IMPACT CRATER/STRUCTURE GLOSSARY
by: Charles O’Dale
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 Iizuka, Takao Yamaguchi, Yuki Hibiy, and Yuri Amelin
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.
The oldest zircons in the solar system
Here we report the occurrence, chemistry, and UThPb isotopic systematics of three meteoritic zircon assemblages, two from the Vaca Muerta mesosiderite and one from the Simmern H5 chondrite. One of the Vaca Muerta zircons occurs in the mesosiderite proper, the other in a eucritic clast associated with chromite, ilmenite, and tridymite, whereas the Simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar. Like terrestrial zircons, the meteoritic zircons are enriched in the heavy rare-earth elements, but unlike terrestrial zircons they do not show a positive Ce anomaly. This feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment: under oxidising conditions Ce4+ (which can substitute for Zr4+ in the zircon structure) is stabilized whereas under relatively reducing conditions Ce3+ is stable. The zircon from Simmern has depletions in the relative abundances of Tm and Yb, a characteristic of volatility fractionation of the REE. The U concentrations of the two Vaca Muerta zircons are quite different at 1 and 50 ppm, respectively. Both zircons have extremely low initial lead and give radiogenic ages that are the same, albeit with relatively large errors on VM-1 zircon because of its much lower U concentration and hence radiogenic Pb concentration. The mean of the two analyses made on VM-2 zircon is concordant with a207Pb/206Pb age of 4563 ± 15 Ma(2σ). The Simmern zircon has an exceptionally low U concentration of around 180 ppb and only a poorly constrained 207Pb/206Pb age of 4100 ± 700 Ma could be obtained.