Review by B.C. Schreiber

1. STRONTIUM ISOTOPES Why and how it is used in estimating stratigraphic position of carbonates and evaporites Review by B.C. Schreiber

2. ONE OF THE MOST USEFUL, CONSERVATIVE, AND STABLE ELEMENTS FOR ISOTOPIC STUDY IN SEDIMENTARY ROCKS (CARBONATES AND EVAPORITES) IT HAS BEEN TREATED AS A GEOLOGICAL “TRACER”

3. RESIDENCE TIME OF STRONTIUM IN THE OCEANS >2 M.Y. MIXING TIME OF STRONTIUM IN THE OCEANS ~103 YEARS

4. BACKGROUND During fractional crystallization, Sr tends to be come concentrated in the first minerals to crystallize, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios occur in pegmatites. Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order. The Rb-Sr dating method has been used extensively in dating rocks. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and 87Sr/86Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.

5. 4 stable naturally occurring isotopes 84Sr (0.56%), 86Sr (9.86%), 81Sr (7.0%) and 88Sr (82.58%). 4 stable naturally occurring isotopes 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%)

6. Strontium is present as a ubiquitous minor element in the crust of the Earth –

7. Strontium is present as a ubiquitous minor element in the crust of the Earth – Present in many rock types.

8. Strontium is present as a ubiquitous minor element in the crust of the Earth – Present in many rock types. Typically found in concentrations of a few hundred parts per million

9. 87Sr 87Rb half-life of 48,800,000 years

12. 4 stable naturally occurring isotopes 84Sr (0.56%), 86Sr (9.86%), 81Sr (7.0%) and 88Sr (82.58%). 4 stable naturally occurring isotopes 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%)

13. 4 stable naturally occurring isotopes 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%)

14. Capo et al., 1998

15. IN A MAGMA, IGNEOUS PROCESSES CONCENTRATE RUBIDUM IN THE RESIDUAL MELT SO THE LATER FORMED MINERALS CONTAIN MORE AND MORE RUBIDIUM SOURCES OF Sr From radioactive decay of 87Rb 85Rb = 72.165% 87Rb = 27.834%

16. Primordial 87Sr/86Sr ratio of 0.699, value derived from meteorites, NOW HIGHER ON EARTH due to the decay of 87Rb Faure, 1991

18. WHAT CONTROLS THE 87Sr/86Sr RATIO IN THE OCEANS?

21. WEATHERING WEATHERING FastWeathering First to Crystallize First to Crystallize Bowen’s Reaction Series Goldrich Stability Series Last to Crystallize SlowWeathering Last to Crystallize Last to Crystallize

27. CONTESSA QUARRY marine carbonates

28. Modern seawater = 87Sr/86Sr = 0.7092

29. CONTROLLED BY THE EXTENT THE Sr2+ CAN SUBSTITUTE FOR Ca2+ IN CALCIUM BEARING MINERALS

30. Two sources of 87Sr in any material: (1) formed primordial nucleo-synthesis along with 84Sr, 86Sr and 88Sr (2) radioactive decay of 87Rb.

31. The ratio 87Sr/86Sr is the parameter typically reported in geologic investigations

32. WHAT CONTROLS THE 87Sr/86Sr RATIO IN THE OCEANS?

35. Elderfield, 1986

36. PRESENT DAY SPREADING CENTERS

39. WHAT EFFECT DOES RAPID/SLOW SEAFLOOR SPREADING HAVE ?

40. Based on results of Alt et al, 1986,

41. WHAT DOES RAPID SEA FLOOR SPREADING DO TO THE OCEANS?

44. Elderfield, 1986

45. Visser, 1989

50. Sr IN GYPSUM REMAINS WITHIN SULPHATE EVEN WHEN IT DEHYDRATES BUT WHEN IT REHYDRATES, MUCH Sr IS LOST ALSO WHEN ARAGONITE GOES TO CALCITE MUCH Sr IS LOST