5  Strucure & Classification

Part 1: The chondrite components (5.1 - 5.4)
Chondrites contain primitive material that directly formed in the protoplanetary disk and was subsequently not at all to strongly altered in the meteorite parent body. Individual meteorite components allow reconstructing the structure, formation, evolution, chronology, etc. of the protoplanetary disk. It is therefore pivotal to know the various chondrite components. Learn about the various chondrite components and describe these. It might be very helpful to do this using sketches.
Learning Goals
Describe the general structure and texture of chondrites. Name and describe their components, including their general mineralogy, and how chondrites vary in their component abundances.

Part 2: The Various Meteorite Classes (5.5 - 5.8)
The various meteorites differ in many of their properties. Have a look how meteorites are classified, and which important criteria are used for this. The meteorite classification is further directly linked to their parent bodies. Understand this link.
Learning Goals
State the criteria for meteorite classification. Draw the general scheme of meteorite classification down to about the clan-level, including their relative abundances. Explain the relation between meteorite classification with the structure and texture of their parent bodies.

Bonus 1: Element composition of chondrules (5.9 - 5.10)
Chondrules of a chondrule population in a chondrite have variable bulk compositions, hence, are not all the same. This is already evident from the chondrule classification into type I, II, and A, AB,B, respectively (-> these need to be known). Bulk chondrule compositions in chondrule populations of individual chondrites have distinct distributions. Also, bulk chondrules have typical REE patterns. Learn about bulk compositions of chondrules in chondrule-populations in chondrites, and what can be learned from these for chondrule formation conditions as well as locations. Use MetBase to construct a plot displaying the distribution of bulk chondrule compositions in various chondrites. Learning Goals
Recall approximate compositional intervals of important elements of a typical chondrule population of a single chondrite. Describe the distribution of bulk chondrule compositions of the chondrule population in a single chondrite. Describe the REE patterns and abundances in bulk chondrules. Explain why chondrules in a single chondrite have variable bulk compositions. Demonstrate from how many different reservoirs chondrules of a single chondrite originate. Apply MetBase to specific problems.

Bonus 2: Isotope composition of the meteorites (5.11 - 5.12)
The isotope composition of bulk meteorites is used to identify various meteorite formation regions. In this way, meteorite bulk isotope compositions are used as classification properties. Two often used schemes are oxygen isotopes and nucleosynthetic anomalies. These two schemes are principally different. Learn the difference between these schemes, explain how these are used, and what can be learned from these for our understanding of the protoplanetary disk. Learning Goals
Explain the various slopes in the the oxygen 3-isotope-plot Explain mass(in)dependent fractionation. Describe how oxygen isotopes are used for meteorite classification, and where meteorites, but also other components as well as the Sun plot in the oxygen 3-isotope plot. Provide arguments to explain the nucleosynthetic meteorite dichotomy, thereby relating this to presolar grains and a possible structuring of the protoplanetary disk, as well as the influence of Jupiter on this hypothetical structuring.

Bonus 3: Sizes and size-distributions of chondrules (5.13 - 5.15)
Chondrules have distinct size distributions and variable mean sizes in the various chondrite groups. Have a look how the chondrule size distributions look like, and what can be learned from these for chondrule formation. Learning Goals
Roughly recall the chondrule sizes in the various chondrites. Describe the chondrule size distributions and argue what can be learned from this from this for chondrule formation. Explain the difference between the 2D and 3D chondrule size distributions.

5.1 The Various Chondrites and their Components

The major components of chondrites are chondrules (typically 20-80 vol%), matrix (also typically 20-80 vol%), and the minor components CAIs (0-3 vol%) and opaque phases (metal + sulphide; typically 0-5 vol%). The sizes and abundances of these components vary in part strongly among the various chondrite groups. Further, traces of numerous other and often quite important components are: presolar grains, organic material, UN/FUN inclusions, Fremdlinge – and more.

Main-components: chondrules: 20-80 vol%; matrix: ca. 20-80 vol% Minor-componens: CAIs: ca. 0-3 vol%; opaque phases: ca. 0-5 vol%.

Presolar grains, organic material, UN/FUN inclusions, Fremdlinge, etc. Organic material might contain information about the building blocks for life, or, more mundane, about the petrologic sub-type of a chondrite. Presolar grain can change the isotope composition of a chondrite and its component characteristically, which is useful for reservoir studies.

✗CI

✗CV

✓CK

✗CO

✗CM

✗CH

✗CB

✓True

✗False

✓Ordinary Chondrites

✗Carbonaceous Chondrites

5.2 Structure of Bulk Chondrites in 3D

Micro computer aided tomography (µ-CT – with ›micro‹ referring to the studied size scale) allows for 3D insights into meteorties. The so produced 3D images stacks can be studied by either moving sequently through the image stack, rendering a 3D object and removing various components individually, or in stereographic images or movies using red/green glasses. Tomography studies are critical to understand e.g., true 3D component sizes, distributions, appearances, or structures. These studies are further highly instructive and illustrative to understand the structure of meteorites.

2D studies are always biased by 2D sectioning effects, which do not exist in 3D. It is therefore possible to obtain true chondrule sizes, modal abundances of opaque phases in chondrules and matrix, or structures of various chondrule types, etc. in 3D.

✓True

✗False

✗Pixel

✗Vixel

✓Voxel

✗Poxel

✗mm

✓µm

5.3 Fundamental Properties of Chondrites

Among the fundamental chondrite properties are the modal abundances of their components: chondrules (plus their average sizes), matrix, Ca,Al-rich inclusions (CAIs) and metal, as well as sulphide. It is often not discriminated between the latter two. Further, it is interesting to know either the type meteorite for a specific chondrite group, or which criteria are else used to name a chondrite group.

Carbonaceous chondrites, except for CI. Also, CK chondrites have low chondrule abundances, likely because of their often higher petrologic types, which recrystallises their initial texture. Further R and K chondrites.

Carbonaceous chondrites (8 groups) contain ›C‹ (for carbonaceous) as first letter, followed by a second letter ,which is the first letter of the type meteorite for the group. An exception are CH chondrites, here ›H‹ and for ›high iron‹. Ordinary chondrite (3 groups) are discriminated according to their iron and metal content in ›H‹ for high iron, ›L‹ vor low iron, and ›LL‹ for low iron, low metal. The two enstatite groups have an ›E‹ as first letter, followed by either ›H‹ or ›L‹, with again standing for high and low iron, respectively. The individual groups R and K are named after their respective type meteorites.

✗<0.1 mm

✓<1 mmx000B<10 mm

✗ca. 3 vol% in CR chondrites

✗ca. 4 vol% in CR chondrites

✗ca. 4 vol% in CK chondrites

✓ca. 3 vol% in CV chondrites

✗ca. 4 vol% in CV chondrites

✓CC

✗OC

✗EC

✗K

✗R

5.4 Structure and Appearance of Chondrite Matrices

Carbonaceous chondrites have the highest matrix modal abundances, in the tens of vol%, and virtually 100 vol% in case of CI chondrites. The origin and re-processing of matrix material is still highly debated. Matrix material initially was either sub-µm tiny interstellar material of the parental cloud from which our Solar System formed. Or, matrix material condensed from an evaporated region of the protoplanetary cloud, likely close to the you Sun. Finally, matrix material could have been fine grained material from collisions and abrasion of some bigger particles. It is, at least, obvious that matrix and chondrules were incorporated together into planetesimals, from which a few million years later the planets formed. It is also clear, that the initial matrix material was aqueously and/or thermally altered on most of the meteorite parent bodies, contributing to a broad range of structure and appearances of chondrite matrices.

CC: 20-80 vol% (100 vol% in CI); OC: 10-15 vol%; EC: 2-15 vol%; R: 36 vol%: K: 73 vol%.

  1. sub-µm tiny, interstellar material; (ii) condensed material from an evapoarted region close to the young Sun; (iii) commuted material from collisions of bigger particles, such are e.g., chondrules.

✗True

✓False

✓phyllosilicate

✓sulphide

✓feldspar

✓magnetite

✓pyroxene

✓serpentins

✓True

✗False

5.5 Meteorite Classification

Meteorite groups are classified based on a number of characteristics, such as component abundances, sizes, and occurrences, or bulk elemental and isotopic compositions.

Parent Body: Undifferentiated – Intermediate – Differentiated Super-Classes: Chondrites – Primitive Achondrites – Achondrites Classes: Carbonaceous Chondrites – Ordinary Chondrites – Enstatite Chondrites – (R, K) Clans: CM-CO, CV-CK, CR-clan – Acapulcoite-Lodranite, Winonaite-IAB-IIICD – Moon, Mars, Vesta, Pallasite, Iron

For some groups it is not really critical, whether these are designated rather to a group or a clan.

Meteorites are classified based on their components, such as chondrules, CAIs, matrix; their bulk mineralogy, the modal abundance and sie of their components/minerals, as well as their bulk element and isotope compositions.

There are additional classification characteristics such as shock, alteration, etc.. There are, however, not necessarily the basis for this particular classification scheme.

These are used to further sub-classify the ordinary and enstatite chondrites, as well as one group of carbonaceous chondrites. The letters stand for: high iron, low iron und low iron low metal.

The only carbonaceous chondrite group not following this classification are the CH-chondrites.

✓Primitive Achondrites

✗Vesta

✗Lodranite

✓CR-clan

✓Undifferentiated

✗IIAB

✗Intermediate

✗CI-CR clan

✗Sun-Chondrites

✓Carbonaceous Chondrites

✗Ordinary Chondrites

✗C stands for ›Casablanca‹

✓C stands for ›carbonaceous‹

✗The second letter specifies the clan

✗The second letter stands for ›carbonaceous chondrite‹ The second letter stands for the type-chondrite

✗These became very hot and are highly metamorphised

✓These were completely molten, but only partly differentiated

✓These still contain some chodnrules

✗The Moon belongs to these

✗The Pallasites belong to these

✓Some meteorite groups represent the core-mantle boundary

✓Mars belongs to these

✓All are differntiated parent bodies

✓Iron meteorites are further sub-divided based on their trace element contents

✓Mesosiderites are a group of the achondrites

5.6 Classification of Ordinary Chondrites

Ordinary Chondrites (OC) are classified based on the fayalite (fa) and ferrosilite (fs) content of chondrule olivine and pyroxene, respectively. 3 main groups are then discriminated, based on their bulk Fe contents: high-metal (H), low-metal (L), low-iron, low-metal (LL). It seems irritating that the fa and fs contents increase with decreasing bulk chondrite Fe, but this is simply because LL chondrites are more oxidised than H chondrites. Further characteristics of OC are: mostly consisting of chondrules (60-80 vol%) and matrix (~10-15 vol%), minor opaques (2-10 vol%) and rare CAIs (0.1-1 vol%). Average chondrules sizes are between 300 and 900 µm. Si/Mg ratios are elevated compared to solar, and volatile elements are variably depleted relative to solar.

H: high Fe (iron); L: low Fe (iron): LL: low Fe (iron), low metal.

The LL chondrites have the highest FeO contents, i.e., highest fa and fs contests. This is curious, as LL chondrites have the lowest Fe content. The reason is that although LL have the lowest Fe content, these are the most oxidised, which is why their silicates contain the most FeO.

✓H3.5

✗CO3

✓L

✓L/LL

✗R3.7

✓L6

✗True

✓False

✓True

✗False

5.7 Classification of Iron Meteorites

Iron meteorites have a long history of classification, among this structural classifications such as ›octahedrite‹ or ›ataxite‹, which are no longer used. Instead, bulk meteorite trace elements are used to discriminate >10 groups, plus a large number of ungrouped irons. The large compositional trends observed in individual groups are interpreted as the result of fractional crystallisation. Irons with extensive trends are called ›magmatic‹, while those with smaller trends are called ›non-magmatic‹. The latter designation is irritating, as all irons were once molten.

Based on their siderophile trace element concentrations. Typical classification plots are Ir vs. Ni and Ge vs. Ni.

Based on their structural characteristics. The meteorites were cut, polished and treated with a weak acid (HNO3). The various minerals react differently to the acid, and their crystallisation structure is revealed (->Widmanstätten pattern). These were then used for classification into e.g., Hexahedrites, Octahedrites, or Ataxites.

✓Nothing genetic, only whether these have large or small compositional ranges.

✗As the name says: one group was produced by magmatic processes, the other was not.

✗Nothing, this only refers to the old structural classification scheme and has no more meaning today.

✗Parent body differentiation

✓Fractional Crystallisation

✗Mixing of various meteorite groups

✗Sub-solidus element re-distribution

✗<10

✓

10

✗unclear

5.8 Abundances of the Various Meteorite Classes

It is interesting to study how many meteorites of a certain type ae present in our collections. For example, most meteorites are undifferentiated (93%). Of these, most are HEDs, only few are pallasites or mesosiderites. Most chondrites are ordinary chondrites (95%) – explaining their name. Only 4% are carbonaceous chondrites. Of these, most are CM, CO, CV and CK, of which CM-CO and CV-CK are combined into clans. Only about 0.4% of the carbonaceous chondrites belong to the utterly important CI chondrites, which have approximately the same composition as the Sun. It would be possible and interesting to study similar insights, e.g., it would be interesting to see how falls and finds are different.

This cannot be said, but likely not very representative. Many meteorites might orginate from only one parent body, as indicated by their CRE ages. This will highly skew the statistics.

CO, CM, CV, and CK. Further, CO-CM and CV-CK are clans, i.e., these have a number of similarities. The CO-CM clan contributes more than half (52%) to the carbonaceous chondrite, and the CV-CK clan still 36%. This means, only 12% are other carbonaceous chondrites.

✓

60.000 10.000 6.000

✓ca. 92% ca. 29% ca. 48% ca. 63%

✗ca. 4% ca. 0.015% ca. 0.4% ca. 0.04%

5.9 Bulk Chondrule Element Distributions

Bulk compositions of chondrules in individual chondrites are variable. Typically, these distributions are unimodal, about normal to log-normal. Unimodal means one peak, i.e., typically chondrules in individual chondrites have no multi-modal distributions. However, depending on the element, multi-modal distributions can occur, e.g., for refractory elements such as Al. This might then represent chondrules to which refractory material, e.g., CAIs were added. The generally unimodal distributions might represent gas-melt exchange during chondrule formation. Mixing variable precursor material is unlikely, as these were µm-sized, and mixing thousands to millions of µm-sized grains into chondrule precursor aggregates will always result in the precursor average composition, i.e., all chondrules should then have the same composition. Chondrules similarly are also variable in their isotope compositions within individual chondrites.

Between about 10 and 30 wt%. Note that this range is significantly higher when MgO is considered (element -> oxide conversion factor: 1.65).

Unimodal and about normal to log-normal. In rare cases and for some elements mutli-modal. For example in case of Al, which is likely because CAI-like material was mixed to a portion of the chondrules.

✓True

✗False

✗mixing with silica

✗chondrules originated from reservoirs of various compositions

✓gas-melt exchange

✗addition of presolar grains

✓True

✗False

5.10 REE Composition of Bulk Chondrules

REE patterns of bulk chondrules from all chondrite groups are typically unfractionated, flat, und mostly enriched relative to CI. Chondrules apparently formed from material that was not fractionated previously and was slightly enriched in REE. CAIs often have high REE concentrations and various fractionated REE patters, and can therefore be excluded as a significant contributor to chondrules.

The REE in chondrules are typically enriched relative to CI by a factor between about 1.2 and 2, mostly around 1.5.

It is likely not true. The REE patterns in chondrules are mostly flat, whereas CAIs often have REE patterns with some kind of fractionation, which should be inherited by the chondrules, but is not seen. Nonetheless, fractionated REE patterns are seen in some chondrules and these chondrules indeed most likely incorporated CAI material. The majority of chondrules, however, received their flat REE patterns from refractory material with no element fractionations. This might be interpreted as evidence that CAIs formed in a different region than chondrules.

✓True

✗False

✓True

✗False

✗True

✗No, the other way round

✓False

5.11 Classification and the Oxygen (O) 3-Isotope Plot

Meteorites differs significantly in their oxygen isotope composition. This is therefore used as a classification characteristic. Meteorites of the same group can, however, scatter across a significant range of O-isotope compositions. Components of a meteorite, such as chondrules or CAIs, also scatter across significant range of O-isotope compositions. It is striking that certain meteorite groups, but also components such as chondrules or CAIs plot along a mass-independent line with a slope close to 1. The solar wind has a highly 16O-rich composition, close to that of CAIs. This likely is the original composition of our solar system, and the presently observed composition in meteorites and their components, as well as the composition of other planetary bodies such as the Earth, the Moon, or Mars developed in subsequent processes. One possibility is mixing of material that initial had a similar composition as the Sun, with a highly 16O-rich component, as it is measured in ›astrosilicates‹. These have delta17/16O compositions around +150 ‰, and plot along the extension of the around slope 1 fractionation line.

5.12 Bulk Meteorite Nucleosynthetic Isotope Dichotomy

Meteorites are separated into two groups based on their nucleosynthetic anomalies (e.g., 54Cr vs 50Ti): carbonaceous chondrites and non-carbonaceous chondrites. One explanation could be that presolar grains with largely different isotope ratios were mixed to carbonaceous chondrites in the outer solar system, while the non-carbonaceous chondrite did not receive these presolar grains, as these were gravitationally shielded by Jupiter.

At least two characteristics are clearly different among various objects. In this case the isotope composition of various meteorites.

The two discriminated groups are called ›carbonaceous‹ and ›non-carbonaceous‹ meteorites. The dichotomy between these two are differences in some nucleosynthetic anomalies, in this case in 50Ti and 54Cr.

✓True

✗False

✗True

✓False

✗The protoplanetary disk was structured in an outer region with, and an inner region without presolar grains during its formation.

✓Presolar grains that were added to the outer, but not the inner solar system.

5.13 Mean Chondrule Sizes in the various Chondrite Groups

The chondrule population in individual chondrite groups have a range of diameters, typically roughly following a normal to log-normal distribution. The mean chondrule diameters of chondrule populations vary among the chondrite groups from around 900 µm in CV and CK chondrites down to about 150 µm in CM chondrites. Clan relationships are observed, e.g., CV and CK chondrule populations have similar mean chondrule diameters. The same is observed for CO and CM chondrule populations.

The chondrules of the various chondrite classes cannot have been mixed, as almost each group has a different, characteristic mean chondrule size. Chondrite groups with similar mean chondrule sites constitute a clan – however, not only based on their mean chondrule sizes. The various mean chondrule sizes indicate variable chondrule forming conditions in the regions where the chondrules of the respective chondrite classes formed.

CV-CK and CM-CO.

✗True

✓False

✓CC

✗OC

✗EC

✗K

✗R

✓CC

✗OC

✗EC

✗K

✗R

5.14 Chondrule Size Distributions

Chondrule sizes in individual chondrites are variable, with about log-normal distributions. The average chondrule sizes among chondrite groups vary significantly and is mostly characteristic for a specific chondrite group. The shapes of the distributions also vary, but only slightly. The chondrule size distributions is an important constraint for chondrule formation. The size distribution might be inherited from chondrule precursor aggregate sizes, which likely aggregated stochastically. Or, the size distribution might be the result from variable loss and gain of material during gas-melt exchange that happened when chondrules formed. Alternatively, it has been suggested that size sorting produced the observed distributions. However, chondrules still must have had various sizes that then could have been sorted.

These are unimodal, normal to log-normal distributions.

  1. size sorting: theoretical calculations successfully reproduce the shape of chondrule size distributions. However, only chondrules with various sizes can be size sorted. So, size sorting alone cannot explain the various sizes itself. Size sorting might also be in conflict single reservoir models for chondrules and matrix. (ii) chondrule precursors: agglomeration of µm-sized, dusty material might have produced variably sized chondrule precursor aggregates, which then became variably sized chondrules. (iii) evaporation & recondensation/chondrule formation: individual chondrules might have experienced slightly variable conditions during the high temperature event that formed them. Slight differences in e.g., peak temperature, temperature duration, evaporation and recondensation rates, etc. could have produced variable chondrule sizes. (iv) multiple origins: chondrules could have formed in various locations, and in each with a different size. These variable chondrules were then mixed. In this case, however, a multimodal size distribution would be more likely.

✗True

✓False

✓… 155 µm

✗… 1550 µm

✗… 155 mm

✓… 0.155 mm

✗True

✓False