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

Mangrove forests are one of the world's most threatened tropical ecosystems with global loss exceeding 35% (ref 1) Juvenile coral reef fish often inhabit mangroves, but the importance of these nurseries to reef fish population dynamics has not been quantified Indeed, mangroves might be expected to have negligible influence on reef fish communities: juvenile fish can inhabit alternative habitats and fish populations may be regulated by other limiting factors such as larval supply or fishing Here we show that mangroves are unexpectedly important, serving as an intermediate nursery habitat that may increase the survivorship of young fish Mangroves in the Caribbean strongly influence the community structure of fish on neighbouring coral reefs In addition, the biomass of several commercially important species is more than doubled when adult habitat is connected to mangroves The largest herbivorous fish in the Atlantic, Scarus guacamaia, has a functional dependency on mangroves and has suffered local extinction after mangrove removal Current rates of mangrove deforestation are likely to have severe deleterious consequences for the ecosystem function, fisheries productivity and resilience of reefs Conservation efforts should protect connected corridors of mangroves, seagrass beds and coral reefs

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Mangroves enhance the biomass of coral reef fish communities in the Caribbean.

Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system

Recycled oceanic crust, with or without sediment, is often invoked as a source component of continental and oceanic alkaline magmas to account for their trace-element and isotopic characteristics Alternatively, these features have been attributed to sources containing veined, metasomatized lithosphere In melting experiments on natural amphibole-rich veins at 15 gigapascals, we found that partial melts of metasomatic veins can reproduce key major- and trace-element features of oceanic and continental alkaline magmas Moreover, experiments with hornblendite plus lherzolite showed that reaction of melts of amphibole-rich veins with surrounding lherzolite can explain observed compositional trends from nephelinites to alkali olivine basalts We conclude that melting of metasomatized lithosphere is a viable alternative to models of alkaline basalt formation by melting of recycled oceanic crust with or without sediment

http://science.sciencemag.org/content/sci/320/5878/916.full.pdf

Metasomatized lithosphere and the origin of alkaline lavas.

Widespread basic magmatism across much of the western United States in the late Cenozoic followed the cessation of subduction along the Pacific coast. This volcanism accompanied lithospheric extension, block faulting and regional uplift In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO>4%) lava samples from all the major volcanic fields in the region. The data were divided into seven sets representing the main tectonomagmatic provinces: Basin and Range (BR), Western Great Basin, Transition Zone (TZ), Colorado Plateau, Snake River Plain, Southern Rocky Mountains, and Great Plains. It was further divided into relatively recent ( 5 Ma) subsets on the basis of field relations and K-Ar data. The 5 Ma) subset shows no great differences between the BR and the other tectonomagmatic provinces; all have high La/Nb and Ba/Nb. Crustal contamination alone cannot be responsible for these variations. We conclude that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. Pelagic sediment, returned to the mantle by subduction, is a possible agent for fluids rich in Ba, radiogenic Sr and unradiogenic Nd, but very poor in Nb. At least some of this enrichment must have accompanied the formation of the Proterozoic crust. It appears that subduction-enriched lithospheric mantle was involved in the generation of all extension-related basic magmas across the western United States until relatively recently. Only in the younger BR and parts of the TZ have asthenosphere-derived magmas, uncontaminated by lithosphere, reached the surface. These observations conflict with models in which uplift and extension are caused by the replacement of mantle lithosphere by asthenosphere. Triey are best explained by the progressive erosion of the lithospheric manue over a plume currently located beneath the Southern Rocky Mountains.

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Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time

Incompatible trace-elements in oib and morb and source enrichment in the sub-oceanic mantle

The Role of Lithospheric Mantle in the Generation of Late Cenozoic Basic Magmas in the Western United States

Incompatible trace elements in OIB and MORB and source enrichment in the sub-oceanic mantle.

Lavas from the eastern Virunga province, Rwanda, are dominated to lie within the continental mantle lithosphere (e.g. Fraser et al., 1986; Nelson et al., 1986; Dudas, 1991; by K-hawaiites, K-basanites and latites. All lavas are shoshonitic with 1 < K2O/Na2O < 2 and strongly enriched in incompatible Rogers et al., 1992; Gibson et al., 1995) of which they are a complementary sample to that provided by peridotite elements. Sr/Sr varies from 0·70586 in the K-basanites to 0·70990 in the latites, Nd/Nd from 0·51254 to 0·51206, xenoliths (e.g. Menzies et al., 1987; Jochum et al., 1989). In addition, their trace element and isotopic charand Pb isotopes define sub-vertical trends on isotope diagrams ( Pb/Pb 19·30–19·51, Pb/Pb 15·69–15·93 and acteristics can be used to infer the temporal evolution of the mantle lithosphere and the processes involved in Pb/Pb 40·28–41·5). Ar/Ar ages of leucite and phlogopite separates suggest that the latites are between 100 and 200 ka and lithosphere stabilization. Potassic magmas are also among the most compositionally extreme products of processes the K-basanites <100 ka. The latites are hybrid magmas produced by mixing between a K-basanite melt with a silicic melt from the that scavenge and fractionate incompatible elements in the upper mantle. Hence their geochemical variations deep crust. The low-silica K-basanites reflect interaction between a mafic K-basanitic melt with Nd/Nd ~0·51204, Sr/Sr can also be used to investigate the time-integrated effects of these processes on the radiogenic isotope evolution of ~0·707, and a nephelinite with Nd/Nd ~0·51267 and Sr/Sr ~0·7045. Both are derived from the mantle lithosphere the mantle lithosphere and to explore links between their lithospheric source regions and those of ocean-island with source ages of 1 Ga and 0·5 Ga, respectively, and the youngest ages correspond to the deepest magma sources. The magma production basalt (OIB) (McKenzie & O’Nions, 1983, 1995; Turner et al., 1996). rate in the Virunga is low (~0·04 km/yr), and reflects prolonged (10–15 My) heating of the lithosphere by the East African mantle The Virunga province of the western branch of the African Rift is a classic example of intra-plate potassic plume. magmatism, and the Ugandan part of the province was first described in remarkable detail by Holmes & Harwood (1937). They presented both excellent field and petrographic descriptions of mineralogically and

D. James

Papers

Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time

Incompatible trace-elements in oib and morb and source enrichment in the sub-oceanic mantle

The Role of Lithospheric Mantle in the Generation of Late Cenozoic Basic Magmas in the Western United States

Incompatible trace elements in OIB and MORB and source enrichment in the sub-oceanic mantle.

The Generation of Potassic Lavas from the Eastern Virunga Province, Rwanda