Impact Induced Dehydration of Serpentine and the Evolution of Planetary Atmospheres

Shock recovery experiments in the 25 planetesimals are released when these bodies hit to 45 GPa range on antigorite serpentine determine the accreting planets, and is continually added to the amount of shock-induced loss of structural a growing proto-atmosphere. Arrhenius et al. water as a function of shock pressure. Infrared [1974] propose, based mainly on noble gas absorption spectra of shock recovered samples abundance patterns in the present terrestrial demonstrate systematic changes in the amount of atmosphere, that the earth's atmosphere and structural water and molecular, surface adsorbed hydrosphere are primarily the result of such an water. These yield qualitative estimates of initial process and have since undergone only shock-induced water loss and demonstrate that a minor alteration. In our earlier models [Lange portion of the shock released structural water is and Ahrens 1980, 1952a] we demonstrated that the readsorbed on interfacial grain surfaces. effects of a major sink for atmospheric water, the Determination of the post-shock water content of hydration of nonhydrous surface minerals (mainly the shocked samples relates shock-induced water pyroxene and olivine) would be negated by repeated loss and shock pressure. Based on the present dehydration of surface layers due to impacting results and theoretical predictions, we conclude planetesimals. Based on a model for the accretion that shock pressures of from 20 to ~60 GPa induce rate of the terrestrial planets and the volatile incipient to complete water loss, respectively. content of the planetesimals, critical parameters This result agrees closely with theoretical for the generation of a primary atmosphere were estimates for total dehydration. The dehydration determined, parameters which have likely been met interval and the activation energies for by the accretional environment on earth [Lange and dehydration in shocked samples decrease Ahrens, 1952a]. systematically with increasing shock pressure as Important parameters for these models are the experienced by the sample. We believe the present physical conditions leading to the dehydration of experiments are applicable to describing hydrous minerals under shock loading (more dehydration processes of serpentine-like minerals generally, to the devola•ilization of in the accretional environment of the terrestrial volatile-bearing minerals), e.g., the critical planets. We conclude that complete loss of impact pressure Pcr for partial or complete water structural water in serpentine could have occurred loss [Lange and Ahrens, 1980, 1952a]. The value from accretional impacts of ~3 km/sec when earth for complete dehydration (P•_ = 60 GPa) was and Venus have grown to about 50% of their final determined theoretically [Lang•and Ahrens, 1979] size. Accreting planetesimals, impacting Mars, using the entropy gain method of Ahrens and never reached velocities sufficient for complete O'Keefe [1972]. However, as Pcr is critically dehydration of serpentine. Our results support a dependent on the GrHneisen parameter of model in which an impact generated serpentine, which is only poorly constrained, the atmosphere/hydrosphere forms while the earth is theoretical value may be somewhat in error. accreting. Boslough et al. [1980] first demonstrated the loss of structural water in shocked antigorite Introduction serpentine in vapor recovery experiments. In the present experiments, shock-induced water loss in We report results of shock recovery experiments antigorite serpentine was quantitatively carried out on antigorite serpentine determined from comparison of the amount of (Mg3Si205(OH)4). The main objective of this study post-shock water in recovered samples with that is the determination of critical shock pressures initially present [Lange and Ahrens, 1952b]. for partial and complete dehydration of serpentine Qualitative estimates for loss of structural water under shock loading. were also obtained from infrared spectra of Serpentine and serpentine-like layer silicates shocked and unshocked samples. are the major water-bearing phases in carbonaceous chondrites [Wilkening, 1978, Barber, 1981]. It Experimental Details appears that these minerals, and a poorly defined cometary contribution, were the most likely The material used in the experiments is water-bearing phases in accreting planetesimals antigorite serpentine (Mg3Si205(OH)4) from which led to the formation of the terrestrial Thurman, New Yo•k, with an initial density of planets. In models proposed by Arrhenius et al. 2.54ñ0.004 g/cm •. The major oxide composition of [1974], Benlow and Meadows [1977], and Lange and the material determined from a 10 point microprobe Ahrens [1980, 1952a] formation of a planetary analysis is given in Table 1. As can be seen, the atmosphere/hydrosphere is envisioned as a primary measured composition is close to the theoretical process, taking place during accretion. Water and value. The initial amount of structural water in other volatiles in minerals of the infalling the samples (12.6 wt. %) was determined by use of Copyright 1982 by the American Geophysical Union. a Mettler Thermoanalyzer TA 2000C on an unshocked reference sample. Paper number 2B!157. Sample discs, 4.4 mm in diameter, 0.5 mm thick, 0148-0227/$2/002B-1157505.00 were polished flat and parallel to within ñ2