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Ronny Schoenberg - One of the best experts on this subject based on the ideXlab platform.
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coupled stable chromium and iron isotopic fractionation tracing Magmatic mineral crystallization in archean komatiite tholeiite suites
Chemical Geology, 2021Co-Authors: Luise J Wagner, Ilka C Kleinhanns, Nadja Weber, Michael G Babechuk, Axel Hofmann, Ronny SchoenbergAbstract:Abstract Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature Magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during Magmatic Differentiation), in a manner similar to Magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox. Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of Magmatic Differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing Magmatic Differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.
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coupled stable chromium and iron isotopic fractionation tracing Magmatic mineral crystallization in archean komatiite tholeiite suites
Chemical Geology, 2021Co-Authors: Luise J Wagner, Ilka C Kleinhanns, Nadja Weber, Michael G Babechuk, Axel Hofmann, Ronny SchoenbergAbstract:Abstract Chromium exists in the two oxidation states Cr2+ and Cr3+ during high-temperature Magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during Magmatic Differentiation), in a manner similar to Magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox. Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of Magmatic Differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and − 0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52Cr SRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing Magmatic Differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.
Paul S Savage - One of the best experts on this subject based on the ideXlab platform.
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potassium isotope fractionation during Magmatic Differentiation of basalt to rhyolite
Chemical Geology, 2019Co-Authors: Brenna Tullerross, Paul S Savage, Heng Chen, Kun WangAbstract:Abstract High-temperature equilibrium and kinetic stable isotope fractionation during partial melting, fractional crystallization, and other igneous Differentiation processes has been observed in many isotope systems, but due to the relative nascence of high-precision analytical capabilities for K, it is still unclear whether igneous processes induce systematic and resolvable K isotope fractionation. In this study, we look to the natural laboratory of Hekla volcano in Iceland to investigate the behavior of K isotopes during Magmatic Differentiation of basalt to rhyolite. Using a novel MC-ICP-MS method, we analyzed 24 geochemically diverse samples from Hekla, including 7 basalts, 8 basaltic andesites, 3 andesites, 4 dacites, and 2 rhyolites, along with 2 additional samples from Burfell, Iceland, for comparison (1 basalt and 1 trachyte). We observed extremely limited variation of 41K/39K ratios throughout our suite of samples, which is not resolvable within the best current analytical uncertainty. The average value of all samples is δ41KNIST SRM3141a = −0.46 ± 0.07‰ (2sd). This value agrees with the Bulk Silicate Earth value previously defined by average global oceanic basalts in literature. The lack of variation throughout this suite of samples from a single volcano system indicates that K does not fractionate during Magmatic Differentiation (of basalt to rhyolite) through processes such as partial melting and fractional crystallization. This conclusion is important to the estimation of the Bulk Silicate Earth K isotope composition, to placing a more robust estimate on the composition bulk continental crust, and to fostering a better understanding of the behavior of K isotopes during Differentiation of the terrestrial planets.
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isotopic fractionation of zirconium during Magmatic Differentiation and the stable isotope composition of the silicate earth
Geochimica et Cosmochimica Acta, 2019Co-Authors: Edward C Inglis, John Creech, Zhengbin Deng, Matthew G Jackson, Martin Bizzarro, Frederic Moynier, Fangzhen Teng, Paul S SavageAbstract:Abstract High-precision double-spike Zr stable isotope measurements (expressed as δ94/90ZrIPGP-Zr, the permil deviation of the 94Zr/90Zr ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in δ94/90ZrIPGP-Zr (0.046 ± 0.037‰; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of >0.7 show a limited range in δ94/90ZrIPGP-Zr (0.053 ± 0.040‰; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in δ94/90ZrIPGP-Zr, from −0.045 ± 0.018 to 0.074 ± 0.023‰. Highly evolved igneous differentiates (>65 wt% SiO2) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53‰. These results suggest that both mantle melt depletion and extreme Magmatic Differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a αzircon-melt of 0.9995 based on the whole rock δ94/90ZrIPGP-Zr values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depleted mantle source (N-MORB) or that underwent zircon saturation (SiO2 > 65 wt%) are removed from the dataset to give an estimate of the primitive mantle Zr isotope composition of 0.048 ± 0.032‰; 2sd, n = 48. These data show that major controls on Zr fractionation in the Earth result from partial melt extraction in the mantle and by zircon fractionation in differentiated melts. Conversely, fertile mantle is homogenous with respect to Zr isotopes. Zirconium mass-dependent fractionation effects can therefore be used to trace large-scale mantle melt depletion events and the effects of felsic crust formation.
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absence of molybdenum isotope fractionation during Magmatic Differentiation at hekla volcano iceland
Geochimica et Cosmochimica Acta, 2015Co-Authors: Jie Yang, Paul S Savage, Christopher Siebert, Jane Barling, Yuhsuan Liang, Alex N HallidayAbstract:This study investigates the behaviour of molybdenum (Mo) isotopes during Magmatic Differentiation. Molybdenum isotope compositions, as well as concentrations of rare earth elements and a selection of trace elements, have been determined for a well characterised sequence of lavas from Hekla volcano, Iceland, covering a compositional range from basalt to rhyolite (46-72wt.% SiO2), and thought to have developed by Differentiation and mixing of melts derived from a cogenetic source. All samples have identical Mo isotopic compositions with an average δ98Mo of -0.15±0.05‰ (2 s.d.; n=23). There is therefore no resolvable Mo isotope fractionation during Magmatic Differentiation at Hekla. This finding is supported by the fact that Mo remains highly incompatible in Hekla lavas, increasing from 1.3 to 4.6μg/g from basalt to rhyolite, indicating that the crystallising phases are extracting only limited amounts of Mo from the magma and therefore that significant fractionation of Mo isotopes is unlikely. It has previously been proposed that cerium (Ce) and Mo have similar bulk distribution coefficients and are equally incompatible during mantle melting. While both Ce and Mo remain incompatible in Hekla lavas, the Ce/Mo ratio decreases from 50 to 36 during Magmatic Differentiation indicating that Mo is more incompatible than Ce. Comparison of Mo with other incompatible trace elements indicates that Mo is as incompatible as La and slightly less incompatible than K. Sulphur (S) decreases strongly from ~200 to as low as ~2μg/g from basalt to andesite and more evolved compositions, yet this has no effect on the Mo isotopes. Therefore, Mo does not exhibit significant chalcophile behaviour in Hekla magmas. The Mo isotopic signature therefore may be used as an indicator of parent magma composition and a potential discriminant of assimilation processes.
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zinc isotope fractionation during Magmatic Differentiation and the isotopic composition of the bulk earth
Earth and Planetary Science Letters, 2013Co-Authors: Heng Chen, Fangzhen Teng, Paul S Savage, Rosalind Tuthill Helz, Frederic MoynierAbstract: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.).
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silicon isotope fractionation during Magmatic Differentiation
Geochimica et Cosmochimica Acta, 2011Co-Authors: Paul S Savage, Bastian R Georg, Helen M Williams, Kevin W Burton, Alex N HallidayAbstract:Abstract The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ 30 Si = −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 30 Si 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 30 Si 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.
Frederic Moynier - One of the best experts on this subject based on the ideXlab platform.
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mare basalt meteorites magnesian suite rocks and kreep reveal loss of zinc during and after lunar formation
Earth and Planetary Science Letters, 2020Co-Authors: Beda A Hofmann, James M D Day, Elishevah M M E Van Kooten, Frederic MoynierAbstract:Abstract Isotopic compositions of reservoirs in the Moon can be constrained from analysis of rocks generated during lunar Magmatic Differentiation. Mare basalts sample the largest lunar mantle volume, from olivine- and pyroxene-rich cumulates, whereas ferroan anorthosites and magnesian-suite rocks represent early crustal materials. Incompatible element enriched rocks, known as ‘KREEP,’ probably preserve evidence for the last highly differentiated melts. Here we show that mare basalts, including Apollo samples and meteorites, have remarkably consistent δ 66 Zn values ( + 1.4 ± 0.2 ‰ ) and Zn abundances (1.5 ± 0.4 ppm). Analyses of magnesian-suite rocks show them to be characterized by even heavier δ 66 Zn values (2.5 to 9.3‰) and low Zn concentrations. KREEP-rich impact melt breccia Sayh al Uhaymir 169 has a nearly identical Zn composition to mare basalts ( δ 66 Zn = 1.3 ‰ ) and a low Zn abundance (0.5 ppm). Much of this variation can be explained through progressive depletion of Zn and preferential loss of the light isotopes in response to evaporative fractionation processes during a lunar magma ocean. Samples with isotopically light Zn can be explained by either direct condensation or mixing and contamination processes at the lunar surface. The δ 66 Zn of Sayh al Uhaymir 169 is probably compromised by mixing processes of KREEP with mafic components. Correlations of Zn with Cl isotopes suggest that the urKREEP reservoir should be isotopically heavy with respect to Zn, like magnesian-suite rocks. Current models to explain how and when Zn and other volatile elements were lost from the Moon include nebular processes, prior to lunar formation, and planetary processes, either during giant impact, or Magmatic Differentiation. Our results provide unambiguous evidence for the latter process. Notwithstanding, with the currently available volatile stable isotope datasets, it is difficult to discount if the Moon lost its volatiles relative to Earth either during giant impact or exclusively from later Magmatic Differentiation. If the Moon did begin initially volatile-depleted, then the mare basalt δ 66 Zn value likely preserves the signature, and the Moon lost 96% of its Zn inventory relative to Earth and was also characterized by isotopically heavy Cl ( δ 37 Cl = ≥ 8 ‰ ). Alternative loss mechanisms, including erosive impact removing a steam atmosphere need to be examined in detail, but nebular processes of volatile loss do not appear necessary to explain lunar and terrestrial volatile inventories.
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isotopic fractionation of zirconium during Magmatic Differentiation and the stable isotope composition of the silicate earth
Geochimica et Cosmochimica Acta, 2019Co-Authors: Edward C Inglis, John Creech, Zhengbin Deng, Matthew G Jackson, Martin Bizzarro, Frederic Moynier, Fangzhen Teng, Paul S SavageAbstract:Abstract High-precision double-spike Zr stable isotope measurements (expressed as δ94/90ZrIPGP-Zr, the permil deviation of the 94Zr/90Zr ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in δ94/90ZrIPGP-Zr (0.046 ± 0.037‰; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of >0.7 show a limited range in δ94/90ZrIPGP-Zr (0.053 ± 0.040‰; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in δ94/90ZrIPGP-Zr, from −0.045 ± 0.018 to 0.074 ± 0.023‰. Highly evolved igneous differentiates (>65 wt% SiO2) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53‰. These results suggest that both mantle melt depletion and extreme Magmatic Differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a αzircon-melt of 0.9995 based on the whole rock δ94/90ZrIPGP-Zr values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depleted mantle source (N-MORB) or that underwent zircon saturation (SiO2 > 65 wt%) are removed from the dataset to give an estimate of the primitive mantle Zr isotope composition of 0.048 ± 0.032‰; 2sd, n = 48. These data show that major controls on Zr fractionation in the Earth result from partial melt extraction in the mantle and by zircon fractionation in differentiated melts. Conversely, fertile mantle is homogenous with respect to Zr isotopes. Zirconium mass-dependent fractionation effects can therefore be used to trace large-scale mantle melt depletion events and the effects of felsic crust formation.
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zinc isotope fractionation during Magmatic Differentiation and the isotopic composition of the bulk earth
Earth and Planetary Science Letters, 2013Co-Authors: Heng Chen, Fangzhen Teng, Paul S Savage, Rosalind Tuthill Helz, Frederic MoynierAbstract: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.).
Rosalind Tuthill Helz - One of the best experts on this subject based on the ideXlab platform.
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stable chromium isotope fractionation during Magmatic Differentiation insights from hawaiian basalts and implications for planetary redox conditions
Geochimica et Cosmochimica Acta, 2020Co-Authors: Rosalind Tuthill Helz, Richard W Carlson, Ji Shen, Jiuxing Xia, Liping Qin, Shichun Huang, Timothy D MockAbstract:Abstract The stable isotope compositions of chromium (Cr) are fractionated during Magmatic Differentiation of lunar mare basalts, which might be attributed to redox-related mineral crystallization. It has yet to be demonstrated whether Magmatic Differentiation fractionates Cr isotope composition of terrestrial samples. Here, we present high-precision stable Cr isotope measurements, reported as δ53Cr relative to NIST SRM 979, of well-characterized Hawaiian tholeiitic basalts from Koolau, Mauna Kea and Kilauea. The studied Makapuu-stage Koolau lavas have MgO ranging from 6.58 to 21.54 wt.%, and they have homogeneous δ53Cr ranging from −0.21‰ to −0.17‰. Similarly, studied Mauna Kea lavas have MgO ranging from 9.11 to 17.90 wt.%, and they also have homogeneous δ53Cr ranging from −0.17‰ to −0.13‰. Some Makapuu-stage Koolau and Mauna Kea lavas experienced subaerial or submarine alteration. The homogenous δ53Cr within each sample suites implies that the post-Magmatic alterations have not significantly changed the Cr isotope compositions of these samples. Conversely, nine Kilauea Iki basalts have MgO ranging from 7.77 to 26.87 wt.% reflecting varying degrees of Magmatic Differentiation, and they show resolvable Cr isotope variations with δ53Cr ranging from −0.18‰ to 0.00‰. The δ53Cr values of the Kilauea Iki samples are positively correlated with indicators of Magmatic Differentiation such as Cr and MgO contents, and Mg# values. The most evolved samples have the lightest isotope compositions, whereas the olivine-spinel cumulates display complementary heavy isotope compositions. This fractionation is most likely generated by the crystallization and accumulation of spinel, which is dominated by Cr3+ and, hence, enriched in heavier Cr isotopes relative to the residual melt. At a given MgO content, Kilauea and Mauna Kea lavas, both Kea-trend volcanoes, have higher δ53Cr than Makapuu-stage Koolau lavas, a Loa-trend volcano. This difference might reflect recycling of altered oceanic crusts or redox differences of their Magmatic sources, with the mantle source of Makapuu-stage lavas being more reducing. To understand the different Cr isotope fractionation behaviors of terrestrial and extraterrestrial basalts, we present a quantitative model that relates the Cr isotope compositions of basalts from the Earth, the Moon and Vesta, to the crystallization assemblage, the degree of fractional crystallization, and the Cr2+/ΣCr ratios of minerals and melts, which are related to the oxygen fugacity during Differentiation. The primitive Hawaiian basaltic magma for Kilauea Iki and Mauna Kea lavas is estimated to have δ53Cr of −0.15‰, which is close to the average value of the BSE (−0.14‰ to −0.12‰). We further speculate that the initial lunar mantle is relatively homogeneous with BSE-like isotope composition (−0.16‰ to −0.09‰). The observed low δ53Cr in lunar mafic rocks is the result of redox-dominated fractional crystallization and accumulation processes of lunar mafic magmas. These magmas might be derived from variable degrees of partial melting of the primitive lunar mantle. Combined with previous results on the variations in Cr valences and contents in silicate melts and minerals related to oxygen fugacity, Cr concentration and isotope composition can serve as a useful oxybarometer for understanding the redox conditions of planetary Differentiation and Magmatic evolution.
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the behavior of chalcophile elements during Magmatic Differentiation as observed in kilauea iki lava lake hawaii
Geochimica et Cosmochimica Acta, 2017Co-Authors: Allison T Greaney, Rosalind Tuthill Helz, Roberta L Rudnick, Richard M Gaschnig, Philip M Piccoli, R D AshAbstract:Abstract We quantify the behavior of Cu, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, W, Tl, Pb, and Bi during the Differentiation of a picritic magma in the Kilauea Iki lava lake, Hawaii, using whole rock and glass Differentiation trends, as well as partition coefficients in Cu-rich sulfide blebs and minerals. Such data allow us to constrain the partitioning behavior of these elements between sulfide and silicate melts, as well as the chalcophile element characteristics of the mantle source of the Kilauea lavas. Nearly all of the elements are generally incompatible on a whole-rock scale, with concentrations increasing exponentially below ∼6 wt% MgO. However, in-situ laser ablation data reveal that Cu, Ag, Bi, Cd, In, Pb, and Sn are chalcophile; As, Ge, Sb, and Tl are weakly chalcophile to lithophile; and Mo, Ga, and W are lithophile. The average Dsulfide/silicate melt values are: DAg = 1252 ± 1201 (2SD), DBi = 663 ± 576, DCd = 380 ± 566, DIn = 40 ± 34, DPb = 34 ± 18, DSn = 5.3 ± 3.6, DAs = 2.4 ± 7.6, DGe = 1.6 ± 1.4, DSb = 1.3 ± 1.5, DTl = 1.1 ± 1.7, DMo = 0.56 ± 0.6, DGa = 0.10 ± 0.3, and DW = 0.11 ± 0.1. These findings are consistent with experimental partitioning studies and observations of Ni-rich sulfide liquid in mid-ocean ridge basalts (MORB), despite the different compositions of the KI sulfides. The KI glasses and whole rocks are enriched in As, Ag, Sb, W, and Bi, relative to elements of similar compatibility (as established by abundances in MORB), mimicking enrichments found in basalts from the Manus back arc basin (Jenner et al., 2012) and the upper continental crust (UCC). These enrichments suggest the presence of terrigenous sediments in the Kilauea mantle source. The KI source is calculated to be a mixture of depleted MORB mantle (DMM) and 10–20% recycled crust composed of MORB and minor terrigenous sediments.
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zinc isotope fractionation during Magmatic Differentiation and the isotopic composition of the bulk earth
Earth and Planetary Science Letters, 2013Co-Authors: Heng Chen, Fangzhen Teng, Paul S Savage, Rosalind Tuthill Helz, Frederic MoynierAbstract: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.).
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iron isotope fractionation during Magmatic Differentiation in kilauea iki lava lake
Science, 2008Co-Authors: Fangzhen Teng, Nicolas Dauphas, Rosalind Tuthill HelzAbstract: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 (‰) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2‰ 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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iron isotope fractionation during Magmatic Differentiation in kilauea iki lava lake
Science, 2008Co-Authors: Fangzhen Teng, Nicolas Dauphas, Rosalind Tuthill HelzAbstract: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 1.2 per 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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absence of molybdenum isotope fractionation during Magmatic Differentiation at hekla volcano iceland
Geochimica et Cosmochimica Acta, 2015Co-Authors: Jie Yang, Paul S Savage, Christopher Siebert, Jane Barling, Yuhsuan Liang, Alex N HallidayAbstract:This study investigates the behaviour of molybdenum (Mo) isotopes during Magmatic Differentiation. Molybdenum isotope compositions, as well as concentrations of rare earth elements and a selection of trace elements, have been determined for a well characterised sequence of lavas from Hekla volcano, Iceland, covering a compositional range from basalt to rhyolite (46-72wt.% SiO2), and thought to have developed by Differentiation and mixing of melts derived from a cogenetic source. All samples have identical Mo isotopic compositions with an average δ98Mo of -0.15±0.05‰ (2 s.d.; n=23). There is therefore no resolvable Mo isotope fractionation during Magmatic Differentiation at Hekla. This finding is supported by the fact that Mo remains highly incompatible in Hekla lavas, increasing from 1.3 to 4.6μg/g from basalt to rhyolite, indicating that the crystallising phases are extracting only limited amounts of Mo from the magma and therefore that significant fractionation of Mo isotopes is unlikely. It has previously been proposed that cerium (Ce) and Mo have similar bulk distribution coefficients and are equally incompatible during mantle melting. While both Ce and Mo remain incompatible in Hekla lavas, the Ce/Mo ratio decreases from 50 to 36 during Magmatic Differentiation indicating that Mo is more incompatible than Ce. Comparison of Mo with other incompatible trace elements indicates that Mo is as incompatible as La and slightly less incompatible than K. Sulphur (S) decreases strongly from ~200 to as low as ~2μg/g from basalt to andesite and more evolved compositions, yet this has no effect on the Mo isotopes. Therefore, Mo does not exhibit significant chalcophile behaviour in Hekla magmas. The Mo isotopic signature therefore may be used as an indicator of parent magma composition and a potential discriminant of assimilation processes.
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silicon isotope fractionation during Magmatic Differentiation
Geochimica et Cosmochimica Acta, 2011Co-Authors: Paul S Savage, Bastian R Georg, Helen M Williams, Kevin W Burton, Alex N HallidayAbstract:Abstract The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ 30 Si = −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 30 Si 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 30 Si 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.
