Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.396m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 396
Other
2
Alteration, Boriskino, Carbonaceous Chondrites, Carbonate, Fluids, Ion Probe, Murchison, Nogoya, Orgueil
Scientific paper
Carbonates in CI and CM chondrites precipitated from aqueous fluids, probably derived from melted ices originally accreted into the parent asteroids [1]. Previous attempts to constrain the proportions and properties (temperature, pH, Eh, and dissolved C) of these fluids have been based on oxygen isotope fractionations [2] and on computer simulations of the alteration process [3]. The concentrations of minor and trace elements in carbonates can also provide insights into the composition of the alteration fluids. However, the carbonate grains are usually very small, and it is difficult to obtain reliable results for element concentrations <500 ppm using the electron microprobe. Utilizing a Cameca 4f ion microprobe at ORNL, we have analyzed calcites and dolomites in four CM chondrites (Boriskino, Murchison, Nogoya, and ALH 83100) and dolomites in one CI chondrite (Orgueil) for Fe, Mg, Mn, Sr, Na, Ba, and B, using Ca concentrations determined by electron microprobe as a reference element. Electron microprobe analyses for Fe, Mg, and Mn [4] (when detectable) are in good agreement with our results. Mean element concentrations (ppm) for 25 calcites in CM chondrites are Fe 2272, Mg 876, Mn 446, Sr 104, Na 685, Ba 8, B 13. Corresponding data (ppm unless otherwise noted) for 5 dolomites in CM (and 10 dolomites in CI) chondrites are Fe 2.55 wt% (1.65%), Mg 10.02% (10.73%), Mn 1.98% (2.69%), Sr 135 (142), Na 410 (286), Ba 10 (7), B 10 (6). Calcites are highly variable in composition, even within the same CM chondrite, and there are no apparent interelement correlations. ALH 83100 is visibly brecciated, but the range of calcite compositions in this sample is no greater than in other meteorites, and calcites within one clast show appreciable compositional scatter. Dolomites in both CM and CI chondrites appear to be more restricted in composition compared to calcites, and CI dolomites are consistently less Fe- rich and usually more Mn-rich than those in CM chondrites. We have estimated element ratios (relative to Ca) for fluids in equilibrium with these carbonates using element/fluid partition coefficients for calcite [5] and dolomite [6]. Such Ds have large uncertainties due to possible influences of temperature, pH, and other factors, but some general properties of the fluids can be ascertained. For CM chondrites, dolomites indicate fluids having consistently higher Fe/Ca, Mg/Ca, Mn/Ca, and Sr/Ca ratios than do co- existing calcites. All these fluids have high Na/Ca, and appear similar in many respects to terrestrial brines. The nearly stoichiometric compositions of dolomites also suggest high salinity. Dolomites in CI chondrites suggest fluids indistinguishable from those in equilibrium with CM dolomites. Fluids in equilibrium with calcite in highly altered meteorites like Nogoya have higher Fe/Ca, Mg/Ca, and Mn/Ca than those calculated for less-altered meteorites like Murchison. Moreover, the fluids that produced calcite in CM chondrites were apparently distinct from those that produced dolomites in CM and CI chondrites. These observations reinforce previous conclusions [4,7,8] that fluid compositions in the parent asteroids were locally and temporally variable. The elevated trace-element contents of the carbonates and the brine-like compositions of the assocated fluids further indicate that dissolution of soluble components may have been more pervasive than suggested by the preservation of bulk compositions thought to be cosmic. References: [1] Grimm R. E. and McSween H. Y. (1989) Icarus, 82, 244-280. [2] Clayton R. N. and Mayeda T. K. (1984) EPSL, 67, 151-161. [3] Zolensky M. E. et al. (1989) Icarus, 78, 411-425. [4] Johnson C. A. and Prinz M. (1993) GCA, in press. [5] Veizer J. (1983) SEPM Short Course No. 10, 3-100. [6] Kretz R. (1982) GCA, 46, 1979-1981. [7] Richardson S. M. (1978) Meteoritics, 13, 141- 159. [8] Fredriksson K. and Kerridge J. F. (1988) Meteoritics, 23, 35-44.
Johnson Annette C.
McSween Harry Y. Jr.
Prinz Martin
Riciputi Lee R.
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