Microtextures of Phyllosilicates in the Mokoia CV Chondrite: Evidence for Aqueous Alteration Prior to Consolidation
The CV and CO chondrites were less affected by aqueous alteration than the CI and CM chondrites; thus they may serve as important indicators of the earliest stages of carbonaceous chondrite formation. Mokoia is one of the rare CV chondrites that contains considerable amounts of phyllosilicates [1,2]. They occur in CAIs, some chondrules, and matrix; they also occur abundantly in some chondrule rims. Tomeoka and Buseck  showed that the phyllosilicates in Mokoia are saponite and Na-rich phlogopite and were formed by aqueous alteration of olivine, pyroxene, anorthite, and glass. However, the mineralogy and occurrence of phyllosilicates in Mokoia are different from those in CI and CM chondrites, suggesting that the alteration of Mokoia occurred in a condition distinct from those for CI and CM chondrites. Most phyllosilicates in CI and CM chondrites probably resulted from alteration of olivine and pyroxene by the activity of liquid water on the meteorite parent bodies (e.g., ). Major questions are (1) What were the conditions responsible for the alteration in Mokoia? (2) Were these conditions different from those experienced by the CI and CM chondrites? (3) Where did the aqueous alteration in Mokoia occur? To address these questions, I performed extensive observations of the Mokoia CV chondrite by using a scanning electron microscope equipped with an EDS analysis system. The Mokoia chondrite is a breccia composed of submillimeter to millimeter clasts. Its matrix consists largely of fine grains (<0.1 to 10 micrometers in diameter) of Fe-rich olivine. Phyllosilicates occur in narrow interstices (<1 micrometer) between the olivine grains. The abundance of phyllosilicates in matrix differs within a clast and among clasts. Some clasts that are rich in phyllosilicates are adjacent to phyllosilicate-poor or -free clasts; the evidence suggests that aqueous alteration of the matrix preceded some of the brecciation events that produced the clasts from earlier matrix. Phyllosilicates are much more abundant in some chondrules and CAIs than in matrix in Mokoia; in some chondrules, phyllosilicates extend over an area of 100 x 100 micrometers. Chondrules and inclusions that were altered in various degrees occur together with those that remain unaltered within the same clasts Because of small sizes, olivines in the Mokoia matrix are more permeable to fluids and thus more reactive to aqueous alteration than the coarser olivines and pyroxenes in chondrules. Therefore, if alteration.of chondrules occurred in situ in the parent body (after consolidation), the matrix olivines should have been altered preferentially to those in chondrules, which is just the opposite of what I observe. These observations suggest that some chondrules and inclusions experienced a distinct environment in terms of hydration from that of the matrix materials. Mineralogy itself does not provide precise constraints on whether the alteration occurred via a solar nebular gas or aqueous solutions. However, based on the present observations, it is likely that some chondrules, inclusions, and portions of matrix in Mokoia have experienced aqueous alteration prior to consolidation into the present configuration. References:  Cohen R. E. et al. (1983) GCA, 47, 1739-1757.  Tomeoka K. and Buseck P. R. (1990) GCA, 54, 1745-1754.  Tomeoka K. (1990) Nature, 345, 138-140.
- Pub Date:
- July 1993
- AQUEOUS ALTERATION;
- CV CHONDRITES;