Mantle plumes from ancient oceanic crust

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.

The genesis of basaltic magmas

Preface 1. Origin and history of the Solar System 2. Composition of the Earth 3. Radioactivity, isotopes and dating 4. Isotopic clues to the age and origin of the Solar System 5. Evidence of the Earth's evolutionary history 6. Rotation, figure of the Earth and gravity 7. Precession, wobble and rotational irregularities 8. Tides and the evolution of the lunar orbit 9. The satellite geoid, isostasy and post-glacial rebound 10. Elastic and inelastic properties 11. Deformation of the crust: rock mechanics 12. Tectonics 13. Convective and tectonic stresses 14. Kinematics of the earthquake process 15. Earthquake dynamics 16. Seismic wave propagation 17. Seismological determination of Earth structure 18. Finite strain and high pressure equations of state 19. Thermal properties 20. The surface heat flux 21. The global energy budget 22. Thermodynamics of convection 23. Thermal history 24. The geomagnetic field 25. Rock magnetism and paleomagnetism 26. Alternative energy sources and natural climate variations: some geophysical background Appendix A. General reference data Appendix B. Orbital dynamics (Kepler's laws) Appendix C. Spherical harmonic functions Appendix D. Relationships between elastic moduli of an isotropic solid Appendix E. Thermodynamic parameters and derivative properties Appendix F. An Earth model: mechanical properties Appendix G. A thermal model of the Earth Appendix H. Radioactive isotopes Appendix I. A geological time scale 2004 Appendix J. Problems References Index.

Physics of the Earth

Strontium isotope analysis of archaeological skeletons has provided useful and exciting results in archaeology in the last 20 years, particularly by characterizing past human migration and mobility. This review covers the biogeochemical background, including the origin of strontium isotope compositions in rocks, weathering and hydrologic cycles that transport strontium, and biopurification of strontium from to soils, to plants, to animals and finally into the human skeleton, which is subject to diagenesis after burial. Spatial heterogeneity and mixing relations must often be accounted for, rather than simply ``matching'' a measured strontium isotope value to a presumed single-valued geologic source. The successes, limitations and future potential of the strontium isotope technique are illustrated through case studies from geochemistry, biogeochemistry, ecology and archaeology.

/pdf/strontium-isotopes-from-the-earth-to-the-archaeological-2c7rwbxe6b.pdf

Strontium Isotopes from the Earth to the Archaeological Skeleton: A Review

This chapter is in part an update of a previous, more abbreviated review ( Hofmann, 1997 ) It covers the subject in greater depth, and it reflects some significant changes in the author's views since the writing of the earlier paper In particular, the spatial range of equilibrium attained during partial melting may be much smaller than previously thought, because of new experimental diffusion data and new results from natural settings Also, the question of ‘layered’ versus ‘whole-mantle’ convection, including the depth of subduction and of the origin of plumes, has to be reassessed in light of the recent breakthroughs achieved by seismic mantle tomography As the spatial resolution of seismic tomography and the pressure range, accuracy, and precision of experimental data on melting relations, phase transformations, and kinetics continue to improve, the interaction between these disciplines and geochemistry sensu stricto will continue to improve our understanding of what is actually going on in the mantle The established views of the mantle being engaged in simple two- or single-layer convection are becoming obsolete In many ways, we are just at the beginning of this new phase of mantle geology, geophysics, and geochemistry

Sampling mantle heterogeneity through oceanic basalts: Isotopes and trace elements

Rubidium and strontium are used as tracers in the history of differentiation of the continental sial, and for this purpose it is assumed that the Rb/Sr ratio follows the aggregate enrichment of sialic components sufficiently well to represent an approximately linear measure of the entire process. Measurements of the Sr87/Sr86 ratios in basalts and other igneous rocks of subsialic origin suggest that the source regions of sial have had a Sr87/Sr86 ratio in the range 0.705 to 0.710 in different areas and at different times in the span of geologic history. Measurements of the Rb/Sr ratio in sialic rocks of the continental crystalline basement have indicated that the weighted average value for this ratio is in the vicinity of 0.25. This means that the Sr87/Sr86 ratio in average sialic rocks of the continental basement would increase at a faster rate than that in the subsialic source regions, by an amount equal to approximately 0.010 per billion years. This is more than five times the standard deviation precision error in measurement. A comparison of the ratio Sr87/Sr86 with the geologic age of the rock, in samples of typical sialic basement rocks leads to the conclusion that there has been a continuous generation of primary sial from subsialic source regions that has caused the continental areas to grow roughly in proportion to the extent of the geological age provinces. In North America this areal growth is estimated to be at the average rate of about 7000 km2/m.y., and it seems to have been operative over most of geologic time.

A model is formulated in which the proportion and age of reworked material that is incorporated into new sialic basement are related to the Sr87/Sr86 development in the various materials involved. The model is generalized with all the parameters left open. An example set of values for the various parameters is tested on the model to indicate where there is need for more definitive data. This example set is derived from data available at present which, although very limited, indicate clearly that if the reworked material is young enough it can constitute a major proportion of a new sialic basement area, but if it is ancient (1 to 2 b.y. in primary age prior to incorporation) it could not be present in more than a minor proportion of the typical new basement.

Radiogenic strontium-87 model of continent formation

An examination of the Sr extracted from the calcareous shells of fresh-water mollusks from localities on the Precambrian shield of North America has revealed significant enrichment of Sr87 relative to primary basalt Sr. This enrichment had been predicted on the basis of the average Rb/Sr ratio and estimated age of the crystalline basement rocks. The average Sr87/Sr86 ratio for the calcareous shells, grouped according to the geologic age province in which they were found, are as follows: Kenoran age province (2.5 b.y), 0.721±0.005; Hudsonian age province (1.7 b.y.), 0.720±0.002; and Grenville age province (1.0 b.y.), 0.717±0.004. The isotopic composition of Sr in calcareous shells from the Great Lakes and the general drainage system of the St. Lawrence River seems to be in reasonable correlation with the bedrock geology of the drainage basin. No evidence of fractionation of Sr isotopes during incorporation in the calcareous shells was found.

An estimate of the isotopic composition of strontium in rocks of the Precambrian Shield of North America

Rb-Sr isotopic analyses of sixteen whole-rock samples from ten localities in the Monteregian igneous province, Quebec, show that the initial Sr87/Sr80 is 0.704, which is in essential agreement with our current estimate for oceanic and nonorogenic continental basalt. An isochron plot of nordmarkite and essexite from adjoining localities, Brome and Shefford, gives (Sr87/Sr80)0 = 0.7039 and an age of 124 m.y. (early Cretaceous). The former agrees with the over-all estimate; the latter is in approximate agreement with four of five K-Ar biotite ages but is significantly higher than five Rb-Sr biotite analyses. A radiogenic Sr87 development plot of biotites and whole rocks from Brome shows that the biotites from the co-magnetic nordmarkite and essexite have comparable ages (84.4 and 88.3 m.y., respectively), thus indicating a postcrystallization Sr diffusion episode late in the Cretaceous.

Initial Ratio of Strontium 87 to Strontium 86, Whole-Rock Age, and Discordant Biotite in the Monteregian Igneous Province, Quebec

The mean initial Sr87/Sr86 ratio of four volcanic rock specimens from Oahu which have low SiO2 concentrations is significantly lower than that of four specimens from Hawaii which have higher SiO2 contents. This difference may reflect the fact that the source regions of magmas under Oahu have lower Sr87/Sr86 ratios than those under Hawaii. Alternatively, it may indicate that the strongly undersaturated rocks of the Hawaiian Islands are formed independently of both tholeiite and alkali olivine basalt magma. The initial Sr87/Sr86 ratios of the individual specimens are (weight % SiO2 shown in parentheses): melilite-nepheline basalt I (36.3), 0.7030; melilite-nepheline basalt II (37.2), 0.7031; nepheline basalt (38.6), 0.7031; basanitoid (43.1), 0.7033; alkali olivine basalt (46.5), 0.7040; hawaiite (48.0), 0.7043; tholeiite (51.9), 0.7041; trachyte (62.0), 0.7043. With one exception corrections for radiogenic strontium were not required. The Sr87/Sr86 ratios are the means of duplicate and triplicate analyses and their standard deviation is estimated to be ±0.0004. All Sr87/Sr86 ratios are normalized to Sr86/Sr88 = 0.1194. A value of 0.7073 was obtained for the Sr87/Sr86 ratio of the Eimer and Amend interlaboratory strontium standard.

G. Faure

Papers

Radiogenic strontium-87 model of continent formation

An estimate of the isotopic composition of strontium in rocks of the Precambrian Shield of North America

Initial Ratio of Strontium 87 to Strontium 86, Whole-Rock Age, and Discordant Biotite in the Monteregian Igneous Province, Quebec

Strontium 87 abundance in a suite of Hawaiian volcanic rocks of varying silica content