Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

The composition of the Earth

Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization

The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle

The long-standing problem of the excess abundances of siderophile elements in the mantle can be resolved by considering an equilibrium core-mantle differentiation in the earth at 3000 to 3500 kelvin. This high-temperature differentiation results in mantle siderophile element abundances that closely match the observed values. Some lithophile (light) elements could enter the core in this process as is necessary to account for its low density. The abundances of siderophile elements in the mantle are consistent with the conclusion derived from the recent physical models that the earth was molten during accretion.

https://science.sciencemag.org/content/sci/253/5017/303.full.pdf

Early differentiation of the Earth and the problem of mantle siderophile elements: a new approach.

The hypothesis that 40K may be a significant radioactive heat source in the Earth's core was proposed on theoretical grounds1,2 over three decades ago, but experiments3,4,5,6,7,8 have provided only ambiguous and contradictory evidence for the solubility of potassium in iron-rich alloys. The existence of such radioactive heat in the core would have important implications for our understanding of the thermal evolution of the Earth and global processes such as the generation of the geomagnetic field, the core–mantle boundary heat flux and the time of formation of the inner core9,10,11,12. Here we provide experimental evidence to show that the ambiguous results obtained from earlier experiments are probably due to previously unrecognized experimental and analytical difficulties. The high-pressure, high-temperature data presented here show conclusively that potassium enters iron sulphide melts in a strongly temperature-dependent fashion and that 40K can serve as a substantial heat source in the cores of the Earth and Mars.

Experimental evidence that potassium is a substantial radioactive heat source in planetary cores.

Nd and Sr isotope compositions of diopsides from micaceous garnet Iherzolite nodules found in kimberlite pipes at Bultfontein and Kimberley, South Africa are reported. The diopside data extend the mantle array, as defined by uncontaminated oceanic and continental basalts, to radiogenic 87Sr/86Sr values. These data are used to demonstrate the ancient enriched, grossly heterogeneous nature of the sub-continental mantle, which complements the predominantly depleted nature of the sub-oceanic mantle.

V. Rama Murthy

Papers

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle

Early differentiation of the Earth and the problem of mantle siderophile elements: a new approach.

Experimental evidence that potassium is a substantial radioactive heat source in planetary cores.

Enriched mantle: Nd and Sr isotopes in diopsides from kimberlite nodules

A Nd isotopic study of the Kerguelen Islands - Inferences on enriched oceanic mantle sources