Showing papers in "Contributions to Mineralogy and Petrology in 1974"

Sea floor basalt alteration: Some chemical and Sr isotopic effects

Abstract: Major element, Sr isotope and trace element data for 16 elements are reported for various weathered zones in three submarine basalt pillows. During the initial stages of alteration, it appears that SiO2, Al2O3, CaO, S, and Ga are lost from the basalt, whereas Fe2O3, total Fe, MnO, K2O, H2O, Cl, B, Rb, and Cs increase. Sr87/Sr86 ratios also increase during weathering. MgO, Na2O, P2O5, Ba, Ni and Cu show significant (10–50%) but less consistent changes. TiO2, Zr, Sr and V show only minor changes (<7%). Zn, Cr, Co, Y and Nb show no changes outside experimental error.

The solubility of alumina in orthopyroxene coexisting with garnet

Abstract: The solubility of alumina in complex orthopyroxenes crystallised in equilibrium with garnet has been determined over the pressure-temperature range 8–30 Kb and 800–1250° C. The results are in good agreement with predictions made using the simple thermodynamic model of Wood and Banno (1973). The model has been refined using a combination of the new data on orthopyroxenes of about En60Fs40 (with variable calcium content) and previously published data on more magnesian systems. Given analyses of coexisting Orthopyroxene and garnet in natural assemblages it is possible to calculate a P-T curve for the rock which should, in most cases, be accurate to within 2 or 3 kbar.

Experimental determination of the temperature and pressure dependence of the Fe-Mg partition coefficient for coexisting garnet and clinopyroxene

Abstract: An experimental study initiated to calibrate the distribution coefficient $$K_D = \frac{{({\text{FeO}}/{\text{MgO}})_{{\text{ga}}} }}{{{\text{(FeO}}/{\text{MgO)}}_{{\text{cpx}}} }}$$ in eclogites as a geothermometer has been done on (a) a mineral mis, (b) a glass of the typical tholeiite composition and (c) a series of glasses of tholeiite compositions with $$6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 93.$$ The mineral mix was found to be unsuitable as reactant due to incomplete equilibration but the minimum K D of the mineral mix and the K D from glass of tholeiite composition are identical within experimental uncertainty. These data constitute a reversal of the garnet/clinopyroxene partition relationship and provide justification of the use of glass as a reactant for the project. To eliminate any uncertainty in interpretation of mineral analyses due to possible variation in Fe+++/Fe++ between runs, experiments were carried out in iron capsules on the nine tholeiite glasses, thus maintaining iron as Fe++. Microprobe analytical techniques yielded mineral analyses of comparable accuracy to analyses of natural phases for experiments within the temperature range from 600° C to 1500° C and a pressure range from 20 kb to 40 kb. It has been shown that for $$6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 85$$ , the bulk chemical composition does not perceptibly affect the K D value. At 30 kb the K D value ranges from 18.0 at 600° C to 1.45 at 1400° C, defining the linear relationship in a ln K D vs 1/T(°K) plot. The pressure dependence of the K D -value has been shown to be greater than previously predicted. There is a straight line relationship in ln K D vs Pressure (Kb) between 20 and 40 kb at constant temperature (1100°C). This enables us to determine K D =fn (T, P) and $$T(^\circ {\text{K}}) = \frac{{3686 + 28.35 \times P({\text{Kb}})}}{{\ln K_D + 2.33}}$$ . This expression uniquely determines the temperature of equilibration of natural eclogites of basaltic bulk composition when the K ga,cpx is known and a pressure estimate can be given.

Al and Ti contents of hornblende, indicators of pressure and temperature of regional metamorphism

Abstract: Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their AlVI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.

Primary basalts and magma genesis

Abstract: Three Eocene lavas from Skye, NW Scotland, have been subjected to anhydrous experimental studies within their melting ranges at pressures up to 30 kb Two of these, an olivine-phyric magnesian alkali basalt and a near-aphyric Mg-poor transitional basalt, appear to show four-phase points on their liquidi at high pressures which are thought to have genetic significance From experimental and mineralogical evidence, the magnesian basalt is postulated to be a primary magma, erupted without significant compositional change from its genesis by slight partial melting of a relatively Fe-rich spinel lherzolite upper mantle at about 60 km depth The liquid seems to have had a reaction relationship with Ca-poor pyroxene (pigeonite) in the residual lherzolite Partial crystallization of batches of this magma, delayed during its ascent at depths of about 40 km, is thought to have given rise to the Mg-poor basaltic liquids The third lava studied experimentally, a sparsely olivine-phyric hawaiite, does not have olivine on the liquidus in any part of its anhydrous P-T diagram and therefore cannot have been derived under anhydrous conditions from olivine-saturated sources The mineralogy and chemistry of the lavas are used to support an hypothesis that the hawaiites are products of partial crystallization of pockets of basalt magma at depths approximating to the crust/ mantle boundary beneath Skye, with $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ rising to sufficient values to make the residual liquids comparatively rich in normative feldspar Finally, the genesis of all other Skye Eocene lavas is reviewed in the light of the new experimental data

The b 0 value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists

Abstract: The b 0 of the potassic white micas was measured in 410 samples of low-grade pelitic schists from the Central Pyrenees, the Abukuma belt, Northern New Hampshire, the Eastern Alps, Otago, the Sanbagawa belt and the Central Pontic Chain. Temperature and rock bulk composition can be considered substantially constant all over the sample range, so that the differences in the b 0 values depend primarily on differences in pressure. The analytical data demonstrate that the b 0 increases for increasing pressure, and that it is practically identical in the micas from very different regions belonging to the same baric type of metamorphism. With the help of other analytical data (about 600), a b 0 scale was prepared, which makes the distinction of six different facies series possible. The writers, proposing this very rapid new method of barometric analysis, draw attention to its limitations and point out the main fields of its application.

The Energetics of Natural Garnet Solid Solution

Abstract: Approximate mixing properties of the dominant calcium silicate end-member components of natural garnets, namely grossularite, andradite and uvarovite, have been derived through theoretical thermodynamic and crystal chemical analysis, and appropriate reduction of the available experimental data. The stability of the solid solution with respect to phase separation in the ternary system has been analyzed. Finally, a general model is presented as to the approximate mixing properties of multicomponent natural garnet solid solution involving substitutions in both eight and six coordinated sites.

O 18 enriched ophiolitic metabasic rocks from E. Liguria (Italy), Pindos (Greece), and Troodos (Cyprus)

Abstract: Low grade hydrothermally metamorphosed ophiolitic basic rocks from E. Liguria (Italy), Pindos (Greece) and Troodos (Cyprus) are enriched in O18 relative to the oxygen isotope ratio of fresh basalt (6.0±0.5‰). The maximum observed δO18 value of +13.22‰ corresponds to a positive isotope shift of 7‰ Enrichments in Sr87 relative to Sr86 correlate with hydrothermal alteration. The δC13 values of secondary calcite from E. Liguria are positive, and fall in the range from +0.2% to +3.6‰ Since ophiolitic rocks are considered to be fragments of the oceanic crust and upper mantle, and since the secondary metamorphic assemblages were produced before mechanical emplacement, it is considered that the hydrothermal metamorphism which affected these rocks occurred in the sub-sea-floor environment. The isotope data are directly consistent with the hypothesis that the alteration was produced by interaction of the basaltic material with introduced sea water. Water: rock ratios were sufficiently large to produce the observed isotope shifts. In the Troodos ophiolite sequence δO18 values decrease steadily downwards and change to progressively larger depletions in the Sheeted Intrusive Complex. The trend of δO18 decrease correlates with the original direction of increasing temperature. The O18 depletions, which have also been observed for oceanic “greenstones” (Muehlenbachs and Clayton, 1972b), resulted from water/rock interaction at temperatures greater than the particular temperature range above which whole rock-water fractionations became less than the isotopic difference between fresh basalt and sea water. Since this isotope geochemistry indicates that the water responsible for hydrothermal metamorphism was of sea water origin, the data support the more general hypothesis that convection of sea water within the upper 4–5 kms of the oceanic crust is a massive and active process at oceanic ridges. This process may be completely or partially responsible for (a.i.), the local scatter and low mean value of the conductive heat flux measured near ridges, (a.ii), the transfer of considerable quantities of heat from spreading oceanic ridges, (b) hydrothermal metamorphism, metasomatism and mineralization of oceanic crust, (c), the production of metal enriched, relatively reduced brines during sea water/basalt interaction, d), the high degree of scatter and low mean value of the compressional wave velocities of oceanic basement layer 2 and (e), the low natural remanent magnetization (NRM) intensity of the lower part of layer 2 and upper part of layer 3 of oceanic crust.

Plagioclase lherzolite-residual mantle relationships within two eastern mediterranean ophiolites

Abstract: The ophiolites of Othris, northern Greece, and Troodos, Cyprus, are of mixed lherzolite-harzburgite and harzburgite sub-type respectively. Within both ophiolites an entire spectrum of harzburgite, plagioclase harzburgite, lherzolite and plagioclase lherzolite interpreted respectively as residual and highly, moderately and slightly modified upper mantle has been recognised. Plagioclase lherzolite frequently contains gabbroic segregations generated by partial melting and incomplete extraction. Othris shows the full range of mantle peridotites while Troodos is dominated by harzburgite with very minor occurrences of plagioclase harzburgite and lherzolite. Chemically, the plagioclase lherzolites have low contents of the basaltophillic minor elements, and a composition slightly more residual than postulated upper mantle compositions, suggestive of a preceding phase of minor depletion. The Othris and Troodos ophiolites seem to have formed under fundamentally different environments — Othris as a marginal ophiolite at the inception on rifting of continental crust, and Troodos later in such an event when spreading was well established.

Role of underthrust oceanic crust in the genesis of a Fijian calc-alkaline suite

Abstract: A test of the proposition that calc-alkaline magmas are direct partial melts of underthrust oceanic crust is presented. It involves numerical evaluations of whether or not the major and trace element and isotopic composition of a Fijian calc-alkaline rock suite is consistent with these rocks representing unfractionated partial melts of oceanic crust at high pressures. Experimental data for one of the samples constrain the calculations. When compositions of liquidus minerals at 27 kb are combined with compositions of the volcanic rocks, close approximations can be made to the composition of oceanic crust only if the degree of partial melting is between ∼20% (dacite) and ∼45% (basaltic-andesite), and if accessory minerals are refractory phases. Concentrations of elements such as K, Rb, Sr, Pb, Th, and U, and 87Sr/86Sr ratios in the Fijian suite can be satisfactorily explained only if the parental material consisted of altered rather than fresh ocean floor basalt. Sediments are not likely to have been involved. Concentrations of Na, Ni, Co, Cr, Sc, V, the REE, Y, Zr, Hf, and Nb cannot be explained unless, or in some cases even if, several accessory phases are partially refractory. Therefore, partial melting of underthrust lithosphere does not seem likely to produce magmas with the composition of at least one quite typical calc-alkaline suite.

Chemical and deformational controls on recrystallization of mica

Abstract: Chemical and microstructural data from one experimentally and two naturally deformed and recrystallized micas are discussed in terms of established metallurgical mechanisms for strain-induced nucleation and growth during recrystallization. It is suggested that none of these nucleation mechanisms is applicable, and that nucleation is driven by energy from both chemical and permanent strain sources. Subsequent growth of the nuclei is influenced by the deformation or by coincidence lattice relationships.

Upper mantle source for some hawaiites, mugearites and benmoreites

Abstract: Chemical analyses of over seventy lavas or dykes containing spinel lherzolite inclusions of high pressure mineralogy, show that most host magmas are of alkali olivine basalt or basanite composition with relatively rare olivine nephelinites, and olivine melilitites. The 100 Mg/Mg+Fe++ ratios of host magmas display a strong maximum at about Mg70 consistent with partial melting of source peridotite with olivine of Fo88–90. In contrast to these primary magmas, there occur some host magmas with 100 Mg/Mg+Fe++<60 and with chemical compositions resembling those of classical hawaiite, mugearite, and nepheline benmoreite magmas. It is inferred that these magmas have been produced by crystal fractionation, within the upper mantle, of parental basanites or alkali olivine basalts. The presence of kaersutitic hornblende xenocrysts accompanying the lherzolite inclusions, and the nature of the chemical variation between associated basanites and nepheline benmoreites suggests that crystal fractionation has been dominated by kaersutitic hornblende, together with olivine and, in some cases, probably clinopyroxene. The mantle-derived nepheline benmoreite magmas also show similarities to some plutonic nepheline syenites.

Isotopic U-Pb ages of monazites and zircons from the crust-mantle transition and adjacent units of the ivrea and ceneri zones (Southern Alps, Italy)

Abstract: The Ivrea zone forms a part of the Southern Alps and is composed of basic rocks interfingered with granulite facies acidic rocks. According to geophysical evidence, this zone represents the transition between crust and uplifted and overthrusted mantle. Towards the Ceneri zone the metamorphic grade changes to amphibolite facies. Paragneisses, migmatites and anatectic gneisses dominate, within which postmetamorphic granites occur.

Liquids in equilibrium with peridotitic mineral assemblages at high water pressures

Abstract: Pressure-temperature conditions for which “andesitic” liquids (∼60% SiO2) may coexist with mineral assemblages of the type magnesian olivine + orthopyroxene + clinopyroxene±amphibole±phlogopite have been investigated, both by means of partial melting experiments on the pyrolite model composition, and by experiments involving addition of olivine to andesite or basaltic andesite compositions at or near their liquidus temperatures. In the latter experiments, reaction relationships between quartz-normative liquid and olivine were made to proceed until olivine persisted. The composition of the final liquid in equilibrium with olivine (plus pyroxenes etc.) was then estimated by microprobe analysis of its quench products (glass, quench crystals). The inferred liquid compositions were tested for equilibrium with coexisting crystals using criteria based on mass balance within the total assemblage, equilibrium element partition relationships, and the requirement that the liquidus temperatures and near-liquidus crystalline phases of these compositions should closely match the temperature of the original olivine-addition experiment and the crystalline phases developed during it. At 1000° C–1050° C, liquids which satisfy these criteria for equilibrium with assemblages which include olivine are “andesitic” (58–60% SiO2, 5–12% normative Qz) only at water pressures ≤ 10kb. At 15 kb, liquids in equilibrium with olivine at 1000° C and 980° C have ∼56% SiO2, high alkali contents, and 5–10% normative olivine. Similar compositions are in equilibrium with orthopyroxene and garnet alone at 20 kb. These results show that andesitic magmas are unlikely to be produced by melting of a peridotitic mantle at pressures >10 kb (depths>35 km). If hydrous, but otherwise geochemically primitive peridotitic compositions are partially melted at pressures<10 kb, then “andesitic” products will be much more magnesian and poorer in alkalies than typical natural andesites. These conclusions raise serious difficulties for models of andesite genesis by the melting of hydrous peridotitic mantle immediately overlying Benioff zones.

Petrology and geochemistry of Easter Island

Abstract: Easter Island has developed around three volcanoes—Poike, an older (3 m.y.) strato-volcano, Rano Kau, a caldera, and the fissure complex of Terevaka and its associated cones. The lavas show a wide compositional spread from tholeiites and olivine tholeiites to hawaiites, mugearites, benmoreites, trachytes and rhyolites (comendites). Hawaiite is by far the most abundant rock type and trachytes and rhyolites are relatively rare. Intermediate and acid rocks are concentrated in the southwestern part of the island on or around Rano Kau. The basaltic rocks, which are plagioclase-phyric or aphyric, are transitional hypersthenenormative types characterized by high contents of Fe, Ti and Zr but low K and Mg. The Poike basalts are marginally lower in Zr, Nb, Y and Zn compared with those of the younger volcanoes, but the trachytes from this centre show anomalously high concentrations of Rb, Zr and Nb. The island's youngest flow, the Roiho basalt, is an olivine tholeiite with distinctly more alkaline affinities: it is olivine-microphyric with relatively high contents of Mg, Ni and K.

Geochemistry of the Rare Earth elements in spilites from the oceanic and continental crust

Abstract: Analytical data including major elements, the Rare Earths, Ni, Cu, Zn, Rb, and Sr are presented for twenty-three spilites from the Mid-Atlantic Ridge, the Hercynian part of the Variscan geosyncline in Germany, and several localities in Switzerland. Low grade metamorphism (up to approximately 400° C) and spilitization of basaltic rocks apparently do not alter the original Rare Earth element (REE) distributions. This fact permits comparison of the spilites and unaltered tholeiitic basalts from the Mid-Atlantic Ridge. The relative REE distributions thus appear suitable for delineating the original basalt types of spilites formed by metamorphism.

Strontium isotope composition and petrogenesis of the Kirkpatrick basalt, Queen Alexandra Range, Antarctica

Abstract: The initial 87Sr/86Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1σ). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO2 (56.50%) and K2O (1.29%) and are depleted in Al2O3 (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts. The initial 87Sr/86Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial 87Sr/86Sr ratios of successive flows and their concentrations of SiO2, FeO (total iron), Na2O, K2O, P2O5, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO2, Al2O3, MgO, CaO and MnO increase upward. The initial 87Sr/86Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial 87Sr/86Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain. Mixtures of two components having different 87Sr/86Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial 87Sr/86Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their 87Sr/86Sr ratios. The chemical composition of the parent basalt (87Sr/86Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (87Sr/86Sr=0.720) is enriched in strontium (173 ppm), SiO2, FeO (total iron) and the alkalies but is depleted in Al2O3, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.

Petrology of some volcanic rock series of the Aeolian arc, Southern Tyrrhenian Sea: Calc-alkaline and shoshonitic associations

Abstract: In the Aeolian island arc two different magmatological associations, calc-alkaline andesite series and shoshonites, occur in close vicinity. Although both associations erupted simultaneously during the last glaciation, there is a general tendency for the calc-alkaline rocks to be older. Shoshonitic activity is still going on.

Geochronology of a polymetamorphic and anatectic gneiss region: The moldanubicum of the area Lam-Deggendorf, eastern Bavaria, Germany

Abstract: Rb-Sr isotopic analyses of whole-rocks and biotite and U-Th-Pb analyses of zircon and monazite reveal regional metamorphic events for the Ordovician (Caledonian metamorphism) and the Carboniferous (Variscan=Hercynian orogeny), both accompanied by anatexis The extent of the Caledonian and Variscan anatexis, however, cannot be evaluated, so far, because the field petrographic criteria are not sufficient to distinguish clearly between early and late Paleozoic anatexites Evidence for a Precambrian metamorphism has not been found Rb-Sr whole-rock isochrons obtained on leucosomes and melanosomes of partially molten paragneisses are interpreted as a minimum age of the second, early Variscan anatexis The alternative explanation of the isochrons as a result of local Sr isotopic redistribution without a melt involved is considered less likely Concordant and nearly concordant zircon ages (318–335 my) of a coarse-grained granite and of “diatexites” are regarded as evidence for an intensive early Variscan granitization and palingenesis Concordant zircon ages of “diorite” dykes, crosscutting the anatexites, establish a lower time limit of 309–312 my for the Variscan anatexis Rb-Sr ages of biotite (310-290 my) indicate the end of the Variscan metamorphism

Electron microprobe measurement of pyroxenes coexisting with H_2O-undersaturated liquid in the join CaMgSi_2O_6-Mg_2Si_2O_6-H_2O at 30 kilobars, with applications to geothermometry

Abstract: Mixtures of synthetic crystalline enstatite and diopside were reacted with small water contents in sealed capsules in piston-cylinder apparatus at 30 kb between 1000° C and 1700° C. The compositions of coexisting enstatite and diopside solid solutions were measured with an ARL-EMX electron microprobe between 1000° C and 1500° C. Between 1100° C and 1500° C the pyroxenes coexisted with H2O-undersaturated liquid which quenched to inhomogeneous pyroxene crystals. The presence of liquid facilitated growth of pyroxene crystals suitable for microprobe determinations. The solvus of Davis and Boyd (1966) is generally used in geothermometry; our enstatite solvus limb is a few mol-% richer in Mg2Si2O6 in the temperature range 1000–1400° C; our diopside solvus limb is a few mol-% richer in Mg2Si2O6 below 1100°C, in close agreement between 1100° C and 1200° C, but richer in CaMgSi2O6 between 1200° C and 1500° C. Estimated equilibration temperatures for a diopside with composition 78.7% Di is 1300° C according to our results compared with 1210° C for the Davis and Boyd solvus.

K-Ar and Rb-Sr Dating of Blue Amphiboles, Micas, and Associated Minerals from the Western Alps

Abstract: The results of 63 new radiometric K-Ar and Rb-Sr measurements on metamorphic minerals from the internal units of the Western Alps show Hercynian, Permian, as well as three Alpine age groups. The first of the Alpine ages cover the period between 78 and 100 m.y. and refer to high pressure parageneses. The second group comprises K-Ar 39 to 50 m.y. ages; these values are affected by some inherited argon, as indicated by Rb-Sr measurements which point to 35–36±4–5 m.y., i.e. similar to the culmination of the Lepontine crystallization. The final group includes 15 to 30 m.y. ages. It is not yet clear which geologic processes have led to this isotope re-equilibration. Large amounts of inherited argon have been found in Alpine metamorphic minerals of the basement rocks.

Sector and oscillatory zoning in calcic augites from M. Etna, Sicily

Abstract: Single whole crystals of calcic augite from M. Etna, Sicily, which display both sector and oscillatory zoning, have been serially sectioned and the form of the sectors mapped out. The principal intersectorial chemical variation is shown to be that of Ti and Al by as much as 39% (cation %) change in each, and the least variation, that of Ca (showing a 0.77 to 3.20% change). For each of three isochronous surfaces the ratio of Ti∶Al is shown to be individually constant no matter what the sector and this is thought to indicate near equilibrium conditions at the crystal/liquid interface. In addition to sector zoning these crystals display two types of concentric banding, having dimensions of about 0.25 mm, 20 μm. The 20 μm banding is shown to be both optically and chemically oscillated. Relative diffusion rates of cationic species in the melt, and growth rates of crystal faces are thought to play an important role in the formation of the oscillations, and the sector zoning.

An olivine-clinopyroxene geothermometer

Abstract: The iron-magnesium exchange reaction between olivine and calcium-rich clinopyroxene is formulated as a geothermometer. It is shown that the clinopyroxene M1 site must be nonideal and it is expressed as a regular solution. The appropriate mixing parameters are calculated from a set of groundmass olivine-clinopyroxene pairs from lavas for which there are groundmass iron-titanium oxide temperatures. The pressure dependence of the geothermometer is calculated from abailable experimental work, and is approximately 5°C per kilobar. Layered gabbros from the Kap Edvard Holm Complex, East Greenland, show no significant variation of temperature with structural height in the intrusion, while those of Skaergaard give temperatures which do not have a consistent variation with height. Continued equilibration during post-crystallisation cooling is a possibility in slowly cooled intrusions. Inclusions in diamond give a pressure-temperature line consistent with formation at 1300°C at 55 kb, 1400°C 72 kb and 1500°C 90 kb. Ultrabasic xenoliths in Basutoland kimberlites have similar pressure-temperature lines. Lavas, including alkali basalts, basanites, andesites and rhyolites give temperatures from 1025°C to 890° C.

238 U/ 206 Pb- 235 U/ 207 Pb- 232 Th/ 208 Pb zircon geochronology in alpine and non-alpine environment

Abstract: Based on regular arrays in a U-Pb diagram of data points of zircons from polymetamorphic rocks of the Alps quantitative models are developed that include several episodic lead losses, continuous lead losses during discrete periods and combinations of both.

Metabasite amphiboles of the Scottish Dalradian

Abstract: Blue-green hornblendes are observed in metabasite assemblages throughout the chlorite, biotite and garnet zones of the southwest Scottish Highlands. Actinolites are common in more Mg-rich metabasites in these zones.

A Calorimetric Investigation of the Stability of Anhydrous Magnesium Cordierite with Application to Granulite Facies Metamorphism

Abstract: The heats of solution of synthetic anhydrous Mg-cordierite and of its high-pressure breakdown assemblage sapphirine + quartz (+ enstatite?) have been measured in a lead borate melt at 694°C. The ΔH of this reaction at this temperature and one atmosphere is 6.1±1 kilocalorie per mole of cordierite.

Rb-Sr whole-rock dating of late diagenetic to anchimetamorphic, palaeozoic sediments in southern France (Montagne Noire)

Abstract: Rb-Sr whole-rock and mineral analyses are reported from Early Paleozoic sediments of the southern rim of the French Central Massif (Montagne Noire).

Calculation of Muscovite-Paragonite-Alkali Feldspar Phase Relations

Abstract: Experimental data on K-Na exchange between NaCl-KCl-H2O fluids and alkali feldspars or white micas, and end-member dehydration reactions of white mica (± quartz to alkali feldspar plus corundum (or Al2SiO5) and H2O have been used to evaluate K-Na exchange potentials between coexisting white mica and alkali feldspar. Calculations using these exchange potentials and Margules parameters for excess molar Gibbs energies of alkali feld-spars and white micas have permitted the construction of a projected binary phase diagram for the white mica dehydration. Extrapolation to higher temperatures and pressures gives a topology wholly consistent with field evidence. The subsolidus univariant curves intersect H2O-saturated minimum-melting curves. At pressures greater than about 6 kbar the common assemblage Ms+Or+Ab+Q could undergo H2O-saturated minimum-melting at lower temperatures than the disappearance of paragonite or the kyanite-sillimanite isograd in more aluminous rocks. The apparent restriction of anatexis to rocks above the second sillimanite isograd may imply that activity of H2O is usually less than unity in high-grade regional metamorphism.

High hafnium members of the zircon-hafnon series from the granite pegmatites of Zambézia, Mozambique

Abstract: A mineralogical nomenclature applicable to the natural binary isomorphous series ZrSiO4-HfSiO4 is proposed. It is suggested that (Hf, Zr)SiO4 with more than 90 mol.-% hafnium component be called hafnon.