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

[1] We present an estimate for the composition of the depleted mantle (DM), the source for mid-ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent-daughter ratios. These estimates form the “backbone” of the abundances of the trace elements that make up the Coryell-Masuda diagram (spider diagram). The remaining elements of the Coryell-Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low-degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2003GC000597

Composition of the depleted mantle

The composition of the primitive mantle derived here shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. These processes occurred at the initial stage of the solar system formation, under conditions thought to be present in the solar nebula. But the stable isotope record excludes chondritic meteorites as the ‘building blocks’ of Earth. Meteorites formed in local environments separated from that part of the inner solar system where much of the material forming the terrestrial planets was sourced.

https://booksite.elsevier.com/brochures/treatiseongeochemistry/contents/sample2.pdf

Cosmochemical Estimates of Mantle Composition

http://www.mantleplumes.org/WebDocuments/Courtillot2003.pdf

Three distinct types of hotspots in the Earth's mantle

A redetermination of the isotopic abundances of atmospheric Ar

https://www.ipgp.fr/~kaminski/web_doudoud/JavoyEA_EPSL_2010.pdf

The chemical composition of the Earth: Enstatite chondrite models

New experimental data on the isotopic variations of neon, argon, and xenon in a popping rock imply that the 40Ar/36Ar ratio of the upper mantle is less than 44,000 and that the 129Xe/130Xe ratio is less than 8.2. The elemental abundance pattern of rare gases is chondritic-like and is quite distinct from the solar pattern. These data imply that Earth accreted from planetesimals that probably underwent a transformation of their rare gas budget from solar- to chondritic-like, leaving the isotopic composition unchanged from the solar pattern.

https://science.sciencemag.org/content/sci/279/5354/1178.full.pdf

Rare Gas Systematics in Popping Rock: Isotopic and Elemental Compositions in the Upper Mantle

Helium signature of the subcontinental lithospheric mantle

Earth’s mantle is shown to display heterogeneous sulphur isotope ratios, with a depleted end-member that is not chondritic as has been thought; the mantle’s inferred composition can be accounted for by fractionation during core–mantle differentiation. The differentiation of early Earth's subsurface material into core and mantle layers should be reflected in the residual mantle composition, as most of the iron-loving elements, presumably including sulphur, would have been scavenged by the liquid core. However, previous analyses of Earth's mantle have identified stable sulphur isotope ratios resembling those seen in chondritic meteorites, perhaps the result of a 'late veneer' meteoritic origin for mantle material. But here Jabrane Labidi et al. provide evidence that the mantle displays heterogeneous sulphur isotope ratios directly correlated to strontium and neodymium isotope ratios. The authors conclude that these results can be reconciled by fractionation during core-mantle differentiation. Core–mantle differentiation is the largest event experienced by a growing planet during its early history. Terrestrial core segregation imprinted the residual mantle composition by scavenging siderophile (iron-loving) elements such as tungsten, cobalt and sulphur. Cosmochemical constraints suggest that about 97% of Earth’s sulphur should at present reside in the core1, which implies that the residual silicate mantle should exhibit fractionated 34S/32S ratios according to the relevant metal–silicate partition coefficients2, together with fractionated siderophile element abundances. However, Earth’s mantle has long been thought to be both homogeneous and chondritic for 34S/32S, similar to Canyon Diablo troilite3,4,5,6, as it is for most siderophile elements. This belief was consistent with a mantle sulphur budget dominated by late-accreted chondritic components. Here we show that the mantle, as sampled by mid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous 34S/32S ratios, directly correlated to the strontium and neodymium isotope ratios 87Sr/86Sr and 143Nd/144Nd. These isotope trends are compatible with binary mixing between a low-34S/32S ambient mantle and a high-34S/32S recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative δ34S of −1.28 ± 0.33‰ that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic 34S/32S ratio for the silicate Earth could be accounted for by a core–mantle differentiation record in which the core has a 34S/32S ratio slightly higher than that of chondrites (δ34S = +0.07‰). Despite evidence for late-veneer addition of siderophile elements (and therefore sulphur) after core formation, our results imply that the mantle sulphur budget retains fingerprints of core–mantle differentiation.

/pdf/non-chondritic-sulphur-isotope-composition-of-the-4w9gpavrws.pdf

Manuel Moreira

Papers

The chemical composition of the Earth: Enstatite chondrite models

Rare Gas Systematics in Popping Rock: Isotopic and Elemental Compositions in the Upper Mantle

Helium signature of the subcontinental lithospheric mantle

Non-chondritic sulphur isotope composition of the terrestrial mantle.

Helium and lead isotope geochemistry of the Azores Archipelago