SHOCK TEMPERATURES IN CALCITE (CaC03): IMPLICATION FOR SHOCK INDUCED DECOMPOSITION

The temperatures induced in crystalline calcite upon planar shock compression (95 160 GPa) are reported from two-stage light gas-gun exp.eriments. The temperatures are obtained fitting 6-channel optical pyrometer radiances in the 450 to 900 nm range, to a Planck radiation law temperature varied from 3300 to 5400 K. Calculations demonstrate that the temperatures are some 400 to 1350 K lower than if either shock-induced melting and/or disproportionation of calcite behind the shock front was not occurring. Here calcite is modeled as disproportionating into a molecular liquid, or a solid CaO plus CO2 gas. For temperature calculations, specific heat at constant volume for one mole of CO2 is taken to be 6.7R as compared to 9R in the solid state; whereas calcite and CaO have their solid state values (15R and 6R). Calculations also suggest that the onset of decomposition in calcite to CaO and CO2 during loading occurs at 75+10GPa, along the Hugoniot whereas decomposition begins upon unloading from _ 18 GPa. The 18 GPa value is based on comparison of VISAR measurements of particle velocity profiles induced upon isentropic expansion with one-dimensional numerical simulation. INTRODUCTION carbonates, raised possibility of an extinction A a Research on the shock-induced devolatilization behavior of calcite is motivated by a need to understand the role the greenhouse gas CO;! played in the formation and evolution of atmosphere of Earth, Mars and Venus. These planets have long thought to contain substantial amount of carbonates in their crusts. In the case of Mars, defination of the shock pressure required to volatilize substantial quantities of CO% from carbonates can be used to define the cratering rate and the period over which Mars operated an efficiently CO2 greenhouse, such that a liquid Hz0 reservoir became available on the surface which might host early life. Shock devolatilization is also important to evaluate the mechanisms of impact induced extinction of marine and terrestrial biota for example at the WI’ boundary. The discovery of the Chicxulub crater as the impact site and studies that indicated -70% of the 3-km-thick section are mechanism involving a long duration ( lo”10” years) global greenhouse event with surface land temperature increasing by 10 degrees at K/T times due to injection of impact-induced CO2 into atmosphere [ 11. Temperature measurements along the Hugoniot are useful for detecting and characterizing phase transitions including molecular dissociation [ 21. In the present work, we measured shock temperatures in single-crystal calcite using optical pyrometery. A two-stage light gas-gun launched impactors shock compressed the samples. Comparison of measurements and calculations demonstrates that temperatures are lower than if either melting and/or devolatilization of calcite are not occurring. Since deficit we have is much more than that expected from melting suggests that shock-induced decomposition of calcite occurs behind the shock front.