Figures 1-2. Two slices of a sample of the Acasta gneiss. The smaller sample was cut from the polished face seen at right. The Canadian flag pin is 13 mm (half an inch) in width. This metamorphic rock is a textbook gneiss, segregated into lighter (quartz- and feldspar-rich, trace pyrite) and darker bands, and flecked with red mm-scale porphyroblasts of garnet. The rock has a strong directional banding (foliation). It has appreciable heft, but it is not appreciably magnetic.

The mineralogy appears to be a mixture of quartz, plagioclase feldspar and dark greenish-black hornblende (amphibole), with minor red almandine garnet, traces of fine-grained pyrite, plus (probably) some iron-titanium oxide such as ilmenite, and a film of biotite mica on late fractures cutting the foliation. Traces of secondary limonite occur around pyrite. The magnetic susceptibility is about 0.25-0.28x10^-3 SI units. The larger slice weighs 443.94 grams, 116x80x20 mm, the smaller end piece is 205.69 grams, 100x60x32 mm.

"Rock of the Month #212, posted for February 2019" ---

Acasta gneiss, oldest-known rock on Earth:

The Story Develops

Careful applications of the science of geochronology, the determination of the ages of rocks, in the past 30 years affirm that the Acasta gneiss outcrops include rocks as old as 4000-4030 Ma. Bowring et al. (1989) analysed the isotopes of lead and uranium in zircon crystals from the region, using an ion microprobe, a mass spectrometer capable of high spatial resolution, to examine the chemistry of individual crystals, and of growth bands (rather like tree rings) within individual crystals. The rock samples had crystallized as tonalite or granite, igneous rocks that might not hold the imagination of non-specialists for very long. The mineralogy of early Archean rocks is summarized by Papineau (2010). Within these igneous rocks, now metamorphosed, recrystallized and deformed into gneiss, zircon crystals indicated that the original magmas had crystallized some 3962 Ma. Further studies on the uranium-lead, samarium-neodymium and lutetium-hafnium isotope systems added further detail to the story (Bowring and Housh, 1995; Amelin et al., 1999; Sano et al., 1999; Amelin et al., 2000; Patchett and Samson, 2005). Zircons as old 4030 Ma were found (Stern and Bleeker, 1998) and data hint at rocks as old as 4060 Ma (Bowring and Williams, 1999).

Table 1 indicates the span of some of the isotopically-determined ages of some of the oldest rock, crystal and meteorite samples, as well as the rise of "modern" life. Earth has a long history! Source: MINLIB database and references therein. Note that there are many primitive meteorites that "set the clock" for the formation of the solar system, including Sun, planets, asteroids and comets. Dating to the late 1970s, there are some reports of apparent ages that predate the solar system. It is to be expected that such dates may, in principle, be possible, since there are about a dozen species of minerals found as presolar grains in meteorites. These quaintly-termed ur-minerals, such as diamond, graphite, corundum, moissanite (silicon carbide), forsterite and enstatite condensed in the atmospheres of large stars that predated the Sun, and thus ARE older than our solar system, though they tend to be tiny, and not amenable to direct dating. Starting from a simple menu of elements, planetary evolution has led to ever-larger numbers of minerals (Hazen, 2015; Hazen et al., 2015), of which we have to date (2022) approaching 6,000 individual species that are considered valid (clearly and uniquely identified)! At this time, the oldest terrestrial material we can recognize is discrete crystals of minerals such as zircon, that preserve isotope systems storing chronological data.

A recent study of presolar grains in carbonaceous chondrites focused on several dozen (relatively) large SiC (moissanite) grains extracted from the Murchison CM2 meteorite. A "large" grain is 10 microns (0.01 mm) in diameter: an exceptional example might reach 30-60 microns. This brings to mind those 1970s studies of grains in the Allende meteorite, which apparently were not replicated. The new study (Heck et al., 2020) builds on decades of astrophysical research and modeling of interstellar dust. The lifetime of interstellar dust is thought to generally be <300 Ma, following its genesis by condensation upon cooling in the outermost atmosphere of a parent star. Cosmic ray exposure age dating using neon isotopes suggests that most of the SiC grains were formed 0-300 Ma before the formation of the solar system, consistent with the models for dust survival. However, some of the larger grains might be as much as 3100 Ma older than the accretion of the Murchison meteorite, dating their formation around the parent star to as long ago as 7600 Ma. This would be, by far, the oldest material recognized in our global museum collections.

York (1993) wrote a popular account of some of these oldest rocks and the means by which their ages are estimated. More recently, an encyclopaedic review comes from Van Kranendonk et al. (2007), and more concise summaries are provided by Sankaran (2000) and Kamber et al. (2001). The oldest rocks, 4000 Ma and older, are now termed Hadean, an allusion to a distant, stark and uncomfortably hot world! An earlier term for the period without any surviving rocks, after the Earth formed, is the Priscoan. Table 1 picks out 36 dates in our universe, mostly on Earth, that range from the Big Bang to the most distant origin of our own species. Our species has thus far persisted, at best, for a mere 0.015% of Earth's history. To use a well-known metaphor, a 24-hour clock, to represent the time elapsed since our planet formed, then if the story of Earth started at midnight, and stretches through the entire day, Homo sapiens, so-called, appeared in the last 15 seconds of the last minute of that day!

Further, if you can imagine the Earth as being a whole year old, not just one day, then an early evidence of our planet-modifying powers, the Great Wall of China, was built in stages starting at 15 seconds to midnight on 31st December of that year! Back to the "one-day world", and saving a last word for the Acasta gneiss, it would have formed at 2:49 in the morning, a terrestrial early-riser.....

A simplified table of selected dates follows --- note that uncertainty (typically expressed as 2 standard deviations) is not shown here: please refer to original literature for the fine print!

You can download the impressively detailed official "International Chronostratigraphic Chart" for yourself, in various languages. As of 07 December 2022, the latest version was 2022/10. Many of the finer subdivisions have changed since I was in college! And even in 2018, alone!

A wee update: a small rock fragment recovered from the Moon by the Apollo 14 mission, in February 1971, has been identified, tentatively, as a rock blasted off the Earth by an ancient impact event. The detailed research is published in Earth and Planetary Science Letters for March 2019 (Bellucci et al., 2019). The rock's origins are summarized by Nicole Mortillaro on the CBC web site.

The sample in question is a cm-sized clast (fragment) of felsite, a fine-grained igneous rock of granitic composition. It occurs in a lunar breccia, Apollo sample 14321. The mineralogy and chemistry of the rock, and the nature of zircon in the rock, determined by ion microprobe analysis, suggest a possible terrestrial origin (crystallization in a magma on Earth) at circa 4011 Ma, i.e., of similar age to the Acasta gneiss. Subsequent modifications would have occurred due to the impact, and the later adventures of the rock following its arrival on the Moon, blending terrestrial and lunar characteristics. There is ample evidence of many impact events in Earth history, some generating huge structures (e.g., Bleeker and Kamo, 2022) and conceivably ejecting debris with sufficient force to escape Earth's gravitational field.

Acknowledgements

This special rock was kindly provided by the office of the Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, in Thunder Bay *. Special thanks to Greg Paju for his generous and dedicated help with sample acquisition and preparation. Great job! The Acasta gneiss is one of a small number of samples, and one of just two from outside Ontario, in preparation for a museum display. This meets a request from the Museu Joias da Natureza in Santos, the port of the great city of Sao Paulo, Brazil. Next month's "Rock" will be another sample from this project, from the Thunder Bay area.

* It is important also to thank the original supplier of the material, the Northwest Territories Geological Survey.

References (n=22)

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