Host Olivine Research Articles

Highly Oxidising Conditions in Volatile-Rich El Hierro Magmas: Implications for Ocean Island Magmatism

Abstract Recent studies investigating magmatic volatile contents indicate widespread enrichment of carbon, sulfur, and halogens in ocean island basalts (OIBs). At El Hierro in the Western Canary Islands, magmas with exceptionally high CO2 and S contents have been erupting throughout the Holocene. High S content of up to 5200 ppm requires an oxidised mantle source, but estimates of initial magmatic oxygen fugacity (fO2) are sparse. Here, we present estimates of fO2 and magmatic temperature for El Hierro together with a global mantle potential temperature dataset to evaluate redox and temperature conditions in the early stages of melt evolution for volatile-rich OIBs. Oxygen fugacities calculated using vanadium partitioning between melt inclusions (MIs) and their olivine hosts are >FMQ + 2.9 (2.9 log10 units above the fayalite-magnetite-quartz buffer), indicating that El Hierro magmas are highly oxidised. MI and matrix glass sulfur speciation data record fO2 between FMQ-1 to FMQ + 2; these values strongly depend on the position of the S2− to S6+ transition relative to the FMQ buffer. Nonetheless, glass sulfur speciation data record lower oxygen fugacity than V partitioning data, indicating MIs were able to maintain Fe3+/ΣFe and S6+/ΣS equilibrium with the surrounding melt during their evolution. The high fO2 of El Hierro magmas is coupled with an average mantle potential temperature estimate of 1443 ± 66°C (1σ, n = 17) for the broader Canary Islands, which is slightly higher than the average potential temperature estimated for adjacent mid-ocean ridge segments (1427 ± 33°C, 1σ, n = 474), albeit the two values are well within error. We find that ~98% of Canary Island rock compositions are not suitable for calculation of mantle potential temperatures using currently available methods. This is caused by the presence of substantial pyroxenite and volatile-enriched peridotite mantle domains under the Canary Islands. A wider compositional calibration of various petrological models is necessary to precisely determine mantle potential temperatures for volatile-rich alkali basalts. Our high oxygen fugacity estimates for El Hierro magmas reflect the fertile, fusible, and volatile-enriched nature of the mantle source beneath the Western Canary Islands.

Deserpentinization and high-pressure (eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua, Greenland

Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt (ISB), hosted in the Itsaq Gneiss Complex of southwestern Greenland. These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record. Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages. Ti-rich humite-group minerals such as titanian chondrodite (Ti-Chn) and titanian clinohumite (Ti-Chu) often occur as accessory phases in the metamorphosed ultramafic rocks. The Ti-rich humite minerals are associated with metamorphic olivine. The host olivine is highly forsteritic (Fo96-98) with variable MnO and NiO contents. The concentrations of the rare-earth elements (REE) and high-field strength elements (HFSE) of the metamorphic olivine are higher than typical mantle olivine. The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting. The close association of olivine, antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between ∼1.3–2.6 GPa within the antigorite stability field (<700 °C). The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.

Formation mechanisms of ringwoodite: clues from the Martian meteorite Northwest Africa 8705

Ringwoodite and wadsleyite are the high-pressure polymorphs of olivine, which are common in shocked meteorites. They are the major constituent minerals in the terrestrial mantle. NWA 8705, an olivine-phyric shergottite, was heavily shocked, producing shock-induced melt veins and pockets associated with four occurrences of ringwoodite: (1) the lamellae intergrown with the host olivine adjacent to a shock-induced melt pocket; (2) polycrystalline assemblages preserving the shapes and compositions of the pre-existing olivine within a shock-induced melt vein (60 μm in width); (3) the rod-like grains coexisting with wadsleyite and clinopyroxene within a shock-induced melt vein; (4) the microlite clusters embedded in silicate glass within a very thin shock-induced melt vein (20 μm in width). The first two occurrences of ringwoodite likely formed via solid-state transformation from olivine, supported by their morphological features and homogeneous compositions (Mg# 64–62) similar to the host olivine (Mg# 66–64). The third occurrence of ringwoodite might fractionally crystallize from the shock-induced melt, based on its heterogeneous and more FeO-enriched compositions (Mg# 76–51) than those of the coexisting wadsleyite (Mg# 77–67) and the host olivine (Mg# 66–64) of this meteorite. The coexistence of ringwoodite, wadsleyite, and clinopyroxene suggests a post-shock pressure of 14–16 GPa and a temperature of 1650–1750 °C. The fourth occurrence of ringwoodite with compositional variation (Mg# 72–58) likely crystallized from melt at 16–18 GPa and 1750–1850 °C. The presence of the four occurrences of ringwoodite was probably due to their very fast cooling rates in and/or adjacent to the thin shock-induced melt veins and small pockets. In addition, the higher Fa-contents of the host olivine (Fa35–39) in NWA 8705 than those in ordinary chondrites (Fa16–32) makes the olivine–ringwoodite transformation prolong to a lower pressure.

Conditions of chondrule formation in ordinary chondrites

Chondrules are sub-millimetric spheroids that are ubiquitous in chondrites and whose formation mechanism remains elusive. Textural and oxygen isotopic characteristics of chondrules in carbonaceous chondrites (CCs) suggest that they result from the recycling of isotopically heterogeneous early-condensed precursors via gas–melt interactions. Here, we report high-resolution X-ray elemental maps and in situ O isotopic analyses of FeO-poor, olivine-rich chondrules from ordinary chondrites (OCs) to compare the conditions of chondrule formation in these two main classes of chondrites. OC chondrules show minor element (e.g., Ti, Al) zonings at both the chondrule and individual olivine grain scales. Considering the entire isotopic data set, our data define a mass-independent correlation, with olivine grains showing O isotopic variations spanning more than 40‰. Though 16O-rich relict olivine grains were identified in OC chondrules, they are much less abundant than in CC chondrules. They appear as two types: (i) those with low minor element abundances and Δ17O < −15‰ and (ii) those with varying minor element abundances and less negative Δ17O values averaging −5.5‰. The host olivine grains exhibit mass-dependent O isotopic variations within individual chondrules.Our results reveal that similar processes (precursor recycling and interactions between chondrule melts and a SiO- and Mg-rich gas) established the observed features of OC and CC chondrules. The mass-dependent isotopic variations recorded by host olivine grains result from kinetic effects induced by complex evaporation/recondensation processes during the gas–melt interactions. This suggests that OC chondrules formed through enhanced recycling processes, in good agreement with the lower abundances of relict olivine grains in OC chondrules compared to CC chondrules. We use the Δ18O = δ18O − δ17O parameter to demonstrate that there is no genetic relationship between CC and OC chondrules, suggesting limited radial transport in the protoplanetary disk. Finally, to the first order, the Δ18O–Δ17O diagram may allow the non-carbonaceous vs. carbonaceous origin of a given chondrule to be deciphered.

Intracrystalline melt migration in deformed olivine revealed by trace element compositions and polyphase solid inclusions

Abstract. Melt transport mechanisms have an important impact on the chemical composition of the percolated host rock and the migrating melts. Melt migration is usually assumed to occur at grain boundaries. However, microstructural studies revealed the occurrence of polyphase inclusions along dislocations, subgrain boundaries and microcracks in single mineral grains. The inclusions are interpreted as crystallized melt pockets suggesting that melts can migrate within deformed crystals. Intracrystalline melt migration and diffusive re-equilibration can lead to significant mineral trace element enrichments when associated with dissolution–precipitation reactions. In this contribution, we study a body of replacive troctolites associated with the Erro-Tobbio ophiolitic mantle peridotites (Ligurian Alps, Italy). The replacive formation of the olivine-rich troctolite involved extensive impregnation of a dunitic matrix, i.e. partial dissolution of olivine and concomitant crystallization of interstitial phases. The olivine matrix is characterized by two distinct olivine textures: (i) coarse deformed olivine, representing relicts of the pre-existing mantle dunite matrix (olivine1), and (ii) fine-grained undeformed olivine, a product of the melt–rock interaction process (olivine2). Previous studies documented a decoupling between olivine texture and trace element composition, namely enriched trace element compositions in olivine1 rather than in olivine2, as would be expected from the dissolution–precipitation process. Notably, the trace element enrichments in deformed olivines are correlated with the occurrence of elongated 10 µm size polyphase inclusions (clinopyroxene, Ti-pargasite, chromite) preferentially oriented along olivine crystallographic axes. These inclusions show irregular contacts and have no crystallographic preferred orientation with the host olivine, and the phases composing the inclusions show similar chemical compositions to the vermicular phases formed at the grain boundaries during late-stage reactive crystallization of the troctolite. This suggests that the investigated inclusions did not form as exsolutions of the host olivine but rather by input of metasomatic fluids percolating through the deformed olivine grains during closure of the magmatic system. We infer that strongly fractionated volatile-rich melts were incorporated in oriented microfractures within olivine1 and led to the crystallization of the polyphase inclusions. The presence of intracrystalline melt greatly enhanced diffusive re-equilibration between the evolved melt and the percolated olivine1, in turn acquiring the enriched character expected in neoformed olivine crystals. Intracrystalline melt percolation can have strong geochemical implications and can lead to efficient re-equilibration of percolated minerals and rocks.

Origins of olivine in Earth’s youngest kimberlite: Igwisi Hills volcanoes, Tanzania craton

Monomineralic millimeter-sized olivine nodules are common in kimberlites worldwide. It is generally thought that such ‘dunitic nodules’ originate from the base of the cratonic lithosphere and that their formation marks the onset of deep-rooted kimberlite magmatic plumbing systems. However, thermobarometric constraints to support such a model have been lacking thus far. This study focuses on the petrography and textures, as well as on pressure–temperature estimations, of well-preserved dunitic nodules from the Quaternary Igwisi Hills kimberlite lavas on the Tanzania craton, with the ultimate goal to constrain their origins. We utilize EBSD-determined textural information in combination with olivine geochemistry data determined by EPMA and LA-ICP-MS methods. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such an association is supported by garnet inclusions within olivine. Projection of Al-in-olivine temperatures onto a regional geotherm suggests that the host olivine grains equilibrated at ~ 100–145 km depth, which points to origins from mid-lithospheric levels down to the lower cratonic mantle if a depth of 160–180 km is considered for the lithosphere–asthenosphere transition beneath the Tanzania craton. These first pressure–temperature estimates for dunitic nodules in kimberlites suggest that their formation also occurs at much shallower depths than previously assumed. Recrystallized olivine grains (i.e., neoblasts) show random crystallographic orientations and are enriched in minor and trace elements (e.g., Ca, Al, Zn, Sc, V) compared to the host olivine grains. These features link neoblast formation to melt-assisted recrystallization of cratonic mantle peridotite, a process that persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths makes these materials texturally weaker and subsequent liberation of mineral grains promotes the assimilation of compositionally ‘unstable’ orthopyroxene in rising carbonate-rich melts, which is considered to be an important process in the evolution of kimberlite magmas. Dunitic nodules in kimberlites and related rocks may form as melt–rock equilibration zones along magmatic conduits within the lower half of the cratonic mantle column all the way up to mid-lithospheric depth. Such an origin potentially links dunitic nodules to olivine megacrysts, which are equally considered as melt/fluid-assisted recrystallization products of peridotitic mantle lithosphere along the ascent pathways of deep-sourced CO2–H2O-rich ultramafic melts.

Clinopyroxene and olivine-hosted spinel inclusions from the Mt Cameroon volcanic area, west-central Africa

Compositions of clinopyroxenes and spinels from the ocean/continent boundary of the Cameroon Line are presented. The spinels occur as minute inclusions in olivine phenocrysts from alkali basaltic bombs collected from the Debunscha maar deposit at the foot of Mt Cameroon. The inclusions are mostly subhedral to euhedral in shape, measuring generally less than 70 μm in diameter. The studied spinels are chemically characterized by: Fe2+/(Fe2++Mg) = 0.47–0.64, Fe3+/(Fe3++Al+Cr) = 0.13–0.18, Cr# = Cr/(Cr+Al) = 0.47–0.65, and Mg# 43.69–56.94. The lack of a systematic correlation between these spinels’ composition and their positions within the host olivine phenocrysts, is in line with the quenching nature of the rocks. The compositions of these spinels show that they are chromites. Comparisons of these chromites with those from other settings show that the Debunscha chromites have Fe3+/(Fe3++Al+Cr) (0.13–0.23) and TiO2 (1.96–5.63 wt %) higher than chromites from most lavas (Fe3#: 0.01–0.16, TiO2: 0.37–2.14). The clinopyroxenes have the following major element compositions SiO2 (45.97–51.62 wt %), MgO (13.84–16.45 wt %), CaO (21.16–23.52 wt %), Al2O3 (3.25–7.45 wt %), FeO (5.50–8.17 wt %), MnO (0.09–0.16 wt %), TiO2 (0.99–2.88 wt %) including Mg# in the range 83.8–96.3 and plot in the diopside-augite boundary on the conventional Wo-En-Fs diagram. These clinopyroxenes are, variably enriched in trace elements. The concentrations of Co are low and are in the range 32.75–42.38 ppm while Cr and Ni concentrations are high and in the 80.60–3557.19 ppm and 38.55–228.62 ppm respectively. Many other important trace elements, for instance, the rare earth elements, Sc, Zr, Sr, Nb, Ba also show variations in the studied clinopyroxenes. The clinopyroxenes equally show very high Zr/Nb ratios (104–708), and plot on the depleted mantle domain in the Zr/Y–Nb/Y graphs. However, primitive mantle normalized trace element patterns for all the clinopyroxene samples are similar with noticeable depletions in Sr and Ba, which is also suggestive of a depleted mantle source. This similarity is an indication that all the samples are genetically related, and have crystallized from petrogenetically related melts as part of fractional crystallization and mixing sequences.

MORB Melt Transport through Atlantis Bank Oceanic Batholith (SW Indian Ridge)

AbstractThe Atlantis Bank Oceanic Batholith is a 660 km2 gabbro massif representing the plutonic foundation of a major ridge magmatic center. It was continuously accreted, emplaced, and exposed in the rift mountains of the paleo-SW Indian Ridge from 13 to 10·3 Ma. Ocean Drilling Program (ODP) Hole 735B, drilled to 1508 m at Atlantis Bank, recovered evolved intercalated olivine and oxide gabbros representing the upper levels of the lower ocean crust. Within this section, ∼5·6 m of primitive chromian-spinel-bearing troctolites (0·1–2 m thick), with sharp modal contacts with the host gabbros, were cored between 410 and 500 m depth. Here we present new mineral (chromian spinel, clinopyroxene, olivine, and plagioclase) major and trace element and petrographic data for the troctolite suites (i.e. troctolites, clinopyroxene-rich troctolites, troctolitic gabbros) from 410 to 528 mbsf (meters below seafloor) and olivine gabbros in the 0–274 mbsf and 410–500 mbsf intervals, and examine the origin of these troctolite layers. Equilibrium melts for the troctolites are primitive to moderately evolved, with Mg# [= 100Mg/(Mg + Fe), 48–68 mol%] comparable with those of Atlantis Bank basalts. By contrast, equilibrium melts for both the 0–274 mbsf and 410–500 mbsf host olivine gabbros are highly to moderately evolved (Mg# 27–63 mol%). The 410–500 mbsf troctolite suites maintain high clinopyroxene Mg# (81–89 mol%) and mineral concentrations of compatible elements (i.e. clinopyroxene Cr2O3 0·1–1·2 wt%, Ni 151–330 μg g–1 and olivine Ni 1055–1559 μg g–1) with the increase in incompatible trace element abundances (i.e. clinopyroxene TiO2 0·3–2·1 wt%, Zr 5·8–112 μg g–1 and olivine Ti up to 293 μg g–1). Combined with abundant dissolution–reprecipitation textures, our results indicate that the troctolites were formed by the reaction between a spinel-bearing, troctolitic mush, from which they inherited the high Mg#, Ni and Cr, and percolating melts adding incompatible trace elements. Moreover, the spinel (NiO versus TiO2) and olivine (Ti versus Y) trace element compositions indicate that the troctolites were affected by low-degree Fe–Ti-rich melt metasomatism. In contrast, both the 0–274 mbsf and 410–500 mbsf olivine gabbros display a prominent decrease in clinopyroxene Mg# (from 88 to 66) and mineral compatible element concentrations (i.e. clinopyroxene Cr2O3 from 1·1 to ∼0 wt%, Ni from 208 to 34 μg g–1 and olivine Ni from 1200 to 136 μg g–1) with increasing incompatible trace element abundances (e.g. clinopyroxene Zr from 4·8 to 157 μg g–1). These features are compatible with the reactive porous flow of slightly to highly evolved melts through the cooling crystal mush zone. Our results, combined with the literature data, indicate that most olivine gabbros between 410 and 500 mbsf were formed prior to the troctolite layers. We document that the troctolites represent conduits for mid-ocean ridge basalt melt transport through the lower oceanic crust, whereas the olivine gabbros represent crystallization of a large crystal mush, recording initial gabbro emplacement, hyper- and sub-solidus deformation, and melt–rock reaction owing to upward penetrative flow of intercumulus melt.

Origin of sulphur in relation to silicate-sulphide immiscibility in Tolbachik primitive arc magma (Kamchatka, Russia): Insights from sulphur and boron isotopes

This study reports the first measurements of sulphur and boron isotopic compositions in olivine-hosted melt inclusions (MI) from basaltic lava and scoria deposits from the 1941 Tolbachik eruption, Kamchatka, Russia. The primary aim of the present study is to constrain the origin of sulphur in relation to silicate-sulphide immiscibility that occurred in the Tolbachik mafic magmas. Melt inclusions fall into two compositionally distinct groups based on their S- and B-isotope systematics, the first is hosted by olivine from lava and the second in olivine from lapilli and scoria. The upper ends of the concentration ranges for CO2 (3660–4200 μg/g) and H2O (4.5–5.9 wt%) imply the onset of magma crystallization between ~25 and ~30 km for both MI groups, i.e., close to the Moho discontinuity. After filtering the compositions of melt inclusions experienced H2O-loss due to the diffusion of H from olivine hosts, both lava- and scoria-related melt inclusions are characterized by similar fractionation depths between ~15 and ~30 km. The inclusions from scoria show no signs of sulphur degassing at depth and, along with the most S-rich inclusions from lava, exhibit enrichment in S. Strong positive correlations of S/K2O with H2O/K2O, Cl/K2O and F/K2O, as well as the correlations of these ratios with δ34S also preclude significant S-loss by degassing and point towards mixing of at least two magmas containing contrasting volatile concentrations and isotopic compositions. Slab-derived 11B-enriched fluids and, to a minor extent, 11B-depleted melts could be major agents controlling the composition of the mantle wedge and, consequently, the chemical and isotopic variability of its derivative melts. A subducted serpentinized peridotite could serve as a possible source of the 11B- and probably the 34S-rich signature of the studied primitive Tolbachik magmas.

A siliceous komatiitic source of Bushveld magmas revealed by primary melt inclusions in olivine

Our experimental thermobarogeochemical study of the primary melt inclusions in olivine (Fo91) from poikilitic harzburgite to dunite of the Uitkomst Complex in the Bushveld Igneous Province revealed a series of melt compositions that are the result of combined fractional crystallization and assimilation. The primary melt inclusions up to 50 μm in size are fully crystallized and composed of predominantly olivine and orthopyroxene with highly subordinate chromite, sulfide, amphibole and plagioclase accompanied by a fluid phase in shrinkage voids. The most primitive melt within the series has a re-homogenization temperature of >1450 °C and contains 22.47 wt% MgO, 12.57 wt% FeO, 51.90 wt% SiO2, 2.50 wt% CaO, 8.27 wt% Al2O3 and 1.69 wt% Na2O recalculated to equilibrium with the host olivine. This is the first direct measurement of the komatiitic parental magma of the Bushveld magmas, inferred in previous studies. The estimated maximum PT parameters of the melt corresponds to 2.5 GPa and 1590 °C implying that the primitive melt inclusions in the olivine were trapped while still in the upper mantle and before further crustal assimilation. This primitive melt shows the steepest slope (La/Yb)PM, the highest Ni and is depleted in volatile components (168 ppm H2O, 264 ppm Cl, 258 ppm F and 114 ppm S according to secondary ion mass spectrometry measurements) compared to more evolved compositions, which were progressively enriched in volatiles up to 7665 ppm Н2О, 1728 ppm Cl, 2288 ppm S, and 549 ppm F as the rehomogenization temperature fell to 1335 °C. The progressive assimilation of crustal material responsible for magma modification is also manifested by the occurrence of CaNa volatile-rich minerals (such as amphibole, phlogopite, albite, sodalite, natrolite, pectolite, Cl-apatite and carbonate) in the interstitial assemblage and in “exotic” polyphase inclusions in cumulus olivine.The concentrations and ratios of the incompatible elements in the melt inclusions in Uitkomst olivine are similar to those in the model compositions of the ultramafic Bushveld parental magma and support a common source of ultramafic melts in the Bushveld Province. The main components of the melts, however, are not in accord with either boninitic or komatiitic lineages indicating a unique nature for the Bushveld magmas.

Re-Os isotope geochemistry of the Palaeoproterozoic Sakatti Cu-Ni-PGE sulphide deposit in northern Finland

The Sakatti Cu-Ni-PGE sulphide deposit is located in the Central Lapland greenstone belt in northern Finland. It comprises three mineralised olivine cumulate units called the main, north-east and south-west bodies. Sulphides form massive, semi-massive, disseminated and vein-type ores in these ultramafic rocks. The ore mineral assemblage is typical for magmatic sulphide deposits, including chalcopyrite, pyrrhotite, pentlandite, pyrite and magnetite. We present Re-Os concentration and isotope data for whole-rock olivine cumulate samples and mineral separates (chromite, olivine) as well as for massive sulphides and associated oxides. Chromite separates contain 2.49–115.4 ppb Os and 0.03–2.85 ppb Re while olivine separates have 3.13–42.1 ppb Os and 0.43–3.89 ppb Re. Their host olivine cumulates show Os and Re concentrations of 1.17–23.7 ppb and 0.04–0.93 ppb, respectively. The sulphide and magnetite separates show a considerable variation and overlap in the Os and Re concentrations, with the total range being 0.69–393.5 ppb for Os and 4.44–246.9 ppb for Re. Among the sulphide separates, the highest Re and Os concentrations were obtained for pyrrhotite and pyrite, respectively. On average, Re/Os ratios increase in the following order: chromite, whole-rock olivine cumulate, olivine, pyrite, pentlandite, chalcopyrite, pyrrhotite, and magnetite. Excluding the most disturbed samples, 30 analyses yielded a Re-Os isochron age of 2063 ± 35 Ma for the Sakatti deposit, being similar to the U-Pb zircon age of 2058 ± 4 Ma determined for the Kevitsa Ni-Cu sulphide ore-bearing intrusion, which is located ~20 km NE of Sakatti. Re-Os dating of massive sulphide samples yields variable Re-Os isochron ages from 1852 ± 200 Ma to 2045 ± 300 Ma, with all sulphide samples together giving an age of 1928 ± 16 Ma. This indicates resetting and open-system behaviour of some sulphide samples, potentially related to Svecofennian orogenic events. In spite of this resetting, the low Re/Os ratios and high Os concentrations of chromite and several sulphide separates allow us to determine their initial γOs values, which turned out to be very close to chondritic for chromite and moderately radiogenic (γOs from +21 to +56) for sulphides, indicating that sulphide saturation took place after chromite crystallisation.

MushPEC: Correcting Post-entrapment Processes Affecting Melt Inclusions Hosted in Olivine Antecrysts

Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB®script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo77–78) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c.125 – 150 MPa) at undersaturated H2O contents ofc. 1 – 2 wt% H2O).

From Explosive Vent Opening to Effusive Outpouring: Mineral Constraints on Magma Dynamics and Timescales at Paricutin Monogenetic Volcano

Abstract Paricutin volcano is the youngest monogenetic cone of the Michoacán–Guanajuato volcanic field (Mexico), with an excellent historical eruption record from February 1943 to March 1952. The magma emitted during the 9 years of activity was compositionally zoned from basaltic andesite to andesite. This eruption has been considered a classic example of magma differentiation controlled by crustal assimilation combined with fractional crystallization. However, a recent geochemical study of the eruption products points to mantle source heterogeneity and fractional crystallization as the key processes involved in the compositional variability of the magmas. Here we present a detailed petrological characterization of the minerals [olivine, plagioclase, pyroxene, Cr-spinel, and (Ti-)magnetite] to shed light on the processes that led to the chemical evolution of the eruptive products. Our sample set includes the early tephra from the first weeks or months of eruption and the whole sequence of lava flows that followed. The mineral assemblage, their texture, and chemical composition show a systematic evolution between the products from the opening of the explosive vent to the effusive stage. The early tephra are basaltic andesites with oscillatory-zoned olivine and plagioclase, zoned Cr-spinel, and rare pyroxene xenocrysts. In contrast, later erupted tephra and post-January 1944 lavas are basaltic andesites and andesites with normally zoned olivine, Cr-spinel inclusions in equilibrium with the host olivine, and frequent orthopyroxene (after December 1947) with minor chemical zoning. Mineral chemistry data and olivine diffusion timescales, together with whole-rock geochemistry, suggest a convective magma regime with large temperature and oxygen fugacity gradients characterized by short timescales (few days) during the opening stage, followed by a steadier magma regime with longer timescales (few months) and including periodic magma recharge, mixing, and fractional crystallization. In addition, the mineralogical evidence we have gathered does not support considerable crustal assimilation at Paricutin. This study shows that monogenetic eruptions are far from being geochemically simple, and instead involve multiple magma batches with complex storage and mixing stages before eruption.

Determination of total CO2 in melt inclusions with shrinkage bubbles

The compositions of volatiles in magmas can be estimated using melt inclusions encapsulated in phenocrysts. Shrinkage bubbles are occasionally observed in natural and rehomogenized melt inclusions. Because of the low solubility of CO2 in melts, CO2 is partitioned in both the melts and shrinkage bubbles in melt inclusions. We developed a method to determine total CO2 in melt inclusions by measuring (1) CO2 concentration in glass with secondary-ion mass spectrometry, (2) CO2 density in bubbles with micro-Raman spectrometry, and (3) bubble/glass volume ratio with micro X-ray computed tomography on rehomogenized melt inclusions. We applied this method to olivine-hosted melt inclusions in high-μ (HIMU)-type ocean island basalts from Raivavae in Austral Islands in French Polynesia.The shape of melt inclusions is predominantly related to their size; larger melt inclusions tend to be more angular than smaller ones. The volume of large melt inclusions may be overestimated by previous approaches in which the dimension of melt inclusions was measured by microscopy and the volume was calculated assuming ellipsoidal shape. CO2 dissolved in glass in rehomogenized melt inclusions ranges up to ~7000 ppm. The glass with a more alkalic composition shows greater CO2 concentration than that with a less alkalic composition in accordance with the composition-dependent CO2 solubility. By contrast, the correlation between the total CO2 content and the chemical composition of the glass is unclear because CO2 in shrinkage bubbles is generally greater than that in the glass. Some melt inclusions demonstrate an extremely high total CO2 content (up to 25,000 ppm). Because these melt inclusions show a large bubble volume proportion in melt inclusions (>12 vol%), they are likely formed by the simultaneous entrapment of melts and fluids during the host olivine growth. Excluding these melt inclusions, the total CO2 content and CO2/Nb ratio in melt inclusions were up to 7000 ppm and 200, respectively. This study offers an accurate determination of CO2 concentration and CO2/Nb of the HIMU basalts from Raivavae.

Petrogenesis of middle Okinawa Trough volcanic rocks: Constraints from lead isotopes in olivine-hosted melt inclusions

Olivine-hosted melt inclusions provide crucial information about their parental magma composition and evolution and represent ideal targets for determining the nature of the poorly understood and still-debated magma sources in the Okinawa Trough, a back-arc basin in the western Pacific. We present the first analyses of the lead (Pb) isotope compositions of olivine-hosted melt inclusions to evaluate the mantle properties and petrogenesis of middle Okinawa Trough volcanic rocks. The melt inclusions have more variable major and trace elements and Pb isotope compositions than the host whole-rock samples. We report the discovery of both high-207Pb/206Pb (>0.865) and low-207Pb/206Pb (<0.865) isotope compositions in melt inclusions in individual volcanic rocks, even within a single host olivine, indicating a compositionally heterogeneous magma source. The trace element and Pb isotope characteristics of the melt inclusions show that the magma source is affected by enriched components. We modeled the injection of enriched components into a magma source to explain the generation of the magma heterogeneity. The results indicate that the mixing of Pacific Ocean-type mantle (MORB), an EMI-like component from recycled lower continental crust and EMII-like material from subducted sediments can explain the low-207Pb/206Pb isotope values observed in the melt inclusions. The discovery of small proportions of melt inclusions with high 207Pb/206Pb ratios, high K2O, P2O5, Rb and U contents and low Pb and Cu contents in the studied andesites suggests that the andesitic magma may have been formed by the mixing of materials with different elemental and isotopic compositions. Our study results suggest that pervasive magma mixing may have occurred in the magma source prior to eruption in the Okinawa Trough.

Comparative Geothermometry in High-Mg Magmas from the Etendeka Province and Constraints on their Mantle Source

Abstract There is still debate whether Large Igneous Provinces (LIPs) are caused by high mantle temperatures induced by thermal plumes or by other factors that enhance melt production from the mantle. A prerequisite for assessing the thermal plume model is a reliable estimate of liquidus temperatures of the magmas produced, preferably based on more than one method of geothermometry. The study reported here compares multiple geothermometers for the Etendeka LIP, which is among the largest Phanerozoic examples and one that shows several features suggestive of a plume origin (continental flood basalt province linked via an age-progressive volcanic ridge to an active hotspot). Magnesium (Mg)-rich magmas emplaced as dikes in NW Namibia are the most primitive rocks known from this province and are thus best suited to determine the composition and melting conditions of their mantle source. Earlier studies of the Etendeka Mg-rich dikes reported high liquidus temperatures based on olivine-melt Mg–Fe equilibria. We extend that work to a larger set of samples and compare the results of olivine-melt Mg–Fe thermometry with other methods based on spinel-melt and spinel–olivine equilibria (Al-in-olivine thermometry), as well as olivine-melt trace-element exchange (Sc/Y thermometry and V oxybarometry). All methods used the same starting assumptions of nominally anhydrous melts and a crystallization pressure of 0·5 GPa. Only mineral-melt or mineral-mineral pairs consistent with compositional equilibrium were used for calculating temperatures. The trace-element compositions of olivine are also used to discuss the relative proportion of peridotite and pyroxenite in the mantle source for these magmas. Twelve dike samples were studied, with whole-rock MgO concentrations ranging from 8·4 to 19·4 wt %. Diagnostic element ratios of transition metals in olivine (e.g., Mn/Fe, Mn/Zn, Zn/Fe) indicate a peridotite-dominated mantle source for the magmas, which is consistent with the other indicators based on whole-rock data e.g., 10 000×Zn/Fe, CaO–MgO trend, FeO/MnO and FC3MS (FeO/CaO–3×MgO/SiO2). The temperature variations show a positive correlation with the Fo-content of host olivines, and values from high-Fo olivine agree well with olivine and spinel liquidus temperatures calculated from thermodynamic models of bulk-rock composition. All methods and most samples yielded a temperature range between 1300 °C and 1400 °C. An exceptional few samples returned temperatures below 1300 °C, the minimum being 1193 °C, whereas several samples yielded temperatures above 1400 °C, the upper range being 1420–1440°C, which we consider to be a robust estimate of the maximum liquidus temperatures for the high-Mg magmas studied. The conversion to mantle potential temperatures is complicated by uncertain depth and degree of melting, but the functional relationship between Tp and primary melt MgO contents, using melt inclusions from olivine phenocrysts with of Fo &amp;gt; 90, indicate a Tp range from 1414 to 1525 °C ( 42 °C), which is 100–150°C higher than estimates of ambient upper mantle Tp in the South Atlantic today.

Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions

The conditions of methane (CH4) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H2) and CH4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H2O. The generation of molecular H2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH4 Once formed, CH4(g) and H2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH4 and H2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.

Investigation of Na-immigration into olivine LiFePO4

In 21th century, rechargeable batteries are main key of modern technology in many applications from portable devices (smartphone, laptop) to large-scale (hydride electric vehicle-HEV, smart grid system). Among the rechargeable batteries, Li-ion battery (LIB) is outstanding member due to the highest gravimetric as well as volumetric capacity; and Sodium-ion batteries (SIBs) can have contribution to alternating LIBs in large-scale application. Li-ion and Na-ion batteries have the same configuration with an insertion/extraction reversible of Li+ ions and Na+ ions into electrode positive and negative during charge-discharge process. This work aimed to investigate Na-immigration into olivine LiFePO4. The olivine phase LiFePO4 was prepared by hydrothermal process. The synthesized LiFePO4 was characterized the structure, morphology and electrochemical properties. The XRD pattern showed the high crystalline and, the Rietveld refinement with X2 = 2.32% confirmed the highly pure olivine phase without impurity. The SEM images exhibited the uniform and good distribution of synthesized olivine in submicrometric scale. The delithiated phase FePO4 was prepared by electrochemical oxidation at low rate C/20. The charge-discharge curves demonstrated the reversible Na-immigration into olivine host with a highest capacity of 80 mAh/g, the cyclability was found out in 73 mAh/g upon 30 cycles. The ex-situ XRD (electrode after electrochemical oxidation, electrode after Na-insertion) revealed the stability of FePO4 framework during Na-immigration.&#x0D;

Grain scale processes recorded by oxygen isotopes in olivine-hosted melt inclusions from two MORB samples

Although olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy for mantle composition, these melt inclusions generally show larger elemental and isotopic compositional variation than their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotope data from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions from different crystals within the same sample show >2.5‰ δ18O variation within each sample, which is nearly eight times the analytical error of 0.3‰ (2 standard deviations) and five times the δ18O range in unaltered MORB. Measured δ18O in melt inclusions do not correlate with common magmatic tracers, and δ18O measured in the host olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions from each sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higher olivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here we discuss several potential processes that could contribute to these observations, but none satisfactorily explain the olivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear that the variability of δ18O in melt inclusion from two MORB samples do not record only common magmatic process(es), but rather a localized grain scale process. Any δ18O variation in melt inclusions should thus be interpreted with caution.

Plume-ridge interaction in the South China Sea: Thermometric evidence from Hole U1431E of IODP Expedition 349

Testing the hypothesis that the Hainan mantle plume triggered the opening of the South China Sea (SCS) and interacted with the spreading ridge is important for the purpose of understanding the dynamics of the evolution of the SCS. This study examined the most primitive olivine (Fo = 83.1–87.1) and spinel (Cr# = 28–60%) in the basalt and glassy samples collected from Hole U1431E of International Ocean Discovery Program (IODP) Expedition 349, with the goal of estimating the crystallization temperature. Spinels mostly enclosed by olivines have low Fe3+ and Ti contents and show compositional equilibrium with the host olivines. The application of the Al-in-olivine thermometer provided crystallization temperatures for the studied samples, which varied from 1208 ± 29 °C to 1287 ± 29 °C. When extrapolating to primitive olivine of Fo = 90, the crystallization temperature of primitive olivine is approximately 1350 °C, which is much higher than that of other primitive mid-ocean ridge basalt (MORB) but comparable to that of Iceland oceanic island basalt (OIB). Therefore, the mantle potential temperature at the extinct ridge of the SCS at ~16 Ma is much higher than that beneath a normal mid-ocean ridge, manifesting the mantle plume-ridge interaction during the last spreading of the SCS. Given the lack of thick oceanic crust, seamount chains or lineaments in the SCS, we propose that this plume impacted on the SCS might be eclogite-rich and of low buoyancy.