Aqueous alteration on the hydrous asteroids: Results of EQ3/6 computer simulations

Abstract

In order to understand the course of aqueous alteration of the hydrous asteroids we constructed two models for carbonaceous chondrite precursor material, one (model A) based upon the anhydrous mineralogy of the abundant CM chondrites and the other (model B) based upon that of the equally abundant CV chondrites. We then proceeded to calculate the likely course of aqueous alteration on asteroids composed of these materials through the use of the EQ3/6 computer algorithm. We find that the alteration mineralogy of the CM chondrites (and by extension the CM parent asteroids) may best be produced by starting with the anhydrous mineralogy of the same CM chondrites, at temperatures of 1 to approximately 25°C, and at total solution carbon concentrations which vary from as little as 10 −8 m up to at least 10 −2 m. A wide range of rock/fluid ratios is permitted by this alteration mineralogy. Under these conditions solution pH is calculated to vary from 7 to just above 12. Solution Eh is calculated to vary from −0.5 to −0.75 V. We calculate that the mineralogy of the important CI chondrites can be well-reproduced by alteration of either CM or CV anhydrous material. This alteration is calculated to occur best for temperatures of 50 to (at least) 150°C, total solution carbon concentrations varying from approximately 10 −3 to (at least) 10 −2 m, and a wide range of rock/fluid ratios. Under these conditions solution pH is calculated to vary from 7 to between 9 and 10, and Eh from −0.3 to −0.8 V, in the direction of increasing alteration. We therefore conclude that these were the principal conditions of aqueous alteration on the CI parent asteroids. The alteration assemblage observed for the CM chondrites is not produced by alteration of the CV chondrites under the modeling conditions imposed by this study, which suggests that the CM chondrites do not necessarily share the same parent asteroids with the CV chondrites. On a purely mineralogical basis, howerver, the CI chondrites could have been produced from either (or both) CM or CV chondrite material, and therefore be present on either type of parent asteroid.