No abstract

The Antarctic CM2 carbonaceous chondrite QUE 93005 contains four compositionally distinct carbonates, namely breunnerite, calcite, dolomite and a Ca-poor dolomite. These carbonates can form monomineralic grains, or may be intergrown as bimineralic grains consisting of dolomite plus breunnerite and dolomite plus calcite, or polymineralic grains containing an intergrowth of breunnerite, Ca-poor dolomite and calcite. Carbonates in all grain types have inclusions of Fe,Ni sulphides and/or Mg-Fe phyllosilicates. In the bimineralic grains dolomite crystallised first to be overgrown by breunnerite or partially replaced by calcite. Polymineralic grains are concentrically layered, with breunnerite crystallising first on pore margins to be later etched, then overgrown and partially replaced by Ca-poor dolomite that was itself partly dissolved prior to being overgrown by calcite. Calcite and dolomite have also cemented fractures that cross-cut the fine-grained rims to aqueously altered chondrules and were formed by expansion of the chondrules during their hydration. Overall, the sequence of mineralisation in QUE 93005 was: (1) dolomite, (2) breunnerite, (3) Ca-poor dolomite then (4) calcite. This secular change in carbonate composition and mineralogy reflects changing solution composition and probably also provenance. Mg-Fe phyllosilicates replaced dolomite, breunnerite and Ca-poor dolomite prior to calcite crystallisation and most or all of the sulphides formed after both the phyllosilicates and calcite. Following sulphide crystallisation the edges of carbonate grains were abraded, either by impact 'gardening' or as a consequence of fluidisation of the matrix during rapid loss of gas or vapour. Determination of the crystallisation age of dolomite via the Mn−Cr system indicates that aqueous alteration of QUE 93005 began on or before 3.93 ± 0.23 Ma after the formation of the solar system. Overall, the water/rock ratio and fO 2 during alteration of QUE 93005 was similar to that of the CM1s and CR1s, but the lower degree of alteration of QUE 93005 overall suggests that alteration timescales were shorter, possibly due to loss of intergranular liquid water during fluidisation.2

No abstract

Abstract-Clasts in the CM2 carbonaceous chondrite Lonewolf Nunataks (LON) 94101 have been characterized using scanning and transmission electron microscopy and electron microprobe analysis to determine their degrees of aqueous alteration, and the timing of alteration relative to incorporation of clasts into the host. The provenance of the clasts, and the mechanism by which they were incorporated and mixed with their host material are also considered. Results show that at least five distinct types of clasts occur in LON 94101, of which four have been aqueously altered to various degrees and one is largely anhydrous. The fact that they have had different alteration histories implies that the main part of aqueous activity occurred prior to the mixing and assimilation of the clasts with their host. Further, the presence of such a variety of clasts suggests complex mixing in a dynamic environment involving material from various sources. Two of the clasts, one containing approximately 46 vol% carbonate and the other featuring crystals of pyrrhotite up to approximately 1 mm in size, are examples of unusual lithologies and indicate concentration of chemical elements in discrete areas of the parent body(ies), possibly by flow of aqueous solutions.

No abstract