Finding challenges long-held assumptions about common material in continental crusts
Evidence from rocks in Yosemite National Park suggests that granite stored in the Earth's crust is partially molten at 500 degrees Celsius, nearly 200 degrees lower than had previously been believed. The finding, published online today in Nature, challenges long-held assumptions that underlie our views about the state of magma in volcanically active regions, the location of economically important ore deposits, and Earth's geothermal gradient.
"In making predictions, geologists have relied on a crystallization temperature for granite that was established more than half a century ago, using the best tools available at the time," said Michael Ackerson, lead author of the paper. Ackerson's adviser and corresponding author E. Bruce Watson, Institute Professor at Rensselaer Polytechnic Institute, added, "with advances in science and technology, our tools have improved. This finding will affect our understanding of where we find molten rock at depth in the Earth - knowledge that impacts several sub-fields of geology."
The finding draws on research establishing the effects of temperature and pressure on the titanium content of quartz, and builds on previous work which, in 2005, used the relationship between the titanium content of zircon and the temperature at which the zircon crystallized to reveal that early Earth contained liquid water near its surface only 200 million years after the solar system formed.
Beneath the surface of the Earth, temperature and pressure increase with depth. Changes in temperature and pressure with depth in the Earth impart unique chemical signatures in minerals that can subsequently be used to unravel the conditions in which the minerals formed. Quartz is primarily four atoms of oxygen arranged around one atom of silicon, but under certain temperatures and pressures, titanium atoms can replace silicon atoms in the quartz structure.
Through extensive experimentation and analysis, the Watson lab calibrated a "thermobarometer" that relates the concentration of titanium in a quartz crystal to the temperature and pressure under which it formed. In general, higher temperature and lower pressure allow more titanium to infiltrate the crystal, whereas lower temperature and higher pressure impede the incorporation of titanium into the crystal.
When applied to the titanium content of quartz crystals from the Tuolumne Intrusive Suite - a series of granites in Yosemite National Park that constitute a portion of the Sierra Nevada Mountains - the thermobarometer indicates a crystallization temperature of 474 to 561 degrees Celsius, well below the prevailing accepted crystallization temperature of 650-700 degrees Celsius.
These findings are supported by a comparison between quartz crystals from the Tuolumne Intrusive Suite and computer models predicting how titanium concentrations in a growing crystal will change as a function of initial crystallization temperature and cooling rate. Ackerson mapped titanium concentrations in cross-sections of quartz crystals using an electron microprobe.
The maps show variations in titanium concentrations as the crystal grew from a central nucleation point, much as tree rings in the cross section of a tree trunk show the growth of a tree across time. Steep gradients in the cross-sections mark areas where titanium concentrations change rapidly. Ackerson extracted titanium concentration profiles from those gradients and compared them to computer diffusion models of titanium in quartz under varying initial crystallization temperatures and cooling rates. Diffusion models with an initial crystallization temperature of 500 degrees matched the gradients in Tuolumne suite quartz, thereby confirming the cool crystallization temperatures of quartz.
"Both tests, the 'thermometer' and the diffusion model, use titanium and quartz, but with two completely independent mechanisms to produce observations that show you these quartz crystals are crystallizing from melt at 500 degrees," said Ackerson. "Once you eliminate all the other possibilities, you're left with cold crystallization. And that is surprising."
The result is sufficiently unexpected that Ackerson and Watson said they are just beginning to consider applications. One will certainly be a new perspective on the geothermal gradient, which describes how temperature changes with depth, and therefore where molten materials will be found in the Earth's crust. Because many economically important ores, like porphyry copper and gold, are underlain by granites, the finding will likely impact the temperature regime under which they are predicted to form. And it may influence how geophysicists interpret data at active magmatic centers like Yellowstone, proving that current interpretations are recording temperatures lower than had been thought possible.
"Low-temperature crystallization of granites and the implications for crustal magmatism" appears in Nature. Watson and Ackerson were joined by co-authors B.O. Mysen of the Carnegie Institution of Washington, and N.D. Tailby of the American Museum of Natural History. Their research was partially funded by the Carnegie Institution of Washington's Postdoctoral Fellowship Program.
Geochemical research fulfills The New Polytechnic, an emerging paradigm for higher education which recognizes that global challenges and opportunities are so great they cannot be adequately addressed by even the most talented person working alone. Rensselaer serves as a crossroads for collaboration -- working with partners across disciplines, sectors, and geographic regions -- to address complex global challenges, using the most advanced tools and technologies, many of which are developed at Rensselaer. Research at Rensselaer addresses some of the world's most pressing technological challenges -- from energy security and sustainable development to biotechnology and human health. The New Polytechnic is transformative in the global impact of research, in its innovative pedagogy, and in the lives of students at Rensselaer.
About Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute, founded in 1824, is America's first technological research university. For nearly 200 years, Rensselaer has been defining the scientific and technological advances of our world. Rensselaer faculty and alumni represent 86 members of the National Academy of Engineering, 18 members of the National Academy of Sciences, 25 members of the American Academy of Arts and Sciences, 8 members of the National Academy of Medicine, 8 members of the National Academy of Inventors, and 5 members of the National Inventors Hall of Fame, as well as 6 National Medal of Technology winners, 5 National Medal of Science winners, and a Nobel Prize winner in Physics. With 7,000 students and nearly 100,000 living alumni, Rensselaer is addressing the global challenges facing the 21st century--to change lives, to advance society, and to change the world. To learn more, go to http://www.
Mary Martialay | EurekAlert!
Scientists discover Earth's youngest banded iron formation in western China
12.07.2018 | University of Alberta
Drones survey African wildlife
11.07.2018 | Schweizerischer Nationalfonds SNF
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
13.07.2018 | Event News
13.07.2018 | Materials Sciences
13.07.2018 | Life Sciences