The composition of the Earth

Basaltic volcanism 'samples' the Earth's mantle to great depths, because solid-state convection transports deep material into the (shallow) melting region. The isotopic and trace-element chemistry of these basalts shows that the mantle contains several isotopically and chemically distinct components, which reflect its global evolution. This evolution is characterized by upper-mantle depletion of many trace elements, possible replenishment from the deeper, less depleted mantle, and the recycling of oceanic crust and lithosphere, but of only small amounts of continental material.

Mantle geochemistry: the message from oceanic volcanism

The Lu–Hf and Sm–Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets

The early Precambrian odyssey of the North China Craton: A synoptic overview

Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate convergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength heterogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

/pdf/evidence-for-deep-mantle-circulation-from-global-tomography-3unqcjsa05.pdf

Evidence for deep mantle circulation from global tomography

[1] Unlike in the ocean basins where the shallow mantle eventually contributes to the destruction of the overlying crust, the shallow mantle beneath continents serves as a stiff, buoyant “root” whose presence may be essential to the long-term survival of continental crust at Earth's surface. These distinct roles for subcrustal mantle come about because the subcontinental mantle consists of a thick section of material left behind after extensive partial melt extraction, possibly from the wedge of mantle overlying a subducting oceanic plate. Melt removal causes the continental mantle to be cold and strong but also buoyant compared to oceanic mantle. These characteristics allow thick sections of cold mantle to persist beneath continental crust in some cases for over 3 billion years. If the continental mantle becomes gravitationally unstable, however, its detachment from the overlying crust can cause major episodes of intracontinental deformation and volcanism.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004RG000156

Physical, chemical, and chronological characteristics of continental mantle

Isotopic variability in barium, neodymium, and samarium in carbonaceous chondrites reflects the distinct stellar nucleosynthetic contributions to the early solar system. We used 148Nd/144Nd to correct for the observed s-process deficiency, which produced a chondrite 146Sm-142Nd isochron consistent with previous estimates of the initial solar system abundance of 146Sm and a 142Nd/144Nd at average chondrite Sm/Nd ratio that is lower than that measured in terrestrial rocks by 21 ± 3 parts per million. This result strengthens the conclusion that the deficiency in 142Nd in chondrites relative to terrestrial rocks reflects 146Sm decayand earlyplanetary differentiation processes.

https://science.sciencemag.org/content/sci/316/5828/1175.full.pdf

Chondrite Barium, Neodymium, and Samarium Isotopic Heterogeneity and Early Earth Differentiation

Introduction and Overview 1. Cosmochemical Estimates of Mantle Composition 2. Seismology Constraints upon Mantle Composition 3. Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements 4. Orogenic, Ophiolitic, and Abyssal Peridotites 5. Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds 6. Noble Gases as Mantle Tracers 7. Mantle Volatiles - Distribution and Consequences 8. Melt Extraction and Compositional Variability in Mantle Lithosphere 9. Trace Element Partitioning under Crystal and Uppermost Mantle Conditions: The Influences of Ionic Radius, Cation Charge, Pressure and Temperature 10. Partition Coefficients at High Pressure and Temperature 11. Subduction Zone Processes and Implications for Changing Composition of the Upper and Lower Mantle 12. Convective Mixing in the Earth's Mantle 13. Compositional Evolution of the Mantle 14. Experimental Constraints on Core Composition 15. Compositional Models for the Earth's Core

The mantle and core

New Rb-Sr, 146,147Sm-142,143Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, 147Sm-143Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial 146Sm/144Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for 146Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd—142Nd/144Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2013.0246

Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation

[1] Chemical and radiogenic isotope (Sr, Nd, Hf, Os, and Pb) data are presented for a variety of mafic-alkalic rocks from the Maymecha-Kotuy section of the Siberian flood-volcanic province. These data are compared to a similar data set for Siberian kimberlites that were emplaced both before and after the flood-volcanic event in order to examine the spatial-temporal evolution of Paleozoic magma sources in the mantle beneath this site of voluminous magmatic activity. As shown in previous studies, the high-Mg, meimechitic composition rocks extend the range in Sr and Nd isotopic composition seen in the flood basalts in the direction of more “depleted” compositions, i.e., higher 143Nd/144Nd and lower 87Sr/86Sr, overlapping values typically observed in intraplate ocean-island basalts. Sr, Nd, Hf, and Pb isotopic compositions show little correlation with major- and trace-element compositions in the Maymecha-Kotuy rocks. Os isotopic compositions, on the other hand, show rough correlations with a number of major-element characteristics of the magmas. The Os data suggest that the magma sources range from peridotite for the meimechitic magmas to a mixture of peridotite and pyroxenite for the nephelinitic, melilititic, and trachybasaltic compositions. The isotopic overlap of both old and young kimberlites with the Maymecha-Kotuy rocks is consistent with all these magmas being derived from mantle sources that were present beneath Siberia long before, and long after, the flood-volcanic event. The isotopic characteristics of the mantle source of these magmas best match the FOZO component observed in ocean-island basalts, which suggests that this mantle composition may be prevalent in the upper mantle outside of ocean basins. The long-lived presence of this source beneath Siberia makes it unnecessary to appeal to a mechanism, such as a plume, to bring this type of mantle into play only during the flood-volcanic episode.

http://www.mantleplumes.org/WebDocuments/Carlsonetal2006.pdf

Richard W. Carlson

Papers

Physical, chemical, and chronological characteristics of continental mantle

Chondrite Barium, Neodymium, and Samarium Isotopic Heterogeneity and Early Earth Differentiation

The mantle and core

Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation

A comparison of Siberian meimechites and kimberlites: Implications for the source of high-Mg alkalic magmas and flood basalts