Olivine Inclusions Research Articles

The Study of Olivine Inclusions in Diamonds from Liaoning, China and the Evaluation of Related Thermometers

Olivine is the most abundant mineral in mantle peridotite and a typical inclusion in diamonds, providing essential evidence for the characterization of the diamondiferous lithospheric mantle. Three olivine inclusions in diamonds (OlDia) from Liaoning in the North China Craton (NCC) were exposed for in situ measurements, and the compositional data of 62 other OlDia from Liaoning were collected based on previous reports. The enrichment of TiO2 (>0.1 wt.%) with high Cr# (>50; Cr# = 100Cr/(Cr + Al) by atom) was revealed, despite the predominance of depleted TiO2 contents and high Mg# (92.8–93.2; Mg# =100Mg/(Mg + Fe) by atom) for OlDia. Silica fluid accompanying olivine still trapped in the host diamond was recognized using Raman spectroscopy. Three thermometers were applied to the OlDia, based on the data from Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Electron Probe Micro-Analysis (EPMA), and a temperature range (TLA and TEPMA) of 1080–1380 °C was yielded. With respect to the TLA, Al-in-olivine thermometers were preferred, although there was a smaller deviation between TEPMA and TLA when using the Cr-in-olivine thermometer. The results of these thermometers show a high correlation with TEPMA, enabling their application based on EPMA data. Projections onto 39–42 mW/m2 model geotherms underline a diamondiferous base of the lithospheric mantle beneath the NCC. The lithospheric mantle is characterized by refractory and depleted sections, where enrichment metasomatism may have occurred at the lower roots (161–178 km).

Chromitites and associated mineralization of the Akkarga ophiolitic massif in the southeastern Urals (Russia)

The Akkarga ultramafic massif exposed in the Trans-Uralian megazone represents the mantle section of an ophiolite assemblage that was emplaced into the upper crust during the Permian collision. Ultramafic rocks include harzburgites and subordinate dunites that have undergone complete serpentinization and host podiform chromitite bodies typical of the ophiolite complexes worldwide. Numerous lens-shaped and podiform occurrences of massive and densely disseminated chromitites are surrounded by envelopes of serpentinized dunites whereas nodular chromitites and lenticular bodies of banded disseminated ores are of subordinate importance. Three ore zones are distinguished on the massif, including the Western, Central and Eastern, but only the sites of the latter two are currently accessible for study. Chromite from chromitites of the Central Ore Zone has higher Cr# (Cr/Cr + Al) (0.81–0.83) than that of chromitites of the Eastern Ore Zone (Cr# = 0.67–0.80) and accessory chromite of peridotites (Cr# = 0.52–0.75). Chromites contain mineral inclusions, which are distributed unevenly. The most abundant are inclusions of high-Mg high-Ni olivine (Fo94–98 and up to 1.5 wt% NiO) and calcic amphibole, while inclusions of pyroxenes and base metal sulfides are less common. Platinum group minerals (PGMs) in chromitites are represented by alloys, sulfides, and sulfoarsenides, which occur in single-mineral and composite inclusions. Ruthenium and Os disulfides typically compose the euhedral single-mineral inclusions in cores of chromite crystals, whereas the composite inclusions, mostly of irregular shapes, are dominated by Ir compounds. PGMs are regularly associated with OH-bearing silicates such as amphibole and, less frequently, chlorite. The setting, morphology and composition of the inclusions seem to support a leading role of subsolidus solid-state exsolution in the formation of primary laurite-erlichmanite mineralization in Akkarga chromitites. Subsequent hydrothermal reworking of podiform chromitites and their ultramafic hosts, which is likely related to the supra-subduction setting, led to the precipitation of more diverse interstitial assemblages, comprising base-metal sulfides, nickeline, sulfoarsenides of the Ir-subgroup platinum group elements (IPGE), REE phosphates, zircon, barite, and baddeleyite. Later granite intrusions likely provided an additional contribution of fluid-mobile incompatible elements to chromitites.

Confocal Raman spectroscopic study of melt inclusions from peridotite xenoliths in economic and barren kimberlites from Kaapvaal Craton

AbstractMelt inclusions trapped in minerals inside xenoliths from kimberlites can help to examine the composition of kimberlite melt and/or metasomatic processes in the subcratonic lithospheric mantle as well as shed more light on the role of these melts in diamond destruction. In this study, confocal Raman spectroscopy of secondary melt inclusions in olivine from xenoliths in five different Kaapvaal Craton kimberlites was used for testing any compositional differences between melt inclusions from economically productive (Bultfontein and Frank Smit) and uneconomic diamond barren (Matsoku, Thaba Putsoa, and Pipe 200) kimberlite pipes. The xenoliths represent a range of pressures (37–45 kbar) and temperatures (1000–1300°C). The 26 daughter minerals identified within melt inclusions include Ca–Mg (±Na, K, P, Cl)‐bearing carbonates, alkali (±Ca, Ba, Cl, F, H2O, CO2)‐bearing sulfates, phosphates, oxides, silicates, and a rare nitrate. The mineral assemblages in melt inclusions are similar in both economic and barren kimberlite pipes from the interior and the edge of the craton, indicating the similar composition of the entrapped melts in all studied samples. However, the petrographic study revealed different metasomatic processes recorded by xenoliths from barren and economic kimberlites. Metasomatism by a melt enriched in K, Ca, and H2O could be instrumental in diamond destruction and the low diamond grade of the three barren kimberlites from Lesotho. Our study revealed no effect of kimberlite melt composition on diamond preservation in the studied kimberlites: instead, diamond grade is most likely affected by diamond destruction in the mantle source prior to kimberlite emplacement with kimberlite ascent.

Melt infiltration in a crystal mush and pegmatoid formation in the platiniferous Merensky Reef, Bushveld Complex, South Africa

AbstractGiant mafic-ultramafic layered intrusions of Archaean-Proterozoic age are the fossilised remnants of huge injections of silicate magma in the Earth’s crust and are our most important repositories of platinum-group elements. Magmatic PGE-rich ore deposits, such as the Merensky Reef, are typically hosted in stratiform reefs at the contacts between ultramafic and feldspathic cumulates. The Merensky Reef is commonly characterised by coarse-grained and pegmatoidal textures that may provide important clues to its origin. We present textural and in situ geochemical data for Merensky pegmatoids at Styldrift Mine (Impala Bafokeng) in the Western Bushveld Complex of South Africa. This region is adjacent to an inferred magmatic feeder zone to the Bushveld. The Merensky pegmatoids are characterised by (i) amoeboid olivine inclusions in zoned orthopyroxene megacrysts with increasing molar Mg# of orthopyroxene towards olivine, (ii) fine-grained chains of orthopyroxene in compositional equilibrium with adjacent orthopyroxene megacrysts, (iii) increasing molar Mg# of orthopyroxene megacrysts and increasing molar An with decreasing 87Sr/86Sri (at 2.06 Ga) of plagioclase oikocrysts in pegmatoids laterally across a 10-km section distal to the feeder, and (iv) highly variable molar An and initial 87Sr/86Sri of interstitial plagioclase proximal to the feeder. We interpret the coarse-grained and pegmatoidal textures, their dissolution-reprecipitation features, and lateral chemical variations as the product of lateral melt infiltration and mixing in a crystal mush. We suggest that the platiniferous Merensky Reef was not formed at the base of a large melt-filled magma chamber but was instead the product of non-sequential magma emplacement that rejuvenated the crystal mush.

Confocal μ-XANES as a tool to analyze Fe oxidation state in heterogeneous samples: the case of melt inclusions in olivine from the Hekla volcano

Abstract. Here we present a confocal Fe K-edge μ-XANES method (where XANES stands for X-ray absorption near-edge spectroscopy) for the analysis of Fe oxidation state in heterogeneous and one-side-polished samples. The new technique allows for an analysis of small volumes with high spatial 3D resolution of <100 µm3. The probed volume is restricted to that just beneath the surface of the exposed object. This protocol avoids contamination of the signal by the host material and minimizes self-absorption effects. This technique has been tested on a set of experimental glasses with a wide range of Fe3+ / ΣFe ratios. The method was applied to the analysis of natural melt inclusions trapped in forsteritic to fayalitic olivine crystals of the Hekla volcano, Iceland. Our measurements reveal changes in Fe3+ / ΣFe from 0.17 in basaltic up to 0.45 in dacitic melts, whereas the magnetite–ilmenite equilibrium shows redox conditions with Fe3+ / ΣFe ≤0.20 (close to FMQ, fayalite–magnetite–quartz redox equilibrium) along the entire range of Hekla melt compositions. This discrepancy indicates that the oxidized nature of glasses in the melt inclusions could be related to the post-entrapment process of diffusive hydrogen loss from inclusions and associated oxidation of Fe in the melt. The Fe3+ / ΣFe ratio in silicic melts is particularly susceptible to this process due to their low FeO content, and it should be critically evaluated before petrological interpretation.

The December 2018 eruption at Etna volcano: a geochemical study on melt and fluid inclusions

This study focus on the Mt Etna December 2018 eruption with the aim of investigating the geochemical characteristics of the feeding magma. New data on major and trace element geochemistry of olivine-hosted melt inclusions (MI) in volcanic products are presented together with the noble gas geochemistry of fluid inclusions (FI) in olivines. The noble gas geochemistry of fluid inclusions (FIs) in olivines was also investigated. The major element composition of MIs is variable from tephrite/trachybasalt to phonotephrite/basaltic trachyandesite, with SiO2 = 45.51–52.72 wt%, MgO = 4.01–6.02 wt%, and CaO/Al2O3 = 0.34–0.72. Trace element patterns of MIs present a typical enrichment in LILE and LREE, depletion in HFSE, and relatively fractionated REE patterns: (La/Lu) N= 18.8–41.08, with Eu/Eu* = (0.5–1.8). Positive anomalies in Sr (Sr/Sr* = 0.8–2.3) and Ba can be ascribed to the assimilation of plagioclase-rich cumulates in the magmatic reservoir. The variable Ba/La (9.8–15.8), K/Nb (260–1037), Ce/Nb (1.9–3.4), Rb/La (0.4–1.6), and Ba/Nb (10.8–25.8) ratios reveal mixing between two types of end-member magmas comparable to those emitted from 1) the 2001 Upper Vents and 2002–03 Northern Fissures (Type-1) and 2) the 2001 Lower Vents and 2002–03 Southern Fissures (Type-2), respectively. Type-2 represents a magma that was under the influence of a crustal component, whereas Type-1 is compatible with a HIMU–MORB-type heterogeneous mantle source. It appears that the 2018 MIs have captured the two different types of magmas, and the lack of homogenization may imply a very fast ascent (a few months). Compatible with the contemporary presence of primordial HIMU–MORB and crust-contaminated end-members are the data on noble gases from FI that highlighted an 3He/4He value of 6.5–6.6Ra. The hypothesis of two different types of magmas, identified by the trace element geochemistry in MIs, is, thus, reinforced by helium isotopic data on FI of the 2018 eruption together with data from other Etnean eruptions and allows the inference of a bicomponent magma mixing.

SO2 emissions from the Timanfaya eruption (1730–36 CE), Lanzarote, Canary Islands

The 1730–36 CE eruption of Lanzarote (Canary Islands) is the third largest basaltic fissure eruption known to have occurred in the last 1000 years, after the Icelandic events of Laki (1783–84 CE) and Eldgja (939 CE). Approximately 5.2 km3 (1.1 × 1012 kg) of alkalic basalt (sensu lato) lava and pyroclastic rocks were emplaced on land, with a further unknown amount entering the sea, The activity occurred in five eruptive phases from vents situated along a 15-km long, E-W trending fissure. Each phase began with Strombolian fire fountain activity, building large spatter and scoria cones, accompanied and followed by effusive ‘a'ā and pāhoehoe lava flow emplacement. Phases I and II were the most explosive and occurred between 1st September 1730 and June 1731. Samples of scoria and lava surfaces were collected from all phases and the major elements, S, Cl and F contents of matrix glasses and glass inclusions in olivine (the dominant phenocryst phase) were analysed by electron microprobe. Water contents of selected glasses were measured on polished wafers by Fourier Transform Infrared spectroscopy. Glass compositions mimic the range of published whole rock compositions, with Phase I being predominantly basanitic and later phases alkali basalt. Whole rock variations in incompatible trace element ratios are mimicked by ratios involving TiO2, K2O and P2O5 in glass. Matrix and glass inclusion analyses in a given sample are similar, indicating that the inclusions formed synchronously with eruption and that most phenocryst crystallization occurred during earlier evolution of the magmas. Matrix glasses in scoria and surface lava samples have 80–300 ppm S and 260–750 ppm H2O, whereas glass inclusions in olivine have 420–2650 ppm S and 0.1 to 0.5 wt% H2O. Glass inclusions with low sulfur contents were trapped during degassing-induced crystallization as the magma approached the surface. The inclusions with the highest sulfur contents, at given P2O5/TiO2 ratio, are inferred to represent un-degassed melt, and their S content and S/K2O ratios correlate very strongly with incompatible element concentrations and P2O5/TiO2 ratios. The original S contents of 131 matrix glasses were calculated from their P2O5/TiO2 ratios and compared to their measured degassed S contents. The mass of S released per unit mass of magma ranged from about 0.25% in Phase I Episode 1to 0.17% in later phases, with propagated uncertainties of about 2 to 5%. About 40 Mt. of SO2 was released overall, with ca. 25 Mt. being released during the first 5 months of the eruption (Phase I). Estimates of average magma discharge rates imply that plumes remained in the troposphere, with the possible exception of those associated with extreme peak discharge rates during Phase I Episode 1 which may have just reached the upper troposphere – lower stratosphere. Lower eruption rates during the later phases would have confined SO2 emissions to the troposphere. Although acid and sulfate peaks dated to 1730 and 1731 have been reported from some Greenland ice cores, the evidence for stratospheric impact, such as cooling at the Earth's surface and increased stratospheric turbidity, is minimal.

EVOLUTION OF THE VICTORIA PIPE ULTRAMAFIC ALKALINE MELT (ANABAR REGION, SIBERIAN CRATON): MELT INCLUSION WITHIN OLIVINE AND GROUNDMASS MINERALS

To provide new insights into evolution of the primary ultramafic alkaline melts, we present an investigation of the primary and secondary melt inclusions within olivine macrocrysts and groundmass minerals from the Victoria monticellite-nepheline damtjernite, Anabar diamondiferous province, Siberia craton. The primary melt inclusions within olivine macrocrysts shown that initial damtjernite melts were K–Na bearing carbonate-silicate melts by composition whereas the K and Na associated within only silicate phases like that phlogopite, nepheline, kalsilite. That distinguishes these inclusions from similar inclusions in olivines from aillikites and kimberlites and emphasizes a more alkaline character of the damtjernitic parental melts. Based on the composition of melt inclusions within spinel and monticellite from magmaclastic groundmass, under the further evolution of the damtjernitic melts, K and Na are not only included in silicate daughter phases, but they can also form alkaline phosphates, carbonates, sulfates, and halides. That led to form the alkaline carbonate and saline sulfate-phosphate-chloride-carbonate liquids. This composition of the evolved ultramafic alkaline melt is common for melt inclusions within different minerals from aillikite, kimberlite and some carbonatites highlighted uniform mechanism of evolution of alkaline-ultramafic melts. Further accumulation of the fluid phase led to its reaction interaction with olivine with the formation of monticellite and degassing processes.

COMPOSITION OF PHENOCRYSTS OF LAMPROITE LAVA, GAUSSBERG VOLCANO, EAST ANTARCTICA

This paper presents numerous new data on the geochemical composition of olivine, clinopyroxene, and leucite phenocrysts, as well as spinel inclusions in olivine and quench glass from lamproites of the Gaussberg Volcano (East Antarctica). Most of the olivine phenocrysts in the Gaussberg lamproites are high Mg varieties (Fo 89–91) with elevated Ni contents (up to 4900 ppm) and high Ni/Co ratios. According to the data of about 320 analyzes of clinopyroxenes, two groups of phenocrysts belonging to the diopside group have been established. Group I consists mainly of high-Mg varieties (Mg#80), while group II clinopyroxenes are less magnesian (Mg# 52–80). The main difference between the clinopyroxenes of the two groups is manifested in the increased contents of Al2O3, FeO and reduced TiO2, Cr2O3, and NiO in the compositions of group II compared to group I, as well as different contents of trace elements, which may reflect their crystallization from different types of primary melts. According to the study of about 550 grains of leucite phenocrysts in Gaussberg lamproites, it was shown that they correspond to the ideal stoichiometry of leucite K[AlSi2O6] and are enriched in Na2O (0.05–0.35 wt %), but depleted in K2O (19.9–20.9 wt %) compared to with leucites from lamproites of other provinces. The BaO content reaches 0.3 wt %. %, SrO –0.04 wt. %. The iron content in most leucite phenocrysts varies within 0.7–1.2 wt % Fe2O3, with individual grains with low Fe2O3 contents (0.5 wt %). In microlites of leucite in the groundmass and rims of phenocrysts, the iron content can reach 2.4 wt % Fe2O3, which may indicate more oxidized conditions at the time of lava eruption.Based on the study of natural samples, existing experimental data and computational models, the order and conditions of crystallization of Gaussberg lamproites were restored. Crystallization proceeded in the following order: chrome spinel - chrome spinel + olivine - olivine + leucite (± chrome spinel) - olivine + leucite + clinopyroxene (± chrome spinel). The near-liquidus association, represented by high-Mg olivine phenocrysts with inclusions of Cr-spinel, was formed in the temperature range from 1180 to 1250°C. Further crystallization of the melt with the formation of an association of olivine+leucite+clinopyroxene phenocrysts could occur at pressures below 2 GPa and temperatures of 1070–1180°C, corresponding to the presence of water in the magmatic system. Estimates of the redox conditions of crystallization of lamproites, obtained using different oxybarometers, vary in a wide range from QFM-0.5 to QFM+2.3.Elevated Ni contents in liquidus olivines of Gaussberg indicate high nickel contents in the source. It is shown that the possible formation of ultraalkaline magmas in the region of Gaussberg Volcano occurred during the melting of the continental lithosphere, which was heterogeneous and included both the peridotite mantle and hydrous pyroxenite fragments.

Composition of Phenocrysts in Lamproites of Gaussberg Volcano, East Antarctica

—This paper presents numerous new data on the geochemical composition of olivine, clinopyroxene, and leucite phenocrysts, as well as spinel inclusions in olivine and quench glass from lamproites of Gaussberg volcano (East Antarctica). Most of the olivine phenocrysts in the Gaussberg lamproites are high Mg varieties (Fo89–91) with elevated Ni contents (up to 4900 ppm) and high Ni/Co ratios. According to data of about 320 clinopyroxene analyses, two groups of diopsidic phenocrysts have been established. Group I consists mainly of high-Mg varieties (Mg#>80), while group II clinopyroxenes are less magnesian (Mg# 52–80). The main difference between the clinopyroxenes of the two groups is the elevated contents of Al2O3, FeO and reduced TiO2, Cr2O3, and NiO in the compositions of group II compared to group I, as well as different contents of trace elements, which may reflect their crystallization from different types of primary melts. According to the study of ~550 grains of leucite phenocrysts in the Gaussberg lamproites, it was shown that they correspond to the ideal stoichiometry of leucite K[AlSi2O6] and are enriched in Na2O (0.05–0.35 wt %), but depleted in K2O (19.9–20.9 wt %) compared to leucites from lamproites of other provinces. The BaO content reaches 0.3 wt %, SrO –0.04 wt %. The iron content in most leucite phenocrysts varies within 0.7–1.2 wt % Fe2O3, but some grains have the low Fe2O3 contents (<0.5 wt %). In leucite microlites of the groundmass and rims of phenocrysts, the Fe2O3 content can reach 2.4 wt %, which may indicate more oxidized conditions at lava eruption. Based on the study of natural samples, existing experimental data and numerical models, the order and conditions of crystallization of the Gaussberg lamproites were obtained. Crystallization proceeded in the following order: chromian spinel → chromian spinel + olivine → olivine + leucite (± chromian spinel) → olivine + leucite + clinopyroxene (± chromian spinel). The near-liquidus assemblage represented by high-Mg olivine phenocrysts with inclusions of Cr-spinel was formed in the temperature range from 1180 to 1250°C. Further crystallization of the melt with the formation of an association of olivine+leucite+clinopyroxene phenocrysts could occur at pressures below 2 GPa and temperatures of 1070–1180°C, corresponding to the presence of water in the magmatic system. Estimates of the redox conditions of crystallization of lamproites obtained using different oxybarometers vary in a wide range from QFM-0.5 to QFM+2.3. The elevated Ni contents in liquidus olivines of Gaussberg indicate the high nickel contents in the source. It is shown that the formation of ultra-alkaline magmas in the Gaussberg volcano area is likely related to melting of the continental lithosphere, which was heterogeneous and included both the peridotite mantle and hydrous pyroxenite fragments.

Evolution of the Victoria Pipe Ultramafic Alkaline Melt (Anabar Region, Siberian Craton): Melt Inclusion within Olivine and Groundmass Minerals

The results of studying primary and secondary melt inclusions within olivine macrocrysts and groundmass minerals from monticellite–nepheline damtjernites of the Victoria pipe (Anabar diamondiferous province, Siberia craton) are presented. They were used to reconstruct the evolution of the ultramafic alkaline melt during the formation of the pipe. It is shown that the primary damtjernite melts were K–Na bearing carbonate–silicate in composition. Here, K and Na in the primary melt inclusions within olivine macrocrysts enter the composition of essentially silicate daughter phases. According to this, they are different in composition from similar inclusions in olivine from aillikites and kimberlites and emphasize the more alkaline character of the damtjernitic parental melts. At the subsequent stages of melt evolution during pipe formation, Na and K in the studied inclusions not only enter silicate daughter phases but can also form alkaline phosphates, carbonates, sulfates, and halides. This leads to the formation of alkaline carbonate and saline sulfate–phosphate–chloride–carbonate solutions. According to this, the evolution of the damtjernitic melt is similar to that of aillikite and carbonatite melts and can serve as a uniform mechanism of evolution of alkaline–ultramafic melts. The further reactionary interaction of the fluid phase with olivine led to the formation of monticellite and processes of degassing.

Behavior of critical metals in cumulates of alkaline ultramafic magmas in the Siberian large igneous province: Insights from melt inclusions in minerals

Ultramafic-alkaline-carbonatite complexes (UACC), which are formed from mantle-derived carbonated alkali-ultramafic melts in large igneous provinces (LIPs), are important resources of Fe, Ti, U, Th, Nb, rare earth elements (REEs), Cu, Ni and platinum group elements (PGEs). Concentration of these metals and ore formation is assumed to be largely controlled by magmatic differentiation and post-magmatic hydrothermal processes. Although basic patterns of the metals’ partitioning during formation of the UACCs are constrained by geochemical and mineralogical features of the rocks, including in experimental studies, our understanding of their pathway “from primitive melt to ore deposit” is far from complete. In order to further constrain the metals’ behavior during differentiation of a carbonated alkali-ultramafic melt, we studied multiphase inclusions in olivine, chromite, perovskite, pyroxene and magnetite in the ultramafic rocks from three UACCs (Guli, Bor-Uryakh and Odikhincha), located in the Siberian LIP. Examination of both unheated and experimentally heated and quenched inclusions reveals a variety of compositions from melanephelinitic through to highly differentiated nephelinitic to alkali-rich carbonatitic. In addition, sulfide minerals, which turn into immiscible sulfide liquids during heating experiments, are widely distributed in the inclusions’ assemblages. We consider these inclusions to be snapshots of intercumulus melts, which were entrained into olivine-rich cumulate mush, and use their compositions to delineate plutonic differentiation of a carbonated alkali-ultramafic melt. Highly differentiated silicate melts, entrapped in Fe-rich chromite (Guli dunites), were rich in U, Th and Nb and crystallized Os-Ir-Ru phases in proximity to the host chromite. Concentrations of U, Th, Nb and REEs in alkali-rich carbonatite liquids, which were present in the intercumulus of Odikhincha and Bor-Uryakh peridotites, approached levels similar to mineralized carbonatites and support the concentration of these metals in an immiscible alkali-carbonatite fraction, which was enriched in S, P and Cl. Minor sulfide liquids, which are closely associated with these carbonatite fractions, were strongly enriched in Cu and Ni, thus explaining the origin of the Phalaborwa-like sulfide ores in carbonatites as a result of magmatic differentiation. Finally, our study provides insights into the formation of peridotite-hosted types of mineralization (perovskite-magnetite ores, mineralized carbonatite veins and PGE-bearing chromitites) and shows that these ore-bearing assemblages can be formed due to the infiltration of the metal-bearing intercumulus melts through the ultramafic matrix.

The multi-component mantle source of Roman province ultrapotassic magmas revealed by melt inclusions

A key effect of subduction is the upward infiltration of slab-derived melts and fluids to create a chemically and lithologically heterogeneous region in the mantle wedge. Magmatism within the post-subduction setting of peninsular Italy is derived from a sediment-metasomatised lithospheric mantle source, with primitive products recording substantial temporal and spatial variations in the nature and extent of metasomatism. The Roman magmatic province, in central Italy, is host to ultrapotassic (HKS), leucite-bearing magmas that require source modification that is not yet well-constrained in terms of mineralogy and sediment provenance. Here, we present the chemistry and Sr-Nd-Pb isotope compositions of melt inclusions in forsterite-rich olivine (Fo88−92) from three key Roman volcanic centres (Vulsini, Sabatini, Alban Hills) to characterise the metasomatic imprint of their mantle sources. The melt inclusion suites (Vulsini HKS, Vulsini melilite-bearing HKS, Sabatini HKS, Alban Hills HKS) have clear differences in major-element (K, Ca, Al, P, Ti), volatile (F, Cl, B, Be), trace-element (e.g., Rb, Sr/Y, Th/Nb), and radiogenic-isotope (87Sr/86Sr) compositions that reflect the diversity of the primary melts beneath the region. For each suite, the extraction of melt batches from different domains in a heterogeneous (veined) source rock with different compositions and mineralogies can explain the chemical characteristics of the melts. At least three metasomatic vein types are required to form the respective parental melts and we infer phase assemblages consisting of phlogopite + Ca-amphibole + clinopyroxene ± apatite. The Sr-isotope differences between Vulsini melilite-bearing HKS and Vulsini and Sabatini HKS require at least two distinct metasomatic events to have occurred beneath the Roman province. These events need not be associated with two distinct subduction systems. Instead, we infer that different metasomatic agents could have originated under different P-T-t conditions from different domains in the same subducting slab.

In situ single-crystal X-ray diffraction of olivine inclusion in diamond from Shandong, China: implications for the depth of diamond formation

Abstract. We have investigated a suite of natural diamonds from the kimberlite pipe of the Changma Kimberlite Belt, Mengyin County, Shandong Province, China, with the aim of constraining pressures and temperatures of formation. Here we report the non-destructive investigation of an olivine inclusion still entrapped within a lithospheric diamond by single-crystal X-ray diffraction. We were able to refine anisotropically its crystal structure to R1= 1.42 % using ionized scattering curves; this allows estimation of the composition of the olivine as Mg1.82Fe0.18SiO4. This composition corresponds to a calculated unit-cell volume equal to V= 292.70 Å3 at room temperature and pressure. We have validated the above-calculated composition and unit-cell volume by releasing the inclusion from the diamond host, resulting in a consistent composition calculated using non-destructive methods of Mg1.84Fe0.16SiO4 and V= 292.80 ± 0.07 Å3. Considering that the unit-cell volume of the olivine still inside its diamond host is V= 289.7 ± 0.2 Å3, we calculated a residual pressure Pinc= 1.4 ± 0.1 GPa with respect to the released crystal and Pinc= 1.3 ± 0.2 GPa with respect to the volume calculated from the “composition” indirectly retrieved by the structure refinement under ambient conditions. The two values of Pinc overlap within experimental uncertainty. We performed Fourier transform infrared (FTIR) analysis on the diamond host in order to calculate its mantle residence temperature, Tres, which resulted in a value of 1189 ∘C (for an assumed diamond age of 3 Ga) and 1218 ∘C (for an age of 1 Ga), with an average Tres equal to 1204 ± 15 ∘C. Using the most up-to-date pressure–volume–temperature equations of state for olivine and diamond, the residual pressure Pinc= 1.4 ± 0.1 GPa and average residence temperature of the diamond host Tres= 1204 ∘C, we retrieved a pressure of entrapment Ptrap= 6.3 ± 0.4 GPa. Using the non-destructive approach and relative Pinc = 1.3 GPa, we obtained a perfectly overlapping Ptrap= 6.2 GPa, within experimental uncertainty. This entrapment pressure corresponds to depths of about 190 ± 12 km. These results demonstrate that for high-quality crystal structure data measured on inclusions still trapped within diamond hosts, even a non-destructive approach can be used to calculate the depth of formation of diamond–olivine pairs. In terms of geological implications, the results from this work show that Changma diamonds formed under a conductive geotherm lying between 35 and 40 mW m−2, at a depth of about 190 km. This value lies within the recently reported upper limit of the average depth of formation of worldwide lithospheric diamonds, which is 175 ± 15 km and is in agreement with P–T data obtained in the literature from kimberlite xenoliths.

Magnesian Basalts of the Medvezhia Caldera: Main Magmas and Their Sources on the Example of the Menshiy Brat Volcano (Iturup Island)

The paper represents new data on the formation conditions of basalts from post-caldera volcano Menshiy Brat (Medvezhia caldera, Iturup Island). Liquidus mineral assemblage is composed of olivine (Fo up to 90.1 mol. %) and chromium spinel (Cr# = 0.46–0.6), which crystallized at 1090–1170°С and oxygen fugacity NNO +0.6 (σ = 0.2) – NNO +0.2 (σ = 0.14). The study of melt inclusions in the liquidus olivine demonstrated that its parental melts have low-alumina, low-potassium compositions with up to 15.5 wt. % MgO and with an average water content of 5.5 wt. %. The data on volatile contents in the olivine-hosted melt inclusions suggest that the basic melts appeared due to partial melting of essentially peridotitic source with a small admixture of an olivine-free component at 1225°С under active influence of the slab-related fluids. These fluids were separated from the subducting slab at 670–705°С and depths of 95–105 km beneath the Iturup Island. The results of this study enhance our understanding of the evolution of basic magmas that serve as a heat and volatile supply during the formation of large calderas.

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.

Origin of high-Cr podiform chromitites from Kabaena Island, Southeast Sulawesi, Indonesia: constraints from mineralogy and geochemistry

ABSTRACT The East Sulawesi Ophiolite, one of the three largest ophiolites in the world, contains important podiform chromitites. However, the origin of these chromite deposits is not well constrained. Here, we present the detailed mineralogy and in situ chemistry of chromite and solid inclusions within chromite for podiform chromitites from the Kabaena Island, Southeast Sulawesi. Chromite grains have low TiO2 (0.17–0.27 wt%) and Al2O3 (13.4–15.1 wt%) contents and high Cr# [Cr/(Cr+Al), molar] of 0.70–0.74 and Mg# [Mg/(Mg+Fe2+), molar] of 0.69–0.75, which resemble those of other high-Cr podiform chromitites worldwide. The MORB-normalized patterns for some selected major-trace elements of chromite show slightly positive slopes from Al2O3 to Mn. Chromite-hosted silicate inclusions include olivine, clinopyroxene, orthopyroxene and amphibole and are characterized by higher Mg/Fe ratios than those of host peridotites. The geothermobarometry of chromite-hosted clinopyroxene-orthopyroxene pairs indicates that the estimated T-P conditions of the parental magma of the Kabaena chromitites are 950–1010°C and 7.0–8.4 Kbar. The extreme-Mg-rich olivine inclusions (Fo >96) with extremely high Ni (5300–8200 ppm) and Cr (1500–6700 ppm) contents are interpreted to have crystallized from high-Mg and Cr boninitic melts, and the Fo contents were then elevated by subsolidus re-equilibration (Fe–Mg and Fe–Ni exchange) with chromite. Sulfide inclusions contain base metal minerals containing millerite with rare pentlandite and chalcopyrite, and platinum-group minerals that include the laurite-erlichmanite series, Ir–Ni monosulfide, irarsite and cuproiridsite. This sulfide assemblage reveals that their parental magmas experienced the evolution of high T and low f(S2) to low T and high f(S2) from the early to late stage. Finally, combined with the palaeogeographic reconstruction in Sulawesi, we propose that the high-Cr chromitites in this region were formed by the reaction of depleted mantle and boninitic magma beneath a juvenile island arc, implying the initiation of a new subduction in the Miocene.

Extreme redox variations in a superdeep diamond from a subducted slab.

The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation1. Here we investigate the redox conditions of the deep mantle recorded in inclusions in a diamond from Kankan, Guinea. Enstatite (former bridgmanite), ferropericlase and a uniquely Mg-rich olivine (Mg# 99.9) inclusion indicate formation in highly variable redox conditions near the 660 km seismic discontinuity. We propose a model involving dehydration, rehydration and dehydration in the underside of a warming slab at the transition zone-lower mantle boundary. Fluid liberated by dehydration in a crumpled slab, driven by heating from the lower mantle, ascends into the cooler interior of the slab, where the H2O is sequestered in new hydrous minerals. Consequent fractionation of the remaining fluid produces extremely reducing conditions, forming Mg-end-member ringwoodite. This fractionating fluid also precipitates the host diamond. With continued heating, ringwoodite in the slab surrounding the diamond forms bridgmanite and ferropericlase, which is trapped as the diamond grows in hydrous fluids produced by dehydration of the warming slab.

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.

Redox evolution of differentiating hydrous basaltic magmas recorded by zircon and apatites in mafic cumulates: The case of the Malayer Plutonic Complex, Western Iran

Mafic-ultramafic cumulates can provide records of basaltic magma chambers' conditions and processes, which are often difficult to determine in areas dominated by crustal-derived felsic intrusions, such as the Malayer Plutonic Complex (MPC), Western Iran. New U-Pb zircon ages for mafic cumulates in the MPC confirm the presence of isolated magma chambers of contrasting compositions during Middle Jurassic. Mafic cumulates found in seven separate zones across the MPC vary from olivine gabbro to anorthosite. While the mineralogical, textural, and geochemical lines of evidence recorded in mafic cumulates indicate pH2O controls on the liquidus phases, the estimated oxygen fugacity (logfO2) using zircon and apatite chemistry suggests a smoothly rising redox state during the fractionation process, consistent with the trend expected for late-stages differentiation of hydrous arc magmas. This trend is further confirmed by sulfur speciation in apatites determined from microbeam sulfur K-edge X-ray absorption near edge structure (μ-XANES) spectra (S6+/∑S = 0.93–0.98 ~ FMQ + 2 to 0.99 ~ FMQ + 3, where ∑S = S6++S4++S2−). The low S content and increasing redox state of the fractionating basaltic melts most likely resulted from preferential removal of sulfur en-route to the magma chambers along with effective assimilation of oxidizing crustal components. The reduced condition in the early basaltic melt is also evidenced by the presence of pyrite and magnetite inclusions in olivines in mafic cumulates. The shift in the prevailing fO2 from sulfide-saturated to sulfate-bearing recorded by MPC mafic cumulates, similar to that in other magmatic arcs, is accompanied by changes in the differentiation path from transitional tholeiitic to calc-alkaline.