Aqueous and Thermal Alteration of Very Primitive Chondrites

Julia Goreva and Dante Lauretta
Lunar and Planetary Lab, University of Arizona, Tucson, AZ

With advances in Space Exploration, the age of using asteroids as potential natural resource for human exploration and development of space is in sight.

Planetary scientists study chemistry, mineralogy and petrology of meteorites, direct samples of asteroids. Here we focus on the very first processes that happened after asteroids accreated from protoplanetary disk.

There is no doubt that aqueous and thermal alteration of asteroids and planetesimals occurred very early in the history of the Solar System. Direct observations of phosphates in unequilibrated ordinary chondrites (UOC) indicate that prior to metamorphism, phosphorus was associated with Fe-based alloys. The alloying of P in Fe-Ni metal is a predicted consequence of gas-solid equilibrium under nebular conditions. Parent asteroid conditions appear to have been sufficiently oxidizing that phosphates formed early even in the most primitive chondrites. Presumably, P reacts with Ca and O on the metal grain boundaries to form Ca-phosphates. Both phosphate minerals - apatite and merrillite - are observed in UOC. However, the conditions and timing of the phosphate nucleation is not constrained. In this study we compare the phosphorus distribution in two least equilibrated ordinary chondrites Semarkona and Bishunpur. In terms of metamorphic history, Semarkona and Bishunpur are very similar. The main difference is a higher degree of aqueous alteration of the Semarkona matrix. Differences in petrographic association of merrillite in two meteorites leads us to believe that from two phosphates so abundant in ordinary chondrites, merrillite nucleated first. Moreover, it could have nucleated through both, the aqueous alteration of metal and sulfides, and through moderate heating of chondrules. The question of in what form, when and where Cl in Cl-apatites came from at the later stages is an area of active research.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas