Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle ...wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt‐Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P‐T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S‐bearing and C‐bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.
Key Points
Iron and zinc stable isotope and elemental data are presented for a prograde suite of metabasalts and metagabbros from Western Alpine ophiolite complexes
Bulk rock δ56Fe and δ66Zn do not vary across metamorphic facies and the eclogitic samples show a MORB‐like isotope composition
Blueschist facies metagabbros preserve evidence for infiltration of sediment derived fluids that impart a light δ56Fe isotope composition to the gabbro
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present titanium stable isotope measurements of terrestrial magmatic samples and lunar mare basalts with the aims of constraining the composition of the lunar and terrestrial mantles and ...evaluating the potential of Ti stable isotopes for understanding magmatic processes. Relative to the OL–Ti isotope standard, the δ49Ti values of terrestrial samples vary from −0.05 to +0.55‰, whereas those of lunar mare basalts vary from −0.01 to +0.03‰ (the precisions of the double spike Ti isotope measurements are ca. ±0.02‰ at 95% confidence). The Ti stable isotope compositions of differentiated terrestrial magmas define a well-defined positive correlation with SiO2 content, which appears to result from the fractional crystallisation of Ti-bearing oxides with an inferred isotope fractionation factor of ΔTioxide–melt49=−0.23‰×106/T2. Primitive terrestrial basalts show no resolvable Ti isotope variations and display similar values to mantle-derived samples (peridotite and serpentinites), indicating that partial melting does not fractionate Ti stable isotopes and that the Earth's mantle has a homogeneous δ49Ti composition of +0.005 ± 0.005 (95% c.i., n=29). Eclogites also display similar Ti stable isotope compositions, suggesting that Ti is immobile during dehydration of subducted oceanic lithosphere. Lunar basalts have variable δ49Ti values; low-Ti mare basalts have δ49Ti values similar to that of the bulk silicate Earth (BSE) while high-Ti lunar basalts display small enrichment in the heavy Ti isotopes. This is best interpreted in terms of source heterogeneity resulting from Ti stable isotope fractionation associated with ilmenite–melt equilibrium during the generation of the mantle source of high-Ti lunar mare basalts. The similarity in δ49Ti between terrestrial samples and low-Ti lunar basalts provides strong evidence that the Earth and Moon have identical stable Ti isotope compositions.
•First study of the Ti stable isotope composition of terrestrial and lunar rocks.•Ti isotopes display large variations during magma differentiation.•The terrestrial and lunar mantles display similar mantle Ti stable composition.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Abstract
The engineered removal of atmospheric CO
2
is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, ...in the case of Iceland’s CarbFix experiment, precipitates dissolved CO
2
as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO
2
injection waters to quantify the amount of carbonate precipitated, and hence CO
2
stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO
2
were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.
The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ
30Si
=
−0.29
±
0.08‰, 2
s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved ...igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.
At an average SiO
2 content of ∼60
wt.%, the predicted δ
30Si value of the continental crust that should result from magmatic fractionation alone is −0.23
±
0.05‰ (2
s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ
30Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Existing strontium radiogenic isotope (87Sr/86Sr) measurements for foraminifera over Quaternary glacial–interglacial climate cycles provide no evidence for variations in the isotope composition of ...seawater at the ±9–13 ppm level of precision. However, modelling suggests that even within this level of uncertainty significant (up to 30%) variations in chemical weathering of the continents are permitted, accounting for the longer-term rise in 87Sr/86Sr over the Quaternary, and the apparent imbalance of Sr in the oceans at the present-day. This study presents very high-precision 87Sr/86Sr isotope data for modern seawater from each of the major oceans, and a glacial–interglacial seawater record preserved by planktic foraminifera from Ocean Drilling Program (ODP) Site 758 in the north-east Indian ocean. Strontium isotope 87Sr/86Sr measurements for modern seawater from the Atlantic, Pacific and Indian Oceans are indistinguishable from one another (87Sr/86Sr = 0.7091792 ± 0.0000021, n=17) at the level of precision obtained in this study (±4.9 ppm 2σ). This observation is consistent with the very long residence time of Sr in seawater, and underpins the utility of this element for high precision isotope stratigraphy. The 87Sr/86Sr seawater record preserved by planktic foraminifera shows no resolvable glacial–interglacial variation (87Sr/86Sr = 0.7091784 ± 0.0000035, n=10), and limits the response of seawater to variations in the chemical weathering flux and/or composition to ±4.9 ppm or less. Calculations suggest that a variation of ±12% around the steady-state weathering flux can be accommodated by the uncertainties obtained here. The new data cannot accommodate a short-term weathering pulse during de-glaciation, although a more a diffuse weathering pulse accompanying protracted ice retreat is permissible. However, these results still indicate that modern weathering fluxes are potentially higher than average over the Quaternary, and such variations through glacial cycles can also account for the longer-term rise in 87Sr/86Sr over this time interval. The very high-precision measurements made for the marine 87Sr/86Sr record in this study place clear limits on the magnitude and timing of changes in the chemical weathering flux during glacial–interglacial cycles. Further, constraints must be sought from even higher precision measurement or elements with shorter residence times in the ocean, such as osmium (Os), that have the capacity to respond to short-term variations in input.
•High-precision glacial–interglacial 87Sr/86Sr record from planktic foraminifera.•Major oceans yield indistinguishable 87Sr/86Sr values at ±5 ppm level of precision.•Foraminifera show no resolvable 87Sr/86Sr variation since last glacial interval.•These data accommodate a ±12% variation for the steady-state weathering flux.•A short-term weathering pulse during de-glaciation is not accommodated by the data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Sensory processing in olfactory systems is organized across olfactory bulb glomeruli, wherein axons of peripheral sensory neurons expressing the same olfactory receptor co-terminate to transmit ...receptor-specific activity to central neurons. Understanding how receptors map to glomeruli is therefore critical to understanding olfaction. High-throughput spatial transcriptomics is a rapidly advancing field, but low-abundance olfactory receptor expression within glomeruli has previously precluded high-throughput mapping of receptors to glomeruli in the mouse. Here we combined sequential sectioning along the anteroposterior, dorsoventral, and mediolateral axes with target capture enrichment sequencing to overcome low-abundance target expression. This strategy allowed us to spatially map 86% of olfactory receptors across the olfactory bulb and uncover a relationship between OR sequence and glomerular position.
The engineered removal of atmospheric CO
is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the ...case of Iceland's CarbFix experiment, precipitates dissolved CO
as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO
injection waters to quantify the amount of carbonate precipitated, and hence CO
stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO
were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.
Lithium isotopes are rapidly becoming one of the most useful tracers of silicate weathering processes, but little is known on their behaviour in groundwaters and hydrothermal springs, and how these ...sources might influence the weathering signal in surface waters. This study presents lithium isotope compositions (δ7Li) for cold groundwaters (3–7°C) and hydrothermal springs that were at geothermal temperatures (200–300°C) but have cooled during transport (17–44°C). Both represent an important source of water and nutrients for Lake Myvatn, Iceland. We also present a time-series from the Laxa River, which is the single outflow from the lake. The δ7Li values in the input springs to Lake Myvatn are highly variable (5–27‰), and correlate inversely with temperature and total dissolved solids. These co-variations imply that even in such waters, the processes controlling δ7Li variations during weathering still operate: that is, the ratio of primary rock dissolution to secondary mineral formation, where the latter preferentially incorporates 6Li with a temperature-dependent fractionation factor. In high-temperature geothermal waters (>300°C) secondary mineral formation is inhibited, and has a low fractionation factor, leading to little δ7Li fractionation. Even in waters that have cooled considerably over several months from their geothermal temperatures, fractionation is still low, and δ7Li values are similar to those reported from waters measured at >350°C. In contrast, cooler groundwaters promote relatively high proportions of clay formation, which scavenge dissolved solids (including 6Li). The time series on the Laxa River, the single outflow from Lake Myvatn, shows little δ7Li variation with time over the 12month sampling period (17–21‰), demonstrating that in contrast to tracers such as Si isotopes, Li isotopes are unaffected by the significant seasonal phytoplankton blooms that occur in the lake. Thus, these results clearly illustrate that Li isotopes are ideally suited to constrain silicate weathering processes, because fractionation by secondary mineral formation operates even when groundwater and hydrothermal inputs are significant, and because Li isotopes are demonstrably unaffected by phytoplankton or plant growth.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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