Abstract

On June 15, 1994 many of the residents of Montreal watched and listened to a fireball crossing the city's sky. The next day 20 meteorite fragments weighing approximately 25 kg were recovered in St Robert, Quebec (Brown et al ,1996). This report presents data collected from one of the pieces of this meteorite purchased by a consortium of Quebec universities.

The meteorite is classed as a H5 ordinary chondrite. Texturally the sample is characterized by approximately 30 % chondrules of varying textures (mainly barred or porphyritic) in a matrix of chondrule fragments and native iron.

The chondrules are composed mainly of olivine and orthopyroxene, in a matrix of extremely fine grained (average grain size less than 5 microns) recrystallized glass. The chondrule matrix is apparently composed of feldspar and diopside, although positive identification is difficult. The chondrules also contain minor amounts of native iron. The matrix of the meteorite is primarily olivine, orthopyroxene and native metal, in order of abundance. Native iron occurs as two distinct types, Ni poor iron (kamacite), with Ni homogeneously distributed through it and Ni rich iron (taenite) with Ni richer areas randomly distributed through it. Minor phases in the matrix (in order of abundance) include feldspar, clinopyroxene, chromite, troilite, whitlockite and chlorapatite. Especially interesting is the presence of several small (less than 50 microns) grains of native copper which are consistently associated with taenite and troilite.

Microprobe analyses of the various phases were carried out with the following results. The composition of olivine and orthopyroxene is consistent over all parts of the meteorite. The olivine is forsteritic (Fo₈₃) and the orthopyroxene is bronzite (En₈₃). The majority of the feldspars are oligoclase (Ab₈₀An₁₆Or₄). Chromite average composition is Mg_0.16Al_0.27Fe_0.87Cr _1.590₄ with possibly significant Mg and Fe variations between the core and rims of the grains. Kamacite averages approximately 5 wt % Ni, while the taenite ranges from 20 to 60 wt % Ni.

Equilibration temperatures of the olivine and orthopyroxene were modelled using TWEEQU (Berman, 1991). The resulting temperatures were approximately 650 °C, in good agreement with calculated temperatures from other H5 chondrites. Temperatures calculated for co-existing orthopyroxene and clinopyroxene are in excess of 1000 °C suggesting that clinopyroxene has not re-equilibrated.

Figure 1. Photographic view of the McGill Sample of the St. Robert Meteorite.

Figure 2. 0.5 mm chondrule of the porphyritic type. The ntermediate phase which makes up most of the chondrule is orthopyroxene. The brighter phase on the bottom of the chondrule is olivine. The dark material between the orthopyroxene is mostly feldspar with some clinopyroxene. This material is believed to be recrystallized glass.

Figure 3. Mg ka x-ray map of a fine grained chondrule. Chondrule is mostly orthopyroxene (brighter areas) with fined grained clinopyroxene and feldspar (dark areas). The bright material outside the chondrule is olivine, while the black areas are kamacite.

Figure 4. Enlargements of chondrule matrices. Panel A shows the centre of a fine grained chondrule. Panel B shows an enlargement of the centre of the chondrule in figure 3.. Panel C shows the edge of the same chondrule as panel A.

Figure 5. Typical inter-chondrule matrix area. White patches are predominantly kamacite. Lighter areas are composed of angular silicates and chondrule fragments with intergranular devitrified glass.

Figure 6. Coloured backscattered image of aggregate of metal and sulphide grains. White grain in lower right is native copper, darkest phase is troilite. Note the striking contrast between the two large kamacite grains in terms of homogeneity. In almost all cases native copper is found with heterogeneous taenite grains.

Fig 7. Minor phases in meteorite matrix. Panel A shows the common relationship between the two phosphate minerals found in the meteorite. Chlorapatite is the brighter phase on the left and the darker phase on the right is whitlockite. Panel B shows a chromite grain on he edge of a chondrite, where they are often found. The mineral in the lower left corner is whitlockite. Panel C shows a mixed taenite - kamicite grain. Taenite is the brighter phase. The small bright speck on the right end of the grain is native copper. Panel D shows an intergrowth of taenite (bright) and troilite (dark).

Figure 8. Plot of MgO vs. FeO for all silicates.

Fig 9. Fe vs Ni wt % for all Fe bearing non-silicate phases. The range of Fe and Ni values in Cu metal are probably the result to secondary fluorescence of Fe and Ni by Cu Ka x-rays.

Figure 10. Plot of maximum and minimum temperatures calculated using various calculations and calibrations. Olivine -Chromite (1) - Wlotzka (1987) Olivine - Chromite (2) Johnson and Prinz (1991). Olivine - Opx, Cpx - Opx and Olivine Cpx - TWEEQU (Berman, 1991). Opx and Cpx - Lindsley (1983).

Figure 11. WDS profile across small taenite grain in matrix. Note smooth increase in Ni and decrease in Fe as rim is approached. See Figure 7, panel C for location of traverse.

Figure 12. Plot of Ni wt% vs. distance (mm) for the rims of some matrix kamacite grains. Note increasing width of diffusion profile with increasing grain size.

Discussion

Attempts to determine the metamorphic temperatures of the St Robert meteorite using mineral thermometry have met with limited success. Olivine and orthopyroxene appear to be equilibrated throughout the entire meteorite (Fig 8). Calculation of temperatures for these two minerals using TWEEQU (Berman, 1991) gives values ranging from 500 to 800 °C. Calculation of clinopyroxene - orthopyrene equilibria, however, give temperatures which are much higher (950 - 1100 °C). McSween et al. (1989) argue that Ca diffusion in CPX was too slow to maintain equilibrium. Temperatures calculated using the calibrations of Lindsley (1983) for Cpx and Opx give results similar to those obtained from TWEEQU. Thermometry based on olivine - spinel compositions (Johnson and Prinz, 1991) suggest temperatures of approximately 500 +/- 100 °C for their calibration and 650 to 750 °C for Wlotzka (1987) . Wlotzka (1987) has suggested that this range is due to different closure temperatures for different grain sizes. Further work is required to determine if this temperature range is in fact correlated to different grain sizes. The strongest statement that can be made about the temperature of equilibration for St Robert is that it was probably somewhere between 600 and 700 °C, and although this is a large range, it is in reasonable agreement with Dodd's estimate of 700 to 750 °C for H5 chondrites (Dodd, 1981).

The metallic phases of the meteorite show a wide range of textures and compositions. The ubiquitous matrix kamacite grains exhibit consistently zoned edges, with the Ni content increasing from 4.4 at the rim to 6.5 - 7 % in the centres of these grains. The width of the profile ranges from 25 to 45 um, with wider profiles observed on larger grains (Fig. 12). This compositional variation is consistent in all kamacite grains and probably represents re-equilibration during cooling from peak temperatures. The values at the grain centres are quite comparable to those measured in H5 and H6 metal grains in ordinary chondrites by Afiattalab and Wasson (1980). The rarer taenite grains show more varied textures. Particular attention was paid to one grain consisting of an intergrowth of taenite (possibly tetrataenite cf. Clarke and Scott, 1980) and kamacite surrounded by a strongly zoned taenite rim (Fig 7c). The rim zone drops from 48 to 30 wt % Ni within 30 um of the edge. The core zone is composed of taenite grains (tetrataenite?) with an average composition of ~ 55 wt % Ni surrounded by kamacite of an average composition of 4.2 wt % Ni. Further work on the timing, and development of these features should shed light on the thermal history of the meteorite.

Overall, the St Robert meteorite is a an ordinary unequilibrated H5 chondrite. with all it's features well within the range of those for previously described meteorites of its class and petrologic type (Frederiksson and Wlotzka, 1985).

References

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Brown, P., Hildebrand A. R., Green D.W.E., Page D., Jacobs C., Revelle D., Tagliaferri E., Wacker J., and Wetmiller B., (1996). The Fall of the St. Robert Meteorite. Meteoritics and Planetary Science. 31 P502 - 507

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