LOW PRESSURE ADSORPTION OF Ar, Kr AND Xe ON KEROGEN: IMPLICATIONS FOR THE TRAPPING OF NOBLE GASES ONTO METEORITIC

Introduction: Carbonaceous chondrites contain trapped noble gases that are present in different carriers. The most abundant carrier, known as "planetary" also referred to as "Q" or "P1" [1] is formed of carbonaceous material, the nature of which remains ill-defined. It appears to be mainly composed of aromatic moieties linked by short aliphatic chains for which the best terrestrial analog may be kerogen [2]. Trapped noble gases in chondrites (Ar, Kr, Xe) share common characteristics. First, they are tightly bound (release temperature up to 1000°C) in the carbonaceous residue obtained after acidic attack (HF, HCl) of chondrites [3]. Second, these gases are easily released upon oxidation (using HNO3) which suggests that they may be located near the external surfaces of the organic materials. [3]. Third, the noble gas elemental abundances patterns are strongly fractionated relative to Solar with marked depletions of the lighter noble gases relative to the heavy ones. The mechanism(s) leading to the trapping of noble gases in carbonaceous carriers remains poorly understood. The elemental abundances of noble gases in Q from different types of meteorites exhibit comparable fractionated pattern, which supports the assumption that one single Q reservoir existed during the accretion of planetary bodies [1] or that common processing determined the characteristics of Q. Gas adsorption could represent one of the mechanisms leading to noble-gas concentration prior to their trapping. Previous adsorption experiments managed to reproduce the requested elemental abundance pattern but lead to very low amounts of adsorbed noble gases [4]. Here we report the results of an experiment designed to study the adsorption of Ar, Kr and of Xe at very low relative pressure, i.e. in pressure conditions more relevant than those used in [4] to the processes that could have occurred in the solar nebula. In addition, such experiments allow us to probe the most energetic surface sites of the materials investigated, i.e. the surface sites that are most likely playing a role in noble gas concentration. Low pressure adsorption isotherms were obtained for different mineral phases known to be present in Orgueil: ferrihydrite, montmorillonite and terrestrial type III kerogen (Q analog) [5]. Here we report the kerogen results as analogs for Q. Analytical: Adsorption isotherms of Ar and Kr at 77K and Xe at 195K were determined using a quasiequilibrium adsorption device [6] equipped with three precision pressure gauges (0-0.1 Torr; 0-10 Torr and 0-1000 Torr Baratron) allowing accurate measurements in the very low pressure range. In this set-up, the adsorbate is introduced through a calibrated microleak at a slow enough flow rate to ensure equilibrium all along the adsorption isotherm [6]. Around 1g of sample was outgassed overnight at room temperature under a residual pressure of 10 Torr. Surface areas were determined using the classical BET treatment, which allows to determine for each equilibrium pressure the surface coverage θ defined as V/Vm where Vm is the adsorbed volume at the monolayer derived from the BET equation. At very low pressure the adsorption isotherm is governed by Henry's law : θ = H*P (1) with :