Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.229g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 229
Other
7
Scientific paper
It is widely held that alteration of Ca Al-rich inclusions (CAIs) in CV3 and CO3 meteorites occurred in the nebula (Hashimoto 1992). The CO3 chondrites, however, appear to define a metamorphic sequence dominated by parent body, and not nebular, metamorphic effects (Scott and Jones, 1990). To investigate the effects of metamorphism on CAIs we have studied inclusions from 4 CO chondrites: Colony (3.0), Felix (3.2), Lance (3.4), and Warrenton (3.6). In a section of Colony (74 mm^2) 81 CAIs, 30-870 micrometers long, comprise 52 nodular spinel-rich inclusions (fragments of Type-A CAI composed largely of spinel), 12 spinel-pyroxene inclusions, 10 melilite-rich inclusions, 2 hibonite-only inclusions, 2 CaAl4O7-bearing inclusions, and 3 spinel-pyroxene- olivine inclusions. Although a find, CAIs in Colony are relatively fresh, melilite in particular being little altered. In 79% of the spinel-bearing inclusions, spinel has <2wt% FeO, which otherwise ranges to 34.8%. Mg isotopic compositions were determined in 5 selected Colony inclusion; evidence of ^26Mg* from decay of ^26Al was found in 4 CAI. A hibonite-only inclusion has the largest ^26Mg* excess, delta^26Mg 32o/oo. Data show no evidence of isotopic disturbance and define a linear array with slope ^26Mg* /^27Al = (3.4+- 0.6) x 10^-5, like that obtained by Davis and Hinton (1986) in a hibonite-bearing spherule from Ornans. Despite Al/Mg ratios of up to 1500, CaAl4O7 in one inclusion shows no evidence of ^26Mg*; ^26Mg* < 4 x 10^-6. All three melilite-bearing inclusions from Colony C21 (angstrom k(sub)8.3-14.3), C56 (angstrom k(sub)10.5-16) and C62 (angstrom k(sub)15-21) show evidence of radiogenic ^26Mg*. Excess ^26Mg positively correlates with the Al/Mg ratios but the data do not define a unique initial value of ^26Al/^27Al. Data for melilite in C21, in particular, show evidence for disturbance of the Al-Mg system, as is common for Allende CAI (Podosek et al. 1991). Melilites in C56 in contrast show no evidence of isotopic disturbance. In one section of Felix (48 mm^2) 15 CAIs, 66-440 micrometers long, comprise 6 nodular spinel-rich inclusions, 1 spinel-pyroxene inclusions, 7 melilite-rich inclusions, and 1 pervasively altered inclusion. Melilite shows partial breakdown to a fine-grained alteration product. Fe-rich monticellite (Fe(sub)0.7Mg(sub)0.3CaSiO(sub)4) occurs along the margins of 3 inclusions, presumably formed by the exchange of Ca for Mg in matrix olivine during melilite breakdown. Spinel in Felix has 0-30 wt% FeO, and many grains are strongly zoned. In one section of Lance (53 mm^2) 15 CAIs, 65-450 micrometers long, comprise 8 nodular spinel-rich inclusions, 1 melilite-rich inclusions, and 6 pervasively altered inclusions. Melilite is present in only 3 inclusions having been otherwise totally replaced. Spinels in Lance are strongly zoned, the range being 3.4-25.9 wt% FeO. In two sections of Warrenton (combined area 70 mm^2) 38 CAIs, 70-1000 micrometers long, comprise 18 nodular spinel-rich inclusions, 18 pervasively altered inclusions, 1 hibonite-only inclusions, and 1 olivine-fassaite inclusions. Spinel contains 19.4-24.1 wt% FeO and individual grains show minor zoning. Perovskite inclusions in spinel have altered to FeTiO3. Melilite has been totally replaced by a fine-grained alteration product. With increasing metamorphic grade in CO3s a sequence of changes occurs in CAIs. Melilite progressively breaks down to become completely altered. Spinel becomes enriched in FeO and in Warrenton (3.6) has almost completely reequilibrated. Perovskite is replaced by FeTiO3. Evidence for increasing severity of alteration in the 4 CO chondrites agrees qualitatively with the metamorphic sequence of Scott and Jones (1990), and suggests that alteration of CAIs occurred within the CO parent body. Disturbance of Al-Mg systematics in Colony CAIs, the lowest petrographic type, indicates that either the inclusions were not pristine before accretion or, more likely, that Colony did not completely escape metamorphism . Davies, A.M. and Hinton, R.W. (1986) Lunar Planet. Sci. (abstract) 17, 154-155. Hashimoto, A. (1992) Geochim. Cosmochim. Acta 56, 511-532. Podosek, F.A. et al. (1991) Geochim. Cosmochim. Acta 55, 1083-1110. Scott, E.R.D. and Jones, R.H. (1990) Geochin. Cosmochim. Acta 54, 2485-2502.
Greenwood Richard C.
Huss Gary R.
Hutcheon Ian D.
Hutchison Robert
No associations
LandOfFree
CAIs in CO3 Meteorites: Parent Body or Nebular Alteration? 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 CAIs in CO3 Meteorites: Parent Body or Nebular Alteration?, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and CAIs in CO3 Meteorites: Parent Body or Nebular Alteration? will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1209497