Chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past. However, Cr is a trace ...metal in seawater and its low concentrations make isotopic measurements challenging. Here we report the first determinations of δCr53 for seawater from open ocean (Argentine Basin) and coastal (Southampton Water) settings, using a double-spike technique. The total chromium concentration in seawater from Southampton Water is 1.85 nM, whereas the Cr content of Argentine Basin samples is 5.8–6.6 nM. The δCr53 value of seawater from the Argentine Basin is 0.491–0.556‰ in intermediate and deep waters, and varies between 0.412 and 0.664‰ in surface waters (<150 m). The δCr53 value of Southampton Water seawater is 1.505‰, which may reflect in situ reduction of Cr(VI) to Cr(III). All of our seawater samples have higher δCr53 than crustal and mantle silicates, and mass balance modelling demonstrates that river water must also be enriched in heavy Cr isotopes, indicating that Cr isotopes are fractionated during weathering and/or during transport to the oceans.
We also show that the Cr isotopic composition of modern non-skeletal marine carbonates (0.640– 0.745‰) encompasses the range that we measure for Argentine Basin seawater. Thus, fractionation of Cr isotopes during precipitation of these marine carbonates is likely to be small (<0.2‰), and they have the potential to provide a record of the Cr isotopic composition of ancient seawater. Phanerozoic carbonates are also characterised by heavy δCr53 and a correlation between δCr53 and Ce/Ce⁎ suggests that the Cr and Ce cycles in the ocean are linked.
•First stable Chromium isotope dataset for seawater.•First stable Chromium isotope dataset for modern marine carbonates.•Positive δCr53 for seawater and modern carbonates.•Correlation between Cr isotopes and Ce anomaly in Phanerozoic carbonates.
Galaxy And Mass Assembly (GAMA): stellar mass estimates Taylor, Edward N.; Hopkins, Andrew M.; Baldry, Ivan K. ...
Monthly notices of the Royal Astronomical Society,
December 2011, Letnik:
418, Številka:
3
Journal Article
Recenzirano
Odprti dostop
This paper describes the first catalogue of photometrically derived stellar mass estimates for intermediate-redshift (z < 0.65; median z= 0.2) galaxies in the Galaxy And Mass Assembly (GAMA) ...spectroscopic redshift survey. These masses, as well as the full set of ancillary stellar population parameters, will be made public as part of GAMA data release 2. Although the GAMA database does include near-infrared (NIR) photometry, we show that the quality of our stellar population synthesis fits is significantly poorer when these NIR data are included. Further, for a large fraction of galaxies, the stellar population parameters inferred from the optical-plus-NIR photometry are formally inconsistent with those inferred from the optical data alone. This may indicate problems in our stellar population library, or NIR data issues, or both; these issues will be addressed for future versions of the catalogue. For now, we have chosen to base our stellar mass estimates on optical photometry only. In light of our decision to ignore the available NIR data, we examine how well stellar mass can be constrained based on optical data alone. We use generic properties of stellar population synthesis models to demonstrate that restframe colour alone is in principle a very good estimator of stellar mass-to-light ratio, M
*/Li
. Further, we use the observed relation between restframe (g−i) and M
*/Li
for real GAMA galaxies to argue that, modulo uncertainties in the stellar evolution models themselves, (g−i) colour can in practice be used to estimate M
*/Li
to an accuracy of ≲0.1 dex (1σ). This 'empirically calibrated' (g−i)-M
*/Li
relation offers a simple and transparent means for estimating galaxies' stellar masses based on minimal data, and so provides a solid basis for other surveys to compare their results to z≲0.4 measurements from GAMA.
We present new mass independent and mass dependent Cr isotope compositions for meteorites measured by double spike thermal ionisation mass spectrometry. Small differences in both mass independent ...53Cr and 54Cr relative to the Bulk Silicate Earth are reported and are very similar to previously published values. Carbonaceous chondrites are characterised by an excess in 54Cr compared to ordinary and enstatite chondrites which make mass independent Cr isotopes a useful tool for distinguishing between meteoritic groups. Mass dependent stable Cr isotope compositions for the same samples are also reported. Carbonaceous and ordinary chondrites are identical within uncertainty with average δ53Cr values of −0.118±0.040‰ and −0.143±0.074‰ respectively. The heaviest isotope compositions are recorded by an enstatite chondrite and a CO carbonaceous chondrite, both of which have relatively reduced chemical compositions implying some stable Cr isotope fractionation related to redox processes in the circumstellar disk. The average δ53Cr values for chondrites are within error of the estimate for the Bulk Silicate Earth (BSE) also determined by double spiking. The lack of isotopic difference between chondritic material and the BSE provides evidence that Cr isotopes were not fractionated during core formation on Earth. A series of high-pressure experiments was also carried out to investigate stable Cr isotope fractionation between metal and silicate and no demonstrable fractionation was observed, consistent with our meteorites data. Mass dependent Cr isotope data for achondrites suggest that Cr isotopes are fractionated during magmatic differentiation and therefore further work is required to constrain the Cr isotopic compositions of the mantles of Vesta and Mars.
•We report high precision mass dependent Cr isotope variations in meteorites.•The average δ53Cr values for chondrites are within error of the Bulk Silicate Earth.•There is no equilibrium fractionation between metal and silicate during HP–HT experiments.•Cr isotopes are not fractionated during core formation on Earth.
We present a detailed investigation into the effects of galaxy environment on their star formation rates (SFRs) using galaxies observed in the Galaxy And Mass Assembly (GAMA) survey. We use three ...independent volume-limited samples of galaxies within z < 0.2 and M
r
< −17.8. We investigate the known SFR-density relationship and explore in detail the dependence of SFR on stellar mass and density. We show that the SFR-density trend is only visible when we include the passive galaxy population along with the star-forming population. This SFR-density relation is absent when we consider only the star-forming population of galaxies, consistent with previous work. While there is a strong dependence of the EWHα on density we find, as in previous studies, that these trends are largely due to the passive galaxy population and this relationship is absent when considering a 'star-forming' sample of galaxies. We find that stellar mass has the strongest influence on SFR and EWHα with the environment having no significant effect on the star formation properties of the star-forming population. We also show that the SFR-density relationship is absent for both early- and late-type star-forming galaxies. We conclude that the stellar mass has the largest impact on the current SFR of a galaxy, and any environmental effect is not detectable. The observation that the trends with density are due to the changing morphology fraction with density implies that the time-scales must be very short for any quenching of the SFR in infalling galaxies. Alternatively, galaxies may in fact undergo predominantly in situ evolution where the infall and quenching of galaxies from the field into dense environments is not the dominant evolutionary mode.
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.
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, suprasubduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0 times 40-0 times 80), very low incompatible element contents, and (with the exception of amphibolebearing 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 times 1 (log units) and FMQ + 0 times 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 times 2 to FMQ + 1 times 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 times 8 and FMQ + 1 times 6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1 times 2 to FMQ + 1 times 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.
Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (
f
O
2
values) calculated from the ...ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by
57
Fe
Mössbauer spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, 53, 1277–1291., yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between
f
O
2
values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between
f
O
2
and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between
f
O
2
and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe
2O
3. This amount of Fe
2O
3 in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H
2O is required which is considerably less than the 0.25% H
2O envisaged by Stolper and Newman Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325., suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in
f
O
2
of the source region. Although it is unclear which model is correct the maximum
f
O
2
of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher
f
O
2
values of primitive arc lavas than arc-peridotites.
This study reports stable and radiogenic strontium isotope behaviour in the dissolved load and suspended sediments from the subglacial outflow of the Lemon Creek glacier (Juneau Ice Field, Alaska) ...over a single melt season. In situ measurements (discharge, total alkalinity, pH and conductivity) are combined with elemental concentrations, X-ray diffraction (XRD) analysis and radiogenic strontium isotope measurements to interpret the variations observed in stable strontium isotopic ratios.
The stable Sr isotope composition (88Sr/86Sr ratio expressed as δ88/86Sr, ‰) of the dissolved load averages 0.31±0.05‰, and is heavier than both the suspended sediment 0.18±0.03‰, as well as local bedrocks ~0.20 to 0.26‰. We attribute the enrichment of heavier isotopes in the dissolved load to the uptake of lighter Sr isotopes by secondary weathering minerals, driving the dissolved load to heavier values. X-ray diffraction (XRD) analysis confirms the presence of clays in the suspended sediments and thermodynamic modelling suggests the presence of iron oxy-hydroxide phases. Although it is not possible to completely rule out the effect of dissolution of primary minerals in controlling Sr isotopic compositions of the dissolved load, our data indicate that the extent of secondary mineral formation likely plays a significant role. The preferential weathering of minerals such as biotite (consistent with the mineralogical assemblages found in the suspended sediments), as well as the potential presence of radiogenic calcites from metacarbonates (derived from the Yukon-Tanana terrain), may be driving the small seasonal shifts in 87Sr/86Sr of the dissolved load to more radiogenic compositions, from 87Sr/86Sr(DL)=0.71048 to 0.710647.
Using the combination of stable and radiogenic strontium isotopes to investigate weathering processes shows that radiogenic Sr isotopes provide information regarding weathering of primary phases. While the stable Sr isotope data appear to record information regarding the extent of secondary mineral formation, where secondary minerals incorporate the light isotopes, driving the dissolved load to heavy values.
•Stable strontium isotopes fractionate during subglacial weathering.•δ88/86Sr of the dissolved load is heavier than bedrock values.•δ88/86Sr of suspended sediments are lighter than bedrock values.•Combined 87Sr/86Sr–δ88/86Sr systems track both weathering source and process.
The decay of
187Re to
187Os provides a unique tracer of recycled lithosphere in the Earth's upper mantle, and the radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) have been ...taken to indicate the presence of recycled oceanic crust in the mantle source. However, others have attributed the data to contamination by seawater or an age effect (due to
187Re decay following igneous crystallisation). This study presents new Re–Os, U–Th–Ra and B isotope data for normal-MORB (N-MORB) glass (including 18 samples analysed previously). The scale of the variation in
187Os/
188Os observed here in N-MORB glass ranges from 0.126 to 0.148, with a mean value of 0.133
±
0.009 (2
σ). Previously published Os isotope data for MORB is shown to have been affected by an analytical artefact, and in particular the very radiogenic Os isotope compositions reported earlier are now shown not to exist. Nevertheless, some radiogenic Os isotope compositions are still seen in the data obtained here. Coupled Th and Ra excesses appear to rule out an age effect, at least, for those samples studied, however,
δ
11B covariations with
187Os/
188Os are consistent with the assimilation of older oceanic crust (affected by seawater contamination). Individual sulphides (less likely to be affected by such contamination) are always less radiogenic than their glass host, with
187Os/
188Os values ranging from 0.123 to 0.131. This indicates that sulphide precipitation occurred prior to contamination of the surrounding melt, but the data also suggest that low-Os sulphides have been affected by contamination through partial equilibration with that melt. Taken together, these indicate that dominant process responsible for the radiogenic Os isotope compositions of N-MORB glass is contamination by older oceanic crust. Although the influence of age effects and the presence of recycled material cannot be completely ruled out, Os-rich sulphides yield sub-chondritic Os isotope compositions which point rather to a long-term depletion in Re in the MORB source likely to have resulted from formation of the continental crust over much of Earth's history.
Cenozoic volcanism within Mongolia forms part of a large central Asian province of intra-plate magmatism. Numerous small-volume volcanic cones and alkali basalt lava flows have been formed since c. ...30 Ma; from c. 12 Ma activity has been focused on the uplifted Hangai dome. A mechanism for melting beneath the dome has, however, thus far remained enigmatic. Some of the oldest basalts on the Hangai dome erupted at its centre at similar to 6 Ma and their geochemistry suggests a garnet lherzolite source region at 90-100 km depth. These lavas have Pb isotope compositions similar to those of depleted Indian mid-ocean ridge basalts (MORB) ( super(206)Pb/ super(204)Pb = 17.822, super(207)Pb/ super(204)Pb = 15.482, super(208)Pb/ super(204)Pb = 37.767), which may be indicative of the involvement of ambient asthenospheric mantle in their petrogenesis. Younger basalts exhibit a gradual shift in isotopic composition towards a source that has less radiogenic Pb and more radiogenic Sr, evidenced by the eruption of lavas with super(206)Pb/ super(204)Pb = 16.991 and super(87)Sr/ super(86)Sr = 0.704704. The youngest lavas, dated as younger than similar to 8 ka, have the highest K sub(2)O contents (up to 5.2 wt %) and are characterized by the most enriched trace-element signatures; they are interpreted to represent melting of a metasomatically altered sub-continental lithospheric mantle containing phlogopite. Concurrent with progressive melting of the lithosphere, melting appears to propagate outwards from the centre of the dome to its margins; by 0.7 Ma the marginal magmatism is interpreted to result from melting of a depleted MORB-source mantle component with a smaller contribution from the lithospheric mantle. The spatial and temporal variations in melting beneath the Hangai dome may be explained by either lithospheric delamination or the presence of a small-scale thermal anomaly in the upper mantle. Although it is not possible to distinguish between these models on the basis of geochemistry alone, the lack of a viable mechanism to generate small-scale upwelling lends support to a model involving delamination of the lithospheric mantle beneath the Hangai dome.
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