Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

The composition of the Earth

Volcanic arc magmas can be defined tectonically as magmas erupting from
volcanic edifices above subducting oceanic lithosphere. They form a coherent
magma type, characterized compositionally by their enrichment in large ion
lithophile (LlL) elements relative to high field strength (HFS) elements. In
terms of process, the predominant view is that the vast majority of volcanic arc
magmas originate by melting of the underlying mantle wedge, which contains
a component of aqueous fluid and/or melt derived from the subducting plate.
Recently, opinions have converged over the key aspects of the physical model
for magma generation above subduction zones (Davies & Stevenson 1992),
namely:
1. that the mantle wedge experiences subduction-induced corner flow (e.g.
Spiegelman & MacKenzie 1987);
2. that the subduction component reaches the fusible part of the mantle wedge
by the three-stage process of (i) metasomatism of mantle lithosphere, followed
by (ii) aqueous fluid release due to breakdown of hydrous minerals at
depth (e.g. Wyllie 1983, Tatsumi et al 1983) and (iii) aqueous fluid migration,
followed by hydrous melt migration, to the site of melting;
3. that slab-induced flow may be locally reversed beneath the arc itself, allowing
mantle decompression to contribute to melt generation (e.g. Ida 1983).
The simplified model in Figure 1 highlights the physical and chemical processes
that have been invoked as being important in controlling the composition of
volcanic arc magmas. Magma compositions (coupled with experimental data
on element behavior) can help us gain further understanding of these physical
and chemical processes. In this review, we first summarize knowledge of the
behavior of elements in the subduction system. We then focus on compositional
evidence for the processes illustrated in Figure 1, which we group as follows:
1. derivation of the subduction component,
2. transport of the subduction component to the melting column,
3. depletion and enrichment of the mantle wedge, and
4. processes in the melting column.

Tectonic Implications of the Composition of Volcanic Arc Magmas

A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantlederived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.

/pdf/the-genesis-of-intermediate-and-silicic-magmas-in-deep-3g95p0zv3p.pdf

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near-fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America-Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n = 0.002–0.05); depletion of Ti (300–1600 ppm), Zr (0.1–10 ppm), and Sr (0.1–10 ppm); and fractionation of Zr relative to Ti (Ti/Zr = 250–4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in ≤0.1% increments to a total of 5–25% melting, a process very close to Rayleigh (fractional) melting; batch melting of a LREE-depleted source cannot account for the observed trace element concentrations in abyssal peridotites. The shapes of some REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Trace element concentrations in calculated integrated fractional liquids approximate the composition of primitive ocean floor basalts, consistent with postsegregation aggregation of small increment melts produced over a depth and melting interval.

http://www.mantleplumes.org/WebDocuments/Johnson1990.pdf

Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites

As magmas rise toward the surface, they traverse regions of the mantle and crust with which they are not
in equilibrium; to the extent that time and the intimacy of their physical contact permit, the melts and
country rocks will interact chemically. We have modeled aspects of these chemical interactions in terms of
ion-exchange processes similar to those operating in simple chromatographic columns. The implications for
trace element systematics are straightforward: the composition of melt emerging from the top of the column
evolves from close to that of the incipient melt of the column matrix toward that of the melt introduced into
the base of the column. The rate of evolution is faster in the incompatible than the compatible elements and,
as a result, the abundance ratios of elements of different compatibilities can vary considerably with time. If
diffusion and other dispersive processes in the melt are negligible and if exchange between melt and solid
rock is rapid, extreme fractionations may occur, and the change from initial to final concentration for each
element can be through an abrupt concentration front. Integration and mixing of the column output in a
magma chamber or dispersive processes within the column, in particular the incomplete equilibration
between matrix and fluid due to the slow diffusion in the solid phases, may lead to diffuse fronts and
smooth trace element abundance patterns in the column output. If the matrix material is not replenished,
the chromatographic process is a transient phenomenon. In some geological situations (e.g., under island
arcs and oceanic islands), fresh matrix may be fed continuously into the column, leading to the evolution of
a steady state. Aspects of the geochemistry of ultramafic rocks, island arc lavas, and comagmatic alkaline
and tholeiitic magmas may be explained by the operation of chromatographic columns.

/pdf/geochemical-consequences-of-melt-percolation-the-upper-2pdcn0owtf.pdf

Geochemical Consequences of Melt Percolation: The Upper Mantle as a Chromatographic Column

Micro-inclusions in diamonds from Zaire and Botswana differ in composition from the more common large inclusions of the peridotitic or eclogitic assemblages. These sub-micrometre inclusions resemble potassic magmas in their composition, but are enriched in H_2O, CO^(2−)_3 and K_2O and depleted in MgO. This composition represents a volatile-rich fluid or melt from the upper mantle, which was trapped in the diamonds as they grew.

Oded Navon

Papers

Geochemical Consequences of Melt Percolation: The Upper Mantle as a Chromatographic Column

Mantle-derived fluids in diamond micro-inclusions

Oxygen isotope variations in phosphate of biogenic apatites, I. Fish bone apatite—rechecking the rules of the game

Bubble nucleation in rhyolitic melts: Experiments at high pressure, temperature, and water content

Hydrous and carbonatitic mantle fluids in fibrous diamonds from Jwaneng, Botswana