Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27s.236h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 236
Other
3
Scientific paper
In Tieschitz, an unequilibrated ordinary chondrite, chondrules, and inclusions have rims of fine-grained, opaque matrix. Between the larger objects, channels <~20 micrometers wide may be filled with Na- and Al-rich, nepheline-normative white matrix. Christophe Michel-Levy (1976) suggested that white matrix was formed by the partial solution and redeposition of chondrule mesostases, but this was disputed by Hutchison et al. (1979), because silica undersaturated white matrix commonly abuts low-Ca pyroxene, with no sign of reaction. Such materials should be incompatible at low pressure and elevated temperature. Alexander et al. (1989) found material resembling white matrix in two Sharps chondrules, but did not find it elsewhere. Furthermore, the major minerals of white matrix are still unidentified. In a thick polished section, the analytical scanning electron microscope revealed several porphyritic chondrules with mesostases composed of two types of material with different electron back-scattering properties. One type lacks internal boundaries and appears brighter than the other, which is blocky on a 5-micrometer scale; the latter resembles white matrix. Bright mesostasis has higher SiO2, TiO2, FeO, MgO, and CaO compared with the dark, blocky material, which is richer in Al2O3 and the alkalis (Table 1, cf nos. 3 and 5). A porphyritic olivine (PO) chondrule 1 mm in diameter has zoned phenocrysts of Fo(sub)90-75 (Table 1 ) and mesostasis containing Ca-pyroxene dendrites of uniform composition close to Wo(sub)38En(sub)42 and with a high MnO/FeO ratio of ~0.07. Both types of mesostasis are present and in contact. Pyroxene dendrites extend across contacts without structural or compositional change. Furthermore, at its contact with a 0.6-mm barred pyroxene chondrule, the PO chondrule is slightly indented and needles of Ca-pyroxene radiate from the point of contact into bright mesostasis. The evidence indicates that in the PO chondrule a single generation of dendritic Ca pyroxene crystallized from bright mesostasis. The pyroxene resisted later alteration (H2O, Cl?) that transformed some of the bright mesostasis into white matrix material. It is concluded that white matrix formed from chondrule mesostases by secondary metasomatism that caused depletion in Si, Ti, Fe, Mg, and Ca and enrichment in Al, Na, K (and Cl?), possibly from opaque matrix, which in Tieschitz is not enriched in these elements over bulk meteorite (Alexander et al., 1989). Christophe Michel-Levy (1976) was largely correct. The metasomatism probably occurred on the parent body, which is compatible with the textural observations that many chondrules aggregated when hot and plastic. TABLE 1: EDS analyses, 15 kV, 1 x 10^-9Amp 1 2 3 4 5 6 SiO2 41.1 38.3 63.3 53.7 53.3 50.8 TiO2 -- -- 0.44 0.41 -- 0 51 Al2O3 -- -- 10.1 1.80 21.1 2.06 Cr2O3 0.10 -- 0.09 0-94 0.15 1.31 FeO 9.6 22.2 8.8 10.5 1.90 10.5 MnO 0.33 0.65 0.20 0.68 -- 0.87 MgO 49.7 38.7 4.8 13.5 1.08 13.1 CaO 0.09 0.12 2.80 16.4 0.80 16.4 Na2O -- -- 7.8 0.95 10 0 1 62 SUM 100.9 100.0 98.2 98.9 *89.3 97.2 Comp. Fo90 Fo75 Wo37En43 Wo38En42 *Includes 0.63%K(sub)2O and 0.28%Cl. 1,2 Olivine core and rim. 3 Bright mesostasis. 4 Px in 3. 5 Blocky mesostasis. 6 Px in 5. REFERENCES: Alexander, C. M. O'D., Hutchison, R. and Barber, D.J. (1989) Earth Planet. Sci. Lett. 95 1, 87-207. Christophe Michel-Levy, M. (1976) Earth Planet. Sci. Lett. 30, 143-150. Hutchison, R., Bevan, A.W.R., Agrell, S.O. and Ashworth, J.R. (1979) Nature 280, 116-119.
No associations
LandOfFree
New Evidence for the Origin of White Matrix in Tieschitz does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with New Evidence for the Origin of White Matrix in Tieschitz, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and New Evidence for the Origin of White Matrix in Tieschitz will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1210084