Some granites are cooler than others
Low crystallisation temperatures of Yosemite granites challenge models of how they form. The finding could have widespread implications that could span Earth’s lifetime. Vhairi Mackintosh reports.
In a paper published in the journal Nature, a team of US-based scientists have shown that 90-million-year-old granites from Yosemite National Park crystallised at much lower temperatures than previously believed possible.
Granite is an intrusive igneous rock — one that solidifies from magma deep within the Earth — and was thought to form when the magma cooled to between 650 and 700 degrees Celsius.
"These [Yosemite] granites tell a different story," says geophysicist lead-author Michael Ackerson of the Carnegie Institution of Washington.
The researchers used a specifically developed thermobarometer, calibrated in co-author Elbert Watson’s lab at the Rensselaer Polytechnic Institute in New York, to deduce the crystallisation temperature of the quartz crystals within the granites.
The thermobarometer determined the pressure and temperature conditions under which they formed using the concentration of titanium, which is a common trace element substitute for silicon in quartz. Essentially, more titanium reflects higher temperature and lower pressure formation conditions, and vice versa.
From the readings, they found that the Yosemite granites crystallised at temperatures between 474 and 561 degrees Celsius, around 200 degrees lower than the commonly accepted range.
They then compared the observed titanium concentration profiles in the quartz crystals, mapped by electron microprobe, with those predicted from computer diffusion models. This method generated a similarly low crystallisation temperature of 500 degrees Celsius.
As temperature can be taken as a proxy for depth below the surface of the Earth, the findings have implications for geothermal gradients – that is, how temperature increases with depth in the Earth’s interior.
“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," explains Watson.
For instance, many economically important ore deposits, such as gold and copper, form in the crust above, due to fluids released from granitic magma chambers. This shift to lower granitic crystallisation temperatures, thus, may warrant a comparable shift in the temperature regime and crustal depths likely to host formation.
Ackerson says that the surprising results could also “rewrite what we think we understand about how Earth's continents form”.
Earth’s continents are mostly made of granite and, due to their composition, are too buoyant to be subducted and recycled back into the mantle at destructive tectonic plate boundaries.
Because of this, “minerals from granites record almost all of our planet's history — from 4.4 billion years ago to today,” explains Ackerson.
He adds, “geologists have relied on a crystallisation temperature for granite that was established more than half a century ago.”
It appears it’s time for an update.