The Broken Belt: Meteorite Concentrations

Introduction: Since the first Antarctic meteorite concentrations were discovered more than 25 years ago, many theories regarding the role of iceflow in the production of meteorite concentrations have been put forward , and most agree on the basic principles [1-3]. These models suggest that as the East Antarctic icesheet flows toward the margins of the continent, meteorites randomly located within the volume of ice are transported toward the icesheet margin. Where mountains or subsurface obstructions block glacial flow, diversion of ice around or over an obstruction reduces horizontal ice movement rates adjacent to the barriers and creates a vertical (upward) component of movement. If local mechanisms for ice loss (ablation) exist at such sites, an equilibrium surface will develop according to the balance between ice supply and loss, and the cargo of meteorites is exhumed on a blue ice surface. The result is a conceptual " con-veyor belt " bringing meteorite-bearing volumes of ice from the interior of the continent to stagnant or slow-moving surfaces where ice is then lost and a precious cargo is left as a lag deposit. Cassidy et al. [4] provides an excellent overview of how this model has been adapted to several Antarctic stranding surfaces. Conveyor Belt Models: This " conveyor-belt " model actually incorporates two distinct meteorite concentration mechanisms, both of which reduce the volume of the ice substrate. Continued precipitation gradually compresses surface snow into ice at depth; typically this transition occurs at about 50 meters [5]. Conversion of snow to ice typically reduces the vertical dimension of a given stratigraphic sequence by a factor of 10x or more. As a result, a volume of deep glacial ice represents a much larger accumulation time than the same volume of surface snow, and thus contains a concentrated meteorite sample. Simple delivery of surface snows from one site to another does not concentrate meteorites; but the delivery of deep glacial ice through upward movement by the " conveyor-belt " models is a true concentration mechanism. Ablation: The second concentration process implicit in " conveyor-belt " models is a continuation of the theme of reducing ice volume , but taken to the extreme through physical loss of ice. In principle, the simple loss of ice through ablation should be enough to produce a meteorite concentration regardless of iceflow or volu-metric conversion of snow to ice [6]. As ablation removes surface snow and ice it leaves behind …