Big-picture geochemistry from microanalyses - my four-decade odyssey in sims

Author Posting. © European Association of Geochemistry, 2019. This article is posted here by permission of [publisher] for personal use, not for redistribution. The definitive version was published in Shimizu, N. Big-picture geochemistry from microanalyses - my four-decade odyssey in sims. Geochemical Perspectives, 8(1), (2019): 1-104, doi:10.7185/geochempersp.8.1.Secondary Ion Mass Spectrometry (SIMS) is now a well established analytical technique in geochemistry. Its developmental history goes back to the 1970s. Here, I tell the story of how I got involved in its applications to geochemistry in 1974 at the Institut de Physique du Globe in Paris (IPGP) with the Cameca IMS 300 instrument and my ensuing struggles with theories of secondary ion formation processes and the eventual development of the energy filtering approach as an effective method for suppressing molecular ion interferences in silicate minerals and glasses. The geochemical applications of the techniques that I developed with my colleagues at IPGP, Massachusetts Institute of Technology (MIT), and Woods Hole Oceanographic Institution (WHOI) are summarised in four different categories: (1) trace element zoning of phenocrysts and the kinetics of magmatic crystallisation processes, (2) trace element abundance patterns and geochemical processes in the mantle, (3) use of trace element abundances in magmatic processes in the mid-ocean ridge system, and (4) use of Sr/Ca ratios in biogenic carbonates in palaeoceanographic studies. Trace element zoning patterns observed in phenocrysts reveal that crystal growth in magmas can occur with non-equilibrium partitioning of trace elements at the crystal-melt interface. Trace element zoning patterns in augite phenocrysts from Gough Island also indicate repeated drastic changes in magma composition, suggesting a turbulent dynamic state of magma bodies beneath eruptive centres. Chondrite normalised rare earth element (REE) patterns measured in clinopyroxenes from mantle rocks (peridotites from the Horoman massif and xenoliths in basalts from both oceanic and continental localities) show strong evidence for melt-rock reactions, indicating that lithospheric peridotites depleted in incompatible elements by melt extraction often show evidence of having been subsequently enriched in these elements through melt-rock reaction. The distribution of Sr in garnet inclusions in peridotitic diamonds from South Africa and Siberia is highly heterogeneous over wide concentration ranges, suggesting growth of inclusion garnets as well as formation of these diamonds, occurred shortly before the diamonds were carried to the surface by kimberlite eruptions. Rare earth element and other trace element abundance patterns measured in clinopyroxenes in abyssal peridotites clearly indicate that melting and melt extraction processes beneath mid-ocean ridges is akin to fractional melting by which small degree melt fractions are progressively extracted from residues as the mantle decompresses and only later mixed, sometimes incompletely, to form mid-ocean ridge basalt lavas erupted on the ocean floor. However, meltrock reactions between residual peridotites and upwelling melt fractions could significantly alter trace element abundance patterns of originally residual clinopyroxenes. In situ analyses reveal very large trace element variations occurring on intra-mineral scales, suggesting that precipitation of clinopyroxene also occurred during melt-rock reactions. It is evident that abyssal peridotites contain complex geochemical histories beyond melt extraction via fractional melting. Sr/Ca variations in coral skeletons (aragonite) reveal strong effects of photosynthesis of symbiont algae in day time growth zones, while those in night time growth zones near centres of calcification record variations of sea surface temperatures (SST) in Porites lutea. In Astrangia poculata, which experience a large temperature range (-2 – 23 °C), non-symbiotic skeletons faithfully record temperature variations, while symbiotic skeletons display ontogenic effects of algal symbionts. Sr/Ca variations in coccolithophores across the Paleocene-Eocene Thermal Maximum (PETM) display different responses of individual species to the sudden greenhouse environment, and the severity of the response also is dependent on the oceanographic conditions of their habitats

Contrasting partition behavior of F and Cl during hydrous mantle melting: implications for Cl/F signature in arc magmas

International audienceWe present the results of five experiments on F and Cl partitioning during hydrous mantle melting under conditions relevant to subduction zone magmatism (1.2–2.5 GPa, 1,180°C–1,430°C). For each experiment, we determined the F and Cl partition coefficients between lherzolitic mineral phases (olivine, orthopyroxene (opx), clinopyroxene (cpx), and garnet), amphibole, and hydrous basaltic melts (0.2–5.9 wt.% dissolved H2O). At constant pressure, View MathML show contrasting response to the combined effects of decreasing temperature from 1,310°C to 1,180°C and increasing H2O content in the melt from 0.2 to 5.9 wt.%: View MathML. decreases from 0.123 ± 0.004 to 0.021 ± 0.014 while View MathML increases from 0.0021 ± 0.0031 to 0.07 ± 0.01. Similar results are observed for clinopyroxene: View MathML decreases from 0.153 ± 0.004 to 0.083 ± 0.004 while View MathML increases from 0.009 ± 0.0005 to 0.015 ± 0.0008. Experimentally determined F and Cl partition coefficients were used in a hydrous melting model of a lherzolitic mantle metasomatized by slab fluid. In this model, we vary the amount of metasomatic slab fluid added into the mantle while its composition is kept constant. Increasing the amount of fluid results in an increase of both the degree of melting (due to the effect of H2O addition) and the F and Cl input in the mantle wedge. Because of the change of F and Cl partition coefficients with the increase of H2O, the observed variation in the F and Cl contents of the modeled melts is produced not only by F and Cl input from the fluid, but also by the changes in F and Cl fractionation during hydrous melting. Overall, the model predicts that the Cl/F ratio of modeled melts increases with increasing fluid fraction. Therefore, a variation in the amount of fluid added to the mantle wedge can contribute to the variability in Cl/F ratios observed in arc melt inclusions

CO2-rich komatiitic melt inclusions in Cr-spinels within beach sand from Gorgona Island, Colombia

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 288 (2009): 33-43, doi:10.1016/j.epsl.2009.09.005.The volatile content of komatiite is a key to constrain the thermal and chemical evolution of the deep Earth. We report the volatile contents with major and trace element compositions of ~ 80 melt inclusions in chromian spinels (Cr-spinels) from beach sands on Gorgona Island, Colombia. Gorgona Island is a ~ 90 Ma volcanic island, where picrites and the youngest komatiites known on the Earth are present. Melt inclusions are classified into three types on the basis of their host Cr-spinel compositions: low Ti (P type), high Ti with high Cr# (K1 type) and high Ti with low Cr# (K2 type). Chemical variations of melt inclusions in the Cr-spinels cover all of the island's lava types. P-type inclusions mainly occur in the picrites, K1-type in high-TiO2 komatiites (some enriched basalts: E-basalts) and K2-type in low-TiO2 komatiites. The H2O and CO2 contents of melt inclusions within Cr-spinels from the beach sand are highly variable (H2O: 0.03–0.9 wt.%; CO2: 40–4000 ppm). Evaluation of volatile content is not entirely successful because of compositional alterations of the original melt by degassing, seawater/brine assimilation and post-entrapment modification of certain elements and volatiles. However, the occurrence of many melt inclusions with low H2O/K2O ratios indicates that H2O/K2O of Gorgona komatiite is not much different from that of modern mid-oceanic ridge basalt (MORB) or oceanic island basalt. Trend of CO2/Nb and Zr/Y ratios, accounted for by two-component mixing between the least degassed primary komatiite and low-CO2/Nb evolved basalt, allow us to estimate a primary CO2/Nb ratio of 4000 ± 2200 or a CO2 content of 0.16 ± 0.09 wt.%. The determined CO2/Nb ratio is unusually high, compared to that of MORB (530). Although the presence of CO2 in the Gorgona komatiite does not affect the magma generation temperature, CO2 degassing may have contributed to the eruption of high-density magmas. High CO2/Nb and the relatively anhydrous nature of Gorgona komatiite provide possible resolution to one aspect of the hydrous komatiite debate.This work is financially supported by grants from the Japan Society for the Promotion of Science

Oxygen isotope constraints on the origin of high-Cr garnets from kimberlites

The association between diamonds and high-Cr, sub-calcic garnets from kimberlites suggests that garnets have experienced high pressure (and presumably, high temperature) conditions in the subcontinental lithosphere mantle (SCLM). The oxygen isotope compositions of these garnets are generally reported to be lower than average values of mantle olivines (~5.2‰)—opposite the sense of oxygen isotope fractionation expected at high-temperature equilibrium, and to correlate negatively with their Cr-contents. These observations were interpreted to indicate the sub-calcic garnets either are derived from melting Cr-enriched but ^(18)O-depleted source in the SCLM, or have experienced cryptic metasomatism by high-Cr, high-Mg#, but ^(18)O-depleted fluids/melts in the SCLM or during their ascent to the surface. We investigate the possibility that oxygen isotope characteristics of these garnets instead reflect an analytical artifact that reduces the measured δ^(18)O of garnets in proportion to its Cr content—a so-called ‘Cr-effect’. Mixtures of garnet standards (UWG-2) plus various amounts of pure Cr-metal were measured for δ^(18)O-values by infrared-laser fluorination technique (ILFT). Our results show that oxygen isotope compositions of sample mixtures vary systematically as a function of Cr-content and O_(2)-yield. Mass-balance analyses indicate that the correlation between the measured δ^(18)O-value and Cr-content is an analytical artifact mostly due to isotopic fractionation between the extracted-O_(2) and chromium oxyfluorides left as residues in the sample chamber during the ILFT analyses, which amounts to a decrease in the measured δ^(18)O values of ~0.064‰ for the increase of every wt.% Cr_(2)O_(3) present in garnets. After applying our experimentally-calibrated correction factor to garnet samples in garnet peridotite xenoliths from South Africa, our study reveals an equilibrium oxygen isotope fractionation between the Cr-poor garnet and coexisting olivine (i.e., garnet≥olivine) with a fractionation factor of ~0.28‰. More importantly, high-Cr garnets (Cr_(2)O_(3)>5%), after correction for the ‘Cr-effect’, are actually enriched, rather than depleted, in 18O compared with low-Cr garnets, suggesting the involvement of ^(18)O-enriched crustal materials during their formation. We hypothesize a fluid-flow model using depthdependent Cr partitioning. This model simultaneously explains relatively high-δ^(18)O-values and Cr and Mg enrichment in garnets, implying that the reduced C\H\O fluids that are responsible for the formation of diamonds in both eclogite and peridotite xenoliths in kimberlites, are also responsible for transporting Cr to oxidized locations where the Cr partition coefficient between garnet and fluids is high

An experimental study of the grain-scale processes of peridotite melting : implications for major and trace element distribution during equilibrium and disequilibrium melting

Author Posting. © Springer, 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 156 (2008): 87-102, doi:10.1007/s00410-007-0275-8.The grain-scale processes of peridotite melting were examined at 1340°C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P– T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal melt interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxenemelt partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements in residual minerals reequilibrate with their surrounding melt.This work was supported by NSF grants EAR-0208141 and EAR-0510606 to Yan Liang

Carbon solution and partitioning between metallic and silicate melts in a shallow magma ocean: Implications for the origin and distribution of terrestrial carbon

The origin of bulk silicate Earth carbon inventory is unknown and the fate of carbon during the early Earth differentiation and core formation is a missing link in the evolution of the terrestrial carbon cycle. Here we present high pressure (P)–temperature (T) experiments that offer new constraints upon the partitioning of carbon between metallic and silicate melt in a shallow magma ocean. Experiments were performed at 1–5 GPa, 1600–2100 °C on mixtures of synthetic or natural silicates (tholeiitic basalt/alkali basalt/komatiite/fertile peridotite) and Fe–Ni–C ± Co ± S contained in graphite or MgO capsules. All the experiments produced immiscible Fe-rich metallic and silicate melts at oxygen fugacity (fO2) between ~IW-1.5 and IW- 1.9. Carbon and hydrogen concentrations of basaltic glasses and non-glassy quenched silicate melts were determined using secondary ionization mass spectrometry (SIMS) and speciation of dissolved C–O–H volatiles in silicate glasses was studied using Raman spectroscopy. Carbon contents of metallic melts were determined using both electron microprobe and SIMS. Our experiments indicate that at core-forming, reduced conditions, carbon in deep mafic–ultramafic magmas may dissolve primarily as various hydrogenated species but the total carbon storage capacity, although is significantly higher than solubility of CO2 under similar conditions, remains low (<500 ppm). The total carbon content in our reduced melts at graphite saturation increases with increasing melt depolymerization (NBO/T), consistent with recent spectroscopic studies, and modestly with increasing hydration. Carbon behaves as a metal-loving element during core-mantle separation and our experimental Dmetal=silicate C varies between ~4750 and P150 and increases with increasing pressure and decreases with increasing temperature and melt NBO/T. Our data suggest that if only a trace amount of carbon (~730 ppm C) was available during early Earth differentiation, most of it was partitioned to the core (with 0.20–0.25 wt.% C) and no more than ~10–30% of the present-day mantle carbon budget (50–200 ppm CO2) could be derived from a magma ocean residual to core formation. With equilibrium core formation removing most of the carbon initially retained in the terrestrial magma ocean, explanation of the modern bulk silicate Earth carbon inventory requires a later replenishment mechanism. Partial entrapment of metal melt in solid silicate matrix, carbon ingassing by magma ocean–atmosphere interaction, and carbon outgassing from the core aided by reaction of core metal and deeply subducted water are some of the viable mechanisms

Post-entrapment modification of volatiles and oxygen fugacity in olivine-hosted melt inclusions

The solubilities of volatiles (H_2O, CO_2, S, F, and Cl) in basaltic melts are dependent on variables such as temperature, pressure, melt composition, and redox state. Accordingly, volatile concentrations can change dramatically during the various stages of a magma's existence: from generation, to ascent through the mantle and crust, to final eruption at the Earth's surface. Olivine-hosted melt inclusions have the potential to preserve volatile concentrations at the time of entrapment due to the protection afforded by the host olivine against decompression and changes to the oxidation state of the external magma. Recent studies, however, have demonstrated that rapid diffusive re-equilibration of H_2O and oxygen fugacity (f_(O_2)) can occur within olivine-hosted melt inclusions. Here we present volatile, hydrogen isotope, and major element data from dehydration experiments and a quantitative model that assesses proposed mechanisms for diffusive re-equilibration of H_2O and f_(O_2) in olivine-hosted melt inclusions. Our comprehensive set of data for the behavior of common magmatic volatiles (H_2O, CO_2, F, Cl, and S) demonstrates that post-entrapment modification of CO_2, and to a lesser extent S, can also occur. We show that the CO_2 and S concentrations within an included melt decrease with progressive diffusive H_2O loss, and propose that this occurs due to dehydration-induced changes to the internal pressure of the inclusion. Therefore, deriving accurate estimates for pre-eruptive CO_2 and S concentrations from olivine-hosted melt inclusions requires accounting for the amount of CO_2 and S hosted in vapor bubbles. We find, however, that Cl and F concentrations in olivine-hosted melt inclusions are not affected by diffusive re-equilibration through the host olivine nor by dehydration-induced pressure changes within the melt inclusion. Our results indicate that measured H_2O, CO_2 and S concentrations and Fe^(3+)/ΣFe ratios of included melts are not necessarily representative of the melt at the time of entrapment and thus are not reliable proxies for upper mantle conditions

Effect of Fluorine on Near-Liquidus Phase Equilibria of Basalts

Volatile species such as H2O, CO2, F, and Cl have significant impact in generation and differentiation of basaltic melts. Thus far experimental work has primarily focused on the effect of water and carbon dioxide on basalt crystallization, liquid-line of descent, and mantle melting [e.g., 1, 2] and the effects of halogens have received far less attention [3-4]. However, melts in the planetary interiors can have non-negligible chlorine and fluorine concentrations. Here, we explore the effects of fluorine on near-liquidus phase equilibria of basalt. We have conducted nominally anhydrous piston cylinder experiments using graphite capsules at 0.6 - 1.5 GPa on an Fe-rich model basalt composition. 1.75 wt% fluorine was added to the starting mix in the form of AgF2. Fluorine in the experimental glass was measured by SIMS and major elements of glass and minerals were analyzed by EPMA. Nominally volatile free experiments yield a liquidus temperature from 1330 C at 0.8GPa to 1400 at 1.6GPa and an olivine(Fo72)-pyroxene(En68)-liquid multiple saturation point at 1.25 GPa and 1375 C. The F-bearing experiments yield a liquiudus temperature from 1260 C at 0.6GPa to 1305 at 1.5GPa and an ol(Fo66)-pyx(En64)-MSP at 1 GPa and 1260 C. This shows that F depresses the basalt liquidus, extends the pyroxene stability field to lower pressure, and forces the liquidus phases to be more Fe-rich. KD(Fe-Mg/mineral-melt) calculated for both pyroxenes and olivines show an increase with increasing F content of the melt. Therefore, we infer that F complexes with Mg in the melt and thus increases the melt s silica activity, depressing the liquidus and changing the composition of the crystallizing minerals. Our study demonstrates that on a weight percent basis, the effect of fluorine is similar to the effect of H2O [1] and Cl [3] on freezing point depression of basalts. But on an atomic fraction basis, the effect of F on liquidus depression of basalts is xxxx compared to the effect of H. Future studies on kimberlitic and subduction zone magmas, which could have significant amount of fluorine, will need to consider the combined effects of F, Cl, and H on their stability and chemical evolution

CO2 content beneath northern Iceland and the variability of mantle carbon

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geology 46 (2018): 55-58, doi:10.1130/G39413.1.Primitive basalt melt inclusions from Borgarhraun, northern Iceland, display large correlated variations in CO2 and nonvolatile incompatible trace elements (ITEs) such as Nb, Th, Rb, and Ba. The average CO2/ITE ratios of the Borgarhraun melt inclusion population are precisely determined (e.g., CO2/Nb = 391 ± 16; 2σM [two standard errors of the mean], n = 161). These data, along with published data on five other populations of undegassed mid-oceanic ridge basalt (MORB) glasses and melt inclusions, demonstrate that upper mantle CO2/Ba and CO2/Rb are nearly homogeneous, while CO2/Nb and CO2/Th are broadly correlated with long-term indices of mantle heterogeneity reflected in Nd isotopes (143Nd/144Nd) in five of the six regions of the upper mantle examined thus far. Our results suggest that heterogeneous carbon contents of the upper mantle are long-lived features, and that average carbon abundances of the mantle sources of Atlantic MORB are higher by a factor of two than those of Pacific MORB. This observation is correlated with a similar distinction in water contents and trace elements characteristic of subduction fluids (Ba, Rb). We suggest that the upper mantle beneath the younger Atlantic Ocean basin contains components of hydrated and carbonated subduction-modified mantle from prior episodes of Iapetus subduction that were entrained and mixed into the upper mantle during opening of the Atlantic Ocean basin.Maclennan is supported by Natural Environment Research Council grant NE/M000427/1. This research was supported by the Carnegie Institution of Washington