Regional averages of the major element chemistry of ocean ridge basalts, corrected for low-pressure fractionation, correlate with regional averages of axial depth for the global system of ocean ridges, including hot spots, cold spots, and back arc basins, as well as “normal” ocean ridges. Quantitative consideration of the variations of each major element during melting of the mantle suggests that the global major element variations can be accounted for by ∼8–20% melting of the mantle at associated mean pressures of 5–16 kbar. The lowest extents of melting occur at shallowest depths in the mantle and are associated with the deepest ocean ridges. Calculated mean primary magmas show a range in composition from 10 to 15 wt % MgO, and the primary magma compositions correlate with depth. Data for Sm, Yb, Sc, and Ni are consistent with the major elements, but highly incompatible elements show more complicated behavior. In addition, some hot spots have anomalous chemistry, suggesting major element heterogeneity. Thermal modeling of mantle ascending adiabatically beneath the ridge is consistent with the chemical data and melting calculations, provided the melt is tapped from throughout the ascending mantle column. The thermal modeling independently predicts the observed relationships among basalt chemistry, ridge depth, and crustal thickness resulting from temperature variations in the mantle. Beneath the shallowest and deepest ridge axes, temperature differences of approximately 250°C in the subsolidus mantle are required to account for the global systematics.

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Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness

Mantle plumes from ancient oceanic crust

A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near-fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America-Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n = 0.002–0.05); depletion of Ti (300–1600 ppm), Zr (0.1–10 ppm), and Sr (0.1–10 ppm); and fractionation of Zr relative to Ti (Ti/Zr = 250–4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in ≤0.1% increments to a total of 5–25% melting, a process very close to Rayleigh (fractional) melting; batch melting of a LREE-depleted source cannot account for the observed trace element concentrations in abyssal peridotites. The shapes of some REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Trace element concentrations in calculated integrated fractional liquids approximate the composition of primitive ocean floor basalts, consistent with postsegregation aggregation of small increment melts produced over a depth and melting interval.

http://www.mantleplumes.org/WebDocuments/Johnson1990.pdf

Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites

A general mixing equation with applications to Icelandic basalts

Heterogeneous mantle domains: signatures, genesis and mixing chronologies

I. The Geochemistry of Rubidium and Strontium.- 1. Introduction.- 2. Rubidium.- 3. Strontium.- 4. The Rubidium/Strontium Ratio.- 5. Summary.- II. Measurement of Geologic Time by the Rubidium-Strontium Method.- 1. Introduction.- 2. Radioactive Decay of 87Rb to 87Sr.- 3. The Half-life of 87Rb.- 4. The Growth of Radiogenic 87Sr in Rocks and Minerals.- 5. The Rubidium-Strontium Isochron Method of Dating.- 6. A Useful Approximation.- 7. Analytical Methods.- 8. Summary.- III. Uses of Strontium Isotopes in Petrogenesis.- 1. Introduction.- 2. Evolution of Strontium in the Continental Crust and the Upper Mantle.- 3. Applications to Petrology.- 4. Summary.- IV. Volcanic Rocks.- 1. Introduction.- 2. The Strontium Isotopic Data for Volcanic Rocks.- a) General Comment.- b) Basaltic Rocks from Oceanic Islands.- c) Sea-Floor Basalts.- d) Andesites of Volcanic Arcs.- e) Antarctic and Tasmanian Dolerites.- 3. Variations in the Initial 87Sr/86Sr Ratio among and within Geologic Provinces.- a) Oceanic Islands.- b) Continental Volcanic Rocks.- 4. Possible Causes of Variations in Isotopic Composition among Cogenetic Rocks.- 5. Summary.- V. Granitic Rocks.- 1. Introduction.- 2. The Granite Problem.- 3. Observed Initial 87Sr/86Sr Ratios of Granitic Rocks.- 4. Interpretation and Discussion of Initial 87Sr/86Sr Ratios of Granites.- a) Granites with Initial Ratios within the Basalt Field.- The Salisbury Pluton, North Carolina.- b) Granites with High Initial 87Sr/86Sr Ratios.- The Heemskirk Granite, Western Tasmania.- c) Granites with Intermediate Initial 87Sr/86Sr Ratios.- The Isle of Skye.- Phanerozoic Batholiths of North America.- 5. Summary.- VI. Alkalic Rocks and Carbonatites.- 1. Introduction.- 2. Carbonatites.- 3. Alkalic Rocks.- a) Potassic Rocks.- b) Sodic Rocks.- c) Comparison of North American and African Alkalic Rocks.- 4. Summary.- VII. Ultramafic and Related Rocks.- 1. Introduction.- 2. 87Sr/86Sr Ratios of Ultramafic Rocks.- a) The Layered Gabbro-Norite-Peridotite Association in Major Intrusions.- Anorthosites.- b) The Alpine-Type Peridotite-Serpentinite Association.- c) Ultramafic Rocks in Differentiated Basic Sills and in Minor Intrusions.- d) Kimberlites.- e) Ultramafic Nodules.- 3. Summary.- VIII. Sedimentary Rocks and the Oceans.- 1. Introduction.- 2. Marine Carbonate Rocks and the Oceans.- 3. Marine Evaporites.- 4. Dating of Detrital Sedimentary Rocks.- 5. Glauconite.- 6. Nonmarine Carbonate Rocks.- 7. Summary.- IX. Isotopic Homogenization of Strontium in Open Systems.- 1. Introduction.- 2. Contact Metamorphism.- 3. Regional Metamorphism.- 4. Open-System Behavior of Total-Rock Samples.- 5. Chemical Weathering.- 6. Summary.- X. Meteorites.- 1. Introduction.- 2. Rubidium and Strontium in Stony Meteorites.- 3. The Ages of Stony and Iron Meteorites.- 4. The Initial 87Sr/86Sr Ratios of Stony Meteorites.- 5. Summary.- XI. The Moon.- 1. Introduction.- 2. Lunar Basalt.- 3. Initial 87Sr/86Sr Ratios.- 4. Luny Rock 1 and Sample 12013.- 5. The Soil.- 6. Luna 16.- 7. Summary.- XII. The Evolution of the Isotopic Composition of Terrestrial Strontium.- 1. Introduction.- 2. Evolution of Strontium in the Mantle.- 3. Initial 87Sr/86Sr Ratios of Gabbro and Basalt.- 4. Continental Growth.- 5. Summary.- References for Appendix.- Author Index.

Strontium Isotope Geology

Strontium isotope ratios in basalts demonstrate two distinct mantle sources under Iceland and the Reykjanes Ridge.

Basalts from Iceland and Along the Reykjanes Ridge: Sr Isotope Geochemistry

Carbonatites discovered in south-west Asia have typical trace elements, but some have unusually high ratios of strontium-87 to strontium-86. Related hydrothermal fluorites have quite different strontium isotope ratios and no distinctive trace elements, emphasizing difficulties in identifying ultimate sources of fluorites.

Trace Elements and Strontium Isotopes in Carbonatites, Fluorites and Limestones from India and Pakistan

THE Kibarides, characterized by radiometric ages in the range 1,300–800 m.y., were first recognized in the Congo Craton1,2 of central and eastern Africa where the shield has not been overprinted by the 600 ± 150 m.y. Pan-African thermotectonic event. They comprise the Kibara and Karagwe-Ankolean metasedimentary cover and older reacted crystalline basement rocks and major events in the Kibaran orogeny have been shown to have occurred between 1,300 and 1,180 m.y. ago3,4. In southern Africa, within the Kalahari Craton1,2, a second cratonic block with respect to the Pan-African event, similar age patterns have been found in the Namaqualand and Natal belts and although these belts are dominantly granitic and gneissic in character and lack metasedimentary sequences equivalent to the Kibara and Karagwe-Ankolean Groups, they have long been considered to be synchronous with the Kibarides5.

Kibaran Metamorphic Belt in Pan-African Domain of West Africa?

The ultramafic rocks are composed mainly of dark-colored mafic or ferro-magnesian minerals such as olivine, pyroxene, amphibole, serpentine, garnet, biotite, and opaque oxides. They are usually, but not always, ultra-basic (having less than 45 percent SiO2). Ultramafic rocks have been comprehensively reviewed in a volume edited by Wyllie (1967), who recognizes 10 ultramafic rock associations. 87Sr/86Sr ratio measurements are available for the following: (1) the layered gabbro-norite-peridotite association in major intrusions, (2) the alpine-type peridotite-serpentinite association,(3) ultrmafic rocks in differentiated basic sills and in minor intrusions,(4) kimberlites, (5) ultrmafic nodules, (6) alkali ultrabasic rocks in ring complexes, and (7) ultrabasic lavas. In the following sections of this chapter we shall review the strontium isotopic data for the first five of these associations; the last two have been covered in Chapter VI.

James L. Powell

Papers

Strontium Isotope Geology

Basalts from Iceland and Along the Reykjanes Ridge: Sr Isotope Geochemistry

Trace Elements and Strontium Isotopes in Carbonatites, Fluorites and Limestones from India and Pakistan

Kibaran Metamorphic Belt in Pan-African Domain of West Africa?

Ultramafic and Related Rocks