Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.569r&link_type=abstract
Meteoritics, vol. 30, no. 5, page 569
Mathematics
Logic
1
Calcium-Aluminum-Rich Inclusions, Classification, Chondrites, Carbonaceous, Extinct Radionuclides, Aluminum-26, Meteorites, Macalpine 87300, Macalpine 88107, Rare Earth Elements
Scientific paper
MAC 87300 and MAC 88107 are similar, ungrouped carbonaceous chondrites with properties that are intermediate between the CM and CO meteorite groups, though they are more closely related to the latter [1,2]. 170 CAIs were observed in a 102 micrometers^2 thin section of MAC 87300, all <500 micrometers and most <300 micrometers in size. Most are composed of pyroxene and spinel, with textures similar to spinel-pyroxene inclusions in CM meteorites [3]. Melilite-bearing inclusions are uncommon. One unusual hibonite-bearing inclusion, #43, is composed of three large (<=75 micrometers) hibonite laths surrounded by thin rims of hibonite + spinel + (outermost) diopside. The hibonite has low TiO2 (1.8 wt%) and MgO (0.8 wt%), reminiscent of Murchison PLACS [4]. Hibonite in #43 has a unusual smoothly fractionated concave-upwards rare-earth (REE) pattern, with Lu enriched ~3x relative to the light REE, a small negative Eu anomaly and a much larger negative Yb anomaly. The largest inclusion (480 micrometers across) in this section, #44, is a Type A consisting of melilite (Angstrom k(sub)7-17) enclosing 1-5 micrometers sized grains of spinel. The inclusion has a well-defined diopside rim and an accretionary mantle. REE in #44 are unfractionated, ~10-30xC1, with a small negative Yb anomaly. No excess 26-Mg was detected in either #43 or #44: for #43, inferred initial 26-Al/27-Al <1x10^(-5) at 27-Al/24-Mg ~105-125; for #44, initial 26-Al/27-Al = (0.5+/-2.0)x10^(-5) (+/-2sigma) at 27-Al/24-Mg up to 32 (see Fig.1). 162 CAIs were located in a 172 micrometers^2 thin section of MAC88107, similar in size distribution to those in MAC87300. The most common CAIs are again spinel- and pyroxene-rich, but irregularly-shaped melilite-rich Type A inclusions (Angstrom k(sub)3-19; FeO in spinel <=0.6 wt%) similar to those in the least-metamorphosed CO3 chondrites [5] are more common than in MAC 87300. Inclusion #14 is a compact intergrowth of melilite, spinel, perovskite and hibonite laths, with an incomplete diopside rim. Inclusion #16 is unrimmed and extensively cracked, consisting of melilite enclosing spinel and perovskite. Inclusion #20 consists of melilite, spinel and perovskite (the latter two phases rimmed with Ti-rich pyroxene), and has numerous round voids. The inclusion has a complete diopside rim. Inclusion #35 is a compact, unrimmed intergrowth of melilite and spinel only. Apart from pyroxene, no secondary minerals were detected in these four inclusions. Each contains an excess of 26-Mg that corresponds to an initial 26-Al/27-Al of ~4.5x10^(-5). REE patterns for #14, #16 and #20 are unfractionated with REE ~50xCI (#14, #16) and ~10xCI (#20). Inclusion #35 has a modified Group II REE pattern. Differences in CAI populations from MAC 87300 and 88107 suggest that the two meteorites are not paired. The CAIs from MAC 87300 and 88107 are most closely related to those from the lowest petrologic type CO3s, and demonstrate that these two meteorites are extremely primitive carbonaceous chondrites. The canonical initial 26-Al/27-Al of CAIs (4-5x10^(-5)), defined by the major carbonaceous chondrite groups [6], also applies to inclusions from MAC 88107, whereas no evidence for 26-Al in MAC 87300 has yet been found. References: [1] Kallemeyn G. (1992) LPS XXIII, 649-650. [2] Sears D. et al. (1990) LPS XXI, 1121-1122. [3] MacPherson G. and Davis A. (1994) GCA, 58, 5599-5625. [4] Ireland T. et al. (1988) GCA, 52, 2827-2839. [5] Greenwood R. et al. (1992) Meteoritics, 27, 229. [6] MacPherson G. et al. (1995) Meteoritics, 30, in press.
Davis Aileen M.
MacPherson Glenn J.
Russell Stuart
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