Geochemistry is the study of Earth’s chemical composition and the chemical processes that shape it, from the separation of the planet’s iron core billions of years ago to the contamination of a village well today. It sits at the intersection of geology and chemistry, applying the tools of one to the questions of the other. The field’s reach is broader than most people expect: geochemists date ancient rocks, trace volcanic plumbing systems, figure out why groundwater in certain regions carries dangerous levels of arsenic, reconstruct temperatures from millions of years ago using the chemistry of cave minerals, and even help explain how life may have started. If a question involves both rocks and reactions, geochemistry probably has something to say about it.
How a Planet Gets Its Chemistry
One of the biggest questions in geochemistry is how Earth ended up with its particular layered structure and the specific chemical inventory we find in the mantle and crust. The planet did not arrive pre-assembled. Early in its history, a global magma ocean allowed heavier metallic liquids to sink toward the center, forming the core, while lighter silicate material floated upward to become the mantle. This process, called core-mantle differentiation, determined which elements ended up where. A 2025 study using machine-learning-driven molecular simulations found that even the amount of water present during this separation mattered: increasing water content promoted magnesium moving into the metallic core while silicon, iron, and hydrogen preferred the silicate mantle. That model estimated Earth’s total water budget at roughly ten ocean masses, most of it locked deep inside the planet rather than sitting on the surface.1PubMed Central. Earth’s core-mantle differentiation shaped by water
The process was not a single clean event. Current models treat core formation as the result of many collisions between the growing Earth and smaller bodies that had already differentiated into their own mini-cores and mantles. Each impact brought fresh metallic liquid into contact with a magma ocean, and the two reacted before the metal sank further. The chemical outcome depended on temperature, pressure, and oxygen availability at each step.2Earth and Planetary Science Letters. Heterogeneous accretion, composition and core–mantle differentiation of the Earth Because those conditions changed as Earth grew, the resulting mantle ended up with a chemical fingerprint that cannot be explained by a single equilibrium event. High temperatures during the segregation of the largest planets pushed more silicon into the core, even when oxygen levels were relatively high.3Tectonophysics. Core formation, mantle differentiation and core-mantle interaction within Earth and the terrestrial planets
Once the mantle existed, further chemical sorting happened as partial melting produced the crust. The continental crust is enriched in elements that do not fit easily into mantle minerals. A classic analysis showed that the most “incompatible” elements, including cesium, rubidium, barium, and thorium, reach concentrations 50 to 100 times their mantle values in the continental crust, while the oceanic crust peaks at about 10 times mantle values for a different set of moderately incompatible elements like sodium and titanium.4Earth and Planetary Science Letters. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust This complementary pattern is one of the tidier results in geochemistry: what is enriched in one reservoir is depleted in the other, because they were separated from the same starting material.
Reading Ancient Thermometers
Geochemists have become remarkably good at reconstructing past climates using the chemistry of minerals that formed long ago. The workhorse technique relies on oxygen isotopes. When carbonate minerals precipitate in caves or lakes, the ratio of heavy to light oxygen atoms in the crystal depends on the temperature at the time of growth and the isotopic makeup of the water. Shifts in oxygen isotope values recorded in lake sediment carbonates can therefore reflect changes in temperature or in the water feeding the lake.5Quaternary Science Reviews. Palaeoclimate interpretation of stable isotope data from lake sediment archives
Cave deposits, or speleothems, are particularly useful because they grow slowly and continuously over thousands of years. A study that farmed calcite inside a monitored cave found that oxygen isotopes in the mineral tracked growth temperature reliably, although the relationship between temperature and isotope fractionation measured in real caves differed from what laboratory experiments predicted. The researchers combined their data with other cave studies to produce an updated equation for how oxygen isotopes and temperature relate in natural cave settings, with a sensitivity of about −0.18 per mil per degree Celsius, slightly lower than the previously accepted value.6Geochimica et Cosmochimica Acta. Speleothem calcite farmed in situ: Modern calibration of δ18O and δ13C paleoclimate proxies in a continuously-monitored natural cave system Carbon isotopes in the same minerals turned out to respond to something unexpected: changes in how air moved through the cave rather than shifts in vegetation or atmospheric carbon dioxide above.
A newer approach called clumped isotope thermometry avoids one of the classic headaches of oxygen isotope work. Conventional methods require you to know the isotopic composition of the water the mineral grew from, which is often unknown for ancient samples. Clumped isotope measurements instead look at how often heavy isotopes of carbon and oxygen bond to each other within the same carbonate molecule, a preference that depends only on temperature.7Annual Review of Earth and Planetary Sciences. Frontiers of Carbonate Clumped Isotope Thermometry Calibration work has shown that the technique works not just for calcite but also for siderite, an iron carbonate common in ancient soils. A laboratory calibration of siderite grown between 21 and 51 degrees Celsius found that its clumped isotope response was indistinguishable from calcite’s, and that natural siderites from the Mississippi Delta recorded warm-season marsh water temperatures rather than annual averages.8Geochimica et Cosmochimica Acta. Siderite ‘clumped’ isotope thermometry: A new paleoclimate proxy for humid continental environments That kind of seasonal bias matters enormously for interpreting past climates accurately.
Earth’s Long-Running Carbon Thermostat
Over millions of years, Earth’s temperature stays within a livable range partly because of a geochemical feedback loop involving silicate rocks. When atmospheric carbon dioxide and temperatures rise, chemical weathering of silicate minerals speeds up. That weathering consumes carbon dioxide, pulling it out of the atmosphere and eventually locking it into carbonate minerals on the seafloor. When carbon dioxide drops and the climate cools, weathering slows down, allowing volcanic carbon dioxide to accumulate again and warm things up.9Earth-Science Reviews. Silicate weathering as a feedback and forcing in Earth’s climate and carbon cycle This silicate weathering feedback is why Earth has avoided permanent runaway warming or snowball freezing for most of its history, though it has come close to both extremes.
The feedback is real, but it is not fast. Modeling work estimated the characteristic timescale for this carbon dioxide drawdown at roughly 240,000 years, with a plausible range between 170,000 and 380,000 years.10Global Biogeochemical Cycles. The time scale of the silicate weathering negative feedback on atmospheric CO2 That is relevant to current climate discussions: the thermostat is effective on geological timescales but far too slow to counteract human carbon dioxide emissions. A separate modeling effort showed that hydrology, the actual movement of water through river basins, regulates the strength of the weathering feedback by controlling how much rock surface the water contacts and for how long.11PubMed. Hydrologic regulation of chemical weathering and the geologic carbon cycle Tectonics and rainfall matter just as much as temperature in determining how effectively weathering scrubs carbon dioxide.
Oxygen, Iron, and the Early Atmosphere
Geochemistry provides the best evidence we have for how Earth’s atmosphere transformed from oxygen-free to oxygen-rich. For roughly the first two billion years, the atmosphere contained vanishingly little free oxygen. The fingerprint for this comes from sulfur isotopes: in sediments older than about 2.3 billion years, sulfur isotopes show a pattern called mass-independent fractionation, which can only be produced by ultraviolet light breaking apart sulfur dioxide in an atmosphere with essentially no oxygen, less than a hundred-thousandth of today’s level. In younger rocks, that signal disappears, because even small amounts of oxygen force all sulfur through the ocean’s sulfate pool, erasing the anomalous pattern.12PubMed. Mass-independent fractionation of sulfur isotopes in Archean sediments: strong evidence for an anoxic Archean atmosphere
The transition was not a smooth ramp. Iron isotope data from sediment cores spanning the period between roughly 2.5 and 2.4 billion years ago show that oceanic iron concentrations increased substantially, with residence times climbing from about 200,000 years to 2.3 million years over that interval, translating into iron concentrations rising from about 6 to 37 millimoles per kilogram.13Earth and Planetary Science Letters. Dynamics of oceanic iron prior to the Great Oxygenation Event Meanwhile, a separate study of seawater oxygenation showed that the redox state of seawater oscillated between oxygen-rich and oxygen-poor conditions before the permanent rise, rather than increasing monotonically.14Nature Geoscience. Fluctuating evolution of seawater oxygen before the Great Oxidation Event The ocean was flickering between two chemical states before it finally tipped toward the oxygenated world we know.
Tracing Volcanic Plumbing With Noble Gases
Helium is one of the more surprising tools in a geochemist’s kit. Most helium on Earth’s surface is helium-4, produced by radioactive decay of uranium and thorium in the crust. But helium-3, a lighter form, is primordial, left over from the planet’s formation. The ratio between the two in volcanic gases tells you something about where the magma is coming from. A study of Fogo volcano in the Cape Verde islands found helium-3/helium-4 ratios of roughly 7 to 8.4 times the atmospheric ratio across different mineral hosts and fumarole gases.15Earth and Planetary Science Letters. The helium and carbon isotopic signature of Ocean island basalts: Insights from Fogo volcano (Cape Verde archipelago) Those values sit near the range typical of mid-ocean ridges, but some ocean island volcanoes produce much higher ratios, suggesting their source contains more primordial material.
A machine-learning analysis of global plume data found that helium-3/helium-4 ratios correlate with the buoyancy flux of mantle plumes: the hottest, most vigorous plumes carry the least radiogenic helium, meaning more primordial helium-3 relative to decay-produced helium-4. One interpretation is that the hottest plumes originate from the core-mantle boundary itself, and that the unradiogenic helium may come from the core rather than from a preserved primordial mantle reservoir.16Geochemistry, Geophysics, Geosystems. Insights Into Mantle Plume Geochemistry From Machine Learning If confirmed, that would mean the core is not just an inert iron ball but an active chemical participant in the planet’s volcanism.
Hydrothermal Vents and the Chemistry of Life’s Origin
Some of the most provocative geochemistry happens where seawater meets hot rock deep on the ocean floor. At hydrothermal vents, water circulates through cracks in the crust, reacting with iron-rich minerals in a process called serpentinization. This oxidizes iron in the rock and produces hydrogen gas, along with microcompartments and chemical gradients that look eerily like the machinery inside living cells.17PubMed Central. Serpentinization as the source of energy, electrons, organics, catalysts, nutrients and pH gradients for the origin of LUCA and life That resemblance is at the heart of one leading hypothesis for how life began: not in a warm pond on the surface, but in alkaline hydrothermal vents where geochemistry was already doing a crude version of what metabolism does today.
The Lost City hydrothermal field on the Mid-Atlantic Ridge is the poster child for this idea. Its alkaline fluids are driven by serpentinization rather than by volcanism, and its spectacular carbonate chimneys host dense microbial communities fueled by the hydrogen and reduced carbon the process provides.18PubMed Central. Fossil evidence for serpentinization fluids fueling chemosynthetic assemblages These communities need no sunlight. Serpentinization generates both the energy and the raw materials, carbon compounds, reducing power, and catalytic mineral surfaces, to support chemosynthetic life that may extend deep into Earth’s subsurface.19Reviews in Mineralogy and Geochemistry. Serpentinization, Carbon, and Deep Life The implications go beyond Earth: any rocky body with water and iron-rich minerals could host similar chemistry, which is partly why the icy moons of Jupiter and Saturn are targets in the search for extraterrestrial life.
Where Ore Deposits Come From
Geochemistry is central to understanding why valuable metals concentrate in mineable deposits rather than being spread thinly through ordinary rock. Many of the world’s major copper, tin, zinc, lead, silver, and gold deposits are associated with granitic (felsic) magmatic systems, but the actual source of the metals has long been debated. A study of mafic enclaves from a giant tin-lead-zinc-silver-copper deposit in northeast China traced the supply chain in detail. Sulfide melt inclusions trapped inside crystals from the mafic (iron- and magnesium-rich) magma contained thousands of parts per million of copper and hundreds of parts per million of zinc, lead, and tin. When fluids exsolved from the magma, they oxidized and dissolved these sulfide droplets, transferring the metals upward into the overlying felsic magma body.20PubMed Central. Felsic hydrothermal deposit formation by chalcophile metal concentration through mafic recharge and fluid-mediated sulfide dissolution The metals people mine from granitic systems may actually originate deeper in the plumbing, in the mafic magmas that fed into them.
The role of fluids in moving metals around is a recurring theme. Partitioning experiments confirm that the affinity of metals for the fluid phase depends heavily on fluid and melt composition: chlorine-rich fluids and highly evolved melts drive the strongest metal partitioning into the fluid.21Chemical Geology. Fluid-melt trace-element partitioning behaviour between evolved melts and aqueous fluids: Experimental constraints on the magmatic-hydrothermal transport of metals This is why porphyry copper deposits, the largest single source of copper on the planet, form in association with particular types of magmatic-hydrothermal systems rather than randomly.
Arsenic in Groundwater
Not all geochemistry is about deep time and planetary evolution. One of the most pressing applications is understanding why groundwater in parts of South and Southeast Asia carries dangerously high levels of arsenic, affecting tens of millions of people. The contamination is natural, not industrial. Arsenic binds to iron oxide minerals in sediments. When oxygen-poor conditions develop underground, microbes reduce the iron from its oxidized form to its more soluble reduced form, and the arsenic hitches a ride into solution.22Chemical Geology. The role of electron donors in arsenic-release by redox-transformation of iron oxide minerals – A review
Field studies in Bangladesh confirmed that arsenic in shallow wells (less than 35 meters deep) was mobilized under iron-reducing conditions and remained dissolved even as the system shifted to sulfate-reducing conditions.23Applied Geochemistry. Redox control of arsenic mobilization in Bangladesh groundwater Work in central China’s Jianghan Plain showed that the organic carbon already buried in the sediment was enough to fuel the microbial activity driving the release, so the process can sustain itself without any external input.24Geochimica et Cosmochimica Acta. Redox controls on arsenic enrichment and release from aquifer sediments in central Yangtze River Basin Understanding this geochemical mechanism is essential for predicting which aquifers are safe and designing treatment strategies, because the problem is not a pollutant you can regulate away but a natural reaction embedded in the sediment itself.
Acid Mine Drainage and Metal Mobility
Mining disrupts the geochemistry of the rock it exposes. When sulfide-bearing minerals like pyrite come into contact with air and water, they oxidize, producing sulfuric acid and releasing metals. The resulting acid mine drainage can contaminate rivers for decades after a mine closes. A study of the Hengshi River in China, affected by acid mine drainage, mapped how iron-bearing minerals and heavy metals changed along the river’s length. Near the source, where pH was as low as 2.5, schwertmannite (an iron oxyhydroxide sulfate mineral) dominated at concentrations up to 35 percent by weight in the sediment, along with high levels of metals. As pH rose downstream toward neutral, schwertmannite gave way to other iron minerals like goethite and ferrihydrite, and metal concentrations declined. The retention ability of the sediments varied by element, with lead held most tightly and nickel and cadmium most loosely.25PubMed Central. Mineralogical characteristics of sediments and heavy metal mobilization along a river watershed affected by acid mine drainage
The broader framework for predicting how metals move from mine waste into waterways involves four factors: whether the mineral source of the contaminant is actually present, how abundant it is, how fast it reacts relative to water flow, and what flow paths the water takes. Iron is the most reactive player because bacteria oxidize it rapidly and it precipitates at very low pH. Aluminum precipitates near pH 5, providing a secondary buffer. Below pH 3.5, almost no metal attenuation occurs, so any water that acidic carries its full metal load downstream.26Applied Geochemistry. Hydrogeochemical processes governing the origin, transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters
Geochemistry Beyond Earth
The same chemical reasoning geochemists apply to Earth works on other worlds. Carbonaceous chondrite meteorites, fragments of asteroids that never melted and differentiated, are considered among the most primitive surviving materials from the early solar system. Their bulk ratios of magnesium to silicon, calcium to silicon, and aluminum to silicon form a distinctive group that helps researchers estimate the original chemical composition of the dust cloud that formed the planets.27Reviews of Geophysics. Are carbonaceous chondrites primitive or processed? A review
Lunar geochemistry entered a new era with China’s Chang’e missions. Samples returned by Chang’e-5 from the Moon’s near side revealed basalts whose parent magma contained at most about 283 micrograms per gram of water, implying a mantle source with only 1 to 5 micrograms per gram of water, the driest end of the range known from lunar basalts. The dehydration appears to have resulted from billions of years of prior volcanic activity extracting water from the mantle.28PubMed Central. A dry lunar mantle reservoir for young mare basalts of Chang’e-5 Chang’e-6 then returned the first sample from the Moon’s far side, yielding a basalt dated at 2.83 billion years old using lead-lead and rubidium-strontium isotope systems. Its strontium, neodymium, and lead isotopes indicated a mantle source depleted in incompatible elements and containing almost no KREEP component, a hallmark chemical signature enriched in potassium, rare earth elements, and phosphorus.29PubMed. A sample of the Moon’s far side retrieved by Chang’e-6 contains 2.83-billion-year-old basalt Calcium isotope work on a broader suite of mare basalts found compositions resembling terrestrial basalts, even though the lunar mantle’s formation history was very different, a result that modeling attributes to the balance between heavy-isotope cumulates in the interior and a light-isotope anorthositic crust.30Earth and Planetary Science Letters. The Ca isotope composition of mare basalts as a probe into the heterogeneous lunar mantle
Mars tells yet another story. Widespread clay minerals in the planet’s oldest terrains indicate extensive water-rock interaction more than 3.7 billion years ago. However, the available geochemical data point toward formation by hydrothermal groundwater circulation rather than surface lakes and rivers, a distinction that reshapes the picture of early Mars from a warm, wet world to one where most of the water action happened underground.31PubMed. Subsurface water and clay mineral formation during the early history of Mars
Geochemistry and Carbon Storage
Applied geochemistry increasingly intersects with efforts to mitigate climate change, particularly through geological carbon storage. The concept involves injecting carbon dioxide underground and letting it react with minerals to form stable carbonates. Basalt formations are promising targets because their calcium- and magnesium-rich minerals react readily with dissolved carbon dioxide. Iceland’s CarbFix project demonstrated that when carbon dioxide is dissolved in water before injection, eliminating its buoyancy and tendency to migrate upward, mineral trapping becomes the dominant storage mechanism within about two years.32Heliyon. Review of in-situ carbon mineralization mechanisms and influencing factors in geological carbon storage – Section: The carbon mineralization in basalt The trade-off is that the mineralization process can weaken the host rock: tests on one type of Indian basalt showed compressive strength dropping by 90 percent after extensive carbon mineralization, a reminder that locking carbon away permanently requires understanding the geochemistry of the rock at a detailed, site-specific level.