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

We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near-constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid-oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U-230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.

/pdf/element-transport-from-slab-to-volcanic-front-at-the-mariana-2lo62mj4d2.pdf

Element transport from slab to volcanic front at the Mariana arc

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

Element transport from slab to volcanic front at the Mariana Arc

Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites

ATTEMPTS to understand the formation and evolution of the sub-continental lithospheric mantle (SCLM) have been hampered by the absence of reliable time constraints, reflecting a lack of appropriate isotopic dating techniques The most commonly used methods, involving strontium, neodymium and lead isotopes, yield ambiguous results in mantle rocks, and show no relationship with magmatic processes, as the low concentrations of these elements make them susceptible to later metasomatic disturbance Osmium, by contrast, is much more abundant in the mantle than in the crust1, so that peridotite Os isotope ratios are largely immune to recent metasomatic imprints This provides a way to date the magmatic processes that determine mantle major-element compositions2 We present here two examples of striking correlations between 187Os/188Os and Al2O3concentration in orogenic peridotites, and argue that these can be used to date the differentiation of the SCLM The old ages obtained agree with associated lower-crustal Nd model ages3–5, and indicate that―in these post-Archaean terrains as well as in Archaean cratons2,6,7―SCLM can remain isolated from the convecting mantle for more than a billion years

Longevity of sub-continental mantle lithosphere from osmium isotope systematics in orogenic peridotite massifs

Os isotope systematics in ocean island basalts

Isotope anomalies provide important information about early solar system evolution. Here we report molybdenum isotope abundances determined in samples of various meteorite classes. There is no fractionation of molybdenum isotopes in our sample set within 0.1‰ and no contribution from the extinct radionuclide 97Tc at mass 97 (97Tc/92Mo < 3 × 10-6). Instead, we observe clear anomalies in bulk iron meteorites, mesosiderites, pallasites, and chondrites characterized by a coupled excess in p- and r-process or a mirror deficit in s-process nuclides (Mo-w). This large-scale isotope heterogeneity of the solar system observed for molybdenum must have been inherited from the interstellar environment where the Sun was born, illustrating the concept of "cosmic chemical memory." The presence of molybdenum anomalies is used to discuss the filiation between planetesimals.

https://iopscience.iop.org/article/10.1086/324597/pdf

Laurie Reisberg

Papers

Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites

Longevity of sub-continental mantle lithosphere from osmium isotope systematics in orogenic peridotite massifs

Os isotope systematics in ocean island basalts

Molybdenum Evidence for Inherited Planetary Scale Isotope Heterogeneity of the Protosolar Nebula

The ReOs systematics of the Ronda Ultramafic Complex of southern Spain