About 400 million years ago, the population of plants with vein systems for transferring water and nutrients, called vascular land plants, exploded. Soon thereafter, rocks from some continental magmas showed notable shifts in their chemical compositions. Geologists have suggested that these magma changes happened worldwide, but some argue that the data might be biased because some geographic regions have more samples to analyze than others. A new research team recently tested whether these magmatic changes occurred on a global scale, versus in isolated mountain belts or volcanic islands. 

Geologists use the chemistry of rocks formed from magma to understand a magma’s history. In particular, a mineral called zircon that forms from cooling magma preserves chemical clues about where the magma came from and what it interacted with. To test whether magma changes were global or local, the authors needed data ranging from the equator to the poles. Continents have shifted over the past 400 million years, so scientists use the latitude a rock had when it formed, called its paleolatitude, to compare samples from different parts of ancient Earth. To understand magma histories worldwide, the team used publicly available chemical data from zircons in magmatic rocks that formed across a wide spread of paleolatitudes.

Chemical elements with the same number of protons but different numbers of neutrons are called isotopes with different masses. To discern how plants influenced magma, the researchers analyzed 2 different isotope signals preserved in the zircons. The first isotope signal comes from the ratio of the heavy to light oxygen isotopes, which increases when sediment mixes into magma. Scientists refer to this value as δ¹⁸O, pronounced “delta 18-O.” 

The second isotope signal comes from the element hafnium, denoted Hf. Geologists use hafnium to estimate how long ago magmas melted and separated from the mantle. Zircon contains 2 Hf isotopes, one of which is stable and one of which is produced by radioactive decay. Because this decay happens over billions of years, the ratio between the 2 Hf isotopes over time shifts only slightly. Geologists express these tiny differences using a shorthand called εHf, pronounced “epsilon hafnium,” which shows how much a magma’s Hf signature has changed from Earth’s original mantle. Lower εHf values indicate magmas that incorporated older crustal rock, while higher εHf values reflect mantle sources.

The researchers found that δ¹⁸O values increased as εHf values decreased in these zircons. They concluded that this trend indicates increasing amounts of land-derived sediment in magmas, corresponding with the evolution of land plants. They suggested that land plants altered the ancient landscape, changing how sediments weathered and moved over land. 

To explore this pattern in detail, the team focused on the Andes Mountains, a region that preserves a long history of magmatic activity across a long span of space and time. Using a database, they accessed isotope data from zircon samples collected in the Andes Mountains by dozens of other research groups. These samples covered 32 degrees of modern latitude and 520 million years of Earth’s history, offering a broad window into how magma chemistry changed during that time.

They found that zircons older than 450 million years had no relationship between their εHf and δ¹⁸O values. However, in zircons younger than 450 million years, δ¹⁸O increased as εHf decreased. The researchers saw this pattern in magmas that formed along the edge of the continent, where one tectonic plate sinks below the other, called a subduction zone. They also saw this pattern in magmas that formed inland, away from the subduction zone, around 200 million years ago during the breakup of the supercontinent Pangaea.

 They found similar results in published zircon isotope data from igneous rocks in China, the Caribbean, Antarctica, Madagascar, and Tasmania. Zircons from each region showed the same relationship as zircons in the Andes. Since paleolatitude can also reflect ancient climate, the researchers compared the ratio of εHf and δ¹⁸O, written as εHf/δ¹⁸O, with paleolatitude to test whether ancient climate zones influenced magma chemistry. They found no link between paleolatitude and εHf/δ¹⁸O. 

With these results in mind, the researchers concluded that the relationship between εHf and δ¹⁸O shifted worldwide after vascular land plants evolved. They argued that as plants spread across the continents, their roots accelerated the breakdown of rocks. This accelerated weathering produced large amounts of sediment that washed into ocean basins and was eventually subducted into the mantle, forever changing the chemistry of magma formed there. They suggested that this chain of events illustrates how life on Earth’s surface can drive changes deep within the planet. 

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The continent of Africa is splitting into 2 tectonic plates in the middle of Ethiopia. In the recent past, geophysicists have improved their understanding of how tectonic plates like these separate. They’ve shown that continents begin to split apart as the crust and upper mantle, known as the lithosphere, crack and shift. Later, magma from deep within the Earth travels upward through these cracks to Earth’s surface, forming volcanoes. Therefore, scientists know that volcanoes form in areas of continental rifting, but not how quickly they form, which complicates efforts to assess volcanic hazards in rift zones.

Researchers led by Kevin Wong sought to answer this question by examining a mineral formed when magma cools, called olivine. They focused on 72 olivine crystals ranging in size from 1 to 4 millimeters (0.04 to 0.16 inches) from rocks collected at the Boku and Ziway volcanic fields in the Main Ethiopian Rift (MER) zone in Africa. They explained that the lithosphere in this area is still about 35 to 40 kilometers (21 to 25 miles) thick. This thick lithosphere suggests that the MER represents an intermediate stage of continental separation and offers a rare opportunity to study how tectonic stretching transitions into magmatic rifting in the process.

Wong and his team analyzed olivine because it’s one of the first minerals to crystallize from magma, and it continues to grow as the magma rises and cools. As magma rises, its composition changes, producing sharp chemical “zones” within the growing crystals, analogous to growth rings in trees. Changing temperatures and magma compositions cause different elements, like magnesium and iron, to diffuse into and out of the crystals at various rates during the magma’s ascent. So scientists can model these chemical zones and their boundaries in olivine crystals to determine how quickly the magma ascended from the upper mantle to erupt in the rift.

Wong and colleagues examined the olivine crystals from the MER volcanic fields using high-magnification imaging and chemical analyses, with an instrument known as an electron microprobe. Within each crystal, the team mapped 10 to 15 points spaced approximately 5 to 15 microns apart (about 10% of the thickness of a human hair) along a transect from the inner core to the outer rim, spanning its growth zones. 

They found 2 different populations of olivine crystals. The first consisted of normal-zoned crystals with magnesium-rich inner cores, and the second consisted of reverse-zoned crystals with lower magnesium cores. They explained that recently-formed magmas in the deep Earth contain higher amounts of the element magnesium relative to iron. The magnesium-rich zone has a sharp boundary with the magnesium-poor zone, but this boundary can get blurred when elements diffuse across it. Diffusion progressively smooths these crystal boundaries over time at known rates, so researchers can use their “blurryness” to extract information on how quickly the crystals equilibrated with the surrounding magma.

The researchers used numerical models to estimate how quickly magnesium and iron would diffuse across these chemical boundaries at different temperatures and surrounding magma chemistries. They compared thousands of simulated diffusion profiles to their measured olivine diffusion profiles. They used this iterative process to estimate that the crystals diffused, on average, for 40 and 17 days in the surrounding magma while ascending from the deep Earth to erupt at Boku and Ziway, respectively. They further tested these estimates using a growth-diffusion model that better represented natural crystal behavior. That model produced ascent times of about 27 days on average and better reproduced the crystal zoning patterns they observed.

Based on these models, the researchers concluded that intermediate-stage rifting events happen on unexpectedly short timescales. Magmas travel up to 40 kilometers (25 miles) from the deep Earth to the surface within, on average, a single calendar month, which is closer to human timescales than geologic timescales. They suggested that this rapid ascent is likely due to highly developed magmatic plumbing systems in the lithosphere that form before much lithospheric thinning. However, they noted that their results still suggest a wider range in ascension timescales than optimal for disaster mitigation and prediction. 

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