The Earth’s geodynamo—the magnetic field created by the roiling inner core—protects our planet from solar radiation and may be integral to Earth’s habitability. The magnetic field leaves its mark in the rock record by forcing iron-bearing minerals to align their magnetic fields, for example, as they precipitate in the pore space of a sedimentary rock or as igneous rocks solidify. These often minuscule magnets, which find north no matter where it might have been, have helped scientists to discover seafloor spreading, trace the path of continents past, and explore just how old the geodynamo is.
Bulk rock paleomagnetic measurements, typically collected from samples the size of a soda bottle cap, should tell scientists the direction and intensity of Earth’s magnetic field when the rock formed, said Sonia Tikoo, an assistant professor at Stanford University. But some rocks have heterogeneous magnetic signatures at fine scales, and others may no longer record the original magnetic imprint because of weathering, erosion, or some other alteration, she explained. The relatively nascent quantum diamond microscope, or QDM, helps scientists like Tikoo read these complicated chapters of a rock’s history.
Originally developed to image magnetic fields at high resolution, these instruments enable micrometer-scale imaging of either thin sections—slivers of rock mounted on glass—or individual crystals that contain magnetic inclusions. By discerning exactly which part of a sample is magnetic, said Tikoo, scientists have used this tool to address a host of questions, from the Hadean to the Holocene.
Lasers, Diamonds, and Microwaves
“This is our QDM lab,” said University of California, Berkeley, doctoral student Yiming Zhang as he unlocked a door with a laser hazard warning sign posted on the wall. Upon entrance to the laboratory, a list of safety protocols greeted Zhang, along with a floor-to-ceiling black curtain shielding the makeshift foyer from the rest of the room. The curtain, said Zhang, protects people as they don red-tinted safety goggles designed to protect their eyes from a laser that produces green light.
Zhang ducked behind the curtain and headed to a long table with a microscope in the middle. The microscope was surrounded by small copper-colored circles that look like hula hoops designed for dolls. These, he said, are Helmholtz coils, arranged in different orientations in part to cancel Earth’s magnetic field in the region where the sample sits.
A square-shaped diamond, machined to have a flat face, is mounted on the microscope lens. Zhang must load his thin section onto the microscope’s stage, ensuring that the diamond and sample sit flush. Too close, and the diamond could scratch or crack the carefully polished thin section. Too far, and the magnetic signal dies away.
The synthetic diamond is designed with a specific defect comprising a nitrogen atom and a void space, or vacancy, in the crystal structure. Each nitrogen-vacancy center swaps out two carbon atoms, said Roger Fu, an assistant professor at Harvard and a progenitor of using the quantum diamond microscope for paleomagnetic work.
Once the sample is properly positioned, analysis begins by shining the green laser’s light on the diamond as a horseshoe-shaped loop emits microwaves near the sample. The diamond will fluoresce, emitting a reddish light. The intensity of that fluorescence changes as the microwave energy changes. “By looking at how the intensity of the fluorescence changes with the microwave you put in, you can convert that to the magnetic field,” said Fu.
A camera mounted atop the microscope captures this fluorescence information across the entire viewing area, which is about 2 square millimeters, said Fu. Each 1- × 1-micrometer pixel is a single measurement of that field, he said, which means the camera captures 2 million separate measurements of the magnetic field at once.
Accumulating sufficient information from a single field of view can take anywhere from 20 minutes to several hours, depending on the sample. Software designed by Fu converts these measurements into a magnetic field map, which can be interrogated for where north was when each magnetic carrier or group of carriers internalized its signature.
Solar System to Rainstorms
When magnetic minerals document differing north directions within a sample, interpretations can be tricky. Meteorites in particular record multiple magnetic directions at the scale of millimeters, said Fu, in part because they’re amalgams of disparate parts of the early solar system. Dating each magnetic event with geochronological methods can help detangle the first 5 million years of our solar system’s history, he said.
In zircon, magnetism ideally comes from inclusions like magnetite, said Tikoo, and this method is “a good way to test whether your magnetic carriers could be secondary.”
Paleomagnetic studies of zircons older than about 3 billion years hinted that the geodynamo could have formed with the zircons themselves. However, using the quantum diamond microscope, Fu and his colleagues found that when similarly old zircons contained magnetic minerals, those paleomagnetic indicators formed later. Any original magnetic signature from when the zircons crystallized, he said, appears to be lost. The debate is ongoing.
At the opposite end of the age spectrum, said Fu, are actively forming cave deposits. Each layer—as thin as a single sheet of paper—has a distinct magnetic signature as floods, wind, and drip waters bring different material into the cave, said Fu. In some environments, extreme rainfall events tend to mobilize magnetite, whereas in others, dry spells bring more soil in, he explained. These fine-scale changes in magnetite, identified using the quantum diamond microscope, help scientists track rainfall and reconstruct paleoclimate, he said.
With an eye toward extraterrestrial craters that could have harbored life, Tikoo is using Berkeley’s quantum diamond microscope to explore the longevity of hydrothermal systems in the Chicxulub impact crater, associated with the demise of the dinosaurs. The rocks in question—made from broken bits of other rock—have complex magnetic signatures. With the quantum diamond microscope, she can pinpoint from where the dominant magnetic signal comes. “It’s a powerful tool for looking at very small things.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer
25 October 2021: This article has been updated to better reflect the ongoing discourse surrounding zircons and use of the quantum diamond microscope.