The mantle isotopic printer: Basic mantle plume geochemistry for seismologists and geodynamicists

High-temperature geochemistry combined with igneous petrology is an essential tool to infer the conditions of magma generation and evolution in the Earth's interior. During the past thirty years, a large number of geochemical models of the Earth, essentially inferred from the isotopic composition of basaltic rocks, have been proposed. These geochemical models have paid little attention to basic physics concepts, broadband seismology, or geological evidence, with the effect of producing results that are constrained more by assumptions than by data or first principles. This may not be evident to seismologists and geodynamicists. A common view in igneous petrology, seismology, and mantle modeling is that isotope geochemistry (e.g., the Rb-Sr, Sm-Nd, U-Th-Pb, U-Th-He, Re-Os, Lu-Hf, and other less commonly used systems) has the power to identify physical regions in the mantle, their depths, their rheological behavior, and the thermal conditions of magma generation. We demonstrate the fallacy of this approach and the model-dependent conclusions that emerge from unconstrained or poorly constrained geochemical models that do not consider physics, seismology (other than teleseismic travel-time tomography and particularly compelling colored mantle cross sections), and geology. Our view may be compared with computer printers. These can reproduce the entire range of colors using a limited number of basic colors (black, magenta, yellow, and cyan). Similarly, the isotopic composition of oceanic basalts and nearly all their primitive continental counterparts can be expressed in terms of a few mantle end members. The four most important (actually “most extreme”, because some are extraordinarily rare) mantle end members identified by isotope geochemists are DMM or DUM (depleted MORB [mid-ocean-ridge basalt] mantle or depleted upper mantle), HIMU (high mu, where mu = μ = ^(238)U/^(204)Pb), EMI, and EMII (enriched mantle type I and type II). Other mantle end members, or components, have been proposed in the geochemical literature (e.g., PHeM, FOZO, LVC, PreMa, EMIII, CMR, LOMU, and C), but these can be considered to be less extreme components or mixtures in the geochemical mantle zoo. Assuming the existence of these extreme "colors" in the mantle isotopic printer, the only matter for debate is their location in the Earth's interior. At least three of them need long-term insulation from convection-driven homogenization or mixing processes. In other words, where these extreme isotopic end members are located needs to be defined. In our view, no geochemical, geological, geophysical, or physical arguments require the derivation of any magma from deep mantle sources. Arguments to the contrary are assumption based. The HIMU, EMI, and EMII end members can be entirely located in the shallow non-convecting volume of the mantle, while the fourth, which is by far the more abundant volumetrically (DMM or DUM), can reside in the transition zone. This view is inverted compared with current canonical geochemical views of the Earth's mantle, where the shallowest portions are assumed to be DMM like (ambient mantle) and the EMI-EMII-HIMU end members are assumed to be isolated, located in the deep mantle, and associated with thermal anomalies. We argue that the ancient, depleted signatures of DMM imply long-term isolation from recycling and crustal contamination while the enriched components are not free of contamination by shallow materials and can therefore be shallow

Geochemical characteristics and mantle sources of the Oligo-Miocene primitive basalts from Sardinia: The role of subduction components.

During the Oligo-Miocene, the Island of Sardinia was covered by the products of voluminous magmatic activity, with a typical subduction-related signature. The mafic rocks of the Montresta (north) and Arcuentu (south) volcanic districts include primitive high MgO basalts whose trace element and Sr-, Nd- and Pb-isotope compositions constrain the nature and role of subduction-related components in the Tertiary Sardinian volcanism. The geochemical and isotopic data require an approximate degree of partial melting of 15% of a MORB-like depleted mantle prior to enrichment, and the input of two subduction components in the mantle wedge consisting of fluids from subducted oceanic crust (altered MORB) and fluids from subducted sediments. Ratios among trace elements which are variably compatible with fluid and melt phases (i.e. Th/Pb, Th/Nd and Sr/Nd) exclude the contribution of melts from the subducted slab. Models based on isotopic ratios indicate that the pre-subduction depleted mantle source of Sardinia magmas was enriched by 0.1-0.5% MORB fluid and less than 0.1% sediment fluid. The geochemical and isotopic compositions of the Montresta volcanic rocks are homogeneous, whereas those of the Arcuentu show quite heterogeneous characters, suggesting variations in mantle source over the long time-span (about 13 Ma) of volcanic activity in this district

Volcanic activity from the neogene to the present evolution of the Western Mediterranean area. a review

The Neogene to Present geodynamic and magmatological evolution of the western Mediterranean area may be summarized as follows: 1) Paleocene to Present volcanic activity in the Massif Central, France, related to the presence of a mantle plume in the Alpine foreland; 2) roughly continuous W-NW subduction of the Adria Plate from the Oligocene under the southern European margin; 3) development of a subduction-related back-arc basin during the Oligocene (Ligurian-Provençal Basin); 4) split of the Sardinia-Corsica Block from the Provençal basement; 5) collapse of the Betic orogen, with rapid exhumation of deep crustal and mantle rocks and development of volcanic activity in SE Spain (~30-2 Ma); 6) subduction-related magmatism in Sardinia (28-15 Ma) and eastern Spain (24-15 Ma); 7) a mid-Miocene 'leap' in the subduction system, from the Ligurian-Provençal Basin to the Tyrrhenian Sea, with a shift from Hercynian to Alpine terrane overthrusts; 8) opening of the Tyrrhenian Sea as a back-arc basin; 9) Neogene-Quaternary eastward-moving distensive and compressive tectonic waves and coeval magmatism along the western and southern margins of the Italian peninsula (Tuscan, Roman and Campanian Provinces and Aeolian Islands); 10) volcanic activity in the Betic foreland (Calatrava Province, ~9-1 Ma); 11) Plio-Pleistocene development of rift systems and coeval magmatism in Sardinia, northern and eastern Sicily (Mt. Etna and Hyblean Mts.) and the Strait of Sicily. Intense volcanic activity accompanied the evolution of the last 30 Ma in the western and central Mediterranean, with a wide range of magmatic products which may be grouped into: a) oceanic floor basalts (Ligurian-Provençal Basin and Tyrrhenian bathyal plain with Magnaghi, Vavilov and Marsili seamounts), whose compositions vary from N-MORB to E-MORB and pure low-K calc-alkaline basalts; b) subalkaline series with both tholeiitic and calc-alkaline affinities; c) alkaline products with extreme compositions ranging from mildly alkaline types with sodic affinity to strongly SiO2-undersaturated with both sodic and potassic to ultrapotassic affinity, possibly including also carbonatitic lithotypes. The Sr-Nd-Pb isotopic compositions of these products comprise virtually all the most common worldwide reservoirs and testify to the extremely heterogeneous compositions of the mantle sources of this sector of European lithosphere/ asthenosphere system

Debated topics of modern igneous petrology

Progress made thanks to the great amount of high-quality geochemical and isotopic data gathered over the last twenty years has allowed important insights into igneous petrology (e.g., Continental Flood Basalt petrogenesis, mantle source characterization, geophysical models of mantle plume systems, primary melt compositions, isotopic systematics of crustal and mantle domains, etc.). This large dataset has also been used to relate the compositional characteristics of igneous rocks with specific tectonic settings and to infer the geodynamic processes involved. However, inferring a tectonic setting mainly on the basis of geochemical constraints may fail, because partial melts with substantial compositional differences can originate from the same source, and the same melts may have been generated in different tectonic settings . Moreover, geochemical characterization of the main mantle components is still hotly debated, even in terms of concepts such as asthenosphere and lithosphere. Asthenospheric mantle is quite often believed to be a geochemically homogeneous convecting domain, whereas the lithospheric one is thought to be a variably enriched, heterogeneous, non-convecting reservoir, capable of retaining geochemical and isotopic gradients for periods of time exceeding 2 Ga. These assumptions are clearly over-simplifications, particularly when relationships between physical and geochemical mantle characteristics are not properly constrained. What emerges from recent literature is the «misuse» of petrological concepts tending towards the most convenient explanations and «effectively shutting out the entire creative thought process of the human mind» (Sheth, 1 999). More appropriate use of petrological data is necessary to stimulate true scientific growth, especially as regards our knowledge of mantle-crust dynamics

What 'anorogenic' igneous rocks can tell us about the chemical composition of the upper mantle: case studies from the circum-Mediterranean area

The composition of the upper mantle bounded by the Canaries, Eastern Anatolia, Libya and Poland is indirectly investigated by means of the chemical composition of igneous rocks with 'anorogenic' geochemical characteristics emplaced during the Cenozoic. The relatively homogeneous composition of these products in terms of incompatible trace-element content and Sr-Nd-Pb isotopic composition is unexpected, considering the variable lithospheric structure of this large area and the different tectono-thermal histories of the various districts. In order to reconcile the geochemical characteristics with a statistical sampling model, it would be necessary to propose volumes of the enriched regions much lower than the sampling volumes for each volcano (that is, less than 10 cubic metres), or alternatively, efficient magma blending from larger areas. The data are consistent with a relatively well-stirred and mixed sub-lithospheric upper mantle, in the solid state, which is also hard to understand. This contrasts with the situation under oceans where magma blending from diverse sources and sampling theory can explain the compositional statistics

Volcanic activity in the western Mediterranean during the last 30 Ma

A strong volcanic activity accompany the last 30 Ma evolution of the western and central Mediterranean sea with wide range of compositions which can be grouped in: a) oceanic floor products (Ligurian-Provencal basin and Tyrrhenian sea bathial plain with Magnaghi, Vavilov and Marsili seamounts), whose compositions vary from N-MORB to E-MORB and pure low-K calcalkaline basalts; b) subalkaline series of both tholeiitic (OIB-like) and calcalkaline affinity; c) alkaline products with the most extreme compositional variability ranging from mildly alkaline types with sodic affinity to strongly SiO2-undersaturated with both sodic and potassic to ultrapotassic affinity, exotic compositions such as lamproites and kamafugites, and also quartz-saturated peralkaline trachytes and rhyolites. The Sr-Nd-Pb isotopic compositions of these products comprise virtually all the most common worldwide reservoirs and testify the extreme heterogeneous compositions of the mantle sources of this sector of European lithosphere/asthenosphere system