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.
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