Carbonaceous Chondrite Parent Bodies Research Articles

炭素質コンドライトは本当に未分化な物質か

The carbonaceous chondrites are the most primitive surviving materials from the early solar system, and thus they may provide important information regarding the early processes in the solar system. Some carbonaceous chondrites, CI and CM types in particular, consist in large part of fine-grained phyllosilicates, which account for most of the water in the meteorites (5-20 wt.% H20). Previous studies suggested that the phyllosilicates were formed by aqueous alteration on the meteorite parent body, but formation by direct condensation from the solar nebula has not been ruled out. This paper reviews, in the first part, results of mineralogical and chemical studies indicating the primitive nature of the carbonaceous chondrites, and reviews, in the second part, studies dealing with aqueous alteration experienced by these meteorites. This paper also discusses important points of information and their implications regarding the carbonaceous chondrite parent body.

Water and the thermal evolution of carbonaceous chondrite parent bodies

Many carbonaceous chondrites have been aqueously altered within their parent bodies. From chemical and textural data on these meteorites and from studies of collision mechanics, we pose two hypotheses for the aqueous alteration environment. In the first model, alteration occurs throughout the parent body interior; in the second, alteration occurs in a postaccretional surface regolith. Both models are based on the assumptions of an initially homogeneous mixture of ice and rock and heating by decay of 26Al. Under the interior-alteration model, linked bounds on the initial ice-to-rock ratio and 26Al abundance are found that satisfy alteration temperatures derived from oxygen isotope studies. We find that water may play a strong role in controlling chondrite evolution by acting as a thermal buffer that allows substitution of low-temperature aqueous alteration instead of high-temperature recrystallization. Additional constraints imposed by the inferred water volume consumed by the alteration reaction and the total water volume that exchanged oxygen isotopes with host rocks are best explained if alteration occured ina regolith. We show quantitatively how liquid water may be introduced there by hydrothermal circulation, by diffusion of vapor from below, or by venting due to fracture when interior pore pressures exceed the parent body strength. A sealed permafrost zone is not required to ensure insulation of water long enough for aqueous alteration. Retention of primordial ice is probably not limited by sublimation or by collisional comminution, but by shock vaporization. If large, C-type asteroids are representative of carbonaceous chondrite parent bodies, they may still contain significant quantities of ice.

Aqueous alteration in carbonaceous chondrites: Mass balance constraints on matrix mineralogy

Bulk chemical compositions of matrix material in Antarctic CM chondrites and other non-Antarctic CM and CI chondrites have been determined using microprobe defocused beam techniques. These are used, along with the results of previously published mineralogical studies, to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices. Results of these calculations indicate differing amounts of PCP (a mixture of approximately 25% tochilinite and 75% cronstedtite) and serpentines (Mg-rich and Fe-rich varieties in varying proportions or intermediate compositional varieties). Additional sulfide phases are also probably necessary to account for excess Ni and S. Fe/Si ratios for matrices of individual meteorites range from 1.21 to 2.77, corresponding to PCP/(PCP + SERF) ratios of 0.16 to 0.58. Progressive aqueous alteration of matrix appears to have occurred by formation of tochilinite, then cronstedtite and Mg-rich serpentine, and finally Fe-rich serpentine and sulfides. CM matrix clearly did not behave as an isolated system during alteration. CI chondrite matrices appear to contain little if any PCP; this may be a natural consequence of the absence of chondrule-associated metal, from which PCP forms, in the unaltered precursor material. These data provide a more quantitative picture of low-temperature aqueous alteration processes in carbonaceous chondrite parent bodies than has heretofore been possible from TEM studies alone.

Shock effects on hydrous minerals and implications for carbonaceous meteorites

New infrared absorption spectra, thermo-gravimetric analyses and optical-and scanning electron microscopy of shock-recovered specimens of antigorite serpentine (Mg3Si2O5(OH)4) from the pressure range between 25 to 59 GPa are reported. The infrared spectra show systematic changes in absorption peaks related to structural and molecular surface absorbed water. H2O absorption peaks increase at the expense of OH peaks with increasing shock pressure. Changes in SiO bond vibrational modes with increasing shock pressure parallel those seen for other, non-hydrous minerals. Thermogravimetric analyses of shock-recovered samples determine the amount of shock-induced water loss. For samples shocked in vented assemblies, the data define a relation between shock-induced water loss versus shock pressure. Results for samples shocked in sealed assemblies demonstrate a dependence of water loss on shock pressure and target confinement. For the vented assembly samples, a linear relation between shock pressure and both the length of dehydration interval and the effective activation energy for releasing post-shock structural water in antigorite is found. Optical and scanning electron miscroscopy of shocked antigorite reveal a number of textures thought to be unique to shock loading of volatile-bearing minerals. Gas bubbles, which probably are the result of shock-released H2O appear to be injected into zones of partial melting. This process may produce the vesicular dark veins which are distributed throughout heavily shocked samples. The present observations suggest several criteria which may constrain possible shock histories of the hydrous matrix phases of carbonaceous condrites. A model is proposed for explaining hydrous alteration processes occurring on carbonaceous chondrite parent bodies in the course of their accretion. We speculate that shock loading of hydrous minerals would release and redistribute free water in the regoliths of carbonaceous chondrite parent bodies giving rise to the observed hydrous alterations.