Showing papers by "Jan Kramers published in 2007"

Hierarchical Earth accretion and the Hadean Eon

Abstract: Geochemical traces from the Hadean Eon and terrestrial siderophile and volatile element data are discussed in the light of the standard, or hierarchical, model of planetary accretion, which envisages growth of the planets by collisions of progressively larger protoplanets. Siderophile element depletion patterns can be explained by partial inheritance from early stages of the process, when metal cores resulted from melting in collisions of small planetesimals. The abundances and isotope data for hydrogen and nitrogen suggest a chondrite-type source rather than nebular gas, in accord with atmosphere loss in the high-temperature aftermath of the Moon-forming Giant Impact, after which late accretion replenished the atmosphere. Lead isotope patterns and anomalous 142 Nd/ 144 Nd ratios in early Archaean metasediments and 176 Hf/ 177 Hf ratios of 3.7–4.4 Ga old detrital zircons point to a vanished crust that persisted through much of the Hadean Eon. A working hypothesis is proposed that links these observations. Following the freezing of a magma ocean caused by the Giant Impact, the mantle would be gravitationally unstable and an overturn would occur, leading to the formation of a huge mafic crust. After the overturn the mantle could be stable and inactive for hundreds of million years.

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

Abstract: 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.