Saturday, September 18th, 2021

Diamond microscope reveals slow crawl of Earth’s ancient crust

A quantum diamond microscope contains a doped diamond slide that is excited by tiny magnetic fields.


On a table inside the magnetically shielded lab of Roger Fu, a planetary scientist at Harvard University, sits a modest contraption that is telling profound stories about the geologic past. The quantum diamond microscope (QDM) consists of a few electromagnetic coils wrapped around a camera, a small laser, and what looks like a rose-tinted sample slide. The slide is not glass, however; it is a diamond, doped with defects sensitive to tiny magnetic fields. Using this diamond sensor, the microscope can map the fields imprinted in rock grains at scales smaller than the width of a human hair, allowing geologists to tease out history that coarser techniques overlook.

Fu’s lab is using the microscope to probe meteorites for clues about the Solar System’s earliest days; chronicle rainfall thousands of years ago from stalactites; and, as detailed this week in Science Advances, detect some of the earliest motions of Earth’s tectonic plates in ancient lavas. “It’s a completely new principle in earth science,” Fu says. And it’s catching on. The National Science Foundation has paid for Fu’s team to build microscopes at the University of California, Berkeley, and the University of Minnesota, while NASA funded a third at the Massachusetts Institute of Technology (MIT). He’s building another for a Dutch lab. “Roger has been the great Johnny Appleseed of QDMs,” says Ronald Walsworth, a physicist at the University of Maryland, College Park, who pioneered the diamond technology some 15 years ago.

The microscope relies on minuscule defects in diamond created when a nitrogen atom knocks two carbons out of the crystal lattice; the impurities exist in nature, but can also be manufactured. The result is a void next to the nitrogen that traps electrons whose quantum states are sensitive: Laser light, microwaves, and magnetic fields can all manipulate their energy levels and spin states. The diamond defects are a promising technology for hosting qubits, the logical elements in quantum computers. But for experiments in basic physics, their hair-trigger magnetic sensitivity could be annoying, Walsworth says. “Then we started thinking, ‘If it’s so sensitive, maybe that’s a feature, not a bug?’” Walsworth eventually connected with Fu, who was eager to exploit the feature.

An analysis of zircon crystals more than 4 billion years old from the Jack Hills in Western Australia provided an early test. Traditional paleomagnetic measurements using superconducting sensors had found faint fields preserved in the zircons, a clue that Earth had a magnetic field half a billion years earlier than expected. But the superconducting sensors could measure only the average field across the zircons, which are as small as motes of dust. To scrutinize the Jack Hill specimens at a finer scale, Fu placed them on the diamond slide and illuminated the slide with a green laser. The nitrogen vacancy centers, primed by the electromagnetic coils, responded by emitting red light at a brightness that depended on the sample’s magnetism.

The finer spatial resolution of the QDM showed the fields came not from the crystals’ interiors, but from rims of iron that had, presumably, formed much later in their history. The fields “are not born with the zircons,” Fu says, fueling debate about the claims of a 4-billion-year-old field.

Now, Fu and his colleagues have applied the technique to other rocks from Western Australia—its 3.2-billion-year-old Honeyeater Basalt. Using traditional superconducting sensors, which still beat the QDM for overall sensitivity, they measured the magnetic field strengths and directions captured in 235 samples of the rocks. But they couldn’t be sure those fields were primordial: Over its lifetime, the Honeyeater Basalt had been deeply altered and buried under the sea floor, where water and immense strains could have allowed later sources of magnetism to contaminate it. It was up to the QDM to deduce whether its magnetic field was a native or immigrant.

The fine-scale magnetic map the instrument produced showed the fields came not from mineral grains, but from halos around them, which had formed underwater, almost immediately after the lava likely oozed out of fissures in the sea floor, says Alec Brenner, lead author of the Science Advances study and Fu’s graduate student. “Technically that means they did not form with the rock,” he says. But for practical purposes, the halos were just as old; the team could say with confidence that the fields were 3.2 billion years old.

Earth’s magnetic field lines plunge into the ground at angles that increase from the equator to the poles. So the field directions preserved in the Honeyeater samples reveal the latitude at which the basalt formed. Last decade, another team of geologists analyzed ancient magnetism in nearby 3.35-billion-year-old rocks to show they formed at a different “paleolatitude.” From the difference in latitudes, Fu’s group calculated that Earth’s crust moved, at a minimum, some 2 centimeters per year during this span of 150 million years. “That’s roughly comparable to modern plate motion,” Brenner says.

 The result “is by itself quite an achievement,” says Jun Korenaga, a geophysicist at Yale University, and suggests studies of even older rocks could pin down when plate tectonics began. Credible arguments based on how fast the infant Earth cooled put the start anywhere from 4.5 billion years ago, soon after Earth’s formation, to 3 billion years ago. “If they can pursue this direction and go deeper in time, that would be interesting,” Korenaga says.

Meteorites, time capsules from the early Solar System, are another natural subject for the QDM, Fu says. In a new study accepted at the Journal of Geophysical Research: Planets, Fu used the microscope to zoom in on the sulfide rims of a crystalized droplet of primordial melt within a meteorite that likely formed beyond Jupiter. Just 100 micrometers wide, the rims are too small for older techniques to isolate and measure, but the QDM revealed a weak relic magnetism. Combined with past measures of a stronger field in a meteorite formed closer to the Sun, the finding indicates that 4.6 billion years ago, the disk of material that gave rise to the planets might have had a patchy magnetic field. Fu says that indicates magnetism, not just gravitational dynamics, may have played a role in the planets’ coalescence out of the disk.

Closer to home, scientists have long used cave formations to gauge rainfall, based on ratios of oxygen isotopes left in stalactites and stalagmites by seeping water. But the isotope ratios can be an unreliable indicator. The water is also known to deposit microscopic magnetic grains picked up from soil and rock, in amounts that vary with rainfall. In samples collected from the Brazilian rainforest, Fu’s team found that the QDM appears capable of measuring those grain abundances over time as a proxy for rainfall, he says. This would “unlock a whole new source of data about past environmental change,” says Joshua Feinberg, a geologist at the University of Minnesota, Twin Cities.

Although it has only migrated to a few labs so far, the QDM could become the go-to tool for resolving controversial claims of paleomagnetism, says Claire Nichols, a geologist at MIT. Nichols has reported finding magnetic fields in 3.7-billion-year-old rocks from Greenland—another sign of an early magnetic dynamo on Earth. A QDM map would bolster that claim. “It’s now going to become the gold standard,” she says.

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