Strontium mantle evolution

Martelli M, Nuccio PM, Stuart FM, Burgess R, Ellam RM, Italiano F (2004) Helium-strontium isotope constraints on mantle evolution beneath the Roman Comagmatic Province, Italy. Earth Planet Sci Lett 224 295-308 Marty B, Trull T, Lussiez P, Basile I, Tanguy JC (1994) He, Ar, O, Sr andNd isotope constraints on the origin and evolution of Mount Etna magmatism. Earth Planet Sci Lett 126 23-39... 

A number of authors (e.g. Faure, 1977) have proposed on the basis of measured initial ratios that the growth of Sr/ Sr in the mantle with time defines a curvilinear path (cf. Figure 6.14, curves 1 and 2) and that this reflects the irreversible loss of Rb from the mantle into the crust during the formation of the continental crust. The loss of Rb from the mantle and its enrichment in the continental crust lead to very different patterns of strontium isotope evolution in the two reservoirs as a consequence of their different Rb/Sr ratios (Figure 6.15). The high Rb/Sr ratios found in the continental crust give rise to an accelerated... 

There are many examples in the literature of authors who have used mantle evolution diagrams of the type illustrated in Figure 6.15 to plot the initial strontium isotope ratios of measured samples relative to mantle and crustal evolution curves, in order to determine their likely source region. For example, it is easy to see from Figure 6.15 that a suite of rocks produced by partial melting of the mantie at 1.0 Ga wUl have a very different initial ratio ( Sr/ Sr = 0.7034) from rocks produced by partial melting of the crust at this time ( Sr/ Sr = 0.7140). It is this principle which may be used to identify the source of magmatic rocks of known age. 

The Sm/Nd ratio of the condnental crust is highly fractionated (<1.0) relative to CHUR and shows retarded Nd/ Nd evoludon with time (Figure 6.16). This is the opposite of the Rb-Sr system where the strontium-isotopic evolution of the crust shows accelerated evoludon with dme reladve to the mantle. 

Godderis, Y. and Francois, L.M. (1995) The Cenozoic evolution of the strontium and carbon cycles relative importance of continental erosion and mantle exchange. Chem. Geol., 126, 169-190. 

Although Sr/ Sr isotopic evolution for the mantle is only poorly constrained at present, this system has tremendous potential for revealing details of cmst-formation processes through time as well as for providing precise timescales on Earth formation. The test of this awaits determination of accurate initial strontium isotopic compositions from Precambrian samples. The potential for acquisition of this type of data is high, as advances in techniques including both in situ measurements (e.g., Christiensen et al., 1996) and low-level solution work (e.g., Mueller et al., 2000) now allow precise determination of isotopic compositions from extremely small amounts (<10 g) of strontium. This ability makes feasible analyses of rare, relict high-strontium, low-mbidium phases which may be preserved within ancient rocks and minerals and may accurately record mantle compositions. 

Halliday AN, Davidson JP, Holden P, DeWolf C, Lee D-C, Fitton JG (1990) Trace-element fractionation in plumes and the origin of HIMU mantle beneath the Cameroon line. Nature 347 523-528 Hamano Y, Ozima M (1978) Earth-atmosphere evolution model based on Ar isotopic data. In Alexander EC, Ozima M (eds) Terrestrial Rare Gases. Center for Academic Publications Japan, Tokyo, p 155-171 Hamelin B, Dupre B, Allegre C-J (1984) Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge a comparative study. Earth Planet Sci Lett 67 340-350 Hanan BB, Graham DW (1996) Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes. Science 272 991-995... 

Figure 6.H The evolution of strontium isotopes with time. The shaded held mdicates the evolution of the bulk earth om 4.6 Ga to the present-day value of between 0.7045 and 0.7052, reflecting Rb/Sr rados of between 0.03 and 0.032. The depleted mande curve (1) is nun Bell et al. (1982) and indicates the separation of a depleted mantle reservoir from the primitive mande at ca 2.8 Ga ben th die Superior Province, Canada. Curve (2) is from Ben Othman et a . (1984) and is based upon the data from MORB, ophiolites, komadites and meteorites. It represents the equation ( Sr/ Sr) - + Bt + C where A = —1.54985776 x 10 , B -...

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