Zircon M127 - A Homogeneous Reference Material for SIMS U-Pb Geochronology Combined with Hafnium, Oxygen and, Potentially, Lithium Isotope Analysis
Faculty of Geosciences, Geography and Astronomy, University of Vienna; Beijing SHRIMP Centre, Chinese Academy of Geological Sciences; European Commission [MEXC-CT-2005-024878]; FWF Austrian Science Fund [P20028-N10, P24448-N19]; U.S. National Science Foundation [EAR 1524336]
Generation of early Archaean grey gneisses through melting of older crust in the eastern Kaapvaal craton, southern Africa
Abstract We report zircon ages, Hf-in-zircon isotopes as well as whole-rock geochemistry and Hf–Nd isotopic systematics for Palaeoarchaean grey gneisses of the Ancient Gneiss Complex of Swaziland, the oldest components of the Kaapvaal craton, southern Africa. The Hf-in-zircon isotopic compositions in these compositionally heterogeneous, multicomponent, migmatitic gneisses are highly variable, even in the oldest zircons dating back to 3.66 Ga, suggesting growth of zircon from melts representing a mix of juvenile and anatectic material derived from differentiated continental crust of Eoarchean to late Hadean age. In contrast, the initial Nd and Hf whole-rock isotopic compositions are frequent…
Generation of early Archaean felsic greenstone volcanic rocks through crustal melting in the Kaapvaal, craton, southern Africa
Abstract High-potassium felsic volcanic rocks interlayered with basalt and komatiite in early Archaean greenstone sequences in the Barberton Greenstone Belt of South Africa and Swaziland, previously considered to be derived from melting of mafic precursors, are shown to be the result of melting of significantly older felsic crust. This is documented by a combination of SHRIMP zircon dating with Hf-in-zircon and whole-rock Lu–Hf and Sm–Nd isotopic data. Zircons from felsic rocks of the oldest Barberton unit, the 3.53 Ga Theespruit Formation, yielded predominantly negative e Hf ( t ) -values, whereas whole-rock e Hf ( t ) - and e Nd ( t ) -data are slightly negative to slightly positive. Simi…
Rutile crystals as potential trace element and isotope mineral standards for microanalysis
Abstract The present paper reports trace element concentrations of 15 elements (V, Cr, Fe, Zr, Nb, Mo, Sn, Sb, Hf, Ta, W, Lu, Pb, Th and U) as well as Pb and Hf isotope data for four relatively homogeneous and large (centimeter size) rutile grains. Methods employed are SIMS, EMP, LA-ICP-MS, ID-MC-ICP-MS and TIMS. For most elements homogeneity is usually within ± 10% and occasionally variations are even narrower (± 5%), particularly in the core of two of the studied grains. The trace element concentrations of the grains span a broad compositional range (e.g., Zr concentrations are ca. 4, 100, 300 and 800 ppm). Provisional concentration values, calculated based on the homogeneity of the eleme…
Petrogenetic evolution of metabasalts and metakomatiites of the lower Onverwacht Group, Barberton Greenstone Belt (South Africa)
A well-preserved sequence, by Archean standards, of mantle-derived metabasalts and metakomatiites forms large parts of the lower Onverwacht Group of the Barberton Greenstone Belt (South Africa). To elucidate the origin of mafic and ultramafic rocks from this 3.55 to 3.45 Ga sequence, we present a comprehensive geochemical dataset including major and trace elements as well as Lu-Hf and Sm-Nd isotope compositions for a variety of metavolcanic rocks. These include metabasalts of the amphibolite-facies Sandspruit and Theespruit Formations as well as metabasalts and metakomatiites of the lower greenschist-facies Komati Formation. Based on their incompatible trace element patterns, the basalts of…
Hafnium-Neodymium isotope, trace element and U-Pb zircon age constraints on the petrogenesis of the 3.44–3.46 Ga Dwalile greenstone remnant, Ancient gneiss Complex, Swaziland
Abstract The geodynamic setting of the 3.44–3.46 Ga Dwalile greenstone remnant (DGR) of the Ancient Gneiss Complex (Swaziland) and its relationship to other Greenstone belts of similar age is investigated in this contribution. We present Hf-Nd isotope, major and trace element data for komatiites, komatiitic basalts, basalts, basaltic andesites as well as felsic volcanic and sedimentary rocks, all metamorphosed at amphibolite-facies grade. Major and trace element compositions of metabasalts and metakomatiites indicate crustal contamination. The best-preserved samples yield 176Lu-176Hf and 147Sm-143Nd isochron ages of ca. 3500 and 3460 Ma, respectively, in line with U-Pb zircon ages of interl…