Review by B.C. SchreiberWhy and how it is usedin estimating stratigraphic position of carbonates and evaporites STRONTIUM ISOTOPES

RESIDENCE TIME OF STRONTIUMIN THE OCEANS

>2 M.Y.MIXING TIME OF STRONTIUMIN THE OCEANS

~103 YEARS

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.

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%)

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

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

Typically found in concentrations of a few hundred parts per millionStrontium is present as a ubiquitous minor element in the crust of the Earth

Present in many rock types.

87Sr

87Rb half-life of 48,800,000 years

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%)

4 stable naturally occurring isotopes

84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%)

Capo et al., 1998

SOURCES OF SrFrom radioactive decay of 87Rb85Rb = 72.165%87Rb = 27.834%

Primordial 87Sr/86Sr ratio of 0.699, value derived from meteorites,NOW HIGHER ON EARTHdue to the decay of 87Rb

Faure, 1991

GoldrichStabilitySeriesFirst toCrystallizeLast toCrystallizeSlow WeatheringFast WeatheringBowensReactionSeriesFirst toCrystallizeLast toCrystallizeLast toCrystallizeWEATHERINGWEATHERING

CONTESSA QUARRYmarine carbonates

Modern seawater = 87Sr/86Sr = 0.7092

Two sources of 87Sr in any material:

(1) formed primordial nucleo-synthesis along with 84Sr, 86Sr and 88Sr

(2) radioactive decay of 87Rb.

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

Elderfield, 1986

PRESENT DAY SPREADING CENTERS

WHAT EFFECT DOES RAPID/SLOW SEAFLOORSPREADING HAVE?

Based on results of Alt et al, 1986,

WHAT DOES RAPID SEA FLOOR SPREADING DO TO THE OCEANS?

Elderfield, 1986

Visser, 1989

CONTESSA QUARRYmarine carbonates

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.

Secondly, differences in the relative mobilities of water at scales ranging from inter-grain pores tothe catchment scale may also profoundly affect 87Sr/86Sr (Bullen et al., 1996). For example, the chemical composition and the resultant 87Sr/86Sr in immobile waters at a plagioclase-hornblende grain boundary versus a quartz-mica boundary will be different from eachother.

Third, a difference in the relative "effective" surface areas of minerals in one portion o f the rock unit will also cause differences in chemistry and isotopic composition; "poisoning" of reactive surfaces by organiccoatings is an example of this kind of process.In a fundamental sense, because the waters in shallowsystems are not in chemical equilibrium with the rocks,it is unrealistic to expect that waters along flowpaths within even a constant-mineralogy unit should have a constant 87Sr/86Sr.

Instead, the waters moving along specific flowpaths slowly react with the rocks and gradually approach

The important concept for isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same 87Sr/86Sr as the mineral itself.

Therefore, differences in 87Sr/86Sr among ground waters require either (a) differences in mineralogy along contrasting flowpaths, or (b) differences in the relative amounts of Sr weathered from the same suite of minerals.

*****Particularly in K feldspars and micas**ABLE TO INCREASE 87Sr OVER TIME***Only 86Sr is RADIOGENIC***B REPRESENTS Sr EVOLUTION FOR A MANTLE DEPLETED IN Rb, WHILE THE CURVATURE TO A APPROXIMATES AN EVOLUTION OF THE MANTLE WITH PROGRESSIVE DEPLETION IN Rb/Sr RATIO. IN GEOLOGICAL TERMS, THE TWO ALTERNATIVES REPRESENT INSTANTANEOUS VERSUS PROGRESSIVE GENERATION OF CONTINENTAL CRUST

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