59 research outputs found
A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge
No full textMantle oxygen fugacity (fO2) governs the physico-chemical evolution of the Earth, however current estimates from commonly used basalt redox proxies are often in disagreement. In this study we compare three different potential basalt fO2 proxies: Fe3+/Fetot, V/Sc and V isotopes, determined on the same submarine lavas from a 700 km section of the Reykjanes Ridge, near Iceland. These samples provide a valuable test of the sensitivities of fO2 proxies to basalt petrogenesis, as they formed at different melting conditions and from a mantle that towards Iceland exhibits increasing long-term enrichment of incompatible elements. New trace element data were determined for 63 basalts with known Fe3+/Fetot. A subset of 19 lavas, covering the geographical spread of the ridge transect, was selected for vanadium isotope analyses. Vanadium is a multi-valence element whose isotopic fractionation is theoretically susceptible to redox conditions. Yet, the δ 51 VAA composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range ( δ 51 VAA = −1.09 to −0.86‰; 1SD = 0.02–0.09) and does not co-vary with fractionation-corrected Fe3+/Fetot (0.134–0.151; 1SD = 0.005) or V/Sc (6.6–8.5; 1SD = 0.1-1.3) ratios. However, on a global scale, basaltic δ 51 VAA may be controlled by the extent of melting. The V/Sc compositions of primitive (MgO > 7.5 wt%) basalts show no systematic change along the entire length of the Reykjanes Ridge. Typical peridotite melting models in which source Fe3+/Fetot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO2 dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe3+/Fetot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe3+/Fetot of Reykjanes Ridge lavas remain nearly constant over the ridge length. This discrepancy is explained by source heterogeneity, where an oxidized mantle pyroxenite component contributes to melting with increasing proximity to Iceland. Comparison of observed and modeled Fe3+/Fetot indicate that source variation in fO2 is present under the Reykjanes Ridge, with higher Fe3+/Fetot closer to Iceland. This source variability in fO2 cannot be resolved by V isotopes and redox-sensitive trace element ratios, which instead appear to record magmatic processes
Compositional trends of Icelandic basalts: Implications for short–length scale lithological heterogeneity in mantle plumes
No full textLithological variations in the mantle source regions under mid-ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we combine compositional observations from Icelandic basalts and modelling of melting of a bi-lithologic peridotite-pyroxenite mantle to demonstrate that, while short-lengthscale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of endmember Icelandic melts, we find enriched melts to be characterised by low SiO2 and CaO, but high FeO. We quantitatively compare endmember compositions to experimental partial melts generated from a range of lithologies, pressures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilised by the addition of up to 40% mid-ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be over-represented in accumulated melts compared with its abundance in the source. Modelling of peridotite-pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations
Control of the symmetry of plume-ridge interaction by spreading ridge geometry
No full textThe Iceland, Gal´apagos and Azores plumes have previously
been identified as interacting asymmetrically with adjacent spreading centres.
We present evidence that the flow fields in these plume heads are radially
symmetric, but the geometry of the mid-ocean ridge systems imparts
an asymmetric compositional structure on outflowing plume material. First,
we quantify the degree of symmetry in geophysical and geochemical observables
as a function of plume centre location. For each plume, we find that
bathymetry and crustal thickness observations can be explained using a single
centre of symmetry, with these calculated centres coinciding with independently
inferred plume centre locations. The existence of these centres of
symmetry suggests that the flow fields and temperature structure of the three
plume heads are radially symmetric. However, no centres of symmetry can
be found for the incompatible trace element and isotopic observations. To
explain this, we develop a simple kinematic model to predict the effect of midocean
ridge geometry on the chemical composition of outflowing plume material.
The model assumes radially symmetric outflow from a compositionally
heterogeneous plume source, consisting of a depleted mantle component
and enriched blebs. These blebs progressively melt out during flow through
the melting regions under spreading centres. Asymmetry in trace element
and isotopic profiles develops when ridges either side of the plume centre receive
material that has been variably depleted according to the length of flow
path under the ridge. This model can successfully explain compositional asymmetry around Iceland and Gal´apagos in terms of an axisymmetric plume interacting
with an asymmetric ridge system
Azimuthal C/O variations in a planet-forming disk
No full textThe elemental carbon-to-oxygen ratio (C/O) in the atmosphere of a giant planet is a promising diagnostic of that planet’s formation history in a protoplanetary disk. Alongside efforts in the exoplanet community to measure the C/O ratio in planetary atmospheres, observational and theoretical studies of disks are increasingly focused on understanding how the gas-phase C/O ratio varies both with radial location and between disks. This is mostly tied to the icelines of major volatile carriers such as CO and H2O. Using ALMA observations of CS and SO, we have found evidence for an entirely unexpected type of C/O variation in the protoplanetary disk around HD 100546: an azimuthal variation from a typical, oxygen-dominated ratio (C/O ≈ 0.5) to a carbon-dominated ratio (C/O ≳ 1.0). We show that the spatial distribution and peculiar line kinematics of both CS and SO molecules can be well explained by azimuthal variations in the C/O ratio. We propose a shadowing mechanism that could lead to such a chemical dichotomy. Our results imply that tracing the formation history of giant exoplanets using their atmospheric C/O ratios will need to take into account time-dependent azimuthal C/O variations in a planet’s accretion zone
Compositional trends of Icelandic basalts: Implications for short-length scale lithological heterogeneity in mantle plumes
No full textScum of the Earth: a hypothesis for prebiotic multi-compartmentalised environments
No full textCompartmentalisation by bioenergetic membranes is a universal feature of life. The eventual compartmentalisation of prebiotic systems is therefore often argued to comprise a key step during the origin of life. Compartments may have been active participants in prebiotic chemistry, concentrating and spatially organising key reactants. However, most prebiotically plausible compartments are leaky or unstable, limiting their utility. Here, we develop a new hypothesis for an origin of life environment, that capitalises upon, and mitigates the limitations of, prebiotic compartments: multi-compartmentalised layers in the near surface environment --- a 'scum'. Scum-type environments benefit from many of the same ensemble-based advantages as microbial biofilms. In particular, scum layers mediate diffusion with the wider environment, favouring preservation and sharing of early informational molecules, along with the selective concentration of compatible prebiotic compounds. Biofilms are among the earliest traces imprinted by life in the rock record: we contend that prebiotic equivalents of these environments deserve future experimental investigation
MSA ROEBLING MEDAL LECTURE: THE EFFECTS OF SOLID-SOLID PHASE EQUILIBRIA AND PARTIAL MELTING ON THE OXYGEN FUGACITY OF THE UPPER MANTLE
No full textThe temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry
No full textNew crystallization temperatures for four eruptions from the Northern Volcanic Zone of Iceland are determined using olivine-spinel aluminum exchange thermometry. Differences in the olivine crystallization temperatures between these eruptions are consistent with variable extents of cooling during fractional crystallization. However, the crystallization temperatures for Iceland are systematically offset to higher temperatures than equivalent olivine-spinel aluminum exchange crystallization temperatures published for MORB, an effect that cannot be explained by fractional crystallization. The highest observed crystallization temperature in Iceland is 1399 ± 20°C. In order to convert crystallization temperatures to mantle potential temperature, we developed a model of multilithology mantle melting that tracks the thermal evolution of the mantle during isentropic decompression melting. With this model, we explore the controls on the temperature at which primary melts begin to crystallize, as a function of source composition and the depth from which the magmas are derived. Large differences (200°C) in crystallization temperature can be generated by variations in mantle lithology, a magma's inferred depth of origin, and its thermal history. Combining this model with independent constraints on the magma volume flux and the effect of lithological heterogeneity on melt production, restricted regions of potential temperature-lithology space can be identified as consistent with the observed crystallization temperatures. Mantle potential temperature is constrained to be 1480−30+37 °C for Iceland and 1318−32+44 °C for MORB
Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source
No full textThe temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry
No full textNew crystallization temperatures for four eruptions from the Northern Volcanic Zone of Iceland are determined using olivine-spinel aluminum exchange thermometry. Differences in the olivine crystallization temperatures between these eruptions are consistent with variable extents of cooling during fractional crystallization. However, the crystallization temperatures for Iceland are systematically offset to higher temperatures than equivalent olivine-spinel aluminum exchange crystallization temperatures published for MORB, an effect that cannot be explained by fractional crystallization. The highest observed crystallization temperature in Iceland is 1399 ± 20°C. In order to convert crystallization temperatures to mantle potential temperature, we developed a model of multilithology mantle melting that tracks the thermal evolution of the mantle during isentropic decompression melting. With this model, we explore the controls on the temperature at which primary melts begin to crystallize, as a function of source composition and the depth from which the magmas are derived. Large differences (200°C) in crystallization temperature can be generated by variations in mantle lithology, a magma's inferred depth of origin, and its thermal history. Combining this model with independent constraints on the magma volume flux and the effect of lithological heterogeneity on melt production, restricted regions of potential temperature-lithology space can be identified as consistent with the observed crystallization temperatures. Mantle potential temperature is constrained to be math formula °C for Iceland and math formula °C for MORB
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