Chondrites are divided into 3 classes, ordinary, carbonaceous, and enstatite, which are further divided into groups, 12 in all. These groups are defined by properties that include bulk compositions, isotopic compositions, oxidation states, and textures. Ordinary chondrites (OCs) are divided into three groups: H group has high total Fe contents (FeO as Fa and metallic Fe), L group has low total Fe, and LL group has lowest metallic Fe as well as low total Fe.
Early in their histories meteorite parent bodies were heated by radioactive decay and overburden pressures, among other sources, and their component rocks were thermally modified (metamorphism), some to the point of melting. Heating of chondritic parent bodies produced levels of metamorphism that range from complete recrystallization to those that show little change. A summary of the diagnostic mineralogic and chemical characteristics of ordinary chondrites is given in the table below.
| H | L | LL | ||
|---|---|---|---|---|
| Examples | Dhajala | Khokar | Semarkona, NWA 1756 | |
| Chemistry1 | Abundance patterns | RLE = 0.8 x CI; Fe2+/Fetotal~0.38 | RLE = 0.8 x CI; Fe and siderophiles ~ 0.75 x CI; Fe2+/Fetotal~0.66 | RLE = 0.8 x CI; Fe and siderophiles ~ 0.6 x CI; Fe2+/Fetotal~0.88 |
| Mean Mg/Si | 0.96 | 0.93 | 0.93 | |
| Mean Al/Si | 0.069 | 0.068 | 0.065 | |
| Mean Ca/Si | 0.052 | 0.050 | 0.048 | |
| Mean δ18O | 4.1 | 4.6 | 4.9 | |
| Mean δ17O | 2.9 | 3.5 | 3.9 | |
| Approximate chondrule abundance (vol. %) | 60-80 (matrix is dominately olivine) | |||
| Chondrule | Size (mm) | 0.3 | 0.7 | 0.9 |
| Types 2 | PO (23), POP (48), PP (10), granular (3), BO (4), RP (7), CC (5) | |||
| Ni-Fe metal (vol. %) | 15-19 | 4-9 | 0.3-3 | |
| Olivine | Fa (mol. %) | 16-20 | 22-26 | 27-32 |
| FeO/MnO (g/g) | 30-40 | 46-55 | 50-63 | Pyroxene composition Fs (mol. %) | 14.5-18.5 | 19-22 | 23-26 |
Van Schmus and Wood (1967) devised a clever classification scheme that combined metamorphic changes with bulk compositions and assigned the petrologic types 1 through 6. Only types 3 through 6 are used for ordinary chondrites, 1 and 2 are theoretical and have not been found. The most primitive of these is type 3 and the remaining types reflect increasing metamorphic grade through type 6. The original classification system did not include a type 7, although researchers have used a 7 designation to account for chondrites that have no relict chondrules. In other words, metamorphism progressed to the point that all chondrules were completely recrystallized. The table below summarizes some of the critical changes that are more easily recognized by observations and analyses.
| Criterion | Petrologic Type | |||
|---|---|---|---|---|
| 3 | 4 | 5 | 6 | |
| Olivine homogeneity | > 5% mean deviations | <5% | homogeneous | |
| Low-Ca pyroxene |
predominantly monoclinic | <20% monoclinic | ≤20% monoclinic | orthorhombic |
| Feldspar | minor primary | secondary <2 μm grains | secondary 2-50 μm grains | secondary >50 μm |
| Chondrule glass | clear, isotropic | devitrified | absent | |
| Matrix | opaque to transparent | transparent, recrystallized (coarsening in 4 to 6) | ||
| Chondrule-matrix definition | sharply defined in 3-4, becoming more diffuse and less abundant in 5-6 | |||
| Carbon (wt.%) | 0.2-1 | <0.2 | ||
| Water (wt.%) | 0.3-3 | <1.5 | ||
Type 3 to 4 chondrites possess characteristics that have the potential for finer tuning of metamorphic effects. Because of the subtle differences in textures and compositions in these inhomogeneous meteorites, classification into subtypes (3.1-3.9) between types 3 and 4 can be made that are useful in estimating heating/cooling rates, temperatures reached, and elemental diffusion rates for materials on or near the surface of the respective parent bodies. For example, FeO contents (Fa molecule, the FeO-rich end member of the dichotomous FeO-MgO series in olivines) in unequilibrated olivines are very heterogeneous and may vary between Fa1 to Fa60 in the least equilibrated subtypes. Fa contents in subtypes < 3.5 show a deviation from the mean of ± Fa15, subtype 3.9 has a deviation of ± Fa3 from the mean. Type 4 olivines have a deviation of <Fa1 or that within the precision microprobe analyses.
The methods used for determining these subtypes in unequilibrated chondrites are rather labor intensive and time consuming. Recently, Grossman and Brearley (2005) refined an analytical technique that is very reliable and uses less instrumental and observational time (figure below from their paper). This technique is based on chromium (Cr) content and distribution in FeO-rich olivine, which is very sensitive to metamorphic conditions and changes dramatically between subtypes 3.0 and 3.2. For example, Semarkona, the only 3.0 ordinary chondrite and the least equilibrated known, has a mean Cr2O3 content in ferroan olivines of 0.50 wt % and a range of 0.4 to 0.5 wt %. The LL3.2 Bishunpur chondrite has a Cr2O3 content of 0.22 ± 0.16 wt % in its ferroan olivines; the more equilibrated LL3.4 Chainpur has 0.06 ± 0.06 wt % Cr2O3. Those > 3.6 are devoid of measurable Cr2O3 (< 0.02 wt % or 200 ppm via the electron microprobe), chromium was entirely driven out of the olivine structure and into the surrounding matrix in chondrules of subtype >3.6 by metamorphic heating.
Compositional zoning in olivine and pyroxenes of unequilibrated chondrites is readily observed in scanning electron microscope (SEM) and electron microprobe (EMP) backscattered electron images. These images are created when high-energy electrons strike the sample surface and are backscattered according to the sample's atomic number or density. Bright areas in the images indicate dense (Fe-rich) material and dark areas indicate areas of less dense or MgO-rich material. The first SEM BSE image below displays dark, MgO-rich cores in olivine and FeO-rich rims in an unequilibrated LL3.1 chondrule. Compositional zoning is absent in subtypes > 3.6 where metamorphic heating has homogenized the FeO- and MgO- rich zones. Additional images below show the gradual homogenization of FeO and MgO in chondrule olivine with increasing petrologic (thermal) grade.