Bulk Mantle Research Articles

Long wavelength convection, Poiseuille–Couette flow in the low-viscosity asthenosphere and the strength of plate margins

SUMMARY Mixed heated 3-D mantle convection simulations with a low-viscosity asthenosphere reveal relatively short and long wavelength regimes with different scalings in terms of surface velocity and surface heat flux and show that mantle flow in the lithosphere–asthenosphere region is a Poiseuille–Couette flow. The Poiseuille/Couette velocity magnitude ratio, D/U, allows us to characterize solid-state flow in the asthenosphere and to predict the regime transition. The transition from dominantly pressure-driven Poiseuille flow at shorter wavelengths to dominantly shear-driven Couette flow at long wavelengths depends on the relative strength of lithosphere and asthenosphere and is associated with a switch in the dominant resistance to convective motion. In the Poiseuille regime significant resistance is provided by plate-bending, whereas in the Couette regime most resistance is due to vertical shear in the bulk mantle. The Couette case corresponds to classical scaling ideas for mantle convection whereas the Poiseuille case, with asthenospheric velocities exceeding surface velocities, is an example of a sluggish lid mode of mantle convection that has more recently been invoked for thermal history models of the Earth. Our simulations show that both modes can exist for the same level of convective vigour (i.e. Rayleigh number) but at different convective wavelengths. Additional simulations with temperature- and yield-stress dependent viscosity show consistent behaviour and suggest an association of the regime crossover with the relative strength of plate margins. Our simulations establish a connection between the strength of plate margins, solid-state flow in the asthenosphere and the wavelength of mantle convection. This connection suggests that plate tectonics in the sluggish lid mode is wavelength dependent and potentially more robust than previously envisioned.

Progressive mixing of meteoritic veneer into the early Earth’s deep mantle

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old (from the Archaean era), that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1. In particular, the platinum group element (PGE) contents of komatiites provide a unique source of information on core formation, mantle differentiation and possibly core–mantle interaction2, 3, 4, 5, 6, 7, 8. Most of the available PGE data on komatiites are from late Archaean (~2.7–2.9 Gyr old) or early Proterozoic (2.0–2.5 Gyr old) samples. Here we show that most early Archaean (3.5–3.2 Gyr old) komatiites from the Barberton greenstone belt of South Africa and the Pilbara craton of Western Australia are depleted in PGE relative to late Archaean and younger komatiites. Early Archaean komatiites record a signal of PGE depletion in the lower mantle, resulting from core formation. This signal diminishes with time owing to progressive mixing-in to the deep mantle of PGE-enriched cosmic material that the Earth accreted as the ‘late veneer’ during the Early Archaean (4.5–3.8 Gyr ago) meteorite bombardment.

The effect of oxygen fugacity on the olivine to wadsleyite transformation: Implications for remote sensing of mantle redox state at the 410 km seismic discontinuity

High-pressure and -temperature multianvil experiments were performed to test the effect of varying oxygen fugacity on the olivine to wadsleyite transformation. Two capsules, containing samples of (FeMg) 2 SiO 4 , were placed in each experiment; the first buffered the oxygen fugacity with an assemblage of Re and ReO 2 . whereas the second ensured the lowest possible ferric iron concentra- tion through the presence of excess Fe metal. Measurements of coexisting olivine, wadsleyite. and ringwoodite compositions from the Fe metal saturated experiments were used to accurately determine the pressure in each experiment using established phase relations. Under the more oxidizing conditions of the Re-ReO 2 buffer, the stability field of wadsleyite was found to expand with respect to both the olivine and ringwoodite stability fields. Mössbauer spectroscopy measurements reveal Fe 3+ /∑Fe ratios for wadsleyite buffered by Re-ReO: of 0.1-0.25. while olivine appears to be FeJ"-free. A thermodynamic model that employs the wadsleyite end-members (Fe 3+ 5/3 □ 1/3 )Fe 3+ O 4 -Fe 2 SiO 4 -Mg 2 SiO 4 is used to examine the effect of varying bulk mantle Fe 3+ /∑Fe ratio on the depth and depth interval of the 410 km seismic discontinuity. Fe 3+ /SFe ratios in the range 0.02-0.12 would cause the depth interval or thickness of the 410 km discontinuity to increase from -8 to 15 km but would haw very little effect on the seismically observable absolute depth. Very large bulk mantle Fe 3+ /∑Fe ratios (>0.2), unrepresented in recovered mantle samples, would be required to explain recent seismic observations that the depth interval of the 410 km may be >20 km beneath certain regions. Such observations are more likely to be explained by moderate local enrichments in both ferric iron and H;0 in the mantle, most likely as a result of slab interaction.

Thermal convection heated both volumetrically and from below: Implications for predictions of planetary evolution

Solid-state thermal convection, as a model for the internal dynamics of planetary mantles, is generated both by volumetric heating and heating from below. A series of 2D and 3D numerical experiments is described where a bottom heat flux is prescribed as well as a constant fraction of volumetric heating with either free-slip or no-slip conditions for the two horizontal boundaries. The assumption that hot plumes rising in the interior of the layer act as a volumetric heat source leads to a first order scaling of thermal boundary layers. Cases with a no-slip boundary agree well with this scaling while cases with free-slip systematically deviate: a decrease of 20–30% is observed for the temperature difference across the boundary layer when the fraction of heating from below is increased from 0 to 1. These differences are attributable to a velocity structure near the boundary varying with the fraction of volumetric heating. The main planetary implications are that (i) the average thermal structure of the bulk mantle and lithosphere are not good indicators of the distribution of heat sources between the interior (e.g. radioactive heating, secular cooling) and the lower interface (e.g. core cooling); (ii) in contrast, the nature of volcanism (whether it is localized, hot spot like or widespread) reflect this distribution and should thus help to constrain the global thermal structure as well as the existence of a magnetic dynamo.

Timing and duration of Palaeoproterozoic events producing ore-bearing layered intrusions of the Baltic Shield: metallogenic, petrological and geodynamic implications

Abstract There are two 300–500 km long belts of Palaeoproterozoic layered intrusions in the Baltic (Fennoscandian) Shield; the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt. New U–Pb (TIMS) ages and radiogenic isotopic (Nd–Sr–He) data have been determined for mafic-ultramafic Cu–Ni–Ti–Cr and PGE-bearing layered intrusions of the Kola Belt. U–Pb zircon and baddeleyite data from gabbronorite and anorthosite bodies of the Fedorovo-Pansky, Monchepluton, Main Ridge (Monchetundra and Chunatundra) and Mt Generalskaya intrusions, and from gabbronorite intrusions and dykes associated with the Imandra lopolith, yield ages from c . 2.52–2.39 Ga. The age range of 130 Ma recorded in the Kola Belt samples, associated with at least four intrusive phases (three PGE-bearing and one barren), is significantly greater than that for intrusions of the Southern (Fenno-Karelian) Belt which clusters at 2.44 Ga. Nd isotopic values for the Kola Belt range from −1.1 to −2.4 and indicate an enriched mantle ‘EM-1 type’ reservoir for these layered intrusions. Initial Sr isotopic data for the Kola intrusions are radiogenic relative to bulk mantle, with I Sr values from 0.703 to 0.704, but geochemical data and 4 He/ 3 He isotopic ratios of various minerals record a significant contribution from a mantle source rather than simply crustal melting. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic‐ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.

Hydrogen partitioning between nominally anhydrous upper mantle minerals and melt between 3 and 5 GPa and applications to hydrous peridotite partial melting

To extend our knowledge of partitioning of hydrogen between nominally anhydrous mantle minerals and basaltic melts to higher pressures, we conducted 8 experiments at 3–5 GPa and 1350–1440 °C on a modified mid-ocean ridge basalt (MORB) + 2 wt.% H 2O. H contents in olivine, orthopyroxene, clinopyroxene, and garnet were measured by counting 16O 1H ions with low-blank SIMS, and total H 2O concentrations in coexisting glasses were measured by FTIR, SIMS, and electron microprobe. The glasses have water contents ranging from 4.1 to 8.9 wt.%. Eleven measured pyroxenes have H 2O concentrations ranging from 430 to 1430 ppm, two garnets have concentrations of 73 and 260 ppm, and one olivine has a concentration of 29 ppm. Mineral/melt hydrogen partition coefficients are as follows: D H cpx/melt ranges from 0.014–0.021, D H opx/melt ranges from 0.009–0.019, D H gt/melt ranges from 0.001–0.0033, and D H ol/melt has a value of 0.0006. Mineral/mineral hydrogen partitioning data are as follows: D H cpx/opxranges from 1.2–2, D H cpx/gt has values of 5.5 and 14, and a single D H cpx/ol has a value of 27. D H pyx/melt correlates with the concentration of Al 2O 3 in pyroxene and with the concentration of Al on tetrahedral sites. Combining the mineral/melt hydrogen partitioning data from this study, and estimates of mineral modes and compositions along the solidus, we estimate that D H perid/melt increases from 0.006 at 1 GPa up to 0.01 near the garnet lherzolite/spinel lherzolite transition, owing to the maximum in Al 2O 3 concentration in pyroxenes. With increasing pressure D H perid/melt for garnet lherzolite diminishes to 0.005 at 5 GPa, as pyroxenes become less aluminous and less abundant. As a result, incipient hydrous melts beneath oceanic ridges with bulk mantle H 2O contents of 50–200 ppm will have solidus partial melts with H 2O contents between 0.5 and 3.8 wt.%, while oceanic island mantle with 300–1000 ppm will have solidus partial melts with H 2O content between 3 and 20 wt.%.

Three‐dimensional mantle convection simulations with a low‐viscosity asthenosphere and the relationship between heat flow and the horizontal length scale of convection

Numerical mantle convection simulations show that depth‐dependent viscosity can increase the flow wavelength. A recent analysis demonstrates that flow channelization into a low‐viscosity region lowers lateral dissipation. This allows long wavelength flow to more efficiently cool the interior mantle. We present three‐dimensional, mixed heating mantle convection simulations with a thin low‐viscosity channel for a range of aspect ratios to test the implications of the theoretical analysis. For reasonable viscosity contrasts between lithosphere, asthenosphere and bulk mantle we find that very large aspect ratios can develop. Velocity profiles quantify the degree of channelization for variable aspect ratios. Internal temperatures are found to decrease with increasing aspect ratio and both surface heat flux and velocity are found to increase with aspect ratio. Our results are consistent with the idea that the asthenosphere channels lateral mantle flow which, in turn, stabilizes long wavelength convection cells and makes long wavelength flow energetically favorable.

The evolution of He Isotopes in the convecting mantle and the preservation of high 3He/ 4He ratios

A key requirement for any model of mantle evolution is accounting for the high 3He/ 4He ratios of many ocean island basalts compared to those of mid-ocean ridge basalts. The early, popular paradigm of primitive, undegassed mantle stored in a convectively isolated lower mantle is incompatible with geophysical constraints that imply whole mantle convection. Thus it has been suggested more recently that domains with high 3He/U ratios have been created continuously from the bulk mantle throughout Earth history. Such models require that the 3He/ 4He ratio of the convecting mantle was at least as high as the highest values seen in OIB at the time the OIB source was generated. These domains must also be created with sufficient He to impart distinctive He isotopic signatures to ocean island basalts. However, the He isotope evolution of the mantle has not been consistently quantified to determine if such scenarios are plausible. Here a simple model of the He evolution of the whole mantle is examined. Using a wide range of possible histories of continental extraction and He degassing, the bulk convecting mantle was found to have had 3He/ 4He ratios as high as those seen in the Iceland hotspot only prior to 3 Ga. Such high 3He/ 4He ratios can only be preserved if located in domains that are not modified by convective mixing or diffusive homogenisation since that time. Further, there are difficulties in producing, with commonly invoked magmatic processes, domains with sufficiently high 3He/U ratios and enough 3He to be able to impart this signature to ocean island basalts. The results are consistent with models that store such He signatures in the core or a deep layer in the mantle, but are hard to reconcile with models that continuously generate high 3He/ 4He domains within the mantle.

Hafnium–tungsten chronometry of angrites and the earliest evolution of planetary objects

Angrites are amongst the oldest basalts in the solar system and their origins are enigmatic, some even proposing the planet Mercury as the parent body (APB). Whatever their exact provenance their chronometry provides insights into early stages of planetary melting and differentiation. We present the first high-precision internal 182Hf– 182W isochrons for such early differentiated objects. Angrites Sahara 99555, D'Orbigny, and Northwest Africa 2999 define ages of 5.1 ± 1.3 Ma, 4.7 ± 1.3 Ma and 9.5 ± 3.3 Ma respectively after formation of calcium–aluminum-rich refractory inclusions (CAIs). These data are in good agreement with 26Al– 26Mg, 53Mn– 53Cr and most 207Pb– 206Pb ages for other angrites and provide evidence for two texturally and temporally well-resolved groups. The quenched angrites (SAH 99555, D'Orbigny and five others) have a weighted mean age of 4562.1 ± 0.4 Ma and are the products of igneous crystallization on the APB ∼ 5 Ma after the formation of CAIs, whereas the more slowly cooled angrites (NWA 2999, Angra dos Reis, LEW 86010, average age: 4557.7 ±0.2 Ma) reflect metamorphic closure ∼ 5 Ma later following second reheating process or a complex cooling history. The concordance obtained between various short-lived chronometers provides evidence that 26Al, 53Mn and 182Hf were homogeneously distributed in the solar nebula, although we cannot rule out the possibility of local small heterogeneities. Contrary to recent proposals, the data are also consistent with the previously determined age of the solar system based on 207Pb– 206Pb systematics of CAIs. The Hf–W data are discussed in the context of two endmember models for the early differentiation of the angrite parent body. In the first model, core formation occurred at 3–4 Ma after CAIs and both groups of angrites formed by two distinct partial melting events from the bulk mantle of the angrite parent body. In the second model, the angrite parent body underwent progressive core formation with an increasing degree of W-depletion over time. In this model, the two groups of angrites derive from distinct reservoirs. The heat sources responsible for such late melting and core formation are unclear. Quenched angrites are coeval with non-magmatic IAB iron meteorites and CB chondrules at ∼ 4562 Ma. However, demonstration of a genetic link between angrite melting and impact events must await the acquisition of still higher resolution chronometry.

Flow and electrical anisotropy in the upper mantle: Finite-element models constraints on the effects of olivine crystal preferred orientation and microstructure

Long-period magnetotelluric (MT) data shows that electrical conductivity in the upper mantle is highly anisotropic. Agreement between high electrical conductivity directions and seismic anisotropy fast directions suggests that anisotropic diffusion of hydrogen along oriented olivine crystals controls the anisotropy of electrical conductivity in the upper mantle, since both seismic waves and hydrogen diffusion are faster along the [1 0 0] axis of olivine. Thus, MT electrical anisotropy data, like seismic anisotropy, may be used to map flow patterns in the upper mantle. However, observed electrical anisotropies are significantly higher than seismic ones. To quantify the influence of strain-induced crystal preferred orientations of olivine on upper mantle bulk electrical conductivities, we calculate the macroscopic electrical conductivity anisotropy of a series of naturally and experimentally deformed peridotites using an anisotropic finite-element model. These models, which fully take into account the microstructure: crystal and shape preferred orientations, based on orientation maps obtained by indexation of electron back-scattered diffraction (EBSD) patterns, show macroscopic electrical anisotropy factors ranging from 3 to 16. The intensity of electrical anisotropy depends to first order on the intensity of the olivine crystal preferred orientations, but the relation saturates for strong crystal preferred orientations. In addition, the spatial distribution of the various crystal orientations may significantly enhance influence the anisotropy in strongly textured mantle rocks. The strongest anisotropy factors (>10) occur in mantle rocks in which deformation by dislocation creep has produced not only crystal but also strong shape preferred orientations, even if the latter is masked by recrystallization. Higher anisotropy factors (>100) observed in a few MT experiments imply however an additional, as yet unknown, mechanism controlling electrical conduction at asthenospheric depths in these regions.

Electrical conductivities of pyrope-almandine garnets up to 19 GPa and 1700 C

Electrical conductivities of polycrystalline garnets ranging in chemical composition from almandine (Fe 3 Al 2 Si 3 O 12 ) to pyrope (Mg 3 Al 2 Si 3 O 12 ) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe 2+ -Fe 3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profile of the mantle, using a recently developed laboratory- derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py 100 to Py 85 Alm 15 . Although only a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle.

“New” lunar meteorites: Implications for composition of the global lunar surface, lunar crust, and the bulk Moon

Abstract— New data for lunar meteorites and a synthesis of literature data have significant implications for the interpretation of global Th data and for the Moon's bulk composition. As presently calibrated (Prettyman et al. 2002), the Lunar Prospector gamma‐ray data imply that the average global surface Th = 1.58 μg/g. However, that calibration yields implausibly high concentrations for the three most Th‐poor documented sampling sites, it extrapolates to a nonzero Lunar Prospector Th, ˜0.7 μg/g, at zero sample Th, and it results in a misfit toward too‐high Th when compared with the global regolith Th spectrum as constrained using mainly lunaite regolith breccias. Another problem is manifested by Th versus K systematics. Ground truth data plot consistently to the high‐Th/K side of the Prospector data trend, offset by a factor of 1.2. A new calibration is proposed that represents a compromise between the Th levels indicated by ground truth constraints and the Prettyman et al. (2002) calibration. Conservatively assuming that the Th versus K issue is mostly a K problem, the average global surface Th is estimated to be ˜1.35 μg/g. The Moon's remarkable global asymmetry in KREEP abundance is even more pronounced than previously supposed. The surface Th concentration ratio between the hemisphere antipodal to the Procellarum basin and the hemisphere centered on Procellarum is reduced to 0.24 in the new calibration. This extreme disparity is most simply interpreted as a consequence of Procellarum's origin at a time when the Moon still contained at least a thin residual layer of a global magma ocean. Allowing for diminution of Th with depth, the extrapolated bulk crustal Th is ˜0.73 μg/g. Further extrapolation to bulk Moon Th yields ˜0.07 μg/g, which is nearly identical to the consensus estimate for Earth's primitive mantle. Assuming chondritic proportionality among refractory lithophile elements implies Al2O3 of approximately 3.8 wt%. The Moon's bulk mantle mg ratio is only weakly constrained by seismic and mare‐basaltic data. KREEP‐and mare‐free lunaite regolith samples, other thoroughly polymict lunar meteorites, and a few KREEP‐free Apollo highland samples manifest a remarkable anticorrelation on a plot of Al2O3 versus mg. This trend implies that an important component of the Moon is highly magnesian. The bulk Moon is inferred to have an Earth‐like oxide mg ratio of ˜87–88 mol%. The close resemblance between the bulk Moon and Earth's primitive mantle extends to moderately volatile elements, most clearly Mn. Unless major proportions of Cr and V are sequestered into deep mantle spinel, remarkably Earth‐like depletions (versus chondrites) are also inferred for bulk Moon Cr and V.

Lower mantle composition and temperature from mineral physics and thermodynamic modelling

SUMMARY A generalized inverse method is applied to infer the radial lower mantle composition and temperature profile from seismological models of density and bulk sound velocity. The computations are performed for a five-component system, MgO‐FeO‐CaO‐Al2O3‐SiO2 and three phases: (Mg,Fe,Al)(Si,Al)O3 perovskite, (Mg,Fe)O magnesiowustite, and CaSiO3 perovskite. A detailed review of the elasticity data set used to compute the elastic properties of mineral assemblages is given. We consider three different a priori compositional models—pyrolite, chondritic and a model based on cosmic abundances of elements—as a priori knowledge for the inversions in order to investigate the sensitivity of any given best-fit solution to the assumed initial composition. Consistent features in all inversions, independent of the a priori model, are a total iron content of X Fe � 0.10 ± 0.06 and a subadiabatic temperature gradient over most of the lower mantle depth range. A peculiar correlated behaviour of the two most sensitive parameters (iron content and temperature) is found below the 660 km discontinuity: over the depth range from 660 km down to 1300 km. Significantly, we find that the bulk composition inferred from any given inversion is strongly dependent on the choice of a priori model. Equally satisfactory fits to the lower mantle bulk sound velocity and density profiles can be obtained using any of the a priori models. However, the thermal structure associated with these compositional models differs significantly. Pyrolite yields a relatively cool geotherm (T 660 � 1800 K and X Pv � 0.64), while perovskite-rich models such as chondritic or cosmic models yield hot geotherms (T 660 � 2500 K and X Pv � 0.84 for the latter), but all of the geotherms are subadiabatic. The results of inversions are virtually unaffected by the partitioning of iron between perovskite and magnesiowustite. Out of the five oxide components considered in our models, the bulk Al2O3 and CaO contents of the mineral assemblages are least well constrained from our inversions. Our results show that a major shortcoming of lower mantle compositional and thermal models based on inversions of bulk sound velocity and density is the strong dependence of the final solution on the a priori model. That is, a wide variety of best-fit compositional and thermal models can be obtained, all of which provide satisfactory fits to global average seismic models. It is, in fact, this non-uniqueness that dominates the resulting a posteriori uncertainties and prevents a clear discrimination between different compositional models. Independent constraints on the thermal structure or on the shear properties of lower mantle assemblages are needed to infer lower mantle composition with a higher degree of certainty.

The influence of the transition zone water filter on convective circulation in the mantle

The “transition zone water filter” model of mantle convection attempts to reconcile geochemical evidence for isolated mantle reservoirs with geophysical evidence for whole mantle circulation by decoupling incompatible elements from bulk mantle circulation via a melt layer above the 410 km discontinuity. This would result in a stratified heating distribution similar to layered convection models, but would permit bulk mass and heat transfer across mantle interfaces. Here we test the basic effect of the water filter mechanism on mantle flow, in particular to see whether it induces layered convection and/or strong upwelling plumes above strongly heated layers. We investigate the influence of ten likely heating distributions on the planform of convection. Excepting an extreme case where all heating occurs in the transition zone, convection is not significantly influenced by the heating distribution. For Earth‐like internal heating, the flow is dominated by downwellings which extend across the entire model mantle, as in uniformly internally heated layers.

Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data

A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.

Iron oxidation state in lower mantle mineral assemblages: II. Inclusions in diamonds from Kankan, Guinea

Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mössbauer spectroscopy to determine Fe3+/ΣFe. Fe3+ concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3+/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53+O (M=Fe3+, Cr3+, Al3+). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and São Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe–(Mg,Fe)O equilibrium, but more reducing than Re–ReO2 equilibrium. Fe2+/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mössbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3+ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3+ concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle.

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in 3He and 4He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in 3He/ 4He should be correlated with larger variations (factor of 5) in 4He/ 40Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s 4He/ 40Ar and 3He/ 4He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of 3He relative to 4He and of 4He relative to 40Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low 4He/ 40Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low 4He/ 40Ar, the complement of the high 4He/ 40Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high 3He/ 4He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.

Plate-kinematic Constraints from Mantle B�nard Convection

General kinematic implications for plate tectonics are determined for Rayleigh-Benard convection of the mantle. The continuum of all possible configurations of Benard polygons is probed by large random samples of global configurations (450,000 to 54,000,000), for each of which the Euler poles are determined on the basis of viscous coupling across the asthenosphere. Two computationally related methods lead first, to Euler pole restrictions for fourteen plates, and second, to restrictions on the Benard cell configuration. Result No. 1: Euler poles occur in global “preference-patterns,” which are determined exclusively by the shape of the plate. The observational HS2-NUVEL1 model poles occur near regions preferred by Benard convection (Eurasia excluded); the agreement is best for the most accurate observational poles. Result No. 2: Seven specific mantle Benard cells are indicated by present-day plate motions. The upwelling centers correlate with hotspot domains; the major global subduction zones correlate with Benard model downwelling. This result is independent of the Euler pole accuracy used in its determination, and is consistent with the distribution of low seismic p-wave propagation velocities determined by tomography, and with shear-wave splitting analysis within the asthenosphere. Conclusions: The results suggest that the bulk mantle is divided into less than ten Benard convection cells globally (cf., Fohlmeister and Renka, 2002), each of which extends from the asthenosphere to the core-mantle boundary; turbulent flow, and other perturbations of the Benard kinematics appear to be limited. These primally poloidal flow kinematics provide basal shear forces as a major component in driving plate tectonics, and are specifically configured for the directions of plate motions. The Benard model is incomplete without a dynamic contribution from the lithosphere, which represents a separate convection layer of the distinct polar kinematics of rigid plates. The complete hybrid mechanism for driving plate tectonics includes lithospheric buoyancy dynamics, specifically from the subducting Pacific plate slabs to compensate for plate-slowing due to the “back-flow sector” of the Hawaiian convection cell, and collision-drag dynamics principally for smaller plates or continental margins.

Copper isotope ratios in magmatic and hydrothermal ore-forming environments

Multi-collector inductively coupled plasma mass spectrometry now provides for precise and accurate measurements of Cu isotope ratios. Copper minerals prepared by direct dissolution with and without chromatographic purification yield identical Cu isotope ratios within analytic precision of about 0.04‰ (1 σ). Cu isotope ratios have been measured for copper minerals from worldwide magmatic and hydrothermal copper deposits, and for several weathered deposits. Natural variations in δ 65Cu values, relative to NBS976, range over 9‰. Chalcopyrite samples from mafic intrusions lie within a narrow range of about 1.5‰, and most cluster tightly between −0.10‰ and −0.20‰. This range lies within the broader black smoker chalcopyrite and iron meteorite ranges, and possibly represents a bulk mantle Cu isotope ratio. Most values for hydrothermal native copper from the Michigan native copper district also show a narrow range just larger than 0.1‰ and suggest a common homogeneous source for Cu in this large hydrothermal system. Later copper sulfide and arsenide minerals from this district range to values more than 2‰ lower than native copper. Ratios for chalcopyrite and bornite from moderate to high-temperature porphyry, skarn, and replacement deposits as a group and within individual deposits exhibit a broad range of values. Variations of nearly 1‰ are observed over distances on the order of 1 m. In some cases, these variations may result from multiple mineralization events or copper remobilization during retrograde or later hydrothermal activity. Fractionations between chalcopyrite and bornite, where they occur in the same sample or in related samples, cluster near 0.4‰, suggesting equilibrium Cu isotope fractionation between them at moderate temperatures. In addition, weathering of hydrothermal copper minerals produces a wide range of values in secondary copper phases. In the supergene environment, cuprite typically has higher values than associated native copper. Therefore, redox states appear to exert a significant control over fractionation at low temperatures. Significant questions remain to be answered. Before the source of copper in hydrothermal environments can be fully addressed, source reservoir Cu ratios need to be determined, and chemical and physical factors that control Cu isotope fractionation must be quantitatively defined.

Li abundances in inclusions in diamonds from the upper and lower mantle

Lithium abundances in inclusions in diamonds of peridotitic, websteritic, eclogitic (including garnets with majoritic component) and lower mantle paragenesis were determined with the ion microprobe. The partitioning of Li both between olivine and clinopyroxene and clinopyroxene and garnet appears to be pressure-dependent in peridotites. More importantly, garnets containing a majorite component show a dramatic increase in Li solubility so that even for mildly majoritic garnets D cpx/majorite is smaller than one. Lower mantle ferropericlase is a sink for lithium, containing several hundred times more Li than coexisting Mg-perovskite. Lithium decreases in olivine inclusions with increasing forsterite content, which we interpret as reflecting a depletion event in the protoliths of the inclusions. We also infer that lithium was not re-enriched in the silicate phases by a metasomatic agent during diamond formation unlike other trace elements like REE and HFSE. Depletion on partial melting and no re-enrichment also seems to hold for the eclogitic suite of inclusions where cpx's (and deduced bulk composition) have similar Li abundances as in the peridotitic suite and are significantly lower than in crustal eclogites. For the lower mantle bulk, lithium estimates give values four to five times higher than primitive mantle. This may relate either to local enrichment through fluid/melt metasomatism similarly as for spinel lherzolites or high Li abundances in lower mantle diamonds sources are inherited from crustal protoliths.