Stony meteorites with compositions reflecting solar abundances of nonvolatile elements are regarded as chondrites, a group that accounts for more than 85% of all meteorite falls. Further studies based on compositional and petrographic trends have distinguished a number of chondrite groups, including 8 carbonaceous (CI, CM, CO, CV, CK, CR, CB, CH), 3 ordinary (H, L, LL), 2 enstatite (EH, EL), and R and K chondrite groups, along with a number of subgroups resolved for some. A group is established when 5 or more members are recognized as sampling a unique parent body through similarities in their taxonomic properties. Some groups are associated through their formation under similar conditions at a narrow range of heliocentric distances, and are therefore grouped into respective clans (Kallemeyn and Wasson, 1981). Furthermore, petrographic trends within the chondrite groups define a metamorphic sequence of types 1–7, as originally outlined in the Van Schmus–Wood (1967) classification scheme, based on both the silicate and the opaque phases:
SILICATE PHASES:
Even though type 3 chondrites have remained essentially unaltered, lower types have experienced progressive aqueous alteration and higher types progressive thermal or shock alteration (type 3 = 250–600°C, type 4 = 600–700°C, type 5 = 700–750°C, type 6 = 750–950°C; Keil, 2000). Type 7 chondrites are recrystallized and transitional to an achondrite classification.
More accurate equilibration temperatures based on the olivine/Cr-spinel thermometry have been calculated by Wlotzka (2005) and Kessel et al. (2007). Type 4 through 6 H chondrites cover a narrow range of peak temperatures (~150°C) and have similar average temperatures. However, the high range of cooling rates at low temperatures are inconsistent with an onion-shell model. These facts are more consistent with a two-stage scenario, in which initial metamorphism occurred within an onion-shell structure(s), which was then followed by breakup and reassembly into a rubble pile structure. Type 4 through 6 H chondrites reached maximum temperatures of 825°C, while type 3.7–3.8 chondrites were located in the cooler outer regions of the rubble pile and did not experience temperatures higher than 660°C. The classification scheme proposed by Binns (1967) may best reflect this new metamorphic scenario; it separates all petrologic types into primitive (type 3), intermediate (type 4), and crystalline (types 5 and 6) groups.
Type 3 chondrites have been further resolved into types 3.0–3.9 by the use of several techniques, including induced thermoluminescence (TL) sensitivity measurements. This technique measures a 1,000-fold or higher variation in the TL sensitivity corresponding to the feldspar abundance, which is increasingly produced through the devitrification (crystallization) of chondrule glass as the degree of metamorphism increases. On the other hand, feldspar is destroyed through shock and reheating processes. Though not useful in distinguishing between the very lowest petrographic types, TL sensitivity measurements have been utilized for the ordinary chondrite groups, and, with reduced degrees of variation, for certain carbonaceous chondrite groups and achondrites (CO, CV, Coolidge grouplet, eucrites, and shergottites).
A new decimal scheme that is more discriminating at the lowest petrologic types for the highly unequlibrated chondrites (3.0–3.1–3.2) was proposed by J. Grossman and A. Brearley (2005). The new classification scheme, based on a sensitive technique utilizing the variation in the distribution of Cr in ferroan olivine, is virtually unaffected by the processes of terrestrial weathering and aqueous alteration. The scale of the new decimal system is extended as follows:
3.00–3.05–3.10–3.15–3.2+
They have identified several parameters, which, when used in combination, are instrumental in determining an accurate classification at the lowest petrologic grades:
Another method to distinguish the least equilibrated chondrites was proposed by Bunch et al. (1967), and more recently by Kimura et al. (2006). They utilized the systematics of spinel group minerals (solid solutions of chromite [FeCr2O4] to spinel [MgAl2O4]) to resolve petrologic types in LL chondrites. They found that spinel minerals in type 3.00–3.3 chondrites contain pure Cr-spinel, Mg–Al-spinel, and chromite, which preserve a very wide compositional range. As an increase in thermal metamorphism results in higher petrologic types of 3.5-3.9, diffusion tends to smooth out any compositional variability, and Mg–Al-spinel is absent. By petrologic types 4–6, there is compositional homogeneity of chromite. In addition, they found a trend of increasing size and abundance of spinel group minerals as petrologic type increased.
Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphic criteria to distinguish between petrologic types 6 and 7:
In further contrast to Dodd et al., Wittke and Bunch (2004) suggest that the relative size of all of the silicates, rather than only the feldspar grains, would provide a better gauge of a petrographic type 7 since silicates attain an equigranular texture only under the highest metamorphism. They have also discovered that simple twinning of plagioclase occurs only in type 7, and suggest that this could be utilized as an additional parameter. Beyond that, it was revealed that modal metal contents decrease significantly during late metamorphic stages; i.e., low-Ni metal, as well as pyroxenes, are consumed to produce olivine, resulting in only small amounts of Ni-rich metal along with lower amounts of orthopyroxene and clinopyroxene compared to those amounts present in lower metamorphic grades.
Metachondrite is a newly recognized term, proposed by Irving et al. (2005) to describe those achondrites which are texturally evolved chondrites. The metachondrites have completely recrystallized textures resulting from high degrees of metamorphism or partial melting, they lack chondrules, and their elemental ratios and O-isotopic compositions show affinities to several existing chondrite groups (e.g., CV, CR, H, L, and LL). Other groups of achondrites having relict chondrule-bearing members may also be more appropriately called metachondrites (e.g., acapulcoites and winonaites, as demonstrated by Monument Draw and NWA 725, respectively.
For those meteorites that experienced metamorphic temperatures high enough for metal–sulfide melting to occur, which most commonly occurs as a result of impact events, an igneously textured partial melt residue would be produced (Mittlefehldt and Lindstrom, 2001). In these cases the use of the Van Schmus–Wood classification scheme would no longer be valid, and these meteorites may be referred to as primitive achondrites or even impact melts. The following characteristics are typically observed in primitive achondrites (Ford et al., 2004):
Recent studies into the metamorphic changes of opaque phases in chondrites have led to the establishment of a calibrated metamorphic scale. From type 3.0 to 3.5, rounded metal and sulfide grains remain associated, but contact becomes less distinct. Sulfide abundance increases inside chondrules. From type 3.5 to 4, metal and sulfide separate from each other and sulfide grains aggregate (more
obvious by 3.7). Opaque grains inside chondrules become more angular. By type 3.8 zoning in taenite begins, and metal grains begin to merge due to grain boundary diffusion. From type 4 to 5, the segregation of metal and sulfide becomes complete and metal grains gradually coalesce, their shapes governed by the spaces between the silicates. Recrystallization continues from type 5 to 6, and by type 6, chondrules have virtually disappeared and metal grains have become smaller and more evenly distributed.
Kimura et al. (2006) have found that FeNi-metal can be used to resolve the petrologic subtypes at the very lowest scale, consistent with the scheme previously proposed by Grossman and Brearley (2005), in which they measured the distribution of Cr in olivine. The classification scheme utilizes a decimal system to extend the petrologic resolution: 3.00–3.05–3.10–3.15–3.2+. The texture and composition of FeNi-metal varies with both its petrologic subtype and its location (e.g., within chondrules, on chondrule rims, and within the matrix). Within chondrules, FeNi-metal systematically progresses from plessite to a coarse-grained intergrown of kamacite and Ni-rich metal.
Chondrules are found in all petrologic types except types 1 and 7, in which either aqueous or thermal alteration, respectively, has left them indistinct from the matrix. Chondrules were likely formed during a time span 1–3 m.y. after CAI formation, in shock waves caused by gravitational instabilities. They crystallized rapidly, within seconds to minutes, from supercooled liquid droplets of CI-like precursor material. They are classified by both textural and compositional type, each associated with a specific thermal history (i.e., number of nucleation sites, melting temperature, degree of undercooling, cooling rate):
Porphyritic chondrules experienced peak temperatures lower than those of barred or radial chondrules, with the range of peak temperatures calculated at between 1500–1850°C, and the duration measured in minutes. These peak temperatures are consistent with the observation that some crystallization nuclei were conserved in porphyritic chondrules, while all were lost in barred and radial chondrules. In general, 84% of the chondrule population is composed of porphyritic chondrules, while radial and barred textures account for 7–9% and 3–4%, respectively. It was determined by Fox and Hewins (2005) that porphyritic chondrules may have required multiple cycles of reheating to develop.
Compound chondrules represent only ~2% of ordinary chondrite chondrules, and ~1.4% of carbonaceous chondrite chondrules. They formed in high density regions characterized by high melting and high cooling rates, some representing multiple melting events, consistent with the preponderance of radial and barred textures resulting from complete melting. The cooling rate for chondrules in a molten state was initially rapid, but following the onset of crystallization it became much slower. Four basic structural types of compound chondrules have been identified (Wasson et al., 1995; Akaki and Nakamura, 2005): 1) enveloping—the enclosure of one chondrule by another resulting from secondary flash melting of a dust layer on the primary chondrule; 2) adhering—the adherence of a small melted chondrule onto a larger solidified chondrule resulting from collision; 3) consorting—two conjoined chondrules of similar size resulting from collision; and 4) blurred boundary—the product of a collision between two partially melted chondrules, resulting in an unrecognizable and texturally blurred boundary.
2. COMPOSITION (McSween, 1977; Cohen et al., 2004; Kunihiro et al., 2004)
olivine vs. pyroxene ratio
The FeO content increases and the crystallization temperature decreases as crystallization proceeds in the following order:
IA => IAB => IB => IIA => IIAB => IIB => CC
Olivine-rich IA chondrules were the earliest to form, crystallizing at temperatures of 1547°K in a solar gas pressure of 0.1 bar at a location ~1 AU from the sun (Blander et al., 2004); from 26Al/27Al ratios, ages were calculated to be as low as 0.7 (±0.2) m.y. after CAI formation. Chondrules in CV chondrites are the oldest known and may be associated with this early stage of formation. They derived their O-isotopic ratios through the mixing of 16O-rich primordial gas with vaporized 16O-poor ices that were transferred from the outer nebular regions. Oxygen isotopic systematics provides evidence showing that after the formation of this early phase of chondrules, they underwent partial dissolution and evaporative loss of SiO. This SiO was subsequently added to the next generation of chondrule precursor material via nebular gas exchange, producing the pyroxene-rich chondrule population that formed as temperatures cooled down to 1435°K. The olivines present in this later generation of chondrules are in isotopic disequilibrium with both the pyroxenes and the mesostasis and are considered to be relict grains (Chaussidon et al., 2008). It was suggested by Fox and Hewins (2005) that the presence in some chondrules of relict grains of a different type—type-I relicts inside of type-II chondrules and vice versa—provides evidence for a simultaneous formation of these chondrule types.
The website of the Northern Arizona University Electron Microprobe Lab hosts a full color photographic exposition of the different
chondrule characteristics.
Specific terminology has been adopted to help distinguish those chondrites that are unique from the others. Certain meteorites are classified with a hyphen separating two different petrographic grades, e.g. L3–6, signifying a breccia containing L3 and L6, and possibly (but not necessarily) everything in between. Others may have a forward slash separating two different classes or petrologic grades, e.g. L/LL5/6, representing a transitional class and/or petrologic grade, or alternatively, the forward slash may be used when the classifier cannot distinguish between the two choices. Another convention utilizes parentheses separating two different groups, e.g. L(LL)3, indicating that the first group listed is the most probable classification, although the group in parentheses may actually be the correct class.
These are primitive, undifferentiated, stony meteorites composed of silicate chondrules set in a fine-grained silicate matrix. Within the matrix, calcium-aluminum inclusions are commonly found, which represent the earliest material that condensed from the hot nebula. In addition, certain isotopes are present that originated within interstellar grains that predate the formation of the solar system. Also found in these meteorites are carbon compounds including long-chain hydrocarbons and amino acids similar to those used in protein synthesis in living organisms. Carbonaceous chondrites formed in an oxygen-rich environment with most metal combined into silicates, sulfides, or other oxides. They formed on the smaller asteroids that retain the oldest record of the solar nebula, containing solar abundances of non-volatile elements. Carbonaceous chondrites constitute 2.5% of all meteorites recovered so far, and they have been divided into the following chemical groups: CI, CM, CR, CO, CK, CV, CH, and CB groups, along with the Coolidge–Loongana grouplet and some unique ungrouped members. The order in which each carbonaceous chondrite parent body accreted has been estimated by the relative ages of their chondrules (Alexander et al., 2007). This is given to be, from oldest (4566.7 ±1.0 m.y.) to youngest (4564.7 ±0.6 m.y.), CV > CM > CO + OC > CR. The CI group contains up to 20% water locked into hydrated minerals. The discovery of new and unique carbonaceous chondrite meteorites helps us to continually revise the record of early solar system processes.
The CV group has been further divided into three subgroups (McSween, 1977; Weisberg et al., 1997):
The following mineralogical relationships have been found to exist among these three subgroups:
Subtypes among CV3 group members, and more recently between chemical classes, have been successfully resolved utilizing Raman spectroscopy to quantify the thermal metamorphic maturity of organic matter, in conjunction with other independent metamorphic tracers (i.e., noble gas and presolar grain abundances, and zoning of olivine phenocryts) (Bonal et al., 2006). In an expansion of this method, Quirico et al. (2006) determined that LL3.0 Semarkona has experienced thermal metamorphism beyond the onset stage, and they proposed a new petrologic scale to provide consistency in the range as follows: Semarkona would become petrologic type (PT) 1, with PT 0 being reserved for the stage of true onset of thermal metamorphism. All other meteorites analyzed to date would have a PT greater than 1.
The ordinary chondrites are composed of varying ratios of mostly olivine and pyroxene with spheroidal chondrules that represent the early condensates of the presolar nebula. The group is subdivided primarily into the H (olivine-bronzite), L (olivine–hypersthene), and LL ("amphoterite") groups based on chemical trends, mainly their iron to silicon ratio. The "H" refers to a high-iron content of 27 wt%, the "L" to a low-iron content of 23 wt%, and the "LL" to both a low-iron content of 20 wt% along with a low-metal content of only 2 wt%. Additionally, there are a number of transitional ordinary chondrites that may be anomalous members of one of the established chondrite groups, but may also represent new chondrite groups. These anomalous meteorites have been given the designations H/L and L/LL. With a few outliers, the majority of ordinary chondrites fall within distinct fayalite and ferrosilite ranges:
Ordinary chondrite material comprises variable petrographic types ranging from 3 to 7. This material likely formed as an onion-shell structure within the parent asteroid, reflecting an increased depth and a reduced cooling rate for a correspondingly higher petrographic type. The fact that the abundance of brecciated members increases as the petrographic type decreases (nearer the surface), is further support for this ordering scheme. Further metamorphic equilibration may have occurred following the breakup and reassembly of the original planetesimal(s), and the subsequent formation of a rubble pile structure. Thermal history constraints predict a diameter for the ordinary chondrite parent bodies of between 160 and 180 km. Based on remote sensing data, a likely asteroid for the parent body of the H chondrites is the S,IV-type, 6 Hebe. Most L chondrites were severely shocked and had their radiometric chronometers reset ~20 m.y. ago, which recorded a disruptive impact on the parent body. Ordinary chondrites represent about 79% of all meteorite falls.
These chondrites are highly reduced with all of the iron visible as metal or troilite (FeS). The silicate consists mainly of the iron-free pyroxene, enstatite. As with the ordinary chondrites, a subdivision is made based on the bulk iron content; the EH group contains ~30% total iron, while the EL group contains only ~25%. Each subgroup comprises a complete range of thermally metamorphosed types, from 3 through 6 (or 7), with the Si and Ni content in kamacite increasing with respect to increasing petrographic type of each series.
The following mineralogical relationships have been found to differentiate these subgroups:
A third grouplet with intermediate mineralogy has recently been identified, represented by the meteorites Y-793225 and QUE 94204. Studies have determined that they were not derived from the EH or EL groups through any metamorphic processes. Oxygen-isotopic data and rare-gas fractionation patterns have led some researchers to suggest that E chondrites may have formed inside the orbit of Venus. However, the identification of E-type asteroids in the inner asteroid belt provides evidence that the asteroid belt was their actual location of origin. Supporting the latter theory, studies of Cr isotopes and their correlation with heliocentric distance place the formation of E chondrites ~1.4 AU from the Sun. Based on N and O systematics, a ratio of EC and OC material of 57:43 has been shown to be most consistent with the composition of the precursor material of Mars. Enstatite meteorites are rare, representing about 1% of all meteorite falls.
A modified version of the Van Schmus–Wood classification scheme has been proposed by Berlin and Stöffler (2004) to accommodate the R chondrite metamorphic variation present in the pyroxene, feldspar, and sulfides, especially the lack of low-Ca pyroxene in types 5 and 6:
OPAQUE PHASES:
1. TEXTURE (Gooding and Keil, 1981)
(e.g. PO, PP, PPO, POP, BO, BP, RP, RPO, GO, GOP, GL)
CARBONACEOUS CHONDRITES
A spectroscopic classification technique—fourier transform infrared spectroscopy—has recently been applied to carbonaceous chondrites (Osawa et al., 2005). This technique utilizes the variation in water-induced absorption bands, related to phyllosilicates and the temperature of aqueous alteration, to distinguish among the different CC groups. The overall spectral characteristics of the O–H stretching band between ~2900 cm-1 and 3692 cm-1 resolve the different CC groups (CI, Tagish Lake, CM, CR, CO, CV, CK, CH, CB), and could serve as a method of classifying these meteorites. However, the anhydrous character of both the CV and CK members prevents this technique from resolving these two groups from each other. Similarly, both the CH and CB members have a sharp absorption band that is distinct from all other CC groups, but fails to resolve these two groups from each other.
ORDINARY CHONDRITES
ORDINARY CHONDRITE COMPOSITIONS
Fa
Fs
H
16–20
14.5–18
L
22–26
19–22
L/LL
25.5–26.5
-
LL
26–32
22–26
It has been proposed by A. E. Rubin (2005) that the H chondrites, having the lowest oxygen state and lightest O isotopes, formed the earliest and incorporated the least amount of Δ17O-rich phyllosilicates, while the L and LL groups formed at increasingly later periods and accumulated higher abundances of Δ17O-rich phyllosilicates. He has also proposed that as the precursor dustballs grew in size over time, the H chondrites were the first to form, resulting in their having the smallest chondrule size; the L and LL groups formed at progressively later periods and thus accumulated progressively larger chondrule sizes. All ordinary chondrites accumulated at ~2 AU from the sun.
ENSTATITE CHONDRITES
OTHER CHONDRITES
R Chondrites:
While this group of meteorites was initially distinguished through studies of the Carlisle Lakes, Australia specimen, its designation is now based on the only fall of the group from Rumuruti, Kenya. The group is highly oxidized, olivine-rich, and metal-poor. They differ greatly in oxidation state, oxygen isotope composition, and mineralogy from ordinary, carbonaceous, or enstatite
chondrites, or silicate inclusions in IAB and IIE irons. The parent body was originally highly unequilibrated but was subsequently thermally metamorphosed and impact-melted to a moderate degree. Most R chondrites are highly brecciated and contain implanted solar wind gases, two features which are indicative of an origin from a surface regolith.
3
4
5
6
Homogeneity
of olivine>5% mean deviation
homogenous
homogenous
homogenous
Pyroxene
predominantly
low-Ca pyroxenelow-Ca and
Ca-rich pyroxeneonly Ca-rich
pyroxeneonly Ca-rich
pyroxene
Feldspar
small glassy
intergrowthsisolated intergrowths
networks forming
well-developed
networks
Sulfides
even distribution
even distribution
even distribution
mobilized
K Chondrites:
The type specimen of this chondrite grouplet, Kakangari, along with two other members, have unique petrographic, bulk chemical, and O isotopic characteristics that distinguish it from other chondrite groups. The grouplet also does not fit into the existing systematics of the E, O, R, or C chondrites as their characteristics relate to heliocentric distance of formation. K chondrites therefore represent a unique, primitive, parent asteroid.
CONTINUE TO
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] CAIs
[PART VI] Trends for Classification
[APPENDECTOMY]
©
1997–2008 by David Weir