87Sr/86Sr Ratio
Definition
The ratio of radiogenic strontium-87 (produced by decay of rubidium-87) to the stable reference isotope strontium-86. Values range from roughly 0.703 in young ocean basalts to above 0.800 in ancient granites. This range encodes the bedrock age and Rb/Sr chemistry of the local geology.
- Radiogenic isotope
- Strontium-87, from rubidium-87 decay
- Reference isotope
- Strontium-86 (stable)
- Typical range
- ~0.703 in young basalt to over 0.800 in old granite
- Field
- Geoforensic provenancing
Common questions
How is this ratio actually used to place a person or item geographically?+
The ratio in local bedrock and soil is absorbed into local water and food chains and ends up in teeth, bone or hair. Comparing a sample's ratio against regional isoscape reference maps can narrow where a person lived or an item originated.
Why does older bedrock give a higher ratio?+
Strontium-87 accumulates over time as rubidium-87 decays, so rock that formed long ago and has had more time for that decay to occur carries a higher proportion of strontium-87 relative to the stable strontium-86.
Related terms
- Isoscape
- A map of isotope ratio variation across a geographic area, derived from systematic sampling of soil, water, plant, or mineral material; used...
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- Mixing Model
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- Radiogenic Ingrowth
- The progressive increase in a daughter isotope such as 87Sr over geological time as the parent nuclide (87Rb) decays. Materials with higher...
- Strontium Isoscape
- A map of 87Sr/86Sr values across a defined geographic area, built from measurements of soil, plants, or water. Used to compare the...