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Breccia

Sedimentary Petrology

Frederick L. Schwab , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

III.C Conglomerates and Breccias (Rudaceous Rocks or Rudites)

The coarsest terrigenous clastic sedimentary rocks, conglomerate and breccia, are consolidated deposits of gravel and rubble, respectively. They both contain significant (certainly 10%) quantities of clasts coarser than sand. Conglomerates and breccias typically differ from one another only in how rounded the individual grains are. A breccia contains mainly angular or subangular clasts. In a conglomerate, most clasts are subrounded or round.

Conglomerate and breccia deposits can be classified in a variety of ways. For example, it is relatively easy to recognize different conglomerates and breccias on the basis of purely descriptive textural characteristics (boulder conglomerate versus cobble conglomerate) or composition (chert pebble conglomerate versus limestone pebble conglomerate). Other schemes differentiate conglomerates and breccias on the basis of depositional agency and environment (river conglomerate, beach conglomerate, glacial breccia). However, the most common classification scheme recognizes several geologically meaningful categories based on objective physical characteristics and genesis. Table VI lists the most common varieties and some of their note worthy properties

TABLE VI. Classification of Conglomerates and Breccias ^a

Mode of genesis	Major varieties	Specific varieties
I. Epiclastic: produced by the breakdown (physical disintegration) of preexisting rocks	A. Extraformational: source rocks lie outside the depositional area in which the deposit occurs	1. Orthoconglomerates: an intact framework composed of boulders, cobbles, pebbles, and sand bound together with chemical cement; the clasts support one another and are in tangential contact (beach deposits, and fluvial channel deposits)
		2. Paraconglomerates: the framework clasts "float" in a finer than sand-sized matrix; essentially a sprinkling of pebbles, cobbles, and boulders in a mudrock matrix (tillites, glacial till deposits and tilloids, submarine land slide debris)
	B. Intraformational: storm waves or brief current slurries rip-up material deposited immediately prior to the event and transport such locally derived detritus for only a few centimeters or meters	Shale pebble conglomerates and breccias; limestone pebble ("edgewise") conglomerates and breccias
II. Pyroclastic: produced by the explosive activity of volcanoes	Volcanic (pyroclastic) breccias (angular) and agglomerates (rounded)
III. Cataclastic: produced by local earth movements (fault zone activity) or solution collapse	Fault and fold breccias; collapse and solution breccias
IV. Meteoritic: produced by the impacting of extraterrestrial bodies (asteroids) on the earth's surface	Impact breccias

a: From Pettijohn, F. J. (1975). "Sedimentary Rocks, 3rd ed." Harper and Row, New York.

Analysis of both modern coarse-grained sediment (gravel and rubble) and ancient conglomerate and breccia. show that epiclastic varieties are by far the most abundant and significant, although even they are not volumetrically important (in comparison to mudrocks, sandstones, and carbonates). There are two types of epiclastic conglomerate and breccia (generated by weathering preexisting rocks), extraformational and intraformational. The extraformational conglomerates and breccias are more widespread. These can be either "ortho" or "para" conglomerates and breccias.

The orthoconglomerates (and orthobreccias) are deposited by highly turbulent water, either high velocity streams (alluvial fan and fluvial channel deposits) or in the upper reaches of winter storm beaches. The beach deposits tend to be composed mainly (90%+) of stable, resistant rock and mineral clasts (quartz, quartzite, and chert, for example) and are commonly referred to as oligomictic (orthoquartzitic) (conglomerates of the Tuscarora Formation, Appalachians). Alluvial fan and fluvial conglomerates and breccias typically contain less than 90% stable clasts, and are referred to as petromictic (Alpine molasse conglomerates; Triassic rift valley deposits, Appalachians).

Paraconglomerates and breccias are not deposited by normal currents of moving water. Some are produced as glacial ice melts, which results in the lowering of a heterogeneous mix of materials to the ground as till (later to become consolidated into tillite) (Gowganda Formation of Canada; Mount Rogers Volcanic Group of Virginia). Some paraconglomerates and parabreccias are generated by chaotic submarine landslides and turbidity currents and are referred to as tilloid (Squantum tilloid, Boston Bay, Massachusetts). Others are produced when ice-rafted coarse clasts tumble into finer grained offshore mudrock. These "dropstones" are released when the clast-bearing ice block melts, forming laminated pebbly (or cobbly or bouldery) mudstone. Subaerial debris flows (formed as alluvial fans are covered by flash flooding), subaqueous debris flows, and subaqueous grain flows can also generate dominantly fine-grained sedimentary rocks which contain a significant fraction of coarser-than-sand components. The terms diamictite and diamixtite are used for such poorly sorted terrigenous rocks in which pebbles, cobbles, and boulders float in a sandy and/or muddy matrix. Obviously distinguishing tillite and tilloid from one another and from various kinds of debris flow deposits is important and significant, but commonly not possible.

Intraformational conglomerates and breccias are quite widespread but volumetrically unimportant. When they occur as laterally continuous bands or horizons within a sequence of shallow water deposits, they generally indicate periodic storms that generate bottom currents capable of ripping up recently deposited, unconsolidated material. Other intraformational conglomerates and breccias consist of a poorly sorted mixture of shallow water sedimentary rock clasts enclosed in a matrix of deeper water material. These deposits form in deep water along the margins of scarps produced by synsedimentary faulting.

The other major conglomerate–breccia types (pyroclastic, cataclastic, and meteoric or meteoritic) are also volumetrically unimportant (with the possible exception of volcanic breccias and conglomerates). Nevertheless, each type gives valuable hints about the geological history of the areas in which they are found.

In summary, the very coarse clastic terrigenous sedimentary rocks are easily analyzed. Their mode of genesis, likely provenance, and probable depositional setting can generally be implied. Unfortunately their limited extent in time and space constrains their usefulness. Far more historical information can be garnered from sandstones.

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Meteorites, Comets, and Planets

D.W. Mittlefehldt , in Treatise on Geochemistry, 2007

1.11.3.5 Mesosiderite Silicates

Mesosiderites are stony irons in which the rocky material is a polymict breccia of crustal rock from a differentiated body—basalt, gabbro, pyroxenite, dunite, and anorthosite. The silicates are very similar to the HED suite meteorites. Thus, mesosiderite silicates are discussed here, but the metallic phase is not.

Petrologic and compositional information on mesosiderites was taken from Delaney et al. (1980, 1981), Floran (1978), Floran et al. (1978b), Hewins (1984), Ikeda et al. (1990), Kimura et al. (1991), Mittlefehldt (1979, 1980, 1990), Mittlefehldt et al. (1979), Powell (1971), Rubin and Jerde (1987, 1988), Rubin and Mittlefehldt (1992), and Simpson and Ahrens (1977).

The silicates consist of mineral and lithic clasts set in a fine-grained fragmental to impact–melt matrix. The most common lithic clasts are basalts, gabbros, and orthopyroxenites, while dunites are minor and anorthosites are rare. The most common mineral clasts are centimeter-sized orthopyroxene and olivine fragments, while millimeter-sized plagioclase fragments are less common.

Olivine clasts are typically single-crystal fragments, varying in composition from Fo₉₂ to Fo₅₈. Mesosiderites also contain fine-grained olivine. Most literature analyses are of unspecified grains, and metamorphic equilibration may have altered the compositions of smaller grains. Coarse-grained olivines have FeO/MnO of 40–45, within the range of main-group pallasite olivines (Mittlefehldt et al., 1998). Low-calcium pyroxene clasts include centimeter-sized orthopyroxene and millimeter-sized pigeonite. The orthopyroxene clasts are compositionally similar to diogenites and orthopyroxenite clasts in howardites. Compositional ranges are from about Fs₂₀ to Fs₄₀ for low-calcium pyroxenes, and orthopyroxenes more ferroan than about Fs₃₀ were inverted from pigeonite (Powell, 1971). Plagioclase grains are calcic, like those of HED meteorites.

Basaltic and gabbroic clasts are generally similar to eucrites and mafic clasts in howardites. They are composed of ferroan pigeonite and calcic plagioclase, with minor to accessory silica, whitlockite, augite, chromite, and ilmenite. Metal and troilite are common, but these may have been added during brecciation. Many of the mafic clasts are distinguishable from similar HED meteorite materials in detail—they generally have higher modal proportions of tridymite and whitlockite (Nehru et al., 1980; Rubin and Mittlefehldt, 1992), and have chromite>ilmenite, opposite to what is found for basaltic eucrites (Delaney et al., 1984; Nehru et al., 1980). Pyroxene compositions in mafic clasts generally have lower FeO contents and lower FeO/MnO ratios than those of HED meteorites (Mittlefehldt, 1990; Mittlefehldt et al., 1998).

The mesosiderite matrix varies from cataclastic texture, with highly angular mineral fragments (texture grade 1) to igneous-textured matrix (texture grade 4) (Floran, 1978). Matrix pyroxene grains were affected by FeO reduction and metamorphic equilibration. They typically have low FeO/MnO ratios and are relatively magnesian, irrespective of metamorphic grade—no matrix pyroxene grains have compositions as ferroan as pyroxenes in the basaltic clasts. This is unlike the case for howardites, where ferroan pigeonite is a common constituent of the matrix (e.g., Reid et al., 1990).

There are very few bulk analyses for coarse-grained olivine clasts from mesosiderites, and observed distinctions between them and pallasite olivines are mostly due to chromite and metal inclusions in the former (Mittlefehldt, 1980). Orthopyroxene clasts are generally similar to diogenites in major-, minor-, and trace-element composition (see Mittlefehldt et al., 1998).

The mesosiderite basalt and the gabbro clasts display wider ranges in trace-element compositions than do similar HED lithologies. All mesosiderite clasts have high nickel and cobalt contents and some have high selenium contents, undoubtedly due to impact mixing with matrix metal and troilite. Some mesosiderite basalts are identical in composition to basaltic eucrites. A clast from Mount Padbury has an mg# of ∼36, a molar FeO/MnO of ∼36, and a flat REE pattern at 9–10×CI chondrite abundances (Mittlefehldt, 1979)—all within the ranges for basaltic eucrites. However, many basaltic clasts are distinct in major-element composition, with higher mg#'s and lower molar FeO/MnO ratios than those of basaltic eucrites, and have LREE-depleted patterns and (Eu/Sm)_CI>1—patterns unknown among unaltered basaltic eucrites. Some gabbro clasts are similar to cumulate eucrites in major- and trace-element contents, but many are distinct in having extreme depletions in the most incompatible elements (Mittlefehldt, 1979; Rubin and Mittlefehldt, 1992). In extreme cases, samarium abundances are only 0.02–0.03×CI chondrites (Rubin and Jerde, 1987; Rubin and Mittlefehldt, 1992), much less than the 1–2×CI typical of cumulate eucrites. These clasts have (Eu/Sm)_CI of 220–260, the most extreme ratios known among solar system igneous rocks (Mittlefehldt et al., 1992).

Mesosiderite silicates are broadly similar to howardites in bulk composition, although there are distinctions. Major differences between mesosiderite silicates and howardites are that the former have generally lower FeO, systematically lower FeO/MnO, and are enriched in P₂O₅ compared with howardites (Simpson and Ahrens, 1977). Mesosiderite silicates typically have lower incompatible trace-element contents, such as the LREE, than do howardites.

Age determinations have been done for many mesosiderites, but some of these are for bulk samples of polymict breccias and thus are difficult to interpret. Brouxel and Tatsumoto (1991) and Prinzhofer et al. (1992) showed that Estherville and Morristown were formed early in the history of the solar system and later metamorphosed. The U–Pb ages on zircons in a basaltic clast from Vaca Muerta yielded an age of 4.563±0.015 Ga (Ireland and Wlotzka, 1992). The petrology of this clast was not given, so its origin is not known. Regardless, this demonstrates that magmatic processes occurred very early on the mesosiderite parent body. Stewart et al. (1994) studied three clasts from Vaca Muerta and one from Mount Padbury. The Sm–Nd isochron ages vary from 4.52 to 4.48 Ga for three igneous clasts, and 4.42 Ga for an impact–melt clast. Wadhwa et al. (2003) determined the chromium isotopic composition of four metaigneous clasts from Vaca Muerta and demonstrated that ⁵³Mn was present at the time of differentiation of the mesosiderite parent asteroid. Comparing mesosiderite and HED Mn–Cr isochrons, Wadhwa et al. (2003) concluded that the mesosiderite parent asteroid differentiated ∼2 Myr after the HED parent asteroid.

Numerous ³⁹Ar–⁴⁰Ar ages have been done for a wide range of whole-rock and igneous clast samples (Bogard and Garrison, 1998; Bogard et al., 1990). Most have stepwise argon- release profiles exhibiting modest increases in calculated age with extraction temperature. The ³⁹Ar–⁴⁰Ar ages for these samples are ∼3.9 Ga, indicating either slow cooling or thermal resetting (Bogard and Garrison, 1998; Bogard et al., 1990).

The silicate fraction of mesosiderites is broadly similar to the HED meteorites, and the original formation of the silicates is generally believed to be the same as that of the HED suite. However, differences in detail between most mesosiderite lithic clasts, and eucrites and diogenites, suggest that the later history of mesosiderites was different. Mittlefehldt (1979, 1990), Mittlefehldt et al. (1992), and Rubin and Mittlefehldt (1992) argued that many of the mesosiderite mafic clasts were remelted after metal–silicate mixing. This process formed magnesian basalts and gabbros with low FeO/MnO, high-modal abundances of tridymite and/or whitlockite, and LREE-depleted patterns with high Eu/Sm. A large fraction of the mafic clasts are from remelted parent rocks, suggesting that a large fraction of the parent-body crust was remelted after metal–silicate mixing.

The iron metal and troilite of mesosiderites are presumed to represent core materials of an asteroid. Mixing of this with crustal silicates requires an unusual formation process. Some have suggested that a naked molten core (a core with the silicate crust and mantle largely stripped off) impacted a differentiated asteroid at low velocity (Wasson and Rubin, 1985). Others have suggested that an impact disrupted the differentiated, mesosiderite parent body, which reaccreted. This process mixed materials from different portions of the parent body, with mesosiderites representing a location where the core and crust were mixed together (Haack et al., 1996; Scott et al., 2001).

Mesosiderites are polymict breccias, as are many HED meteorites, thus presenting evidence for impact mixing on the surfaces of their parent bodies. Impact mixing on asteroidal bodies is expected to be parent-body-wide (e.g., Housen et al., 1979), leading some researchers to argue that mesosiderites and HEDs were formed on different parent bodies (see Mittlefehldt et al., 1998; Rubin and Mittlefehldt, 1993).

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Lunar Rocks

Arden L. Albee , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.C Highland Breccias and Ancient Rocks

The repeated impact episodes are reflected in the complexities of the fragmental rock (breccia) samples returned from the lunar highlands. The shock of the impact results in intense fragmentation and melting. The molten material may quench to glass during the ejection, or the hot mixture may remelt, sinter, and recrystallize during deposition and cooling in a thick ejecta blanket. The breccia samples range from friable aggregates to hard, sintered material with spherical vesicles that were bubbles filled by a gas phase prior to solidification. Many samples show multiple generations of impacts; fractured fragments of ancient rocks are within irregular fragments of breccia that are themselves contained in a mixture of fragments and melt rock.

The major minerals within the highland breccias are anorthite-rich plagioclase (CaAl₂Si₂O₈), orthopyroxene ([Mg,Fe]SiO₃), and olivine ([Mg,Fe]₂SiO₄); these occur both as mineral fragments and as plutonic rocks made up predominantly of these minerals. The high content of anorthitic plagioclase and the low abundance of iron and titanium oxide minerals is responsible for the light color and for the characteristically high calcium and aluminum composition of the lunar highlands. The words anorthosite, norite, and troctolite are used in various combinations as adjectives or nouns to describe coarse-grained rocks made up of various combinations of these three minerals. Hence the acronym ANT is commonly used to describe this suite of rocks. Such rocks are found on the earth in layered igneous bodies that have crystallized from a silicate melt or magma very slowly deep beneath the surface. The term magma includes not only the complex silicate melt, but the various crystallizing minerals, and may include bubbles of volatiles and globules of sulfide or metal melt. Plagioclase-rich rocks such as the ANT suite do not form by simple crystallization of magma, but represent accumulation of early crystallizing minerals by floating or settling, as evidenced by terrestrial examples of cumulate rocks. Remote sensing maps indicate that anorthosite is the dominant rock type of the highlands.

Despite the complex history, a number of fragments of ANT rocks collected from the breccia have yielded isotopic ages greater than 4.4 billion years (Gy), indicative of crustal formation dating back almost to the origin of the solar system. The existence of an early crust was also inferred from geochemical evidence. The rare earth element europium, unlike the other rare earth elements, is highly concentrated in plagioclase during crystallization of a silicate melt. This element has a relatively high abundance in the highlands rocks and is relatively underabundant in the lunar basalts. These complementary anomalies are ascribed to extensive early differentiation of the primitive lunar material into a plagioclase-rich crustal cumulate of crystals and a more mafic melt, which eventually became the source of the lunar basalts. Hence, it is inferred that much of the outer part of the moon was molten that is, a magma ocean during the early part of lunar history.

This early differentiation seems also to have been responsible for another compositional class of material rich in K, rare earth elements, and P (KREEP). These elements are representative of the "incompatible elements" (which also include Ba, U, Th, and Rb) that do not enter the crystal structure of the major lunar rock-forming minerals and hence become concentrated in the residual liquid during final crystallization of a magma. The abundance of KREEP ranges greatly in the samples of highland breccias and regolith, occurring as both small rock fragments and glass. However, the uniformity of abundance pattern and the isotopic systematics, albeit partially disturbed in some cases, suggest a rather homogeneous source, one that was enriched in the incompatible elements at about 4.4 Gy. Orbital measurements of gamma rays have shown that material rich in K, Th, and U is concentrated in the region of Mare Imbrium and Oceanus Procellarum. The KREEP-rich material may have been distributed from these regions into the regolith by impact scattering.

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Meteorites, Comets, and Planets

C. Koeberl , in Treatise on Geochemistry, 2007

1.28.4.1 Meteorite Craters: Source Rocks and Impactites

It is important to understand how the compositions of various impact lithologies (i.e., melt-matrix breccia, black-matrix breccia, defined below, and suevites) can be explained as mixtures of different proportions of target rocks. A good example from Meteor Crater was given by Hörz et al. (2002). This usually involves mixing calculations. In the case of Meteor Crater, Hörz et al. (2002) found that preserved impact glasses are generally clear and texturally homogeneous, but unlike typical impact melts, they have unusually heterogeneous compositions, both within individual particles and from sample to sample. The meteoritic content (at Meteor Crater an iron meteorite) is unusually high and it is distributed bimodally, with specific samples containing either 5–10 or 20–30% FeO. According to Hörz et al. (2002) these compositional heterogeneities may reflect the high carbonate content of the target rocks and the release of CO₂ during melting. The high projectile content and the CO₂-depleted residue of purely sedimentary rocks produced mafic melts that show three compositional groups, reflecting variable proportions of the major target rock formations (the Moenkopi, Kaibab, and Coconino formations). Least-square mixing calculations indicated that one group of impact glasses was made from 55% Moenkopi, 40% quartz-rich upper Kaibab, and 5% meteorite, suggesting a source depth of <30 m from the preimpact surface. The other two impact melt groups have higher contents of meteorite (15–20%) and Kaibab (50–70%), implying melt depths >90 m or <30 m, respectively. The work by Hörz et al. (2002) is a good example of the usefulness of mixing calculations.

For such calculations, the average compositions (with standard deviations) of the main target rock types present should be used. To illustrate the procedure in more detail, I describe another example of an application of mixing calculations, for the Gardnos impact crater, Norway (French et al., 1997). At Gardnos, three basement (target) lithologies have been distinguished (French et al., 1997): (1) a variable suite of granitic gneisses that form the majority (>75–80 area%) of the outcrop area; (2) amphibolite, generally present as crosscutting dikes in the granitic gneisses; and (3) a coarse-grained metamorphic orthoquartzite, which is white and massive outside the structure and becomes black and highly fractured within it. Impactites at Gardnos are: (1) shocked quartzite, which is black and highly fractured within the structure; (2) lithic breccias: (a) the well-known subcrater "Gardnos Breccia," which consists of fragments of white granitic gneiss in a pulverized black matrix; (b) a "black-matrix breccia," which is similar to the "Gardnos Breccia," but contains fewer fragments and a higher percentage of generally darker (carbon-rich) matrix; and (3) melt-bearing breccias: (a) suevite, in which fragments of melt and basement rock occur in the clastic matrix; (b) melt-matrix breccias (or impact melt breccias) in which crystallized melt forms a matrix to rock and mineral clasts. In the region there are several carbon-rich shales (Alum shale and Biri shale) that could be the source of the high carbon contents of the impactites (Gilmour et al., 2003).

Mixing calculations were performed with the Harmonic Least-Squares (HMX) mixing calculation program (Stöckelmann and Reimold, 1989), which allows the usage of any number of components (in this case, target rock components) and component or mixture parameters (here elemental abundances). An advantage of this particular program is that analytical uncertainties can be included in the model computation. Furthermore, it is possible to select from a number of so-called refinement control parameters for the purpose of setting different constraints on such calculations. It is, for example, possible to constrain any results (component proportions) to total exactly to 100% or, if this option is not chosen, it is possible to determine whether additional, not yet considered, components are needed to obtain a result of 100%. It is also possible to determine a priori that a so-called pivot component, one of the offered components, is considered a "fixed" component of the mixture.

Four calculation runs were performed for each of the mean mixture compositions and with all the average component compositions (French et al., 1997). All of these led to the conclusion that gneiss is the dominating target rock component of all three impactites. Before the first calculation was attempted, the average compositions were scrutinized to determine which parameters show large-enough variations between the target rock groups to be useful for distinguishing the resulting mixtures. Some element abundances were likely to have undergone postimpact changes, making them of little use for constraining target rock proportions. Some calculations were performed with major element parameters only, others with major elements in combination with a number of trace elements that appeared to be sufficiently different in abundance from component to component. The conditions for the various computation runs were: Run 1, nine major elements without P₂O₅, all component proportions were forced to total to 100%; Run 2, seven major elements without P₂O₅, Na₂O, and K₂O, but including carbon, totals=100%; Run 3, seven major elements, as in Run 2, but without carbon, and including scandium, yttrium, lanthanum, ytterbium, hafnium, and thorium, totals=100%; Run 4, as in Run 2, but without carbon and with quartzite as a forced component; Run 5, seven major and eight trace elements, without carbon, total=100%. The carbon-bearing shale component was inferred from the high carbon content of the breccias, but is not observed to occur in any of the analyzed target rocks. Results are summarized in Table 2a.

Table 2a. Comparison of measured breccia compositions from the Gardnos Crater with those obtained from mixing calculations

	Impact melt breccia		Black-matrix breccia		Suevites
	Run 2	Δ_obs–calc	Run 2	Δ_obs–calc	Run 3	Δ_obs–calc
SiO₂	66.21	−0.11	74.31	−0.33	63.31	−0.70
TiO₂	0.834	0.006	0.425	−0.05	0.92	0
Al₂O₃	12.92	0.14	10.88	0	13.31	0.04
Fe₂O₃	5.07	−0.009	3.43	−0.13	6.52	−0.02
MnO	0.075	<0.001	0.052	0.006	0.135	0.014
MgO	1.32	0.02	0.57	0.14	2.38	0.57
CaO	1.41	−0.06	0.76	−0.02	1.94	−0.25

From French et al. (1997).Note: All data in wt.%. The runs giving the best results were used for this comparison. Δ_obs–calc=observed value (Table 1) minus calculated value.

In Table 2b, the observed and calculated mixture compositions for the three main impactite groups are compared, and the discrepancies observed for the various parameters are listed. Discrepancy factors, as given in Table 2b, are a calculated measure for the validity of the results: the better and statistically more valid a result, the closer the corresponding discrepancy value approaches 0. Some results are not satisfactory. For example, due to the (postimpact) mobility of the alkalis, the calculations containing Na₂O and K₂O concentrations (Run 1) may give unreliable results. Addition of trace-element data (scandium, chromium, cobalt, lanthanum, ytterbium, hafnium, throrium, and uranium) to the parameter list led to a slight increase in the discrepancy factor. For the melt-matrix breccias and the black-matrix breccias, runs with good discrepancy factors, and for the suevites, with reasonable discrepancy factors, were obtained. The high carbon content in the impactites can be reproduced by adding a carbon-shale component similar to Biri shale. For the best runs, the discrepancies between the calculated and observed compositions are small for all elements in the melt- and black-matrix breccias, with somewhat higher deviations for CaO and MgO in the suevites. Discrepancy factors do not change significantly when a carbon-rich shale component is included. The results show that gneiss is the dominant target rock component for all three impactites, with significant (10–30%) amphibolite contributions for the melt-bearing breccias, and 3–19% carbon-shale contributions in all three breccia types.

Table 2b. Mixing calculations to reproduce Gardnos impact melt breccia and suevite compositions

		Target rock components				Discrepancy factor
		Quartzite	Amphibolite	Gneiss	C-Shale	Discrepancy factor
Impact melt breccia	1	0.0	6.5±1.8	93.5±2.9	—	6.5
Impact melt breccia	2	0.0	12.1±1.1	84.9±1.2	2.9±0.08	1.06
Impact melt breccia	3	4.6±1.7	17.1±1.8	78.3±2.7	—	4.6
Black-matrix breccia	2	7.7±1.2	<0.5	80.4±2.2	11.9±1.2	0.38
Black-matrix breccia	3	12.8±1.8	<0.5	87.2±1.8	—	1.3
Suevites	2	<0.5	25.1±2.3	56.1±3.2	18.8±0.5	6.7
Suevites	3	<0.2	25.7±2.6	74.3±2.6	—	8.9
Suevites	4	1.2±1.4	28.0±2.9	70.8±3.8	—	5.2

From French et al. (1997).

Run numbers: 1, all major elements, components total 100%; 2, major elements, except Na and K, plus C, components total 100%; 3, major elements, except Na and K, plus Sc, Y, La, Yb, Hf, Th; 4, as 2, but with quartz as a forced component.

Such calculations allow the determination of the rock types that can explain the compositions of breccias and melt rocks. This is important if the indigenous compositions need to be determined for the study of possible meteoritic components, as described in the next section.

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Meteorites*

Michael E. Lipschutz , Ludolf Schultz , in Encyclopedia of the Solar System (Second Edition), 2007

2.4.3 BRECCIAS

Even though most chondrites are readily pigeonholed, a few consist of two or more meteorite types, each readily identifiable in the lithified breccia. Noblesville, for example, consists of light H6 clasts embedded in dark H4 matrix ( Fig. 1a). Such an assemblage—two petrographic types of the same chondritic chemical group—is a genomict breccia. A polymict breccia contains two or more chemically distinct meteorite types, implying the mixing of materials from 2 (or more) parent bodies, each with its own history. The most striking such case is Cumberland Falls where black forsterite chondrite inclusions as large as 3 cm × 5 cm are embedded in an 8 cm × 11 cm white enstatite achondrite.

Of the other sorts of breccias, perhaps the most important is the regolith breccia. Noblesville (Fig. 1a) is such a meteorite, and its typically dark and fine-grained matrix contains large quantities of light noble gases—He and Ne—of solar origin (cf. Section 5.1). In addition to these gases, radiation damage in present as solar-flare tracks (linear solid-state dislocations) in a 10-nm-thick rim on the myriad matrix crystals. However, solar gases and flare tracks are absent in the larger, lighter-colored clasts of regolith breccias. Clearly, dark matrix is lithified fine dust originally dispersed on the very surface of the meter-thick regolith or fragmental rocky debris layer produced by repeated impacts on bodies with no protective atmosphere. (The lunar regolith is both thicker, ∼1 km, and more mature and gardened, or better mixed by impacts than are asteroidal regoliths.) This dust acquired its gas- and track-component from particles with keV/nucleon energies streaming outward as solar wind or solar flares with MeV energies [see The Solar Wind] so that the dust sampled the solar photospheric composition. The irradiated dust, often quite rich in volatile trace elements from another source, was mixed with coarser, unirradiated pebble-like material and formed into a breccia by mild impacts that did not heat or degas the breccia to any great extent. Regolith breccias occur in many meteoritic types but are especially encountered as R (and H) chondrites, aubrites, and howardites.

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The Apollo program

Eric A. Jerde , in Sample Return Missions, 2021

Abstract

Six landings were made during the Apollo program and a total of 376 kg of samples was returned. These samples are varied, including basalt, breccia, glass, and unexpected amounts of anorthosite. While much of the returned material remains unstudied in any detail, the basic nature of lunar history has been deciphered. Anorthosite represents the original crust of the Moon, forming at 4.5–4.1 Ga through plagioclase crystallization and flotation from a global magma ocean. Large impacts during this time and lasting until about 3.8 Ga formed large basins all over the Moon. Secondary melting of deeper portions of the crystallized magma ocean resulted in basalts, which erupted and filled many of the basins, resulting in the circular features observable from Earth. Although this volcanism appears to have mainly occurred prior to about 3 Ga, more recent studies indicate that it might have continued at least sporadically to much more recent times.

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IMPACT STRUCTURES

R.A.F. Grieve , in Encyclopedia of Geology, 2005

Impact Melting

Impact-melted lithologies occur as glass bombs in crater ejecta, as dykes within the crater floor and walls, as glassy to crystalline lenses within the breccia lenses of simple craters, or as coherent annular sheets ( Figure 8) lining the floor of complex craters. When crystallized, impact-melt sheets have igneous textures, and may, therefore, resemble endogenic igneous rocks. An important textural characteristic, however, of impact-melt rocks is the presence of mineral and rock fragments, which exhibit shock metamorphism to different degrees. The size of such fragments ranges from millimetres to several hundreds of metres and gradational changes in fragment content are observed, with highest concentrations towards the lower and upper contacts of coherent impact-melt sheets.

The composition of impact-melt rocks reflects the wholesale melting of a mix of target rocks, as opposed to partial melting and/or fractional crystallization relationships for endogenous igneous rocks. The composition of impact-melt rocks can be reproduced by a mixture of the various target rock types, in their appropriate geological proportions. Such parameters as ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios also reflect the preexisting target rocks, although other isotopic systems, e.g., Ar³⁹/Ar⁴⁰, reflect remelting at the time of impact. In general, even relatively thick impact-melt sheets are chemically homogeneous over distances up to tens of kilometres. Differentiation is not a characteristic of impact-melt sheets (with the exception of the extremely thick, ≥2.5 km, Sudbury igneous complex, at the Sudbury Structure, Canada).

Enrichments above target rock levels in siderophile elements and Cr have been identified in some impact-melt rocks. These represent an admixture of up to a few percent of meteoritic material from the impacting body. In some melt rocks, the relative abundances of the various siderophiles have constrained the composition of the impacting body to the level of meteorite class. In other melt rocks, no siderophile anomaly has been identified. The latter may be due to the inhomogeneous distribution of meteoritic material or to differentiated and, therefore, non-siderophile-enriched impacting bodies, such as basaltic achondrites. High-precision chromium and osmium-isotopic analyses have also been used to detect a meteoritic signature at terrestrial impact structures.

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SHOCK METAMORPHISM

P.S. DeCarli , in Encyclopedia of Geology, 2005

Shock Metamorphic Effects

The impact event depicted in Figure 1 can produce numerous shock metamorphic effects. Ejected material, including solid, solidified melt, and condensed vapour, can serve as a stratigraphical marker. The breccia lens provides most of the evidence to distinguish an impact crater from a volcanic crater. As noted above, the breccia lens is a heterogeneous mixture of high-pressure and lower pressure material. This is advantageous in that the lower pressure clasts have been subjected to only minor shock heating and serve to quench the more strongly shock-heated and shock-melted material. Quenching helps to preserve more fragile high-pressure phases, such as stishovite. Some common shock metamorphic effects are presented in Table 2.

Table 2. Shock metamorphic effects

Effect	Source material	Pressure (GPa) (single shock)	Comments
Melting	Iron	>170	Melts on release of pressure
Melting	Olivine, pyroxene	>100	Melts on release of pressure
Melting	Quartz, granite	>50	Melts on release of pressure
Melting	Sand, soil	>20, possibly as low as 7	Energy increase on shock compression much greater for porous materials
Diaplectic glass	Quartz, feldspars	>15, possibly as low as 7	Diaplectic glass forms by solid-state transformation. It is amorphous, but retains original crystal form and usually has a higher refractive index than melt glass. Lower bound pressure from PDF formation
Stishovite, hollandite	Quartz, feldspars	>15, possibly as low as 7	Stishovite, hollandite, polymorphs of quartz, and feldspar found in impact craters and meteorites. Lower bound pressure from PDF formation
Coesite	Quartz	>15, possibly as low as 3	Coesite found in impact craters in association with diaplectic glass, implying that it formed on release of pressure. Could conceivably be found in a pseudotachylite that solidified under pressure
Ringwoodite, wadsleyite	Olivines	>15	Found in meteorite melt veins; pseudotachylite-like structures that were quenched at high pressure
Akimotite, majorite	Pyroxenes	>17	Found in meteorite melt veins; pseudotachylite-like structures that were quenched at high pressure
Diamond, cubic and hexagonal mixture	Graphite	>25	Found in meteorites; P ∼ 100 GPa from graphite in iron meteorites. Found in impact craters; P ∼ 30 GPa from graphite in gneiss. Made in laboratory shock experiments
Cubic diamond	Porous carbon	>15	Made in laboratory shock experiments
Planar deformation features (PDFs)	Quartz, feldspar predominantly. Also other minerals	>7	PDFs in quartz are a primary diagnostic for impact. A PDF is a lamellar feature aligned with a low-index crystallographic plane. A number of different orientations may appear in the same grain. There is evidence that the lamellae contain high-pressure phases that invert to low-pressure forms during electron microscopy
Fractures	All rocks	>∼0.1	Laboratory shock experiments show dynamic fracture strength comparable (∼1.5 times) to static strength
Pseudotachylite formation	All rocks	>∼0.1	Pressure estimate based on observation of pseudotachylites in the fractured zone

Numerous other high-pressure minerals have been observed in meteorites and impact craters. The most common and readily observed are listed. The book by French (see Further Reading) contains numerous micrographs of shock metamorphosed quartz.

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Asteroid Impacts and Extinctions

Carolyn S. Shoemaker , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

III.C Characteristics of Craters

Craters come in varying sizes and shapes, mostly round but sometimes elongate, depending upon the angle of impact. Oblique impacts are the most probable, but, unless this angle is less than 15–20°, the craters produced are circular. (Meteor Crater, Arizona, is somewhat square.) Beneath the crater floor is a lens-shaped body of breccia (rock smashed by the shock wave) lining the cavity. Near the base of the breccia there is shock-melted rock called impact glass, sometimes interfingered with the breccia. In ancient, highly eroded structures, both the breccia and impact glass may have disappeared. Some of the impact glass which lined the cavity walls is ejected at the time of impact into the atmosphere in the form of tektites (glassy spherules of impact melt), which may be thrown above the atmosphere and travel for many thousands of miles. There are four known tektite-strewn fields: the moldavites from the Ries crater in Germany, the Ivory Coast microtektites associated with the Bosumtwi Crater, tektites in the North American strewn field resulting from a huge impact in Chesapeake Bay, and the Australasian tektites, which comprise one of the largest strewn fields from an impact either in the vicinity of Cambodia or in the ocean nearby. They have been found in the Indian Ocean, clear across Australia, and farther north in southeast Asia and in the Philippines, as shown in Fig. 10. Microtektites, which are small enough to require a microscope to be seen, may circulate even farther, and many have been found in deep-sea drilling cores. Noble metals, including platinum, iridium, gold, osmium, and palladium, are relatively abundant in meteorites and depleted in the crust of the Earth; these contaminate the impact glass. Iridium is easier to measure than some of these other metals. It is in low abundance on the Earth's surface because it has an affinity for iron and much of it has been extracted into Earth's core. The amount brought in by impact, then, provides an excellent tracer of the source. In addition, there are macroscopic and microscopic deformations of unmelted glass with shocked quartz showing deformed planar lamellae in multiple sets readily distinguished from those tectonically deformed. The shock-formed minerals of coesite, stishovite, and diamond formed from high pressure may occur at some impact sites in the breccia or in the suevite.

Diagnostic of cratering events are shatter cones, a horse-tailing, striated shock effect producing divergent grooves upon a conical rock surface and formed only in nature close to the center of impact sites. Shatter cones are most often found in place in the rocks below the crater floor and usually in the central uplifts of complex craters, but they have been discovered as far away from the center of impact as one third the radius of the whole structure and may occur in crater-floor breccias even farther away and in crater ejecta. In size, they may range from millimeters to meters in length. Laboratory and field experiments demonstrate that shatter cones are formed by compression in the direction of shock propagation. They are especially useful in the field as indicators of impact because they can be so easily recognized.

Impact melt-rocks are the most intensely shocked materials, due to passage of the shock wave and to viscous heating. Most of the displaced rocks underlying impact structures and craters are intensely fractured in comparison with the rocks in the surrounding terrain. This decreases their density and is often reflected in a negative Bouguer gravity anomaly, although dense rocks brought up from great depths may give a high gravity reading, surrounded by a low gravity reading. Gravity surveys have proved useful in the investigation of buried structures or those with limited exposures.

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Crust and Lithosphere Dynamics

B.D. Marsh , in Treatise on Geophysics, 2007

6.07.7.1 The Sudbury Igneous Complex (SIC)

The Sudbury impact melt sheet is the closest example on Earth of a massive magmatic laboratory bench experiment. This is a body that was emplaced instantaneously free of crystals. These are the exact initial conditions postulated by early petrologists. In this context alone, it is enormously valuable to examine the end product. The SIC was produced 1.85 Ga in a matter of minutes (∼5) by impact of 10–12 km bolide (e.g., Grieve et al., 1991). Perhaps originally containing as much as 35 000 km³ of magma ∼3 km deep and spread across a crater ∼200 km in diameter, the melt sheet had the aspect ratio of a compact disk. Moreover, the initial temperature, which can be ascertained by several methods, was ∼1700 °C, well beyond the liquidus (∼1200 °C) of the magma (Zieg and Marsh, 2005). The melt was intensely superheated, destroying all existing crystals. For all intents and purposes, this is an infinite sheet of magma. The body today (see Figure 21 ) has been deformed by continent-to-continent collision during Grenville time, which has folded it from south to north much as one would fold a sandwich. This folding, along with glaciation, has made the stratigraphy particularly accessible as has extensive drilling associated with mining (∼1.5 billion metric tons) of massive deposits of Ni–Cu sulfide ores along the crater floor.

The final section of igneous rock is an upper layer of ∼2 km of granophyre (∼70 wt.% SiO₂) and a lower ∼1 km layer of norite (∼57 wt.% SiO₂) separated by a transition zone of quartz gabbro of several hundred meters, which is the densest rock in the entire section ( Figure 21 ). The most surprising aspect of this body is the degree of homogeneity of the norite and granophyre. The same stratigraphy is laterally continuous across the entire body. There is no modal layering anywhere. Trace element and isotopic ratios are much too similar to ascribe to pure chance or a protracted igneous process. For example, isotopic ratios, such as ⁸⁷Sr/⁸⁶Sr (Dicken et al., 1999) and La/Sm, Gd/Yb, and Th/Nb, are nearly identical in the two main units (Lightfoot et al., 1997). Although sill-like in form, which is similar to other large bodies, like Bushveld, Dufek, and Stillwater, it has none of the features of major sills. The average composition is ∼64% SiO₂ but this material is not found as extensive chilled margins at the top and bottom, and there is no systematic chemical progression inward to suggest any form of differentiation by crystal fractionation. These features, among many others (see Zieg and Marsh, 2005), along with unusually uniform textural homogeneity, suggests that, once formed, the body did not evolve at all along the lines long assumed for magmatic bodies. In fact, to a close first approximation, once formed, the main units did not evolve at all. They simply crystallized in place. All of these features are likely natural consequences of the impact process.

Impacts of this size generate an outward moving shock wave of ∼500 GPa (5 Mb), which is equivalent to the pressure at Earth's center. Temperatures reached about 4000 °C at the center of the impact, vaporizing the impactor and adjacent silicate crust. Beyond the vaporization zone an extensive melting front existed and beyond this a fracture front. The production of breccia of all sizes (tens of meters to nanometers), in states of vapor, liquid, and solid, characterizes all aspects of the cratering event. As the impactor penetrated downward, it produced within a few minutes a transient cavity ∼30 km deep and ∼90 km in diameter. The target materials from which the breccias were made consisted of highly evolved continental crust. The crust was granitic overall, but in detail it consisted of granitic plutons, gneisses, swarms of diabase dikes and sills, gabbroic plutons, quartzose sediments, and untold other lithologic varieties. These materials form an extensive 3-km-thick sequence of 'fallback' breccia (Onaping Formation) that cascaded in from the crater rim and atmosphere as the crater relaxed over a few minutes into the final shallow impact melt sheet (∼6 × 200 km). The magmatic part of the sheet is the molten breccia (Zieg and Marsh, 2005). This molten breccia is more properly described as a magmatic viscous emulsion. That is, it consisted of blobs of silicate melt of a wide spectrum of chemical composition and thus density and viscosity. Although certainly not chemically immiscible, small density and compositional differences gave rise to an exceedingly heterogeneous melt where each blob had the freedom to sink or rise. Large volumes of near-identical composition rapidly coalesced into an extensive 'continuous phase' within which were other smaller chemically and physically distinct blobs, which formed a 'dispersed phase'. Because of the inherent granitic nature of the crust here, the continuous phase was granitic and the dispersed phase basaltic. Once the sheet formed, this superheated magmatic emulsion immediately began to separate and coalesce into an upper granitic layer and a lower basaltic layer. The separation and coalescence process was rapid, taking place within a few years. Although a rapid process, this process also allowed extensive intimate chemical exchange between the dispersed and continuous phases.

Melt parcels smaller than a certain critical size (∼1 cm) rose or fell so slowly that they were chemically resorbed by diffusion into the surrounding melt. Moreover, the vast surface area available to chemical exchange allowed the entire melt sheet to chemically interact by diffusion during coalescence of the emulsion. And once the two layers had separated, each superheated layer went into vigorous thermal convection, which thoroughly homogenized each layer. Prior to this time, the strong upward and downward flow of melt parcels had suppressed thermal convection. This convective phase of cooling lasted at most a few tens of years (Zieg and Marsh, 2005). The net result is two layers of strongly contrasting chemical composition that have intimately and thoroughly exchanged trace chemical components to a high degree and, then, each has been thoroughly homogenized internally by exceedingly vigorous convection. The high temperature and the vigorous convection stripped the entire body of volatiles, making it unusually dry for magmas of these compositions.

Once the superheat had been dissipated from each layer through thermal convection, convection ceased and crystallization commenced in the form of inward propagating solidification fronts from the top and bottom. These fronts were unusual in that the contact temperatures at the base and roof were each near the solidus temperature of the respective magmas, granophyre and norite. This pinned the solidus at or near the boundaries while the liquidus, defining the leading edge of the front, freely propagated inward to meet, perhaps coincidentally, at the juncture of the two layers. This allowed for unusually thick solidification fronts to form. This made thick crystal mushes at the top and bottom that acted to retard the descent and rising of solid breccia debris from, respectively, the roof and floor. Rafts of Onaping breccia foundering from the roof fall in the less dense granitic magma and descend until reaching the lower layer of basaltic magma. At the same time, rafts of breccia and individual large blocks from granitic plutons forming the crater floor rose until encountering the upper granitic magma. The net result of the movement of all this debris is a zone of collection at the interface between the two layers. This debris, some of which further disaggregated and dissolved in surrounding magma, and mafic melt squeezed from slight compaction of the lower solidification combined in the end to make the transition zone. The collection of all this debris formed an unusual environment in terms of chemical compositions and also oxidation state and collection of volatiles. This debris carried hydrous minerals that dehydrated upon heating, further shattering the breccias into smaller fragments and freeing volatiles to collect and form miarolitic cavities. This all produced an oxidized environment of heterogeneous composition of strong density contrasts that may have allowed internal gravity waves to propagate along this interface. The culmination of this collection process occurred as the lower solidification front reached the interface, structurally supporting this unit, which is the densest horizon in the entire body. This strong density gradient and overall structure of the transition zone is remarkably consistent everywhere it has been measured (see Figure 22 ).

The net result of generating a large body of magma instantaneously and free of all crystals is to produce two essentially homogeneous layers with no internal modal sorting or layering. This is a dramatic confirmation of the null hypothesis, which states that given a sheet-like magma free of crystals it will crystallize to homogeneous rock with only slight internal variations (Zieg and Marsh, 2005). Magmas do not become exotically layered and show strong differentiation trends from basaltic to granitic by crystal fractionation within such bodies. Then, how does it happen? It happens by injections of magma laden with large crystals. The extent and degree of layering is a direct indication of the extent of injection of crystal-laden magma.

The Sudbury record is also valuable in showing what happens to true granite target rock once it is heated beyond its liquidus and then allowed to crystallize as a dry high silica magma. The surprising result is that the final texture is not a coarse-grained typical granite. It is a fine grained granophyre. This is likely due to the loss of its textural template due to superheating and higher viscosity, and thus slower diffusion rates, under anhydrous conditions.

Sudbury is also a prime example of how the continental crust became organized early in Earth history. Ongoing impacts extensively melted the fledgling crust, allowing it to continually reorganize through emulsion sorting and coalescence.

As mentioned above, Sudbury is critically important in clearly showing what happens to magma when the initial conditions are explicitly known. In this regard, it is an end-member example. Many thinner diabase sills the world over are examples of this type of magmatic body.

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