (CC) and ordinary chondrites (OC), (2) Howardite-Diogenite meteorites though to originate from Vesta, (3) Martian meteorites (shergottite, nackhlites) and (4) lunar basalt meteorite, They will be compared with Ge isotopic composition of iron meteorites and Earth silicate mantle (peridotites, basalts) [3,4] to assess parent bodies composition. Samples and methods: Variable quantities of sample, from 80 mg for CC (CI, CM, CV, CO), OC (H, L, LL) and martian meteorites (shergottite, nackhlites), to up to 2 g for Ge-depleted HED (unbrecciated and brecciated eucrites, diogenite) and lunar basalt, were analyzed for Ge concentrations at the SARM (ICP-MS, CRPG-Nancy), and for bulk Ge isotopic compositions by using Ge chemistry and MC-ICPMS techniques (Hydride Generator System coupled to NeptunePlus) developed at CRPG (δ 74/70 Ge NIST3120a ≤0.1‰, 2σSD) [1,2,3]. Results and discussion: OC and CC type chondrites present fundamental Ge isotopic dichotomy that follow O and Cr isotopic anomalies [4]. Bulk ordinary chondrites values display negative values from-0.51±0.16 ‰ in H OC,-0.29±0.05‰ in L to-0.33±0.14‰ in LL, that agree within error with data on metal phase of OCs (Florin et al., 2020), then confirming that the metal phase hosts the Ge isotopic budget. By contrast, carbonaceous chondrites have positive δ 74 Ge values and show exceptional large variations of ≈1‰, from CI (Orgueil) with the heaviest composition (δ 74 Ge=0.901±0.06‰) toward lighter composition in CV (Allende) (δ 74 Ge=+0.096±0.12‰). The δ 74 Ge values and matrix fraction (%) of OCs and CCs are positively correlated and describe a mixing line between CI composition and expected chondrule composition (Fig.1). In addition large Ge isotopic composition in CC are exceptionally well correlated with Δ 17 O (Fig. 1) and ε 54 Cr, then constraining the origin and/or processes that lead to Ge isotopic signatures in the Solar System. Planetary mantles, except the Moon, have Ge isotopic compositions that are lighter than the iron meteorites, thus confirming the positive Δ 74 Ge metal/silicate during core formation [2]. In addition, distinct Ge isotopic signatures are recorded: Mars has δ 74 Ge values that overlap terrestrial mantle and crustal values, and reveals a dichotomy between shergottites with higher δ 74 Ge, lower Ge contents (+0.84‰, 0.73 ppm) than nakhlites (+0.35‰, 2.2 ppm), thus emphazing degazing processes [6]. The highly depleted HED and lunar samples (Ge = <0.03 to 0.17ppm; 0.2 ppm, respectively) have higher δ 74 Ge values than the Earth: the δ 74 Ge of +1.07‰ of the non-brecciated HED samples would be consistant with evaporation during intense impact activity identified on Vesta, and the highest δ 74 Ge=+1.74‰ of the lunar sample could also include volatile loss during Moon formation
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