Alan B. Woodland

Variation in oxygen fugacity with depth in the upper mantle beneath the Kaapvaal craton, Southern Africa

Oxygen fugacity (fO2) is an important parameter in many geochemical processes in the Earth’s mantle. To assess how fO2 varies with depth, Fe3+ contents in garnet and spinel from peridotite xenoliths were determined by Mossbauer spectroscopy. A total of 49 xenoliths were investigated from localities on the southern flank of the Kaapvaal craton in South Africa (Kimberley, Jagersfontein, Frank Smith Mine, Monastery) and Lesotho (Letseng-la-Terae, Liqhobong, Matsoku). These samples provide a depth coverage from ∼80 to 220 km. For the Lesotho and South African xenoliths there is a systematic, but not monotonic, decrease in fO2 with depth. Between about 80 and 150 km depth there is a decrease of ∼3 log units. At shallow depths, where spinel peridotites are stable, ΔlogfO2 values of ∼FMQ−1 (i.e. one log unit below the fayalite–magnetite–quartz reference oxygen buffer) are obtained, similar to other worldwide occurrences. At greater depths the decrease in fO2 is less, amounting to ∼0.8 log units over the depth interval of 150–220 km. At ∼220 km depth the ΔlogfO2 lies just below FMQ−4 and is expected to decrease further with depth, reaching conditions of metal saturation near the 410 km discontinuity. Under such fO2 conditions a coexisting fluid phase would be dominantly composed of H2O and CH4. Thus in the deeper portions of the upper mantle the necessary conditions for ‘redox melting’ are met, namely a region where CH4-rich fluids can exist and migrate upward into more oxidised peridotite. The lowering of fO2 with depth follows in part from a negative ΔV for the reaction describing the incorporation of Fe3+ in garnet. The change in the rate of fO2 decrease is attributable to a change in the bulk composition, the sheared peridotites at depth being relatively fertile compared to the overlying depleted peridotites. Variable degrees of oxidation appear to have attended metasomatism, as recorded in samples from Kimberley. Our results emphasise that the upper mantle cannot be treated as monolithic in terms of redox state. The early development of the Earth’s atmosphere was directly influenced by degassing of the mantle via volcanic activity. Since the majority of magmas, such as MORB, are produced under conditions within the spinel peridotite field, this implies that mantle degassing will be dominated by CO2 and H2O, although S should be present mostly as sulphide. This is consistent with recent data on Cr and V abundances in lavas. However, hydrothermal or volcanic emissions with fO2 values in the range of FMQ should not be seen as ‘oxidising’ since they had a capacity to react with O2 and inhibit the build-up of free O2 in the early atmosphere.

Olivine water contents in the continental lithosphere and the longevity of cratons

Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180–250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere–asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

The distribution of lithium in peridotitic and pyroxenitic mantle lithologies — an indicator of magmatic and metasomatic processes

Lithium concentrations in orthopyroxene, clinopyroxene, olivine, garnet or spinel from equilibrated spinel peridotite, garnet peridotite and garnet pyroxenite xenoliths and from metasomatised peridotites and pyroxenites were measured using the ion-microprobe (SIMS). With respect to changes in physical and chemical parameters, we present new data on lithium contents in mantle minerals and its partitioning behaviour in (1) equilibrated and (2) metasomatised samples. Lithium is preferentially incorporated into olivine, ranging between 1 and 2 ppm in equilibrated unmetasomatised peridotites. Pyroxenes from peridotitic xenoliths have Li concentrations on the order of several hundred ppb up to 1.3 ppm, while pyroxenes from pyroxenites have somewhat higher abundances (1–3 ppm with a maximum of 19 ppm). The following partitioning relationships have been established: ol>cpx≥opx≫grt or sp for garnet and spinel peridotites, respectively, and cpx≥opx>grt for garnet pyroxenites. The intercrystalline partitioning of Li is independent of T, P and bulk composition (for ultramafic to mafic compositions), making Li a suitable tracer element for chemical processes such as metasomatism. We estimate a bulk Li content of 1.0–1.5 ppm for fertile to moderately depleted lithospheric mantle. Low Li abundances in mantle peridotites and pyroxenites emphasise its incompatibility during partial melting and fractional crystallisation. However, elevated Li concentrations are present in some pyroxenites, presumably due to complete crystallisation of trapped partial melts. Metasomatised samples from peridotite massifs (Pyrenees and Ivrea Zone) and mantle nodules from two Victorian volcanic fields (Australia) clearly show enrichment of Li in both olivine and clinopyroxene, whereby the distribution of Li between olivine and clinopyroxene has generally not achieved equilibrium. Disequilibrium is manifested by preferential Li enrichment in either olivine or clinopyroxene depending on the type of metasomatic agent involved (carbonatite vs. mafic silicate melt). Differences in absolute Li abundances and in its partitioning behaviour allow the identification not only of metasomatic overprints, but also of magmatic processes such as partial melting, crystal fractionation and accumulation. The sensitivity of Li as such a chemical tracer gives an additional criterium for recognising cryptic metasomatism.

The orthorhombic to high‐P monoclinic phase transition in Mg‐Fe Pyroxenes: Can it produce a seismic discontinuity?

The orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO 3 pyroxene with a mantle-relevant composition (X Fs = 0.1) is expected to occur at ∼300 km depth [Woodland and Angel, 1997]. However, the divariant nature of the phase transition in the Mg-Fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity. New experimental results with binary Mg-Fe pyroxenes constrain the divariant loop to be 0.2 GPa wide at the composition of X Fs = 0.4 and on the order of 0.15 GPa for a mantle-relevant composition. This implies that the phase transition will be complete over a depth interval of about 5-6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO 3 pyroxene is indeed narrow enough to produce a jump in seismic velocities. The experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition. The expected location of this phase transition coincides with a small magnitude seismic discontinuity, the X-discontinuity, occasionally observed in seismic profiles at ∼300 km depth, and thus provides a viable petrologic explanation for the origin of this discontinuity, if it truly exists.

Metasomatic interactions in the lithospheric mantle: petrologic evidence from the Lherz massif, French Pyrenees

Abstract To better understand the processes of metasomatism in the lithospheric mantle, we have undertaken a detailed petrologic study of wallrock adjacent to late stage mafic dikes at the Lherz massif. Traverse I contains a garnet-pyroxenite dike, along with hornblendite dikelets which cut harzburgite. Traverses II and III contain an anhydrous garnet-pyroxenite dike and a hornblendite dike in lherzolite, respectively. There is evidence for three successive metasomatic events in these samples: (1) crystallisation of Al-rich grain-boundary spinels ± associated phases from a silicate melt, (2) formation of apatite related to carbonatite metasomatism, (3) FeTi metasomatism related to the intrusion of garnet-pyroxenite dikes and hornblendite dikes. The first two events were diffuse in character, and although they penetrated a relatively large volume of peridotite, interaction with the peridotite was not pervasive. The result was the development of localised chemical gradients at the mm scale. The carbonatite metasomatism caused a net oxidation and chemical enrichment of the peridotite. Metasomatic effects related to the intrusion of the garnet-pyroxenite and hornblendite dikes are spatially restricted. Primary and secondary spinel and clinopyroxene compositions indicate that melt migrating from the hornblendite dikes was mostly responsible for pervasive metasomatism at the scale of Successive overprinting of one metasomatic event by another has led to the production of enriched peridotites with complex geochemical signatures. The variable extent of melt-peridotite interaction can lead to geochemical heterogeneities at a variety of length scales. Textural relations reveal little, if any, difference in the mechanism of infiltration of a silicate or carbonatite melt through harzburgite or lherzolite.

Excess methane in continental hydrothermal emissions is abiogenic

Thermogenic hydrocarbons entirely deriving from the thermal degradation of organic matter usually exhibit methane to ethane plus propane ratios smaller than 100. We present hydrocarbon distribution data of continental hydrothermal gases, whose methane has been independently identified to derive from the abiogenic reduction of CO2. We find that excess amounts of methane with respect to thermogenic hydrocarbon distributions are characteristic for the investigated gases. A similar pattern is observed for well discharges whose temperatures are too high to support any microbially mediated methanogenesis. These findings strongly suggest that abiogenic methane production in continental-hydrothermal systems is a more widespread process than previously assumed. The maximum contribution of such emissions to the modern atmospheric CH4 budget is estimated at ~1%.

Experimental determination of the solubility of the assemblage paragonite, albite, and quartz in supercritical H2O

Abstract The concentrations of Na, Al, and Si in an aqueous fluid in equilibrium with natural albite, paragonite, and quartz have been measured between 350°C and 500°C and 1 to 2.5 kbar. Si is the dominant solute in solution and is near values reported for quartz solubility in pure H 2 O. At 1 kbar the concentrations of Na and Al remain fairly constant from 350°C to 425°C but then decrease at 450°C. At 2 kbar, Na increases slightly with increasing temperature while Al remains nearly constant. Concentrations of Si, Na, and Al all increase with increasing pressure at constant temperature. The molality of Al is close to that of Na and is nearly a log unit greater than calculated molalities assuming Al(OH) 0 3 is the dominant Al species. This indicates a Na-Al complex is the dominant Al species in solution as shown by Anderson and Burnham (1983) at higher temperature and pressure. The complex can be written as NaAl ( OH ) 0 4 ± nSiO 2 where n is the number of Si atoms in the complex. The value of n is not well constrained but appears to be less than or equal to 3. The results indicate Al can be readily transported in pure H 2 O solutions at temperatures and pressures as low as 350°C and 1 kbar.

A crystallographic and mössbauer spectroscopy study of Fe 3 2+ Al2Si3O12-Fe 3 2+ Fe 2 3+ Si3O12, (almandine-“skiagite”) and Ca3 Fe 2 3+ Si3O12-Fe 3 2+ Fe 2 3+ Si3O12 (andradite-“skiagite”) garnet solid solutions

AbstractThe crystal chemistry of garnet solid solutions on the Fe32+Al2Si3O12-Fe32+Fe23+Si3O12 (almandine-“skiagite”) and Ca3Fe23+Si3O12-Fe32+Fe23+Si3O12 (andradite-“skiagite”) joins have been investigated by single-crystal X-ray structure refinements and Mössbauer spectroscopy. Together, these two solid solution series encompass the complete range in Fe3+/ΣFe from 0.0 to 1.0. All garnets are isotropic and were re0fined in the Ia % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqaqFfpeea0xe9Lq-Jc9% qqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabG4mayaara% aaaa!3622!

High-pressure phase transition in CaTiOSiO4 titanite

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