Impact - Induced Devolatilization and Implications for Planetary Accretion Hydrogen Isotopic Fractionation of Serpentine

Impact-induced devolatilization of porous serpentine was investigated using two independent experimental methods, the gas recovery method and the solid recovery method, each yielding nearly identicd results. For shock pressures near incipient devolatilization (Pinitid = 5.0 GPa), the hydrogen isotopic composition of the evolved H20 is very close to that of the starting material. For shock pressures at which up to 12% impact-induced devolatilization occurs (up to Ps t ia l = 7.7 GPa), the bulk evolved gas is siflicantly lower in deuterium than the starting material. There is also significant reduction of H20 to H2 in gases recovered at these higher shock pressures, probably caused by reaction of evolved H20 with the metal gas recovery fixture. Gaseous H20-Hz isotopic fractionation suggests high temperature (1 100-1200 K) isotopic equilibrium between the gaseous species, indicating initiation of devolatilization at sites of greater than average energy deposition (shear bands). Bulk gas-residual solid isotopic fractionations indicate nonequilibrium, kinetic control of gas-solid isotopic ratios. Impact-induced hydrogen isotopic fractionation of hydrous silicates during accretion can strongly affect the long-term planetary isotopic ratios of planetary bodies, leaving the interiors enriched in deuterium Depending on the model used for extrapolation of the isotopic fractionation to devolatilization fractions greater than those investigated experimentally, planetary interior enrichments of 4 to 30 per cent in DM relative to that of the incident material can result from this process. INTRODUCTION Impact-induced devolatilization of volatile-bearing rocks and minerals is an important process influencing planetary accretion, early atmosphere formation, and chemical weathering of planetary surfaces [1,2,3,4,5]. Previous experimental investigations of this process have focussed on the shock pressure (impact velocity) dependence of the extent of impact-induced devolatilization in hydrous minerals, carbonates, and meteorites [6,7,8,9,10] and on the shock enhancement of chemical reactivity of silicates [11,12]. In this study, we perform a direct comparison of the two experimental techniques for measuring impact-induced devolatilization, namely, the gas-recovery and solid-recovery techniques. In addition, we have examined the hydrogen isotopic composition of the gaseous products of impact-induced devolatilization of serpentine. Early experimental investigations of impact-induced devolatilization employed gas recovery techniques, in which the gas driven from a sample during impact was recovered directly and analyzed gravimetrically [6J or manometrically and isotopically [7]. The purpose of these early studies was to establish the existence of the impact-induced devolatilization process. Accordingly, shock pressures were investigated only up to the point of incipient devolatilization, the shock pressure at which the material just begins to release volatiles upon impact. Subsequent studies that more fully delineated the shock pressure dependence of the extent of impact-induced devolatilization utilized solid recovery methods, in which solid material was recovered after the impact and analyzed for its post-shock volatile content [8,9,10]. Differences h volatile content between unshocked material and recovered, shocked material were attributed to the impact process. A potential difficulty with the solid recovery method is the problem of the handling of the materials between the shock event and the volatile analysis. The shocked assemblies must be machined open to recover the sample, and given the extremely reactive nature of these materials, the question can be raised whether even brief exposure to the ambient atmosphere compromises the samples. . This study was performed 1) to directly compare impact-induced devolatilization results using