The phenomenon of thermal diffusion (mass diffusion driven by a temperature gradient, known as the Ludwig-Soret effect) has been investigated for over 150 years, but an understanding of its ...underlying physical basis remains elusive. A significant hurdle in studying thermal diffusion has been the difficulty of characterizing it. Extensive experiments over the past century have established that the Soret coefficient, S(T) (a single parameter that describes the steady-state result of thermal diffusion), is highly sensitive to many factors. This sensitivity makes it very difficult to obtain a robust characterization of thermal diffusion, even for a single material. Here we show that for thermal diffusion experiments that span a wide range in composition and temperature, the difference in S(T) between isotopes of diffusing elements that are network modifiers (iron, calcium and magnesium) is independent of the composition and temperature. On the basis of this finding, we propose an additive decomposition for the functional form of S(T) and argue that a theoretical approach based on local thermodynamic equilibrium holds promise for describing thermal diffusion in silicate melts and other complex solutions. Our results lead to a simple and robust framework for characterizing isotope fractionation by thermal diffusion in natural and synthetic systems.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the ...first combined Fe and Mg isotopic study of the upper mantle. δ
56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01
±
0.18‰ (2SD,
n
=
18), opx from 0.24‰ to −0.22‰ with an average of 0.04
±
0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10
±
0.19‰. δ
26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34
±
0.10‰ (2SD,
n
=
18), opx from −0.19‰ to −0.34‰ with an average of −0.25
±
0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24
±
0.18‰. Although current precision (∼±0.06‰ for δ
56Fe; ±0.10‰ for δ
26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ
56Fe
ol–cpx
=
−0.10
±
0.12‰ (2SD,
n
=
18); Δ
56Fe
ol–opx
=
−0.05
±
0.11‰; Δ
26Mg
ol–opx
=
−0.09
±
0.12‰; Δ
26Mg
ol–cpx
=
−0.10
±
0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ
56Fe of peridotites in this study is 0.01
±
0.17‰ (2SD,
n
=
18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ
26Mg is −0.30
±
0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ
56Fe and δ
26Mg.
The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ
56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral–mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ
56Fe, the larger deviations from average δ
56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ
56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ
56Fe and slightly heavy δ
26Mg, which may reflect Fe–Mg inter-diffusion between peridotite and percolating melt.
We conducted laboratory experiments to investigate isotopic fractionations during oxidation of tetravalent uranium, U(IV), by dissolved oxygen. In hydrochloric acid media with the U(IV) dissolved, ...the δ238U value of the remaining U(IV) increased as the extent of oxidation increased. The δ238U value of the product U(VI) paralleled, but was offset to 1.1±0.2‰ lower than the remaining U(IV). In contrast, oxidation of solid U(IV) by dissolved oxygen in 20mM NaHCO3 solution at pH=9.4 caused only a weak fractionation (∼0.1‰ to 0.3‰), with δ238U being higher in the dissolved U(VI) relative to the solid U(IV). We suggest that isotope fractionation during oxidation of solid U(IV) is inhibited by a “rind effect”, where the surface layer of the solid U(IV) must be completely oxidized before the next layer is exposed to oxidant. The necessity of complete conversion of each layer results in minimal isotopic effect. The weak shift in δ238U of U(VI) is attributed to adsorption of part of the product U(VI) to the solid U(IV) surfaces.
Uranium groundwater contamination due to U mining and processing affects numerous sites globally. Bioreduction of soluble, mobile U(VI) to U(IV)-bearing solids is potentially a very effective ...remediation strategy. Uranium isotopes (238U/235U) have been utilized to track the progress of microbial reduction, with laboratory and field studies finding a ∼1‰ isotopic fractionation, with the U(IV) product enriched in 238U. However, the isotopic fractionation produced by adsorption may complicate the use of 238U/235U to trace microbial reduction. A previous study found that adsorption of U(VI) onto Mn oxides produced a −0.2‰ fractionation with the adsorbed U(VI) depleted in 238U. In this study, adsorption to quartz, goethite, birnessite, illite, and aquifer sediments induced an average isotopic fractionation of −0.15‰ with the adsorbed U(VI) isotopically lighter than coexisting aqueous U(VI). In bicarbonate-bearing matrices, the fractionation depended little on the nature of the sorbent, with only birnessite producing an atypically large fractionation. In the case of solutions with ionic strengths much lower than those of typical groundwater, less isotopic fractionation was produced than U(VI) solutions with greater ionic strength. Studies using U isotope data to assess U(VI) reduction must consider adsorption as a lesser, but significant isotope fractionation process.
We experimentally determined the magnitude of uranium isotopic fractionation induced by U(VI) reduction by metal reducing bacterial isolates. Our results indicate that microbial U(VI) reduction ...induces isotopic fractionation; heavier isotopes (i.e., 238U) partition into the solid U(IV) products. The magnitudes of isotopic fractionation (expressed as ε=1000‰*(α−1)) for 238U/235U were 0.68‰±0.05‰ and 0.99‰±0.12‰ for Geobacter sulfurreducens strain PCA and strain IFRC-N, respectively. The ε values for Anaeromyxobacter dehalogenans strain FRC-W, strain FRC-R5, a novel Shewanella isolate, and Desulfitobacterium sp. strain Viet1 were 0.72‰±0.15‰, 0.99‰±0.12‰, 0.96‰±0.16‰ and 0.86‰±0.06‰, respectively. Our results show that the maximum ε values of ∼1.0‰ were obtained with low biomass (∼107cells/mL) and low electron donor concentrations (∼500μM). These results provide an initial assessment of 238U/235U shifts induced by microbially-mediated U(VI) reduction, which is needed as 238U/235U data are increasingly applied as redox indicators in various geochemical settings.
U isotope fractionation may serve as an accurate proxy for U(VI) reduction in both modern and ancient environments, if the systematic controls on the magnitude of fractionation (ε) are known. We ...model the effect of U(VI) reduction kinetics on U isotopic fractionation during U(VI) reduction by a novel Shewanella isolate, Shewanella sp. (NR), in batch incubations. The measured ε values range from 0.96 ± 0.16 to 0.36 ± 0.07‰ and are strongly dependent on the U(VI) reduction rate. The ε decreases with increasing reduction rate constants normalized by cell density and initial U(VI). Reactive transport simulations suggest that the rate dependence of ε is due to a two-step process, where diffusive transport of U(VI) from the bulk solution across a boundary layer is followed by enzymatic reduction. Our results imply that the spatial decoupling of bulk U(VI) solution and enzymatic reduction should be taken into account for interpreting U isotope data from the environment.
The Sonju Lake Intrusion (SLI) is a 1200-meter thick layered mafic intrusion that directly underlies an equally large silicic pluton, the Finland granophyre (FG) within the Beaver Bay Complex of the ...Mid-Continent Rift (MN, USA). The SLI, with a simple mineralogical and compositional stratigraphy, provides an excellent case study for examining the changes in iron isotope ratios (δ
56
Fe). Here new Fe isotope data along with
87
Sr/
86
Sr for a set of stratigraphically controlled samples from the SLI and FG are presented. The Fe isotope data show systematic changes within two differentiation sequences found in the lowermost FG as well as the upper portion of the SLI. Specifically, δ
56
Fe is observed to start at low values and increase to heavy values going stratigraphically up through each differentiation sequence. Within the middle portion of the SLI, δ
56
Fe varies between 0 and 0.1. Two samples from the SLI bottom are isotopically lighter than the middle SLI. The origin of the Fe isotope variations is discussed in terms of recently proposed explanations. A quantitative model shows that the observed spatial variation is consistent with the prediction of a temperature gradient model. Using present constraints on equilibrium phase partitioning, the iron isotope variations do not appear consistent with production by fractional crystallization. Based on these observations, a top-down sill emplacement process coupled with in situ differentiation remains a viable alternative model for forming this layered intrusion.
Melt compositions in equilibrium with quartz and albite at 0.1 GPa and temperatures from 819 to 330 °C are reported for cold seal experiments in the Na2O‐Al2O3‐SiO2‐H2O system. At 60–70 °C intervals, ...melt compositions continuously change from rhyolite‐like melts with <10 wt.% H2O at the quartz‐albite eutectic to melts of nearly endmember Na2Si2O5 having >30 wt.% H2O at 330 °C (vapor undersaturated). Melt Na2O increases while SiO2 and Al2O3 decrease down temperature. Experiments performed at T > 600 °C are vapor saturated with <9.9–21 wt.% H2O while those T < 600 °C are vapor undersaturated with 18–37 wt.% H2O. Because of the continuous change in the liquid's composition down temperature, all compositions can be called melts; in reality, their high water contents may also lead to being termed hydrous fluids. These results indicate that igneous processes continue to temperatures >300 °C below the haplogranite solidus. The water‐rich melts at low temperature have low viscosities and densities suggesting that they buoyantly ascend by reactive flow at low melt porosities (<5%). Formation of low‐temperature melt will occur along the sides of gabbro bodies of mid‐ocean ridges, resulting in large‐scale convection that removes heat from the lower crust. This circulation may move heat off axis influencing observed heat flow. Melt reaction processes result in silicic differentiates (plagiogranites) and formation of greenschist metabasalts. This frees calcium from silicate bonding, providing an important sink for CO2 in Earth's carbon cycle. Sr isotopes of the ocean crust change with ocean crust production rates indicating mid‐ocean ridge systems are a major control on Earth's climate over time.
Plain Language Summary
The lowest temperature that silicate melt exists in the Earth is critically relevant to how silica‐rich igneous rocks form and how heat is transported in magmatic systems. This work presents experiments that show that silicate melt can coexist with quartz and sodium feldspar at temperatures as low as 330 °C. The existence of this melt may play a crucial role in how tectonic plates cool. The interaction between this melt and existing crust results in release of Ca from rock, which can strongly influence Earth's climate by bonding with CO2.
Key Points
Continuous change of melt in equilibrium with quartz and albite from 819–330 °C and 0.1 GPa
Melt at 400–550 °C will form in thermal gradient zones around magma bodies and circulate, providing a means to cool the lower oceanic crust
Reaction of melt at 450 °C with basaltic rock produces plagiogranites and greenschist metamorphosed rocks, potentially playing a role in Ca release and planetary climate control
U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th ...and U concentrations and (
238U)/(
232Th), (
230Th)/(
232Th), (
230Th)/(
238U) and (
234U)/(
238U) for 43 samples, most of which are minimally differentiated, along with (
226Ra)/(
230Th) and (
231Pa)/(
235U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial (
230Th)/(
238U) ranges from 1.1 to 1.6. (
226Ra)/(
230Th)
o ranges between 0.9 and 1.8 while (
231Pa)/(
235U)
o ranges between 1.0 and 2.0.
Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume.
Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia.
The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented ...geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit.
Results show large variations in
7Li/
6Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average δ
7Li for single thin sections along the traverse reveal a low in δ
7Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ∼10 per mil in the low δ
7Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in
7Li/
6Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction.
We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in δ
7Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.
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