Geochemical data have been interpreted as requiring that a significant fraction of the melting in MORB source regions takes place in the garnet peridotite field, an inference that places the onset of melting at ≥80 km. However, if melting begins at such great depths, most models for melting of the suboceanic mantle predict substantially more melting than that required to produce the 7 ± 1 km thickness of crust at normal ridges. One possible resolution of this conflict is that MORBs are produced by melting of mixed garnet pyroxenite/spinel peridotite sources and that some or all of the “garnet signature” in MORB is contributed by partial melting of garnet pyroxenite layers or veins, rather than from partial melting of garnet peridotite. Pyroxenite layers or veins in peridotite will contribute disproportionately to melt production relative to their abundance, because partial melts of pyroxenite will be extracted from a larger part of the source region than peridotite partial melts (because the solidus of pyroxenite is at lower temperature than that of peridotite and is encountered along an adiabat 15–25 km deeper than the solidus of peridotite), and because melt productivity from pyroxenite during upwelling is expected to be greater than that from peridotite (pyroxenite melt productivity will be particularly high in the region before peridotite begins melting, owing to heating from the enclosing peridotite). For reasonable estimates of pyroxenite and peridotite melt productivities, 15–20% of the melt derived from a source region composed of 5% pyroxenite and 95% peridotite will come from the pyroxenite. Most significantly, garnet persists on the solidus of pyroxenite to much lower pressures than those at which it is present on the solidus of peridotite, so if pyroxenite is present in MORB source regions, it will probably contribute a garnet signature to MORB even if melting only occurs at pressures at which the peridotite is in the spinel stability field. Partial melting of a mixed spinel peridotite/garnet pyroxenite mantle containing a few to several percent pyroxenite can explain quantitatively many of the geochemical features of MORB that have been attributed to the onset of melting in the stability field of garnet lherzolite, provided that the pyroxenite compositions are similar to the average composition of mantle-derived pyroxene-rich rocks worldwide or to reasonable estimates of the composition of subducted oceanic crust. Sm/Yb ratios of average MORB from regions of typical crustal thickness are difficult to reconcile with derivation by melting of spinel peridotite only, but can be explained if MORB sources contain ∼5% garnet pyroxenite. Relative to melting of spinel peridotite alone, participation of model pyroxenite in melting lowers aggregate melt Lu/Hf without changing Sm/Nd ratios appreciably. Lu/Hf-Sm/Nd systematics of most MORB can be accounted for by melting of a spinel peridotite/garnet pyroxenite mantle provided that the source region contains 3–6% pyroxenite with ≥20% modal garnet. However, Lu/Hf-Sm/Nd systematics of some MORB appear to require more complex melting regimes and/or significant isotopic heterogeneity in the source. Another feature of the MORB garnet signature, (^(230)Th)/(^(238)U)>1, can also be produced under these conditions, although the magnitude of (^(230)Th)/(^(238)U) enrichment will depend on the rate of melt production when the pyroxenite first encounters the solidus, which is not well-constrained. Preservation of high (^(230)Th)/(^(238)U) in aggregated melts of mixed spinel peridotite/garnet pyroxenite MORB sources is most likely if the pyroxenites have U concentrations similar to that expected in subducted oceanic crust or to pyroxenite from alpine massifs and xenoliths. The abundances of pyroxenite in a mixed source that are required to explain MORB Sm/Yb, Lu/Hf, and (^(230)Th)/(^(238)U) are all similar. If pyroxenite is an important source of garnet signatures in MORB, then geochemical indicators of pyroxenite in MORB source regions, such as increased trace element and isotopic variability or more radiogenic Pb or Os, should correlate with the strength of the garnet signature. Garnet signatures originating from melts of the garnet pyroxenite components of mixed spinel peridotite/garnet pyroxenite sources would also be expected to be stronger in regions of thin crust.

A possible role for garnet pyroxenite in the origin of the “garnet signature” in MORB

Like residual peridotites from mid-ocean ridges, peridotites from the mantle section of the Oman ophiolite are far from equilibrium with mid-ocean-ridge basalt (MORB). By contrast, dunites from Oman are close to equilibrium with MORB, indicating that they were conduits for focused melt flow. Formation of dunite conduits by porous flow is sufficient to explain extraction of MORB from the mantle, and fracture mechanisms may not be necessary in this process.

Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels

We review physical and chemical constraints on the mechanisms of melt extraction from the mantle beneath midocean ridges. Compositional constraints from MORB and abyssal peridotite are summarized, ...

A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges

Many mantle peridotite samples are too rich in Si02 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

Osmium isotopic compositions of mantle xenoliths: A global perspective

Detailed study of a 65 cm harzburgite section perpendicular to an amphibole pyroxenite vein from the Lherz massif reveals a strong mineralogical and chemical zonation with distance from the vein-host boundary. At less than ~ 20 cm, the host peridotite is modally metasomatized and displays patterns of increasing Fe, Ti, Mn, Al, Ca, Na, and HREE, and decreasing Mg and Ni toward the vein contact This zone is also relatively impoverished in Cr but is enriched in K and Sr. It is characterized by relatively unfractionated, mainly convex-upward, chondrite-normalized REE patterns. At a distance over ~ 20 cm, the host peridotite displays the typical feature of cryptic metasomatism, i.e., selective LREE enrichment in otherwise anhydrous mineralogy. The chondrite-normalized REE patterns vary from U-shaped in the range 15—25 cm to strongly fractionated in the range 25-65 cm. These variations encompass the whole range reported from metasomatized peridotite nodules in alkali basalts. They may be accounted for by a single, silicate-melt, metasomatic event associated with infiltration of the Pyrenean alkali basalts into the most refractory peridotites, during their ascent through the subcontinental lithosphere, ~ 100 Ma ago. The proposed model involves a chemical evolution of the infiltrated melt with increasing distance in the host. At < 15-20 cm, the melt composition would be strongly influenced by the proximity of the vein conduit, because of the existence of advective chemical fluxes through grain boundaries (short-range porous flow; distance of percolation < 1 m) and into small branching cracks, and a possibly dominant diffusive flux within the infiltrated melt. This may explain the reactivity of the melt towards the anhydrous peridotite mineralogy, the existence of chemical gradients for most elements, and the lack of REE chromatographic fractionation. At a distance of > 20-25 cm, chemical exchange with the conduit would be negligible and the melt composition would be mainly controlled by re-equilibration of the peridotite matrix during long-range (> 1 m) porous flow percolation. Thus the melt would be buffered by the amphibole peridotite mineralogy, except for LREE. This may explain the lack of mineralogical reaction and chemical enrichments (except for REE) in this zone, and the chromatographic fractionation of REE. We propose a quantitative model of diffusion and percolation-controlled metasomatism associated with infiltration of alkali basalts into peridotites hosting vein-conduits. We also suggest that silicate-melt percolation may explain mineral disequilibrium features observed in mantle xenoliths.

J. Fabries

Papers

Mechanisms of Mantle Metasomatism: Geochemical Evidence from the Lherz Orogenic Peridotite

Petrogenesis of layered pyroxenites from the Lherz, Freychinéde and Prades ultramafic bodies (Ariége, French Pyrénées)

Evolution of the Upper Mantle beneath the Pyrenees: Evidence from Orogenic Spinel Lherzolite Massifs

Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees

REE and Sr-Nd Isotopic Geochemistry of Eastern Pyrenean Peridotite Massifs: Sub-Continental Lithospheric Mantle Modified by Continental Magmatism