Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle sh...

The Redox State of Earth's Mantle

Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.

Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for Mantle Melting and Melt-Mantle Interaction in a Supra-Subduction Zone Setting

Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc–basin system and for the nature of interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling shows that they represent the residues from 15–20% melting at oxygen fugacities close to the QFM buffer. The dunites have U-shaped REE patterns, low incompatible element abundances and high Cr# (0.66–0.77). Petrography and geochemistry indicate that the latter are the product of intense interaction between peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of lesser interaction between peridotite and melt saturated with olivine ± clinopyroxene. The data demonstrate that the harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centre), and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich arc magmas. The second dredge locality, near the South Sandwich Trench–Fracture Zone intersection, yielded rocks ranging from lherzolite to harzburgite that could similarly have resulted from a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and island arc tholeiite. The South Sandwich peridotites have a similar petrogenetic history to those from Conical Seamount in the Mariana forearc in the sense that both involved interaction between arc magma and pre-existing mantle lithosphere of different provenance. However, the precise compositions of the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, data from the South Sandwich and Izu–Bonin–Mariana systems emphasise the potential significance of peridotite geochemistry in unravelling the complex tectonic histories of forearcs past and present.

Geochemistry and tectonic significance of peridotites from the South Sandwich arc-basin system, South Atlantic

The redox state of subduction zones: insights from arc-peridotites

material in or, most probably, below the asthenospheric mantle, Petrological and geochemical data are presented for a suite of brought down in a subducting slab at a former convergent plate ultramafic xenoliths (lherzolites and wehrlites) from La Grille margin. volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up

Johan Nell

Papers

High-temperature cation distributions in Fe3O4-MgAl2O4- MgFe2O4-FeAl2O4 spinels from thermopower and conductivity measurements

High temperature electrical measurements and thermodynamic properties of Fe3O4-FeCr2O4-MgCr2O4-FeAl2O4 spinels

Jonker-type analysis of small polaron conductors

Macroscopic and microscopic thermodynamic properties of oxides