The analysis of an acid-soluble and acid-insoluble fraction of one systematically sampled, partially dolomitized limestone bed is reported. The original lime sediment, composed principally of skeletal remains, pellets, occasional intraclasts, detrital quartz, illite, and muscovite has been modified by the addition of calcite, dolomite, and dickite cements and has been patchily replaced by dolomite, quartz, and pyrite. Rapid chemical and mineralogical variations occur over short distances, i.e. within 10 m. Fe and Mn are both present in greater concentration in dolomite than in calcite. Dolomite itself varies in overall composition within the limestones and dolostones but is always Sr-deficient. The insoluble residue is dominated by quartz. Authigenic quartz occurs but its distribution is controlled by pre-existing detrital quartz so SiO2 distribution corresponds to a detrital arrangement. TiO2 and K2O follow the distribution of SiO2 and are believed to be sited in detrital minerals, muscovite, and other unidentified minerals. Sulphur, present in pyrite, has a patchy distribution but is deficient in totally dolomitized areas suggesting removal of pyrite during dolomitization.

Portions of the carbonatitic fluids responsible for the deposition of apatite from the Tororo car-bonatite complex are trapped and preserved as minute (< 100 µm) inclusions within the apatites. These fluids consist predominantly of alkali-carbonate-bearing brines and demonstrate that water and alkalis played an important role in the formation of the carbonatites from this complex. Furthermore, the opinion that carbonatite ‘magmas’ are richer in alkalis and water than the chemistry of the carbonatites themselves reveal is clearly substantiated. The mean minimum formation temperature of the apatites from five separate carbonatite samples were determined from the homogenization temperatures of 200 primary aqueous inclusions. The results from Limekiln Hill samples (328 °C 321 °C 359 °C) are similar to those from samples of the separate carbonatite mass at Tororo Rock (353 °C, 365 °C).

Ten rather uniform tholeiitic basalt flows totalling a thickness of 190 m from Sagar, India, contain two stratigraphical breaks. Major element variation allows the flows to be grouped into two eruption cycles. Due to intraflow variation, however, chemical composition is unlikely to be useful in widespread correlation of flow sequence. The initial 87Sr/86Sr ratios vary from 0·7039 to 0·7084 with Rb and Sr averaging 6·5 and 184 ppm respectively and Rb/Sr ratios ranging from 0·002 to 0·224, with no systematic relationship between the initial 87Sr/86Sr ratios and other chemical parameters. Bulk crustal contamination is considered unlikely; selective introduction of 87Sr from the granitic basement is postulated.

The petrological interpretation of the variation of K2O content with depth to the Benioff zone in orogenic magmas is complicated by several factors: (1) Magmas may not be true primary liquids but have under-gone differentiation. K2O contents, at a specific SiO2 level, will depend upon the SiO2 content of the parental magma and the natureof the crystallizing phases more than on original K2O content. (2) The depth to the Benioff zone may not give a true reflection of the depth of melting. Magmas may not be derived from the uppermost layer of subducted oceanic crust but rather the overlying upper mantle. Earthquakes may not occur in the uppermost layer of the subducted plate, but in its colder, central parts. Hence, earthquake depth may not be related to the source of the magma. (3) It is difficult to envisage how the temporal increase in K2O may occur in certain areas (e.g. Aleutian I., Fiji) if it is primarily controlled by the depth to the Benioff zone.

There may be some correlation between K2O content of magmas and earthquake depth, but its cause is difficult to determine. None of the existing explanations is entirely satisfactory, and it is suggested that varying degrees of fractional crystallization may play an important role in controlling K2O contents.

Isolated pockets of pegmatitic appinite characterized by hollow-shell, prismatic amphiboles are common in the Pre-Cambrian metagabbros and metasomatic diorites of Jersey. Field relationships and petrography indicate a liquid phase in the formation of these appinitic pockets, which are chemically distinct from the associated gabbros and diorites. Close chemical ties between appinites and host rocks, however, prove a replacive, metasomatic, rather than intrusive origin for the pockets. Significant enrichment in SiO2, K2O, and Na2O suggest that surrounding granite provided the metasomatic agents. The localized changes in composition of the basic rocks resulted in the formation of partially molten pockets from which the appinites crystallized. This mechanism probably necessitates a temperature in the region of 900 °C at 2–5 Kb PH2O: Fractured, hollow-shell, prismatic amphiboles of the pockets are consistent with quench crystallization, possibly due to the sudden loss of volatiles. An increase in the oxygen fugacity may have played a major role in inducing the rapid crystallization of kaersutitic amphibole. The envisaged conditions under which these changes took place are those of a high-level, sub-volcanic environment.

Fragments of a hydrothermal deposit, partly moulded on pieces of basalt lava, were dredged from the edge of the median valley in the centre of the Gulf of Aden. The deposit consists of two main components, manganese oxide in the form of spongy brown to hard black lumps and coatings, and friable green massive smectite. There are smaller amounts of an iron-oxide component, and mixed manganese-oxide-smectite material. Contents of the less mobile trace elements in the smectite are too low for it to have formed by alteration of basalt glass, and it is interpreted as a direct precipitate from hydrothermal solution. None of the components is enriched in Cu or Zn, or contains appreciable amounts of sulphur. If there are sulphides in this deposit, they must lie beneath the parts sampled.

Pontlyfni and Mount Morris (Wisconsin) are briefly described. Chondrule structure is absent from both, the latter is coarsely crystalline and both are sulphide-rich and contain forsterite and enstatite. Kakangari has well-defined chondrules; its silicate and sulphide minerals are unequilibrated, but similar to those of Pont-lyfni and Mount Morris (Wisconsin). New bulk chemical analyses are presented; Pontlyfni has 33·84 % total Fe and 7·04 % S; Mount Morris (Wisconsin) has 19·88 % total Fe and 4·72 % S; and Kakangari has 22·79 % total Fe and 5·30 % S. These three meteorites, together with Winona, have ordinary chondritic Mg/Si ratios, which result in their having abundant forsterite; this distinguishes them from E-chondrites. The four meteorites have Mg/Si ratios lower than those of C-chondrites, and they are more reduced than C or ordinary chondrites. These four unusual stones, therefore, have some chemical similarities, for example all have Mg/Si (atomic) of about 0·95, mean olivine composition ranging from Fa1 to Fa5, and significant Cr in the sulphide. Cumberland Fails chondritic xenoliths and other exotic fragments in polymict meteorites may be related to the four stones, which may ultimately prove to belong to a distinct chemical group of chondrites related to the irons of Group IAB.

These meteorites are described and discussed together because they have similarities in mineralogy, bulk composition, and oxidation state. All have more than 10 % forsterite (i.e. an olivine in the range Fa0·10). In our study we investigated the mineral chemistry of, first, Kakangari (Graham and Hutchison, 1974), and then Mount Morris (Wisconsin), with cursory examination of the previously described Winona (Mason and Jarosewich, 1967) and Cumberland Falls chondritic fraction (Binns, 1969; Jarosewich, 1967). By chance, the British Museum (Natural History) acquired the Pontlyfni stone during the course of this work. Pontlyfni fell in Wales in 1931, but is so far undescribed; it proved to be chemically and mineralogically similar to Kakangari. We thank Mr. J. R. Owen, the original owner, for its timely addition to the Museum's Collection.

Paragonite has been found in two chloritoid-chlorite-mica schists from the Dalradian, north of Stonehaven. The principal bulk-chemical constraints on the occurrence of this mineral within the chloritoid zone appear to be the ratios of soda to potash and the Al2O3 contents of the rock. The composition of paragonite coexisting with muscovite indicates a crystallization temperature of 550 °C for rocks some way above the chloritoid isograd. Although this result is supported by other mineral equilibria, the muscovite compositions suggest a temperature of 690 °C which is too high because the mineral assemblage chloritoid-paragonite is unstable above about 600 °C By combining data on the breakdown of the chloritoid-paragonite assemblage with a mineral equilibrium for the assemblage garnet-plagioclase-muscovite-biotite, it has been deduced that rocks close to the staurolite isograd crystallized at about 7·5 kb and 600 °C.

Sea-water from Deception Island was found to contain 0·55–1·48 mg Mn 1−1 and 10·2-64·3 mg Si 1−1. Reaction of Deception Island basalt and sea-water at 190 °C and 500 bars simulated the measured water composition but leaching experiments suggest that much of the Mn etc. may derive from local volcanic ash. Mn and Mg in thermal waters at Reykjanes have concentrations compatible with equilibrium with components of montmorillonite present as an alteration mineral of average composition . However, Fe is controlled by equilibrium with sulphides. Flux calculations based on these and other data place upper limits on hydrothermal Mn input to the oceans of 5–36×1011 g yr−1 and Mn accumulation in metalliferous sediments of 2–9×1011 g yr−1.

The compositions of apatite crystals from seven Skaergaard rocks were obtained by electron-probe microanalysis. These span the range of occurrence from the base to the top of the exposed zones of the layered series. Apatite occurs as an intercumulus phase in the lower zones, but becomes a cumulus phase at a structural height of 1850 m (c. 98 % crystallized), defining the base of Upper Zone b. All apatite analyses show a high F:Cl ratio. There is a slight but significant increase in F and decrease in Cl when the apatite becomes a cumulus phase, the F/Cl values changing from < 10 to > 30. Variations in F:Cl:OH are attributed to differential volatile migration from trapped intercumulus liquid sites. The apatite data provide new support for ferrodiorite-granophyre liquid immiscibility.

Gypsum and anhydrite from the Neogene of the central Jordan valley associated with basalt bodies in the Menahamiya area contain abundant euhedral magnetite and diopside crystals and rarer grossular. The original calcareous shales above the gypsum were transformed into hydrogrossular cemented by secondary calcite and gypsum. The mineralization is assumed to be due to low-temperature, high water-pressure metasomatic activity similar to the process of rodingization. The described occurrence is unusual in that the hydrothermal solutions may have been partly formed in situ by thermal dehydration of the gypsum. In a nearby locality, Tel Shaharit, the mineralization occurred in a calcareous environment producing thomsonite, natrolite, and hydrogrossular.

Basalt composition is defined as that chemical composition which when crystallized at one atmosphere under standard conditions will yield olivine, clinopyroxene, and plagioclase as the principal silicate phases. This gives six classes of basalt defined by their order of crystallization but only four of these are encountered in rocks: class 1, olivine→clinopyroxene→plagioclase; class 2, olivine→plagioclase→clinopyroxene; class 3, plagioclase→olivine→clinopyroxene; and class 4, plagioclase→linopyroxene→olivine. Classes 5 and 6 would have clinopyroxene as the first phase. The class can be determined from the rock composition from multivariate discriminant function equations but Si, Mg, Al, and Na provide a more simple diagnosis via dis-criminant diagrams. Rock series plotted on these diagrams show systematic trends that allow the series them-selves to be classified. Each class corresponds with a commonly recognized basalt type and all four classes relate to tectonic setting groups.

The plagioclase-alkali-feldspar geothermometer formulated by Stormer (1975) does not take into account the effect of small amounts of calcium in the alkali feldspar. This geothermometer has been reformulated using ternary regular solution model equations, and it is shown that as the Ca content of the alkali feldspar increases the temperature calculated using the Stormer geothermometer will be increasingly too high.

Equilibrium thermodynamic methods are used to develop an independent iron-titanium oxide geothermometer and oxygen barometer from the experimental data of Buddington and Lindsley (1964). The geothermometer and oxygen barometer are presented graphically for coexisting ilmenite solid solution and magnetite solid solution in the system FeO-Fe2O3-TiO2. Equations are also given for calculating temperature and oxygen activity for natural coexisting iron-titanium oxides containing other components. Large departures in composition from the system FeO-Fe2O3-TiO3 result in large uncertainties in the calculated temperature and activity of oxygen.

Published analyses for 14 trace and minor siderophilic elements have been interpreted to indicate a close relationship between the IIIAB iron meteorites and the main group of pallasites. These two groups respectively account for about 32 and 75 % of all irons and pallasites. The composition of the metal in the main group pallasites is very close to that calculated for the liquid after 80 % of a IIIAB iron melt has fractionally crystallized. The pallasites probably formed by the intrusion of residual molten metal from the core into a surrounding olivine mantle.

Dykes from four sections across the axis of the Skye regional swarm have been analysed for major elements, Nb, Rb, Sr, Y, Zr, and rare-earth elements. 70 % of the dykes are characterized by relatively high CaO (11–12%), and low abundance of elements such as TiO2 (∼1%), K2O (0·05–0·20 %), Zr (30–60 ppm), with a light-rare-earth depleted chondrite normalized pattern. The remainder of the swarm comprises dykes equivalent in composition to the Skye Main Lava Series and dykes belonging to a further magma type distinguished by a flat chondrite normalized rare-earth pattern. The high-calcium, low-alkali tholeiite dykes are confined to a narrow persistent zone forming the major axis of dilation of the swarm; other basaltic dykes are axially less persistent and have a wider lateral distribution. The more evolved dykes tend to be restricted to central Skye.

A phosphate-rich carbonate nodule from carboniferous mudstones has been studied by wet chemistry, X-ray diffraction, and electron microprobe. Partial heavy-liquid separation has enabled characterization of the phosphate phase as francolite (carbonate fluorapatite) with a composition Ca10(PO4)5·59(CO3)1·00((OH)1·49F2·10). Diffractometer data for this mineral is presented and cell dimensions are calculated as a = 9·349 Å, c = 6·887 Å. with c/a = 0·737. The problem of the structural positioning of carbon in francolite is discussed.

The francolite is seen in thin section as a structureless groundmass enclosing siderite spheru-lites. Electron microprobe X-ray distribution photographs for P, Al, Si, Mg, and Fe show the relationship of these two phases to quartz, pyrite, and at least two clay minerals. One of the latter is intimately mixed with francolite.

Possible origins of phosphatic nodules are discussed. Textural evidence in the present case strongly suggests that the francolite is concretionary and formed during early diagenesis possibly by replacement of earlier calcite. Phosphate was probably derived from protein degradation, which also maintained the alkaline pH necessary for francolite formation.

The accessory minerals rutile and apatite have been studied in 77 known porphyry copper deposits. Their value as indicators has been well established on the basis of specific chemical and paragenetic variations which they show.

Rutile occurs as the only Ti-mineral in the quartz-sericite zone, is dominant in the biotite-orthoclase zone, and is generally found in the inner fringes of the chlorite-epidote zone. It forms in these zones mainly as a result of the destruction of sphene, but also from biotite and hornblende.

The length: width ratio of rutile crystals is 1·5:1 in the centre of a porphyry system, increasing gradually outward to 2:1. A characteristic red colour displayed in thin section is attributed to a high copper content ranging from 100 to 500 ppm. The ratio of Cr+V:Nb+Ta is also unusually high.

Apatite shows evidence of a complicated history of corrosion and redeposition accompanied by outward migration during the life of the porphyry system. The migration parallels that of copper and typically extends far into the host rocks. The apatite is enriched in chlorine. A plot of a versus c shows a clear separation of apatites of various genetic types, including tin and molybdenum porphyries.