Showing papers by "Jan Kramers published in 2001"

Determination of molybdenum isotope fractionation by double‐spike multicollector inductively coupled plasma mass spectrometry

Abstract: [1] Molybdenum isotopic compositions are precisely determined by MC-ICP-MS measurements using a Mo double spike. The double spike is added prior to chemical purification, so that laboratory and instrumental mass fractionations are separated from natural mass-dependent fractionation. Fractionation is determined on four Mo mass ratios, providing an internal consistency check. The external standard reproducibility is at 0.06 per mil on the 98Mo/95Mo ratio (2 standard deviation)). Using a normal microconcentric nebuliser with a cyclonic spray chamber, the minimum quantity of Mo is ∼1 μg for high-precision results. A hydrothermal molybdenite shows fractionation of −0.3 per mil on the 98Mo/95Mo ratio relative to our standard (Johnson Matthey, 1000 μg/mL (±0.3%) ICP standard solution, lot 602332B). Fine-grained sediments show fractionation of 0.1 and −0.3 per mil on the 98Mo/95Mo ratio. The observed Mo isotope fractionation is small but resolvable with the presented high-resolution technique.

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

Abstract: 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.

The Smile of the Cheshire Cat

Abstract: Little is known about geological processes on Earth during the Hadean, over 4000 million years ago. In his Perspective, Kramers discusses evidence for crust formation during this early period. The new calibration of the lutetium-176 decay constant reported by Scherer et al. necessitates a new interpretation of existing data. It appears that continental crust existed as long as 4400 million years ago but that it was rare.