Planetary Impact Breccias

Lunar Impact Breccias

During the impact of a meteorite, old rocks are broken apart and new rocks are formed. Most lunar meteorites, many HED meteorites (howardites, eucrites, and diogenites), one martian meteorite, and some chondrites are breccias – rocks composed of fragments of older rocks that have been broken apart and relithified (“glued” back together), often many times, by impacts of meteorites. Impact brecciation results in rock that consists of clasts (rock fragments) of a range of sizes imbedded in a matrix of smaller clasts, crystallized impact melt, or perhaps impact glass.

Photo of a thin section of Apollo 17 polymict (mixture of many rock types) breccia sample 72275, about 3.8 cm wide. The breccia consists of fragments of a variety older rocks, many of which are smaller breccias, embedded in a matrix of still finer fragments.

In school, we learn that there are three kinds of rocks – sedimentary, metamorphic, and igneous. An impact breccia can be any of the three but the distinction among them is sometimes difficult to determine. Some impact breccias are sedimentary rocks in that they are fine-grained, near-surface material (regolith) that was consolidated (“glued together”) into a coherent rock by the heat and shock pressure of an impact. NWA 12604 (below) is such a rock, a regolith breccia. A few lunar and HED breccias are metamorphic in they they formed from fragmental material, perhaps several kilometers deep. A large impact occurred that was not close enough (laterally or vertically) to melt the material but was close enough to “cook” and recrystallize the minerals (thermal metamorphism). NWA 3163 and NWA 8687 (below) are metamorphic breccias, which are usually termed granulitic breccias because of their sugar-crystal like texture. Finally, meteorite impacts will melt target material below the point of impact. Some of the liquid will pool in the crater while the rest is ejected to form an ejecta deposit. The chaotic turbulence during and after the impact causes hot impact melt to mix with cold rock clasts. The clasts cool the melt and the melt heats the clasts sometimes to the point where the exteriors melt. This is the only mechanism of which I am aware that can lead to rare, rounded clasts in lunar impact breccias. Liquid impact melt that forms in small craters cools quickly to form glassy-matrix breccias. Crystalline-matrix breccias form in large craters and basins where the melt cools slow enough to crystallize. Both kinds of breccias are igneous in that they cooled and solidified from a liquid. Dhofar 1085, Dhofar 1443, and Shişr 166 (below) are clast-laden, impact-melt breccias. Veins of liquid impact melt injected into the space between rock fragments during an impact, which cool quickly to glass, are common in lunar breccias as are vesicles (gas bubbles) that get trapped in impact glass before it solidifies.

Sawn faces of 6 lunar meteorite breccias up close. Dhofar 1085 and Shişr 166 (the only lunar meteorite to have been found at night with a flashlight) are from Oman and rusty red from terrestrial hematite staining. Shişr 166 has some vesicles in the dark grayish veins of impact melt. Several of these vesicles are amygdules – vesicles now filled with whitish secondary (terrestrial) minerals such as zeolites, calcite, quartz, or chalcedony. In Oman, barite and celestine also occur in fractures and amygdules. The pink clast – a spinel grain – on the lower left side of NWA 8687 is the largest reddish thing that I have ever seen in a lunar rock. The color of the rock is a bit greenish because it contains 15-25% olivine, a greenish mineral olivine (think olives). In Dhofar 1443, the large, light-colored clast at the top is rounded. Rounded clasts are uncommon in lunar breccias because there is no wind and water to cause abrasion. NWA 8687 and NWA 3163 are granulitic breccias. The processes that lead to this metamorphic texture tends to erase the distinction between clasts and finer-grained matrix. There are many veins from fracturing during impacts while on the Moon in most of these meteorites but notice that none of the are linear. The matrix of Dhofar 1443 and NWA 12604 is dark because it is mostly glassy impact melt. Image credits: Randy Korotev

Nonlunar Meteorite Breccias

Some chondrites are regolith and impact-melt breccias. (I do not study chondrites, so I do not have photos.) Below are photos of a few HED (howardite, eucrite, diogenite) breccias.

NWA 7496 (polymict eucrite, 788 g). Some clasts are reddish from terrestrial weathering. Eucrites do not contain the whitish anorthosite clasts that are common in lunar meteorite breccias. Image credit: Randy Korotev
Two sides of a slice of Dhofar 007 (21.3 kg), a brecciated cumulate eucrite. In this coarse-grained rock, the white parts are grains of plagioclase. There are many rusty grains of iron-nickel metal. Dhofar 007 has several unusual features for a eucrite (Yamaguchi et al., 2006). Millimeter ticks for scale. Image credit: Randy Korotev
NWA 6072 (granulitic eucrite, 333 g). Granulitic breccias are often not obviously breccias except in thin section. See, for example, the NWA 3163 clan of granulitic lunar meteorites. Millimeter ticks for scale. Image credit: Randy Korotev
NWA 6073 (howardite, 25 g). The clasts are eucrites and diogenites. Millimeter ticks for scale. Image credit: Randy Korotev
NWA 6074 (polymict diogenite, 49 g). Some areas are reddish from terrestrial weathering and there are fractures filled with terrestrial secondary minerals. Millimeter ticks for scale. Image credit: Randy Korotev
NWA 7475, one of the several NWA 7034 pairs. At this time (June, 2021), this meteorite is the only brecciated meteorite from Mars. It is also the only meteorite of which I am aware that has large “round things” in it. Some of the clasts have rims. Image credit: Luc Labenne

Impact Breccias Are Fractal

Impact breccias are fractal objects – they look the same regardless of the scale at which you look at them. The clasts have a large range in size. On the left, below, is a photograph of a sawn face of lunar meteorite NWA 5000, about 12 cm high and 8 cm wide. In the middle is an enlarged image of the portion within the yellow rectangle on the left. Similarly, on the right is an enlargement of the area within the yellow rectangle of the middle. This fractal characteristic is important because in some terrestrial sedimentary rocks clasts sizes have been sorted.

See: Rocks That Look Like Impact Breccias But Are Not