Olivine Inclusions Research Articles

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.

Magnetic Ordering of Magnetite Inclusions in Olivine at Mantle Depths in Subduction Zones

Magnetic Ordering of Magnetite Inclusions in Olivine at Mantle Depths in Subduction Zones

A Combined Re-Os and Pt-Os Isotope and HSE Abundance Study of Ru-Os-Ir Alloys from the Kunar and Unga Placer Deposits, the Taimyr Peninsula, Polar Siberia

In order to provide further insights into the origin of Ru-Os-Ir alloys, this study presents new highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundance and 187Re-187Os and 190Pt-186Os isotope data for detrital grains of native Ru-Os-Ir alloys in placer deposits of the Kunar and Unga Rivers, which display a close spatial association with the Kunar dunite–harzburgite complex in the northern part of the Taimyr Peninsula in the Polar Siberia. The study utilized electron microprobe analysis, negative thermal ionization mass-spectrometry (N-TIMS) and laser ablation multiple-collector inductively coupled plasma mass-spectrometry (LA MC-ICP-MS). The primary nature of the Ru-Os-Ir alloys is supported by the occurrence of euhedral inclusions of high-Mg olivine (Fo92–93) that fall within the compositional range of mantle olivine. The LA MC-ICP-MS data show similar average initial 187Os/188Os and γ187Os(740 Ma) values for PGM assemblages from the Kunar and Unga deposits of 0.1218 ± 0.0010, −0.18 ± 0.85, and 0.1222 ± 0.0025, +0.10 ± 2.1, respectively. These values are identical, within their respective uncertainties, to the initial 187Os/188Os value of the Ru-Os-Ir alloy grain measured by N-TIMS (0.1218463 ± 0.0000015, γ187Os(740 Ma) = −0.1500 ± 0.0012). The combined 187Re-187Os isotopic data for all studied grains (γ187Os(740 Ma) = −0.02 ± 1.6) indicate evolution of the Kunar and Unga mantle sources with a long-term chondritic 187Re/188Os ratio of 0.401 ± 0.030. In contrast to the 187Os/188Os data, the initial 186Os/188Os value of 0.1198409 ± 0.0000012 (µ186Os(740 Ma) = +34 ± 10) obtained for the same Ru-Os-Ir alloy grain by N-TIMS is suprachondritic and implies evolution of the Kunar and Unga mantle source(s) with a long-term suprachondritic 190Pt/188Os ratio of 0.00247 ± 0.00021. This value is ~40% higher than the average chondritic 190Pt/188Os ratio of 0.00180 and indicates long-term enrichment of the Kunar source in Pt over Os. Establishing the source of this enrichment requires further investigation.

MELT INCLUSIONS IN OLIVINE AS A SOURCE OF INFORMATION ON THE COMPOSITION AND EVOLUTION OF DEEP MELTS OF AILLIKITES (ULTRAMAFIC LAMPROPHYRES) OF THE ILBOKICHI UPLIFT, THE SW SIBERIAN PLATFORM

Aillikites are kimberlite-like rocks, important for understanding the composition and processes occurring in the mantle. Melt inclusions represent a reliable source of information. The paper provides the first results of studies (Raman, EDS) on primary and secondary melt inclusions in olivine from the Ilbokich uplift aillikites. The composition of primary inclusions is close to that of parent melt of aillikites. It was significantly enriched in CO2, H2O, phosphorus and titanium. Phlogopite, diopside, dolomite, calcite, apatite, Ti-containing phases (brookite, perovskite, Ti-magnetite) and lizardite were identified in these inclusions. The similarity of the composition and ratios of the daughter phases with the aillikite matrix indicates a slight change in the parent melt when it is rising to the surface. As to the secondary inclusions, there are wide variations in compositions and a smaller amount of silicates, as compared to the primary ones. The main daughter phases are carbonates, e.g. dolomite, calcite, magnesite and alkaline carbonates. In addition, phlogopite, clinopyroxene, apatite, halite, pyrrhotite and magnetite, graphite and CO2 were discovered. The variability of the compositions of the secondary inclusions might be due to the silicate-carbonate immiscibility that appeared during the rising of the aillikite melt at pressures <4 GPa.

Age and Petrogenesis of Ultramafic Lamprophyres of the Arbarastakh Alkaline-Carbonatite Complex, Aldan-Stanovoy Shield, South of Siberian Craton (Russia): Evidence for Ultramafic Lamprophyre-Carbonatite Link

Abstract In this study, we discuss mineral chemistry data, melt inclusion study results, and report Ar–Ar phlogopite age for the aillikite dikes of the Arbarastakh alkaline-carbonatite complex on the Aldan-Stanovoy shield, Russia. Aillikite was crystallized at 631 ± 8.5 Ma, coeval with the intrusion age of the Arbarastakh rocks. The Arbarastakh complex was formed during the late Neoproterozoic epoch of rare earth element-Nb ore-bearing alkaline-carbonatite magmatic activity that was widespread on the southwestern and southern margins of the Siberian craton, related to rifting processes during the breakup of the supercontinent Rodinia. The aillikites show mineralogical characteristics of primitive magmas such as highly forsteritic olivine, Mg-ilmenite, and Cr-rich spinel. The variance in olivine zonation, morphologies, and chemical element distribution indicate that olivine in the aillikites is represented by several genetic types: xenogenic olivines (Fe-poor cores) from the sheared peridotite, olivine antecrysts (Fe-rich cores) related to mantle metasomatism by preceding proto-aillikite melt, and olivine phenocrysts formed during crystallization of aillikite melt. The latter shows decreasing Ni and Cr due to fractional crystallization of olivine, ilmenite, and chromite, along with increasing Mn and Ca concentrations that are consistent with enrichment of these elements in the residual melt. The olivine phenocrysts chemistry shows variations that are characteristic of the presence of phlogopite and carbonate in the mantle source (low 100*Ca/Fe (0.4–1.2) and 100*Mn/Fe (1–2), moderate 100*Ni/Mg (1.4–0.4)). Spinel shows a wide compositional variation with two compositional zoning trends, one of which follows the titanomagnetite trend, while the other follows the qandilite-rich magnesio-ulvöspinel-magnetite one. The latter trend indicates an increase in fO2 and attendant Fe oxidation to Fe3+ during crystallization. Ilmenite composition evolution (from Mg-rich to Mn-rich) also reflects the carbonate-rich nature of aillikite melt. We identify primary melt inclusions hosted in phlogopite and secondary melt inclusions in olivine; both melt inclusions types have daughter minerals dominated by dolomite, calcite, Na-Ca carbonates, phosphates, and phlogopite, consistent with the carbonate-rich nature of aillikite melt. The calculated temperatures reflect the early stage of aillikite crystallization, with values ranging from 1169 to 1296°C and fO2 values (olivine-spinel pair) varying from +0.40 to +1.03 ΔFMQ and from ΔNNO −0.9 to ΔNNO −2.0 (perovskite oxygen barometer); in contrast, the homogenization temperature of the secondary melt inclusions in olivine (700–720°C) characterizes late-stage aillikite melt evolution. The carbonate-rich nature of the Arbarastakh aillikite and its similar age to the carbonatites are consistent with a genetic relationship between them.

Electron Probe Microanalysis and Microscopy of Polishing-Exposed Solid-Phase Mineral Inclusions in Fuxian Kimberlite Diamonds

Solid-phase mineral inclusions in diamond (1–3 mm in diameter) from the No. 50 kimberlite diatreme of Liaoning Province, China, were exposed by polishing. A variety of silicate, carbonate and sulfide inclusions were recovered in the diamond. The common solid-phase inclusions are olivine, chromite, garnet and orthopyroxene; the rare phases include Ca carbonate, magnesite, dolomite, norsethite, pyrrhotite, pentlandite, troilite, a member of the linnaeite group, an unknown hydrous magnesium silicate and an Fe-rich phase. Abundance and composition of the solid-phase inclusions in diamond indicate that they belong to the peridotitic suite and are mainly harzburgitic. No eclogitic mineral inclusions were found in the diamond. The slightly lower Mg # of the olivine inclusions (peak at 93) than that of harzburgitic olivine inclusions worldwide (Mg # peak at 94), the higher Ni content (0.25–0.45 wt. %) of the olivine inclusions than those of olivine inclusions worldwide (0.30–0.40 wt. %), the higher Ti contents (up to 0.79 wt. %) in some chromite inclusions in diamond than those in chromite inclusions worldwide, the existence of carbonate inclusions in diamond, and the possible presence of hydrous silicate phases in diamond all indicate a metasomatic enrichment event in the source region of diamond beneath the North China craton, suggesting that the diamond probably formed by solid-state growth under metasomatic conditions with the presence of a fluid. Solid-state growth of diamond is also supported by abundant graphite inclusions in the diamond. Sulfide inclusions in diamond often coexist with chromite and olivine or are rich in Ni content, indicating that the sulfide inclusions belong to the peridotitic suite. From the chemical compositions, most sulfide inclusions in diamond from the No. 50 kimberlite were probably trapped as monosulfide crystals, although some may have been entrapped as melts.

Highly heterogeneous mantle caused by recycling of oceanic lithosphere from the mantle transition zone

Geochemical heterogeneities observed in the mantle are usually attributed to recycling of oceanic lithosphere through subduction. However, it remains hotly debated where recycled material stagnates, and how quickly it can be liberated back to surface. This knowledge gap hinders our understanding of mantle circulation and the chemical evolution of the Earth. Here we address these questions using a combination of geochronology and geochemistry from South China Sea (SCS) seamounts. The Shixingbei seamount lavas formed during active seafloor spreading at c. 19.1 Ma show limited geochemical variability, whereas the Zhenbei-Huangyan seamount chain formed during the post-spreading stage at c. 7.8 Ma and displays a wide range of compositions. However, melt inclusions in olivine and plagioclase from the Zhenbei-Huangyan basalts show considerably greater isotopic variability than seen in the whole rock compositions of both the SCS syn- and post-spreading lavas. A previously unidentified third mantle source component (FOZO) revealed by olivine-hosted melt inclusions along with both depleted (DMM) and enriched (EMII) mantle components is required in the source region to explain the observed isotopic and chemical variability. On the basis of our results, the age of the recycled ocean crust and sediments in this region are estimated to be c. 120 – 350 Ma. We infer that these enriched components in the SCS lavas come from the mantle transition zone. Variations in mantle source heterogeneity coupled with melting process control spatial–temporal (spreading vs. post-spreading stage) geochemical variations of lavas from the SCS and surrounding areas. Together with the results from published studies, we propose that marginal basins are one of the major locations on Earth where oceanic and/or continental lithosphere is transferred into the upper mantle and transition zone, representing an important source of upper mantle heterogeneity. We provide a simple conceptual model linking plate subduction and upper mantle heterogeneity and the volcanism in the SCS and surrounding areas.

Carbon compounds in the West Kimberley lamproites (Australia): Insights from melt and fluid inclusions

Petrological and geochemical studies of lamproites can provide useful insights into the nature of their lithospheric mantle sources, but their geochemical and mineralogical diversity has complicated our understanding of their primary/parental melt composition, volatile (CO2, H2O) inventory and magmatic evolution. To help address this issue, we present a detailed study of different generations (primary, pseudosecondary, secondary) of crystal, and melt and fluid inclusions in olivine, Cr-spinel and perovskite from three olivine lamproites in the Ellendale Field of the West Kimberley Province (Australia) in order to understand the composition and evolution of their parental magmas.Melt inclusions in the different host minerals and from each of these localities are broadly similar to each other and consist of glass, alkali/alkali-earth (Mg-Ca-K-Na-Ba) carbonates, phosphates and chlorides, in addition to minerals typical of lamproite groundmass (fluorapatite, perovskite, phlogopite, diopside, wadeite, Mg-ilmenite, Fe-Mg-Ti-Cr spinel). The dominant volatile species in the melt and fluid inclusions is CO2 based on Raman data. Heating experiments of melt/fluid inclusions in olivine show significant phase transformations in which the carbonate may separate into an immiscible carbonate-rich sulphate-bearing fraction or exsolve a CO2 fluid.Our results indicate that carbonates, along with alkali/alkali-earths, halogens and sulphur, became progressively concentrated in the West Kimberley lamproitic magmas during crystallisation, leading to the entrapment of a complex array of daughter minerals, some not previously reported from lamproites and, in some inclusions, immiscible carbonate melt. The widespread occurrence of daughter carbonates in melt/fluid inclusions in lamproite minerals is at odds with their apparent paucity in the lamproite groundmass. The presence of carbonate and the abundance of CO2-rich and H2O-poor melt and fluid inclusions are attributed to the preferential partitioning of CO2 into the vapour and retention of H2O in the magma during degassing, coupled with H2O loss by post-entrapment modification of the inclusions through H+ diffusion.

Formation of mixed paragenesis diamonds during multistage growth – Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds

Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% − an unusually high proportion − contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth.Based on the δ13C–δ15N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; δ13C = –33.2 to −14.4‰ and δ15N = −5.3 to +10.1‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; δ13C = −6.0 to −1.1‰ and δ15N = −4.2 to +9.7‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; δ13C = −7.8 to −3.6‰ and δ15N = −7.9 to −2.1‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.

The chromian spinels of the Lyavaraka ultrabasic complex, Serpentinite Belt, Kola Peninsula, Russia: Patterns of zoning, hypermagnesian compositions, and early oxidation

ABSTRACTThe maximum value of Mg# [= 100Mg/(Mg + Fe2+ + Mn)] in chromium-bearing spinel-group minerals (Chr) in the Ultrabasic Core Zone (UCZ) of the Lyavaraka orthopyroxenite – harzburgite – dunite complex of the Serpentinite Belt in the Kola Peninsula is 54.5–67.5. Such highly magnesian compositions of spinel are associated with notable enrichments of ferric iron (Fe3+# 58–63). There are two generations of accessory Chr in the UCZ unit. The first generation occurs as inclusions in olivine that is not unusually magnesian (Mg# 90.3), and the second is closely associated with serpentine. The compositional series of Chr at Lyavaraka attains more aluminous compositions than was observed in nearby intrusive bodies. The anomalously high level of Mg in Chr, also manifest in ilmenite, is mainly a result of the high intrinsic fugacity of oxygen attained locally in the melt. A progressive buildup in H2O and increase in fO2 likely resulted from efficient vesiculation and selective loss of H2 from the Al-undepleted komatiitic magma crystallizing in a shallow setting. The chromian spinel forming in such a modified magma is virtually unzoned in Mn, and a minor quantity of Mn is also present in olivine and orthopyroxene. In contrast, zinc is strongly partitioned in the core of Chr, as it is relatively incompatible in the coexisting olivine and orthopyroxene at that stage. Zinc efficiently partitioned into the H2O-enriched melt, which crystallized as the pegmatitic orthopyroxenite near the contacts at Lyavaraka. A high potential of oxidation appears to be characteristic of all orthopyroxenite – harzburgite – dunite suites of the Serpentinite Belt formed from a primitive melt of komatiitic composition.

Enhanced weathering in the seabed: Rapid olivine dissolution and iron sulfide formation in submarine volcanic ash

Abstract In basaltic volcanic ash recovered from a seamount at 3000 m water depth, we discovered marcasite and pyrite precipitation within cavities that formed by partial to complete dissolution of olivine. In places, these cavities are reminiscent of negative crystal shapes; elsewhere they apparently continue along cracks. In strong contrast, adjacent volcanic glass shows little, if any, evidence for dissolution. The FeS2 precipitates were commonly found to be conjoined and planar aggregates, occurring in the center of the voids. Their maximum volume fraction in relation to the void space as determined by 2D and 3D imaging techniques corresponds to the amount of iron released by olivine dissolution. Almost all occurrences of FeS2 precipitation are related to Cr-spinel inclusions in the former olivine. We propose that rapid olivine dissolution was initiated by reduced H2S-bearing fluids at olivine grain boundaries or surfaces exposed by cracks. Many of these cracks are connected to spinel grains, where the iron liberated from olivine is mineralized as FeS2, initially facilitated by heterogeneous nucleation. Subsequent pyrite and/or marcasite precipitation occurred as overgrowths on existing FeS2 aggregates. The particular chemical environment of low-pH, hydrogen sulfide-bearing fluids may have enhanced olivine dissolution by (1) keeping Fe in solution and (2) sequestering important quantities of Fe as FeS2. The in situ oxidation of ferrous Fe and precipitation of ferric hydroxides at the olivine surface commonly observed in oxic environments were obviously impeded. It would have slowed down olivine dissolution to rates more similar to the dissolution of basaltic glass. We have no direct indication that the process of rapid olivine dissolution was aided by subseafloor life. However, the presence of fibrous structures with small sulfide particles could indicate late colonization of sulfate-reducing bacteria that may add an additional path of iron fixation.

Oxygen Isotopic Compositions in a Plagioclase‐Olivine Inclusion from Ningqiang Similar to Those in Al‐rich Chondrules

AbstractWe report the petrology and oxygen isotopic composition, using a Cameca NanoSIMS 50L ion microprobe, of a plagioclase–olivine inclusion, C#1, found in the Ningqiang carbonaceous chondrite. In addition to major phases (plagioclase, spinel and olivine), C#1 is also surrounded by a pyroxene rim (64 vol % Ca‐rich and 36 vol % Ca‐poor pyroxenes). On a three‐isotope oxygen diagram, δ17O vs. δ18O, the compositions of individual minerals analyzed in C#1 fall along the carbonaceous chondrite anhydrous mineral line (CCAM), and oxygen isotopic compositions in C#1 show significant variability in δ18O and δ17O. The oxygen isotopic compositions of the pyroxene rim minerals are similar to those of the other host minerals, which suggests that the rim likely formed from the same melting process as the host. The rim is considered to have formed as a result of interaction between an 16O‐poor gas and a melt. Some spinel grains are typically 16O‐rich and likely of relict origin, which is similar to 16O‐rich Ca‐, Al‐rich inclusions, which are probably a precursor of C#1. The inclusion then likely melted in an 16O‐poor region where chondrules form, accompanied by oxygen isotope exchange with an 16O‐poor gas. Some anorthite, pyroxene and spinel might have undergone fluid‐assisted thermal metamorphism on the Ningqiang chondrite parent body. The oxygen isotope data and evolution of the C#1 plagioclase–olivine inclusion are similar with those of Al‐chondrules in chondrites.

Structural and Compositional Features of Garnet-Containing Samples Synthesized in a System with Samarium at High Pressure and High Temperature

A distinctive feature of garnets associated with diamonds is specific containing of “light” rare earth elements. In the paper, the garnet-containing samples obtained at high pressure and high temperature in the system introduced with samarium (Sm) are studied. The experiments are carried out using a multianvil high-pressure apparatus of the “split-sphere” type (BARS) at a pressure of 5 GPa and a temperature of 1300 °С. The accuracy of measuring the pressure and temperature is ± 0.2 GPa и ± 25 °С, respectively. As a result, pyrope grains are synthesized with a CaO content no higher than 0.15 wt.% and Cr2O3 concentration within the range of 3.61-7.55 wt.%. The garnets are characterized by the stable presence of an impurity in the form of the Sm constituent. The garnets contain a significant amount of olivine inclusions. Crystals of the synthesized spinel are observed mainly in the interstices. This study demonstrates that the interaction of the components in the serpentine — chromite — corundum — Sm system leads to the crystallization of pyrope garnet, which forms large intergrowths of individual grains. The zoning observed in garnet is due to the transfer of components by fluid during the experiment. It is concluded that the Sm content in garnet can significantly increase depending on its content in the system.

Amphibole geochemistry of the Baima layered intrusion, SW China: Implications for the evolution of interstitial liquid and the origin of Fe-Ti oxide ores

The Baima layered intrusion hosting a giant Fe-Ti oxide ore deposit in the Panxi region (SW China) is mainly composed of troctolite and olivine gabbro in the lower zone (LZ) and isotropic gabbro, olivine gabbro and apatite olivine gabbro in the upper zone (UZ). Amphibole, as a late-stage crystalized phase, occur throughout the Baima intrusion in several forms and can be used to trace the evolution of interstitial melt. The brown amphibole commonly occurs as rims on Fe-Ti oxides, interstitial granular grains, and inclusions in olivine and clinopyroxene. They are abundant in the Fe-Ti oxide ore layers. The different occurrences have similar major and trace element compositions (1.60–4.71 wt% TiO2, 68–160 ppm REE; 2.35–5.25 wt% TiO2, 55–159 ppm REE; and 5.24–5.33 wt% TiO2, 158–177 ppm REE, respectively). They crystallized in the magmatic stage (948–1033 °C, 954–1041 °C, and 983–1015 °C, respectively). The olive amphibole commonly occurs as granular grains at the base of the UZ. These crystals have 0.04–1.17 wt% TiO2 and 24–80 ppm REE, which are lower than those of the brown amphibole, and they crystallized at 634–755 °C. The green amphibole commonly occurs as granular grains, with <1.59 wt% TiO2, 5–144 ppm REE, and a formation temperature of 649–814 °C.The higher REE contents of the brown amphibole compared to the olive amphibole suggest that immiscibility occurred in the interstitial melt at the late magmatic stage of the Baima intrusion. The absence of negative Ti anomalies in the brown amphibole and their textural relationship indicate that the Fe-Ti oxides crystallized at almost the same time as the brown amphibole. Both the brown amphibole and the Fe-Ti oxides crystallized from an interstitial, immiscible Fe-rich liquid and then formed the Fe-Ti oxide ores. The evolved Si-rich melt migrated upward and mixed with the upper melts, which ended the olivine crystallization at the base of the UZ. The olive amphibole crystallized from the mixed melts. The green amphibole exhibits various REE patterns, suggesting secondary alteration.

In situ quantification of the nitrogen content of olivine-hosted melt inclusions from Klyuchevskoy volcano (Kamchatka): Implications for nitrogen recycling at subduction zones

Assessing the N content of arc magmas and their mantle source remains a challenge because the volatile element composition of melts and gases can be modified during magma ascent, storage, and eruption. Given that melt inclusions (MIs) in Mg-rich olivine represent the best proxies for primary arc melts, we applied, for the first time, an in situ high-resolution secondary ion mass spectrometry (SIMS) method to determine the N concentration in olivine-hosted MIs from Klyuchevskoy volcano in Kamchatka. To reverse the effects of post-entrapment modification processes (i.e., exsolution of volatiles into a fluid bubble), the MIs were partially to completely homogenized at high temperatures (1150–1400 °C) and pressures ranging from 0.1 to 500 MPa under dry to H2O-saturated conditions at variable oxygen fugacities (CCO to QFM + 3.3). After the experiments, N concentrations in water-rich MI glasses correlate positively with H2O and CO2 contents as well as with N/CO2 ratios, and negatively with the volume of the remaining fluid bubble. Glasses of completely homogenized (fluid bubble-free) MIs contain up to 25.7 ± 0.5 ppm N, whereas glasses of three unheated (natural, bubble-bearing) MIs have significantly lower N concentrations of ~1 ± 0.3 ppm. The N-CO2-Nb characteristics of completely homogenized MIs indicate that melts feeding Klyuchevskoy volcano have high absolute concentrations of both N and CO2, as well as large excess of these volatiles relative to Nb, compared to primary mid-ocean ridge melts. This implies that large amounts of N and CO2 in Klyuchevskoy melts and their mantle source are derived from the subducting slab, and that these subducted volatiles are (partially) returned to the crust and atmosphere by arc-related magmatism.

Melilite-olivine neрhelinites of Mt. Tabaat (Makhtesh Ramon, Israel): Geological, petrographic and geochemical characteristics and conditions of genesis

Research subject. The melilite-olivine nephelinite subvolcanic body Tabaat, which includes melilite rocks found for the first time on the territory of Levant.Materials and methods. The chemical composition of minerals (about 400 analyzes) was determined out on a CAMECA SX-100 microanalyzer equipped with five wave spectrometers with crystal analyzers TAP, LPET and LLIF. The elemental composition was measured at an accelerating voltage of 15 kV, an electron beam current of 40 nA. The oxygen concentration was calculated from the condition of stoichiometric composition of silicate minerals and chromite. In addition, the results of earlier studies of minerals (150 analyses) and data on melt inclusions were used.Results. The Tabaat subvolcanic body, which is part of the Early Cretaceous olivine-basalt-basanite-nephelinite association Makhtesh Ramona (Negev, Israel), has a complex concentrically-zonal structure, with olivine melanephelinites in the peripheral zone, melilite-olivine melanephelinites in the central and connecting zones normative and, less often, melilite-containing melanephelinites. Prismatic separation is widely manifested in the rocks. The fold-like bending and concave-curved edges of the prisms are a reflection of the plastic state of the cooling body and its ability to compress and accommodate a high fluid pressure, which develops during the formation of melilite nephelinites.Conclusion. All mineral diversity of rocks of the Mt. Tabaat is a derivative of a single portion of magmatic melt under conditions of its adiabatic cooling at the place of stabilization. A special role in the course of crystallization of the massif belongs to minerals with a high water content – analcime, zeolites, iddingsites, bowlingites and saponite-celadonites, which indicate the deuteric stage of its development. The study of melt inclusions in olivine and clinopyroxene showed the continuity of their composition with the composition of host melilite nephelinites and the importance of incongruent melting during the formation of melilite, which is a product of the reaction of nepheline with olivine or clinopyroxene.

Mafic-ultramafic intrusion formed by multi-stage evolution of hydrous basaltic melts

AbstractThe magmatic processes beneath the active continental margins are very complicated and affect structures and compositions of the arc roots. Neoproterozoic igneous rocks are widely distributed around the margins of the Tarim Block in NW China. The Xingdier mafic-ultramafic intrusion is a composite body, located at the northern margin of the Tarim Block, and consists of gabbro, pyroxenite, and peridotite units. The gabbro unit has a secondary ion mass spectrometry zircon U-Pb age of 727 ± 5 Ma. Rocks from the Xingdier intrusion have a large range of MgO (12.9–32.8 wt%) and SiO2 (43.0–57.9 wt%), and low K2O + Na2O (0.11–2.25 wt%) contents. They have right inclined chondrite-normalized rare earth element patterns with (La/Yb)N ratios of 2.2–8.6. Their primitive mantle normalized trace element patterns show arc-affinity geochemical features characterized by enrichment in Rb, Ba, Th, U, and Pb and depletion in Nb, Ta, and Ti. They have variable initial 87Sr/86Sr ratios (0.7063–0.7093), εNd(t) values (−2.9 to −7.8), 206Pb/204Pb (17.08–17.80), 207Pb/204Pb (15.42–15.49), and 208Pb/204Pb ratios (37.48–38.05), forming an evolution trend from the peridotite unit to the gabbro and pyroxenite units. Clinopyroxene in the three units is chemically similar to those formed in hydrous magmas. The spinel inclusions in olivine from the peridotite unit show unmixing texture and have high Al contents and oxygen fugacity of ~FMQ+1. Therefore, the parental magma was probably derived from a lithospheric mantle enriched by slab-derived fluids. Rocks from the gabbro and peridotite units are proposed to have been derived from olivine-normative melts, whereas rocks from the pyroxenite unit are cumulates from the quartz-normative melts. Such contrasting parental magmas resulted from variable degrees of crustal contamination and fractional crystallization in the arc root.

The petrologic and degassing behavior of sulfur and other magmatic volatiles from the 2018 eruption of Kīlauea, Hawaiʻi: melt concentrations, magma storage depths, and magma recycling

Kīlauea Volcano’s 2018 lower East Rift Zone (LERZ) eruption produced exceptionally high lava effusion rates and record-setting SO2 emissions. The eruption involved a diverse range of magmas, including primitive basalts sourced from Kīlauea’s summit reservoirs. We analyzed LERZ matrix glasses, melt inclusions, and host minerals to identify melt volatile contents and magma storage depths. The LERZ glasses and melt inclusions span nearly the entire compositional range previously recognized at Kīlauea. Melt inclusions in Fo86-89 olivine from the main eruptive vent (fissure 8) underwent 70–170 °C cooling during transport in LERZ carrier melts, causing extensive post-entrapment crystallization and sulfide precipitation. Many of these melt inclusions have low sulfur (400–900 ppm) even after correction for sulfide formation. CO2 and H2O vapor saturation pressures indicate shallow melt inclusion trapping depths (1–5 km), consistent with formation within Kīlauea’s Halemaʻumaʻu and South Caldera reservoirs. Many of these inclusions also have degassed δ34S values (− 1.5 to − 0.5‰). Collectively, these results indicate that some primitive melts experienced near-surface degassing before being trapped into melt inclusions. We propose that decades-to-centuries of repeated lava lake activity and lava drain-back during eruptions (e.g., 1959 Kīlauea Iki) recycled substantial volumes of degassed magma into Kīlauea’s shallow reservoir system. Degassing and magma recycling from the 2008–2018 Halemaʻumaʻu lava lake likely reduced the volatile contents of LERZ fissure 8 magmas, resulting in lower fountain heights compared to many prior Kīlauea eruptions. The eruption’s extreme SO2 emissions were due to high lava effusion rates rather than particularly volatile-rich melts.

Petrologic evolution of boninite lavas from the IBM Fore-arc, IODP Expedition 352: Evidence for open-system processes during early subduction zone magmatism

Abstract Boninite samples from several intervals within Hole U1439C, recovered during IODP Expedition 352, show highly variable mineral chemistries that imply complex crystallization histories. Small pyroxene grains show oscillatory zoning with cores and zones ranging from pigeonite to augite. Late crystallizing augite has highly variable Al2O3 contents (1.9–13.7 wt%) and Ca-Tschermak component contents (3–13 mol%), which reflect disequilibrium conditions. Large, euhedral, low-Ca pyroxene (i.e., enstatite/clinoenstatite) crystals exhibit complex sector and oscillatory zoning patterns. Cr-rich spinel is found as inclusions both in olivine and low-Ca pyroxene. Early crystallized olivine phenocrysts have embayed and reacted margins, and some early crystallized olivines exhibit zoning. A few olivines have multiple zones, with both normal and reverse zoning between Fo86–92. Olivine xenocrysts also have embayed and reacted margins; however, xenocrysts do not exhibit chemical zoning patterns and have consistent Fo88 compositions. Disequilibrium crystallization of pyroxene rims reflects rapid cooling during an eruption. Sector zoning in low-Ca pyroxenes is the result of crystallization during periods of moderate undercooling between mixing events. Oscillatory, normal, and reverse zoning in olivine and pyroxene appears to have formed in response to multi-stage magma mingling or mixing processes, which introduced additional Ca, Fe, Ti, and Al into parental boninitic melts. The presence of olivine xenocrysts and orthopyroxene indicate equilibrium at 2–4 kbar (Whattam et al. 2020) indicates that boninite magma mixing events likely occurred within shallow magma chambers containing olivine-rich cumulate piles. Large mixing events probably destabilized the magma chamber, resulting in devolatilization and eruption. In contrast, small mixing events lacked the energy to destabilize the chamber, resulting in repeated compositional oscillations in minerals affected by multiple small mixing events.

Origin of Ru-Os Sulfides from the Verkh-Neivinsk Ophiolite Massif (Middle Urals, Russia): Compositional and S-Os Isotope Evidence

This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.