Chondrites

Posted on by skyfallmeteorites

PART I

CHONDRITES

CONTINUE TO
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] Refractory Phases
[PART VI] Trends for Classification
[APPENDECTOMY]

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. The components composing a specific chondrite group reflect localized formation in the solar nebula, probably <3–7 AU from the Sun, followed by rapid accretion into an asteroidal body (Alexander, 2015; Nagashima et al., 2015). Some groups are associated through their formation under similar conditions within a narrow range of heliocentric distances, and accordingly, 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) might best reflect this new metamorphic scenario by separating 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 analytical 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. Although not useful in distinguishing among 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 analytical 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:

At the onset of thermal metamorphism, 1) Cr is exsolved from ferroan olivine forming fine Cr-rich precipitates (possibly chromite), which, with progressive metamorphism, become coarser within the olivine cores and form rims on the olivine surfaces; 2) very fine-grained FeS grains in chondrule rims and in fine-grained matrix become coarser, and secondary sulfides form within chondrules; 3) Fe and Mg in olivine are homogenized and metal grains are equilibrated; 4) abundances of presolar grains are diminished; 5) Na and other alkalis are initially lost from the matrix and enter type-I chondrules, causing zonation, only to reverse direction with progressive metamorphism; 6) albite crystallizes from type-II chondrule glass causing blue CL and increased TL sensitivity.

By studying chromite zoning profiles along with the chromite content of individual ferroan olivine grains, Grossman (2008) was able to further resolve the petrologic type for chondrites at the lowest metamorphic stages. These two petrographic features provide a reference for a sequencial history of increasing thermal metamorphism which is consistent among olivine grains within each meteorite. For metamorphic types 3.00–3.03, chromite zoning profiles are smooth and correlate with igneous FeO zoning profiles. In addition, at this lowest metamorphic stage chromite contents account for 0.3–0.5 wt% in the chondrite groups studied. While chromite contents in type 3.05–3.10 chondrites still reflect the lowest degrees of metamorphism, chromite now exhibits igneous zoning profiles which are no longer smooth. Upon reaching a degree of metamorphism equivalent to type 3.15, chromite zoning has diminished considerably, and chromite abundance is now only 0.1–0.2 wt%. With metamorphic types of at least 3.2, no zoning is observed and chromite abundance is mostly less than 0.1 wt%.

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.

Following the scheme of J. Grossman and A. Brearley (2005), the LL chondrite Semarkona and the ungrouped carbonaceous chondrite Acfer 094 (Kimura et al., 2006) were assigned to the least equilibrated subtype 3.00. However, Semarkona has more recently been determined to represent a petrologic subtype 3.01. This specific metamorphic type for Semarkona is also consistent with findings based on the FeNi-metal component, the features of which provide one of the most sensitive indicators for the onset of thermal metamorphism. The technique reveals that primary martensite decomposes to fine-grained plessite during very low degrees of thermal metamorphism in Semarkona, but which did not occur in Acfer 094 (Kimura et al., 2008). Furthermore, they found that metal in and around Semarkona chondrules does not show a solar ratio of Co/Ni like that in Acfer 094, reflecting the greater degree of metamorphism that affected Semarkona. Moreover, low temperature aqueous alteration has occurred in Semarkona as attested by the presence of secondary alteration products such as smectite.

Kimura et al. (2008) also argue for the inclusion of the carbonaceous chondrites of groups CR, CH, CB, and CM as 3.00 type specimens, notwithstanding their general designation as type 2 due to aqueous alteration features. In light of this petrologic typing paradox, they propose that a separate scale be adopted to describe aqueous alteration distinct from that which describes thermal metamorphism. To that end, researchers have proposed a number of aqueous alteration classification scales (hydration scale) distinct from the thermal metamorphic classification scale (e.g., Rubin et al. [2007]; Howard and Alexander [2013]). See the Murchison page for more details about these scales.

Type 4 Grossman et al. (2009) have identified more accurate parameters which are useful for the quantitative distinction of ordinary chondrites belonging to the metamorphic transitions of type 3/4 and type 4/5. The percent mean deviation of FeO in olivine grains (PMD-ol) and pyroxene grains (PMD-px) has served as a measure of the degree of heterogeneity of a chondrite, i.e., PMD-ol ≥5% are defined as unmetamorphosed or unequilibrated type 3. Other parameters have been found that are correlated with PMD-px. As metamorphism increases (PMD decreases) across types 3 and 4 in both olivine and pyroxene, the Fe/Mg ratio equilibrates and the FeO content increases. The petrologic transition from low type 4 towards type 5 can be distinguished by a trend of depletion in olivine CaO, by a trend of depletion in olivine and pyroxene Cr2O3, and by a trend of decreasing PMD of CaO in low-Ca pyroxene. In addition, as metamorphism increases throughout the span of type 4, a significant increase in pyroxene Al2O3 occurs that corresponds to the decrease in Cr2O3. The petrologic transition at type 4/5 can be accurately established as the point of transformation of clinopyroxene to orthopyroxene.

Type 5–6 In an onion shell model the cooling rate should be inversely correlated with the petrologic type, but this is not observed. Another method sometimes used to discriminate between type 5 and 6 utilizes feldspar grain size; e.g., type 4: <2 µm, type 5: 2–50 µm, type 6: >50 µm. However, these quidelines were shown to be inaccurate. In addition, estimated peak temperatures for OCs 500–800°C for types 4 and 5 and 800–1000°C for type 6 [H6: 725–742°C, L6: 808–820°C, and LL6: 800°C]) do not always correlate with petrologic type. Studies of feldspar composition by Kovach and Jones (2010) led them to conclude that equilibration in LL chondrites corresponds to the rate of heating rather than the peak temperature, with type 5 being heated more rapidly than type 6. Furthermore, because cooling rates do not always obey an inverse trend with peak temperatures, they argue that there is no simple metamorphic progression from LL4 to LL5 to LL6. For H chondrites, they found a lack of correlation between feldspar equilibration and petrologic type, leading to the conclusion that feldspar crystallized after chondrule mesostasis had equilibrated. The presence of fluids in the OCs is also thought to have affected metamorphic heating and alkali element redistribution.

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:

  1. the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
  2. low-Ca pyroxenes contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo) in type 6, but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
  3. feldspar grains gradually coarsen to reach a size of at least 0.1 mm in type 7

In the intervening years since Dodd et al. proposed their classification parameters, additional type 7 chondrites have been found and studied. As a result of more recent studies, it was proposed by Wittke and Bunch (pers. comm., 2004) that a type 7 category should not comprise meteorites containing any relict chondrules, but rather, should represent a metamorphic extreme in which no sign of chondrules remains. This would lump those meteorites containing ‘poorly defined’ chondrules and ‘relict’ chondrules into the type 6 category.

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 proposed term supplanting type 7 of the metamorphic recrystallization sequence. The term metachondrite was proposed by Irving et al. (2005) to describe those achondrites which are texturally evolved chondrites. First applied to NWA 3133, with affinities to the CV group, the metachondrites have thoroughly recrystallized textures resulting from high degrees of metamorphism or partial melting. They have chondritic bulk compositions but are completely devoid of chondrules. Their elemental ratios and O-isotopic compositions show affinities to several existing chondrite groups (e.g., CV, CO, H, L, and LL). Other established groups of achondrites, such as the evolved ‘primitive achondrites’ with relict chondrule-bearing members, may also be more appropriately termed metachondrites. The chondrule-bearing members have been referred to as ‘AC chondrites’ or ‘W chondrites’ for those meteorites associated with acapulcoites and winonaites, respectively (as demonstrated by Monument Draw and NWA 725, respectively). In a similar manner, if a chondrule-bearing meteorite is eventually discovered which is associated with the unique metachondrite NWA 2788, for which evidence indicates it derives from a carbonaceous chondrite parent body, it was proposed by Bunch et al. (2006) that it be termed a ‘CT chondrite’ (see NWA 2788 photos and abstract #P51E-1246).

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):

  1. an equigranular (igneous) texture with no extensive segregation
  2. experienced temperatures to levels necessary for FeNi-metal, FeS, and silicate partial melting (~1200°C—perhaps by shock melting)
  3. migration of free metal from olivine fayalite and chromite as a result of reduction processes (i.e., by reaction with graphite), resulting in Mg-rich olivine and chromite and low-Ni metal
  4. Cr acting as a chalcophile element during reduction leading to its incorporation into troilite
  5. may retain close to chondritic isotopic and bulk chemical compositions

OPAQUE PHASES:

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. Based on Pb–Pb ages, chondrules most likely formed during a time period spanning 1–4 m.y. after CAI formation, probably originating through gravitational instabilities by shock waves, rather than originating in an x-wind (Krot et al., 2009). Features cited in support of the shock-wave model include the O-isotopic similarity and the compositional complimentarity between chondrules and matrix material, as well as other thermal processing characteristics such as a high abundance of crystalline silicates, a high cooling rate for the matrix material, and low shielding in the protoplanetary disk as evidenced by large excesses of 26Mg; all of these features are inconsistent with an x-wind model. In addition, the finding of age differences among different components in members within the same chondrite group demonstrates that the components formed asynchronously, contrary to the manner predicted by the x-wind model in which all components (chondrules and CAIs in this case) of a single group should form concurrently. It is widely accepted that CAIs formed at least 1 m.y. before chondrules.

Another plausible model for chondrule formation gaining acceptance is molten planetesimal splashing (Sanders, 2009, 2010). This model is consistent with many features observed in chondrules and chondrites. A chondrule formation scenario begins with the total melting of small-sized planetesimals, at least 60 km in diameter, by heating from the decay of radiogenic elements. These molten planetesimals accreted within 0.5–2 m.y. after CAIs during a period termed the ‘Meltdown Era’ by Sanders (2010), when large abundances of the short-lived radionuclide 26Al were extant. These planetesimals were insulated below a crust of ~5–10 km thick, resulting in total melting and core differentiation. Collisions between such molten planetesimals would have resulted in an immense volume of incandescent, chondrule-forming spray. Some of the crustal material that became trapped in some chondrules is observed today as dunite fragments. Any chondrules that were produced earlier than ~1.5 m.y. after CAIs would not be expected to survive due to melting of their host objects, a concept that is consistent with the ages we observe (>2 m.y. after CAIs). Chondrules that accreted to chondritic parent bodies later than ~1.5 m.y. after CAIs had a good chance of survival, because the energy from the remaining 26Al was no longer enough to melt cold planetesimals. In other studies modelling chondrule formation from shock waves, Fedkin et al. (2012) found that the predicted isotopic variation of Mg, Fe, and Si within chondrules did not exist, and they concluded that the formation of chondrules was instead more consistent with impact events of ice-rich planetesimals which generated clouds of liquid and vapor.

For additional chondrule formation hypotheses, read the PSRD article by G. Jeffrey Taylor: ‘Ancient Jets of Fiery Rain‘, April 2015.

It is generally accepted that type-I chondrules crystallized from supercooled liquid droplets in equilibrium with a gas of solar composition (CI-like precursor material), at pressures of ~0.1–1.0 bar (near the Sun at a distance consistent with the orbit of Mercury). The temperature was lower, the pressure was higher, and the environment was more oxidizing than those conditions associated with CAI/AOA formation. Condensation of the more refractory elements such as Ca, Al, Mg, and Si occurred first (see CAIs), and as temperatures decreased, some of these Ca,Al,Mg,Si-oxide droplets were gravitationally removed from the condensation region. As temperatures continued to decrease, FeNi-metal eventually precipitated.

Contrariwise, Grossman et al. (2012) determined that oxidized-Fe-rich silicate (fayalite), which both composed the precursor material of type-II chondrules and constitutes chondrule matrix olivine grains, could not have formed by similar condensation processes as type-I chondrules. Neither a significant enrichment of nebula water ratio, nor an environment in which the supersaturation of metallic FeNi would have occurred, could have permitted a reasonable quantity of fayalitic olivine to form prior to the nucleation of FeNi-metal. Rather, they conclude that the first FeO was produced inside water-rich planetesimals and/or within water-rich vapor plumes generated by planetesimal collisions.

Chondrules of various textures and compositions were formed, associated with a specific thermal history (i.e., number of nucleation sites, condensation temperature, degree of undercooling, cooling rate, etc.):

1. TEXTURE (Gooding and Keil, 1981)

Porphyritic (P); Barred (B); Radial (R); Granular (G)

O—olivine-rich
P—pyroxene-rich
PO or OP—contains both olivine and pyroxene
GL—glassy

(e.g. PO, PP, PPO, POP, BO, BP, RP, RPO, GO, GOP, GL)

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 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. Their relatively low abundance of ~2% in ordinary and carbonaceous chondrites attests to a low chondrule density and/or a low chondrule velocity in agglomeration regions (Hezel et al., 2013).

The cooling rate for chondrules in a molten state was initially rapid, but following the onset of crystallization cooling 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)

  • Type-I: FeO-poor (a division between FeO-poor and FeO-rich has been established at Fa9 by some researchers), metal-rich, lower mass, lower abundance of moderately volatile elements from evaporative loss due to longer heating times and/or higher crystallization temperatures in regions of low chondrule/dust concentrations; most formed earlier than type-II chondrules, but the actual sequence reflects a continuum
  • Type-II: FeO-rich, metal-poor, higher mass, higher abundance of moderately volatile elements due to shorter heating times and/or lower crystallization temperatures in regions of high chondrule/dust concentrations; 16O-poor compared to type-I chondrules; agglomeratic olivine (AO) objects show transitional variations in texture (grain size due to melting stage) and chemistry (degree of volatile loss) for type-II chondrules; could have been derived from type I chondrules by subsequent oxidation of metallic iron in type-I chondrules which then diffused into the olivine grains forming type-II chondrules
  • Al-rich (>10 wt% Al2O3): thought to have formed by melting of Type-C (spinel-anorthite-pyroxene) CAI precursor material mixed with type-I chondrule precursor material (Krot et al., 2006); in addition, they experienced O-isotopic exchange with an evolving, 16O-poor nebular gas; in contrast to 16O-poor type-I and -II ferromagnesian chondrules, a significant percentage of Al-rich chondrules exhibit O-isotopic heterogeneity due to inclusion of 16O-rich relict CAI material; both olivine- and plagioclase-dominant types are known

    • olivine vs. pyroxene ratio

  • A: silica-poor; contain only, or predominantly, olivine (<10 vol% pyroxene)
  • AB: contain between 10 and 90 vol% pyroxene
  • B: silica-rich; >90 vol% pyroxene
  • CC: cryptocrystalline; contain mainly pyroxene

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 gas of solar composition at pressures of 0.1–1.0 bar at a location ~1 AU from the Sun (Blander et al., 2004, 2009). From 26Al/27Al ratios, chondrule ages were calculated to be as low as 0.7 (±0.2) m.y. after CAI formation, but typically, Al–Mg and Pb–Pb dating systems yield slightly younger ages of ~2–4 m.y. relative to CAIs. Chondrules in CV chondrites are the oldest known and may be associated with the earliest stage of formation, while those in CR chondrites appear to date among the later formed chondrules (Burkhardt et al., 2008). These chondrules 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 provide 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 below 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 diverse chondrules.

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.

CARBONACEOUS CHONDRITES

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 as relatively small asteroids that retain the oldest record of the solar nebula, and contain solar abundances of non-volatile elements.

Carbonaceous chondrites constitute ~2.5% of all meteorites recovered, and they have been divided into the following chemical groups: CI, CM, CR, CO, CV/CK, 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 (4.5667 [±0.0010] b.y.) to youngest (4.5647 [±0.0006] b.y.), CV/CK > CM > CO + OC > CR. The CI group contains up to 20% water locked in hydrated minerals. It was determined by Macke et al. (2011) that porosity in the carbonaceous chondrite groups follows an inverse trend in which porosity decreases with increasing petrologic type. The discovery of new and unique carbonaceous chondrite meteorites helps us to continually revise the record of early Solar System processes.

The CV3 group has been subdivided into three subgroups (McSween, 1977; Weisberg et al., 1997):

  1. Reduced subgroup: e.g., Arch, Efremovka, Leoville, Vigarano, and QUE 93429

  2. Oxidized-Allende subgroup: e.g., Allende, Axtell, Tibooburra, and ALH 84028

  3. Oxidized-Bali subgroup: e.g., Bali, Grosnaja, Kaba, and Mokoia

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. Recent geochemical, mineralogical, and isotopic studies conducted by Greenwood et al. (2009) and other investigators lead them to conclude that CK chondrites originated on the CV parent body, and that the combined groups form a metamorphic progression from the unequilibrated, lower subtype-3 CV chondrites to the higher subtype-3 CK chondrites and beyond, to include the equilibrated type 6 chondrites. It was proposed by Wasson et al. (2013) to merge the CK and CV groups into a single unified group, CV3-6, and that the CK3 members would be designated CV3oxK. A subsequent study was conducted by Dunn et al. (2016) which compared magnetite in a number of CK and CV chondrites. They presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies; details of their study can be found on the Dhofar 015 page.

The following mineralogical relationships have been found to exist among these CV subgroups:

  • matrix abundance: oxB > oxK > oxA > red
  • metal to magnetite ratio: red > oxA ≥ oxK > oxB
  • fayalitic olivine range: oxK (Fa~30), oxA (Fa32–60), red (Fa32–60), oxB (Fa10–90+)

  • abundances of fayalite, ferrous olivine, and magnetite cannot be used to discriminate among the subgroups
  • phyllosilicates are absent from red (evaded significant aqueous alteration)
  • metal in oxA and oxK is Ni-rich, in oxB it is largely Ni-rich, and in red it is largely Ni-poor
  • metal abundances are greater in red in which sulfide contains less Ni
  • low-Ca pyx is found in red while Ca–Fe-pyx is found in oxA, oxK and oxB
  • nepheline, sodalite, wollastonite, andradite, kirschsteinite, and grossular are found only in oxA

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.

Both the CH and CB members have a sharp absorption band that is distinct from all other CC groups. However, that feature does not resolve these two groups from each other.

ORDINARY CHONDRITES

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. All ordinary chondrites accreted ~2 AU from the sun. 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 COMPOSITIONS
Fa Fs
H 16–20 14.5–18
H/L 19.5–21.8 17.2–21.2
L 22–26 19–22
L/LL 25.5–26.5 –
LL 26–32 22–26

It has been proposed by 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.

CHONDRULE SIZES (µm)
after K. Metzler (2018)
Mean
2D (3D)		 Median
2D (3D)		 Min
2D (3D)		 Max
2D (3D)
H4 (NWA 2465) 450 (490) 370 (420) 95 (90) 5400 (2360)
L4 (Saratov) 500 (610) 450 (530) 130 (180) 2160 (2520)
LL4 (NWA 7545) 690 (830) 580 (730) 190 (245) 3810 (2880)
after D. W. Hughes (1978)
L/LL4 (Bjurböle) — (750) — (688) 200 (250) — (—)

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, the S-IV type asteroid 6 Hebe has been considered a likely candidate for the parent body of the H chondrites. However, hydrocode models show inconsistencies exist between expected and observed CRE ages based on the scenario of direct injection into resonances. The steady delivery of H chondrite material from 6 Hebe to Earth also remains unexplained. Current studies by Rubin and Bottke (2009) have led to the conclusion that family-forming events resulting in large meteoroid reservoirs, which have homogenous compositions and locations near dynamical resonances such as the Jupiter 3:1 mean motion resonance, are the likely source of the most prevalent falls including H chondrites and HED achondrites (especially howardites). As a matter of fact, a number of asteroids having H-like mineralogies have been observed near the 3:1 and 5:2 resonances at 2.82 AU (Burbine et al., 2015 and references therein). See further details on the Abbott page. 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 75% of all meteorite falls.

ENSTATITE CHONDRITES

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%. However, Macke et al. (2010) determined that a wide sampling of both finds and falls from the two subgroups exhibits similar values for the physical properties of grain and bulk density, porosity, and magnetic susceptibility, reflecting a similar total Fe quantity in each. Each subgroup comprises a complete range of thermally metamorphosed types consistent with the onion shell model, from type 3 through 6 (or 7), with the Si and Ni content in kamacite increasing with respect to an increasing petrographic type for each series. The Van Schmus and Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011).

The following mineralogical and petrographic relationships have been found to distinguish these subgroups:

  • EH subgroup has a higher Si content in kamacite (EH: 1.9–3.8 wt% vs. EL: 0.3–2.1 wt%)
  • EH subgroup has a lower Mn content in daubreelite (EH: 0.4–1.1% vs. EL: 1.4–4.0%)
  • EH subgroup has a lower Ti content in troilite (EH: <4.8 wt% vs. EL: >5.5 wt%)
  • EH subgroup has a lower An content in plagioclase (EH: <3 mol% vs. EL: 13–17 mol%)
  • EL subgroup cooled more slowly that the EH subgroup

Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).

ENSTATITE CHONDRITE SUBGROUPS
Weyrauch et al., 2018
EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite Ni <6.5 wt% Ni >6.5 wt% Ni <6.5 wt% Ni >6.5 wt%

A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids.

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. Still, advanced computer modeling (Blander et al., 2009) indicates formation at pressures of 0.1–1.0 bar, consistent with a distance of 0.3 AU from the Sun near the orbit of Mercury. 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, and the Fe-rich EC material is consistent with that which constitutes Mercury. Enstatite meteorites are rare, representing about 1% of all meteorite falls.

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.

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:

Modified Van Schmus-Wood Classification Scheme For R Chondrites
3 4 5 6
Homogeneity
of olivine		 >5% mean deviation homogenous homogenous homogenous
Pyroxene predominantly
low-Ca pyroxene		 low-Ca and
Ca-rich pyroxene		 only Ca-rich
pyroxene		 only Ca-rich
pyroxene
Feldspar small glassy
intergrowths		 isolated 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.

F Chondrites: Forsterite (F) chondrite material was first discovered as highly magnesian silicate inclusions in the Cumberland Falls aubrite, while complete F-group chondrite meteorites have only recently been identified and analyzed. See the NWA 7135 page for further details about this unique chondrite grouplet.

G Chondrites: Two likely genetically-related meteorites, GRO 95551 and NWA 5492, have been studied in-depth by various investigators. These are highly-reduced, metal-rich chondrites that plot in a unique location in ε54Cr–Δ17O isotope space, situated between ordinary and enstatite chondrites. See the NWA 5492 page for further details about this unique chondrite duo.

CONTINUE TO
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] Refractory Phases
[PART VI] Trends for Classification
[APPENDECTOMY]

© 1997–2019 by David Weir

Leave a Reply