Search

Inclusions of basaltic melt trapped inside of olivine phenocrysts during igneous crystallization provide a rich, crystal-scale record of magmatic processes ranging from mantle melting to ascent, eruption, and quenching of magma during volcanic eruptions. Melt inclusions are particularly valuable for retaining information on volatiles such as H2O and CO2 that are normally lost by vesiculation and degassing as magma ascends and erupts. However, the record preserved in melt inclusions can be variably obscured by postentrapment processes, and thus melt inclusion research requires careful evaluation of the effects of such processes. Here we review processes by which melt inclusions are trapped and modified after trapping, describe new opportunities for studying the rates of magmatic and volcanic processes over a range of timescales using the kinetics of post-trapping processes, and describe recent developments in the use of volatile contents of melt inclusions to improve our understanding of how volcanoes work. ▪ Inclusions of silicate melt (magma) trapped inside of crystals formed by magma crystallization provide a rich, detailed record of what happens beneath volcanoes. ▪ These inclusions record information ranging from how magma forms deep inside Earth to its final hours as it ascends to the surface and erupts. ▪ The melt inclusion record, however, is complex and hazy because of many processes that modify the inclusions after they become trapped in crystals. ▪ Melt inclusions provide a primary archive of dissolved gases in magma, which are the key ingredients that make volcanoes erupt explosively.

Erebus volcano, Antarctica, is the southernmost active volcano on the globe. Despite its remoteness and harsh conditions, Erebus volcano provides an unprecedented and unique opportunity to study the petrogenesis and evolution, as well as the passive and explosive degassing, of an alkaline magmatic system with a persistently open and magma-filled conduit. In this chapter, we review nearly five decades of scientific research related to Erebus volcano, including geological, geophysical, geochemical and microbiological observations and interpretations. Mount Erebus is truly one of the world9s most significant natural volcano laboratories where the lofty scientific goal of studying a volcanic system from mantle to microbe is being realized.

Description, Incorporation of variable amounts of oxidizing slab fluids leads to globally heterogeneous oxidation state of the subarc mantle., Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system’s thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones.

The processes that controlled accretion of water and volatiles to the inner solar system remain enigmatic, because it is difficult to determine the absolute concentrations of volatile elements in planetary bodies. In this contribution we study rare unequilibrated eucrites derived from the asteroid 4 Vesta, to determine the water and fluorine content of this asteroid by measuring the volatile content of pyroxene. Common thermal metamorphism in most equilibrated eucrites would have diffusively reset magmatic volatile contents. The unequilibrated eucrites are, therefore, the most suitable samples to determine primary magmatic volatile contents of 4 Vesta. We find H2O and F contents in pyroxenes of 4–11 µg/g and 0.12–0.23 µg/g. We also determine a H2O partition coefficient for clinopyroxene and melt equilibrated at 0.1 MPa of DH2O = 0.1, which is higher than values previously reported for higher pressures. The higher compatibility of H2O in this experiment could partially be due to high OH/H2O ratio at the low total water contents in this experimental charge, but only further more detailed experiments will fully delineate the reasons for the more compatible behavior for water at lower pressures. However, given the lack of H2O partitioning data at low pressures we conclude that our 0.1 MPa experiment is the most appropriate to calculate magmatic water contents for melt in equilibrium with eucrite pyroxene. After using appropriate partition coefficients we calculate melt concentrations of 50–70 µg/g H2O and 1.5–2.4 µg/g F. In turn, these are converted into bulk 4 Vesta water and F contents of 10–70 µg/g H2O and 0.3–2 µg/g F by assuming eucrite formation via either mantle partial melting or extraction from a magma ocean. We also measure the D/H of the clinopyroxenes and show that these are identical to the results of previous studies that reported D/H in eucrite apatite. These values match those found in carbonaceous chondrites suggesting that water in 4 Vesta accreted from carbonaceous chondrites and not from cometary material.

Mineral-hosted melt inclusions have become an important source of information on magmatic processes. As the number of melt inclusion studies increases, so does the need to establish recommended practice guidelines for collecting and reporting melt inclusion data. These guidelines are intended to ensure certain quality criteria are met and to achieve consistency among published melt inclusion data in order to maximize their utility in the future. Indeed, with the improvement of analytical techniques, new processes affecting melt inclusions are identified. It is thus critical to be able to reprocess any previously published data, such that reporting the raw data is one of the first “recommended practices” for authors and a publication-criteria that reviewers should be sensitive to. Our guidelines start with melt inclusion selection, which is a critical first step, and then continue on to melt inclusion preparation and analysis, covering the entire field of methods applicable to melt inclusions. Dedication In March of 2000, a melt inclusion workshop was held at the Chateau de Sassenage in Grenoble and a companion issue of Chemical Geology entitled “Melt Inclusions at the Millennium” was published. Erik Hauri was heavily involved with the meeting and contributed two landmark papers to the topical issue of Chemical Geology on the use of secondary ion mass spectrometry to analyze volatiles in melt inclusions. When the melt inclusion community re-convened at Woods Hole Oceanographic Institution (WHOI) in August of 2018, we were saddened that Erik was unable to join us due to his failing health. Less than a month later came the devastating news of his passing at only 52 years of age. In recognition of his incredible contributions to science in general and to the in situ analysis of melt inclusions in particular, the participants and organizers of the WHOI melt inclusion workshop dedicate this collegial paper to Erik Hauri, our colleague, mentor and friend. Thank you Erik.

The Iheya Graben is a back-arc spreading centre in the middle part of the Okinawa Trough. It is also located in the centre of an anomalous volcanic zone (volcanic arc migration phenomenon, or VAMP) and is characterized by bimodal volcanism, unusually high heat flow and active hydrothermal circulation. The subvolcanic magma plumbing system and the magmatic processes related to the formation of rare erupted intermediate lavas in this area remain uncertain. In this study, we conducted systematic mineralogical analyses (in situ major element, trace element and Sr isotopes) and whole rock geochemical analyses (major element, trace element and Sr–Nd isotopes) on an andesite (T5-2; type C andesite) and a rhyolite (C11; type 2 rhyolite), and present evidence for magma mixing in the origins of these lavas. Andesite T5-2 contains a mafic mineral assemblage and a silicic mineral assemblage, which are derived from a basaltic melt and a type 2 rhyolitic melt, respectively. A 4:6 mixture of basalt and type 2 rhyolite from the Iheya Graben reproduces the whole-rock major element, trace element, and Sr–Nd isotope compositions of T5-2. Rhyolite C11 contains a group of disequilibrium minerals that crystallized from a less evolved rhyolitic melt with relatively more enriched Sr–Nd isotope compositions, suggesting mixing of this melt with a more evolved and isotopically more depleted rhyolitic melt. This mixing process could produce a series of rhyolitic melts with a negative correlation between SiO2 concentrations and 87Sr/86Sr ratios (or a positive correlation for 143Nd/144Nd ratios), which are recorded by the whole group of type 2 rhyolites. The results from mineral-based thermobarometers suggest that the premixing storage temperatures of the basaltic and rhyolitic melts are ∼1100 °C and 870–900 °C, respectively. The hybrid andesitic melt has temperatures of ∼950 to ∼980 °C. The magma storage pressures are not well constrained, ranging from ∼400 MPa to ∼100 MPa. We show that magma mixing plays a significant role in the origins of diverse volcanism in the middle Okinawa Trough; more specifically, two of the three types of andesites (types B and C) and one of the two types of rhyolites (type 2) are associated with magma mixing. We thus propose a complex magma plumbing system with multichamber magma storage and frequent magma mixing beneath the Iheya Graben.

The redox state of silicate melts influences crystallization, element partitioning, and degassing behavior. Synchrotron-based micro-X-ray absorption near edge structure (μXANES) spectroscopy has emerged as a powerful tool for determining redox conditions through the direct measurement of speciation of multivalent elements such as iron and sulfur in silicate glasses. In particular, the high spatial resolution afforded by synchrotron μXANES makes it one of the few techniques available for determining redox conditions in melt inclusions, which can provide insights into pre-eruptive melt properties. However, the small size of melt inclusions, the deep penetration of X-rays, and irradiation-induced beam damage make μXANES measurements in melt inclusions challenging. Here we present data showing the rapid occurrence of Fe- and S-μXANES beam damage in experimental glasses, mid-ocean ridge basalt glasses, and olivine-hosted melt inclusions from the southern Cascade arc and Kīlauea Volcano and develop approaches to recognize and correct for beam damage through repeated rapid analyses. By applying a time-dependent correction to a series of rapid measurements (~82 s/scan) of Fe-μXANES pre-edge centroid positions, irradiation-induced photo-oxidation (Fe2+ to Fe3+) can be corrected back to undamaged initial Fe3+/ΣFe even in damage-susceptible hydrous glasses. Using this beam damage correction technique, hydrous basaltic melt inclusions from the southern Cascades have Fe3+/ΣFe values that are ~0.036 lower (corresponding to -0.5 log units lower oxygen fugacity) than would have been indicated by standard Fe-μXANES measurements. Repeated, rapid analyses (150–300 s/scan) were also used to identify S-μXANES beam damage (photo-reduction of S6+ to S4+), which was corrected with a peak fitting method to restore initial S6+/ΣS. We observe that S-μXANES beam damage can occur rapidly even in low-H2O mid-ocean ridge basaltic glasses and melt inclusions from Kīlauea Volcano, which are otherwise stable during even prolonged Fe-μXANES analyses. By mitigating and correcting for sulfur photo-reduction, we conclude that some mid-ocean ridge basaltic glasses contain 0.08–0.09 S6+/ΣS, which is more sulfate than might be expected based on the reduced oxidation state of these glasses (near the fayalite-magnetite-quartz oxygen buffer). Using beam damage identification and correction techniques, the valence states of iron and sulfur can be accurately measured even in beam damage-susceptible glasses and melt inclusions. Finally, using Fe-μXANES, we demonstrate the presence of Fe-oxide nanolites within otherwise glassy, naturally quenched melt inclusions, which can complicate determination of iron valence state in affected glasses.

Earth and the other rocky bodies that make up the inner solar system are systematically depleted in hydrogen (H) and other cosmochemically volatile elements (e.g., carbon (C), fluorine (F), chlorine (Cl), and thallium (Tl)) relative to primitive undifferentiated meteorites known as carbonaceous chondrites. If we are to understand how and when Earth gained its life-essential elements, it is critical to determine the timing, flux, and nature of the delivery of condensed volatiles into the presumed hot and dry early inner solar system. Here we present evidence preserved in ancient basaltic angrite meteorites for an addition of volatiles to the hot and dry inner solar system within the first two million years of solar system history. Our data demonstrate that the angrite parent body was enriched in highly volatile elements (H, C, F, and Tl) relative to those predicted on the basis of the angrite parent body’s overall volatile depletion trend (e.g., H is enriched by up to a factor of 10 6 ). This relative enrichment is best explained by mixing of extremely volatile-depleted material, located well inside the snow line, with volatile-rich material derived from outside the snow line.

Turning up the mantle temperature The temperature at which Earth's mantle begins to melt is a long-standing question in geology. Sarafian et al. present a clever set of experiments to determine the impact of small amounts of water on the melting temperature of mantle rock (see the Perspective by Asimow). This allowed them to reinterpret geophysical observations of melting in the mantle and revise estimates of mantle temperature upward. A hotter mantle has a multitude of implications for mantle melting and geodynamic processes. Science , this issue p. 942 ; see also p. 908

Coral skeletons are valuable archives of past ocean conditions. However, interpretation of coral paleotemperature records is confounded by uncertainties associated with single element-ratio thermometers, including Sr/Ca. A new approach, Sr-U, uses U/Ca to constrain the influence of Rayleigh fractionation on Sr/Ca [DeCarlo et al., 2016]. Here, we build on the initial Pacific Porites Sr-U calibration to include multiple Atlantic and Pacific coral genera from multiple coral reef locations spanning a temperature range of 23.15-30.12 °C. Accounting for the wintertime growth cessation of one Bermuda coral, we show that Sr-U is strongly correlated with the average water temperature at each location (r2 = 0.91, P

Measurements of B/Ca ratios in marine carbonates have been suggested to record seawater carbonate chemistry, however experimental calibration of such proxies based on inorganic partitioning remains limited. Here we conducted a series of synthetic aragonite precipitation experiments to evaluate the factors influencing the partitioning of B/Ca between aragonite and seawater. Our results indicate that the B/Ca ratio of synthetic aragonites depends primarily on the relative concentrations of borate and carbonate ions in the solution from which the aragonite precipitates; not on bicarbonate concentration as has been previously suggested. The influence of temperature was not significant over the range investigated (20–40 °C), however, partitioning may be influenced by saturation state (and/or growth rate). Based on our experimental results, we suggest that aragonite B/Ca ratios can be utilized as a proxy of [CO 3 2– ]. Boron isotopic composition (δ 11 B) is an established pH proxy, thus B/Ca and δ 11 B together allow the full carbonate chemistry of the solution from which the aragonite precipitated to be calculated. To the extent that aragonite precipitation by marine organisms is affected by seawater chemistry, B/Ca may also prove useful in reconstructing seawater chemistry. A simplified boron purification protocol based on amberlite resin and the organic buffer TRIS is also described.

Observations of dunite channels in ophiolites and uranium-series disequilibria in mid-ocean ridge basalt suggest that melt transport in the upper mantle beneath mid-ocean ridges is strongly channelized. We present experimental evidence that spatial variations in mineralogy can also focus melt on the grain-scale. This lithologic melt partitioning, which results from differences in the interfacial energies associated with olivine-melt and orthopyroxene-melt boundaries, may complement other melt focusing mechanisms in the upper mantle such as mechanical shear and pyroxene dissolution. We document here lithologic melt partitioning in olivine/orthopyroxene-basaltic melt samples containing nominal olivine to orthopyroxene ratio of 3 to 2 and melt fractions of 0.02 to 0.20. Experimental samples were imaged using synchrotron-based X-ray micro-computed tomography at a resolution of 700 nm per voxel. By analyzing the local melt fraction distributions associated with olivine and orthopyroxene (opx) grains in each sample, we found that the melt partitioning coefficient, i.e., the ratio of melt fraction around olivine to that around orthopyroxene grains, varies between 1.1 and 1.6. The permeability and electrical conductivity of our digital samples were estimated using numerical models and compared to those of samples containing only olivine and basaltic melt. Our results suggest that lithologic melt partitioning and preferential localization of melt around olivine grains might play a role in melt focusing, potentially enhancing average melt ascent velocities.

This study presents an internally consistent model for the behavior of Fe3+/∑Fe during partial melting of spinel lherzolite. The Fe3+/∑Fe ratio for olivine is calculated on the basis of point defect thermodynamics, and the oxidation states of iron in the other solid phases are calculated using Fe3+/Fe2+ distribution between olivine and orthopyroxene, clinopyroxene, or spinel. Conservation of mass is used to relate the Fe3+/Fe2+ ratio of partial melt to the concentrations of Fe3+ and Fe2+ in the initial and residual solids as a function of pressure, temperature, and oxygen fugacity. Results from isobaric batch melting calculations demonstrate that the Fe3+/∑Fe ratio of the partial melt decreases with increasing melt fraction. Conversely, the Fe3+/∑Fe ratio of the partial melt increases with increasing melt fraction during decompression batch melting. The relative oxygen fugacity of the upper mantle depends on both the oxidation state of iron and mantle potential temperature. Results from incremental decompression melting calculations in which 1% melt is produced for each 100 MPa of decompression and then removed from the residual solid indicate that relative oxygen fugacity calculated from the oxidation state of iron in basaltic glass does not represent a unique value for the oceanic upper mantle but, rather, reflects conditions in the lower portion of the melting regime. A 100 °C change in mantle potential temperature produces a change in relative oxygen fugacity of ∼0.8 log units, similar to the global range inferred from mid-ocean ridge basalt glasses. It is necessary, therefore, to compare relative oxygen fugacity calculated from basaltic glass with proxies for potential temperature before drawing conclusions on heterogeneity of the oxidation state of iron in the oceanic upper mantle. Results from model calculations also suggest that the sub-arc mantle is intrinsically more oxidizing than the oceanic mantle because it is cooler. The global correlation between Fe3+/∑Fe and H2O in basaltic glasses may, in part, reflect the solidus-lowering influence of the latter. Secular variations in relative oxygen fugacity of the upper mantle due to cooling of the Earth over geologic time do not appear to be large enough to have influenced the nature of volcanic outgassing significantly enough to have contributed to the Great Oxidation Event.

We have measured the partition coefficient of Pb (KdPb) between FeS melt and basalt melt at temperatures of 1250–1523 °C, pressures of 1.0–3.5 GPa and oxygen fugacities at iron–wustite and wustite–magnetite. The total observed range of KdPb is 4.0–66.6, with a strong negative dependence on pressure and a strong negative dependence on FeO of the silicate melt (Fe+2 only). The FeO control was constrained over a wide range of FeO (4.2–39.5%). We found that the effect of oxygen fugacity can be subsumed under the FeO control parameter. Prior work has established the lack of a significant effect of temperature (Kiseeva and Wood, 2015; Li and Audetat, 2015). Our data are parameterized as: KdPb = 4.8 + (512 − 119*P in GPa)*(1/FeO − 0.021). We also measured a single value of KdPb between clinopyroxene and basalt melt at 2.0 GPa of 0.020 ± 0.001. This experimental data supports the “natural” partitioning of Pb measured on sulfide globules in MORB (Patten et al., 2013), but not the low KdPb of ∼3 inferred from sulfides in abyssal peridotites by Warren and Shirey (2012). It also quantitatively affirms the modeling of Hart and Gaetani (2006) with respect to using sulfide to buffer the canonical Nd/Pb ratio for MORB and OIB (Hofmann, 2003). For the low FeO and pressure of segregation typical of MORB, KdPb ∼ 45, and the Nd/Pb ratio of erupted basalts will be the same as the Nd/Pb ratio of the mantle source. The remaining puzzle is why MORB and OIB have the same Nd/Pb when they clearly have different FeO and pressure of melt segregation.

Coral skeletons archive past climate variability with unrivaled temporal resolution. However, extraction of accurate temperature information from coral skeletons has been limited by “vital effects,” which confound, and sometimes override, the temperature dependence of geochemical proxies. We present a new approach to coral paleothermometry based on results of abiogenic precipitation experiments interpreted within a framework provided by a quantitative model of the coral biomineralization process. DeCarlo et al. (2015a) investigated temperature and carbonate chemistry controls on abiogenic partitioning of Sr/Ca and U/Ca between aragonite and seawater and modeled the sensitivity of skeletal composition to processes occurring at the site of calcification. The model predicts that temperature can be accurately reconstructed from coral skeleton by combining Sr/Ca and U/Ca ratios into a new proxy, which we refer to hereafter as the Sr-U thermometer. Here we test the model predictions with measured Sr/Ca and U/Ca ratios of 14 Porites sp. corals collected from the tropical Pacific Ocean and the Red Sea, with a subset also analyzed using the boron isotope (δ11B) pH proxy. Observed relationships among Sr/Ca, U/Ca, and δ11B agree with model predictions, indicating that the model accounts for the key features of the coral biomineralization process. By calibrating to instrumental temperature records, we show that Sr-U captures 93% of mean annual temperature variability (26–30°C) and has a standard deviation of prediction of 0.5°C, compared to 1°C using Sr/Ca alone. The Sr-U thermometer may offer significantly improved reliability for reconstructing past ocean temperatures from coral skeletons.

The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (∼0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have 87Sr/86Sr ranging from 0.702907 to 0.703147, e Nd ranging from +4.3 to +6.3, e Hf ranging from +5.6 to +8.6, and 206Pb/204Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet. Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ∼1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism. Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.

The U/Ca ratio of aragonite coral skeleton exhibits coherent patterns of seasonal and interannual variability. In field-sampled corals and those grown in controlled culture experiments, strong correlations have been found between coral skeleton U/Ca and water temperature, pH, carbonate ion concentration, and salinity. However, the mechanism(s) underlying these different correlations remain unclear. We performed abiogenic precipitation experiments designed to evaluate the sensitivity of U partitioning between aragonite and seawater to temperature, pH, and the concentration of carbonate ion in seawater. Aragonite was precipitated from seawater by addition of carbonate alkalinity at rates set to maintain stable carbonate chemistry during precipitation. Experiments were conducted at 20–40 °C, with pH 7.8–9.0 and carbonate ion concentrations of 600–2600 μmol kg−1. U/Ca ratios of the bulk precipitate and fluid were determined by inductively coupled plasma mass spectrometry. Our results show that the U/Ca ratio of aragonite precipitated from seawater decreases with increasing carbonate ion concentration, and is independent of pH and temperature. We use these results as a framework to interpret the skeletal composition of coral aragonite precipitated from a calcifying fluid that is semi-isolated from the external seawater environment. Accordingly, coral U/Ca ratios are consistent with calcifying fluid carbonate ion concentrations that are several times greater than those of ambient seawater. Correlations between coral U/Ca ratios and seawater temperature, carbonate chemistry, and other environmental variables arise indirectly, via the impacts of these variables on the carbonate ion concentration of the coral calcifying fluid.

The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5°C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3°C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.

This study presents a new experimental approach for determining H2O solubility in basaltic melt at upper mantle conditions. Traditional solubility experiments are limited to pressures of ~600 MPa or less because it is difficult to reliably quench silicate melts containing greater than ~10 wt% dissolved H2O. To overcome this limitation, our approach relies on the use of secondary ion mass spectrometry to measure the concentration of H dissolved in olivine and on using the measured H in olivine as a proxy for the concentration of H2O in the co-existing basaltic melt. The solubility of H2O in the melt is determined by performing a series of experiments at a single pressure and temperature with increasing amounts of liquid H2O added to each charge. The point at which the concentration of H in the olivine first becomes independent of the amount of initial H2O content of the charge (added + adsorbed H2O) indicates its solubility in the melt. Experiments were conducted by packing basalt powder into a capsule fabricated from San Carlos olivine, which was then pressure-sealed inside a Ni outer capsule. Our experimental results indicate that at 1000 MPa and 1200 °C, the solubility of H2O in basaltic melt is 20.6 ± 0.9 wt% (2 × standard deviation). This concentration is considerably higher than predicted by most solubility models but defines a linear relationship between H2O fugacity and the square of molar H2O solubility when combined with solubility data from lower pressure experiments. Further, our solubility determination agrees with melting point depression determined experimentally by Grove et al. (2006) for the H2O-saturated peridotite solidus at 1000 MPa. Melting point depression calculations were used to estimate H2O solubility in basalt along the experimentally determined H2O-saturated peridotite solidus. The results suggest that a linear relationship between H2O fugacity and the square of molar solubility exists up to ~1300 MPa, where there is an inflection point and solubility begins to increase less strongly with increasing H2O fugacity.

Earth's carbon budget is central to our understanding of the long-term co-evolution of life and the planet. Direct observations of surface reservoirs allow for the detailed quantification of their carbon content. However, the carbon content of Earth's deep interior remains poorly constrained. Here we study olivine-hosted melt inclusions from two Icelandic eruptions, with those from the Miðfell eruption allowing us to investigate the carbon content of the deep mantle. Comparison with the previously studied Borgarhraun eruption highlights the presence of deep, plume-sourced mantle material within the Miðfell source region. Miðfell contains trace element-depleted melt inclusions undersaturated in CO2, which have high CO2/Ba ( = 396 ± 48 ) and CO2/Nb ( = 1832 ± 316 ), though some inclusions preserve even greater relative carbon enrichment. These observations allow us to reconstruct the CO2 content of the bulk Miðfell source as being > 690 ppm . By identifying that Miðfell is a mixture of depleted and deep mantle components, we can estimate a CO2 content for the deep mantle component of 1350 ± 350 ppm ; a concentration that is over ten times higher than depleted MORB mantle estimates. Assuming that the deep mantle component identified in Miðfell is representative of a global reservoir, then with our new CO2 estimate and by considering a range of representative mantle fractions for this reservoir, we calculate that it contains up to 14 times more carbon than that of the atmosphere, oceans, and crust combined. Our result of elevated CO2/Ba and CO2/Nb ratios, and carbon enrichment support geochemical bulk Earth carbon models that call for the presence of carbon-rich deep mantle domains to balance Earth's relatively carbon-poor upper mantle and surface environment.

Melt percolation in mantle rocks is currently poorly constrained, especially at low melt fractions. At mid-ocean ridges, for example, geochemical and geophysical observations produce divergent estimates of how much melt is present in the mantle and how quickly it moves. Accurate estimates of permeability and grain-scale melt distribution in mantle rock are necessary to reconcile these observations. We present three-dimensional (3-D), 700 nm-resolution images of olivine–basalt aggregates, containing nominal melt fractions ( ϕ n ) between 0.02 and 0.20. Samples were prepared from a powdered mixture of San Carlos olivine and high-alumina basalt and hot-pressed in a solid-medium piston–cylinder apparatus at 1350 °C and 1.5 GPa. Images were obtained using synchrotron X-ray microtomography (SXμT) from the Advance Photon Source at Argonne National Laboratory. Stokes flow simulations, conducted using the digital melt volume as the numerical domain, determine that the permeabilities of experimental charges range from 2 × 10 − 16 to 5 × 10 − 13 m 2 for ϕ n = 0.02 to 0.20, respectively. The simulation results are well represented by the power-law relation between permeability (k) and melt fraction (ϕ), k = ϕ n d 2 / C , where n = 2.6 ± 0.2 , and assuming a grain size of 35 μm in the experiments, C = 58 − 22 + 36 . These results place important new constraints on rates of melt migration and melt extraction within partially molten regions of the mantle.

The magnesium isotope composition (δ26Mg) of sub-annually and bulk-sampled Porites corals is presented and suggests that the coral–seawater fractionation factor (∆26Mg) is temperature-dependent with a sensitivity of ~ 0.03‰ °C− 1. Although the apparent temperature dependence of ∆26Mg in Porites corals could be used empirically to reconstruct paleoclimate, its temperature sensitivity is about three times that of abiogenic aragonites, which indicates that kinetic and/or biologically-induced processes (collectively known as vital effects) are also important. Comparison with data from abiogenic aragonite precipitation experiments suggests that ∆26Mg vital effects vary seasonally in Porites. The ∆26Mg of skeleton deposited in winter is within the analytical error of abiogenic aragonite, but that of skeletal aragonite grown during summer exhibits significantly smaller fractionations that may be associated with light-enhanced calcification. The observed vital effect is consistent with

Oxygen isotope fractionation factors between aragonite and seawater are studied at T = 25–55 °C and pH = 7.4–8.1 in a set of ‘free-drift’ precipitation experiments with various CO2-degassing rates (0–75 cc/min). The measured fractionation factors correlate weakly with degassing rate, but strongly with temperature in the following form (R2 = 0.998): 1000lnαaragonite–seawater=22.5(±0.5)(103/T)-46.1(±1.6) where α is the fractionation factor and T is the temperature in Kelvin. Along with results from previous studies, these experiments help calibrate two extreme cases of a new kinetic model – extremely fast (e.g., during spontaneous precipitation) and slow aragonite precipitation processes (e.g., during slow ‘free-drift’ precipitation experiments) – at pH >7.5, T = 0–55 °C and salinity = ∼5–39.6‰. The model assumes little isotopic fractionation between DIC species and aragonite during their attachment and detachment on aragonite surfaces at low temperatures and the δ18O value of aragonite equals the average δ18O value of all contributing DIC species. During the fast precipitation of aragonite, the contribution of each DIC species to the δ18O value of aragonite is proportional to its concentration, whereas in a slow precipitation process when the system reaches a steady state, the contribution is determined by both its concentration and a modifying factor. Although the physical meaning of this modifying factor depends on the method of derivation and other assumptions made, three different derivations based on precipitation/dissolution kinetics, isotope exchange reactions, and isotope disequilibrium among DIC species all lead to the same formulation. At pH >7.5, the natural logarithm of the modifying factor (kHCO3-) for adjusting the contribution of the bicarbonate ion relative to the carbonate ion is calibrated, using experimental data covering a range of temperature, salinity, pH, and precipitation rate, as follows: lnkHCO3-=(2.22±0.08)×(pH-pH0)+(846±78)·S-(40.3±2.1)×103RT-(0.30±0.03)·S-(4.7±1.2) where S is the salinity, R is the gas constant, pH0 is the pH difference between free scale and the scale used to calculate the concentration of DIC species. Quantitative evaluation of these two extreme cases using our model suggests that oxygen isotope fractionation between aragonite and water varies significantly with a pH change between 7.5 and 10 (e.g., ∼4‰ at 25 °C), but little with a salinity change between 5‰ and 35‰ (

The detectable magnesium (Mg) isotope fractionation between biogenic aragonite (including aragonitic corals, bivalves, scaphopod, and sclerosponges) and seawater can potentially be applied to reconstruct sea surface temperature (SST) in the past. To calibrate this thermometer, eight sets of inorganic precipitation experiments (‘free-drift’) in seawater (Mg/Ca = 5 or 10) have been carried out at 25–55 °C over a range of degassing rate. A cleaning procedure was adopted to remove Mg contamination by sea salt, surface absorbed Mg and silicate dust as nucleation centers. The Mg isotope fractionation between cleaned aragonite and seawater-like aqueous solution varies insignificantly with Mg/Ca ratios and Mg isotope compositions of the initial solution, and the CO 2 -degassing rate (0–75 cc/min), but decreases noticeably with increasing temperatures having a temperature sensitivity of ∼0.008–0.01‰/°C in the following form: Δ aragonite – Mg ( aq ) ≈ 1000 ln α aragonite – Mg ( aq ) = 1.67 ( ± 0.36 ) - 0.82 ( ± 0.11 ) × 1000 T where α aragonite-seawater is the fractionation factor, and T is the absolute temperature in Kelvin. It is consistent with equilibrium fractionation between Mg 2+ aquo-complex and magnesite predicted by one theoretical calculation. Qualitative comparison among Mg-bearing carbonates based on Mg–O bond strengths show the relative sequence of 26 Mg enrichment is aragonite > dolomite > magnesite > calcite. Thus, the surprising agreement indicates either the calculation overestimated Mg fractionation between magnesite and fluid, or both theoretical calculation and our calibration represent Mg isotope fractionation between MgCO 3 0 –H 2 O cluster and Mg 2+ aquo complexes. Comparison of our calibration with the Mg isotope fractionation between biogenic aragonite and seawater suggests Mg and oxygen isotope fractionations of some biogenic aragonites (e.g., Porites sp . corals) agree with our calibration within analytical uncertainty, whereas others deviate significantly, indicating biological and/or kinetic isotope fractionation effects. Thus, although the precision of this temperature proxy is ∼ ± 3–4 °C (2 σ ) given our current analytical precision and temperature sensitivity of the calibration, several factors are limiting the application of this proxy to extract precise sea-surface temperatures (SSTs) from the biogenic aragonite, including the requirement for high analytical precision, proper sample cleaning procedures, uncertainty in δ 26 Mg values of seawater, and possible kinetic and/or biological isotope fractionation.

This study presents a new approach to coral thermometry that deconvolves the influence of water temperature on skeleton composition from that of “vital effects”, and has the potential to provide estimates of growth temperatures that are accurate to within a few tenths of a degree Celsius from both tropical and cold-water corals. Our results provide support for a physico-chemical model of coral biomineralization, and imply that Mg2+ substitutes directly for Ca2+ in biogenic aragonite. Recent studies have identified Rayleigh fractionation as an important influence on the elemental composition of coral skeletons. Daily, seasonal and interannual variations in the amount of aragonite precipitated by corals from each “batch” of calcifying fluid can explain why the temperature dependencies of elemental ratios in coral skeleton differ from those of abiogenic aragonites, and are highly variable among individual corals. On the basis of this new insight into the origin of “vital effects” in coral skeleton, we developed a Rayleigh-based, multi-element approach to coral thermometry. Temperature is resolved from the Rayleigh fractionation signal by combining information from multiple element ratios (e.g., Mg/Ca, Sr/Ca, Ba/Ca) to produce a mathematically over-constrained system of Rayleigh equations. Unlike conventional coral thermometers, this approach does not rely on an initial calibration of coral skeletal composition to an instrumental temperature record. Rather, considering coral skeletogenesis as a biologically mediated, physico-chemical process provides a means to extract temperature information from the skeleton composition using the Rayleigh equation and a set of experimentally determined partition coefficients. Because this approach is based on a quantitative understanding of the mechanism that produces the “vital effect” it should be possible to apply it both across scleractinian species and to corals growing in vastly different environments. Where instrumental temperature records are available, a Rayleigh-based framework allows the effects of stress on coral calcification to be identified on the basis of anomalies in the skeletal composition.

This experimental study examines the influence of tetrahedrally coordinated Al 3+ ( IV Al 3+ ) on the partitioning of H + between high-Ca clinopyroxene and silicate melt. Experiments were carried out at 1.5 GPa and 1275 to 1350 °C on a natural high-alumina basalt and a compositionally similar synthetic basalt that is nominally alumina free. The results extend the compositional range of clinopyroxene for which partitioning has been determined experimentally to both higher and lower IV Al 3+ , thereby clarifying its role in maintaining charge neutrality during H + incorporation. Clinopyroxene-melt partition coefficients for H + ( D H 2 O Cpx − Melt ) determined for the high-alumina basalt are among largest ever reported ( D H 2 O Cpx − Melt = 0.0228 to 0.0477), while those determined for the nominally alumina-free starting composition are the smallest ( D H 2 O Cpx − Melt = 0.00445 to 0.0071). Our results confirm that H + is incorporated into clinopyroxene through two independent mechanisms: (1) the creation of metal vacancies and coupled hydroxyl defects and (2) a coupled substitution involving IV Al 3+ that creates isolated hydroxyl defects. The relative importance of each of these incorporation mechanisms was quantified by formulating D H 2 O Cpx − Melt as the sum of a metal vacancy-related partition coefficient ( D H 2 O V Me ) and an Al-coupled substitution-related partition coefficient ( D H 2 O Al ) and fitting the resulting expression to all available experimental data. The contribution of D H 2 O V Me to D H 2 O Cpx − Melt is relatively constant at 0.006 ± 0.002, but is sensitive to pressure, temperature, H 2 O fugacity and pyroxene composition. The contribution of D H 2 O Al varies from ∼ 0 to 0.038 and is strongly dependent on phase composition, but insensitive to pressure and temperature. Two parameterizations of D H 2 O Cpx − Melt that account for the potential effects of temperature, pressure and phase composition were calibrated using our data and experimentally determined partition coefficients from the literature. The first parameterization considers only compositional effects and can be used to estimate D H 2 O Cpx − Melt in cases where pressure and/or temperature are unknown. This equation was used to demonstrate that the pre-eruptive H 2 O contents of arc lavas calculated on the basis of H + in clinopyroxene phenocrysts are systematically higher than those inferred from olivine-hosted melt inclusions, so that the former provide a record that is closer to primary (undegassed) values. The second parameterization considers the effects of pressure and temperature in addition to phase composition and provides a moderately better fit to the experimental data. This equation was used to re-evaluate the influence of H 2 O on the depth at which peridotite partial melting begins beneath oceanic spreading centers. Our calculations indicate that partial melting begins at depths that are shallower than suggested by previous estimates. For a potential temperature of 1350 °C, peridotite containing 50, 100, 150, and 200 ppm H 2 O dissolved in nominally anhydrous minerals begins melting at depths of 75, 79, 83, and 87 km, respectively.

This experimental study examines the solubility of rutile in silicate melts and presents a model for rutile saturation as a function of temperature, pressure and melt composition. Rutile saturation experiments were carried out in the system SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O at 1 bar to 35 kbar and 1150 to 1450 °C on model rhyodacite (~ 69 wt.% SiO2) and haplobasalt (~ 54 wt.% SiO2) base melt compositions. At rutile saturation, the concentration of TiO2 in the model rhyodacite base melt increases at 1 bar from 3.27 ± 0.03 wt.% at 1150 °C to 13.88 ± 0.04 wt.% at 1450 °C. At 1350 °C it decreases from 8.89 ± 0.08 wt.% at 1 bar to 2.06 ± 0.13 wt.% at 35 kbar. Rutile solubility is significantly higher in the haplobasalt base melt at a given pressure and temperature. The concentration of TiO2 in rutile-saturated haplobasalt increases at 1 bar from 20.9 ± 0.3 wt.% at 1300 °C to 39.0 ± 0.3 wt.% at 1450 °C. At 1350 °C it decreases from 25.8 ± 0.3 wt.% at 1 bar to 15.67 ± 0.16 wt.% at 15 kbar. Results from these experiments were combined with data from the literature to formulate a model for rutile saturation in silicate melts. Application of this model to partial melting of MORB-type eclogite indicates that beneath volcanic arcs low-degree, hydrous partial melts of rutile-bearing subducted oceanic crust contain only not, vert, similar 600 ppm TiO2. Therefore, rutile will remain as a residual phase in the eclogite and the amount of TiO2 that will be transferred to the mantle wedge will be small because so little TiO2 is dissolved in the melt. Partial melting of recycled oceanic crust in an upwelling mantle plume can exhaust rutile from the residual solid at moderate degrees of partial melting (~ 22%). The retention of rutile in subducted oceanic lithosphere during dehydration and/or partial melting, combined with exhaustion of rutile during partial melting of eclogite in mantle plumes suggests that HFSE enrichments in recycled crust that were established during subduction may be detectible in OIB.

Olivine is abundant in Earth’s upper mantle and ubiquitous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are, therefore, not in equilibrium with olivine, and will react with peridotite as they migrate through the upper mantle. If such melts erupt at Earth’s surface, their compositions will be highly modified and they may be olivine-saturated. We investigated experimentally the reaction between olivine and siliceous eclogite partial melt, and determined element partitioning between olivine and the melt produced by this reaction. Our results demonstrate that mixing of reacted eclogite partial melt with primitive basalt is capable of producing the positive correlation between melt SiO2 content and olivine Ni content observed in some Hawaiian lavas. Experiments were carried out by equilibrating eclogite partial melt or basalt with San Carlos olivine at 1 bar and 1,201–1,350°C. Our results show that eclogite partial melts equilibrated with mantle olivine retain their high SiO2, low FeO and MgO characteristics. Further, olivine-melt partition coefficients for Ni measured in these experiments are significantly larger than for basalt. Mixing of these melts with primitive Hawaiian tholeiitic lavas results in crystallization of high-Ni olivines similar to those in Makapuu-stage Koolau lavas, even though the mixed magmas have only moderate Ni contents. This results from a hyperbolic increase of the Ni partition coefficient with increasing polymerization of the mixed melt. Note that while eclogite partial melt in contact with peridotite will equilibrate with pyroxene as well as olivine, this will have the effect of buffering the activity of SiO2 in the reacted melt at a higher level. Therefore, an eclogite partial melt equilibrated with harzburgite will have higher SiO2 than one equilibrated with dunite, enhancing the effects observed in our experiments. Our results demonstrate that an olivine-free “hybrid” pyroxenite source is not required to explain the presence of high-Ni olivines in Hawaiian lavas and, therefore, indicate that the proportion of eclogite in the Hawaiian plume is less than has been estimated in recent studies.

Uranium series disequilibria in ocean island basalts (OIB) provide evidence for the presence of garnet in their source region. It has been suggested that enriched OIB signatures derive from mantle lithologies other than peridotite, such as eclogite or pyroxenite, and, in particular, that silica-poor garnet pyroxenite is the source lithology for alkali basalts. To test the ability of such a source to produce the U–Th disequilibria observed in alkali OIB, we determined experimentally clinopyroxene-melt and garnet-melt partition coefficients for a suite of trace elements, including U and Th, at 2.5 GPa and 1420–1450 °C. The starting composition for the experiments was a 21% partial melt of a silica-poor garnet pyroxenite. Experimentally determined clinopyroxene-melt partition coefficients range from 0.0083 ± 0.0006 to 0.020 ± 0.002 for Th and from 0.0094 ± 0.0006 to 0.024 ±0.002 for U, and garnet-melt partition coefficients are 0.0032 ± 0.0004 for Th and 0.013 ± 0.002 for U. Comparison of our experimental results with partition coefficients from previous experimental studies shows that the relative compatibilities of U and Th in both garnet and clinopyroxene are different for different mineral compositions, leading to varying degrees of U/Th fractionation with changing lithology. For a given melting rate and extent of partial melting, mafic lithologies tend to produce larger 230Th excesses than peridotite. However, this effect is minimized by the greater overall extents of melting experienced by eclogites and pyroxenites relative to peridotite. Results from chromatographic, batch, and fractional melting calculations with binary mixing between partial melts of pyroxenite and peridotite, carried out using our new partitioning data for the pyroxenite component and taking into account variable productivities and different solidus depths for the two lithologies, suggest that OIB are not the product of progressive melting of a source containing a fixed quantity of garnet pyroxenite. Melting a peridotite with enriched signatures, and mixing those melts with melts of a depleted, “normal” peridotite, is an alternative explanation for the trends seen in Hawaiian, Azores and Samoan lavas.

This study presents the results from precipitation experiments carried out to investigate the partitioning of the alkaline earth cations Mg2+, Ca2+, Sr2+, and Ba2+ between abiogenic aragonite and seawater as a function of temperature. Experiments were carried out at 5 to 75 °C, using the protocol of Kinsman and Holland [Kinsman, D.J.J., Holland, H.D., 1969. The coprecipitation of cations with CaCO3 IV. The coprecipitation of Sr2+ with aragonite between 16 and 96 °C. Geochim. Cosmochim. Acta 33, 1–17.] The concentrations of Mg Sr and Ba were determined in the fluid from each experiment by inductively coupled plasma-mass spectrometry, and in individual aragonite grains by secondary ion mass spectrometry. The experimentally produced aragonite grains are enriched in trace components (“impurities”) relative to the concentrations expected from crystal-fluid equilibrium, indicating that kinetic processes are controlling element distribution. Our data are not consistent with fractionations produced kinetically in a boundary layer adjacent to the growing crystal because Sr2+, a compatible element, is enriched rather than depleted in the aragonite. Element compatibilities, and the systematic change in partitioning with temperature, can be explained by the process of surface entrapment proposed by Watson and Liang [Watson, E.B., Liang, Y., 1995. A simple model for sector zoning in slowly grown crystals: implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks. Am. Mineral. 80, 1179–1187] and Watson [Watson, E.B., 1996. Surface enrichment and trace-element uptake during crystal growth. Geochim. Cosmochim. Acta 60, 5013–5020; Watson, E.B., 2004. A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals. Geochim. Cosmochim. Acta 68, 1473–1488]. This process is thought to operate in regimes where the competition between crystal growth rate and diffusivity in the near-surface region limits the extent to which the solid can achieve partitioning equilibrium with the fluid. A comparison of the skeletal composition of Diploria labyrinthiformis (brain coral) collected on Bermuda with results from precipitation calculations carried out using our experimentally determined partition coefficients indicate that the fluid from which coral skeleton precipitates has a Sr/Ca ratio comparable to that of seawater, but is depleted in Mg and Ba, and that there are seasonal fluctuations in the mass fraction of aragonite precipitated from the calcifying fluid (“precipitation efficiency”). The combined effects of surface entrapment during aragonite growth and seasonal fluctuations in “precipitation efficiency” likely forms the basis for the temperature information recorded in the aragonite skeletons of Scleractinian corals.

We present the results of an experimental study of the partitioning of water between common upper mantle minerals (olivine, orthopyroxene, clinopyroxene, garnet) and silicate melt, consisting of 352 measurements in 23 hydrous melting experiments conducted over a broad range of melt water contents (1.7–25 wt.% H2O) at upper mantle conditions (1–4 GPa; 1000–1380 °C). Our data show that, at water-undersaturated conditions, incorporation of H2O into olivine and pyroxenes is accomplished through two independent mechanisms: a coupled substitution in which H+ and Al3+ replace Si4+ in the mineral structure, and the substitution of 2H+ for Mg2+ previously identified in minerals hydrated at water-saturated conditions. At upper mantle temperatures and pressures Our data permit a new evaluation of the maximum storage capacity of water in nominally anhydrous upper mantle peridotite and eclogite. The water storage capacity of peridotite increases gradually with pressure to a maximum of 0.6 wt.% H2O at 410 km depth; the storage capacity of eclogite is 0.4 to 0.5 wt.% H2O from 2–5 GPa, dropping gradually to ∼0.2 wt.% just above the transition zone as majorite is formed at the expense of pyroxene. We show that the water abundances inferred for mid-ocean ridge and hotspot magma sources are not consistent with the composition of water-rich mantle residues emerging from a hydrous melt layer at the top of the transition zone. Regional variations observed in the H2O–LREE systematics of oceanic basalts can result from derivation of these magmas from depleted mantle sources having polybaric melting histories, with high-H2O/Ce sources being residues of shallow (garnet-absent) melting, and low-H2O/Ce sources being the residues of deep melting in the stability field of garnet lherzolite.

There is growing evidence that the budget of Pb in mantle peridotites is largely contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt) are very fast. Given the possibility that sulfide melt “wets” sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. There is only sparse experimental data relating to Pb partitioning between sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of Pb behavior in sulfide may hold the key to several long-standing and important Pb paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all known mantle reservoirs lie to the right of the Geochron, with no consensus as to the identity of the “balancing” reservoir. We propose that long-term segregation of sulfide (containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is greater than bulk earth, and constant at a value of 25. The constancy of this “canonical ratio” implies similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. in Earth Planet Sci Lett 79:33–45, 1986), whereas most experimental studies show that Pb is more incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the sulfide melt/silicate melt partition coefficient for Pb has a value of ∼ 14. Modeling shows that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle sources for these basalts. Sulfide may play other important roles during magmagenesis: (1) advective/diffusive sulfide networks may form potent metasomatic agents (in both introducing and obliterating Pb isotopic heterogeneities in the mantle); (2) silicate melt networks may easily exchange Pb with ambient mantle sulfides (by diffusion or assimilation), thus “sampling” Pb in isotopically heterogeneous mantle domains differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an apparent “de-coupling” of these systems.

This experimental study examines the mineral/melt partitioning of Na, Ti, La, Sm, Ho, and Lu among high-Ca clinopyroxene, plagioclase, and silicate melts analogous to varying degrees of peridotite partial melting. Experiments performed at a pressure of 1.5 GPa and temperatures of 1,285 to 1,345 °C produced silicate melts saturated with high-Ca clinopyroxene, plagioclase and/or spinel, and, in one case, orthopyroxene and garnet. Partition coefficients measured in experiments in which clinopyroxene coexists with basaltic melt containing ~18 to 19 wt% Al2O3 and up to ~3 wt% Na2O are consistent with those determined experimentally in a majority of the previous studies, with values of ~0.05 for the light rare earths and of ~0.70 for the heavy rare earths. The magnitudes of clinopyroxene/melt partition coefficients for the rare earth elements correlate with pyroxene composition in these experiments, and relative compatibilities are consistent with the effects of lattice strain energy. Clinopyroxene/melt partition coefficients measured in experiments in which the melt contains ~20 wt% Al2O3 and ~4 to 8 wt% Na2O are unusually large (e.g., values for Lu of up to 1.33±0.05) and are not consistent with the dependence on pyroxene composition found in previous studies. The magnitudes of the partition coefficients measured in these experiments correlate with the degree of polymerization of the melt, rather than with crystal composition, indicating a significant melt structural influence on trace element partitioning. The ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T) in the melt provides a measure of this effect; melt structure has a significant influence on trace element compatibility only for values of NBO/T less than ~0.49. This result suggests that when ascending peridotite intersects the solidus at relatively low pressures (~1.5 GPa or less), the compatibility of trace elements in the residual solid varies significantly during the initial stages of partial melting in response to the changing liquid composition. It is unlikely that this effect is important at higher pressures due to the increased compatibility of SiO2, Na2O, and Al2O3 in the residual peridotite, and correspondingly larger values of NBO/T for small degree partial melts.

The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H2O contents. The experimental results are used to explore the influence of H2O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H2O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO2. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H2O-rich (>4.5--6 wt% H2O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H2O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H2O saturated conditions producing the distinctive high SiO2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H2O contents (>10 wt% H2O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.

This experimental study examines the mineral/melt partitioning of incompatible trace elements among high-Ca clinopyroxene, garnet, and hydrous silicate melt at upper mantle pressure and temperature conditions. Experiments were performed at pressures of 1.2 and 1.6 GPa and temperatures of 1,185 to 1,370 °C. Experimentally produced silicate melts contain up to 6.3 wt% dissolved H2O, and are saturated with an upper mantle peridotite mineral assemblage of olivine+orthopyroxene+clinopyroxene+spinel or garnet. Clinopyroxene/melt and garnet/melt partition coefficients were measured for Li, B, K, Sr, Y, Zr, Nb, and select rare earth elements by secondary ion mass spectrometry. A comparison of our experimental results for trivalent cations (REEs and Y) with the results from calculations carried out using the Wood-Blundy partitioning model indicates that H2O dissolved in the silicate melt has a discernible effect on trace element partitioning. Experiments carried out at 1.2 GPa, 1,315 °C and 1.6 GPa, 1,370 °C produced clinopyroxene containing 15.0 and 13.9 wt% CaO, respectively, coexisting with silicate melts containing ~1–2 wt% H2O. Partition coefficients measured in these experiments are consistent with the Wood-Blundy model. However, partition coefficients determined in an experiment carried out at 1.2 GPa and 1,185 °C, which produced clinopyroxene containing 19.3 wt% CaO coexisting with a high-H2O (6.26±0.10 wt%) silicate melt, are significantly smaller than predicted by the Wood-Blundy model. Accounting for the depolymerized structure of the H2O-rich melt eliminates the mismatch between experimental result and model prediction. Therefore, the increased Ca2+ content of clinopyroxene at low-temperature, hydrous conditions does not enhance compatibility to the extent indicated by results from anhydrous experiments, and models used to predict mineral/melt partition coefficients during hydrous peridotite partial melting in the sub-arc mantle must take into account the effects of H2O on the structure of silicate melts.

This paper presents a detailed description of an approach for modeling the major element evolution of olivine-hosted melt inclusions. Numerical simulations constrained by experimental data on olivine/melt equilibrium and interdiffusion rates of Fe and Mg in olivine quantify the post-entrapment processes (crystallization, dissolution, Fe–Mg exchange with the host) that influence the major element compositions of included melts. Equilibrium at the olivine/melt interface is described by expressions for FeO and MgO partitioning calibrated using olivine–liquid pairs from the literature spanning temperatures of 1064 to 1950 °C and pressures of 1 bar to 70 kbar. The liquidus temperature and equilibrium olivine composition are calculated simultaneously for melt of a given composition using the partitioning expressions and olivine stoichiometry. Diffusion in the host olivine is modeled by the explicit finite-difference method. The main inputs for a simulation are the initial composition and size of the melt inclusion, the size of the host olivine, and the desired cooling path expressed as a constant cooling rate. The rate of olivine crystallization (d F /d T ) within an inclusion is dynamically adjusted with each time step so that the observed Fe–Mg exchange coefficient ( K D Fe–Mg ) at the inclusion/host interface matches the equilibrium value predicted by the partitioning equations. Application of the approach to model the major element evolution of an inclusion formed within the melting regime beneath an oceanic spreading center indicates that the composition of the included melt is significantly modified during transport to the surface along the mantle geotherm. Further, the compositional path followed by the included melt is dependent upon the capacity of the host olivine to maintain the inclusion at the pressure of entrapment.

Inner Solar System bodies are depleted in volatile elements relative to chondrite meteorites, yet the source(s) and mechanism(s) of volatile-element depletion and/or enrichment are poorly constrained. The timing, mechanisms and quantities of volatile elements present in the early inner Solar System have vast implications for diverse processes, from planetary differentiation to the emergence of life. We report major, trace and volatile-element contents of a glass bead derived from the D'Orbigny angrite, the hydrogen isotopic composition of this glass bead and that of coexisting olivine and silicophosphates, and the 207 Pb– 206 Pb age of the silicophosphates, 4568 ± 20 Ma. We use volatile saturation models to demonstrate that the angrite parent body must have been a major body in the early inner Solar System. We further show via mixing calculations that all inner Solar System bodies accreted volatile elements with carbonaceous chondrite H and N isotope signatures extremely early in Solar System history. Only a small portion (if any) of comets and gaseous nebular H species contributed to the volatile content of the inner Solar System bodies. This article is part of the themed issue ‘The origin, history and role of water in the evolution of the inner Solar System’.

It is commonly assumed that an olivine-hosted melt inclusion is chemically isolated from the melt surrounding its host crystal. This assumption is probably valid for slow-diffusing, incompatible trace elements, but for readily exchanged major elements such as Fe and Mg, it is much more tenuous. We used numerical simulations constrained by experimental data on olivine–liquid equilibrium and interdiffusion rates of Fe and Mg in olivine to assess the extent to which post-entrapment processes alter the major element compositions of olivine-hosted melt inclusions. The results indicate that extensive diffusive communication between a melt inclusion and the melt surrounding its host olivine occurs at cooling rates as rapid as 1–2°C/yr. An included melt undergoes significant and irreversible compositional changes in order to maintain Fe–Mg exchange equilibrium with the fractionating external melt. When a magnesian olivine containing an exotic liquid, such as an ultra-depleted melt, becomes entrained in a basalt and equilibrates with the new external melt by diffusive exchange, the ensuing compositional changes to the included melt cannot be removed simply by adding olivine back into the final (measured) melt composition. Observable disequilibrium between an inclusion and its host olivine is likely to have been produced during cooling of the erupted lava. Simulations, supplemented by experiments carried out on inclusion-bearing olivines, demonstrate that laboratory heating can reverse the effects of syn-eruptive crystallization on melt inclusion composition, and the presence or absence of compositional zoning in the immediately adjacent olivine can provide an indication of how closely the experiment reproduced the pre-eruptive temperature.

This study investigates the effects of variations in the relative fugacities of oxygen and sulfur on the wetting of mantle olivine by molten sulfide. Experiments were performed on mixtures of San Carlos olivine and synthetic FeS at 1 bar and 1350°C. Crucibles were fabricated from San Carlos olivine, and the fugacities of oxygen and sulfur were controlled by mixing CO2, CO, and SO2 gases. Experimental conditions ranged from logfO2=−7.9 to −10.3 and from logfS2=−1.5 to −2.5. Our experimental results demonstrate that, at a given temperature and pressure, the olivine–sulfide melt dihedral angle is controlled by the concentration of O dissolved in an anion-rich melt. Trace amounts of O dissolve in sulfide melt at fO2 conditions near the iron–wustite oxygen buffer and the dihedral angle is 90°. At fO2 conditions near the fayalite–magnetite–quartz oxygen buffer the concentration of dissolved O is near 9 wt% and the dihedral angle is 52°, allowing small amounts of sulfide melt to form an interconnected network in olivine-rich rocks and to migrate via porous flow. These results indicate that sulfide melt is likely to be mobile at current upper mantle fO2 and fS2 conditions. In mantle peridotite, the addition or removal of sulfide melt by porous flow will variably fractionate Re/Os, U/Pb, and Th/Pb ratios because Os and Pb are more chalcophile than Re, U, and Th. The Re/Os ratio of the peridotite is especially sensitive to this process. The mobility of sulfide melt at oxidizing conditions implies that the addition of oxidized chondritic material during the later stages of the accretion of the Earth may have facilitated the segregation of core-forming material by porous flow if temperatures were in excess of the sulfide solidus.

This experimental study examines the effects of variable concentrations of dissolved H2O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet. Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the concentrations of SiO2, FeO, MgO, and CaO. The concentration of Al2O3 is unaffected. The lower SiO2 contents of the hydrous melts result from an increase in the activity coefficient for SiO2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated SiO2/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the peridotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle wedge from the subducted slab.

The incorporation of H into olivine is influenced by a significant number of thermodynamic variables (pressure, temperature, oxygen fugacity, etc.). Given the strong influence that H has on the solidus temperature and rheological behavior of mantle peridotite, it is necessary to determine its solubility in olivine over the range of conditions found in the upper mantle. This study presents results from hydration experiments carried out to determine the effects of pressure, temperature, and the fugacities of H2O and O2 on H solubility in San Carlos olivine at upper mantle conditions. Experiments were carried out at 1–2 GPa and 1,200 °C using a piston-cylinder device. The fugacity of O2 was controlled at the Fe0–FeO, FeO–Fe3O4, or Ni0–NiO buffer. Variable duration experiments indicate that equilibration is achieved within 6 h. Hydrogen contents of the experimental products were measured by secondary ion mass spectrometry, and relative changes to the point defect populations were investigated using Fourier transform infrared spectroscopy. Results from our experiments demonstrate that H solubility in San Carlos olivine is sensitive to pressure, the activity of SiO2, and the fugacities of H2O and O2. Of these variables, the fugacity of H2O has the strongest influence. The solubility of H in olivine increases with increasing SiO2 activity, indicating incorporation into vacancies on octahedral lattice sites. The forsterite content of the olivine has no discernible effect on H solubility between 88.17 and 91.41, and there is no correlation between the concentrations of Ti and H. Further, in all but one of our experimentally hydrated olivines, the concentration of Ti is too low for H to be incorporated dominantly as a Ti-clinohumite-like defect. Our experimentally hydrated olivines are characterized by strong infrared absorption peaks at wavenumbers of 3,330, 3,356, 3,525, and 3,572 cm−1. The heights of peaks at 3,330 and 3,356 cm−1 correlate positively with O2 fugacity, while those at 3,525 and 3,572 cm−1 correlate with H2O fugacity.

This study investigates the effects of Variations in the fugacities of oxygen and sulfur on the partitioning of first series transition metals (V, Cr, Mn, Fe, Co, Ni. and Cu) and W among coexisting sulfide melt, silicate melt, and olivine. Experiments were performed at 1 atm pressure, 1350 C, with the fugacities of oxygen and sulfur controlled by mixing CO2, CO, and SO2 gases. Starting compositions consisted of a CaO-MgO-Al2O3-SiO2-FeO-Na2O analog for a barred olivine chondrule from an ordinary chondrite and a synthetic komatiite. The f(sub O2)/f(sub S2), conditions ranged from log of f(sub O2) = -7.9 to - 10.6, with log of f(sub S2) values ranging from - 1.0 to -2.5. Our experimental results demonstrate that the f(sub O2)/f(sub S2) dependencies of sulfide melt/silicate melt partition coefficients for the first series transition metals arc proportional to their valence states. The f(sub O2)/f(sub S2) dependencies for the partitioning of Fe, Co, Ni, and Cu are weaker than predicted on the basis of their valence states. Variations in conditions have no significant effect on olivine/melt partitioning other than those resulting from f(sub O2)-induced changes in the valence state of a given element. The strong f(sub O2)/f(sub S2) dependence for the olivine/silicate melt partitioning of V is attributable to a change of valence state, from 4+ to 3+, with decreasing f(sub O2). Our experimentally determined partition coefficients are used to develop models for the segregation of sulfide and metal from the silicate portion of the early Earth and the Shergottite parent body (Mars). We find that the influence of S is not sufficient to explain the overabundance of siderophile and chalcophile elements that remained in the mantle of the Earth following core formation. Important constraints on core formation in Mars are provided by our experimental determination of the partitioning of Cu between silicate and sulfide melts. When combined with existing estimates for siderophile element abundances in the Martian mantle and a mass balance constraint from Fe, the experiments allow a determination of the mass of the Martian core (approx. 17 to 22 wt% of the planet) and its S content (approx.0.4 wt%). These modeling results indicate that Mars is depleted in S, and that its core is solid.

The solubilities of volatiles (H 2 O, 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 2 O 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 2 O and f O 2 in olivine-hosted melt inclusions. Our comprehensive set of data for the behavior of common magmatic volatiles (H 2 O, 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 2 O 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 2 O, 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.