Host Olivine Research Articles

Experimental investigation of hydrogen isotope fractionation during hydration of olivine-hosted melt inclusions: Implications for D/H in Baffin Island picrites

Olivine-hosted melt inclusions are used to investigate the hydrogen isotope compositions (D/H) of Earth's mantle reservoirs. Studies have shown, however, that hydrogen in melt inclusion can equilibrate rapidly in response to changes to the external environment via the diffusion of protons (H+) and deuterons (D+) through the host olivine. Given that protons diffuse faster than deuterons, a kinetic fractionation of hydrogen isotopes is expected to accompany both the hydration and dehydration of melt inclusions. Other volatile species in the melt inclusion may also be affected by changes to the internal pressure and/or oxygen fugacity (fO2). Here we report results from experiments designed to investigate the behaviors of volatiles and hydrogen isotopes during the hydration of olivine-hosted melt inclusions. We show that the concentration of H2O in initially H2O-poor inclusions increases rapidly (up to ∼4 wt.% within 24 h) when the host olivine is in contact with aqueous fluid at 1200 °C and 300 MPa. The extent of hydration is controlled by time, temperature, melt inclusion volume, and H+ diffusion distance. Hydrogen isotopes initially become lighter (i.e., D/H decreases) as hydration proceeds, defining a negative correlation with H2O concentration. This trend reverses with increasing hydration as the inclusions must eventually equilibrate with the external fluid. These experimental results agree well with diffusion calculations carried out using a spherical geometry and a lattice diffusivity of 10–11.2±0.2 m2/s for H+ at 1200 °C. Therefore, anomalously light hydrogen isotopes in olivine-hosted melt inclusions from Baffin Island may not be taken as representative of the composition of the mantle source unless post-entrapment hydration can be excluded as a possibility. An increase in CO2 concentration and a significant drop in sulfur concentration accompany hydration of the melt inclusions in our experiments. The former is consistent with an observed decrease in vapor bubble size and results from a hydration-induced internal pressure increase. The latter is ascribed to the exsolution of molten sulfide from the silicate melt, which might be related to a lower fO2 in the experiments as compared to the starting materials. We find no evidence for exchange of F or Cl between the melt inclusion and external fluid.

Symplectite formation in ultramafic achondrites by impact percolation of a sulfide melt

The ungrouped dunitic achondrite Northwest Africa (NWA) 12217 contains symplectic spinel-pyroxene veins that are mineralogically identical to symplectites in other ultramafic planetary materials. The morphology and amount of chromite present in these features relative to the Cr in their olivine hosts suggest an exogenous origin. Petrological experiments show that a Cr laden sulfide liquid reacts with olivine to produce pyroxene by scavenging Mg and Fe from olivine to crystallize chromite. The liquid infiltrates cracks and grain boundaries within the olivine and produces a vein-like symplectic chromite-pyroxene mineralogy similar to that observed in NWA 12217. This process is likely responsible for forming the symplectites in the related ultramafic achondrites NWA 12319, 12562, and 13954, along with many other achondrites. The nucleosynthetic Cr isotopic composition of chromites appears to be in disequilibrium with that of silicates in NWA 12217, suggesting that the liquids responsible for the symplectite forming reaction are at least partially sourced from a different parent body and result from an impact.

Why are oceanic arc basalts Ca-rich and Ni-poor? Insights from olivine-hosted melt inclusions from Kibblewhite Volcano in the Kermadec arc

Ankaramites, which are clinopyroxene-rich basalts with primitive whole-rock compositions (Mg# >65), are common in oceanic arcs and are characterized by high whole-rock CaO/Al2O3 (>1.0) ratios and olivine crystals with anomalously low nickel contents (<0.2 wt% NiO). These geochemical characteristics cannot be explained by the melting of ordinary mantle peridotite. However, their origin is critical for understanding the formation of primary magmas in oceanic arcs. Here, we investigated olivine-hosted melt inclusions (MIs) from ankaramites and magnesian andesites of the Kibblewhite Volcano in the Kermadec arc. The MIs from the ankaramites have similar major and trace element characteristics to the host rocks, indicating that the ankaramites did not result from an accumulation of mafic minerals but rather represent the primary magma in the Kibblewhite Volcano. The MIs from the magnesian andesites were hosted in forsteritic olivine xenocrysts with a wide range of NiO contents (Fo90–92; 0.13–0.39 wt% NiO) and have similar major element compositions to the ankaramites but exhibit a wide range of CaO/Al2O3 (0.85–1.54). The trace element characteristics of the MIs from the magnesian andesites do not match those of the host rocks, indicating that they are not primary melts of the magnesian andesites but primitive basaltic melts generated before the magnesian andesites formed.Interestingly, the CaO/Al2O3 ratio of MIs from the magnesian andesites was negatively correlated with the NiO content of their host olivines. This correlation suggests that the composition of the primary basaltic magmas of the Kibblewhite Volcano changed continuously from peridotite-derived to ankaramitic. This correlation could not be explained by grain-scale process, crustal anatexis, or contribution of slab-derived carbonate-rich fluids. Instead, we propose that this correlation can be explained by the interaction of the ascending primary basaltic melts with the lithospheric mantle. During melt-mantle interaction, the assimilation of clinopyroxene and fractionation of olivine and orthopyroxene caused the CaO/Al2O3 ratio to increase in the melt and the Ni content to decrease. Furthermore, because the magnesian andesites have low CaO/Al2O3 ratios and could be derived from a clinopyroxene-poor mantle lithology, the interaction between the melt and mantle may also be closely related to the origin of the magnesian andesites at Kibblewhite Volcano. This interpretation provides a new perspective on the origin of the oceanic arc ankaramites and why primary andesitic and basaltic magmas coexist in the Kibblewhite Volcano.

Geochemistry and volatile contents of olivine-hosted melt inclusions from Mt. Etna tholeiitic and alkaline magmatism

The analysis of olivine-hosted melt inclusions (MIs) from the whole sub-alkaline and alkaline magmatic suites of Mt. Etna provides fundamental information about the composition of undifferentiated magmas and their pristine volatile content. Olivine crystals (Fo88-66) were selected for Secondary Ion Mass Spectrometry (SIMS) analysis of volatile species (H2O, CO2, F, Cl and S) contained in their host MIs, after preliminary high-pressure/high-temperature re-homogenization, which allowed to reduce the developing of cracks in the host olivine and diffusion-driven outgassing of volatiles from the melt inclusions. This permitted to explore the compositional variability of volatiles of undifferentiated melts and the degassing behavior through the feeding system. The studied MIs show significant major elements compositional heterogeneities (44.57–52.37 wt% SiO2; 3.60–7.51 wt% Na2O + K2O). Fractionation modelling was performed with Rhyolite-MELTs under variable fO2 regimes (∆FMQ + 1.5 to + 3), starting from the less evolved MIs compositions and ultimately reproducing most of the observed compositional trends. Mantle melting modelling was used to replicate the observed MIs composition, starting from a spinel-lherzolitic source, accounting for the alkalinity and Fe content of reproduced melts by varying the eutectic contribution of Amph/Phlog and Opx/Cpx respectively. Although most of the studied MIs were degassed in an open-conduit regime, the observed range of volatile concentration in MIs (2.42–6.14 wt% H2O; 308–8474 ppm CO2; 132–697 ppm F; 221–1766 ppm Cl and 16–1992 ppm S) is correlated with a slight decrease in the molar H2O/(H2O + CO2) ratio from early tholeiites to the recent 2015 alkaline products. This observation allows to estimate a minimum 12,250 ppm CO2 and a maximum of 6.14 wt% H2O in primary melts of the current activity.

Methane genesis within olivine-hosted fluid inclusions in dolomitic marble of the Hida Belt, Japan

Abiotic synthesis of hydrocarbon-bearing fluids during geological processes has a significant impact on the evolution of both the Earth's biosphere and the solid Earth. Aqueous alteration of ultramafic rocks, i.e., serpentinization, which forms serpentinite, is one of the geological processes generating abiotic methane (CH4). However, abiotic CH4 generation is not limited to the serpentinization of mafic and ultramafic lithologies. Metasedimentary dolomitic marble from the Hida Belt, Japan, is characterized by the presence of forsterite-rich olivine (Fo~89–93), and olivine crystals contain abundant fluid inclusions (<1 to 10 μm in size). Raman spectroscopic analyses of olivine-hosted fluid inclusions found that both primary and secondary fluid inclusions contain CH4, lizardite/chrysotile, and brucite. This indicates that micro-scale interactions between COH fluid and host olivine produced CH4 through the reduction of CO2 by H2 released during local serpentinization within inclusions. Our observation implies that the dolomitic marble has the potential to be a key lithology for the synthesis and storage of abiotic CH4 in a shallower crustal portion of orogenic belts.

Review of melt inclusions in lunar rocks: constraints on melt and mantle composition and magmatic processes

Abstract. Mineral-hosted melt inclusions provide a window into magmatic processes and pre-eruptive liquid compositions. Because melt inclusions are small (typically &lt; 100 µm), the study of lunar melt inclusions is enabled by advancements of microbeam instrumental techniques. In the 1970s immediately following the Apollo and Luna missions, major and minor oxide concentrations of lunar melt inclusions were measured using electron microprobes. The data were used to understand magma evolution, and they revealed the immiscibility of two silicate liquids in the late stage of lunar magma evolution. More recently, the development of secondary ion mass spectrometry as well as laser ablation–inductively coupled plasma–mass spectrometry has enabled the measurement of key volatile elements and other trace elements in lunar melt inclusions, down to about the 0.1 ppm level. The applications of these instruments have ushered in a new wave of lunar melt inclusion studies. Recent advances have gone hand in hand with improved understanding of post-entrapment loss of volatiles. These studies have provided deep insights into pre-eruptive volatiles in lunar basalts, the abundance of volatiles in the lunar mantle, the isotopic ratios of some volatile elements, and the partition of trace elements between host olivine and melt inclusions. The recent studies of lunar melt inclusions have played a critical role in establishing a new paradigm of a fairly wet Moon with about 100 ppm H2O in the bulk silicate Moon (rather than a “bone-dry” Moon) and have been instrumental in developing an improved understanding of the origin and evolution of the Moon.

ВЫСОКОГЛИНОЗЕМИСТЫЕ ДОЧЕРНИЕ ПАРАГЕНЕЗИСЫ ИЗ РАСПЛАВНЫХ ВКЛЮЧЕНИЙ В ОЛИВИНЕ ВУЛКАНОВ КУДРЯВЫЙ И МЕНЬШИЙ БРАТ (КАЛЬДЕРА МЕДВЕЖЬЯ, О. ИТУРУП)

For the first time, high-alumina mineral associations including rare minerals of the rhönite-kuratite series were found in post-caldera lava flows of the Kudryaviy and Menshiy Brat volcanoes (Kuril Islands). These lava flows are composed of olivine-bearing basalts and andesite-basalts of the calc-alkaline series. Partially crystallized olivine-hosted melt inclusions in these rocks contain high-alumina daughter phases. Residual glass with up to 25 wt% Al2O3, Al-pyroxenes and spinel with minerals of the rhönite subgroup were identified. The rhönite-kuratite series minerals are characterized by strong variations in Al, Fe2+, Fe3+, Mg, Si, as well as very low Ti, which makes it possible to name it a low-Ti Fe2+ analogue of rhönite with the formula (Ca,Mg,Na)4(Mg,Fe,Fe, Al,Ti,Mn)12O4[Si8.2Al3.8O36] – (Ca,Na,Mg)4(Fe,Mg,Al,Fe,Ti)12O4[Si6.78Al5.22O36]. It is assumed that high-alumina daughter phases form in olivine-hosted melt inclusions through water accumulation in the trapped melts during crystallization of the host olivine on the walls of melt inclusions. Water-rich melts can inhibit early crystallization of plagioclase, which promotes enrichment of the residual melt in Al2O3.

Deciphering recycling processes during solar system evolution from magnesium-rich relict olivine grains in type II chondrules

Ferromagnesian chondrules present a remarkable dichotomy between reduced (type I) and oxidized (type II) varieties. How these formed, and how they may be related remains contentious. Many type II chondrules, especially in carbonaceous chondrites, contain forsteritic grains in disequilibrium with FeO-rich host olivine grains, which must be relicts of precursor material. In this study, we analyzed the oxygen isotopic composition of magnesian relict and host olivine grains in type II chondrules in CO and CR chondrites. The analyzed Mg-rich relicts are generally more 16O-rich than ferroan olivine (mostly host) grains and plot in the range (in term of chemistry and isotopic composition) of type I chondrules in carbonaceous chondrites. Remarkably, they tend to cluster around the dominant Δ17O peaks of the type I chondrules in their host chondrites, viz. –6 ‰ and –2 ‰ for CO and CR, respectively. With the occurrence of relatively intact type I chondrules within some type II chondrules, this corroborates that local type I chondrules were among the precursors of type II chondrules, and that chondrule formation occurred within the accretion reservoir of the eventual chondrites. This supports the nebular brand of chondrule-forming scenarios. Since not all previous generations of chondrules (or other precursor objects) have been recycled, chondrule formation events must also have been extremely localized.

Constraining the volatile evolution of mafic melts at Mt. Somma–Vesuvius, Italy, based on the composition of reheated melt inclusions and their olivine hosts

Abstract. Mount Somma–Vesuvius is a stratovolcano that represents a geological hazard to the population of the city of Naples and surrounding towns in southern Italy. Historically, volcanic eruptions at Mt. Somma–Vesuvius (SV) include high-magnitude Plinian eruptions, such as the infamous 79 CE eruption that occurred after 295 years of quiescence and killed thousands of people in Pompeii and surrounding towns and villages. The last eruption at SV was in 1944 and showed a Volcanic Explosivity Index (VEI) of 3 (0.01 km3 of volcanic material erupted). Following the 1944 eruption, SV has been dormant for the past nearly 79 years, with only minor fumarolic and seismic activity. During its long history, centuries of dormancy at SV have ended with Plinian eruptions (VEI 6) that signal the beginning of a new cycle of eruptive activity. Thus, the current dormancy stage demands a need to better understand the mechanism involved in high-magnitude eruptions in order to better predict future eruption magnitude and style. Despite centuries of research on the SV volcanic system, many questions remain, including the evolution of magmatic volatiles from deep primitive magmas to shallower more evolved magmas. Developing a better understanding of the physical and chemical processes associated with volatile evolution at SV can provide insights into magma dynamics and the mechanisms that trigger highly explosive eruptions at SV. In this study, we present new data for the pre-eruptive volatile contents of magmas associated with four Plinian and two inter-Plinian eruptions at SV based on analyses of reheated melt inclusions (MIs) hosted in olivine. We correct the volatile contents of bubble-bearing MIs by taking into account the volatile contents of bubbles in the MIs. We recognize two groups of MIs: one group hosted in high-Fo olivine (Fo85–90) and relatively rich in volatiles and the other group hosted in low-Fo olivine (Fo70–69) and relatively depleted in volatiles. The correlation between volatile contents and compositions of host olivines suggests that magma fractionation took place under volatile-saturated conditions and that more differentiated magmas reside at shallower levels relative to less evolved/quasi-primitive magmas. Using the CO2 contents of corrected MIs hosted in Fo90 olivine from SV, we estimate that 347 to 686 t d−1 of magmatic CO2 exsolved from SV magmas during the last 3 centuries (38–75 Mt in total) of volcanic activity. Although this study is limited to only few SV magmas, we suggest that further study applying similar methods could shed light on the apparent lack of correlation between the volatile contents of MIs and the style and age of eruptions. Further, such studies could provide additional constraints on the origin of CO2 and the interaction between the carbonate platform and ascending magmas below SV.

Platinum-group minerals and the genesis of the sulfide PGE-Cu-Ni deposit “Ore Horizon 330” of the Monchegorsk Pluton, Kola Region, Russia

The sulfide PGE-Cu-Ni deposit “Ore horizon 330” (OH330) of the reef type is located within the orthopyroxenite of the Sopcha intrusion, which is part of the Paleoproterozoic layered Monchegorsk pluton (Monchepluton) in the Kola Region. OH330 is a sill-like body with a thickness of 4–6 m, traced along the entire perimeter of the Sopcha intrusion at a distance of 3.3 km and a width of 1.2 km. The OH330 deposit was studied in two well-exposed sections of its western flank. Here, OH330 is a thinly layered horison that lies among the host olivine orthopyroxenite. It consists of interlayers of dunite, harzburgite, and orthopyroxenite containing fine sulfide dissemination with increased contents of Ni (1290–5220 ppm), Cu (up to 2320 ppm), and platinum group elements (PGE) (0.43–1.40 ppm of the PGEtot). It is assumed that the OH330 parental magma was significantly contaminated with crustal material, saturated with sulfur and chalcophile elements. The separation of immiscible sulfide droplets during cooling led to the development of ore sulfide concentrations. The association of platinum group minerals (PGM) in harzburgite is represented by the predominant PtFe alloys and subordinate PGE bismuth-tellurides, arsenides, and tellurides. Compared to harzburgite, PGE bismuth-tellurides and tellurides are widely distributed in orthopyroxenite, whereas arsenides, PtFe alloys, and sulfides are minor. The PGE-Cu-Ni ore formed in a relatively wide temperature range from 1000 to 900 °C (base metal sulfides) to 900–600 °C (PGE sulfides and arsenides, and PtFe alloys) and ended at a temperature of 600–400 °C with a crystallization bismuth-tellurides and tellurides of Pt and Pd.

Achieving Ultra-Wideband NIR-II Emission in Cr3+-Doped Li(Sc,In)(Si,Ge)O4 Phosphors Based on Host Composition Engineering.

Realizing ultra-wideband and tunable near-infrared (NIR) emission remains a great challenge in NIR phosphor development. The luminescence of most reported NIR phosphors exhibits a peak wavelength shorter than 1000 nm and the corresponding FWHM is <200 nm. Here, a series of Cr3+-activated Li(Sc,In)(Si,Ge)O4 phosphors with ultra-wideband and tunable NIR-II emission are successfully developed based on the host composition engineering strategy. Significant spectral engineering in the NIR-II region is achieved with a peak wavelength changing from 1110 to 1253 nm. The olivine host structure could provide Cr3+ activator a highly distorted octahedral site with very weak crystal field strength, which results in NIR-II ultra-wideband emission with FWHM > 300 nm. A detailed discussion on the relationship between structural variation, crystal field splitting, and NIR luminescence has been applied. As far as we know, it is the first report about Cr3+ NIR luminescence engineering in such a long wavelength and wide range. The application of these NIR-II phosphors is demonstrated in intensity-based luminescent thermometry with a relative sensitivity of >2.0% K-1 in the physiological temperature range.

First measurements of the Fe oxidation state of spinel inclusions in olivine single crystals from Vulture (Italy) with the in situ synchrotron micro-Mössbauer technique

Abstract. The redox state of the Earth's upper mantle (i.e., oxygen fugacity, fO2) is a key variable that influences numerous processes occurring at depth like the mobility of volatile species, partial melting, and metasomatism. It is linked to the oxidation state of peridotite rocks, which is normally determined through the available oxythermobarometers after measuring the chemical composition of equilibrated rock-forming minerals and the Fe3+ in redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe3+ / ∑Fe in peridotites have been limited to those peridotites (e.g., harzburgites and lherzolites) for which an oxythermobarometer exists and where spinel (or garnet) crystals can be easily separated and measured by conventional 57Fe Mössbauer spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a lherzolite with carbonatitic melts and, therefore, have recorded the passage of (metasomatic) fluids at mantle conditions. However, no oxythermobarometer exists to determine their equilibrium fO2. The aim of this study was to retrieve the fO2 of the mantle beneath Mt. Vulture volcano (Italy) through the study of a wehrlitic lapillus emitted during the last eruption (∼ 140 kyr ago) that contain olivines with multiple tiny spinel inclusions with sizes &lt; 40 µm. To our knowledge, the Fe oxidation state of these inclusions has been never determined with the Mössbauer technique due to their small sizes. Here, we present measurements of the Fe3+ / ∑Fe using in situ synchrotron Mössbauer spectroscopy coupled with chemical and spectroscopic analysis of both host olivine and spinel inclusions. The results show Fe3+ / ∑Fe ratios of 0.03–0.05 for olivine and 0.40–0.45 for the included spinels, the latter of which appear higher than those reported in literature for mantle spinel harzburgites and lherzolites. Given the evidence of the mantle origin of the trapped spinels, we propose that the high fO2 (between 0.81 and 1.00 log above the fayalite–magnetite–quartz buffer; FMQ) likely results from the interaction between the pristine spinel lherzolite and a CO2-rich metasomatic agent prior to the spinel entrapment in olivines at mantle depths.

Mineral Assemblage of Olivine-Hosted Melt Inclusions in a Mantle Xenolith from the V. Grib Kimberlite Pipe: Direct Evidence for the Presence of an Alkali-Rich Carbonate Melt in the Mantle Beneath the Baltic Super-Craton

This report deals with the first mineralogical examination of secondary crystallized melt inclusions (CMIs) in healed cracks within olivine in a mantle peridotite xenolith from the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province). In contrast to micro/nano-inclusions in diamonds, the studied CMIs are quite large (up to 50 µm), so that the mineral composition of the CMIs can be determined via conventional analytical approaches, e.g., Raman spectroscopy and scanning electron microscopy. Garnet peridotite is a coarse-grained mantle rock that equilibrates at 3.3 GPa and 750 °C (corresponding to a depth of ~100 km). The CMIs are therefore tiny snapshots of melt that existed in the shallow lithospheric mantle and were entrapped in olivine. In total, nineteen mineral species were identified among the daughter magmatic minerals of the CMIs. Various Na-K-Ca-, Na-Ca-, Na-Mg-, Ca-Mg-, Mg- and Ca-carbonates; Na-Mg-carbonates with the additional anions Cl−, SO42− and PO43−; alkali sulfates; chlorides; phosphates; sulfides; oxides; and silicates were established. Within the mineral assemblage, carbonates were predominant, with their abundance being more than 62 vol.%. The CMIs contained twelve alkali-rich minerals; nine of them were Na-bearing and showed bulk molar (Na + K)/Ca ≥ 1. The CMIs’ parental melt was an alkali-rich carbonate liquid that contained low amounts of SiO2 (≤9.6 wt%) and H2O (≤2.6 wt%). According to our estimates, the time of complete equilibration between olivine within the healed cracks and host olivine in the mantle at the calculated P-T parameters for the studied xenolith should be no more than several years. Based on this geologically short time span, a genetic link between the studied CMIs and the magmatism that formed the V. Grib kimberlite pipe is suggested.

Epitactic magnetite growth in fluid inclusions as driving force for olivine oxidation coupled with hydrogen production at high pressure

We disclose the crystallisation evolution of magnetite-bearing multiphase inclusions hosted in metamorphic olivine of harzburgites from the Cerro de Almirez (Betic Cordillera, Spain), which have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration of former serpentinites. The chemical exchange between inclusion fluid and olivine started soon after entrapment, at peak P-T conditions of 1.6–1.9 GPa and 650–700 °C, and continued during cooling along the retrograde path, with the coexistence of olivine and magnetite with orthopyroxene, chlorite, talc, antigorite and the destabilisation of olivine and antigorite into brucite and low-temperature chrysotile serpentine, as recognised by Raman analyses. Thermodynamic modelling and mass balance calculations demonstrate that the water component of fluid trapped in the inclusions of metamorphic olivine is expected to trigger the oxidation of the fayalite component in olivine, producing a mineral assemblage made of magnetite + orthopyroxene and molecular hydrogen, where the elemental redox processes are Fe2+ of olivine that oxidizes to Fe3+ and H+ of water that reduces to H2. Probable H2 trapped in the olivine host close to the inclusion wall has been detected by Raman spectroscopy. To corroborate its presence, we performed quantitative mass spectrometry analyses of the fluid phase trapped in the multiphase inclusions and of the olivine crystals hosting the inclusions, revealing that 1 kg of olivine matrix contains 6.2 ± 0.1 mmol of H2. We identify two synergistic driving forces of the whole process, which has the peculiarity to produce molecular hydrogen at apparently oxidising conditions: i) the building up of an epitaxial interface between olivine and magnetite, and ii) the olivine ability to trap H2 at high pressures. The olivine + H2O system of these natural microreactors simulates a process of oxidation of the mantle olivine by water, with production of H2 at pressure and fO2 conditions (FMQ + 2) at which water reduction is considered an unlike mechanism.

Carbon Enrichment in the Lithospheric Mantle: Evidence from the Melt Inclusions in Mantle Xenoliths from the Hainan Basalts

AbstractIt is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs. However, the location of carbon storage in Earth's interior and the reasons for carbon enrichment remain unclear. In this study, we report CO2‐rich olivine‐hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area, Hainan Province, which may shed some light on the carbon enrichment process in the lithospheric mantle. We also present a detailed petrological and geochemical investigation of the late Cenozoic basalts and mantle xenoliths from northern Hainan Island. The collected samples of late Cenozoic Hainan Island basalts belong to both alkaline and subalkaline series, showing fractionated REE patterns with high (La/Yb)Nvalues of 3.52–11.77, which are typical for OIB. Based on Al‐in‐olivine thermometry, the temperatures estimated for the mantle xenoliths can be divided into two groups. One group has temperatures of less than 1050°C, and the other group has temperature ranging from 1050°C to 1282°C. Clinopyroxene (La/Yb)N–Ti/Eu and clinopyroxene Ca/Al–Mg#diagrams indicate that the mantle peridotite experienced metasomatism from both silicate and carbonate melts. Melt inclusions in the olivine of mantle xenoliths include (1) CO2bubble–rich melt inclusions; (2) multiphase melt inclusions (glass + CO2bubble + daughter minerals); (3) pure glass melt inclusions. Magnesite is a daughter mineral in the olivine‐hosted melt inclusions, which could be interpreted as a secondary mineral formed by the interactions of CO2‐rich fluids with an olivine host, due to post‐entrapment effects. The glasses in olivine‐hosted melt inclusions have high SiO2contents (60.21–77.72 wt%). Our results suggest that a considerable amount of CO2‐rich melt inclusions are captured in the lithospheric mantle during metasomatism. The lithospheric mantle can therefore act as is a ‘carbon trap’, with much CO2being absorbed by the lithospheric mantle in this way.

A melt inclusion approach to reconstructing sulfur contents and sulfide saturation of primitive basaltic melts

The study of melt inclusions in minerals is the only direct approach to estimate the contents of volatile components in silicate melts, which are essential to constrain the processes of magma generation and evolution. In particular, the content of sulfur in silicate melts is closely connected with the P-T-fO2 conditions of mantle melting and the formation of sulfide deposits. Inclusions in high-Mg olivine can provide access to the composition of the most primitive melts, but these are the most susceptible to post-entrapment modification such as ‘Fe-loss’. The loss of Fe consequently decreases sulfur solubility and leads to silicate-sulfide immiscibility and formation of daughter sulfide globules, significantly affecting the initial proportions of S contained within the silicate part (glass) of melt inclusions. In this study, we investigated the budget of sulfur in heated and quenched melt inclusions hosted in primitive olivine (87–90 mol% Fo) by accounting for sulfur stored in silicate glass (melt), daughter sulfide globules and shrinkage bubbles. Melt inclusions from MORB-like olivine-phyric rocks (Kamchatsky Mys, Far East Russia) heated for 30 min at 1200 °C and 1300 °C were compared with similar experiments on olivine-hosted melt inclusions over a 5 min duration (at 1200 °C and 1350 °C). Sulfur contents were calculated in the following types of melt inclusions, containing: (i) both sulfide globules and shrinkage bubbles, (ii) only sulfide globules, (iii) only shrinkage bubbles, and (iv) completely homogenized. Our findings indicate that S content in shrinkage bubbles and its precipitates is negligible in reheated water-poor melt inclusions, whereas daughter sulfide globules are significant repositories of initial S. Therefore, when estimating initial S contents in parental melts, the amounts stored in both daughter sulfide and silicate glass should be considered. Our experiments with MORB-type melt inclusions in forsteritic olivine phenocrysts demonstrate that effective homogenization is best achieved by briefly overheating melt inclusions prior to rapid quenching. Although this technique does not allow for the MgFe equilibrium between melt inclusions and their host olivine to be fully achieved, it promotes the dissolution of daughter sulfide globules and shrinkage bubbles, and thus the release of initial sulfur back to the melt/glass.

Silica-rich melts originating from melt-hydrated peridotite reactions

Interactions between melts and hydrated peridotite may control production of silica-rich melts, fluids and magmas in the upper mantle and may be predominant mechanism for producing felsic crust in shallow ultramafic settings of rocky planets. This research investigated composition of olivine-hosted mineral, melt and multi-phase inclusions in dunite xenoliths from the 1800–1801 Kaupulehu flow of Hualalai Volcano. The host olivine (forsterite Fo 82–89 , mol. %) appears with mineral inclusions of crystalline chromite having compositions similar to those found in alkaline series rocks. Abundant silica-rich glasses (SiO 2 = 63 ± 2.9 wt%, Al 2 O 3 = 21 ± 1.2 wt%; K 2 O = 4.3 ± 0.5 wt%, Na 2 O = 3.6 ± 0.7 wt%, 38 analyses) found in the olivine-hosted melt and multiphase inclusions are interpreted as quenched silicate liquids entrapped due to nephelinite melt percolation in hydrated harzburgite in the presence of free aqueous fluid at pressures of 0.7 to 0.1 GPa and a melt-to-peridotite mass ratio of 0.2 to 0.3. This work also re-interprets the metasomatic origin of global examples of silica-rich glass inclusions in peridotite xenolith minerals according to results from recent experiments on melt-hydrated peridotite interaction processes. As a planetary process, production of silica-rich melts and fluids within series of associated magmas and their crystallized differentiates can occur due to reaction of primitive mafic (e.g., ocean island basalt, OIB) melts with hydrated peridotite (or serpentinite) in the presence of aqueous fluids at pressures below 1 GPa. • Si-rich glasses are produced due to reaction of mafic melts with hydrated peridotite.

The capturing and decompression of melt inclusions in olivine and ilmenite megacrysts from the Monastery kimberlite, South Africa

Ilmenite and olivine megacrysts from the 89 Ma Monastery kimberlite (Kaapvaal Craton, South Africa) captured abundant and large melt inclusions containing quenched Si-Mg-rich melt, calcite, spinel, perovskite, phlogopite, and serpentine. Textural observations and 3D X-ray tomography of ilmenite and olivine megacrysts show melt inclusion shapes, sizes and distribution patterns indicative of melt capture during primary crystal growth near the base of the subcontinental lithospheric mantle (SCLM). Patterns supporting secondary melt injection along fractures or veins, such as planar arrays of melt inclusions, are absent. Melt inclusions in olivine, in some examples reaching the dimension of centimetres, likely were captured in skeletal voids forming in fast growing, up to decimetre-sized olivine megacrysts. These large melt inclusions commonly decrepitated, forming apophyses, radial fractures, and veins, along which residual volatile- and Si-Mg-rich melt was extracted. We attribute the decrepitation of melt inclusions in olivine to the rapidly increasing difference between the melt pressure in the inclusions, captured at mantle depth, and the decreasing confining stress to which the host olivines were exposed during magma ascent and after emplacement. In ilmenite, melt inclusions up to ∼ 6 mm in diameter remained commonly intact during the kimberlite ascent from its mantle source to the shallow crust. The quenched silicate melt in olivine- and ilmenite-hosted melt inclusions, in some places preserved as unaltered hydrous and CO3-bearing glass, shows systematic major element compositional variations that suggest that this melt formed by similar fractionation and depletion processes, irrespective of the hosting megacryst phase. Apparent modal variations in quenched silicate melt, calcite, and oxide contents suggest that the melt batches captured as inclusions in ilmenite and olivine either record different evolution stages in the megacryst magma, or document compositional heterogeneities in this magma at the time of megacryst growth.

Defining the challenges of Li extraction with olivine host: The roles of competitor and spectator ions

The lithium supply issue mainly lies in the inability of current mining methods to access lithium sources of dilute concentrations and complex chemistry. Electrochemical intercalation has emerged as a highly selective method for lithium extraction; however, limited source compositions have been studied, which is insufficient to predict its applicability to the wide range of unconventional water sources (UWS). This work addresses the feasibility and identifies the challenges of Li extraction by electrochemical intercalation from UWS, by answering three questions: 1) Is there enough Li in UWS? 2) How would the solution compositions affect the competition of Li+ to major ions (Na+/Mg2+/K+/Ca2+)? 3) Does the complex solution composition affect the electrode stability? Using one-dimensional olivine FePO4 as the model electrode, we show the complicated roles of major ions. Na+ acts as the competitor ion for host storage sites. The competition from Na+ grants Mg2+ and Ca2+ being only the spectator ions. However, Mg2+ and Ca2+ can significantly affect the charge transfer of Li+ and Na+, therefore affecting the Li selectivity. We point to improving the selectivity of Li+ to Na+ as the key challenge for broadening the minable UWS using the olivine host.

Melt inclusion formation during olivine recrystallization: Evidence from stable isotopes

Melt inclusions are often used to infer melting processes or to determine source magmas that are usually overprinted in bulk rocks due to late stage mixing or near surface contamination. Here we present the first investigation of oxygen (O) isotope equilibrium between melt inclusions and their host olivines from arc samples. Olivines in all but one sample record either magma mixing or fractional crystallization. All six melt inclusions from Vulcano, 83% of seven from Sukumoyama, 44% of 21 from St Vincent, 37% of four from Iwate, and 21% of 13 from Aoba are not in isotopic equilibrium with their olivine host, despite the other major elements being in apparent equilibrium. A detailed study of some of the olivines shows that only a small volume around the melt inclusions is in equilibrium with its host. This strongly suggests that in these olivines melt inclusions are trapped in partly recrystallized olivines, highlighting the importance of magma mixing and crystal recycling in the magmatic plumbing system of these volcanoes. Oxygen isotope fractionation between melt inclusions and their host olivines, as well as phosphorus-δ18O systematics, could be used to better understand the formation of melt inclusions and crystal history. It could also provide valuable information to help characterize the magmatic plumbing system that the inclusions and their olivine hosts formed in (e.g., crystal rich-mush versus crystal poor melt lenses).