Rubidium–strontium (Rb–Sr) dating is a radiometric chronometer that exploits the beta decay of 87Rb to 87Sr (half‑life 49.23 billion years) to determine rock and mineral ages. Strontium occurs naturally as four stable isotopes; 87Sr is unique among them because its abundance can increase in a closed system by in‑situ decay of 87Rb while the non‑radiogenic isotopes (commonly referenced through 86Sr) remain essentially constant. Analytical determinations use mass spectrometry to measure present‑day 87Sr/86Sr ratios; if the initial Sr isotopic composition at the time of formation is known, the radiogenic increase in 87Sr/86Sr can be converted quantitatively into an absolute age using the decay law.
The isochron method removes the need for independent knowledge of initial Sr composition by analysing co‑genetic materials that shared a common initial 87Sr/86Sr. Minerals or whole‑rock separates that crystallized from the same melt inherit the melt’s initial Sr ratio but incorporate Rb and Sr in different proportions. Because these phases develop different present‑day 87Sr/86Sr values according to their original Rb/Sr, plotting multiple samples on an 87Sr/86Sr versus 87Rb/86Sr isochron yields a line whose slope corresponds to elapsed time and whose intercept gives the initial 87Sr/86Sr.
Mineral chemistry controls the original Rb/Sr partitioning because Rb commonly substitutes for K in crystal lattices and minerals vary widely in K/Ca and K/Na. Empirical ordering of increasing Rb/Sr (and, with time, increasing 87Sr/86Sr) commonly runs plagioclase → hornblende → K‑feldspar → biotite → muscovite. At the scale of mantle‑crust differentiation, Rb behaves as a highly incompatible element during partial melting and therefore preferentially enters melts, producing relative enrichment of Rb (and elevated 87Sr/86Sr) in crustal reservoirs compared with the mantle. Consequently, Sr isotopic systematics are widely used to distinguish mantle‑derived from crustally contaminated or crust‑sourced magmas and to constrain the timing of crustal extraction and growth even in rocks that have experienced subsequent metamorphism or partial remelting, provided the Rb–Sr system has behaved coherently since the event dated.
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The development of radiochemical methods that underpins Rb–Sr geochronology was advanced in the early 20th century by chemists such as Otto Hahn and Fritz Strassmann, whose work in radioisotope chemistry also contributed historically significant discoveries in nuclear science.
Example — Rubidium–Strontium systematics in granitoids
Granite typically contains a suite of Sr‑bearing minerals (plagioclase, K‑feldspar/orthoclase, hornblende, biotite, muscovite) whose initial Rb/Sr ratios differ because mineral chemistry, the composition of the parental melt, and crystallization conditions determine how much Rb and Sr each phase incorporates. Rubidium substitutes for potassium in crystal lattices, so a mineral’s capacity to take up Rb is largely controlled by its K content and by the Rb and K concentrations in the residual melt; consequently, Rb uptake scales with the melt’s Rb availability. Conversely, Sr commonly substitutes for Ca and is concentrated in Ca‑rich phases such as plagioclase and hornblende, which can therefore be relatively Sr‑rich while remaining low in K (and hence low in Rb).
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Under Bowen’s reaction‑series crystallization of a granitic system, early accumulation of plagioclase ± hornblende produces a cumulate (compositional analogue: tonalite or diorite) that is characteristically low in K and Rb but enriched in Sr. The extraction of these Sr‑rich, K‑poor minerals from the melt progressively concentrates K and Rb in the residual liquid. As crystallization proceeds, K‑rich phases such as orthoclase and biotite precipitate from that evolved melt and preferentially incorporate Rb, producing late‑forming rocks and minerals with much higher Rb/Sr ratios than the early cumulates.
This sequential partitioning generates pronounced contrasts in absolute Rb and Sr concentrations and in Rb/Sr ratios both among mineral phases within a pluton and between early cumulate whole rocks and later K‑feldspar/biotite‑rich granitoids. Because 87Rb decays to radiogenic 87Sr, those compositional differences drive distinct temporal patterns of radiogenic Sr build‑up in separate rocks and minerals. Recognizing these mineralogical and whole‑rock Rb/Sr contrasts is therefore essential for interpreting Sr isotopic data in petrogenetic studies and for applying the Rb–Sr chronometer accurately in geochronology.
Calculating the age
Rubidium–strontium ages are derived by measuring Rb and Sr isotopic compositions in several mineral subsamples from a single rock and plotting their isotopic ratios on an isochron diagram (y: 87Sr/86Sr; x: 87Rb/86Sr). The method rests on the beta‑decay of 87Rb to 87Sr, a process controlled by the decay constant λ, and on the predictable radiogenic accumulation of 87Sr over time.
The growth of radiogenic 87Sr can be written as
87Sr(t) = 87Sr(0) + 87Rb(0)·(e^{λt} − 1),
which, after normalization to the stable, non‑radiogenic isotope 86Sr, becomes
(87Sr/86Sr)_t = (87Sr/86Sr)_0 + (87Rb/86Sr)_t·(e^{λt} − 1).
On an isochron plot the common initial ratio (87Sr/86Sr)_0 appears as the y‑intercept and the line slope m equals (e^{λt} − 1). The age is therefore obtained from the best‑fit slope by
t = (1/λ) · ln(m + 1),
where m is determined by linear regression of the subsample points and associated uncertainties are propagated from the fit and the analytical errors.
Reliable ages require precise, accurate measurements of Rb and Sr (or their isotope ratios), selection of multiple subsamples representing different textural or mineralogical components, and rigorous statistical fitting to quantify slope, intercept and their uncertainties. The isochron method implicitly assumes closed‑system behaviour for Rb and Sr since formation, a common initial (87Sr/86Sr)_0 among the subsamples, and the constancy of 86Sr as a non‑radiogenic reference. A tight linear array of points supports these assumptions and a meaningful age; significant scatter or a poor fit typically signals post‑formation open‑system alteration, inherited initial heterogeneity, or analytical/sample selection problems that undermine the inferred age.
Sources of error in Rb–Sr dating
Rb–Sr ages rely on two accurate measurements for each sample: the bulk rubidium-to-strontium ratio (Rb/Sr) and the present 87Sr/86Sr isotopic ratio, because the calculated age depends on the in‑situ accumulation of radiogenic 87Sr from the decay of 87Rb relative to the initial 87Sr/86Sr composition. The fundamental assumption underpinning this chronometer is closed‑system behaviour for Rb and Sr since the time of crystallization or since the system cooled below its closure temperature. For most relevant minerals the Rb–Sr closure temperature is on the order of 650 °C, below which diffusion of these elements is effectively arrested.
Isochron construction further assumes that the analyzed minerals or whole‑rock fractions shared a common initial 87Sr/86Sr (i.e., formed in chemical equilibrium or were co‑deposited), so that observed variations in Rb/Sr and 87Sr/86Sr reflect radiogenic ingrowth rather than differing starting compositions. Any post‑formation perturbation that changes Rb or Sr concentrations or redistributes isotopes—whether by reheating above the closure temperature (thermally resetting diffusion profiles) or by chemical addition/removal (metasomatism)—invalidates an age as a primary formation age.
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Rb and Sr are relatively mobile in most geologic fluids, and their mobility is particularly enhanced by hot, often carbonated hydrothermal fluids associated with metamorphism or magmatism. Common alteration styles such as potassic alteration or albitisation can introduce, remove, or redistribute Rb and Sr, so isotopic signatures preserved in altered minerals may record the timing of those alteration events rather than primary crystallization. Consequently, an Rb–Sr age that is discordant with other geochronometers is not necessarily meaningless; it frequently marks the timing of a subsequent metasomatic, hydrothermal, or thermal resetting episode.
Interpreting Rb–Sr results therefore requires integrating isotopic data with petrography and regional geology: reconstructing the sample’s metasomatic and thermal history, identifying metamorphic or alteration events in the field and microscope, and cross‑checking ages with other isotopic systems to distinguish primary emplacement ages from later disturbances. The relative mobility of Rb and Sr, while a principal limitation for obtaining unambiguous crystallization ages, can be exploited advantageously to date episodes of fluid flow, hydrothermal alteration, or thermal resetting when the sampled mineralogy preserves an isotopic record of those specific events.
Rubidium–strontium (Rb–Sr) geochronology determines the age of rocks and minerals by measuring concentrations of rubidium and strontium and the isotopic ratio 87Sr/86Sr; converting parent–daughter proportions into elapsed time requires either knowledge of the initial strontium isotopic composition or a reliable extrapolation from the sample population (e.g., an isochron). Accurate ages therefore depend on precise Rb and Sr analyses and a defensible estimate of initial 87Sr/86Sr.
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A central assumption of Rb–Sr dating is that the studied system has remained closed since formation: no post‑formation gain or loss of Rb, Sr, or 87Sr/86Sr. Chemical weathering and other alteration processes on Earth can mobilize strontium, but the mobilized Sr inherits the 87Sr/86Sr signature of its source mineral; consequently, weathering can introduce secondary Sr with the same isotopic ratio into soils, waters, or co‑existing phases, producing open‑system behavior or isotopic contamination that biases calculated ages.
Extraterrestrial materials—lunar rocks and meteorites—are largely exempt from terrestrial chemical‑weathering processes in their native settings, so the specific problem of weathering‑derived Sr carrying source 87Sr/86Sr is not operative. For these materials, Rb–Sr age interpretations are therefore less vulnerable to that particular post‑formation alteration, although the closed‑system requirement and analytical precision remain critical for reliable geochronology.
Isotope geochemistry uses initial 87Sr/86Sr ratios as a robust provenance tracer because the strontium isotopic composition incorporated into biological tissues and cultural materials reflects the local bedrock and soils of the landscape in which those materials formed or organisms lived. Skeletal remains, marine shells and clay artefacts reliably archive such signals; their measured 87Sr/86Sr values can be directly compared with the isotopic composition of candidate source rocks and sediments. In practice, researchers determine the present-day 87Sr/86Sr of a specimen and treat that value as a proxy for the original, landscape-derived signature where diagenetic alteration or contamination is negligible. Provenance and mobility reconstructions are strengthened by multi‑isotope approaches—most commonly pairing 87Sr/86Sr with 143Nd/144Nd—which increase discrimination among geologically distinct source areas. Systematic comparison of sample isotope ratios to mapped or sampled source‑rock signatures thus permits construction of geological fingerprints and inference of origin and movement across geographic space.
Strontium isotope stratigraphy
Strontium isotope stratigraphy uses temporal variations in the seawater 87Sr/86Sr ratio, as captured by marine minerals, to correlate and date sedimentary sequences. In practice the method is applied almost exclusively to carbonate-bearing materials because long-term, high-quality records of seawater 87Sr/86Sr have been established primarily from marine carbonates; these archives provide the reference curve against which samples are compared.
The technique is most powerful for the Cenozoic, where a well-constrained, high-resolution seawater 87Sr/86Sr curve permits relatively precise correlation and numerical assignment of carbonate sediments and fossils. For Mesozoic and older intervals, however, the reference curve is both sparser and less reliably defined. Many older carbonate sequences are incompletely sampled or poorly preserved, reducing data density and the robustness of inferred isotopic trends.
Two related factors particularly weaken Sr-isotope stratigraphy in pre‑Cenozoic applications. First, diagenetic overprinting—post‑depositional chemical and mineralogical alteration—can modify original 87Sr/86Sr values and thus obscure the contemporaneous seawater signal. Second, there is generally limited overlap with independent absolute dating methods (for example, coeval U–Th or other radiometric controls), which hampers anchoring isotope variations to precise numerical ages. Together, poor preservation, diagenetic modification, and sparse absolute age constraints increase uncertainty in the detailed shape and timing of the seawater 87Sr/86Sr curve for Mesozoic and older periods, limiting the method’s resolution and reliability at fine temporal scales in those intervals.