New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts ...per million lower. The tungsten isotopic data negatively correlate with ³He/⁴He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low ³He/⁴He.
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Magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks. The extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for ...some elements. We report analyses of iron isotopes in basalts from Kilauea Iki lava lake, Hawaii. The iron isotopic compositions (⁵⁶Fe/⁵⁴Fe) of late-stagemeltveins are 0.2 permil (per thousand) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2per thousand lighter than those of whole rocks. These results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales. This characteristic of iron relative to the characteristics of magnesium and lithium, for which no fractionation has been found, may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation.
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The zinc stable isotope system has been successfully applied to many and varied fields in geochemistry, but to date it is still not completely clear how this isotope system is affected by igneous ...processes. In order to evaluate the potential application of Zn isotopes as a proxy for planetary differentiation and volatile history, it is important to constrain the magnitude of Zn isotopic fractionation induced by magmatic differentiation. In this study we present high-precision Zn isotope analyses of two sets of chemically diverse, cogenetic samples from Kilauea Iki lava lake, Hawaii, and Hekla volcano, Iceland, which both show clear evidence of having undergone variable and significant degrees of magmatic differentiation.
The Kilauea Iki samples display small but resolvable variations in Zn isotope composition (0.26‰<δ66Zn<0.36‰; δ66Zn defined as the per mille deviation of a sample's 66Zn/64Zn compositional ratio from the JMC-Lyon standard), with the most differentiated lithologies exhibiting more positive δ66Zn values. This fractionation is likely a result of the crystallization of olivine and/or Fe–Ti oxides, which can both host Zn in their crystal structures. Samples from Hekla have a similar range of isotopic variation (0.22‰<δ66Zn<0.33‰), however, the degree of fractionation caused by magmatic differentiation is less significant (only 0.07‰) and no correlation between isotope composition and degree of differentiation is seen. We conclude that high temperature magmatic differentiation can cause Zn isotope fractionation that is resolvable at current levels of precision, but only in compositionally-evolved lithologies. With regards to primitive (ultramafic and basaltic) material, this signifies that the terrestrial mantle is essentially homogeneous with respect to Zn isotopes. Utilizing basaltic and ultramafic sample analyses, from different geologic settings, we estimate that the average Zn isotopic composition of Bulk Silicate Earth is δ66Zn=0.28±0.05‰ (2s.d.).
► We report Zn isotope values of two suites of volcanic rocks. ► The Kilauea Iki samples display small but resolvable variations in Zn isotope composition. ► Earth mantle is homogeneous with respect to Zn isotopes. ► The average Zn isotope composition of Bulk Silicate Earth δ66ZnBSE=0.28±0.05‰.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Mineral zoning is used in diffusion-based geospeedometry to determine magmatic timescales. Progress in this field has been hampered by the challenge to discern mineral zoning produced by diffusion ...from concentration gradients inherited from crystal growth. A zoned olivine phenocryst from Kilauea Iki lava lake (Hawaii) was selected for this study to evaluate the potential of Mg and Fe isotopes for distinguishing these two processes. Microdrilling of the phenocryst (∼300μm drill holes) followed by MC-ICPMS analysis of the powders revealed negatively coupled Mg and Fe isotopic fractionations (δ26Mg from +0.1‰ to −0.2‰ and δ56Fe from −1.2‰ to −0.2‰ from core to rim), which can only be explained by Mg–Fe exchange between melt and olivine. The data can be explained with ratios of diffusivities of Mg and Fe isotopes in olivine scaling as D2/D1=(m1/m2)β with βMg ∼0.16 and βFe ∼0.27. LA-MC-ICPMS and MC-SIMS Fe isotopic measurements are developed and are demonstrated to yield accurate δ56Fe measurements within precisions of ∼0.2‰ (1 SD) at spatial resolutions of ∼50μm. δ56Fe and δ26Mg stay constant with Fo# in the rim (late-stage overgrowth), whereas in the core (original phenocryst) δ56Fe steeply trends toward lighter compositions and δ26Mg trends toward heavier compositions with higher Fo#. A plot of δ56Fe vs. Fo# immediately distinguishes growth-controlled from diffusion-controlled zoning in these two regions. The results are consistent with the idea that large isotopic fractionation accompanies chemical diffusion in crystals, whereas fractional crystallization induces little or no isotopic fractionation. The cooling timescale inferred from the chemical-isotope zoning profiles is consistent with the documented cooling history of the lava lake. In the absence of geologic context, in situ stable isotopic measurements may now be used to interpret the nature of mineral zoning. Stable isotope measurements by LA-MC-ICPMS and MC-SIMS can be used as standard petrologic tools to identify samples for diffusion-based geospeedometry.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The separation of immiscible liquids has significant implications for magma evolution and the formation of magmatic ore deposits. We combine high-resolution imaging and electron probe microanalysis ...with the first use of atom probe tomography on tholeiitic basaltic glass from Hawaii, the Snake River Plain, and Iceland, to investigate the onset of unmixing of basaltic liquids into Fe-rich and Si-rich conjugates. We examine the relationships between unmixing and crystal growth, and the evolution of a nanoemulsion in a crystal mush. We identify the previously unrecognised role played by compositional boundary layers in promoting unmixing around growing crystals at melt-crystal interfaces. Our findings have important implications for the formation of immiscible liquid in a crystal mush, the interpretations of compositional zoning in crystals, and the role of liquid immiscibility in controlling magma physical properties.
In order to understand the mechanism of vanadium (V) isotope fractionation during magmatic differentiation, we analyzed the V isotopic compositions of eruptive pumices, olivine-phyric basalts, and ...differentiated segregation veins from the Kilauea Iki lava lake, Hawaii. Most olivine-phyric samples contain whole rock δ51V ranging from −0.95‰ to −0.80‰ similar to the average value of the bulk silicate Earth (BSE; −0.91‰ ± 0.09‰), reflecting the composition of melt coexisting with olivine. One eruptive pumice is similar to most of the olivine-phyric samples with a δ51V value of −0.89‰, while the other eruptive pumice and the two highest-MgO olivine-phyric samples have δ51V shifting to slightly higher values of −0.81‰ to −0.63‰. For the segregation veins, the whole rock δ51V values vary from −0.81‰ to −0.53‰, suggesting an average mineral-melt fractionation factor (Δ51Vmineral-melt) of about −0.15‰ during fractional crystallization. The late differentiated veins exhibit the highest δ51V, δ56Fe, and δ49Ti values, showing isotope fractionation driven by the crystallization of Fe–Ti oxides. The estimated Δ51Vmineral-melt value for Kilauea Iki samples is much smaller than that reported for Hekla basalts from Iceland and Anatahan lavas from Mariana (−0.5‰ to −0.4‰; Prytulak et al., 2017), which may reflect differences in magma differentiation in terms of the timing of Fe–Ti oxide saturation, and the amount and mineralogy of the Fe–Ti oxides.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The behavior of nickel (Ni) isotopes during magmatic differentiation is not adequately explored. Here, we find that tholeiitic rocks in the Kīlauea Iki (KI) lava lake, Hawai'i, show progressively ...lighter Ni isotopic compositions with increasing magmatic differentiation, whereas calc‐alkaline rocks from the thick Kamchatka arc (30–45 km), located at the convergent boundary of the Eurasian and Pacific plates show increasing Ni isotope values as MgO and Ni decrease. Forty‐three global intermediate‐felsic continental igneous rocks analyzed in this study display large Ni isotopic variations, with the Eoarchean samples having light Ni isotopic compositions that fall in the trend defined by the KI lavas, and the post‐Eoarchean samples showing systematically heavier Ni isotopic compositions overlapping those of Kamchatka arc rocks. The isotopic dichotomy results from the crystallization of isotopically heavy magnetite during low‐pressure differentiation of KI lavas, whereas the participation of sulfide separation that removes isotopically light Ni during high‐pressure differentiation of magmas traversing thick continental crust. Combined with Rhyolite‐MELTS and sulfur concentration at sulfide saturation simulations, we demonstrate that the Ni isotope fractionation during magmatic differentiation is mainly controlled by the tempo of magnetite crystallization and sulfide formation, which is a function of pressure, oxygen fugacity, and water activity. High‐pressure calc‐alkaline differentiation usually suppresses magnetite crystallization while stabilizing sulfide, leading to heavy Ni isotopic compositions for the evolved magmas, significantly deviating from the low‐pressure fractionation trend seen in the KI lavas. Ni isotopes can be used in the future as a tracer of magmatic differentiation and processes of continent formation and differentiation.
Plain Language Summary
Magmatic differentiation plays an important role in generating the distinctive calc‐alkaline compositions of the continental crust. We show that low‐pressure differentiation of the Kīlauea Iki tholeiitic lavas displays progressively lighter Ni isotopes with differentiation, whereas high‐pressure differentiation of calc‐alkaline volcanic rocks from the thick Kamchatka arc leads to heavier Ni isotopes. This difference may result from the fact that higher pressure may have suppressed magnetite crystallization while stabilized sulfide relative to low‐pressure differentiation. The degree and direction of Ni isotope fractionation during magmatic differentiation are mainly controlled by the differentiation pressure, oxygen fugacity, and water activity. Thus, Ni isotopes can be a novel tracer of magmatic differentiation and processes of continent formation and differentiation.
Key Points
Nickel isotopic compositions of Kīlauea Iki lavas become lighter with increasing magmatic differentiation
Nickel isotopic compositions of Kamchatka arc lavas become heavier with increasing magmatic differentiation
Nickel isotope fractionation during magmatic differentiation is controlled by the tempo of magnetite crystallization and sulfide formation
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Recent work has demonstrated that titanium (Ti) isotopes undergo mass-dependent isotope fractionation during magmatic differentiation, leaving evolved silicic melts preferentially enriched in heavy ...Ti isotopes. Preferential incorporation of light Ti isotopes in crystallizing Fe-Ti oxides is thought to be the mechanism responsible for this fractionation in magmatic rocks. To test this hypothesis, we present Ti isotope measurements of Fe-Ti oxide mineral separates of Kilauea Iki lava lake samples. We find that the Ti in Fe-Ti oxides is isotopically light while Ti in the residual melt and minerals is isotopically heavy. This result is consistent with the results of density functional theory (DFT) calculations in other studies, which show progressive heavy isotope enrichment for Ti from 6-fold, 5-fold, through 4-fold coordinated minerals. We therefore conclude that Ti isotopes in silicate melts undergo isotope fractionation during the crystallization of Fe-Ti oxides because Ti in oxides is primarily in 6-fold coordination whereas Ti in silicate melts is in 5- or 4-fold coordination (Ti in more evolved magmas tends to be in lower coordination). Based on our mineral separate results, we estimate the fractionation factor at 1000 °C between silicate and oxide Δ49Tisilicate-oxide to be 0.39 ± 0.06‰. This result is consistent with the fractionation factors inferred in previous studies based on Ti isotopic analyses and modeling of bulk rock measurements. We use this fractionation factor and the fractionation factors proposed by previous workers in Rhyolite MELTS to model the δ49Ti evolution of plume lavas. We find the model to generally predict the fractionations observed in Kilauea Iki, as well as the fractionations previously observed in volcanics from Hekla, Iceland and Afar, East Africa.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In order to investigate possible Ca isotopic fractionation during basaltic magma differentiation, we measured Ca isotopic compositions of lavas recovered from Kilauea Iki lava lake at Hawaii. This ...set of lavas record the whole crystal fractionation history of basaltic magma, ranging from olivine accumulation/fractionation to multiple phase crystallization, including plagioclase and clinopyroxene. Our results show no detectable Ca isotopic variation in all measured Kilauea lavas at a precision of ±0.07‰ for 44Ca/40Ca (δ44/40Ca = 0.80 ± 0.08, 2 SD, n = 19). Using such observation and published intermineral Ca isotopic fractionation factors, a Monte Carlo approach is used to estimate the mineral‐melt 44Ca/40Ca fractionation factors. We found that Ca isotopic fractionation between clinopyroxene and basaltic melt is small, with Δ44/40Cacpx‐melt = 0.04 ± 0.03 at 1200 °C. To the best of our knowledge, this is the first estimated mineral‐melt Ca isotopic fractionation factor reported. We use this estimated Δ44/40Cacpx‐melt and intermineral Ca isotopic fractionation factors to investigate Ca isotopic effects during mantle partial melting under 1–2 GPa. Our simulations show that the largest 44Ca/40Ca effect, up to +0.3‰, is achieved in large degree melting residues during fractional and dynamic melting. In contrast, partial melts show negligible 44Ca/40Ca isotopic effect, <0.07‰.
Key Points
Kilauea Iki lava lake basalts show no detectable Ca isotopic variation at a precision of ±0.07‰ for 44Ca/40Ca
The clinopyroxene‐melt Ca isotopic fractionation factor is estimated at (0.09 ± 0.07)/(T/1,000)2 (T in kelvin)
Small 44Ca/40Ca fractionation in melts during crystal fractionation and melting of spinel peridotite, but large 44Ca/40Ca (0.3‰) fractionation in residues
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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