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IMPACT CRATER/STRUCTURE GLOSSARY

by: Charles O’Dale

The petrographic and geochemical study of actual rocks from the potential impact structure will bring final confirmation of the presence of an impact structure. In case of a structure that is not exposed on the surface, drill-core samples are essential. Good materials for the recognition of an impact origin are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or a contamination from the extraterrestrial projectile.

ZIRCON

Zircon, zirconium orthosilicate (ZrSiO4), is found in most igneous rocks and some metamorphic rocks as small crystals or grains, mostly widely distributed and rarely more than 1% of the total mass of the rock. It is also found as alluvial grains in some sedimentary rocks due to its high hardness. Zircon has a high refraction index and, when the crystals are large enough, is often used as a gemstone.

Two important traits:

  • They are incredibly durable. The rocks in which they initially formed may weather away, but the zircons survive as tiny grains of sand that may later be incorporated into the next generation of rocks.
  • They aren’t pure zirconium silicate. They contain trace amounts of other elements, most importantly uranium, trapped within them as they crystalize. Over the eons, that uranium slowly decays to lead. By comparing the amounts of uranium and lead, scientists can determine the date at which the crystal formed.

In geology, zircon is used for radiometric dating of zircon-bearing rocks (using isotopes of U which is often present as an impurity element, as is Th, radiogenic Pb, Hf, Y, P, and others). Zircon contains the radioactive element uranium, which converts to the element lead at a specific rate over a long span of time, “the most reliable natural chronometer that we have when we want to look at the earliest part of Earth history.”

In a 2017 study in Science Advances, geophysicists used zircons in Moon rocks brought back by Apollo astronauts to determine that the Moon’s crust solidified 4.51 billion years ago, only 60 million years after the formation of the first protoplanets. And zircons in meteorites blasted off the surface of Mars are being studied to peer nearly as far back into the Red Planet’s early history.

Zircon transforms into reidite when meteorites slam into the ground because shock waves from the impact cause a dramatic increase in temperature and pressure at the site. The high pressures cause the building blocks of the mineral to rearrange, becoming tightly repacked. The resulting mineral is similar in composition to zircon, but around 10% more dense. Reidite can also be formed under high-pressure or shock recovery laboratory experiments. In fact, reidite was only known from lab-made samples for around 30 years before it was first discovered in nature in 2001 (Reidite was finally identified in nature starting in 2001, at three impact sites: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany and Xiuyan Crater in China.).

Reidite is a rare mineral,  a dense form (polymorph) of the fairly tough gemstone zircon, which is produced when the latter is subjected to very high pressures.  Reidite has been found only in four crater impacts: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany, Xiuyan Crater in China, and Rock Elm Crater in Wisconsin in the United States (Wiki).

Meteorite zircon constraints on the bulk Lu−Hf isotope composition and early differentiation of the Earth 

Tsuyoshi IizukaTakao YamaguchiYuki Hibiy, and Yuri Amelin

Significance

The radioactive decay of lutetium-176 to hafnium-176 has been used to study Earth’s crust−mantle differentiation that is the primary agent of the chemical and thermal evolution of the silicate Earth. Yet the data interpretation requires a well-defined hafnium isotope growth curve of the bulk Earth, which is notoriously difficult to reconstruct from the variable bulk compositions of undifferentiated chondrite meteorites. Here we use lutetium–hafnium systematics of meteorite zircon crystals to define the initial hafnium isotope composition of the Solar System and further to identify pristine chondrites that are the best representative of the lutetium–hafnium system of the bulk Earth. The established bulk Earth growth curve provides evidence for Earth’s crust−mantle differentiation as early as 4.5 billion years ago.

Back-scattered electron image showing a zircon grain AG6-Zrn#01 in the eucrite Agoult. Mineral abbreviations are as follows: Ilm, ilmenite; Pl, plagioclase; Px, pyroxene; Trd, tridymite; Tro, troilite; Zrn, zircon. Reprinted from ref. 28, with permission from Elsevier;

 

The oldest zircons in the solar system

Cathodoluminescence image from a scanning electron microscope of a typical igneous zircon crystal from samples studied by the QUT research team, revealing growth rings of the zircon. Yellow circles enclose ablation sites by a laser from which isotopic data is measured to determine the age of zircon growth. The analytical spots here show this zircon had two main growth periods approximately 20 million years apart in different magmas. Credit: QUT