Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.306w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 306
Mathematics
Logic
4
Scientific paper
Introduction: Spectacular, large (up to 1 cm) concentrically layered chondrules are conspicuous components of the CR2 chondrites [1-5]. To gain a better understanding of the formation of layered chondrules and their implications, we carried out a petrologic investigation of these chondrules in all CR2 chondrites, and a combined petrologic-oxygen isotopic study of the cores and rims of three layered chondrules from El Djouf 001. Study of separated chondrules from other CR2 chondrites is under way. Results: Layered chondrules in CR2 chondrites generally consist of (A) a silicate-rich core, surrounded by (B) a continuous or discontinuous FeNi mantle followed by (C) an anhydrous silicate-rich rim, +- (D) a serpentine-rich layer, +- (E) a matrix-like layer. Cores (A) consist of olivine (ol) and/or pyroxene (pyx) phenocrysts +- blebs of FeNi set in a feldspathic (feld) glassy or altered (chloritic) mesostasis. Core silicates are magnesian (Fa <=6) and, in some cases, zoned with an increase in FeO (up to 3 wt%), MnO and Cr2O3, at grain margins. Margins approach or equal the composition of silicates in the associated rim (C). Metal blebs have up to 14.3% Ni and a solar Ni:Co ratio. The metallic mantle (B) typically has lower Ni (up to 8.2%) and Co, and also has a solar Ni:Co ratio. Rims (C) consist of ol and/or pyx, a feld component +- FeNi metal; Pyx poikilitically encloses ol. Rims (C) have important general characteristics in comparison to their associated cores (A): (1) they are finer grained; (2) ol has higher (or similar) Fe/(Fe+Mg), Mn and Cr; (3) bulk compositions have higher Na, K and lower Al, Ca; (4) FeNi metal has lower Ni and Co, even lower than the associated metallic mantle, and has a solar Ni:Co ratio. Many layered chondrules contain an outermost serpentine-rich (D) and/or a matrix-like (E) layer. The outermost matrix-like layer (E) is petrologically similar to interstitial matrix and dark inclusions [5]. Cores and rims of three layered chondrules separated from El Djouf 001 have layers A, B and C; none have layers D or E. Oxygen isotopic compositions of the cores (A) and associated rims (C) of these chondrules (figure) lie on the CR2 mixing line (slope=0.7) [5], with each rim having heavier oxygen than its core. CR matrix compositions generally have even heavier oxygen than the rims (C). All of this is analogous to layered Allende chondrules [6], which lie on the Allende mixing line, have rims with heavier oxygen, and some rims have lighter oxygen than the cores of other chondrules. Conclusions: Chondrule cores (A) and rims (B-E) are derived from the same reservoir of materials. Rim (C) materials accreted onto solidified cores (A) and were subsequently melted (or partially melted). Since these rims (C) have higher volatile contents than the cores (A), they may have experienced lesser volatile loss or formed from more volatile-rich precursors. CR chondrite formation may be reflected in the CR layered chondrules, and may be a process of progressive accretion (in time and possibly in space) of chondritic materials at continuously lower temperatures, with decreasing volatile loss or increasing volatile enrichment, and evolution of oxygen toward heavier compositions. References: [1] Mason B. and Wiik H.B. (1962), Amer. Mus. Novit. 2106, 11 pp. [2] McSween H.Y. (1977), GCA 41, 1777-1790. [3] Prinz et al. (1985) LPSC XVI, 677-678. [4] Kring (1991) EPSL 105, 65-80. [5] Weisberg et al. (1992) GCA, in press. [6] Rubin et al. (1990) EPSL 96, 247-255.
Clayton Robert N.
Mayeda Toshiko K.
Prinz Martin
Weisberg Michael K.
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