Replacement of olivine by serpentine in the Queen Alexandra Range 93005 carbonaceous chondrite (CM2): Reactant-product compositional relations, and isovolumetric constraints on reaction stoichiometry and elemental mobility during aqueous alteration

Solvent and solute species diffused through the serpentine between the olivine-serpentine interface and the aqueous solution outside the isovolumetrically replaced volume. In QUE 93005, some of the Fe released from ferroan olivine in excess of the amount required to form serpentine reacted with S sourced from outside the pseudomorphs to form Fe-sulfide decorating the margins of the pseudomorphs of serpentine after fayalitic olivine. Such Fe-sulfide-decorated outlines after fayalitic olivine do not occur in ALH 81002 or Nogoya, indicating different Fe and S mass transfer regimes in different CM2 chondrites. Mg, Fe, Si, and S in the aqueous solution, including the excess Mg and Si exported from all serpentine pseudomorphs after olivine of any composition, were available to be incorporated into other phases spatially separate from the pseudomorphs after olivine, including regularly interstratified serpentine-tochilinite. Serpentines that replaced coarse olivines in QUE 93005 and ALH 81002 are less magnesian than those in Nogoya, indicating that the Nogoya aqueous-alteration environment was more evolved toward Mg-rich solutions. This easily located and characterized phase assemblage may be potentially useful for characterizing clasts of varying degrees of alteration in brecciated and heterogeneous CM chondrites, and future returned samples from mineralogically similar asteroids.