Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts

Mineralogical variations in the Earth's mantle and the relative proportions of peridotitic versus enriched and potentially crustally-derived pyroxenitic domains within the mantle have important implications for mantle dynamics, magma generation, and the recycling of surface material back into the mantle. Here we present iron (Fe) stable isotope data (δ 57Fe, deviation in 57Fe/54Fe from the IRMM-014 standard in parts per thousand) for peridotite and garnet–pyroxenite xenoliths from Oahu, Hawaii and explore Fe isotopes as tracer of both peridotitic and pyroxenitic components in the source regions of oceanic basalts. The pyroxenites have δ 57Fe values that are heavy (0.10 to 0.27‰) relative to values for mid-ocean ridge and ocean island basalts (MORB; OIB; View the MathML source) and the primitive mantle (PM; View the MathML source). Pyroxenite δ 57Fe values are positively correlated with bulk pyroxenite titanium and heavy rare earth element (REE) abundances, which can be interpreted in terms of stable isotope fractionation during magmatic differentiation and pyroxene cumulate formation. In contrast, the peridotites have light δ 57Fe values (−0.34 to 0.14‰) that correlate negatively with degree of melt depletion and radiogenic hafnium isotopes, with the most depleted samples possessing the most radiogenic Hf isotope compositions and lightest δ 57Fe values. While these correlations are broadly consistent with a scenario of Fe isotope fractionation during partial melting, where isotopically heavy Fe is extracted into the melt phase, leaving behind low-δ 57Fe peridotite residues, the extent of isotopic variation is far greater than predicted by partial melting models. One possibility is derivation of the samples from a heterogeneous source containing both light-δ 57Fe (relative to PM) and heavy-δ 57Fe components. While pyroxenite is a viable explanation for the heavy-δ 57Fe component, the origin of the depleted light-δ 57Fe component is more difficult to explain, as melting models predict that even large (>30%) degrees of melt extraction do not generate strongly fractionated residues. Multiple phases of melt extraction or other processes, such as metasomatism, melt percolation or the assimilation of xenocrystic olivine with light δ 57Fe values may need to be invoked to explain these light δ 57Fe values; a caveat to this is that these processes must either preserve, or generate correlations between δ 57Fe and Hf isotopes. Published variations in δ 57Fe in mantle melting products, such as MORB and OIB, are also greater than predicted by melting models assuming derivation from δ 57Fe-homogeneous mantle. For example, OIB from the Society and Cook-Austral islands, which have radiogenic Pb and Sr isotope compositions indicative of recycled components such as subduction modified, low-Pb oceanic crust and terrigenous sediments have heavy mean δ 57Fe values (∼0.21‰) significantly distinct to those of other OIB and MORB, which could explained by the presence of heavy-δ57Fe pyroxenite cumulate or pyroxenitic melt components, whereas large degree partial melts, such as komatiites and boninites, display light Fe-isotopic compositions which may reflect sampling of refractory, light-δ57Fe mantle components. Iron stable isotopes may therefore provide a powerful new means of fingerprinting mineralogical variations within the Earth's mantle and identifying the mineralogy of depleted and enriched components within the source regions of volcanic rocks

Water in the Oceanic Lithosphere: Salt Lake Crater Xenoliths, Oahu, Hawaii

Water can be present in nominally anhydrous minerals of peridotites in the form of hydrogen bonded to structural oxygen. Such water in the oceanic upper mantle could have a significant effect on its physical and chemical properties. However, the water content of the MORB source has been inferred indirectly from the compositions of basalts. Direct determinations on abyssal peridotites are scarce because they have been heavily hydrothermally altered. Here we present the first water analyses of minerals from spinel peridotite xenoliths of Salt Lake Crater, Oahu, Hawaii, which are exceptionally fresh. These peridotites are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. A few have unradiogenic Os and radiogenic Hf isotopes and may be fragments of an ancient (~2 Ga) depleted and recycled lithosphere. Water contents in olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) were determined by FTIR spectrometry. Preliminary H_{2}O contents show ranges of 8-10 ppm for Ol, 151-277 ppm for Opx, and 337-603 ppm for Cpx. Reconstructed bulk rock H_{2}O contents range from 88-131 ppm overlapping estimates for the MORB source. Water contents between Ol minerals of the same xenolith are heterogeneous and individual OH infrared bands vary within a mineral with lower 3230 cm^{-1} and higher 3650-3400 cm^{-1} band heights from core to edge. This observation suggests disturbance of the hydrogen in Ol likely occurring during xenolith entrainment to the surface. Pyroxene water contents are higher than most water contents in pyroxenes from continental peridotite xenoliths and higher than those of abyssal peridotites. Cpx water contents decrease with increasing degree of depletion (e.g. increasing Fo in Ol and Cr# in spinel) consistent with an incompatible behavior of water. However Cpx water contents also show a positive correlation with LREE/HREE ratios and LREE concentrations consistent with refertilization. Opx water contents increase with increasing degree of depletion and decrease with LREE/HREE ratios which is inconsistent with the incompatible behavior of H. Calculated water contents of melts in equilibrium with Cpx or Opx range from 1.4 to 3.8 wt % which is higher than that of all Hawaiian lavas. Calculated melts in equilibrium with Cpx and Opx have variable but mostly high H_{2}O/Ce ratios (194 to 1146) consistent with those of rejuvenated stage lavas from Niihau and the South Arch volcanic field, but unlike the drier shield building stage tholeiites. Whether the high water contents recorded in Salt Lake Crater xenoliths were acquired before and/or during interaction of the oceanic lithosphere with the Hawaiian plume will be discussed

The Paradox of a Wet (High H2O) and Dry (Low H2O/Ce) Mantle: High Water Concentrations in Mantle Garnet Pyroxenites from Hawaii

Water dissolved as trace amounts in anhydrous minerals has a large influence on the melting behavior and physical properties of the mantle. The water concentration of the oceanic mantle is inferred from the analyses of Mid-Ocean Ridge Basalt (MORB) and Oceanic Island Basalt (OIB). but there is little data from actual mantle samples. Moreover, enriched mineralogies (pyroxenites, eclogites) are thought as important sources of heterogeneity in the mantle, but their water concentrations and their effect on the water budget and cycling in the mantle are virtually unknown. Here, we analyzed by FTIR water in garnet clinopyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. These pyroxenites are high-pressure (>20kb) crystal fractionates from alkalic melts. The clinopyroxenes (cpx) have 260 to 576 ppm wt H2O, with the least differentiated samples (Mg#>0.8) in the 400-500 ppm range. Orthopyroxene (opx) contain 117-265 ppm H2O, about half of that of cpx, consistent with other natural sample studies, but lower than cpx/opx equilibrium from experimental data. The pyroxenite cpx and opx H2O concentrations are at the high-end of on-and off-craton peridotite xenolith concentrations and those of Hawaiian spinel peridotites. In contrast, garnet has extremely low water contents (<5ppm H2O). There is no correlation between H2O in cpx and lithophile element concentrations. Phlogopite is present in some samples, and its modal abundance shows a positive correlation in Mg# with cpx, implying equilibrium. However, there is no correlation between H2O concentrations and or the presence of phlogopite. These data imply that cpx and opx may be at water saturation, far lower than experimental data suggest. Reconstructed bulk rock pyroxenite H2O ranges from 200-460 ppm (average 331 +/- 75 ppm), 2 to 8 times higher than H2O estimates for the MORB source (50-200 ppm), but in the range of E-MORB, OIB and the source of rejuvenated Hawaiian magmas. The average bulk rock pyroxenite H2O/Ce is 69 +/-35, lower than estimates of the MORB source (approx 150) or FOZO, C (200-250) mantle component, but consistent with "dry" EM sources (<100). These data suggest that a metasomatized, refertilized oceanic lithosphere that contains pyroxenitic veins (e.g. the lower part of an oceanic plate, where ascending melts can become trapped and crystallize), will have both higher water concentrations and low H2O/Ce, and may contribute to EM-type OIB sources, like that of Samoa basalts. Therefore, a low H2O/Ce mantle source may not necessarily be "dry"

Deepwater expansion and enhanced remineralization in the eastern equatorial Pacific during the last glacial maximum

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 563-578, doi:10.1029/2017PA003221.Published estimates of the radiocarbon content of middepth waters suggest a decrease in ventilation in multiple locations during the last glacial maximum (LGM; 24.0–18.1 ka). Reduced glacial ventilation would have allowed respired carbon to accumulate in those waters. A subsequent deglacial release of this respired carbon reservoir to the atmosphere could then account for the observed increases in atmospheric CO2 and decline in atmospheric radiocarbon content. However, age model error and a release of 14C‐depleted mantle carbon have also been cited as possible explanations for the observed middepth radiocarbon depletions, calling into question the deep ocean's role in storing respired carbon during the LGM. Joint measurements of benthic foraminiferal carbon isotope values (δ13C) and cadmium/calcium (Cd/Ca) ratios provide a method for isolating the air‐sea component of a water mass from changes in remineralization. Here we use benthic foraminiferal δ13C and Cd/Ca records from the eastern equatorial Pacific to constrain changes in remineralization and water‐mass mixing over the last glacial‐interglacial transition. These records are complemented with elemental measurements of the authigenic coatings of foraminifera to monitor postdepositional changes in bottom water properties. Our results suggest an increase of deep waters at midwater depths consistent with a shoaling of the boundary between the upper and lower branches of Southern Ocean overturning circulation. Additionally, our records demonstrate increased organic matter remineralization in middepth waters during the LGM, suggesting that respired carbon did accumulate in middepth waters under periods of reduced ventilation.National Science foundation Grant Number: OCE‐09563682018-11-1

H Diffusion in Olivine and Pyroxene from Peridotite Xenoliths and a Hawaiian Magma Speedometer

Hydrogen is present as a trace element in olivine and pyroxene and its content distribution in the mantle results from melting and metasomatic processes. Here we examine how these H contents can be disturbed during decompression. Hydrogen was analyzed by FTIR in olivine and pyroxene of spinel peridotite xenoliths from Salt Lake Crater (SLC) nephelinites which are part of the rejuvenated volcanism at Oahu (Hawaii) [1,2]. H mobility in pyroxene resulting from spinel exsolution during mantle upwelling Most pyroxenes in SLC peridotites exhibit exsolutions, characterized by spinel inclusions. Pyroxene edges where no exsolution are present have less H then their core near the spinel. Given that H does not enter spinel [3], subsolidus requilibration may have concentrated H in the pyroxene adjacent to the spinel exsolution during mantle upwelling. H diffusion in olivine during xenolith transport by its host magma and host magma ascent rates Olivines have lower water contents at the edge and near fractures compared to at their core, while the concentrations of all other chemical elements appear homogeneous. This suggests that some of the initial water has diffused out of the olivine. Water loss from the olivine is thought to occur during host-magma ascent and xenolith transport to the surface [4-6]. Diffusion modeling matches best the data when the initial water content used is that measured at the core of the olivines, implying that mantle water contents are preserved at the core of the olivines. The 3225 cm(sup -1) OH band at times varies independantly of other OH bands, suggesting uneven H distribution in olivine defects likely acquired during mantle metasomatism just prior to eruption and unequilibrated. Diffusion times (1-48 hrs) combined with depths of peridotite equilibration or of magma start of degassing allow to calculate ascent rates for the host nephelinite of 0.1 to 27 m/s

Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts

Mineralogical variations in the Earth's mantle and the relative proportions of peridotitic versus enriched and potentially crustally-derived pyroxenitic domains within the mantle have important implications for mantle dynamics, magma generation, and the recycling of surface material back into the mantle. Here we present iron (Fe) stable isotope data (δ57Fe, deviation in 57Fe/54Fe from the IRMM-014 standard in parts per thousand) for peridotite and garnet–pyroxenite xenoliths from Oahu, Hawaii and explore Fe isotopes as tracer of both peridotitic and pyroxenitic components in the source regions of oceanic basalts. The pyroxenites have δ57Fe values that are heavy (0.10 to 0.27‰) relative to values for mid-ocean ridge and ocean island basalts (MORB; OIB; View the MathML sourceδFe57∼0.16‰) and the primitive mantle (PM; View the MathML sourceδFe57∼0.04‰). Pyroxenite δ57Fe values are positively correlated with bulk pyroxenite titanium and heavy rare earth element (REE) abundances, which can be interpreted in terms of stable isotope fractionation during magmatic differentiation and pyroxene cumulate formation. In contrast, the peridotites have light δ57Fe values (−0.34 to 0.14‰) that correlate negatively with degree of melt depletion and radiogenic hafnium isotopes, with the most depleted samples possessing the most radiogenic Hf isotope compositions and lightest δ57Fe values. While these correlations are broadly consistent with a scenario of Fe isotope fractionation during partial melting, where isotopically heavy Fe is extracted into the melt phase, leaving behind low-δ57Fe peridotite residues, the extent of isotopic variation is far greater than predicted by partial melting models. One possibility is derivation of the samples from a heterogeneous source containing both light-δ57Fe (relative to PM) and heavy-δ57Fe components. While pyroxenite is a viable explanation for the heavy-δ57Fe component, the origin of the depleted light-δ57Fe component is more difficult to explain, as melting models predict that even large (>30%) degrees of melt extraction do not generate strongly fractionated residues. Multiple phases of melt extraction or other processes, such as metasomatism, melt percolation or the assimilation of xenocrystic olivine with light δ57Fe values may need to be invoked to explain these light δ57Fe values; a caveat to this is that these processes must either preserve, or generate correlations between δ57Fe and Hf isotopes. Published variations in δ57Fe in mantle melting products, such as MORB and OIB, are also greater than predicted by melting models assuming derivation from δ57Fe-homogeneous mantle. For example, OIB from the Society and Cook-Austral islands, which have radiogenic Pb and Sr isotope compositions indicative of recycled components such as subduction modified, low-Pb oceanic crust and terrigenous sediments have heavy mean δ57Fe values (∼0.21‰∼0.21‰) significantly distinct to those of other OIB and MORB, which could explained by the presence of heavy-δ57Fe pyroxenite cumulate or pyroxenitic melt components, whereas large degree partial melts, such as komatiites and boninites, display light Fe-isotopic compositions which may reflect sampling of refractory, light-δ57Fe mantle components. Iron stable isotopes may therefore provide a powerful new means of fingerprinting mineralogical variations within the Earth's mantle and identifying the mineralogy of depleted and enriched components within the source regions of volcanic rocks

Transition-Metal Ion Exchange Using Poly(ethylene glycol) Oligomers as Solvents

Poly(ethylene glycol) oligomers have been found to be effective as alternative solvents for the ion exchange of Mn2þ , Fe2 þ , and Co2þ into hydrated and dehydrated Zeolite X (Na80Al80Si112- O384 3 nH2O). When attempted in aqueous solutions, the exchange of these cations quickly leads to destruction of the zeolite structure within 1-2 exchange cycles. However, in PEG oligomer solvents, the structure can be maintained and exchanges of 48% (Co2þ ), 80% (Mn2þ ), and 91% (Fe2þ ) are observed after one cycle under hydrated conditions. When rigorous steps are taken to remove all water from the zeolite before exchange, absorption of the oligomers into the zeolite pores is promoted, which hinders ion exchange; a maximum of 6% exchange is seen under dehydrated conditions. However, improved catalytic efficiency toward NO decomposition is observed for these samples, with turnover frequencies of 0.0237 s-1 for Dehyd Na/Mn-X oligomer, 0.0213 s-1 for Dehyd Na/Fe-X oligomer, and 0.0190 s-1 for Dehyd Na/Co-X oligomer. Use of these PEG oligomers as reaction media has also been expanded to the ion exchange of layered oxides such as Dion-Jacobson perovskite phases

Mg isotope systematics during magmatic processes: Inter-mineral fractionation in mafic to ultramafic Hawaiian xenoliths

© 2018 Elsevier Ltd Observed differences in Mg isotope ratios between bulk magmatic rocks are small, often on a sub per mill level. Inter–mineral differences in the 26Mg/24Mg ratio (expressed as δ26Mg) in plutonic rocks are on a similar scale, and have mostly been attributed to equilibrium isotope fractionation at magmatic temperatures. Here we report Mg isotope data on minerals in spinel peridotite and garnet pyroxenite xenoliths from the rejuvenated stage of volcanism on Oahu and Kauai, Hawaii. The new data are compared to literature data and to theoretical predictions to investigate the processes responsible for inter–mineral Mg isotope fractionation at magmatic temperatures. Theory predicts up to per mill level differences in δ26Mg between olivine and spinel at magmatic temperatures and a general decrease in Δ26Mgolivine-spinel (=δ26Mgolivine – δ26Mgspinel) with increasing temperature, but also with increasing Cr# in spinel. For peridotites with a simple petrogenetic history by melt depletion, where increasing depletion relates to increasing melting temperatures, Δ26Mgolivine-spinel should thus systematically decrease with increasing Cr# in spinel. However, most natural peridotites, including the Hawaiian spinel peridotites investigated in this study, are overprinted by variable extents of melt-rock reaction, which disturb the systematic primary temperature and compositionally related olivine–spinel Mg isotope systematics. Diffusion, subsolidus re-equilibration, or surface alteration may further affect the observed olivine–spinel Mg isotope fractionation in peridotites, making Δ26Mgolivine-spinel in peridotites a difficult–to–apply geothermometer. The available Mg isotope data on clinopyroxene and garnet suggest that this mineral pair is a more promising geothermometer, but its application is restricted to garnet–bearing igneous (garnet pyroxenites) and metamorphic rocks (eclogites). Although the observed δ26Mg variation is on a sub per mill range in bulk magmatic rocks, the clearly resolvable inter–mineral Mg isotope differences imply that crystallization or preferential melting of isotopically distinct minerals such garnet, spinel, and clinopyroxene should cause Mg isotope fractionation between bulk melt and residue. Calculated Mg isotope variations during partial mantle melting indeed predict differences between melt and residue, but these are analytically resolvable only for melting of mafic lithologies, that is, garnet pyroxenites. Contributions from garnet pyroxenite melts may thus account for some of the isotopically light δ26Mg observed in ocean island basalts and trace lithological mantle heterogeneity. Consequently, applications for high-temperature Mg isotope fractionations are promising and diverse, and recent advances in analytical precision may allow the full petrogenetic potential inherent in the sub per mill variations in δ26Mg in magmatic rocks to be exploited