Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr53d..05p&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR53D-05
Physics
1155 Extinct Radionuclide Geochronology, 3924 High-Pressure Behavior, 3944 Shock Wave Experiments, 5430 Interiors (8147), 8125 Evolution Of The Earth (0325)
Scientific paper
The metal-silicate fractionation in celestial bodies effectively separates siderophile 182W from lithophile 182Hf into the core and mantle, respectively, making the Hf-W chronometer ideal for dating core-formation in differentiated planetary bodies. It is generally believed that there was equilibration of the Hf-W system during primary metal-silicate fractionation of small, initially chondritic, parent bodies. However, the accretion of larger objects like Moon and terrestrial planets generally involves giant impacts, with both the target and projectile probably being differentiated. Then, use of the Hf-W system assumes metal-silicate re-equilibration at high T and P while metal droplets rain through the magma ocean of a growing planet. Currently no experimental data at such conditions exist. Here we report the first results of experiments aimed at studying partitioning of Fe and Ni (proxy for W) between metal and silicate melt formed at high P and T by laser shocks of powered mixtures of pure Fe metal and Ni-bearing ALM-2 dunite. The initial targets with variable metal/dunite ratios were lightly pressed into 6.3 mm pellets (1-3 mm thick) and subjected to single laser pulses (~120-600 J, ~1 nsec). The details of experiments are described in the accompanying abstract by Remo et al. Some shots produced ~ 1 mm round craters in partially preserved targets which have been studied by optical microscopy, SEM and EPMA. The craters have rather rough surfaces and blackened appearance. The SEM images show thin (1-3 microns) films or pockets (3-8 microns) of silicate melt with or without tiny metal blebs which weld together angular grains of olivine and metal. The olivine contains no Al2O3 and ~0.4 wt% NiO; the metal is pure Fe. The melt, besides being lighter in the BSE images, shows distinct compositional differences (higher Al2O3 and FeO, lower MgO and SiO2) from olivine. The NiO content in metal-free melt films and pockets is similar to that in the host olivine. Metal in the metal-bearing melt pockets typically contains 0.2-0.7 wt% Ni and 0.6-1.5 wt% Si, while the NiO content in the silicate melt is close or below the EPMA detection limit of ~0.05 wt%. Thus, our results provide clear evidence for rapid extraction of Ni from silicate melt into metal on a timescale of microseconds. Chemical analyses and further experimental details will be reported at the meeting.
Adams R. G.
Jacobsen Stein B.
Petaev Michael I.
Remo John L.
Sasselov Dimitar D.
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