FTIR spectroscopy of OH in olivine: A new tool in kimberlite exploration

Our study of olivines from Canadian kimberlites shows that the application of FTIR spectroscopy significantly improves the reliability of olivine as a kimberlite indicator mineral (KIM). We have developed an algorithm that yields the water concentration and the normalized intensity of the OH IR absorption band at 3572 cm⁻¹ from unpolished olivine grains of unknown thickness. For 80% of kimberlitic olivines these two parameters are significantly higher than those for olivines from non-kimberlitic magmas and consequently, olivines with water concentrations >60 ppm and a strong absorption band at 3572 cm⁻¹ can be reliably classified as being kimberlitic.We have identified two major spectral features in the OH absorption bands of kimberlitic olivines that allow for a more detailed classification: (a) the presence of three types of high-requency OH absorption bands (Group 1A, 1B and 1C) and (b) the proportion of low-frequency OH absorption bands (Group 2) relative to high-frequency bands (Group 1). Comparison of our results with experimental studies suggests that differences within Group 1 OH absorption bands are due to different pressures of crystallization or hydrogenation. The three identified types of Group 1 OH absorption bands approximately correspond to high (P > 2 GPa, Group 1A), moderate (2-1 GPa, Group 1B), and low (<1 GPa, Group 1C) pressures of hydrogenation. Group 2 OH IR absorption bands in olivines with NiO > 3500 ppm are interpreted to reflect olivine-orthopyroxene equilibria and hence are indicative of xenocrystic olivine derived from lherzolitic or harzburgitic mantle sources. Interaction of xenocrystic olivine with hydrous kimberlitic melts with low silica activity likely will cause a gradual increase in Group 1 absorption bands. Therefore, FTIR spectra of olivine can be used to obtain qualitative estimates of the duration of interaction between mantle material and a kimberlitic melt.In addition to applications in kimberlite and diamond exploration, FTIR spectra of olivine phenocrysts, combined with mineral chemical data, may also provide insights into kimberlite evolution. Our data suggest that in some instances the ascent of kimberlitic magmas could have been interrupted at or near the Moho, followed by olivine crystallization and exsolution of aqueous fluids.     

Searching for parental kimberlite melt

Constraining the composition of primitive kimberlite magma is not trivial. This study reconstructs a kimberlite melt composition using vesicular, quenched kimberlite found at the contact of a thin hypabyssal dyke. We examined the 4 mm selvage of the dyke where the most elongate shapes of the smallest calcite laths suggest the strongest undercooling. The analyzed bulk compositions of several 0.09-1.1 mm² areas of the kimberlite free from macrocrysts were considered to be representative of the melt. The bulk analyses conducted with a new “chemical point-counting” technique were supplemented by modal estimates, studies of mineral compositions, and FTIR analysis of olivine phenocrysts. The melt was estimated to contain 26-29.5 wt% SiO₂, ∼7 wt% of FeO_T, 25.7-28.7 wt% MgO, 11.3-15 wt% CaO, 8.3-11.3 wt% CO₂, and 7.6-9.4 wt% H₂O. Like many other estimates of primitive kimberlite magma, the melt is too magnesian (Mg# = 0.87) to be in equilibrium with the mantle and thus cannot be primary. The observed dyke contact and the chemistry of the melt implies it is highly fluid (η = 10¹-10³ Pa s at 1100-1000 °C) and depolymerized (NBO/T = 2.3-3.2), but entrains with 40-50% of olivine crystals increasing its viscosity. The olivine phenocrysts contain 190-350 ppm of water suggesting crystallization from a low SiO₂ magma (a_SiO2 below the olivine-orthopyroxene equilibrium) at 30-50 kb. Crystallization continued until the final emplacement at depths of few hundred meters which led to progressively more Ca- and CO₂-rich residual liquids. The melt crystallised phlogopite (6-10%), monticellite (replaced by serpentine, ∼10%), calcite rich in Sr, Mg and Fe (19-27%), serpentine (29-31%) and minor amounts of apatite, ulvöspinel-magnetite, picroilmenite and perovskite. The observed content of H₂O can be fully dissolved in the primitive melt at pressures greater than 0.8-1.2 kbar, whereas the amount of primary CO₂ in the kimberlite exceeds CO₂ soluble in the primitive kimberlite melt. A mechanism for retaining CO₂ in the melt may require a separate fluid phase accompanying kimberlite ascent and later dissolution in residual carbonatitic melt. Deep fragmentation of the melt as a result of volatile supersaturation is not inevitable if kimberlite magma has an opportunity to evolve.     

Origin of complex zoning in olivine from diverse,diamondiferous kimberlites and tectonic settings: Ekati (Canada), Alto Paranaiba (Brazil) and Kaalvallei (South Africa)

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78–95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.

     

Dissolution Rates of Upper Mantle Minerals in an Alkali Basalt Melt at High Pressure: An Experimental Study and Implications for Ultramafic Xenolith Survival

The dissolution rates of the major upper mantle minerals olivine,orthopyroxene, clinopyroxene, spinel, and garnet have been determinedin an alkali basalt melt at superliquidus temperatures and 5,12, and 30 kb. At low pressure where olivine is the liquidusphase of the basalt, olivine has a slower dissolution rate thanclinopyroxene; however, at higher pressure where clinopyroxeneis the liquidus phase, clinopyroxene has a slower dissolutionrate than olivine. The relative rates of dissolution of olivineand clinopyroxene at each pressure are, therefore, governedby their relative stabilities in the melt and hence by the structureof the melt. As the degree of superheating above the liquidusincreases at each pressure, the dissolution rates of olivineand clinopyroxene converge, suggesting that the melt undergoestemperature-induced structural changes. Orthopyroxene has a dissolution rate similar to olivine at highpressure and similar to clinopyroxene at low pressure. Spinelhas the slowest dissolution rate at each pressure. Garnet dissolvesvery rapidly at 12 kb and at a comparable rate of olivine at30 kb. The dissolution rates determined in the experiments varyfrom 9.21 ? 10^–9cm s^–1 for spinel at 5 kbar and1250?C to 3.83 ? 10^–5cm s^–1 for garnet at 30 kband 1500?C. Textures produced during the dissolution experiments are relatedto mineral stability in the melt at each pressure and are independentof the degree of superheating. The mineral phases that are stableon or near the liquidus exhibit no reaction; whereas complexreaction textures and crystallization characterize dissolutionof minerals that are relatively unstable in the melt. Concentration profiles in the melt adjacent to the same crystalfor different experimental durations are identical, indicatingthat dissolution is time-independent and a steady-state process.However, cation diffusion coefficients calculated for single-componentoxides in the melt reveal that dissolution may not be completelycontrolled by diffusion of cations away from the crystal/meltinterface. The apparent diffusivities positively correlate withthe dissolution rate, which suggests that the stability of themineral is an important factor to consider when deriving diffusioncoefficients from these experiments. Other factors that maybe involved are multi-component effects and the nature of thediffusing species in the melt. A simple model has been constructed that predicts the survivalof ultramafic xenoliths in alkali basalt magmas as a functionof xenolith radius, magma ascent time and superheating. Theresults of the model suggest that the relative proportions ofperidotite and pyroxenite xenoliths brought to the surface inalkali basalts are generally representative of their proportionsas constituents of the upper mantle. Further experiments usingdifferent melt compositions are required to extend the model.     

The formation of peridotitic suite inclusions in diamonds

Olivine, orthopyroxene and garnet grains belonging to the peridotitic suite of mineral inclusions in natural diamonds typically show compositions poorer in Ca and Al and richer in Mg and Cr than the same minerals in peridotite nodules in kimberlite. Other features suggest the crystallisation of diamonds from magmas of kimberlitic affinities, and it is suggested that the genesis of peridotitic suite diamonds is linked with that of a CO₂-bearing magma. It is shown that the generation of kimberlitic magma from common garnet-peridotite (with 5 wt.% clinopyroxene) in the presence of CO₂ may rapidly remove by melting all Ca-rich solid phases (clinopyroxene and/or carbonate). Further melting may form liquids in equilibrium with olivine, orthopyroxene, and garnet with the distinctive compositions of the diamond inclusions. The amount of melting and CO₂ necessary for the loss of clinopyroxene (and/or carbonate) are estimated at approximately 5.0 wt.% and 0.5 wt.% respectively.     

辽南斑状金伯利岩岩相学特征及其成因研究

辽南金伯利岩岩区是我国最大的原生金刚石矿产区,该区金刚石主要寄主岩石类型为斑状金伯利岩。橄榄石是金伯利岩中最重要的造岩矿物,根据其结构特征可以分为橄榄石粗晶、橄榄石斑晶以及基质中微细粒三个世代。本文将岩相学特征和前人研究成果相结合,构建辽南斑状金伯利岩岩浆起源、上升、喷发和成岩模型,探讨各世代矿物的形成过程。具体包括：深部交代地幔部分熔融,形成初始碳酸盐岩浆;初始岩浆上升过程中捕获的岩石圈地幔橄榄岩不断溶解(形成橄榄石粗晶),岩浆成分发生改变,成为金伯利岩岩浆;金伯利岩岩浆迅速上升侵位,至地表处爆破喷发,最后冷却固结形成包含粗晶及其他两个世代橄榄石的斑状金伯利岩。     

The origin of reaction textures in mantle peridotite xenoliths from Sal Island, Cape Verde: the case for “metasomatism” by the host lava

Reaction zones around minerals in mantle xenoliths have been reported from many localities worldwide. Interpretations of the origins of these textures fall into two groups: mantle metasomatic reaction or reaction during transport of the xenoliths to the surface. A suite of harzburgitic mantle xenoliths from Sal, Cape Verde show clear evidence of reaction during transport. The reactions resulted in the formation of olivine–clinopyroxene and Si- and alkali-rich glass reaction zones around orthopyroxene and sieve-textured clinopyroxene and sieve textured spinel, both of which are associated with a Si- and alkali-rich glass similar to that in the orthopyroxene reaction zones. Reaction occurred at pressures less than the mantle equilibration pressure and at temperatures close to the liquidus temperature of the host magma. In addition, there is a clear spatial relation of reaction with the host lava: reaction is most intense near the lava/xenolith contact. The residence time of the xenoliths in the host magma, determined from Fe–Mg interdiffusion profiles in olivine, was approximately 4 years. Our results cannot be reconciled with a recent model for the evolution of the mantle below the Cape Verde Archipelago involving mantle metasomatism by kimberlitic melt. We contend that alkali-rich glasses in the Sal xenoliths are not remnants of a kimberlitic melt, but rather they are the result of reaction between the host lava or a similar magma and xenolith minerals, in particular orthopyroxene. The formation of a Si- and alkali-rich glass by host magma–orthopyroxene reaction appears to be a necessary precursor to formation of sieve textured spinel and clinopyroxene.     

Experimental estimation of the rate of gravitation fractionating of xenocrysts in kimberlite magma at high P-T parameters

This report considers experimental studies of the gravitation fractionating of xenocrysts (diamond, garnet, and olivine) in kimberlite magma at 4.0 GPa and 1400°C. The values obtained (0.6–0.7 m/h for 0.3-mm diamond crystals, 0.36 m/h for garnet grains, and 0.6–0.29 m/h for olivine grains) point to a high rate of xenocryst sinking in the ultralow-viscous kimberlite magma (as high as 1 m/h and more, depending on the densities and grain sizes of minerals). A high rate of xenocryst sinking in kimberlite magma results in the impossibility of preservation of heterogeneity in these melts for a sufficiently long time.     

The P3 kimberlite and P4 lamproite,Wajrakarur kimberlite field,India: mineralogy,and major and minor element compositions of olivines as records of their phenocrystic vs xenocrystic origin

Distinctly different groundmass mineralogy characterise the hypabyssal facies, Mesoproterozoic diamondiferous P3 and P4 intrusions from the Wajrakarur Kimberlite Field, southern India. P3 is an archetypal kimberlite with macrocrysts of olivine and phlogopite set in a groundmass dominated by phlogopite and monticellite with subordinate amounts of serpentine, spinel, perovskite, apatite, calcite and rare baddeleyite. P4 contains mega- and macrocrysts of olivine set in a groundmass dominated by clinopyroxene and phlogopite with subordinate amounts of serpentine, spinel, perovskite, apatite, and occasional gittinsite, and is mineralogically interpreted as an olivine lamproite. Three distinct populations of olivine, phlogopite and clinopyroxene are recognized based on their microtextural and compositional characteristics. The first population includes glimmerite and phlogopite–clinopyroxene nodules, and Mg-rich olivine macrocrysts (Fo 90–93) which are interpreted to be derived from disaggregated mantle xenoliths. The second population comprises macrocrysts of phlogopite and Fe-rich olivine (Fo 81–89) from P3, megacrysts and macrocrysts of Fe-rich olivine (Fo 85–87) from P4 and a rare olivine–clinopyroxene nodule from P4 which are suggested to have a genetic link with the precursor melt of the respective intrusions. The third population represents clearly magmatic minerals such as euhedral phenocrysts of Fe-rich olivine (Fo 85–90) crystallised at mantle depths, and olivine overgrowth rims formed contemporaneously with groundmass minerals at crustal levels. Close spatial association and contemporaneous emplacement of P3 kimberlite and P4 lamproite is explained by a unifying petrogenetic model which involves the interaction of a silica-poor carbonatite melt with differently metasomatised wall rocks in the lithospheric mantle. It is proposed that the metasomatised wall rock for lamproite contained abundant MARID-type and phlogopite-rich metasomatic veins, while that for kimberlite was relatively refractory in nature.

     

Djerfisherite in xenoliths of sheared peridotite in the Udachnaya-East pipe (Yakutia): origin and relationship with kimberlitic magmatism

Djerfisherite, a Cl-bearing potassium sulfide (K₆Na(Fe,Ni,Cu)₂₄S₂₆Cl), is a widespread accessory mineral in kimberlite-hosted mantle xenoliths. Nevertheless, the origin of this sulfide in nodules remains disputable. It is usually attributed to the replacement of primary Fe–Ni–Cu sulfides when xenoliths interact with a K-and Cl-enriched hypothetical melt/fluid. The paper is devoted to a detailed study of the composition and morphology of djerfisherite from a representative collection (22 samples) of the deepest mantle xenoliths—sheared garnet peridotite, taken from the Udachnaya-East kimberlite pipe (Yakutia). Four types of djerfisherite were distinguished in the mantle rocks on the basis of morphology, spatial distribution, and relationships with the rock-forming and accessory minerals in the nodules. Type 1 was found in the rims of polysulfide inclusions in the rock-forming minerals of the xenoliths; there, it was younger than the primary sulfide assemblage pyrrhotite + pentlandite ± chalcopyrite. Type 2 formed rims around large polysulfide segregations (pyrrhotite+ pentlandite) in the xenolith interstices. Type 3 formed individual grains in the xenolith interstices together with other sulfides, silicates, oxides, phosphates, and carbonates. Type 4 was present as a daughter phase in the secondary melt inclusions which occurred in healed cracks in the rock-forming minerals of the xenoliths. Along with djerfisherite, the inclusions contained silicates, oxides, phosphates, carbonates, alkaline sulfates, chlorides, and sulfides. The results indicate that djerfisherite from the xenoliths is consanguine with kimberlite. Djerfisherite both in the sheared-peridotite xenoliths from the Udachnaya-East pipe and in different xenoliths from other kimberlite pipes worldwide formed owing to the interaction between the nodules and kimberlitic melts. Djerfisherite forming individual grains in the melt inclusions and xenolith interstices crystallized directly from the infiltrating kimberlitic melt. Djerfisherite bounding the primary Fe–Ni ± Cu sulfides formed by their replacement as a result of a reaction with the kimberlitic melt.     

Melt Inclusions in Olivine Phenocrysts: Using Diffusive Re-equilibration to Determine the Cooling History of a Crystal, with Implications for the Origin of Olivine-phyric Volcanic Rocks

A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 3–5 months, corresponding to coolingrates faster than 1–2°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion     

A review and assessment of experiments on Kimberlites,Lamproites and Lamprophyres as a guide to their Origin

Liquidus experimental studies on kimberlite, lamproite and lamprophyre compositions are reviewed with respect to the information they carry on the mantle origin of these rock types. This information is coupled with melting experiments on peridotite in the presence of H₂O and mixed H₂O+CO₂ volatile species. The origin of most lamproites is explained by the melting of mica-harzburgite assemblages at depths ranging from 40km for leucite lamproites to more than 150km for olivine lamproites. Clinopyroxene-rich, silica-poor lamproites remain enigmatic, but are possibly derived by the melting of a mica-bearing ultramafic source richer in clinopyroxene and under more oxidized, CO₂-bearing conditions. There are insufficient experimental studies on kimberlite to reasonably constrain their origin, and what remain are only general indications of the compositions of partial melts of mantle under volatile-bearing conditions. Melt compositions are not sufficiently well known to prevent very conceptual use of melt ‘names’ such as ‘kimberlitic’ or ‘carbonatitic’, and melts similar to alkaline and ultramafic lamprophyre may be hidden under this shroud. Clearer definition of the origins of alkaline melt compositions such as kimberlites and various lamprophyre types badly needs more exact bracketing of melt compositions of a variety of possible mantle mineral assemblages. The recently-developed sandwich reversal technique is ideally suited to study small degrees of partial melting, and could usefully be applied to lherzolitic and non-lherzolitic materials with hydrous and/or carbonate minerals.     

Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins

Olivine is the principal mineral of kimberlite magmas, and isthe main contributor to the ultramafic composition of kimberliterocks. Olivine is partly or completely altered in common kimberlites,and thus unavailable for studies of the origin and evolutionof kimberlite magmas. The masking effects of alteration, commonin kimberlites worldwide, are overcome in this study of theexceptionally fresh diamondiferous kimberlites of the Udachnaya-Eastpipe from the Daldyn–Alakit province, Yakutia, northernSiberia. These serpentine-free kimberlites contain large amountsof olivine (50 vol.%) in a chloride–carbonate groundmass.Olivine is represented by two populations (olivine-I and groundmassolivine-II) differing in morphology, colour and grain size,and trapped mineral and melt inclusions. The large fragmentalolivine-I is compositionally variable in terms of major (Fo_85–94)and trace element concentrations, including H₂O content (10–136ppm). Multiple sources of olivine-I, such as convecting andlithospheric mantle, are suggested. The groundmass olivine-IIis recognized by smaller grain sizes and perfect crystallographicshapes that indicate crystallization during magma ascent andemplacement. However, a simple crystallization history for olivine-IIis complicated by complex zoning in terms of Fo values and traceelement contents. The cores of olivine-II are compositionallysimilar to olivine-I, which suggests a genetic link betweenthese two types of olivine. Olivine-I and olivine-II have oxygenisotope values (+ 5·6 ± 0·1 VSMOW, 1 SD)that are indistinguishable from one another, but higher thanvalues (+ 5·18 ± 0·28) in ‘typical’mantle olivine. These elevated values probably reflect equilibriumwith the Udachnaya carbonate melt at low temperatures and ¹⁸O-enrichedmantle source. The volumetrically significant rims of olivine-IIhave constant Fo values (89·0 ± 0·2 mol%),but variable trace element compositions. The uniform Fo compositionsof the rims imply an absence of fractionation of the melt'sFe²⁺/Mg, which is possible in the carbonatite melt–olivinesystem. The kimberlite melt is argued to have originated inthe mantle as a chloride–carbonate liquid, devoid of ‘ultramafic’or ‘basaltic’ aluminosilicate components, but becameolivine-laden and olivine-saturated by scavenging olivine crystalsfrom the pathway rocks and dissolving them en route to the surface.During emplacement the kimberlite magma changed progressivelytowards an original alkali-rich chloride–carbonate meltby extensively crystallizing groundmass olivine and gravitationalseparation of solids in the pipe. KEY WORDS: kimberlite; olivine; partial melting; carbonatitic melt; oxygen isotopes; H₂O     

辽宁永宁地区含金刚石闪长质岩石显微特征及金刚石来源探讨

辽宁永宁地区发育一套闪长质火山岩，该岩石中曾选获金刚石及其指示矿物，因此成为辽宁省金刚石第Ⅳ成矿带。通过对该岩石矿物组成及其显微结构构造特征研究，表明：(1)闪长质岩石中的斑晶斜长石具有环带结构；(2)橄榄石及金云母具有多世代性，且目估橄榄石+金云母含量在10%以上，局部更为富集；(3)出现“橄榄石+金云母+石榴石”与“斜长石+辉石+角闪石”不平衡的矿物组合；(4)发现橄榄石有碎斑结构和金伯利岩角砾。初步认为，永宁地区的闪长质岩浆在上升过程中捕获了早先已形成的金伯利岩并与之发生熔融混合作用，金刚石来自于深部的金伯利岩。这一认识对该地区寻找金刚石原生矿具有启示意义。     

辽宁瓦房店30号金伯利岩管地球化学特征及金刚石矿床成因

对瓦房店地区30号金伯利岩管岩石学、地球化学和成矿地质条件等研究认为,30号岩管金伯利岩岩浆来源于深部地幔,部分遭到壳源混染,碳来源于深源,属于富含碱性组分正常系列超基性岩;岩管中金伯利岩SiO_2、MgO含量较低,而CaO、CO_2含量较高;微量元素含量与地幔中石榴二辉橄榄岩微量元素含量相似;矿床形成于晚古生代;郯庐断裂为导矿构造,金伯利岩浆在北东东向和东西向断裂交汇处侵入成矿,岩管是由爆发与侵入交替作用形成。     

First-principles calculations of equilibrium fractionation of O and Si isotopes in quartz,albite, anorthite,and zircon

In this study, we used first-principles calculations based on density functional theory to investigate silicon and oxygen isotope fractionation factors among the most abundant major silicate minerals in granites, i.e., quartz and plagioclase (including albite and anorthite), and an important accessory mineral zircon. Combined with previous results of minerals commonly occurring in the crust and upper mantle (orthoenstatite, clinoenstatite, garnet, and olivine), our study reveals that the Si isotope fractionations in minerals are strongly correlated with SiO₄ tetrahedron volume (or average Si–O bond length). The ³⁰Si enrichment order follows the sequence of quartz > albite > anorthite > olivine ≈ zircon > enstatite > diopside, and the ¹⁸O enrichment follows the order of quartz > albite > anorthite > enstatite > zircon > olivine. Our calculation predicts that measurable fractionation of Si isotopes can occur among crustal silicate minerals during high-temperature geochemical processes. This work also allows us to evaluate Si isotope fractionation between minerals and silicate melts with variable compositions. Trajectory for δ³⁰Si variation during fractional crystallization of silicate minerals was simulated with our calculated Si isotope fractionation factors between minerals and melts, suggesting the important roles of fractional crystallization to cause Si isotopic variations during magmatic differentiation. Our study also predicts that δ³⁰Si data of ferroan anorthosites of the Moon can be explained by crystallization and aggregation of anorthite during lunar magma ocean processes. Finally, O and Si isotope fractionation factors between zircon and melts were estimated based on our calculation, which can be used to quantitatively account for O and Si isotope composition of zircons crystallized during magma differentiation.     

The significance of groundmass ilmenite and megacryst ilmenite in kimberlites

Criteria are suggested for distinguishing xenocrystic ilmenites from those indigenous to the host kimberlite. For instance, in contrast to groundmass grains, ilmenite xenocrysts usually are larger, have reaction rims of leucoxene and perovskite, exhibit strong magnesium enrichment outward, and sometimes have exsolution lamellae and deformation features. Most of the abundant ilmenite macrocrysts found in kimberlite appear to have been phenocrysts in a crystal mush unrelated to kimberlite. On the other hand, kimberlitic groundmass ilmenite is rare, but consistently more magnesian than the cores of macrocrysts. Strong Mg-enrichment patterns evident in the ilmenite macrocrysts probably developed during their attempt to equilibrate with the more magnesian, fractionating kimberlitic liquid. The hypothesis of extensive reaction of ilmenite with kimberlite melt/ fluid has implications with regard to the following: (1) the degree of differentiation of kimberlite melts; (2) the genesis of mantle megacrysts; (3) the reactivity of kimberlite; and (4) the usefulness of groundmass ilmenite as a petrogenetic indicator.     

Melt Inclusions in Primitive Olivine Phenocrysts: the Role of Localized Reaction Processes in the Origin of Anomalous Compositions

Melt inclusions are small portions of liquid trapped by growingcrystals during magma evolution. Recent studies of melt inclusionshave revealed a large range of unusual major and trace elementcompositions in phenocrysts from primitive mantle-derived magmaticrocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-Anplagioclase]. Inclusions in phenocrysts crystallized from moreevolved magmas (e.g. olivine Fo < 85 mol %), are usuallycompositionally similar to the host lavas. This paper reviewsthe chemistry of melt inclusions in high-Fo olivine phenocrystsfocusing on those with anomalous major and trace element contentsfrom mid-ocean ridge and subduction-related basalts. We suggestthat a significant portion of the anomalous inclusion compositionsreflects localized, grain-scale dissolution–reaction–mixing(DRM) processes within the magmatic plumbing system. The DRMprocesses occur at the margins of primitive magma bodies, wheremagma is in contact with cooler wall rocks and/or pre-existingsemi-solidified crystal mush zones (depending on the specificenvironment). Injection of hotter, more primitive magma causespartial dissolution (incongruent melting) of the mush-zone phases,which are not in equilibrium with the primitive melt, and mixingof the reaction products with the primitive magma. Localizedrapid crystallization of high-Fo olivines from the primitivemagma may lead to entrapment of numerous large melt inclusions,which record the DRM processes in progress. In some magmaticsuites melt inclusions in primitive phenocrysts may be naturallybiased towards the anomalous compositions. The occurrence ofmelt inclusions with unusual compositions does not necessarilyimply the existence of new geologically significant magma typesand/or melt-generation processes, and caution should be exercisedin their interpretation. KEY WORDS: melt inclusions; olivine; geochemistry; mush zones; MORB; subduction-related magmas     

Fast kimberlite ascent rates estimated from hydrogen diffusion profiles in xenolithic mantle olivines from southern Africa

Fourier transform infrared spectrometry (FTIR) analyses of olivines from peridotite xenoliths found in southern African kimberlites indicate 0 to 80 ppm H₂O concentrations. OH absorbance profiles across olivine grains show homogeneous H contents from core to edge for most samples. In one sample the olivines are H-free, while another has olivines characterized by lower H contents at the grain edges compared to the cores, indicating H loss during transport of the xenolith to the surface. Flat or near-flat H profiles place severe constraints on the duration of H loss from olivine grains, with implications for kimberlite magma ascent rates. Diffusion equations were used to estimate times of H loss of about 4 h for the sample with heterogeneous olivine H contents. Resulting kimberlite ascent rates are calculated to be 5-37 m s⁻¹ minimum, although these estimates are highly dependent on volatile contents and degassing behavior of the host kimberlite magma. Xenolithic olivines from alkali basalts generally have lower H contents and more pronounced H diffusion profiles than do those from kimberlites. This difference is likely caused by higher magma temperatures and lower ascent rates of alkali basalts compared to kimberlites.     

Compositional classification of “kimberlitic” and “non-kimberlitic” ilmenite

Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources.

Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO₂ graphs as well as MgO–Cr₂O₃ relationships.