17.3: Earth's Oldest Rocks

The Jack Hills Zircon

What followed the formation of thin, perturbed, komatiite crust is highly debated and an area of fervent geological research. Insight into the evolution of the earliest crust was unknown until the discovery of Hadean age zircons from the Jack Hills area in southwest Australia (see map). Zircon is a small, durable mineral with the chemical formula \(\ce{ZrSiO4}\). It is a common mineral in felsic rock with the composition similar to that of the continental crust, like granite. It is not a common mineral of the rock of the ocean crust, like basalt and does not occur in the earliest rock of the crust, komatiite.

Figure \(\PageIndex{1}\): Archean Cratons (light grey) and locations of Earth’s oldest rocks on a world map. By Jonathan O’Neil. (With permission from: UOttawa Early Earth Blog)

Figure \(\PageIndex{2}\): Quartz-pebble metaconglomerate (Jack Hills Quartzite, Archean, 2.65 to 3.05 Ga; Jack Hills, Western Australia) (Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA) (CC BY 2.0; James St. John on Flickr)

In the mid-1980s field geologists sampled a metamorphosed sedimentary rock (metaconglomerate) from the Jack Hills. The metaconglomerate is dated as Archean, approximately 3.6 Ga. The detrital sedimentary particles found within the metaconglomerate are, of course, older than their deposition as gravel. The environment of deposition of the original conglomerate is thought to be an alluvial fan, where sediment derived from the weathering and erosion of mountains were moved by water and deposited in an adjacent valley. Small zircon crystals were extracted from the metaconglomerate for analysis. The zircons were originally part of the rock that composed these earlier mountains. Remnants of these mountains have not been discovered and have most likely been erased from Earth’s surface by erosion.

Figure \(\PageIndex{3}\): This image shows an array of Jack Hill zircons arranged on a piece of tape. Largest zircons ~ 1mm. (CC BY-NC-SA 3.0; Science Education Resource Center)

Zircon is a small but mighty mineral. It is one of Earth’s little timekeepers. Zircon typically forms during the crystallization of magma where radioactive uranium can substitute for zirconium in the mineral lattice. Following crystallization, the radiometric clock starts ticking. The unstable radioactive uranium atoms break down through a process known as “decay.” The atoms lose subatomic particles and emit energy. Particle loss includes a decrease in the number of protons which ultimately changes the uranium to lead. The rate of this decay is well known and allows scientists to very accurately date the zircon. Radiometric dating analysis of the Jack Hills detrital zircon grains yield dates as old as 4.404 Ga! This is the oldest Earth material discovered to date, formed merely ~150 Ma after the inception of Earth. That is amazing!!! This tells us that in those 150 million years, the entire Earth was largely formed, its interior differentiated, it cooled enough to have a solid crust of komatiite, and, remelting produced evolved magma where the zircon crystallized in solid rock similar to the composition of our continental crust today. WOW!!! (The method of dating the zircon is discussed more fully in the chapter on Geologic Time).

This discovery has significantly advanced our understanding of the Hadean environment and the evolution of the crust during the Hadean. The discovery of the zircon means that within the first several hundred million years of Earth’s existence, crust of varying composition existed, some of which was more felsic in composition, very much like the continental crust that exists today. This provides significant insight into the processes at work in the Hadean as this type of rock is continuously being formed and reformed through plate tectonic processes today.

Analysis of oxygen isotopes data from the zircon have revealed even more incredible evidence about the environment that existed during the very early Hadean. Oxygen has several isotopes, \(\ce{^{16}O}\) is most abundant with 8 protons and 8 neutrons existing in the nucleus. \(\ce{^{17}O}\) and \(\ce{^{18}O}\) also exist in much less abundance. \(\ce{^{18}O}\) is concentrated in an aqueous environment, such as oceans, as the lighter \(\ce{^{17}O}\) and \(\ce{^{16}O}\) will preferentially evaporate. Analysis of the relative amounts of different isotopes of oxygen \(\ce{^{16}O}\) and \(\ce{^{18}O}\) (ratio denoted with the lowercase Greek delta \(\delta \ce{^{18}O}\)) in the Jack Hills zircon are skewed toward “heavy” \(\ce{^{18}O}\), as opposed to the more common “light” \(\ce{^{16}O}\). This heavy oxygen signature in rock is an indication that it formed by cool, wet, sedimentary processes at the Earth’s surface. Thus, the magma that eventually gave rise to the zircons is theorized to have been formed from what had once been sediments deposited on the floor of an ancient ocean [3]. So not only was the very young Earth capable of making felsic-composition crust, it also was cool enough to have liquid water in oceans. These are surprisingly familiar conclusions about a “hellish” young planet.

Figure \(\PageIndex{4}\): Hadean age zircon discovery locations on a world map. (CC BY 4.0 International; By karaclc on Wikimedia.)

Since the discovery of the Hadean age zircon in the Jack Hills area of Australia, other detrital zircons have also been discovered in Archean age rock in other parts of the world. See the map at right.

Did I Get It? - Jack Hills Zircon Quiz

Exercise \(\PageIndex{1}\)

The mineral zircon is commonly found in which type of igneous rock?

a. komatiite

b. ultramafic

c. basalt

d. granite

Answer

d. granite

Exercise \(\PageIndex{2}\)

How old are the zircon from the Jack Hills of southwest Australia?

a. 4.040 Ga

b. 4.040 Ma

c. 4.404 Ma

d. 4.404 Ga

Answer

d. 4.404 Ga

Exercise \(\PageIndex{3}\)

Which of the following statements is NOT true?

a. The Jack Hills zircon were dated through a process that uses the decay of radioactive elements.

b. The Jack Hills zircon crystals were extracted from sedimentary rock, not from the original rock in which they formed.

c. The Jack Hills zircon crystals originally crystallized in the Earth's first komatiite crust.

d. Oxygen isotope analysis of the Jack Hills zircon crystals indicate that water was present on Earth's surface at their time of formation.

Answer

c. The Jack Hills zircon crystals originally crystallized in the Earth's first komatiite crust.

The Acasta Gneiss Complex

While the Jack Hills detrital zircons tell us that rocks were definitely around at 4.404 Ga, actual rocks that old have not yet been found. In fact, some controversy exists in the scientific community as to the oldest rocks discovered to date. The Acasta Gneiss Complex located in the Slave Craton of Northwest Canada (see map of Archean Cratons, above) preserve the oldest unquestioned isotopically dated rock thus found anywhere on Earth. This complex includes a variety of highly deformed and metamorphosed tonalite-trondhjemite-granodiorite (TTG) rock. This type of rock is discussed in more detail in the Case Study on Greenstone Belts, Primordial Tectonics. TTGs are similar to granite with some chemical and mineralogical differences. They are typical igneous rocks produced as intrusive bodies along tectonically active continental margins today.

Figure \(\PageIndex{5}\): A fragment of the Acasta Gneiss, the oldest known rock on our planet. In exhibition at the Natural History Museum in Vienna. (CC BY-SA 3.0; By Pedroalexandrade on Wikimedia.)

The Acasta Gneiss has been dated using U/Pb isotopic dating techniques on zircon formed during the crystallization of these rocks in their original igneous environment. The presence of zircon in an igneous rock indicates that the magma has “evolved” – it was formed through the remelting of a pre-existing rock. The isotopic dates yield a record of several different intrusive magmatic events ranging in age between 2.94 Ga and 4.02 Ga. The oldest episode of igneous activity recorded in the gneiss complex occurred between 3.92 and 4.02 Ga which straddles the arbitrary Hadean/Eoarchean division on the geologic time scale (above).[8]

Figure \(\PageIndex{6}\): Acasta gneiss in exposure. Notice the tight folds from tectonic deformation. (CC BY NC ND 4.0 International; By: The Traveling Geologist)

Interpretation of the Acasta Gneiss zircon ages provides verification that continental crust existed in the Hadean. Extensive geochemical analysis of many different isotopic pairs and element/isotope concentration comparisons have led to the interpretation that the oldest rock of the Acasta Gneiss Complex was derived from the partial melting of mafic Hadean crust that was 4.3 billion years old [8] [9].

The Nuvvuagittuq Greenstone Belt

Figure \(\PageIndex{7}\): Modern pillow lava forming underwater.

Dating of the rock from the Nuvvuagittuq Greenstone Belt (NGB) has been controversial but, if accepted, may actually provide a glimpse of Earth’s earliest crust. (Read more about Greenstone Belts here). The NGB is located in northern Quebec, on the eastern shore of the Hudson Bay (See Archean Craton map above). Metamorphosed mafic and ultramafic volcanic rock dominate the NGB. The fact that these are volcanic rocks erupted in an ancient ocean is documented by the presence of lava pillows. Lava that erupts underwater forms conspicuous pillow shapes as a crust solidifies instantly around the oozing lava as it spills out underwater.

Another rock type included in the NGB is banded iron formation. BIFs (as they are affectionately known) are sedimentary rocks that also indicate an ocean environment as these sedimentary iron minerals form and settle out of the water column. (Read more about the formation of BIFs here). Intrusive mafic and ultramafic dikes also occur within the NGB.

Figure \(\PageIndex{8}\): Exposure of the metamorphosed rocks of the Nuvvuagittuq Greenstone Belt from the Porpoise Cove locality, Quebec, Canada. Some of these rocks have Sm/Nd ages in excess of 4.0 Ga and may be the oldest rocks on Earth. (Public Domain; NASA.)

The NGB is bounded by a felsic intrusive rock known as a tonalite (see map below). Both the NGB and the tonalite are cut across by a similar type of felsic rock called a pegmatite. The tonalite and pegmatite intrusive bodies contain zircon needed for U/Pb radiometric dating which has provided an accurate date of 3.77 Ga. Due to the cross-cutting relationship of the intrusive rock to the NGB, this provides only a minimum age for the NGB of 3.77 Ga [10] [11]. Further investigation is necessary to understand the maximum age.

The greenstone rocks of the NGB are remnants of ancient ocean crust with a mafic to ultramafic composition (see composition chart above). Rocks of ultramafic composition do not contain zircon and mafic rocks rarely do. Zircon is a mineral that is typically found in igneous rock more felsic in composition, like granite. Uranium-lead (U/Pb) isotopic dating is the “gold standard” for accuracy, particularly in very old rock. A lack of zircon in the NGB will call into question any other method used to provide dates.

Figure \(\PageIndex{9}\): Exposure of the rock in question as the oldest rock found to date from the Nuvvuagittuq Greenstone Belt. It is described as a “faux” amphibolite – a basalt rock that has been subjected to medium grade metamorphism. (With permission by: Jonathan O’Neil from: Guest Post: Oldest Rocks Could Weigh A Man Down)

In 2008, Jonathan O’Neil, a young PhD student from McGill University in Quebec, Canada, challenged the assumed date for NGB by investigating an odd looking amphibolite patch in the NGB (picture). An amphibolite is a metamorphosed mafic rock therefore, zircon is not present. O’Neil used a dating technique and isotopic ratio comparisons of the rare but ubiquitous elements of samarium (Sm) and neodymium (Nd). Using this technique, O’Neil determined the actual age of the NGB to be 4.28 Ga [12]. Further analysis of additional samples since then has pushed the date back even further to 4.31 Ga [11]. If true, the NGB rock would represent the oldest preserved Hadean age rock found on the planet.

Not all geologists investigating these rocks concur with O’Neil’s findings. Their interpretation of the isotopic data and comparisons propose that the isotopic signatures represent only that the NGB was derived from pre-existing rock of Hadean age and that the NGB age of formation is Eoarchean (3.7 Ga). This controversy is sure to continue until zircon is found within the NGB. Are you intrigued by the search for the Earth’s oldest rocks? Consider pursuing graduate study in geology. Much of the NGB extent has yet to be sampled.

Figure \(\PageIndex{10}\): Geologic map of the Nuvvuagittuq Greenstone Belt (with permission by Jonathan O’Neil.)