Hydroxyl in mantle olivine xenocrysts from the Udachnaya kimberlite pipe

The incorporation of hydrogen in mantle olivine xenocrysts from the Udachnaya kimberlite pipe was investigated by Fourier-transform infrared spectroscopy and secondary ion mass spectrometry (SIMS). IR spectra were collected in the OH stretching region on oriented single crystals using a conventional IR source at ambient conditions and in situ at temperatures down to −180°C as well as with IR synchrotron radiation. The IR spectra of the samples are complex containing more than 20 strongly polarized OH bands in the range 3,730–3,330 cm⁻¹. Bands at high energies (3,730–3,670 cm⁻¹) were assigned to inclusions of serpentine, talc and the 10 Å phase. All other bands are believed to be intrinsic to olivine. The corresponding point defects are (a) associated with vacant Si sites (3,607 cm⁻¹ E || a, 3,597 E || a, 3,571 cm⁻¹ E || c, 3,567 E || c, and 3,556 E || b), and (b) with vacant M1 sites (most of the bands polarized parallel to a). From the pleochroic behavior and position of the OH bands associated with the vacant M1 sites, we propose two types of hydrogen—one bonded to O1 and another to O2, so that both OH vectors are strongly aligned parallel to a. The O2–H groups may be responsible for the OH bands at higher wavenumbers than those for the O1–H groups. The multiplicity of the corresponding OH bands in the spectra can be explained by different chemical environments and by slightly different distortions of the M1 sites in these high-pressure olivines. Four samples were investigated by SIMS. The calculated integral molar absorption coefficient using the IR and SIMS results of 37,500±5,000 L mol H₂O cm⁻² is within the uncertainties slightly higher than the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003) (28,450±1,830 L mol H₂O cm⁻²). The reason for the difference is the different distributions of the absorption intensity of the spectra of both studies (mean wavenumber 3,548 vs. 3,570 cm⁻¹). Olivine samples with a mean wavenumber of about 3,548 cm⁻¹ should be quantified with the absorption coefficient as determined in this study; those containing more bands at higher wavenumber (mean wavenumber 3,570 cm⁻¹) should be quantified using the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003).

Composition of olivine,silica activity and oxygen fugacity in kimberlite

Two generations of primary olivine are present in kimberlite, rounded phenocrysts (Fo₉₄Fo₉₁) and euhedral groundmass olivines (Fo₉₁Fo_88-5). Rounded phenocrysts less magnesian than Fo₈₈ are considered to be mantle derived xenocrysts; such crystals comprise up to 40% of the phenocrysts material in kimberlite. Calculated silica activity ranges from 10^?1,5 at 1200 °C, 50 kbs, to between 10^?1,6 and 10^?2.4 at 600 °C, 0.5–1.0 kb. Silica buffers involving olivines, pyroxenes and garnet are considered. Oxygen fugacities on the order of 10^?20 bars indicate that kimberlite magmas are highly reduced at the time of groundmass formation.     

OH point defects in olivine from Pakistan

The infrared (IR) spectra of gem-quality olivine crystals from Pakistan, formed in serpentinised dunitic rocks, are characterised by strongly pleochroic absorption bands at 3,613, 3,597, 3,580 and 3,566 cm^?1. These bands are assigned to O-H stretching vibrations of OH point defects corresponding to H₂O concentrations of about 35 wt ppm. Unlike other olivine spectra, the dominating bands are strongly polarised parallel to the b-axis. The unusual spectra type, excludes the presence of planar defects. This finding is supported by transmission electron microscopy. The 3,613 cm^?1 band is related to vacant Si sites, the slightly lower energetic bands preferentially to vacant M2 sites. The exclusive presence of these bands is not only a characteristic feature of olivines treated under high P,T conditions equivalent to mantle environment, the presence of these bands in untreated natural olivine also indicates formation conditions equivalent to crustal rocks.     

A computer simulation study of (OH) defects in olivine

Recent experiments (Miller et al. 1987; Bell and Rossman 1992; Bai and Kohlstedt 1992) have shown that olivine, the dominant mineral in the Earth's upper mantle, can contain substantial amounts of water in the form of OH. There is uncertainty, however, as to the mechanisms by which water dissolves into the mineral structure and as to the site it occupies in the host lattice. We have therefore used atomistic computer simulation techniques based on the Born model of solids, to investigate the structures and energies of OH defects in olivine. Our calculations suggest that the most favourable route for incorporation of OH into olivine involves reactions with water accompanied by the reduction of ferric iron for which we obtain a solution energy of 0.46 eV. We propose therefore that the OH content will be largely controlled by the concentration of Fe³⁺.     

The timing of kimberlite magmatism in North America: implications for global kimberlite genesis and diamond exploration

Based on a compilation of more than 100 kimberlite age determinations, four broad kimberlite emplacement patterns can be recognized in North America: (1) a northeast Eocambrian/Cambrian Labrador Sea province (Labrador, Québec), (2) an eastern Jurassic province (Ontario, Québec, New York, Pennsylvania), (3) a Cretaceous central corridor (Nunavut, Saskatchewan, central USA), and (4) a western mixed (Cambrian-Eocene) Type 3 kimberlite province (Alberta, Nunavut, Northwest Territories, Colorado/Wyoming). Ten new U–Pb perovskite/mantle zircon and Rb–Sr phlogopite age determinations are reported here for kimberlites from the Slave and Wyoming cratons of western North America. Within the Type 3 Slave craton, at least four kimberlite age domains exist: I-a southwestern Siluro-Ordovician domain (450 Ma), II-a SE Cambrian domain (540 Ma), III-a central Tertiary/Cretaceous domain (48–74 Ma) and IV-a northern mixed domain consisting of Jurassic and Permian kimberlite fields. New U–Pb perovskite results for the 614.5±2.1 Ma Chicken Park and 408.4±2.6 Ma Iron Mountain kimberlites in the State Line field in Colorado and Wyoming confirm the existence of at least two periods of pre-Mesozoic kimberlite magmatism in the Wyoming craton.

A compilation of robust kimberlite emplacement ages from North America, southern Africa and Russia indicates that a high proportion of known kimberlites are Cenozoic/Mesozoic. We conclude that a majority of these kimberlites were generated during enhanced mantle plume activity associated with the rifting and eventual breakup of the supercontinent Gondwanaland. Within this prolific period of kimberlite activity, there is a good correlation between North America and Yakutia for three distinct short-duration (10 my) periods of kimberlite magmatism at 48–60, 95–105 and 150–160 Ma. In contrast, Cenozoic/Mesozoic kimberlite magmatism in southern Africa is dominated by a continuum of activity between 70–95 and 105–120 Ma with additional less-prolific periods of magmatism in the Eocene (50–53 Ma), Jurassic (150–190) and Triassic (235 Ma). Several discrete episodes of pre-Mesozoic kimberlite magmatism variably occur in North America, southern Africa and Yakutia at 590–615, 520–540, 435–450, 400–410 and 345–360 Ma. One of the surprises in the timing of kimberlite magmatism worldwide is the common absence of activity between about 250 and 360 Ma; this period is even longer in southern Africa. This >110 my period of quiescence in kimberlite magmatism is likely linked to relative crustal and mantle stability during the lifetime of the supercontinent Gondwanaland.

Economic diamond deposits in kimberlite occur throughout the Phanerozoic from the Cambrian (Venetia, South Africa; Snap Lake and Kennady Lake, Canada) to the Tertiary (Mwadui, Tanzania; Ekati and Diavik in Lac de Gras, Canada). There are clearly some discrete periods when economic kimberlite-hosted diamond deposits formed globally. In contrast, the Devonian event, which is such an important source of diamonds in Yakutia, is notably absent in the kimberlite record from both southern Africa and North America.     

An image analysis method to determine crystal size distributions of olivine in kimberlite

Crystal size distributions (CSDs) are a standard method of describing populations of crystals within magmatic rocks. Olivine is the dominant phase in kimberlite (∼40–50% by volume) and features a diverse range of sizes, shapes and origins. CSDs of olivine provide a logical means of semi-quantitatively characterising kimberlite. The CSDs can then be used to distinguish or correlate between kimberlite bodies or to investigate processes related to ascent, emplacement and eruption. In this paper, we present an automatic image analysis technique that provides efficient quantification of olivine CSDs within digital images of polished slabs of kimberlite. This technique relies on a combination of algorithms for detecting regions of interest (ROI) and for segmentation of ROIs in order to identify individual olivine crystals that are used for size distribution datasets. The detection process identifies regions expected to be olivine using a model-based colour detection technique using Mahalanobis distance combined with texture analysis based on local standard deviation and greyscale foreground enhancement techniques. The segmentation process separates adjacent domains to identify individual crystals using an iterative marker-based watershed algorithm to separate adjoined structures of varying sizes. We demonstrate the utility of automatic image analysis by comparing CSDs for olivine derived from this method versus results from manual digitisation of olivine grains. The automatic detection system correctly identified ∼86% of the manually detected olivine domains; ∼88% of the automatically detected regions correctly correlate to manually defined olivine grains. Discrepancies between the two methods are mostly the result of oversimplification of crystal margins (i.e. rounding) by manual tracing whereas automatic boundary recognition shows clear advantages in identifying irregularities in crystal edges. Closer examination of the results shows that both methods suffer from under-representation of smaller crystals due to: (1) human subjectivity and error in manual tracing and (2) noise removal processes in automatic detection. Automatic detection of olivine grains is much more efficient than conventional manual tracing; manual detection requires ∼6 h per sample versus ∼1 min for automatic analysis of the same sample.     

A model of the OH positions in olivine,derived from infrared-spectroscopic investigations

Polarized infrared (IR) spectroscopy of olivine crystals from Zabargad, Red Sea shows the existence of four pleochroic absorption bands at 3,590, 3,570, 3,520 and 3,230 cm^?1, and of one non pleochroic band at 3,400 cm^?1. The bands are assigned to OH stretching frequencies. Transmission electron microscopy (TEM) shows no oriented intergrowths in this olivine; it is concluded that OH is structural. On the basis of the pleochroic scheme of the absorption spectra it is proposed that [□O(OH)₃] and [□O₂(OH)₂] tetrahedra occur as structural elements, assuming that the vacancies are on Si sites. If M2 site vacancies were assumed [SiO₃(OH)] and [SiO₂(OH)₂] tetrahedra occur as structural elements.     

OH absorption coefficients of rutile and cassiterite deduced from nuclear reaction analysis and FTIR spectroscopy

Summary The OH content of four rutile and two cassiterite single-crystals was studied by nuclear reaction analysis (NRA) and by polarised FTIR microspectroscopy. The OH absorption bands of both minerals are centered around 3300 cm⁻¹ with different absorption features. The analytical H₂O content determined by NRA ranges from 70 to 820 wt.ppm. The integrated molar absorption coefficients deduced from the total integrated OH absorbances are equal to 38000 lċmol⁻¹ _H2Oċcm⁻² for rutile and 65000 lċmol⁻¹ _H2Oċcm⁻² for cassiterite. For both minerals the absorption coefficients are significantly smaller than those expected from the linear calibration curves given by Paterson (1982) and by Libowitzky and Rossman (1997).

---

Zusammenfassung OH-Absorptionskoeffizienten von Rutil und Cassiterit ermittelt durch Kernreaktions-Analyse und FTIR Spektroskopie Der OH-Gehalt von vier Rutil- und zwei Cassiterit-Einkristallen wurde mittels Kernreaktions-Analyse (NRA) und polarisierter FTIR Mikrospektroskopie untersucht. Die OH Absorptionsbanden beider Minerale sind um 3.300 cm⁻¹ zentriert, mit unterschiedlichen Absorptionserscheinungen. Der analytische H₂O-Gehalt, der mit NRA bestimmt wurde, schwankt von 70 bis 820 Gew.ppm. Die integrierten molaren Absorptionskoeffizienten, die auf den gesamten integrierten OH-Absorptionen basieren, betragen etwa 38.000 lċmol⁻¹ _H2Oċcm⁻² für Rutil and 65.000 lċmol⁻¹ _H2Oċcm⁻² für Cassiterit. Für beide Minerale sind die Absorptionskoeffizienten signifikant kleiner als die, die auf Grund der linearen Kalibrationskurven von Paterson (1982) und Libowitzky und Rossmann (1997) zu erwarten sind.

---

---

Received January 4, 2000; revised version accepted April 10, 2000     

Geochemical hydrocarbon exploration — a new/old exploration tool

The search for petroleum has evolved into a highly sophisticated technology where today almost every scientific discipline known is being brought to bear upon the endeavour. Yet, the use of geochemical hydrocarbon exploration remains a peripheral exploration tool. The trend toward scientific integration has led the petroleum explorationist to the point of being a specialist. It would seem that our petroleum scientists have focussed their interests mainly on the investigation of principles and less on their ultimate purpose of discovering new and larger oil and gas reserves. So, it is not by chance, that leading geochemists have been speaking more and more freely of the necessity to integrate our tools of exploration and thereby do a better job. The theoretical basis for hydrocarbon geochemistry is complex, and, as with all exploration tools, the problems and difficulties of interpreting the data will never be completely eliminated.This article considers the importance of using the ΔC method in geochemical hydrocarbon exploration which has been employed successfully for over 40 years. The addition of carbon-isotope ratios and trace-element analysis to this method has added a new dimension to geochemical hydrocarbon exploration. The theoretical basis of the ΔC method has been presented earlier by the author and will only be touched upon briefly here.Very simply, the basis of all geochemical hydrocarbon exploration is based on the much debated premise that the lighter hydrocarbon gases and their components migrate vertically from a trap through the overlying sedimentary pile to the surface. Upon reaching the surface, through oxidation, they leave their signatures in one form or another that can be detected by physicochemical methods. These physicochemical signatures are discernable as “geochemical haloes”.From soil samples, collected from 2–3 m deep, what is measured is the result of absorption and adsorption by soil particles that are altered to CO₂ by oxidation and form a unique, stable, carbonate system with the surface and near-surface material. This is unlike other carbonate systems and when subjected to a differential thermal technique, dissociates into CO₂ surface material is cumulative and indicates where maximum hydrocarbon leakage has taken place over the life span of the material sampled. It is durable and unaffected by pressure and temperature variation or recent hydrocarbon contamination.Values are expressed in terms of millivolts which are proportional to the CO₂ given off by the dissociation of the carbonate system under standard conditions. Frequency curves are constructed for all values for the determination of significant contour levels above the normal geochemical background for mapping.After significant ΔC anomalies are located, they can be further verified by use of carbon-isotope ratios. As methane migrates to the surface from underlying hydrocarbon accumulations, there is a progressive selection or fractionation that causes enrichment of the carbon-13 isotope. The methane, thus reaching the near-surface, is isotopically lighter. When oxidized in accordance with the equation CH₄ + 2O₂ → 2H₂O + CO₂, the carbon having been converted to carbon dioxide, is taken up in the pore-filling carbonate cements that are found in the near-surface soils and sediments.High carbon dioxide values (ΔC) in the geochemical halo are related the δ ¹³C carbon-isotope ratios from underlying hydrocarbon accumulations. This is observed over fields containing hydrocarbon accumulations where δ ¹³C values in the pore-filling carbonate cements become increasingly negative (lighter) toward the crests of traps (i.e. exhibiting lower ΔC values). This indicates enrichment of ¹²C relative to the PDB standard. Whereas, positive values of δ ¹³C indicate depletion in ¹²C or enrichment in ¹³C (i.e. exhibiting higher ΔC values away from the crests of the traps).The observed ΔC anomalies and δ ¹³C anomalies leave an indelible pattern in the near-surface sediments and soils which are herein referred to as geochemical hydrocarbon haloes.Trace-element associations, that form organometallic compounds, are found “haloed” or concentrated over or around underlying hydrocarbon reservoirs. These associations seem to have occurred from vertically migrating methane that has acted as a “carrier” sweeping up the trace elements on the pathways to the surface. Vanadium, nickel, chromium, iron, cobalt, copper, manganese, strontium, barium are various trace element ratios seen to also halo and indicate subsurface hydrocarbon accumulations.An example presented from the Ocho-Juan Field, a producing reef field, located in Scurry and Fisher Counties, Texas shows that the combination of ΔC, δ ¹³C and trace-element analysis from near-surface soil sampling is a significant step forward in improving geochemical hydrocarbon exploration methods.     

FTIR spectroscopy of Ti-chondrodite,Ti-clinohumite,and olivine in deeply subducted serpentinites and implications for the deep water cycle

Separated olivine grains from a deeply subducted serpentinized wehrlite (Changawuzi in the western Tianshan ultrahigh-pressure belt, China) were analysed with unpolarized transmission FTIR and the Ti contents were determined using LA-ICP-MS. The major bands in the olivine spectra display striking similarities to Ti-clinohumite and are interpreted as OH in lamellae. The quantification of the water content in lamellae requires calibration of the IR-absorption for such bands. We have obtained a new absorption coefficient for Ti-clinohumite of 0.125+/?0.017 ppm cm² based on polarized FTIR measurements on three orthogonal sections through a large single crystal of Ti-clinohumite from Val Malenco, which has a known water content of 1.53 wt%. The resulting water content in olivine using this calibration factor ranges from 440 to 2,600 ppm and correlates positively with the Ti content that ranges from 130 to 1,400 ppm. For the quantification of the water content in Ti-chondrodite and Ti-clinohumite that are associated with olivine, we developed a new method using attenuated total reflectance FTIR spectroscopy. Ti-chondrodite and Ti-clinohumite display similar IR bands at ~3,562, 3,525 and ~3,583–3,586 cm^?1. As in olivine, the water content and Ti content correlate in both Ti-clinohumite and Ti-chondrodite, indicating an intergrowth of these minerals, which has been confirmed by TEM analyses. Our results confirm previous suggestions that there is a strong correlation between the Ti content of ultramafic rocks and their capacity to transport water to the deeper mantle in subduction zones beyond conditions where hydrous phases are stable.     

一种冻土区天然气水合物地球化学勘查新技术——惰性气体氦氖分析

开发不受沼泽微生物影响的地球化学勘查技术是提高中纬度冻土区天然气水合物探井预测成功率的重要课题之一。本文选择在祁连山聚乎更天然气水合物已知区进行惰性气体勘查技术实验,研究了氦氖的测试方法,实验区为高寒沼泽景观,面积150km~2,采样密度2点/km~2,采样深度60cm,采集土壤顶空气样品300件和DK-3井岩芯样品400件,应用色谱反吹技术对顶空气样品进行了惰性气体氦氖的分析。结合地质和地球化学勘查成果进行了综合解释,认为惰性气体异常与天然气水合物矿藏关系密切,与烃类异常浓度范围一致,为顶部异常模式。实验区天然气水合物矿藏11个水合物发现井有10个位于He、Ne异常内,1个井位于异常外。分析了天然气水合物岩芯顶空气轻烃和氦氖指标的垂向分布特征,提出了天然气水合物矿藏上方土壤惰性气体的地气迁移机理。研究区近地表氦氖异常源于深部水合物矿藏和断裂构造,不受沼泽微生物的影响,是冻土区天然气水合物勘查的一种有效技术。     

Prediction of rock alteration patterns: A potential tool in mineral exploration

Hydrothermal alteration of a quartz‐K‐feldspar rock is simulated numerically by coupling fluid flow and chemical reactions. Introduction of CO₂ gas generates an acidic fluid and produces secondary quartz, muscovite and/or pyrophyllite at constant temperature and pressure of 300°C and 200 MPa. The precipitation and/or dissolution of the secondary minerals is controlled by either mass‐action relations or rate laws. In our simulations the mass of the primary elements are conserved and the mass‐balance equations are solved sequentially using an implicit scheme in a finite‐element code. The pore‐fluid velocity is assumed to be constant. The change of rock volume due to the dissolution or precipitation of the minerals, which is directly related to their molar volume, is taken into account. Feedback into the rock porosity and the reaction rates is included in the model. The model produces zones of pyrophyllite quartz and muscovite due to the dissolution of K‐feldspar. Our model simulates, in a simplified way, the acid‐induced alteration assemblages observed in various guises in many significant mineral deposits. The particular aluminosilicate minerals produced in these experiments are associated with the gold deposits of the Witwatersrand Basin.     

The behavior of protons during wet olivine deformation under the conditions of the kimberlite process

This paper reports the results of optical and electron microscopic investigations of mantle olivine samples with H₂O contents of tens-hundreds ppm weight. Samples were obtained from the xenoliths and xenocrysts of the Udachnaya pipe. At the scale of optical microscope magnification, a peculiar banded microstructure was observed in thin sections prepared parallel to the olivine (010) plane. It is formed by cross-hatched bands parallel to four crystallographic directions of the olivine structure: [100], [001], [101], and [−101]. At the scale of electron optical magnifications, the banded microstructure is observed as nanometer-sized heterogeneities of various types which are related to olivine deformation: (a) planar defects parallel to (100) and (001) corresponding to the (100)[010] and (001)[100] dislocation glide systems, respectively; they are occasionally transformed into lamellae or decorated by nanoinclusions; and (b) nanometer-sized heterogeneities formed by nanoinclusion arrays not related to planar defects and oriented along the same directions of the olivine structure as the optically visible bands. The deformation structures are decorated by coupled point OH-bearing defects, which were initially present in the olivine. The crystallographically oriented arrays of nanoinclusions of high-pressure hydrous silicates are considered as a result of olivine deprotonization (elimination of OH-bearing defects from the olivine structure) in the zones of previous deformation compression in the crystal. Light refraction effects on the nanoinclusions make these zones optically visible and produce the banded microstructure, which is a consequence of previous deformation.     

The significance of termites on the future of kimberlite exploration in Botswana

The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline.

     

Results of study of crystallographic orientation of olivine and diamond from Udachnaya kimberlite pipe,Yakutia

The crystallographic orientation of three diamonds and 19 olivine inclusions from Udachnaya kimberlite pipe was studied using monocrystal X-ray diffractometry. No epitaxial olivine inclusions were found.     

Infrared spectroscopy of Triassic conodonts: a new tool for assessing conodont diagenesis

The Fourier transform infrared spectra of individual conodonts with various CAI values indicate definite changes in conodont francolite during diagenesis. Steady decarbonation can be observed in the increasing intensity of the band with the wavenumber 2340 an^-1, which is assigned to trapped molecular CO₂. Carbon dioxide originates from decomposing CO^2-₃-ions occupying the B-site in francolite. Furthermore, the intensity of the water-deformation band at 1644 cm^-1 decreases with higher CAI values. These changes of intensity can be quantified by calculating the maximum extinction (E_max) for the corresponding absorption maxima and correlated with the CAI. Unaltered conodonts have a relative variable water 'content' and very little CO₂. Up to a CAI value of 4 conodont francolite continuously expells water and carbon dioxide is trapped. Conodonts with CAI = 5 have similar water 'contents' to CAI = 4 conodonts, but decarbonation continues to take place during this stage of diagenesis.     

OH in natural orthopyroxene: an in situ FTIR investigation at varying temperatures

In situ unpolarized and polarized Fourier transform infrared spectra of a natural orthopyroxene at varying temperatures were obtained using a heating stage attached on an Infrared microscope. The three main bands (3,595, 3,520 and 3,410 cm⁻¹) at room temperature are ascribed to OH fundamental stretching bands. With increasing temperature from room temperature to 500 °C, the 3,595 cm⁻¹ band shifts 20 cm⁻¹ to lower frequency. The total integral absorbance decreases with increasing temperature. These changes are reversible. Excluding the influences of dehydration, proton migration, thermal expansion, and changes in OH dipole direction, the change of integral absorbance with temperature reflects the temperature dependence of absorption coefficient due to the anharmonicity of OH vibration. Based on the integral absorption coefficient at room temperature (14.84 ppm⁻¹ cm⁻²) from Bell et al. (Am Mineral 80:463–474, 1995), the integral absorption coefficients at other temperatures are calculated. The variation of integral absorption coefficient between room temperature and 500 °C obtained in this study is about 18.5 % and may be greater at higher temperature according to the proposed linear relationship.     

A classification scheme for mantle-derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds

A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.     

Zoning of diamonds from the Mir kimberlite pipe: Results of Fourier-transformed infrared spectroscopy

Plates prepared of diamonds from the Mir kimberlite pipe were examined with FTIR spectroscopy. It is shown that B1 defects were formed by annealing during crystal growth, whereas B2 centers arose after growth cessation. The development of B2 centers in a natural diamond is the secondary process with respect to the aggregation of the nitrogen admixture. The kinetics of this process is related to the breakdown of an oversaturated solid solution. The results obtained make it possible to estimate the temperature and duration of natural diamond growth.     

巨型矿床勘查新战略一一信息找矿

直接信息是最可靠的找矿信息,在矿产勘查中必须起先导的作用。直接信息与间接信息在一定条件和环境下可以互相转化的,只有在信息具有直接指示矿床存在和分布的特性时,它才会发挥实际的找矿效能。因此,多学科信息的收集和综合分析,是信息找矿战略实施的核心。信息找矿战略可以表述为“针对巨型矿床勘查,我们应当瞄准稳陷伏矿和难识别矿,以直接找矿信息（化探资料）为先导,综合地质和地球物理信息,迅速掌握全局,逐步缩小靶区