The Archaean (c. 2800 Ma) ultramafic rocks in eastern Sierra Leone cut basalt lavas and are mostly olivine-rich cumulates either iron-rich (Fo85–86) and derived from a basaltic or picritic parent, or more magnesian (Fo92–93) derived from an ultramafic melt with c. 18–25 wt. % MgO. In central Sierra Leone the ultramafic rocks are lavas predating tholeiitic basalts.

The basalts show a wide variation in Zr/Y, suggesting that garnet was present in the source region of some of these rocks but not others. This implies that melting took place at different depths in the mantle. The REE evidence for basaltic rocks in the upper part of the Nimini belt succession suggests that they were derived from a mantle source region which had already suffered melt extraction. Ti/Zr ratios in the basaltic rocks are also variable and individual belts define different trends on a Ti vs. Zr plot implying that the basaltic rocks evolved in geographically distinct magma chambers. It is likely that the basaltic rocks evolved from a parental liquid with Ti/Zr = 90 via shallow level crystal fractionation. The source region for these rocks therefore had a lower than chondritic Ti/Zr.

There are two possible models for the basaltic and ultramafic magmas in the Sierra Leone greenstone belts. First that the ultramafic and basaltic liquids were derived from mantle diapirs of differing size, but originating in the same region of the mantle. Ultramafic liquids were produced in small diapirs, which store large melt fractions, and basaltic liquids in larger diapirs which segregate larger melt fractions. A second model is based upon the double diffusion process suggested for magma chambers at mid-ocean ridges and involves a transient magma chamber from which basalts, derived from parental ultramafic liquids, are erupted, with ultramafic liquids rising directly to the surface when the magma chamber is frozen. The available data does not discriminate between these two models.

Major element, trace element, and REE data for metasedimentary granulites and their retrogressed derivatives formed from Archaean parent-rocks at two localities in the Lewisian complex of north-west Scotland are presented.

The metasedimentary rocks are enclosed in and intruded by metatonalites and related calc-alkaline rocks and have been highly deformed and metamorphosed along with these rocks. They are geochemically distinct from this meta-igneous suite and range from highly aluminous to highly siliceous types. Comparisons of major and trace element data with those for unmetamorphosed Archaean and post-Archaean sediments suggest that they are derived from a detrital shale-greywacke assemblage, which may have included a volcaniclastic component. High ΣREE and ΣLREE suggest that the source-rocks included fractionated felsic igneous rocks.

Evidence that depletion in the large ion lithophile elements K, Rb, Th accompanied high-pressure granulite metamorphism supports the view, based on data derived from the associated meta-igneous rocks, that depletion was effected by active fluids rather than by abstraction of a partial melt and suggests that removal of K and Th began only when a considerable reduction of Rb had taken place.

Experimental melting equilibria at 1 kbar PH2O are reported for three early Tertiary granites from Skye and one from Rhum, together with a quartz monzonite from Ardnamurchan. The Hebridean Province leucogranites show cotectic melting behaviour, whilst associated ‘primitive’ (mostly adamellitic) acid plutons have silicate liquidi ∼ 925 °C and plagioclase liquidi ∼ 875 °C. At such high temperatures the morefusible crustal rock types of the region would be almost completely molten. If the Tertiary ‘primitive’ acid magmas were simple crustal melts, they would inherit the incompatible element ratios of their sialic progenitors. This is demonstrably not so. Minor element ratios in these plutons are consistent with extensive fractional crystallization during the evolution of their magmas. But it is, in most individual instances, extremely difficult to specify, either by thermal or geochemical arguments, whether (1) fractional crystallization followed sialic contamination of basic magmas or (2) mixing occurred between fractionation residua and sialic melts. Both the minor element ratios and cotectic melting behaviour of the Hebridean Province leucogranites are consistent with the view that they are products of extreme fractional crystallization of (sial contaminated) more-basic magmas. Nevertheless, it is clear—when the sequence and timing of emplacement of acid magmas within individual intrusive centres is considered—that very complex polycyclic combinations of fusion, fractionation, and re-fusion were involved in the evolution of these granites.

The solidi of the samples studied experimentally, together with published data, show a progressive rise in temperature as the compositions of the rocks diverge from the minimum melting composition in NaAlSi3O8-KAlSi3O8-SiO2 at 1 kbarH2O. This relationship is important when considering the likely nature of sialic contamination of basic to intermediate continental magmatic suites fed through reservoir systems which are swarms of dykes and, or, sills. Because salic low-solidus crustal rock types also show near-cotectic melting behaviour, the composition of the contaminant may be strongly biased towards these (usually minor) crustal units, rather than the ‘bulk’ or average crustal composition.

Phlogopite occurs as an early crystallizing mineral in many ultrapotassic lavas of basaltic affinities. Based on high-pressure experiments in lavas of these compositions, the early crystallization of phlogopite is controlled in large part by the bulk compositions of the liquids from which it crystallizes but also by the total pressure and by the aH2O, with early phlogopite forming under a narrow range of aH2O, less than that represented by H2O-saturated conditions. Variations in fO2 do not appreciably affect phlogopite crystallization but high aCO2 suppresses its crystallization. In ultrapotassic magmas, phlogopite will preferentially incorporate K2O, TiO2, MgO, and Al2O3 relative to the coexisting early silicate minerals, olivine and clinopyroxene, and thus, on fractionation of these minerals, phlogopite will be more effective in reducing these oxides in residual liquids. Phenocrysts and microphenocrysts of phlogopite in ultrapotassic lavas are directly related with respect to their K/Ti, K/Al, K/(K + Na), and Mg/(Mg + Fe) ratios. Textural relations suggest phlogopite may form by reaction relationships involving liquid with olivine, and/or clinopyroxene. Such relationships are supported by the experimental studies on ultrapotassic rock compositions.

The Strathy complex is a fault-bounded block of the sub-Moine succession basement characterized by a marked aeromagnetic anomaly. XRF analyses of 94 rocks for 26 elements are used to illustrate the geochemistry of the complex. It comprises siliceous grey gneisses (50 analyses), subordinate amphibolite (13 analyses), hornblende gneiss (13 analyses), and ultramafites (1 analysis), with scapolite-diopside marble and calc-silicate rocks (3 analyses from Strathy, 14 analyses of other calc-silicates). Geochemically the Strathy gneisses are quite distinct from the adjacent Moine lithologies, being unusually low in K, Rb, La, and Ce, and high in SiO2 and Na2O, whilst differences in immobile elements such as Ti and Zr preclude their derivation from a Moine source. The Strathy complex is unlike other high-grade gneiss terrains and apparently acquired its characteristic geochemical features by the removal of an anatectic melt under amphibolite-facies conditions leaving a refractory residue of quartz, plagioclase, Ca-poor amphibole, and garnet ± staurolite in siliceous gneisses. Subsequent influx of water, retrogression to biotite-bearing assemblages, production of sillimanite, and anateclic trondhjemitic melts took place during the maximum metamorphism of the surrounding Moine. The complex is considered to have originated as a dacitic supracrustal sequence with minor sediments and mafic-ultramafic intrusives. REE and Y abundances suggest a Proterozoic age, possibly as part of the Laxfordian cycle.

Karroo basaltic lavas of southern Malawi are divided into two groups on the basis of stratigraphy, petrography, and the data from 20 newly analysed rocks. A lower group of porphyritic lavas is notably rich in plagioclase phenocrysts and has contents of K, Ti, P, Ba, and Zr unusually high for tholeiitic basalts. They are overlain by an upper group of aphyric or sparsely microphyric lavas whose chemistry is more normal for tholeiites and is comparable with that of the Karroo dolerite dykes in Malawi. Though they outcrop in the northern part of the Karroo province, the lavas and dykes have chemical characteristics more similar to southern province rocks, adding some complexity to the concept of geochemical provinces established by earlier workers. Inter-suite variations in incompatible trace element contents and ratios indicate that the basalts of each group were derived from heterogeneous mantle sources.

Over 200 sediment samples taken from ten holes drilled in the Galapagos Hydrothermal Mounds Field during D.S.D.P. Leg 70 have been analysed for twenty-one elements. The three main sediment lithologies recognized are siliceous carbonate ooze, Mn-oxide crust, and an Fe-rich silicate (nontronite), the latter two being of hydrothermal origin. The major element composition of the hydrothermal deposits is similar in each mound hole, suggesting that formation conditions and hydrothermal solution composition have been constant both geographically and with time. The large variations which occur in the concentration of some trace elements in the Mn-oxide crusts and in transition metal ratios in the nontronite compared with the pelagic ooze suggests a hydrothermal supply to the mounds of Li, Mo, Pb, and Ba in addition to Mn, Fe, and silica.

The data are compatible with suggestions that the nontronite formed at depth in the pelagic sediment blanket by replacement of biogenic ooze, whilst the Mn crusts formed at or near the sediment-water interface. Pelagic sediments in the mounds which have not been replaced are similar in composition to pelagic sediments from non-mounds holes.

X-ray diffraction, electron microprobe and STEM EDAX analyses of basalts from the South Pennines (Derbyshire) indicate that Fe-rich smectites are the dominant alteration products, occurring either as pseudomorphs or infilling veins and vesicles. Previously chlorite has been identified as the common alteration product but its occurrence is very infrequent.

A reported occurrence of metamorphic rocks containing sodic pyroxenes, in the transition zone between the Pelagonian Crystalline Massif and the Vardar Ophiolite Belt, in Macedonia, Yugoslavia, has been re-investigated, and pyroxenes, micas, plagioclase, and clinozoisite analysed by microprobe. The presence of the high-pressure metamorphic assemblage quartz + omphacite+albite is confirmed, giving the first definite indication of blueschist-facies metamorphic conditions in Yugoslavia.

Examples of application of equilibria in the systems Fe-Zn-S, Cu-Fe-Zn-S, Fe-As-S, and Fe-Zn-As-S are drawn from modern sulphide-forming hydrothermal vents on the East Pacific Rise, from ancient vein deposits and from metamorphosed sulphide ores. The ZnS content (1.2 and 1.3 mole %) of Cu-Fe-S intermediate solid solution (iss) from the sea-floor deposits gives temperatures of equilibration of 331° and 335 °C from experimental curves, in excellent agreement with the 350 °C expected from actual measurement of the hot springs. Heterogeneities in sphalerite in these deposits do not necessarily represent disequilibrium during deposition but can be explained simply by small fluctuations in aS2. Diagrams of log aS2vs. 1000/T, K for sphalerite and arsenopyrite are useful for estimating temperature and activity of sulphur in hydrothermal or metamorphosed deposits provided that equilibrium can be demonstrated and the systems are properly buffered.

The sphalerite geobarometer has had wide applications, some successful (mostly vein deposits) and some not (particularly metamorphosed ores in which chalcopyrite is in contact with sphalerite). Compositions of sphalerites which are totally enclosed within metablastic pyrites represent preserved high P-T equilibria which have been isolated from further reaction during subsequent retrograde conditions by the inert encapsulating pyrite and may provide more reliable estimates of pressure.

Very large crystals of magnetite, up to 2 cm, are found in pure magnetitite layers in the upper zone of the Bushveld Complex. Detailed electron microprobe analysis of one of these indicates a vertical compositional zoning of Cr (a highly compatible element in magnetite) quite different from the concentric zonation often found, for example, in feldspars in intrusive rocks. It is shown that these crystals could not have grown to their present size in suspension in the magma chamber. Annealing of many small crystals into a single grain could occur either by the Ostwald ripening process at temperatures close to the liquidus or by subsolidus recrystallization. Alternatively, these grains could have formed directly from the magma by upward growth from the floor of the magma chamber, by a process analogous to crescumulate or heteradcumulate growth but in the absence of other crystallizing phases. All of these models require the observed Cr gradient to be produced during the initial crystallization event and not as a secondary effect.

Published data from pyrrhotine-vapour equilibrium experiments are used to make inferences concerning the high-temperature thermodynamic mixing properties of pyrrhotine. Darken's quadratic formalism provides the justification for plotting activity of sulphur data against (1−X)2, where X is used in Fe1−XS to express the composition of non-stoichiometric pyrrhotine. Such plots show that the thermodynamic behaviour of pyrrhotine appears to be different on either side of X = 0.125, the Fe7S8 composition.

Prior to serpentinization, the ultramafic rocks of the ophiolitic Highland Border Suite were depleted harzburgites, dunites, and orthopyroxenites. They may have been the residual mantle after melting of primary lherzolite that produced the basaltic rocks of the Suite. Serpentinization of these peridotites occurred at low temperatures (75±50°C) and produced pseudomorphic-textured lizardite serpentinites in which the minor-element compositions of the primary phases were preserved in the lizardite grains. Olivine was almost completely serpentinized before orthopyroxene was hydrated. Concomitant peripheral alteration of chromespinels, during serpentinization of orthopyroxene, caused release of Cr and Al cations which were incorporated into the lizardite. Under greenschist-facies conditions (c. 350°C at 3–4 kbar), the lizardite serpentinites were subsequently metamorphosed and recrystallized to nonpseudomorphic-textured antigorite serpentinites. Postmetamorphic processes included high-level brittle deformation that resulted in fracturing and cross-fibre chrysotile vein-formation, and ductile shearing (T ∼ 200 °C, P < 2 kbar) which produced talc-chrysotile shear zones.

A previously suggested simple relationship in a diagram of(Al2O3vs. MgO + FeO + Fe2O3)opx for the distinction between igneous and metamorphic orthopyroxenes is shown to be inadequate. A revised diagram is presented and its limitations are discussed. A diagram showing Fe2+/(Fe2+ + Mg) vs. 100Ca/(Fe2+ + Mg + Ca) for orthopyroxenes coexisting with Ca-rich clinopyroxenes proves to be useful for the discrimination of igneous and metamorphic orthopyroxenes and for evaluation of the extent and physical conditions of regional orthopyroxene re-equilibration.

The limits and effect on cell size of silver substitution into synthetic tetrahedrite, Cu10(Zn,Fe,Cd2) Sb4S13, are investigated for comparison with natural tetrahedrite. The limit of Ag substitution into natural zincian tetrahedrite is ∼ 4 atoms per half unit cell and into iron tetrahedrite ∼ 6.5 atoms (with rare exceptions). The cell size of natural tetrahedrite increases with increasing Ag content up to 4 atoms but decreases with further Ag substitution. The highest Ag substitution achieved in synthetic tetrahedrites was 4.7 atoms in Zn2-tetrahedrite, 7.02 atoms in Cd2-tetrahedrite and 6.80 atoms in Fe2-tetrahedrite. The cell size of synthetic tetrahedrites shows a continuous increase with increasing Ag content, the largest cell size of a = 10.927 Å being in the Cd2-tetrahedrite with 7.02 atoms Ag. The iron content of tetrahedrite systematically increased from 1.1 to 2.0 atoms per half unit cell tetrahedrite with increase from 0 to ∼ 4 atoms Ag. The different limit of Ag substitution between Zn2 and Cd2 tetrahedrite can be explained by size constraints on the expanding structure. An explanation is given for a limit of 7 atoms Ag substitution in tetrahedrite using a combined electron band/molecular orbital approach.

In the Forland Complex chloritoid is restricted to the grey and green phyllites which characterize the Scotia and Peachflya groups. The textures and compositions of the rocks and constituent minerals are described and discussed in an attempt to assess how metamorphism, deformation, and chemistry have influenced the stability of chloritoid.

Pyroxene phase relations in the Mg-rich corner of the pyroxene quadrilateral, at 1 atmosphere, have been reinvestigated. Experimental studies on sixteen selected compositions in the systems CMS and CFMS were undertaken in the temperature range 1100–1400 °C. The results of this study clarify our understanding of the pyroxene stability relations at low pressure. In particular, the demonstration that there is a high-temperature stability field of orthoenstatite denies the existence of a stable (or real) invariant point defined by the reactions OE = PE + DI, PE + DI = PI, and OE + DI = PI, in the system CaMgSi2O6-Mg2Si2O6. New phase relations, consistent with the experimental findings of this and other studies, for the Mg-rich corner of the pyroxene quadrilateral are presented. These new phase relations may be of use in interpreting the origin of volcanic rocks containing magnesian pigeonite.

Gortdrumite superficially resembles chalcocite but is more distinctly anisotropic, and is present in a vein containing ferroan dolomite and baryte which cuts dolomitized limestones from the Gortdrum Orebody of County Tipperary, Ireland. It has an empirical formula of approximately (Cu,Fe)6Hg2S5. A distinctive powder diffraction X-ray pattern has been obtained which indicates a probable orthorhombic crystal lattice with a 14.96, b 7.90, and c 24.1 Å. The reflectivity at 589 nm is 25.1 to 27.9% and the VHN10 hardness range 186 to 230.