Formation Of Olivine Research Articles

Crystallization conditions of olivine in dunites the Guli massif, Siberian platform

Crystallization conditions of olivine in dunites of the Guli massif (Siberian Platform) are established using petrological, geochemical, mineralogical and thermobarogeochemical methods. The formation of olivine and chromite from picrite-meimechite ultramafc magmas was followed by the fractionation of clinopyroxenes from basaltic systems with the formation of residual high-alkaline melts. The calculations in PETROLOG and COMAGMAT programs based on the compositions of inclusions and minerals, as well as using olivine-spinel geothermometer, showed that a magmatic system, which was responsible for the formation of dunites of the Guli massif, evolved during the crystallization of olivine in a wide temperature range of 1520–1250 °C in a magma chamber at a depth of about 17 km.

The influence of olivine settling on the formation of basaltic cumulates revealed by micro-tomography and numerical simulations

The speed at which crystals settle in magmatic reservoirs affects the solidification rate of magmas and their differentiation. Despite extensive prior work on the subject, most of our quantitative understanding of the process is still restricted to treating crystals as spherical particles and does not address the geometric complexities of natural crystals. Here, we use three-dimensional (3D) X-ray microcomputed tomography (X-μCT) observations on olivine crystals from Kīlauea Volcano (Hawai‘i) to document their highly intricate and variable geometries, and the textural growth relationships between the olivine crystals and their inclusions (melt, spinel, and fluid/vapor bubbles). Olivine crystals generally have clustered polyhedral or skeletal shapes, which reflect variable magmatic conditions (or growth rates) during their formation. The cumulative presence of spinel, melt, fluid, and vapor inclusions affects the density of the host crystals by up to 6% relative, and thus plays a limited role on modifying crystal settling rate. In contrast, the overall crystal shape plays a major role. We performed numerical simulations employing a finite element method to investigate the effect of crystal morphology on settling rate and the evolution of the particle volume fraction in a magmatic convective layer. We show that for all olivine geometries investigated, the settling velocity is highest when the long axis of the crystal is aligned with the flow direction of the melt. Increasing the aspect ratio of the olivine tends to decrease its settling velocity and results in an increase in the influence of its orientation on the terminal velocity (UT). Extrapolation of the simulation results to variable particle volume fractions (Φ = 0–0.5) indicates high crystal settling rates (UT = ∼9.7 × 10−6–1.4 × 10−5 m/s) that are used to estimate the timescales for the formation of olivine cumulates in natural melt-dominated basaltic systems. The formation of olivine cumulates is therefore rapid, potentially leading to the accumulation of a crystal layer at the bottom, where the frictional contacts between the crystals exert a rheological lock-up acting against further convection. Crystal accumulation in the locked layer (parametrized with the solid/liquid volume ratio in the reservoir) is a function of the reservoir size and crystal fraction, and takes a few years in small reservoirs (<1 km thick) and a few decades in larger reservoirs (several kms thick). We propose a calibration of olivine suspension timescales for mafic magma reservoirs (based on the knowledge of the particle volume fraction, reservoir height, and olivine morphology). This calibration is used to estimate the rate of cumulate build-up, and can help interpret crystal size distributions in the framework of crystal suspension times.

Olivine formation processes and fluid pathways in subducted serpentinites revealed by in-situ oxygen isotope analysis (Zermatt-Saas, Switzerland)

Dehydration of serpentinites plays a crucial role in mass transfer into the Earth's interior by releasing aqueous fluids and forming new minerals. These minerals, such as metamorphic olivine, can serve as tracers of fluid-related processes. High-pressure (HP) antigorite, metamorphic olivine, and coexisting magnetite in serpentinites from a continuous, km-scale outcrop within the Zermatt-Saas HP ophiolite were analyzed in situ for trace elements and oxygen isotopes to identify differences in the initial serpentinization conditions and to investigate fluid pathways during subduction-related metamorphism. The oxygen isotopic composition, and As and Sb concentrations in antigorite reveal two distinct serpentinization conditions within the studied region: i) high As and Sb (1–25 μg/g and 0.5–5 μg/g, respectively), coupled with δ18O of +6 to +7 ‰, suggesting serpentinization at relatively low temperatures near the seafloor, and ii) low As and Sb (0.03–5 μg/g and ≤ 0.1 μg/g, respectively), coupled with mostly lower δ18O of +4 to +6 ‰, suggesting serpentinization at higher temperatures by interaction with fluids deeper below the seafloor.Olivine produced in situ by the brucite + antigorite dehydration reaction during subduction shows isotopic equilibrium with antigorite, and coexisting magnetite with ∆18OAtg-Ol of +1.5–2.5 ‰ and ∆18OOl-Mt of ∼+3 ‰ at reaction temperature conditions of 550–600 °C. The obtained isotopic signatures of metamorphic olivine with δ18O values of +1 to +2 ‰ and + 4 to +5 ‰ correspond to two different isotopic compositions of the released fluid of +5 to +6 ‰ and + 8 to +9 ‰ at these temperature conditions. This suggests that fluids released from subducted serpentinites may have variable δ18O under forearc conditions. The presence of fluids with variable δ18O can cause olivine in structures associated with fluid flow (e.g., shear bands, shear zones and veins) to be in isotopic equilibrium with magnetite, but in either isotopic equilibrium or disequilibrium with antigorite. Isotopic equilibrium with antigorite is achieved when the fluid responsible for olivine crystallization is internally derived. Isotopic disequilibrium is due to an externally derived fluid released by dehydration of serpentinite with a different isotopic composition than the serpentinite with which the fluid interacts. The restricted occurrence of non-equilibrated olivine only in shear bands and nearly pure Ol-veins indicates channelized fluid flow in subduction zone settings and demonstrates that isotopic disequilibrium can be used as a tracer for fluid infiltration.

Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting

Abstract Global volcanic and plutonic olivines record the compositional characteristics and physicochemical conditions of the parental magmas. Thus, they have significant potential for use as petrogenetic discriminators of the olivine formation environment and prospecting indicators for potential host rocks of magmatic sulfide deposits. Several data visualization approaches have been proposed by researchers to determine olivine origins. However, they can only discriminate specific olivine populations and require the incorporation of trace elements for which data are lacking globally. In this study, a machine-learning method consisting of the random forest algorithm and the synthetic minority oversampling technique (SMOTE) is used to discriminate the crystallization environments of olivine and predict the sulfide potential of olivine-bearing mafic-ultramafic intrusions. We employ a global data set of 24 341 olivine samples from 12 environments to determine the contents of MgO, FeO, Ni, Ca, Mn, and Cr and the Fo number [100 × Mg/(Mg+Fe)]. The results indicate that the proposed method can classify olivine into genetically distinct populations and distinguish olivine derived from mineralized intrusions from that derived from sulfide-barren intrusions with high accuracies (higher than 99% on average). We develop a dimensionality reduction algorithm to visualize the olivine classifications using low-dimensional vectors and an olivine classifier (accessible at http://101.33.204.62:8080/olivine_web/main.html, China University of Geosciences, Beijing). The model is used successfully to identify the contributions of distinct sources to regional magmatism using olivines from the late Permian picrite and basalt along the western margin of the Yangtze block (SW China) and to predict the sulfide potential of the newly discovered Qixin mafic-ultramafic complex in the southern Central Asian Orogenic Belt (NW China). The findings suggest that the proposed approach enables the accurate identification of olivine origins in different formation environments and is a reliable indicator suitable for global Ni-Cu-platinum group element (PGE) exploration.

Physicochemical Parameters of Ultrabasic Magmatic Systems of the Northeastern Siberian Platform (Data on Melt Inclusions in Chromian Spinel)

Abstract —Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part of chromian spinel from the Triassic deposits of the northeastern Siberian Platform. The compositions of melt inclusions in the chromian spinel show the participation of alkaline (potassic) magmas in their crystallization. Most data indicate the presence of magmatic systems similar to melts in the Guli ultrabasic massif in the northern Siberian Platform. Studies of the distribution of trace and rare-earth elements in the melt inclusions show the existence of several magmatic systems. These are, first of all, magmas that formed Guli-type ultrabasic massifs and gave rise to meimechites and picrites in the Maimecha–Kotui province. There are also plume-related magmatic systems with kimberlites, lamprophyres, and/or other continental “hot spots”. The composition of the melt inclusions suggests the existence of several types of the primary sources of chromian spinel in the northeastern Siberian Platform, which confirms the earlier data on the heterogeneous composition of the deposits of the diamondiferous Carnian (Upper Triassic) Stage. Applying computer modeling with the well-known COMAGMAT, PETROLOG, and WinPLtb programs as well as the Ol–Sp geothermometers based on the melt inclusions in chromian spinel from the Triassic deposits of the northeastern Siberian Platform, we have determined the P–T conditions of crystallization of minerals in the igneous rocks being the sources of the examined chromites. The temperature of liquidus crystallization of chromian spinel is 1324–1275 ºC. The P–T conditions of formation of olivine and clinopyroxene inclusions in it are estimated at ca. 4.5–4.1 kbar, 1510–1150 ºC and 3.2–1.0 kbar, 1285–1200 ºC, respectively.

Mineralogy and genesis of ultramafic rocks from the Kurtushiba ophiolite belt (Western Sayan)

The studies of minerals and melt inclusions in ultramafc rocks of the Kurtushiba ophiolite belt (Western Sayan) allowed us to establish the conditions of mineral-forming processes during their formation. . The features of the compositions of minerals of ultramafc rocks of Idzhim, Kalny and Ergaki massifs are considered. The presence of melt inclusions directly indicates a magmatic origin of some Cr-spinels from dunites of Idzhim massif. A close assemblage of Cr-spinels from chromitites with magmatic Cr-spinels is important because of the possible formation of chromite ore from melts. The study of melt inclusions revealed the evolution of melts (picrites – picrobasalts – basalts – basaltic andesites) during the consecutive crystallization of minerals from the Western Sayan ophiolites. Computational modeling based on the compositions of melt inclusions and olivine indicates that olivine of ultramafc rocks of the Kurtushiba ophiolites crystallized at a decreasing pressure from 9.2 to 6.2 kbar and decreasing temperature from 1550 to 1430 °С from picritic melts, which evolved to picrobasaltic melts. With a decrease pressure to 3.8–2.3 kbar and temperature of picrobasaltic melts (1240–1230 °C), the formation of olivine terminated following by the crystallization of clinopyroxene from basaltic magmas at 2.7–1.9 kbar and about 1215 °C. The postmagmatic stage was characterized by plastic deformations of the Western Sayan ultramafc rocks at a decreasing temperature from 850 to 640 °C, which resulted in a successive change from protogranular to mesogranular structures and then to porphyroclastic and porphyroleist structures.

Co-Conversion Mechanisms of Boron and Iron Components of Ludwigite Ore during Reductive Soda-Ash Roasting

Ludwigite ore is a typical intergrown mineral resource found in China. Reductive soda-ash roasting followed by water leaching is an innovative process for the high-efficiency separation and recovery of boron and iron. In this study, the co-conversion mechanism of boron activation and iron reduction during soda-ash reductive roasting for boron-bearing iron concentrate was clarified. When the boron-bearing iron concentrate was reduced in the presence of Na2CO3, szaibelyite (Mg2(OH)(B2O4) (OH)) was activated to sodium metaborate (NaBO2) and, meanwhile, magnetite (Fe3O4) was reduced to metallic iron (MFe). Boron activation promoted iron-oxide reduction effectively, while the latter could only slightly influence the former. The promotion occurred through (1) a facilitated generation of sodium magnesium silicate (Na2MgSiO4) and a hindering of the formation of olivine (MgxFe2-x(SiO4)). (2) The newly generated NaBO2 promoted iron-oxide reduction. (3) The low melting point of the NaBO2 (966 °C) favored particle migration, which accelerated metallic iron particle aggregation.

The role of the antigorite\u2009+\u2009brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

Metamorphic olivine formed by the reaction of antigorite + brucite is widespread in serpentinites that crop out in glacier-polished outcrops at the Unterer Theodulglacier, Zermatt. Olivine overgrows a relic magnetite mesh texture formed during ocean floor serpentinization. Serpentinization is associated with rodingitisation of mafic dykes. Metamorphic olivine coexists with magnetite, shows high Mg# of 94–97 and low trace element contents. A notable exception is 4 µg/g Boron (> 10 times primitive mantle), introduced during seafloor alteration and retained in metamorphic olivine. Olivine incorporated 100–140 µg/g H2O in Si-vacancies, providing evidence for low SiO2-activity imposed by brucite during olivine growth. No signs for hydrogen loss or major and minor element diffusional equilibration are observed. The occurrence of olivine in patches within the serpentinite mimics the former heterogeneous distribution of brucite, whereas the network of olivine-bearing veins and shear zones document the pathways of the escaping fluid produced by the olivine forming reaction. Relic Cr-spinels have a high Cr# of 0.5 and the serpentinites display little or no clinopyroxene, indicating that they derive from hydrated harzburgitic mantle that underwent significant melt depletion. The enrichment of Mg and depletion of Si results in the formation of brucite during seafloor alteration, a pre-requisite for later subduction-related olivine formation and fluid liberation. The comparison of calculated bulk rock brucite contents in the Zermatt-Saas with average IODP serpentinites suggests a large variation in fluid release during olivine formation. Between 3.4 and 7.2 wt% H2O is released depending on the magnetite content in fully serpentinized harzburgites (average oceanic serpentinites). Thermodynamic modelling indicates that the fluid release in Zermatt occurred between 480 °C and 550 °C at 2–2.5 GPa with the Mg# of olivine varying from 68 to 95. However, the majority of the fluid released from this reaction was produced within a narrow temperature field of < 30 °C, at higher pressures 2.5 GPa and temperatures 550–600 °C than commonly thought. Fluids derived from the antigorite + brucite reaction might thus trigger eclogite facies equilibration in associated metabasalts, meta-gabbros, meta-rodingites and meta-sediments in the area. This focused fluid release has the potential to trigger intermediate depths earthquakes at 60–80 km in subducted oceanic lithosphere.

Geology of Grimaldi Basin on the Moon: Evidence for volcanism and tectonism during the Copernican period

The Grimaldi Basin, centered at (5.2° S, 68.6° W), is a ~400 km diameter highly degraded, Pre-Nectarian double-ring impact structure on the Moon located near the western edge of the Oceanus Procellarum. Hyperspectral data from Moon Mineralogy Mapper (M3), FeO estimates from Kaguya Lunar Multiband Imager (MI) data and TiO2 estimates from WAC 321/415 nm ratio have been used to study the compositional make-up of the Grimaldi mare basalt. Additionally, morphological studies and crater chronology have been carried out using moderate to very high-resolution images from Lunar Reconnaissance Orbiter (LRO) to decipher the geological evolution of the Grimaldi Basin. Mare Grimaldi are dominantly composed of clino-pyroxenes with olivine and/or of feldspathic mixing with lateral as well as depth-wise variations in composition. In the south central part, the basin experienced Copernican aged volcanism ~700 Ma ago resulting in the formation of olivine bearing basalts with high FeO and TiO2 content. Cross-cutting of small Copernican craters by fresh wrinkle ridges and lobate scarps has been observed at several places in the basin suggesting that tectonic activities occurred in the basin within the past ~50 Ma-1 Ga. Thus, similar to the Oceanus Procellarum region, Grimaldi Basin was also geologically active during the Copernican period.

Characterization and evaluation of magnesite ore mining by-products of Gerakini mines (Chalkidiki, N. Greece)

The qualitative and quantitative characterization of several mining by-product samples, were collected from the magnesite mine of “Grecian Magnesite SA” company (Gerakini, Chalkidiki, North Greece), was aiming to evaluate the possibility of upgrading their refractory properties by applying thermal treatment. The concentration range of main components for the selected best qualified samples was 38.7–43 wt% for MgO, 37.5–44.1 wt% for SiO2 and 6.5–7.3 wt% for FeO. The mineralogical characterization revealed the presence of olivine, pyroxenes and serpentine, with the concentration of the latter positively correlated to LOI. Microprobe analyses clarified the presence of olivine [(Mg1.79Fe0.19Ni0.01)SiO4], consisting mainly of 90 wt% of forsterite (Mg2SiO4) and 10 wt% of fayalite (Fe2SiO4), as well as of pyroxene group minerals [(Mg0.87Fe0.08Ca0.01Cr0.01)(Si0.98Al0.04)O3], consisting mainly of 91 wt% enstatite (MgSiO3) and 9 wt% of ferrosilite (FeSiO3), respectively. The thermal treatment of the qualified samples demonstrated that at the temperature of 650–680 °C serpentine is almost totally decomposed and at the temperature of 850 °C it has been totally recrystallized to olivine and pyroxenes. At higher temperature treatment (1300 °C), it seems that there is a recrystallization process that favors the deformation of olivine and the further formation of pyroxenes, due to the excess of Si available from the initial decomposition of serpentine, while the presence of magnesite resulted to the restriction of olivine deformation through the partial capture of available Si. For increasing the olivine percentage and, subsequently, the improvement of refractory properties of this material, at temperature > 1300°C, the ideal theoretical addition dose of wt% MgO for optimizing the formation of olivine was calculated, ranging from 7.4 to 17.5 wt%. The latter calculations are reported for the first time in the literature regarding this kind of materials.

Analyzing the Magnesium (Mg) Number of Olivine on the Lunar Surface and Its Geological Significance

Olivine formation is directly related to Mg/Fe content. It is also significant in estimating the geological evolution of the moon. In this study, an estimation model of relative Mg number (Fo#) for lunar olivine was presented through multiple linear regression statistics. Sinus Iridum, the Copernicus Crater, and the pyroclastic deposit in the volcanic vents in the southeast of Orientale Basin were selected as the study areas. Olivine distribution was surveyed, and the relative Fo# calculation of olivine was implemented based on Moon Mineralogy Mapper (M3) data. Results demonstrated that olivine in the crater wall of Sinus Iridum and the Copernicus Crater had relatively high Fo#, which reflected the primitive melt. However, the difference in olivine spectral features between Sinus Iridum and the Copernicus Crater indicated different crystallization modes. The olivine in the pyroclastic deposit in the volcanic vents in the southwest of Orientale Basin also presented high Fo#, which indicated that the olivine was formed via rapid cooling crystallization and was accompanied by volcanic glass substances. As a result, the olivine relative Fo# calculated from the estimation model exhibited an important constraint implication for explanation of its causes.

Extremely magnesian olivine in igneous rocks

Published data on extremely magnesian olivine (>96 mol.% forsterite) in igneous rocks were generalized and compared with data of new high-precision electron probe microanalyses of olivine from oxidized lavas of the Tolbachik Volcano (Kamchatka), chromitites from the Ray-Iz deposit (Russia), alkaline ultrabasic lavas from San Venanzo volcanoes (Italy), and skarns from the Kuh-i-Lal deposit (Tajikistan). All the found olivines resulted from low-temperature processes, such as subsurface oxidation, interaction with carbonates, and subsolidus re-equilibration. Low-temperature formation of olivine is reflected in its structure (hematite lamellae and abundance of inclusions of ore minerals) and abnormal contents of minor components (Mn, Ni, and Ca). The Mg content of olivine increases under the influence of postmagmatic processes and can be manifested in different rocks. This gives grounds to refine the genesis of olivine of exotic composition (93-96 mol.% forsterite) in some kimberlites, komatiites, and peridotites.

Sulfide Formation as a Result of Sulfate Subduction into Silicate Mantle (Experimental Modeling under High P,T-Parameters)

Ca,Mg-sulfates are subduction-related sources of oxidized S-rich fluid under lithospheric mantle P,T-parameters. Experimental study, aimed at the modeling of scenarios of S-rich fluid generation as a result of desulfation and subsequent sulfide formation, was performed using a multi-anvil high-pressure apparatus. Experiments were carried out in the Fe,Ni-olivine–anhydrite–C and Fe,Ni-olivine–Mg-sulfate–C systems (P = 6.3 GPa, T of 1050 and 1450 °C, t = 23–60 h). At 1050 °C, the interaction in the olivine–anhydrite–C system leads to the formation of olivine + diopside + pyrrhotite assemblage and at 1450 °C leads to the generation of immiscible silicate-oxide and sulfide melts. Desulfation of this system results in the formation of S-rich reduced fluid via the reaction olivine + anhydrite + C → diopside + S + CO2. This fluid is found to be a medium for the recrystallization of olivine, extraction of Fe and Ni, and subsequent crystallization of Fe,Ni-sulfides (i.e., olivine sulfidation). At 1450 °C in the Ca-free system, the generation of carbonate-silicate and Fe,Ni-sulfide melts occurs. Formation of the carbonate component of the melt occurs via the reaction Mg-sulfate + C → magnesite + S. It is experimentally shown that the olivine-sulfate interaction can result in mantle sulfide formation and generation of potential mantle metasomatic agents—S- and CO2-dominated fluids, silicate-oxide melt, or carbonate-silicate melt.

Petrological and Geochemical Constraints on the Protoliths of Serpentine‐Magnetite Ores in the Zhaoanzhuang Iron Deposit, Southern North China Craton

AbstractThe uncommon Mg‐rich and Ti‐poor Zhaoanzhuang serpentine‐magnetite ores within Taihua Group of the North China Craton (NCC) remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore‐forming processes. Iron ores mainly contain low‐Ti magnetite (TiO2 ∼0.1wt%) and serpentine (Mg#=92.42–96.55), as well as residual olivine (Fo=89–90), orthopyroxene (En=89–90) and hornblende. Magnetite in the iron ores shows lower Al, Sc, Ti, Cr, Zn relative to that from ultramafic Fe‐Ti‐V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine (Fo=83–87), orthopyroxene (En=82–86), humite (Mg#=82–84) and hornblende [XMg=0.87–0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite (Th‐rich oxide) and monazite (LREE‐rich phosphate) are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO2 (4.77–25.23wt%), Fe2O3T (32.9–80.39wt%) and MgO (5.72–27.17wt%) and uniformly, those of the wall rocks are also SiO2 (16.34–48.72wt%), MgO (16.71–33.97wt%) and Fe2O3T (6.98–30.92wt%). The striking high Fe‐Mg‐Si contents reveal that protolith of the Zhaoanzhuang iron deposit was more likely to be chemical sedimentary rocks. The distinct high‐Mg feature and presence of abundant anhydrite possibly indicate it primarily precipitated in a confined seawater basin under an evaporitic environment. Besides, higher contents of Al, Ti, P, Th, U, Pb, REE relative to other Precambrian iron‐rich chemical precipitates (BIF) suggest some clastic terrestrial materials were probably input. As a result, we think the Zhaoanzhuang iron deposit had experienced the initial Fe‐Mg‐Si marine precipitation, followed by further Mg enrichment through marine evaporated process, subsequent high‐grade metamorphism and late‐stage hydrothermal fluid modification.

The Stillwater Complex, Montana – Overview and the significance of volatiles

AbstractThe geology of the 2.7 Ga Stillwater Complex in South-Central Montana is reviewed with a focus on the role of volatiles in locally modifying both the crystallization sequence of the evolving parent magma and the initially precipitated solid assemblages to favour olivine ± chromite. A secondary origin for these two minerals is particularly probable for the olivine-bearing rocks of the Banded series and, at a minimum, also increasing their modal abundance in the Peridotite zone of the Ultramafic series. Direct evidence for volatiles includes the presence of high-temperature fluid inclusions in pegmatoids and hydrous melt inclusions (now crystallized) in chromite and olivine from both the Ultramafic and the Banded series rocks. Indirect evidence includes the boninitic character of the parent magma, the presence of volatile-bearing minerals including high-temperature carbonates, rock textures, and Cl / F variations in apatite. Mechanisms which favour the formation of olivine (± chromite) over pyroxene include volatile phase boundary shifts induced by added H2O, incongruent melting of pyroxene by hydration of a partly-molten mush, and the near- to sub-solidus replacement of pyroxene by olivine and chromite by silica-undersaturated fluids. These mechanisms cast doubt that magmas with different liquid lines of descent were involved in the crystallization of the Stillwater Complex. A dry Stillwater magma would have been mineralogically and modally much less varied and lacking in high-grade platinum-group element and chromium deposits.

Physicochemical conditions of crystallization of dunites of the Nizhnii Tagil Pt-bearing massif (Middle Urals)

Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 °C and then olivine formation continued to 1280 °C during the evolution of melts.

Phosphoran olivine overgrowths: Implications for multiple impacts to the Main Group pallasite parent body

Phosphoran olivine (1–7 wt% P2O5) is a metastable phase known from fewer than a dozen meteoritic or terrestrial occurrences. We have thoroughly examined phosphoran olivine in the Springwater pallasite to characterize its distribution, textural relationships, and geochemistry. Phosphoran olivine is abundant in Springwater as randomly distributed millimeter-scale partial overgrowths on the P-free olivine crystals. Geochemical analyses support the substitution mechanism of P into the tetrahedral Si site with octahedral site vacancies for charge balance; observed trace element variations, on the other hand, are not related to P substitution. Element mapping reveals fine-scale oscillatory P zoning in unusual serrate patterns, indicating rapid crystal nucleation from a melt as proposed by Boesenberg and Hewins (2010) and a subsequently variable rate of crystallization. The timing of phosphoran olivine formation in Springwater is constrained to after the period of macroscopic olivine rounding but before the cooling of the metal matrix; because the phosphoran overgrowths overprint specific host grain boundary modifications, we suggest that the episode of extremely rapid cooling necessary to crystallize and preserve this rare phase may have been triggered by an additional impact to the parent body.

Physicochemical parameters of crystallization of dunite from the Guli ultrabasic massif (Maimecha Kotui province)

On the basis of analysis of molten inclusions in chrome-spinelide, physicochemical parameters of dunite crystallization were defined. Experimental, analytical studies directly indicate that dunite was formed from high-temperature melts close in petrochemical composition and high-temperature characteristics to meimechite magmas. Successive evolution of magmatic systems compositions in a course of intra-chamber crystallization of dunite was established: from picriteâ “meimechite (with olivine formation at 1590–1415°C and chrome-spinelide crystallization at 1405–1365°C) to picrate-basalt and basalt.

Methane-bearing melt inclusion in olivine phenocryst in Cenozoic alkaline basalt from Eastern China and its geological significance

Alkaline basalts originate from the partial melting of the upper mantle or deeper mantle region, and their study can contribute important knowledge of mantle composition. Melt inclusions in these basalts are small natural droplets trapped during the formation of olivine, and can be isolated from later alterations. Therefore, they can effectively record information from the early stages of magma formation and evolution. In this study, the volatile composition of 73 melt inclusions in olivine phenocrysts of alkaline basalts from Yangzhuang in Shandong Province were analyzed by Laser Raman Spectroscopy. The major and trace element compositions of some host olivines were measured by LA-ICP-MS. The results show that these melt inclusions contain amounts of methane (CH4) and other hydrocarbons, and some carbonaceous minerals such as graphite and carbonate. These findings prove that CH4 can stably exist in the upper-mantle environment of high pressure and temperature, and indicate that such a mantle region with low oxygen fugacity does exist below Eastern China. The host olivine phenocrysts are characterized by high Ni content and a high Fe/Mn ratio, implying the presence of a pyroxenite component in the mantle source. The subduction of the Western Pacific Plate beneath the lithospheric mantle of Eastern China is a possible source for the pyroxenite, combined with other geochemical and geophysical data. CH4 is an important form of deep carbon and the subduction of oceanic crust is responsible for the recycling of deep carbon.

Olivine from the Pionerskaya and V. Grib kimberlite pipes, Arkhangelsk diamond province, Russia: Types, composition, and origin

We report the first systematic study of different textural varieties of olivine (olivine from peridotite xenoliths, macrocryst-type Ol-I, and phenocryst-type zoned Ol-II) from two diamondiferous kimberlite pipes of the Arkhangelsk diamond province (V. Grib and Pionerskaya) differing in geologic setting, geochemical and isotopic characteristics, and diamond content. Approximately 550 olivine analyses were obtained by the EPMA technique using the precise method of Sobolev et al. (2007) adapted at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences (Kargin et al., 2014). Olivines from the V. Grib moderate-Ti kimberlites, which are similar to Group I kimberlites in geochemical and Sr-Nd isotopic characteristics and rich in diamond, are dominated by high-Mg# low-Ti Ol-I formed owing to the fractional crystallization of a carbonate-rich protokimberlite melt interacting with orthopyroxene-bearing peridotite material; the fraction of high-Ti Ol-I produced by the metasomatic alteration of peridotite under the influence of silicate aqueous fluids is significantly lower; and xenocrysts weakly affected by metasomatic agents (melts and fluids) occur in minor amounts. Olivines from the low-Ti Pionerskaya kimberlites, which are similar to Group II kimberlites in geochemical and Sr-Nd isotopic characteristics and show a moderate diamond content, are dominated by high-Ti Ol-I, and xenocrysts weakly affected by metasomatic agents are also abundant. In the kimberlites of both pipes, the cores of Ol-II crystals are usually composed of low-Ti olivine similar in composition to Ol-I; both high-Ti and low-Ti olivine cores occur in the Pionerskaya pipe; whereas cores corresponding to high-Ti Ol-I were never found in the V. Grib pipe. The outer zones of olivine and small olivine grains in the groundmass show considerable variations in minor element contents within a narrow Mg# range. It is suggested that the high-Ti rims of Ol-II from the V. Grib and Pionerskaya kimberlites were produced by the late crystallization of kimberlite melt, and the low-Ti rims on the outer zones of Ol-II in the Pionerskaya kimberlites were formed by late-stage equilibration with an aqueous fluid separated from the kimberlite melt and/or possible kinetic effects. Our study revealed the diversity of olivine origin in the kimberlites and showed that there is no single mechanism of olivine formation.