Mineralogy of beneficiation problems involving fluorspar concentrates from carbonatite-related fluorspar deposits

Summary The World's three largest producers of fluorite from carbonatite-related fluorspar ore deposits are located at Okorusu (Namibia), Amba Dongar (India), and Mato Preto (Brazil). Beneficiation problems involving fluorite concentrates from those three deposits share similar characteristics that are directly related to the mineralogy and textures of the ores. The most important of these beneficiation problems involves their phosphorus, silica, and lime contents.Because the fluorite-depositing hydrothermal fluids were partly or largely derived from carbonatite sources, and carbonatites typically are rich in phosphorus, carbonatite-related fluorite deposits would be expected to be characterized by significant amounts of phosphorus mineral in the form of apatite. In the beneficiation products from those ore deposits, apatite occurs as free particles and especially as particles locked with fluorite. The presence of apatite in fluorite concentrates may contribute significant amounts of phosphorus, a deleterious constituent in fluorspar concentrates used in the steelmaking industry.Fluorite concentrates from some carbonatite-related fluorspar deposits are characterized by significant amounts of silica. The silica occurs especially in the form of quartz, potash feldspar, and sericite. Quartz occurs in both free particles and in particles where it is locked with fluorite. Quartz was deposited late in the paragenetic sequence and typically fills small vugs between the fluorite crystals. Potash feldspar formed during early potassic fenitization associated with the magmatic carbonatite emplacement. Potassic feldspar forms intricate intergrowths with fluorite that result in locked feldspar-fluorite particles in the fluorspar concentrates. The potash feldspar is intensely altered to sericite.Fluorite deposits that occur within a carbonatite host, such as those deposits at Amba Dongar and some deposits at Mato Preto, may have the grades of their fluorite concentrate diluted by the presence of calcite. The calcite commonly is present as binary locked calcite-fluorite particles in those fluorspar concentrates.Although the beneficiation problems concerning fluorite concentrates from carbonatite-related fluorspar deposits may be effectively studied by petrographic and ore microscopic techniques, cathodoluminescence microscopy has been found to be uniquely suited to rapid mineral recognition and the study of those minerals involved in fluorspar beneficiation problems.

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Mineralogische und aufbereitungstechnische Probleme von karbonatitischen Fluoritlagerstätten

Zusammenfassung Die weltgrößten Produzenten von Fluorit aus karbonatitischen Fluoritlagerstätten sind Okoruso (Namibia), Amba Dongar (Indien) und Mato Preto (Brasilien). Die Aufbereitungsprobleme der Fluoritkonzentrate aller drei Lagerstätten sind sehr ähnlich und stehen in direktem Zusammenhang mit der Mineralogie und Textur der Erze. Die wichtigsten Aufbereitungsprobleme sind durch die Phosphor-, Silizium- und Kalkge halte bedingt. Weil die hydrothermalen Fluide teilweise bzw. größtenteils aus phosphorreichen karbonatitischen Quellen stammen, ist zu erwarten, daß karbonatitische Fluoritlagerstätten beträchtliche Mengen an Phosphor in Form von Apatit führen. In den Aufbereitungsprodukten dieser Erze tritt Apatit sowohl in Form freier Partikel als auch verwachsen mit Fluorit auf. Die durch die Präsenz von Apatit verursachten erhöhten Phosphorgehalte sind bei Fluoritkonzentraten, die in der Stahlindustrie eingesetzt werden, störend.Fluoritkonzentrate einiger karbonatitischer Fluoritlagerstätten sind durch beträchtliche Siliziumgehalte charakterisiert, die durch Quarz, Alkalifeldspat und Serizit bedingt sind. Quarz tritt auf in Form freier Partikel, aber auch in engster Verwachsung mit Fluorit. Quarz wurde spät in der paragenetischen Abfolge ausgeschieden und füllttypischerweise kleine Hohlräume zwischen Fluoritkristallen. Alkalifeldspat bildete sich während der frühen potassischen Fenitisierung, die die Platznahme der Karbonatite begleitete. Alkalifeldspat ist engstens mit Fluorit verwachsen, was zu nicht trennbaren Feldspat — Fluoritpartikelen in den Konzentraten führt. Alkalifeldspat ist intensiv zu Serizit alteriert.Einigen Fluoritlagerstätten in karbonatitischen Wirtsgesteinen, wie etwa Amba Dongar und einige Lagerstätten bei Mato Preto, lassen eine Verdünnung des Fluoritkonzentrates durch die Anwesenheit von Calcit erkennen. Calcit kommt in den Konzentraten üblicherweise in Form binär verwachsener Calcit — Fluoritpartikel vor.Obwohl die Aufbereitungsprobleme von Fluoritkonzentraten aus karbonatitischen Fluoritlagerstätten sehr effektiv durch petrographische und erzmikroskopische Methoden studiert werden können, hat sich der Einsatz von Kathodenlumineszenz zur raschen Identifikation und für das Detailstudium der bei der Aufbereitung von Fluorit beteiligten Minerale bestens bewährt.

     

多接收器等离子体质谱法Zn同位素比值的高精度测定

详细报道了Zn同位素比值的多接收器等离子体质谱(MC-ICP-MS)高精度测定方法,包括:MC-ICP-MS Zn同位素测量过程中的质量歧视校正、同质异位素干扰评估、基质效应调查和同位素测量的长期重现性检验.研究表明,在测定条件下,运用标样一样品交叉法能有效地进行仪器质量歧视校正.同质异位素干扰的评估通过3种方式进行,即:在高分辨状态下同质异位数干扰信号的直接测定,低分辨状态下Zn同位素原始数据间相关关系的检验和低分辨下浓度梯度效应研究.结果表明,在低分辨模式下,尽管66Zn、67Zn、68Zn的同质异位素干扰信号很小,但的确存在,要获得准确同位素比值,必须使标样和样品的浓度在合适的范围内匹配.在基质效应方面,主要考察Fe对Zn同位素比值测定的影响.结果表明,当溶液中Fe/Zn(质量比)不大于0.2时,Fe对Zn同位素比值测定无影响.重复性测定中,δ66ZnGSB-Romil=6.96‰±0.11‰(2sd),δ67ZnGSB-Romil=10.4‰±0.20‰(2sd),δ68ZnGSB-Romil=13.8‰±0.22‰(2sd),达到国际同类实验室先进水准.运用所建立的方法,对地质岩石成分分析国家标准物质GBW 07270(闪锌矿)进行了Zn同位素平均成分测定为:δ66Zn=6.71‰±0.03‰(20),δ67Zn=10.08‰±0.05‰(20),δ68Zn=13.37‰±0.07‰(2σ).     

The application of ICP-MS methods to tephrochronological problems

The accurate recognition of tephra deposits is of great value to Earth scientists because they facilitate stratigraphic correlation. The most useful tephra deposits form from violent volcanic eruptions; they are isochronous and widespread. Most are dacitic and rhyolitic in composition, and can be difficult to identify unequivocally using major element chemistry alone. Distal tephras are typically thin and are prone to contamination and thus are awkward to analyse by bulk methods. Here, the authors review their previous work in the development of analytical techniques for the analysis of small volumes of glass separates from tephra deposits, both by solution nebulisation and by laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS), placing particular emphasis on the precision and accuracy of the various methods. In solution nebulisation ICP-MS, accurate data can be obtained from samples as small as 0.025 g. LA-ICP-MS methods are described for the analysis of small bulk samples and single glass shards as small as 40 μm in diameter. Accurate and reproducible analyses can be achieved by ICP-MS by both solution and laser ablation methods on homogeneous materials. Solution analyses are normally accurate to ±5% and have typical precisions (1 σ) of around ±4% for abundant trace elements (e.g. Zr, Rb) but this can deteriorate to about ±20% for rare elements in small samples (e.g. HREE in a 25 mg sample). Laser ablation methods are slightly less accurate (typically ±5–10%) and precision decreases from about ±3% at concentrations of a few hundred ppm, to about ±10% at 1 ppm and about ±30% at 0.05 ppm. An apparent lack of precision in the bulk analysis of small volumes of glass shards by LA-ICP-MS often represents within sample heterogeneity (and not analytical error), inter-shard variation becoming abundantly clear in some tephra deposits when individual glass shards are analysed. Single grain analysis on shards as small as 40 μm can provide an accurate analysis of the pure glass phase, which may not be achieved in solution or bulk sample LA-ICP-MS methods. Analyses affected by micro-phenocryst phases, such as feldspar or zircon can be easily removed following careful inspection of the data. Single shard LA-ICP-MS also allows any compositional variation within the parental magma to be defined.     

On some problems of the genesis of hydrothermal deposits

Some important problems of the theory of hydrothermal ore formation that were discussed as early as in the works of F.I. Wolfson are briefly considered.     

Ore mineralization in naphtides and problems of its genesis

The paper presents several examples of metalliferous naphtide mineralization hosted in sedimentary and magmatic rocks and discusses different models of their origin. Geological, geochemical, experimental, and thermodynamic data were used to support a hypothesis assuming that metalliferous naphtide deposits were formed during cold and hot (with melt generation) mantle degassing with subsequent condensation of heavy hydrocarbons and organometallic compounds (OMC) in fault zones. Important criteria for discrimination between metalliferous naphtide mineralization of different origin include endo- and biomarkers, Nd and Sr isotopic data, and chondrite-normalized REE distribution patterns. The formation of endogenic metalliferous naphtide deposits does not rule out the possible assimilation of organic components from the host rock. Juvenile water and inorganic gases were found in the hydrothermal solutions contributing to the formation of these deposits.     

The application of finite elements to heat conduction problems involving latent heat

SummaryThe Application of Finite Elements to Heat Conduction Problems Involving Latent Heat A finite element method is developed for the solution of heat conduction problems which involve latent heat. The method is superior to other wellknown approaches to these problems in that it allows a wider range of material properties and boundary conditions to be dealt with, such as are encountered in complex engineering operations like ground freezing. The method is applied to simple triangular finite elements in this paper, although it could be extended to other types of element including three-dimensional ones. Several example problems are discussed and illustrated, and comparisons are made with other approaches where these can also be used.     

Silicon isotope abundance ratios and atomic weights of NBS-28 and other reference materials

The silicon isotope abundance ratios and atomic weights of NBS-28 and several other silicon isotope reference materials were determined in this study. For the calibration of the measurement procedure, two new synthetic isotope mixtures were prepared gravimetrically from highly enriched silicon isotope materials in the form of SiO₂. All materials were converted into SiF₄ gas and subsequently their silicon isotope ratios were measured on the SiF₃⁺ species by using a gas source mass spectrometer MAT-253. The calibrated isotope abundance ratios of NBS-28 are 0.0507446 (26) for ²⁹Si/²⁸Si and 0.0341465 (15) for ³⁰Si/²⁸Si. The corresponding isotopic abundances are 92.17515(28)% for f(²⁸Si), 4.67739(24)% for f(²⁹Si) and 3.14746(14)% for f(³⁰Si). The silicon molar mass of NBS-28 is determined to be 28.08653(11), which is 0.001 larger than previously reported values. In the meanwhile, the ²⁹Si/²⁸Si and ³⁰Si/²⁸Si isotope abundance ratios and atomic weights of SRM-990, IRMM-017, IRMM-018, GBW-04421 and GBW-04422 are also calibrated. These new calibrated data can improve the reliability and comparability of silicon isotope results using above calibrators.     

A revaluation of the Co/Ni ratio in pyrite as geochemical tool in ore genesis problems

On the basis of a sound evaluation of literature's data, the Co/Ni ratio in pyrite proves a more reliable indicator in ore genesis than previously retained. This is particularly true for volcanogenic pyritic ores which are constantly characterized by Co/Ni values (>5 but more often >10) definitely higher than those attained by sedimentary or hydrothermal pyrites. Moreover, once stated that an appropriate sampling has been performed, such geochemical studies, extended to as many deposits as possible from a same area, may provide relevant insights into the metallogenic history of the given area. On the basis of their Co/Ni ratios the Southern Tuscany ores fit in a two-stage metallogenic model: a primary (Paleozoic) emplacement of volcanogenic massive-sulfide deposits, in a sub-marine volcanic-exhalative environment, followed by later (Mio-Pliocenic) deposition of smaller, prevalently replacement deposits through hydrothermal partial mobilization of the older ones. The research of new ores may benefit from these geochemical studies too.     

昔格达组地层研究中需要注意的若干关键问题

昔格达组与下伏、上覆冲积卵石层之间的垂向叠置关系具有重要的沉积环境指示意义。主要由水平纹层状杂色细粒沉积构成的昔格达组常夹有数十厘米厚的冲积砂卵石层,而粉细砂层中还常见交错纹理和交错层理,成因研究应充分考虑其冲湖积特征和湖底底流;不同地区昔格达组细粒沉积的矿物成分和卵石层的岩石类型各异,基本无可比性。有些地区昔格达组地层中发育有同生变形构造,有些还见有较多的地质构造形迹。昔格达组地层露头区的平面离散性很大,但对现今河流体系高度依赖,要么追踪不同序次干流及支流形成树枝状结构,要么仅沿干流分布、不受支流影响,形成棒状结构,表明昔格达组地层形成于现今河流之后,而且与河流密切相关;垂向上,不同昔格达组地层露头区之间的最大高差达2 290 m,不同露头之间常存在标高突变。昔格达组地层的沉积环境主要是不同序次河流岸坡失稳形成的滑坡坝堰塞湖,其次为沿断裂带发育的河流局部下陷演变而来的过水断陷湖泊。     

Nephrites of East Siberia: geochemical features and problems of genesis

The largest nephrite-bearing province of Russia is located on the southern folded periphery of the Siberian craton. Deposits of two formation types were established here: apoultrabasic (East Sayan and Dzhida areas, Parama massif) and apocarbonate (Vitim area). Nephrites compose schlieren and lenticular bodies usually in the zones of contact of serpentinous (lizardite–antogorite) dunite–harzburgite rocks and dolomitic marbles with aluminosilicate rocks of different compositions. Significant difference in composition has been established for nephrites of different formations. Apocarbonate nephrites are more magnesian and fluoric and less ferroan. The contents of Li, Be, Rb, and Cs in them are two orders of magnitude higher and the contents of Sc, Ti, and Mn are lower than those in apoultrabasic nephrites. The isotope data evidence that the fluid phase of apoultrabasic nephrites was released from serpentinites during metamorphism, whereas the fluid phase of apocarbonate nephrites is only meteoric water. Oxygen in minerals of metamorphosed carbonate rocks was borrowed from the matrix subjected to replacement, sometimes with the participation of formation waters.     

The genesis of the west shropshire orefield: Evidence from fluid inclusions,sphalerite chemistry,and sulphur isotopic ratios

The Pb + Zn + Ba veins of West Shropshire, England, occupy fractures in Ordovician and Precambrian rocks of the Shelve Inlier. Precipitation of sphaleritic ores was succeeded by galena + baryte mineralization, with chalcopyrite also occurring late in the mineralizing episode. Three generations of sphalerite are recognized, and second being chemically zoned with distinct Fe + Cd-rich growth zones. Associated with these iron-rich bands are smaller (∼ 10 μm) zones rich in indium (>1.0 wt%) and copper, electron electron probe microanalyses suggesting coupled substitution of Cu⁺ + In⁺³ in the ZnS. Based on the sequence of growth zones and their chemistry a sphalerite stratigraphy can be recognized within the orefield. Fluid inclusions studies reveal the mineralizing fluids to be highly saline (18–30 wt% CaCl₂ equivalent) Na + Ca-richbrines, with mineralizing temperatures in the range 200–120°C. A trend from higher temperature-low salinity to low temperature-high salinity fluids with time is recognized. Sulphide sulphur isotopic ratios are consistent and suggest a ΔS³⁴S_H2S of the mineralizing fluid of 10%, while ³⁴S_baryte values are in the range + 14 to + 19%, indicating separate sulphate and sulphide sulphur sources. The mineralogical, fluid inclusion, and isotopic data suggest the saline fluids rose into an open plumbing system where mineral precipitation was mainly controlled by fluid cooling. The baryte however, formed due to mixing with oan overlying sulphate-bearing reservioir. Theree possible fluid sources are considered, namely: Lower Carboniferous seawater, basinal brines, and metamorphic fluids. However, the information available does not allow the source to be positively identified.     

Interpretation of soil nitrite-ammonium ratios with possible application to pedogeochemical exploration for platinum mineralization

The nitrogenous compounds ammonia, ammonium, nitrite, nitrate, and diatomic nitrogen play important roles in the geochemical cycle of nitrogen in the near-surface environment. Ammonia or ammonium (ammine species), depending on the pH, is the stable reduced species, nitrate is the stable oxidized species, and nitrite is a metastable intermediate species. Nitrogen is considered unimportant in the present context because of the impracticality of attempting to detect subtle variations in diatomic nitrogen abundances in Earth's highly nitrogenous atmosphere.Platinum is known to be an effective catalyst of the ammine-nitrite oxidation step. It is shown in laboratory experiments, and field studies involving rocks and soil that the nitrite-ammonium ratio is higher in the presence of Pt than in its absence. In addition, Pd also catalyses this oxidation reaction, but only one-fourth as effectively as Pt. Ammonium and nitrite are significantly more mobile in the weathering environment than are the platinoid elements, suggesting that the nitrite-ammonium ratio my have some value as a pathfinder for platinum group element (PGE) mineralization.Soils collected across a platiniferous vein at Crescent Peak, Clark County, Nevada, serve to illustrate the potential of this technique. Copper, Zn, and Au in soils indicate the presence of mineralized bedrock. Platinum and Pd values are quite low, but the high nitrite to ammonium ratio above the mineralized zone suggests the presence of platinoid elements in underlying bedrock. Background nitrite-ammonium ratios do not exceed 0.6, whereas soils above the PGE-bearing rock have values greater than 6.0.Preliminary bedrock data from the Stillwater Intrusive, Montana, indicate nitrite-ammonium ratios in unmineralized anorthosite and mineralized J-M Reef similar to those of barren and mineralized soils, respectively, at Crescent Peak. This result not only supports the Crescent Peak findings but suggests the possibility of applying this methodology to lithogeochemical exploration.     

The genesis of basaltic magmas

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO₂ (48 to 50%) increasingly high Al₂O₃ (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes.     

金刚石成因之谜

叙述了金刚石的四种成因说,研究了各种成因的可能性,并对金刚石成因作为初步评述。     

The use of 18O/16O ratios to study the formation and chemical origin of coal

The ${}^{18}\text{O}{}^{16}\text{O}$ ratios of some New Zealand peat, lignite and coal samples were measured and compared with those of various coal precursors (cellulose, lignin and plant resins). The results showed that the major source of oxygen in all cases (except that of high rank coal) was from cellulose with the contribution from lignin and plant resins being insignificant.     

The genesis of mid-ocean ridge basalt

The tholeiitic volcanics erupted at mid-ocean ridges (mid-ocean ridge basalts or MORB) constitute the dominant volcanic lithology on Earth. Analyses of tachylites from Atlantic, Pacific and Indian Ocean spreading centres range widely in 100 Mg/(Mg + Fe²⁺) ratios (= M) and M varies from 70 to 30. Glasses with M = 55?65 are the most common variants and only a small percentage of glass analyses has M approaching 70. The latter defines the M -value of basaltic melts in equilibrium with residual upper-mantle source peridotites with M ～ 88. The frequency histogram of the M -values of average compositions of MORB glasses at 88 ocean floor localities is similar in analysis distribution to the frequency histograms depicting variation in the M -values of glasses from the various spreading centres.M -values and nickel contents of MORB and the nature and compositions of the near-liquidus phases crystallized experimentally from MORB melts at elevated pressures have been applied to identify primary (unfractionated) melts erupted in a mid-ocean ridge environment. However, Ni abundances and high-pressure phase relationships are not necessarily unique or definitive parameters of primary melts. The latter are generally linked genetically with Mg-rich lherzolitic source rocks of ‘pyrolite’ type (M ～ 90. The spectrum of M -values displayed by MORB glasses, with a definite bias towards relatively Fe-rich compositions (average M of approximately 600 MORB glasses is 58.6), suggests that the melts may have evolved either via ferromagnesian fractionation of relatively Mg-rich parental melts (M = 70?80), or by partial melting of a heterogeneous upper mantle with variable M values, or as a result of magma mixing of already fractionated melts and primitive magma batches.For a number of reasons fractonation models based on the extraction of olivine or one or more of olivine, plagioclase and clinopyroxene, either from picritic melts (M > 75 or ‘primitive’ basaltic melts with M ～ 70, are questionable as prime controls of MORB chemistry. These include: (1) the extreme rarity of ‘quenched’ picritic or Mg-basaltic melts in ocean ridge environments; (2) the lack of adequate evidence of the appropriate (of necessity voluminous) complementary cumulates (dunites, allivalites, troctolites, anorthosites) demanded by olivine, plagioclase, or olivine + plagioclase fractionation models; and (3) the aberrent frequencies of glass M -values whereby the assumed derivatives (M = 55?65 are much more abundant (and presumably much more voluminous) than the alleged parents or transitional derivatives (65 < M < 75). The nature of the trends of Na₂O, CaO and Al₂O₃ in Galapagos Spreading Centre tachylites of extended composition (M = 65?30) indicates the ‘gabbroic’ fractionation is also unlikely to exert important controls on MORB chemistry.As their M -values increase, mid-ocean ridge basalts increase in Al, Ca, Ni, Co, Cr and decrease in Ti, Mn, Na, K and P. Except for Al and Ca, these trends are similar to those displayed by upper-mantle peridotites increasing in M, i.e., becoming more refractory following one or more partial melting episodes. It is suggested that at least a majority of mid-ocean ridge basalts is intrinsically primary and generated by variable degrees of partial melting of heterogeneous lherzolitic upper mantle (80 < M < 90) with variable abundances of elements such as Ti, Al, Ca and Na and also depleted in large ion lithophile (LIL) elements. Negative europium anomalies in the rare-earth patterns of some oceanridge basalts (ferrobasalts with low M) are ascribed mainly to the persistence of residual plagioclase in relatively Fe-rich plagioclase lherzolite source rocks, following low degrees of partial melting. The partial melting events leading to the generation of mid-ocean ridge basalts took place over a relatively modes pressure range (approximately 8–15 kb) which encompassed the transition of plagioclase lherzolite to spinel lherzolite. This proposal appears consistent with the nature and occurrence of megacrysts (xenocrysts) of tschermakitic Cr-diopside (Ca₄₃Mg₅₂Fe₅), olivine (mg 89–91), plagioclase (An_92-85) and spinel (Fe₂Al₆₀Cr₃₈) in some MORB. The megacryst compositions suggest that these phases represent disaggregated plagioclase peridotite or spinel lherzolite acquired by melts during their passage through the oceanic upper mantle.