Chemistry, Petrography, and Mineralogy of the Tonk CI Chondrite: Preliminary Results
Abstract
Chemically, CI chondrites are considered to be the most primitive material available for meteoritic studies; however, CI chondrites are highly brecciated and altered as well [1, references cited therein]. Here, we report on a study of two thin sections (2 x 2 mm, each) and a bulk sample of Tonk. Mineralogy: Our study revealed that Tonk has a very low degree of brecciation, and mineralogically, appears rather homogeneous at least on a millimeter scale. As reported from other CI chondrites, single grains or clusters of magnetites (pure Fe3O4) are homogeneously distributed throughout the studied sections [1,2] and very often associated with small Ca-phosphate grains [2]. Sulfides (pyrrhotite) are much less abundant in Tonk compared to other CIs, forming either laths up to 100 micrometers long or hexagonal euhedral crystals. Carbonates (dolomite and breunnerite) are rare. We did not observe carbonate fragments like those previously reported in Ivuna and Orgueil [3]. Ca-sulfates occurring as vein fillings are less abundant in Tonk than in other CIs. Coarse-grained phyllosilicate fragments (CPFs; probably saponite) are often intergrown with small sulfide laths, similar to CPFs observed in Alais. Olivines and pyroxenes, frequently detected in Orgueil and Ivuna, are apparently lacking in Tonk. Characteristically, Tonk sections exhibit a huge amount of rounded shaped objects, tentatively referred to as fine-grained phyllosilicate aggregates (FPAs; similar to Fe-rich antigorites). Bulk chemistry: We carried out INA analyses on a ~21-mg sample of Tonk. Although the abundances of most major and trace elements analyzed so far are close to the cosmic value, certain differences exist for the elements Br, Au, Mn, Co, and Ni (Table 1). Bromine is highly variable in all CI chondrites [4]. The relative enrichment of Br (~2.5X CI) is probably attributed to hydrothermal alteration processes that occurred on the CI parent body, as previously suggested [4]. Very likely the Au depletion reflects hydrothermal activity as well [4]. The depletion of Mn (0.7X CI) is best explained by the much lower abundance of carbonates in Tonk, which are the main carriers of Mn in CI chondrites. The Co depletion may be due to the low abundance of sulfides. A possible explanation for the Ni depletion (~0.7X CI) are the low Ni-contents of matrix phyllosilicates, the major phases in CI chondrites. For example, the Ni content of CPFs (~0.2 wt% NiO) in Tonk is about 65% lower than in CPFs from other CI chondrites (0.6 wt% NiO). Conclusions: Based on mineralogical and textural characteristics Tonk is more closely related to Alais than to Orgueil and Ivuna. Basically, Tonk exhibits an unfractionated chemical signature, although a certain variability in several major- and trace-element concentrations exists on a <0.1-g-sample scale, which partly can be explained by hydrothermal activities on the CI parent body as suggested by Ebihara et al. [4]. It is noteworthy that some of the elemental variations mentioned above strongly correlate with petrographic observations. References: [1] Kerridge J. F. and Bunch T. E. (1979) In Asteroids I, 745-764. [2] Endress M. and Bischoff A. (1993) Meteoritics, 28, 345. [3] Endress M. and Bischoff A. (1994) LPSC XXV, 349-350. [4] Ebihara M. et al. (1982) GCA, 46, 1849-1861. [5] Anders E. and Grevesse N. (1989) GCA, 53, 197-214. Table 1 shows INAA data of the Tonk CI chondrite (21.41-mg sample).
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29..462E
- Keywords:
-
- Chemical Composition;
- Meteoritic Composition;
- Mineralogy;
- Petrology;
- Tonk Meteorite;
- Abundance;
- Sulfides;
- Textures;
- Trace Elements;
- Lunar and Planetary Exploration;
- CARBONACEOUS CHONDRITES; CI-CHONDRITES; HYDROTHERMAL ALTERATION; METEORITES;
- COMPOSITION; NEUTRON ACTIVATION; PRIMITIVE METEORITES; TONK