Adiabatic Shock Waves in Icy Regions of the Solar Nebula: Implications for Origins of Phyllosilicate Minerals in Primitive Meteorites

Introduction: Phyllosilicate minerals are present in many primitive meteorite classes including CI, CM, and CV carbonaceouschondrites and type-3 ordinary chondrites [1-4]. Their origin has been a scientific puzzle for many years. Thermochemical equilibrium calculations show that serpentine, a phyllosilicate mineral that is abundant in the CM chondrites, is stable below 225 K in the canonical solar nebula [5]. However, kinetic considerations suggest that the rate of serpentine formation is too slow to occur within the lifetime of the solar nebula under these conditions [5]. Because of this kinetic consideration, it has long been accepted that serpentine and other phyllosilicate minerals, particularly those found in chondrule rims, formed during aqueous alteration on a small body in the early solar system. Petrologic studies of the CM chondrites show that phyllosilicate minerals occur as fine-grained rims around chondrules and other coarse-grained components [2,6]. The rims have many features that suggest formation by accretion of fine-grained minerals. This association between chondrules and fine-grained rims is inconsistent with in situ phyllosilicate formation on the CM chondrite parent asteroid. To resolve this problem, a complex history for the origin of fine-grained phyllosilicate minerals has been proposed [2]. Since serpentine formation is inhibited in the solar nebula, aqueous processes on a precursor parent body have been invoked. This body was then disrupted by a catastrophic impact, dispersing the finegrained material back into free space. Chondrules and other coarse-grained minerals then encountered this dust, accreted their fine-grained rims, and then became incorporated into the final parent body. In this study we present an alternative formation mechanism for fine-grained phyllosilicate-rich chondrule rims. We investigate the environmental conditions that result from an adiabatic shock wave in an icy region of the solar nebula. Such a mechanism is a leading candidate for chondrule formation [7]. Shock Model: The thermal evolution of silicates encountering a shock wave in the solar nebula has been studied by many authors to explain the rapid heating required to form chondrules [8-11]. We have modified the model of [10] to study the evolution of solids made of water ice and to model the rate of vaporization of the ice. The rates of sublimation and condensation are taken from [12], and evaporative cooling is considered as in [13]. We present the results for the case of a nebula initially at 10 5 atm and 150 K (slightly below the condensation point of water ice). We assume that the nebula gas is composed of H2 and H2O with water ice uniformly suspended throughout at a solar composition density, such that the water vapor pressure is in equilibrium with the ice. The ice particles are assumed to initially be 1 mm spheres. The shock velocity is chosen as 8.8 km/s because this is what we find to be the minimum required to completely melt silicates to form chondrules in a nebula of similar structure. Figure 1 shows the vapor pressure of water with distance behind the shock. The PH2O reaches a maximum of 5.5 10 7 bars at a distance of 10 km behind the shock, which is the point at which the ice particles completely sublime. This takes place 1.3 seconds after passing through the shock. The gas reaches a maximum temperature of 2754 K. The gas cools through dissociation and molecular vibrations, but the pressure remains constant during this process. Thus, the water vapor pressure also remains constant until condesation begins (while we ignore the effects of water dissociation, which would be important at the high temperatures, most of the water would have reformed once the system began to cool down to the temperatures at which phyllosilicates are stable).