Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.469z&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 469-470
Computer Science
Chondritic Idps, Interplanetary Dust Particles, Olivine, Pyroxenes
Scientific paper
Introduction: A principal goal of meteoritics is to characterize the mineralogy, mineral chemistry, and microstructures of primitive extraterrestrial materials, and to use this information to identify source bodies and (more importantly) characterize origin and evolution of these bodies. We report here analyses of olivines and pyroxenes, and petrofabrics of 26 chondritic IDPs, comparing those from anhydrous and hydrous types. Materials and Methods: All the particles described here were collected in the stratosphere, and had bulk compositions near chondritic, as determined by energy dispersive X-ray spectrometer and INAA analyses. All particles were embedded in EMBED 812 low-viscosity epoxy, and microtomed in preparation for TEM analysis. Microtoming was halted approximately half-way through each particle, yielding "potted butts" from which electron backscattered (BSE) images were obtained. Results: Olivine and pyroxene classes have the same reported mineralogy, and are considered here together. We also consider hydrous IDPs to be those that contain any amount of phyllosilicates. Examination of the BSE images of chondritic IDPs reveals that some anhydrous chondritic IDPs show low porosity, and that some hydrous ones show relatively high porosity, as has been noted previously, but which may not be fully appreciated. We are currently measuring particle porosities, and relating these to interparticle granular arrangements. Olivines and/or pyroxenes are a common constituent of most IDPs, and their compositional range should be useful as an indicator of the degree of their metamorphism. We have previously reported that there exists no significant difference in the compositions of olivines from olivine vs. pyroxene-dominated IDPs [1]. The degree of heterogeneity of these minerals from anhydrous IDPs is approximately Fo52-100 and En46-100 (see Fig. 1). In contrast, olivines in the hydrous IDPs only range in composition from Fo76-100, while the enstatite range is En76-100. We note that 36% (4/11) of hydrous IDPs contain diopside, in contrast to only 6% (1/15) of anhydrous IDPs. Comparisons of Hydrous to Anhydrous IDPs: Are anhydrous and hydrous IDPs genetically related by simple hydrolysis of the former material? The presence of primarily Mg-rich (Fe-poor) olivines and pyroxenes in the hydrous IDPs could be due to the preferential early dissolution of Fe-rich olivines and pyroxenes during aqueous alteration. The observed rate of fayalite (Fo6) dissolution in a reducing atmosphere at a pH of 2-7 at 25 degrees C is six times higher than for forsterite (Fo91) [2]. This relationship probably continues in the pH range 7-12 (the range appropriate to asteroid alteration [3]), although the absolute rate of olivine dissolution should be adversely affected by the change in pH [4]. While pyroxenes weather at a generally slower rate than olivines [5], it is possible that dissolution of Fe-rich pyroxenes is similarly more rapid than Mg-rich varieties. However, the presence of diopside in some IDPs (most frequently in hydrous ones) may indicate slow cooling, probably requiring a parent body, or a more Ca-rich source. We conclude that olivine and pyroxene major-element compositions can be used to discriminate between (1) pristine IDPs, from anhydrous or icy (no liquids), primitive parent bodies, and (2) metamorphosed and (3) aqueously altered IDPs, from various processed bodies (probably hydrous and anhydrous asteroids). References: [1] Zolensky and Barrett (1993) Microbeam Analysis, in press. [2] Wogelius and Walther (1992) Chem. Geol., 97, 101-112. [3] Zolensky et al. (1989) Icarus, 78, 411-425. [4] Grandstaff (1977) GCA, 41, 1097-1104. [5] Eggleton (1986) in Rates of Chemical Weathering of Rocks and Minerals, 21-40.
Barrett Richard
Burkett Patti Jo
Zolensky Michael
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