With rocks, it’s all about provenance

This is the cover of the new Special Paper, 'Mineralogical and Geochemical Approaches to Provenance.' -  Cover design by Eric Christensen, GSA.
This is the cover of the new Special Paper, ‘Mineralogical and Geochemical Approaches to Provenance.’ – Cover design by Eric Christensen, GSA.

Major technical advances in the analysis of individual minerals and whole rocks allow greater insight into the source of sediments and sedimentary rocks, thus unroofing the histories of the landscapes from which they came. This new book from The Geological Society of America combines work in North America, Southeast Asia, the East Antarctic Ice Sheet, the western Alps, Chile, and the North Sea into a comprehensive volume covering the latest techniques for determining sedimentary provenance.

Editors E. Troy Rasbury of Stony Brook University, Sidney R. Hemming of the Lamont-Doherty Earth Observatory, and Nancy R. Riggs of Northern Arizona University have organized the book’s eleven chapters into three general categories, while noting that many of the chapters combine these approaches to address provenance questions: (1) isotope and fission-track dating of minerals, and additional insights from geochemistry and radiogenic isotopes; (2) uses of heavy minerals, relative abundance, isotope fingerprinting, and compositions of the minerals; and (3) automated point counting.

One study focuses on Byrd Glacier, which has a drainage basin that covers more than a million square kilometers of East Antarctica, transporting ice and debris to the Ross Sea. This chapter studies till samples collected adjacent to the Lonewolf Nunataks there in order to determine what lies beneath the East Antarctic Ice Sheet.

Another chapter examines the Malay Peninsula, Sumatra, West Java, and western Borneo. The authors note that while only two percent of all global land area is located in Southeast Asia, the area is estimated to yield 20 to 25 percent of the sediment supplied to the world’s oceans. They find that this tropical, tectonically active region, with its deep basins (up to 15 km deep in the Malay Basin) and high sediment yield, is an ideal natural laboratory for interpreting detrital sedimentary processes and provenance.

Even Canadian rocks are different

Andrew Leier examined zircons from Lower Cretaceous sandstone near the Sulphur river in the Grande Cache, Alberta area. The prominent sandstone cliff is the Cretaceous sandstone. -  University of Calgary
Andrew Leier examined zircons from Lower Cretaceous sandstone near the Sulphur river in the Grande Cache, Alberta area. The prominent sandstone cliff is the Cretaceous sandstone. – University of Calgary

Canadians have always seen themselves as separate and distinct from their American neighbors to the south, and now they have geological proof.

New research published in April’s edition of Geology shows that rock formations roughly along the same political boundary as the two North American countries formed as early as 120 million years ago.

Dr. Andrew Leier, of the Department of Geoscience at the University of Calgary, set out to prove what he thought was the obvious: because the mountains are continuous between the U.S. and Canada, the ancient river systems that flowed from these uplands were likely interconnected. In other words, during Cretaceous Period,120 million years ago, rivers should have flowed north and south between the countries, paying no mind to the modern day political border.

“I thought that I could easily show that in my research,” says Leier who published a paper in Geology with co-author Dr. George Gehrels at the University of Arizona and, Leier adds, a lot of help from Cassandra Frosini, an undergraduate in geoscience at the University of Calgary.

But Leier was wrong. “I was surprised to learn the opposite, in fact, was true,” he says.

A tiny piece of sediment found in sandstone called zircon helped the researchers locate where the sediments had originally formed. Knowing its current location, Leier was able to determine just how far the rivers moved it and the direction from which it came.

During the Cretaceous Period, mountains were being created all along western North America, in both Canada and the United States.

“I thought the sediment transported by ancient rivers in Montana and Utah would flow out of the mountain ranges and then north into Alberta. This is similar with how the Ganges River runs parallel to the Himalayas. Our research shows this wasn’t the case,” says Leier.

Leier and Gehrels used recently developed laser-based techniques to reconstruct the origin of individual sand grains that were deposited during this period in western North America. This technique has applications to the petroleum industry as well, where it can be used to aide in determining drilling locations.

Researchers found slightly different rocks, when eroded, produced slightly different zircons.

“Cretaceous sediment in the United States have a clear American signature; whereas those in the Canadian Rockies have a different and definable Canadian signature,” says Leier.

“The demarcation is pretty much coincidental with the modern day border.”

Also the implication of the data suggests that the rivers that flowed west to east from the mountains in the United States stayed in the United States, and those in Canada stayed in Canada.

“In other words, there is no evidence that rivers in western North America were crossing what is today the border,” says Leier.

Viscous cycle: Quartz is key to plate tectonics

Quartz may play a major role in the movements of continents, known as plate tectonics. -  USGS
Quartz may play a major role in the movements of continents, known as plate tectonics. – USGS

More than 40 years ago, pioneering tectonic geophysicist J. Tuzo Wilson published a paper in the journal Nature describing how ocean basins opened and closed along North America’s eastern seaboard.

His observations, dubbed “The Wilson Tectonic Cycle,” suggested the process occurred many times during Earth’s long history, most recently causing the giant supercontinent Pangaea to split into today’s seven continents.

Wilson’s ideas were central to the so-called Plate Tectonic Revolution, the foundation of contemporary theories for processes underlying mountain-building and earthquakes.

Since his 1967 paper, additional studies have confirmed that large-scale deformation of continents repeatedly occurs in some regions but not others, though the reasons why remain poorly understood.

Now, new findings by Utah State University geophysicist Tony Lowry and colleague Marta Pérez-Gussinyé of Royal Holloway, University of London, shed surprising light on these restless rock cycles.

“It all begins with quartz,” says Lowry, who published results of the team’s recent study in the March 17 issue of Nature.

The scientists describe a new approach to measuring properties of the deep crust.

It reveals quartz’s key role in initiating the churning chain of events that cause Earth’s surface to crack, wrinkle, fold and stretch into mountains, plains and valleys.

“If you’ve ever traveled westward from the Midwest’s Great Plains toward the Rocky Mountains, you may have wondered why the flat plains suddenly rise into steep peaks at a particular spot,” Lowry says.

“It turns out that the crust beneath the plains has almost no quartz in it, whereas the Rockies are very quartz-rich.”

He thinks that those belts of quartz could be the catalyst that sets the mountain-building rock cycle in motion.

“Earthquakes, mountain-building and other expressions of continental tectonics depend on how rocks flow in response to stress,” says Lowry.

“We know that tectonics is a response to the effects of gravity, but we know less about rock flow properties and how they change from one location to another.”

Wilson’s theories provide an important clue, Lowry says, as scientists have long observed that mountain belts and rift zones have formed again and again at the same locations over long periods of time.

But why?

“Over the last few decades, we’ve learned that high temperatures, water and abundant quartz are all critical factors in making rocks flow more easily,” Lowry says. “Until now, we haven’t had the tools to measure these factors and answer long-standing questions.”

Since 2002, the National Science Foundation (NSF)-funded Earthscope Transportable Array of seismic stations across the western United States has provided remote sensing data about the continent’s rock properties.

“We’ve combined Earthscope data with other geophysical measurements of gravity and surface heat flow in an entirely new way, one that allows us to separate the effects of temperature, water and quartz in the crust,” Lowry says.

Earthscope measurements enabled the team to estimate the thickness, along with the seismic velocity ratio, of continental crust in the American West.

“This intriguing study provides new insights into the processes driving large-scale continental deformation and dynamics,” says Greg Anderson, NSF program director for EarthScope. “These are key to understanding the assembly and evolution of continents.”

Seismic velocity describes how quickly sound waves and shear waves travel through rock, offering clues to its temperature and composition.

“Seismic velocities are sensitive to both temperature and rock type,” Lowry says.

“But if the velocities are combined as a ratio, the temperature dependence drops out. We found that the velocity ratio was especially sensitive to quartz abundance.”

Even after separating out the effects of temperature, the scientists found that a low seismic velocity ratio, indicating weak, quartz-rich crust, systematically occurred in the same place as high lower-crustal temperatures modeled independently from surface heat flow.

“That was a surprise,” he says. “We think this indicates a feedback cycle, where quartz starts the ball rolling.”

If temperature and water are the same, Lowry says, rock flow will focus where the quartz is located because that’s the only weak link.

Once the flow starts, the movement of rock carries heat with it and that efficient movement of heat raises temperature, resulting in weakening of crust.

“Rock, when it warms up, is forced to release water that’s otherwise chemically bound in crystals,” he says.

Water further weakens the crust, which increasingly focuses the deformation in a specific area.

Banded rocks reveal early Earth conditions, changes

Pictured in 2008, a banded iron formation about 2.5 billion years old near Soudan Underground Mine State Park in Minnesota shows alternating layers of silica-rich (red) and iron-rich (gray) minerals. This type of ancient rock formation dominated the global ocean floors for more than two billion years, but abruptly disappeared 1.7 billion years ago. A study by researchers at UW-Madison and elsewhere describes a new model of how these ancient rocks formed and what they reveal about the geology, oceans and atmosphere of the Earth’s early environment. - Photo: Huifang Xu
Pictured in 2008, a banded iron formation about 2.5 billion years old near Soudan Underground Mine State Park in Minnesota shows alternating layers of silica-rich (red) and iron-rich (gray) minerals. This type of ancient rock formation dominated the global ocean floors for more than two billion years, but abruptly disappeared 1.7 billion years ago. A study by researchers at UW-Madison and elsewhere describes a new model of how these ancient rocks formed and what they reveal about the geology, oceans and atmosphere of the Earth’s early environment. – Photo: Huifang Xu

The strikingly banded rocks scattered across the upper Midwest and elsewhere throughout the world are actually ambassadors from the past, offering clues to the environment of the early Earth more than 2 billion years ago.

Called banded iron formations or BIFs, these ancient rocks formed between 3.8 and 1.7 billion years ago at what was then the bottom of the ocean. The stripes represent alternating layers of silica-rich chert and iron-rich minerals like hematite and magnetite.

First mined as a major iron source for modern industrialization, BIFs are also a rich source of information about the geochemical conditions that existed on Earth when the rocks were made. However, interpreting their clues requires understanding how the bands formed, a topic that has been controversial for decades, says Huifang Xu, a geology professor at the University of Wisconsin-Madison.

A study appearing today (Oct. 11) as an advance online publication in Nature Geoscience offers a new picture of how these colorful bands developed and what they reveal about the composition of the early ocean floor, seawater, and atmosphere during the evolution of the Earth.

Previous hypotheses about band formation involved seasonal fluctuations, temperature shifts, or periodic blooms of microorganisms, all of which left many open questions about how BIFs dominated the global marine landscape for two billion years and why they abruptly disappeared 1.7 billion years ago.

With Yifeng Wang of Sandia National Laboratories, Enrique Merino of Indiana University and UW-Madison postdoc Hiromi Konishi, Xu developed a BIF formation model that offers a more complete picture of the environment at the time, including interactions between rocks, water, and air.

“They are all connected,” Xu explains. “The lithosphere affects the hydrosphere, the hydrosphere affects the atmosphere, and all those eventually affect the biosphere on the early Earth.”

Their model shows how BIFs could have formed when hydrothermal fluids, from interactions between seawater and hot oceanic crust from deep in the Earth’s mantle, mixed with surface seawater. This mixing triggered the oscillating production of iron- and silica-rich minerals, which were deposited in layers on the seafloor.

They used a series of thermodynamic calculations to determine that the source material for BIFs must have come from oceanic rocks with a very low aluminum content, unlike modern oceanic basalts that contain high levels of aluminum.

“The modern-day ocean floor is basalt, common black basalt like the Hawaiian islands. But during that time, there was also a strange kind of rock called komatiites,” says Xu. “When ocean water reacts with that kind of rock, it can produce about equal amounts of iron and silica” – a composition ideally suited to making BIFs.

Such a mixture can create distinct alternating layers – which range in thickness from 10 micrometers to about 1 centimeter – due to a constantly shifting state that, like a competition between two well-matched players, resists resolving to a single outcome and instead see-saws between two extremes.

BIFs dominated the global oceans 3.8 to 1.7 billion years ago, a time period known to geologists as the Archaean-Early Proterozoic, then abruptly disappeared from the geologic record. Their absence in more recent rocks indicates that the geochemical conditions changed around 1.7 billion years ago, Xu says.

This change likely had wide-ranging effects on the physical and biological composition of the Earth. For example, the end of BIF deposition would have starved iron-dependent bacteria and shifted in favor of microbes with sulfur-based metabolisms. In addition, chemical and pH changes in the ocean and rising atmospheric oxygen may have allowed the emergence and spread of oxygen-dependent organisms.

The new study was partly funded by the NASA Astrobiology Institute, and Xu hopes to look for biosignatures trapped in the rock bands for additional clues to the changes that occurred 1.7 billion years ago and what may have triggered them.

Researchers find oldest rocks on Earth

Discovery of rocks as old as 4.28 billion years pushes back age of most ancient remnant of Earth’s crust by 300 million years

McGill University researchers have discovered the oldest rocks on Earth – a discovery which sheds more light on our planet’s mysterious beginnings. These rocks, known as “faux-amphibolites”, may be remnants of a portion of Earth’s primordial crust – the first crust that formed at the surface of our planet. The ancient rocks were found in Northern Quebec, along the Hudson’s Bay coast, 40 km south of Inukjuak in an area known as the Nuvvuagittuq greenstone belt. Their results will be published in the September 26 issue of the journal Science.

The discovery was made by Jonathan O’Neil, a Ph.D. candidate at McGill’s Department of Earth and Planetary Sciences, Richard W. Carlson, a researcher at the Carnegie Institution for Science in Washington, D.C., Don Francis, a McGill professor in the Department of Earth and Planetary Sciences, and Ross K. Stevenson, a professor at the Université du Québec à Montréal (UQAM).

O’Neil and colleagues estimated the age of the rocks using isotopic dating, which analyzes the decay of the radioactive element neodymium-142 contained within them. This technique can only be used to date rocks roughly 4.1 billion years old or older; this is the first time it has ever been used to date terrestrial rocks, because nothing this old has ever been discovered before.

The data from these findings will give researchers a new window on the early separation of Earth’s mantle from the crust in the Hadean Era, said O’Neil.

“Our discovery not only opens the door to further unlock the secrets of the Earth’s beginnings,” he continued. “Geologists now have a new playground to explore how and when life began, what the atmosphere may have looked like, and when the first continent formed.”

‘Time Machine’ Lab Could Propel New Research

Like something from the classic H.G. Wells’ novel “The Time Machine,” a high-precision thermal ionization mass spectrometer will help Texas A&M University geoscientists Franco Marcantonio, Brent Miller and Debbie Thomas explore the ancient worlds of the deep geological past and allow these researchers to determine the ages of rocks that are millions to billions of years old, providing insight into past climates and oceanic circulation patterns.

The thermal ionization mass spectrometer is a key component in the College of Geosciences ‘ new R. Ken Williams ’45 Radiogenic Isotope Geosciences Laboratory, which is set to open in June. The lab will be used for interdisciplinary research and teaching in marine geology and global tectonics.

Philosophers, physicists and astronomers often compare the flow of time to water flowing down a stream – an irretrievable past, a fleeting present and an unknowable future.

“But the past is not entirely erased from existence; it is in some cryptic ways preserved on the Earth in the form of rocks,” said Miller, assistant professor of geology and geophysics. “All we need are tools to determine the amount of time represented by the rocks and a way to decode the subtle chemical stories contained within the rock record.”

High-tech analytical instruments like thermal ionization mass spectrometers play a key role in revealing those stories. This instrument can detect minute differences in the sub-atomic makeup of many different elements.

“The basic principle behind the mass spectrometer is quite simple and elegant,” noted Marcantonio, an associate professor in geology and geophysics. “Atoms of the same element have the same number of protons in their nucleus, but the number of neutrons determines the atomic weight of the atom, and that number can vary. This type of mass spectrometer accelerates atoms along a curved path and through an intense magnetic field. The lighter atoms, or those with relatively fewer neutrons in the nucleus, are deflected more by the magnetic field than the heavier atoms, essentially sorting out the atoms by atomic weight.

“As the sorted atoms exit the magnetic field, they are focused into an array of very sensitive detectors that produce an electrical current in precise proportion to the number of atoms hitting the detector,” he added.

Thomas, an assistant professor of oceanography, will use the thermal ionization mass spectrometer to measure minute amounts of the rare element Neodymium in fossil fish teeth, bones, scales and shells of ancient marine microorganisms. According to Thomas, “The chemical signatures of ancient ocean waters are inherited by creatures that lived in those oceans,”

Each Neodymium sample is so small that, in the form of a solid metal particle, 6,000 samples could fit on the head of a pin. Moreover, the mass spectrometer is so sensitive that each sample is plenty for a precise analysis.

Thomas and her students use these measurements to discern ocean circulation patterns from the past when those patterns were driven by differences in climate and in the plate tectonic configuration of the ocean basins. “By understanding past ocean circulation, we can better understand what factors affect or are affected by the oceans, and in doing so gain insight into what may lie ahead as the global climate evolves,” said Thomas.

Marcantonio and his students are also examining circulation patterns, including changes in past atmospheric circulation patterns. The distribution of radioactive elements and their stable byproducts in sediments of the deep oceans and shallow coastal regions form a large part of Marcantonio’s research. By studying changes in the composition of wind-blown dust in ancient sediments, Marcantonio and his students are able to use the data provided by thermal ionization mass spectrometry to trace changing patterns of atmospheric circulation through time. According to Marcantonio, “Marine sedimentary deposits play an important role in shedding light on past climate change and its effects on past oceanographic and atmospheric processes.”

Miller focuses his research on using the mineral zircon to determine the ages of rocks. Zircons are nature’s tiny time capsules, acting as a type of clock, faithfully ticking away from the time they are formed until their ratios of radioactive uranium to radiogenic lead are finally measured in a mass spectrometer. Zircons are found in rocks like granite, formed by the crystallization of magma bodies deep in the Earth’s crust, and in volcanic ash deposits, erupted from volcanoes like Mt. St. Helens. The former rock type can be used to test ideas about the formation of Earth’s great mountain ranges like the Himalayas or the Rockies . The latter is often found interlayered with fossil-bearing sedimentary rocks and can constrain the ages of exotic and extinct life forms.

“Geology is a historical science,” said Miller. “It is crucial that we be able to not only put geological events in their correct relative order, but that we can also determine the absolute time spans and rates of those geological events.” According to Miller, the new mass spectrometer will allow him to do that.

In contrast to the main character in Wells’ novel who traveled into time to explore past worlds, Marcantonio, Miller and Thomas travel the world to explore past times as preserved in rocks and sediments. Those collected samples may reveal different stories but they share the same destiny – chemical separation and purification in the R. Ken Williams ’45 Radiogenic Isotope Geosciences Laboratory followed by precise analysis in the thermal ionization mass spectrometer. They will likely propel these researchers and their students a step closer to answering the primal question: How did the Earth work in the deep geological past?

Rock Fracture Dynamics Lab Opens

Earthquakes, volcanoes, water, mining and radioactive waste can all impact rock strength and stability. Now, a cutting-edge facility at the University of Toronto will help researchers accurately understand and predict how rocks will react to these different types of stress. The new Rock Fracture Dynamics Laboratory is the only laboratory in the world where rock samples can be tested under true earth-like stress and temperature conditions while imaging deformation.

“The facility enables us to perform geophysical imaging on samples of rocks so we can now visualize what’s going on inside the rock as it is happening,” says Professor Paul Young, Keck Chair of Seismology and Rock Mechanics and vice president (research) at the University of Toronto. “It will also boost partnerships and be a strong catalyst for collaboration with the top international researchers in the fields of rock mechanics and geophysics.”

A key part of the facility is the advanced computer system called the high performance computing cluster consisting of 64 quad-core 64-bit processors and 4-8GB RAM per processor. In near-real time, the computing cluster processes and displays results from 400 megabytes of data being collected from geophysical acquisition computers. As well as experimental data processing for imaging, it will allow much larger and higher resolution models to be produced then ever before.

The laboratory was made possible through $5 million funding from the Canadian Foundation for Innovation (CFI), Ontario Innovation Trust, Ontario Ministry of Research and innovation and the U.S. Keck Foundation as well as industry contributions including MTS Systems Corporation, Dell Canada Inc, and Microsoft.