A Time Capsule in the Deep Crust: Ancient Groundwater Sealed for 1.5 Billion Years

Deep beneath Earth's surface lies a realm that has long remained largely unknown. Unlike the dynamic water cycle at the surface, water in the deep continental crust can become trapped within rock fractures and mineral structures, potentially remaining isolated for immense spans of geological time.

A research team working in a mine near Timmins in Ontario, Canada collected water and gas samples from fractures in rocks roughly 2.4 kilometres below the surface. These rocks belong to the ancient Archean crust and formed about 2.7 billion years ago. Geological records indicate that the region experienced episodic volcanic eruptions intercalated with both terrigenous and marine sedimentary deposits formed in a shallow seafloor environment. The deposit is relatively undeformed and was metamorphosed to greenschist grade about 2.67–2.69 billion years ago. Greenschist-grade metamorphism refers to a low-to-moderate metamorphic process typically occurring at temperatures of roughly 300–450 °C and pressures of several kilobars, producing mineral assemblages characterized by green minerals such as chlorite, actinolite, and epidote. Later, around 2.64 billion years ago, the region experienced a metasomatic event, during which fluids—commonly hydrothermal fluids—migrated through the rocks and altered their chemical composition and mineralogy. These geological processes created favourable conditions for the formation and long-term preservation of subsurface fluids.

Researchers collecting groundwater from a borehole（感謝 K. Voglesanger 提供）

Water flowing from the boreholes contains significant amounts of hydrogen, methane, nitrogen, and helium. Such gas compositions are not unusual in deep crustal groundwater. Previous research has shown that hydrogen can be generated through the radiolysis of water, in which high-energy radiation splits water molecules, or through serpentinization, a reaction in which ultramafic rocks rich in minerals such as olivine and pyroxene react with water to form serpentine and related minerals. These processes produce reduced gases that can serve as chemical energy sources for microbial life.

Such chemical energy can sustain unique ecosystems composed of chemoautotrophic microbes. These microorganisms do not rely on sunlight; instead, they obtain energy by metabolizing reduced gases such as hydrogen. As a result, microbial communities can survive even in environments completely isolated from the surface for extremely long periods of time.

To determine how long these groundwater systems have remained trapped within the crust, researchers analysed isotopes of several noble gases, including helium, neon, argon, and xenon. Noble gases are valuable geochemical tracers because they rarely participate in chemical reactions, allowing their isotopic compositions to preserve records of geological history. Some isotopes of helium and argon are produced through radioactive decay processes within rocks. For example, ⁴He can be generated from the decay of ²³⁸U, ²³⁵U, and ²³²Th, while ⁴⁰Ar is produced by the decay of ⁴⁰K. In a closed system these radiogenic gases gradually accumulate over time. By measuring their concentrations, scientists can estimate how long fluids have remained isolated within the rock.

The helium isotope ratios measured in the samples show extremely low ³He/⁴He values, indicating that more than 99% of the helium present originates from radiogenic decay rather than from mantle sources. Neon isotope data also suggest that the fluids reflect a radiogenic crustal component rather than any significant mantle contribution. Together, these observations indicate that the fluids have resided within the local rock system for a very long time.

The xenon isotopes provide particularly striking evidence of this ancient origin. The groundwater contains xenon isotope ratios that differ from those in the modern atmosphere. In particular, the lighter xenon isotopes—such as ¹²⁴Xe, ¹²⁶Xe, and ¹²⁸Xe—are enriched relative to present atmospheric compositions. This pattern resembles the isotopic composition of xenon in the ancient Archean atmosphere. Xenon isotopes in Earth's atmosphere have not remained constant through time. In the early history of the planet, processes such as solar radiation and atmospheric escape gradually altered their relative abundances. If groundwater once equilibrated with the ancient atmosphere before becoming trapped within the crust, the xenon isotopic signature of that atmosphere could remain preserved within the fluid.

Using these xenon isotopic deviations, researchers estimated that the average residence time of these fracture fluids is at least about 1.5 billion years. In other words, the water may have been trapped within crustal fractures since the late Archean or early Proterozoic era and remained isolated ever since. This timescale is far longer than previous estimates of deep groundwater residence times, which generally ranged from millions to tens of millions of years.

Another remarkable observation is the enrichment of ¹²⁹Xe. This isotope is typically produced by the radioactive decay of ¹²⁹I, which has a half-life of only about 15.7 million years. Over billions of years, nearly all of the original iodine-129 would have decayed. Therefore, the presence of excess xenon-129 must be linked to ancient geological processes that generated or concentrated this isotope long ago, after which the fluid system remained sealed.

If groundwater can remain isolated within the crust for billions of years, then deep subsurface ecosystems could also persist over comparable timescales. Microorganisms inhabiting these environments rely not on sunlight but on chemical energy generated by interactions between water and rock. Even in complete darkness, such reactions can sustain stable microbial communities.

This discovery also has implications for the search for life beyond Earth. Mars, like Earth's ancient continental shields, is dominated by very old geological terrains that have remained tectonically quiet for billions of years. If ancient fluids capable of supporting microbial ecosystems can persist deep within Earth's crust, similar environments might also exist beneath the surface of Mars. As long as liquid water and chemical energy sources remain available underground, life could potentially survive there for immense spans of planetary history.

Author: Shui Ye-You

Reference:

Holland G et al. (2013). Deep fracture fluids isolated in the crust since the Precambrian era. Nature.