Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..345e&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 345-346
Mathematics
Logic
7
Alteration, Aqueous Alteration, Breccia, Carbonaceous Chondrites, Ci Chondrites, Gardening, Impact Gardening, Magnetite, Matrix, Petrography, Phyllosilicates, Primitive Meteorites, Regolith Breccia
Scientific paper
The textural and mineralogical properties of the CI-chondrites Orgueil, Ivuna, and Alais were studied by electron microscopy in order to obtain new insights into the history and evolution of the CI parent body. Degree of Brecciation. The electron optical investigation of Orgueil, Ivuna, and Alais revealed that the three samples experienced different degrees of brecciation: (1) Orgueil is highly brecciated. It consists of abundant clastic matrix (clasts usually <10 micrometers) surrounding fragments of variable sizes (10-700 micrometers). (2) Alais is much less brecciated than Orgueil and in this manner more closely related to Ivuna (see below); fragments (100-500 micrometers in size) are embedded in clastic matrix. (3) Ivuna shows the lowest degree of brecciation and contains only small areas of clastic matrix. The major part of the sections studied consists of huge fragments (up to 1 x 3 mm in their largest dimension) representing at least three distinct lithologies: The first lithology can be characterized by a high modal abundance of carbonates (up to 10 vol%) and a lack of sulfates; the second is enriched in sulfates, but contains no carbonates, and the third lithology can be characterized by its high abundance of coarse-grained phyllosilicate fragments. Mineralogy. As phases >5 micrometers in size we observed magnetite, sulfides, carbonates, Ca-phosphate, olivine (Fa<5 14 grains), pyroxene (Fs<1 6 grains), sulfates, phyllosilicates, and as accessory phases Mn-rich chromite (15 wt% MnO), Mn-rich ilmenite (11 wt% MnO), Fe,Ni-metal grains in olivine relics and a yet unidentified Na,K-rich silicate (7 wt% Na2O; 6 wt% K2O; only in Ivuna). Magnetites (pure Fe3O4) occur as platelets, framboids, and spherules. As sulfides pyrrhotite, pentlandite (partly exsolved in pyrrhotite) and cubanite were observed. Carbonates (dolomite, breunnerite, calcite) either occur as single grains (usually <40 micrometers) or as big polycrystalline chunks (up to 300 micrometers in size; especially in Ivuna). The Ca-phosphates contain small amounts of FeO (1-2 wt%), MnO (1 wt%), and Na2O (up to 3 wt%). Sulfates mostly occur as vein fillings in Alais (mainly gypsum) and Orgueil (mainly epsomite), but such veins are lacking in our Ivuna sample. Here, single sulfate grains (<10 micrometers) are rarely dispersed throughout the matrix. Aqueous alteration. Based on a study of Orgueil and Y-82162, [1,2] proposed a model, that during late stage aqueous alteration Fe-rich solutions readily decompose coarse-grained phyllosilicate clusters to fine-grained phyllosilicates and ferrihydrite and that contemporaneously sulfates were formed. Since Y-82162 contains abundant phyllosilicate clusters, but no ferrihydrite and no sulfate, Y-82162 should be less altered than Orgueil [1]. Considering this model we can distinguish between Orgueil, Ivuna, and Alais based on the degree of alteration. The abundance of phyllosilicate fragments is low in Orgueil, but high in Ivuna and Alais. In contrast to Orgueil, Ivuna contains no ferrihydrite [3,4] and very little sulfate. Therefore, the degree of aqueous alteration appears to be increasing in the order: Y-82162 << Ivuna < Alais << Orgueil. Furthermore, preliminary studies may indicate a link between the degree of aqueous alteration and the degree of brecciation. TEM studies are in progress in order to resolve this question. References: [1] Tomeoka K. and Buseck P. R. (1988) GCA, 52, 1627- 1640. [2] Tomeoka K. et al. (1989) Proc. NIPR Symp. Antarct. Met., 2, 36-54. [3] Zolensky M. et al., GCA, submitted. [4] Brearley A. J. (1992) LPSC XXIII, 153-154.
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