Showing papers in "Contributions to Mineralogy and Petrology in 1981"

On the crystallinity, probability of occurrence, and rheology of lava and magma

Abstract: Given a set of comagmatic lavas of similar composition but varying crystallinity, a diagram can be constructed using only the modes of the phenocrysts that quantitively shows the sequence of crystallization This is done by plotting the amount of each phenocryst against the total crystallinity or percentage of melt of the lava itself A histogram of the total phenocryst content measures the probability of the magma to be erupted as lava This eruption probability (P E ) is the product of the probability of finding the magma at any state of crystallinity (thermal probability, P T ) and the rheological probability (P R ) of the magma being physically able to erupt (ie P E =P T P R ) It is shown that P E is given by dX/dT, where X is the crystallinity of the magma as a function of temperature (T) Because crystal production is generally nonlinear—in most rocks it is step-like—P E is a bellshaped curve stradling the temperature at which the magma is one half crystallized Near the liquidus it is most favorable rheologically for the magma to erupt But the probability is small of sampling a magma near its liquidus, because it cools quickly there It is maximum when there are high rates of crystal production, because it then cools slowly As the crystallinity increases, it reaches a critical point of maximum packing (ie lowest porosity) around 50–60% crystals where it becomes rheologically impossible to erupt The magma looses its potential to become a lava and it becomes a pluton From a histogram of crystallinity and P T ,P R can be found This technique, as well as the construction of the mode-crystallization (M-C) diagram, is illustrated using a set of Aleutian lavas These lavas also show that the point of critical crystallinity decreases with increasing silica content of the lava Because this critical crystallinity is much lower for granitic magmas, they are much more probable than basaltic magmas to become plutons Beyond this point, granitic magmas can only erupt as ash flows This correlation of critical crystallinity and silica content is used to show a method by which the viscosity of the magma can be estimated as a function of crystallinity This variation is found to compare favorably with Roscoe's equation of the dependence of viscosity on the concentration of suspended solids These results show that differentiation probably can not normally take place beyond this critical crystallinity The extraction of melt beyond this critical point by filter pressing is unlikely because the assemblage dilates upon stressing Only if the phenocrysts deform viscously can additional melt be extracted, and this can probably only occur with large (−30km) bodies

The effect of fluorine on liquidus phase relationships in the system Qz-Ab-Or with excess water at 1 kb

Abstract: Liquidus phase relationships have been determined experimentally for the system Qz-Ab-Or with excess water and 1, 2, and 4 wt.% added fluorine at 1 kb pressure. With increasing fluorine content the position of the quartz-alkali feldspar field boundary moves away from the quartz apex. The position of the minimum melting composition and the minimum liquidus temperature change progressively from Qz37Ab34Or29 and 730° C for the fluorine free system (Tuttle and Bowen 1958) to Qz15Ab58Or27 and 630° C for the system with 4 wt.% added fluorine. Exploratory experiments have been carried out below the liquidus, and have indicated that for certain bulk compositions an assemblage consisting of two alkali feldspars, quartz, melt and vapour can exist at temperatures as low as 550° C at 1 kb.

The transition between spinel lherzolite and garnet lherzolite, and its use as a Geobarometer

Abstract: The equilibrium between spinel lherzolite and garnet lherzolite has been experimentally determined in the CaO-MgO-Al2O3-SiO2 system between 800° and 1,100° C. In confirmation of earlier work and predictions from thermodynamic data, it was found that theP-T slope of the reaction was close to zero, the equilibrium ranging from 16.1 kb at 800° C to 18.7 kb at 1,100° C (±0.3 kb).

Ferric-ferrous equilibria in natural silicate liquids at 1 bar

Abstract: Ferric and ferrous iron concentrations have been measured in 57 silicate liquids equilibrated at temperatures (1,200°–1,330°C) above the liquidus and at oxygen fugacities close to those defined by quartz-fayalite-magnetite. The experimental results reported here span virtually the entire known compositional range of lavas. An empirical equation relating the mole fraction of Fe2O3 and FeO to oxygen fugacity, absolute temperature and liquid composition at 1 bar has been formulated, based on the present experimental results and published data. Extrapolating the proposed empirical relationship over several hundred degrees, for instance below the solidus of the 1965 Makaopuhi tholeiitic lava lake, yields calculated oxygen fugacities which are a little lower than those measured directly in drill holes; at 1,100°C this discrepancy is 0.4 log unit. However, the agreement between the oxygen fugacities calculated from our empirical equation and those indicated by the composition of Fe-Ti oxides in andesites and siliceous obsidians is much closer.

The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense ultrabasic magma

Abstract: This paper describes a fluid dynamical investigation of the influx of hot, dense ultrabasic magma into a reservoir containing lighter, fractionated basaltic magma. This situation is compared with that which develops when hot salty water is introduced under cold fresh water. Theoretical and empirical models for salt/water systems are adapted to develop a model for magmatic systems. A feature of the model is that the ultrabasic melt does not immediately mix with the basalt, but spreads out over the floor of the chamber, forming an independent layer. A non-turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion. Both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock. The convection continues until the two layers have almost the same temperature. The compositions of the layers remain distinct due to the low diffusivity of mass compared to heat. The temperatures of the layers as functions of time and their cooling rate depend on their viscosities, their thermal properties, the density difference between the layers and their thicknesses. For a layer of ultrabasic melt (18% MgO) a few tens of metres thick at the base of a basaltic (10% MgO) magma chamber a few kilometres thick, the temperature of the layers will become nearly identical over a period of between a few months and a few years. During this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling. Olivines only settle after the two layers have nearly reached thermal equilibrium. At this stage residual basaltic melt segregates as the olivines sediment in the lower layer. Depending on its density, the released basalt can either mix convectively with the overlying basalt layer, or can continue as a separate layer. The model provides an explanation for large-scale cyclic layering in basic and ultrabasic intrusions. The model also suggests reasons for the restriction of erupted basaltic liquids to compositions with MgO<10% and the formation of some quench textures in layered igneous rocks.

A routine high-precision method for Lu-Hf isotope geochemistry and chronology

Abstract: A method for chemical separation of Lu and Hf from rock, meteorite and mineral samples is described, together with a much improved mass spectrometric running technique for Hf This allows (i) geo- and cosmochronology using the176Lu→176Hf+β − decay scheme, and (ii) geochemical studies of planetary processes in the earth and moon Chemical yields for the three-stage ion-exchange column procedure average 90% for Hf Chemical blanks are <02 ng for Lu and Hf From 1 μg of Hf, a total ion current of 05×10−11 Ampere can be maintained for 3–5 h, yielding 001–003% precision on the ratio176Hf/177Hf Normalisation to179Hf/177Hf=07325 is used Extensive results for the Johnson Matthey Hf standard JMC 475 are presented, and this sample is urged as an international mass spectrometric standard; suitable aliquots, prepared from a single batch of JMC 475, are available from Denver Lu-Hf analyses of the standard rocks BCR-1 and JB-1 are given The potential of the Lu-Hf method in isotope geochemistry is assessed

The magma mixing origin of mantled feldspars

Abstract: The key to mantled feldspar genesis is epitaxial nucleation of plagioclase on K-feldspar or K-feldspar on plagioclase. Once this nucleation takes place there is a relatively straightforward process of crystal growth yielding “rapakivi” and “antirapikivi” textures. The most common mantling is plagioclase on K-feldspar which occurs in both volcanic and plutonic environments. In the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic, though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non-cellular shell of plagioclase. In the plutonic environment, early stages of plagioclase overgrowth also tend to be dendritic, although with coarser-grained characteristics. Dendritic morphology is thus a common denominator in rapakivi genesis. Since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected. Equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed. Internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma. Such magma mixing yields a heterogeneous system at first, one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on K-feldspar. Mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals.

Diffusion in silicate minerals and glasses: A data digest and guide to the literature

Abstract: The rates of diffusion of cations, oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems. In order that the Earth Scientist may readily apply the available experimentally-determined diffusion rates to such problems, a compilation of data for diffusion in silicate minerals and glasses is presented along with a “working guide” for its use. Published data and experimental conditions are listed in tables, and these are preceded by an outline of diffusion mechanisms, terminology, equations and units, diffusion regimes, factors controlling diffusion rates, measurement techniques, and diffusional behaviour.

Trace elements behavior in granite genesis: A case study The calc-alkaline plutonic association from the Querigut complex (Pyrénées, France)

Abstract: The different granitoids of the zoned Querigut complex (Hercynian Pyrenees) are associated with a series of basic to intermediate rocks ranging from hornblende-bearing peridotites to quartz-diorites. The whole complex appears as a calc-alkaline plutonic suite typical of orogenic zones. The distribution of lanthanides and other trace elements amongst coexisting minerals indicate they are essentially held by accessory phases, particularly in granitoids. This restricts the use of those elements in the calculation of petrogenetic models for acidic plutonic rocks. Magmatic differentiation, mainly by hornblende + plagioclase fractionation, can produce the basic series. This differentiation cannot directly produce the different granitoids, which require a preponderant contribution of crustal melts. The sequence of different granitoids can be explained either by an heterogeneity in the source region, or by magmatic differentiation. The most plausible interpretation of the whole complex calls for the emplacement of a mantle-derived magma into a wet, anatectic continental crust, with interactions between basic rocks and the soproduced acidic melts.

Geobarometry and geothermometry of plagioclase-biotite-garnet-muscovite assemblages

Abstract: The assemblage, plagioclase-biotite-garnet-muscovite is widespread in garnet to lower sillimanite zone metasedimentary rocks. The equilibria, (1) pyr+gr+mu=3an+phl and (2) alm+gr+mu=3an+ann, involve a change in Al coordination from 6 to 4 and in Mg-Fe coordination from 8 to 6 and should be strongly pressure dependent. Using an ionic solution model we can define solid activity products for (1) and (2). Using a linear least squares analysis of activity products and estimated P-T from natural assemblages we can derive equilibrium constant equations for (1) and (2). Combining either of these equations with that for Mg-Fe exchange for garnet-biotite from Ferry and Spear (1978) allows estimation of P and T of metamorphism. Pressures estimated from these equilibria compare favorably to pressures estimated from garnet-plagioclase-Al2SiO5-quartz equilibria.

Chemical control of Al-solubility in orthopyroxene and its implications on pyroxene geothermometry

Abstract: Two suites of spinel peridotites of the Westeifel/West Germany were found to have equilibrated in a narrow temperature interval each. Temperatures calculated from the CaO-solubility in orthopyroxene using experimental data of Lindsley and Dixon (1976) are 945° C to 980° C for the amphibole bearing Ia-suite and 1,150° C to 1,165° C for the Ib-suite. From the study of phase equilibria in both series it appears that Al-solubility in orthopyroxene is controlled by the composition of the coexisting spinel. Temperatures calculated from Al-solubility in orthopyroxene using an equation of Fujii (1976) derived from the univariant reaction en+sp↔fo+Al-en in the MgO-Al2O3-SiO2 system show a clear dependence on the FeCr2O4 component in the spinel phase, although ideal solution correction allowing for additional components was made. Temperatures obtained for the most chromite rich spinel — orthopyroxene pairs are 1,370° C and 1,240° C, respectively. These temperatures are by 220° C higher than those from Cr-poor pairs and those obtained from Ca solution in orthopyroxene. For practical purposes of geothermometry in spinel peridotites, an empirical equation $$T({}^oC) = \frac{{4.59 + ln K - 1.552Y_{C_r }^{sp} }}{{0.0025}}$$ allowing for the mole fraction of FeCr2O4 in spinel has been derived by a linear least squares fit to the orthopyroxene — spinel compositional data and temperatures calculated from the CaO-solubility of orthopyroxene in the present paper.

An experimental study of the effect of boron on a water saturated haplogranite at 1 Kbar vapour pressure

Abstract: An experimental study of the effect of boron in the water saturated Q-Or-Ab-B2O3-H2O system has been performed at P=1 Kbar to provide experimental data and explain the role of boron in some late magmatic and early hydrothermal events. Experiments were conducted between 500° C and 800° starting from a gel, or a previously crystallized gel, and variable amounts of boron (0 to 18% B2O3) added to water. The phases obtained were: quartz, sanidine, albite, silicate liquid quenched to glass, and aqueous vapour phase. Boric acids, borates and isotropic low index materials were found in the quenched vapour phase. An aluminium silicate-like mineral, not yet fully identified, is also present. The solidus temperature of the Q-Or-Ab composition is lowered by 60° C when 5 wt. % B2O3 is added and by more than 130° C when 17wt. % B2O3 is added. Compositions of equilibrated silicate melts and vapours were obtained between 780° C and 750° C for various B2O3 concentrations. The vapour phase is B and Si rich. It is also enriched in Na with respect to K, and in alkalis with respect to Al. Its silicate solute content is higher than in experiments with pure water. The solubility of water is increased by the addition of boron in Q-Or-Ab melts. Microprobe data show that the melts equilibrated with vapour phases become hyperaluminous and more potassic than sodic. The partition coefficient of boron is in favour of the vapour (k D=B2O3% in melt/B2O3% in vapour=0.33±0.02). The effect of the interaction between the silicate phases and the vapour is discussed. Comparison is made between the behaviour of boron and that of chlorine and fluorine. Geological applications are also provided, which concern the influence of boron on minimum melting, on muscovite stability and on the hypersolvus-subsolvus transition.

Oxygen isotope thermometry of basic lavas and mantle nodules

Abstract: Measurements have been made of the oxygen isotope and chemical composition of glass and phenocrysts in lavas and coexisting minerals in mantle nodules. Temperatures of formation of these assemblages have been estimated from various chemical thermometers and range from 855° to 1,300° C. The permil fractionations between coexisting orthopyroxene and clinopyroxene in the lavas and nodules are all near zero. The fractionations between pyroxene and olivine vary from +1.2 to −1.4 and are a smooth function of temperature over the entire range. This function is given by T(° C)=1151-173Δ (px-d)-68Δ2(px-d) and has an uncertainty of ±60° (2σ). At temperatures above 1,150° C, olivine in the nodules becomes more18O-rich than coexisting clinopyroxene, orthopyroxene, and plagioclase. In combination with the experimental work of Muehlenbachs and Kushiro (1974), the olivine-pyroxene fractionations indicate that olivine also becomes substantially more18O-rich than basaltic liquids above 1,200° C. Geothermometers based on the oxygen isotope equilibration of basaltic liquid with olivine, pyroxene, and plagioclase are presented.

Chemical evolution of a pleistocene rhyolitic center: Sierra La Primavera, Jalisco, México

Abstract: The late Pleistocene caldera complex of the Sierra La Primavera, Jalisco, Mexico, contains well-exposed lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. All eruptive units are high-silica rhyolites, but systematic chemical differences correlate with age and eruptive mode. The caldera-producing unit, the 45-km3 Tala Tuff, is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Hf, Ta, Nb, Sb, HREE, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. The earliest post-caldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, moremafic, ring domes. This sequence of ash-flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Since that time, the lavas that erupted 75,000, 60,000, and 30,000 years ago have become decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U. They represent successive eruption of the uppermost magma in the post-95,000-year magma chamber. Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≧quartz >ferrohedenbergite >fayalite>ilmenite±titanomagnetite. Whereas major-element compositions of sanidine, clinopyroxene, and fayalite phenocrysts changed only slightly between eruptive groups, concentrations of many trace elements changed by factors of 5 to 10, resulting in crystal/glass partition coefficients that differ by factors of up to 20 between successively erupted units. The extreme variations in partitioning behavior are attributed to small changes in bulk composition of the melt because major-element compositions of the phenocrysts and temperature, pressure, and oxygen fugacity of the magma all remained essentially constant. Crystal settling and incremental partial melting by themselves appear incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-caldera lavas. Transport of trace metals as volatile complexes within the thermal and gravitational gradient in volatilerich but water-undersaturated magma is considered the dominant process responsible for compositional zonation in the Tala Tuff. The evolution of the post-caldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced trace-metal-site availability in the silicate melt.

The Colima volcanic complex, Mexico: Part II. Late-quaternary cinder cones

Abstract: Since the late Pleistocene, eleven cinder and lava cones have erupted on the floor of the southern Colima graben, NE and NW of the large, active, andesitic volcano Colima. Scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites (9), leucite-basanites (3), and minettes (15), the commonest variety of mica lamprophyre. These samples represent primitive, high temperature magmas with 47.6–50.3% SiO2, 7.4–15.3% MgO, 2.5–4.4% K2O, and 2.2–9.9% normative nepheline. All members of this basic alkalic suite contain Mg-olivine (Fo75–94), chromite, augite, and late plagioclase and titanomagnetite. The petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile-bearing phases phlogopite, apatite, and analcime during late stages of crystallization. As these phases increase in abundance, presumably reflecting a rise in magmatic volatile content, there are corresponding increases in the whole rock concentrations of 16 incompatible elements. Although these incompatible elements are relatively abundant even in the basanites, many are highly concentrated in the minettes: Ba≦ 4,200 ppm, Sr≦3,100 ppm, Zr≦ 550 ppm, Ce≦190 ppm, Hf ≦18 ppm. Among the incompatible elements, the degrees of enrichment in the minettes relative to the basanites decrease in the order: H, Th, Ce, La, Nd, Zr, Hf, U, Ba, Sm, Eu, Pb, P, Nb, Sr, Ti. These enrichments may reflect the increasing importance of minor, incompatible element rich mantle phases during partial melting. The concentrations of alkali metals K, Na, Rb, and Cs do not correlate with these other elemental enrichments. The leucite-basanties have similar incompatible element contents to many minettes, differing from them only in the presence of leucite rather than analcime, and Ti-F-rich groundmass phlogopite rather than hydrous phlogopite phenocrysts; thus the leucite-basanites represent relatively dry equivalents of minettes. Two of the eleven cinder cones are calc-alkaline in nature and do not belong to the basanite-minette group; the easternmost cone is constructed of high-Al basalt, and the northernmost of basaltic andesite. The high-Al basalt (49.5% SiO2, 9.3% MgO, 221 ppm Ni) closely approximates a parental magma to the post-caldera andesitic suite of V. Colima (56.5–61.6% SiO2). The basaltic-andesite is relatively enriched in incompatible elements compared to the high-Al basalt — V. Colima trend. The ne-normative basanite-minette samples are highly enriched in incompatible elements, while the contemporaneous hy — qz-normative calc-alkaline suite, encompassing the high-Al basalt and V. Colima's andesites, is characterized by relatively low abundances of these elements. No likely mineral assemblage can relate the alkaline and calc-alkaline suites through crystal fractionation; they probably represent fundamentally different melting events. During the Quaternary, the main focus of andesitic volcanism in the southern Colima graben has migrated southward with time. Volcan Colima marks its present position, 5 km south of the Pleistocene volcano Nevado de Colima, and another 15 km from the still older Volcan Cantaro. The eruptions of basic alkalic magma probably occurred during the late stages of Nevado's life and through the life of V. Colima. They generally migrated from west to east with time, towards V. Cantaro. The most recent cone, V. Apaxtepec, is the only one east of the andesitic Colima-Cantaro axis. The oldest and the two youngest cones produced basanites, while minettes dominated at cones of intermediate ages. The cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of V. Colima. The southern end of the Colima graben can be viewed, then, as the volcanic analog of many classical, post-plutonic, hypabyssal lamprophyre localities.

Volatile control of contrasting trace element distributions in peralkaline granitic and volcanic rocks

Abstract: Anomalous enrichments of Zr (>500 ppm), Zn (> 100 ppm), Nb (>25 ppm), Y (>60 ppm), Th (>20 pm), U (> 5 ppm), LREE (>230 ppm) and HREE (>35ppm), and high Rb/Sr (>5) characterize peralkaline granites, in contrast to their peraluminous and calc-alkaline equivalents. Within the peralkaline suite, comenditic and pantelleritic volcanics exhibit two- to five-fold increases in the concentrations of these trace elements over comagmatic granites. These cannot be explained by crystal- liquid fractionation processes, and require the evolution of a sodium-enriched fluid. Corresponding trace element increases in the granites in areas of alkali metasomatism support this argument, and reflect the partial confinement of this volatile phase within the high-level magma chambers. REE studies in particular might eventually allow an evaluation of the role of Cl− versus F− and CO3-complexing in the evolution of the volatile fluid.

Water activity changes across an amphibolite-granulite facies transition, Broken Hill, Australia

Abstract: Phase compositions in pelitic and mafic gneisses place tight constraints on pressure (ranging from 3 up to 6 kb), and, to a lesser extent, on temperature (500° up to 800° C) during prograde regional metamorphism of the Willyama Complex, Broken Hill, SE Australia. These limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses. The stability of cummingtonite and biotite over much of the terrain places upper limits on temperature, and the presence of syn-metamorphic partial melts in the metasediments places lower limits on a(H2O). Garnet-biotitesillimanite-K feldspar-quartz relations combined with the partial melting data suggest a decrease in water activity from near 1.0 in the lower grade zones to 0.5±0.2 in the Broken Hill — Little Broken Hill part of the two pyroxene zone. This result is compatible with less precise hornblende-orthopyroxene-clinopyroxene-quartz relations.

The Scourie dyke suite: Petrogenesis and geochemical nature of the Proterozoic sub-continental mantle

Abstract: The petrogenesis of bronzite-picrite, olivine-gabbro, norite and quartz-tholeiite dykes, which make up the 2.39 b.y. Scourie dyke swarm cutting the Archaean Lewisian gneisses of N.W. Scotland, is interpreted on the basis of their major and trace element geochemistry. Most of the dykes bear primary amphibole and/or phlogopite and, with one exception, are all hypersthene- or quartz-normative. Apart from one tholeiite dyke which shows relative light rare-earth element depletion, all the dykes show enrichment in light rare-earths and large-ion lithophile elements. They do not however show an equivalent enrichment in other incompatible high field strength ions such as Nb and Ta, and in this respect resemble island arc and calc-alkaline basalts. The different dyke types have distinctive rare-earth patterns and other trace element ratios which are maintained over a range of major element compositions. Petrogenetic modelling of the major and trace element compositions of the various dykes demonstrates that very few can be related by fractional crystallisation. Indeed, even with partial melting mechanisms at least two different mantle sources, with different major and trace element compositons, are required to explain the compositional differences between the major dyke types. The high degrees of mantle melting implied for the generation of the magnesium-rich bronzite-picrites suggests that their rare earth and other trace element patterns closely reflect those of their mantle source. Similar arguments, though less well constrained, can be advanced for the other dyke types. The results suggest that the sub-continental mantle source feeding the dykes was heterogeneous with respect to both major and trace elements, and that their mantle sources must have been enriched in lithophile elements. Enrichment at the time the Lewisian gneisses were generated (i.e. 2.92 b.y. ago) would be compatible with the initial 87Sr/86Sr ratios of the dykes and the inferred Rb/Sr ratios of their mantle sources. The sub-continental mantle sources have thus retained the geochemical signature of the crustgenerating processes some 500 m.y. earlier.

Surface tension of natural silicate melts from 1,200°–1,500° C and implications for melt structure

Abstract: The surface tension between silicate liquid and gas has been measured for four lava compositions (limburgite to andesite) from 1,200° to 1,500° C. The magnitude of surface tension (γ) is in the range 350–370 dynes/cm. Variations found in γ as a function of liquid composition were small and had no obvious relation to liquid composition. γ was also found to vary little with furnace atmosphere — air, Ar, CO2, H2, CO and their mixtures. A relaxation time of hours to days, depending on temperature, is required before reproducible results can be obtained from originally crystalline starting material.

Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon pluton, Scotland

Abstract: The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. Petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved, each magma controlled by crystallization of plag-opx-cpx-bio. Late stage rocks related to the second magma include the cordierite microgranites and aplites, which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles. Analyses of whole rocks and minerals for REE's and other elements of moderate-high ionic potential indicate that these elements are strongly controlled by minor phase crystallization; apatite, zircon, sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions. It is probable that within each magma the mechanisms of crystal settling and filter pressing operated, the former being initially dominant, and the latter becoming more important with increasing degree of fractional crystallization.

Geochemical characteristics of potassic volcanics from Mts. Ernici (Southern Latium, Italy)

Abstract: Major elements, trace elements and 87Sr/86Sr data are reported for the Quaternary potassic alkaline rocks from the Mts. Ernici volcanic area (Southern Latium — Italy). These rocks are represented by primitive types which display high Mgv, low D.I., variable degrees of silica undersaturation and different K2O contents which allowed the distinction of a potassium series (KS) and a high potassium series (HKS). All the analyzed samples have high LIL element contents and high 87Sr/86Sr which ranges between 0.707–0.711. They also have fractionated REE patterns. The KS rocks have lower LIL element concentrations and 87Sr/86Sr ratios than the HKS rocks with a large compositional gap between the two series. Minor but still significant isotopic and trace element variations are also observed within both KS and HKS. The genesis cannot be completly explained either by crystal liquid fractionation, mixing or assimilation processes or by different degrees of equilibrium partial melting from a homogeneous source, thus indicating that both the KS and HKS consist of several geochemically and isotopically distinct magma types. The data suggest that the KS and HKS magmas originated by low degrees of melting of a garnet peridotite mantle heterogeneously enriched in LIL elements and radiogenic strontium, possibly accompanied by disquilibrium melting of some accessory phases. The occurrence of a geochemical anomaly within the mantle is believed to be due to fluid metasomatism probably generated by dehydration of a lithospheric slab subducted during the Late Tertiary development of the Apennine Chain.

Fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application

Abstract: Fluorine contents in about 300 samples of various types of basalts and related rocks from continental (southwestern U.S.A.; Zaire; Deccan and South Africa) and oceanic regions (Hawaii and Mid-Atlantic Ridge between 23° N and 40° N) were determined by a selective ion-electrode method.

The Al 2 O 3 contents of enstatite in equilibrium with garnet in the system MgO-Al 2 O 3 -SiO 2 at 15–40 kbar and 900°–1,600° C

Abstract: Forty-six reversed determinations of the Al2O3content of enstatite in equilibrium with garnet were made in the P/T range 15–40 kbar/900–1,600° C in the MgO-Al2O3-SiO2 system. Starting materials were mixtures of synthetic pyrope+Al-free enstatite and pyrope+enstatite (5–12% Al2O3). Al2O3 contents in reversal run pairs closely approached common values from both the high- and low-Al sides. Most experiments were done in a piston-cylinder device using a NaCl medium; some runs at very high temperatures were made in pyrex/NaCl or pyrex/talc assemblies. The measured enstatite compositions, expressed as mole fractions of Mg2(MgAl)(AlSi3)O12(X Opy En ) were fitted by a Monte-Carlo method to the equilibrium condition: $$\begin{gathered} \Delta H_{970}^0 - 970\Delta S_{970}^0 \hfill \\ + \mathop \smallint \limits_1^P \Delta V_{970}^0 dP - \mathop \smallint \limits_{970}^T \Delta S_T^0 dT + RT\ln X_{Opy}^{En} = 0 \hfill \\ \end{gathered}$$ where the best fit parameters of ΔH, ΔS and ΔV (1 bar, 970 K) for the reaction pyrope=opy are 2,040 cal/mol, 2.12 eu and 9.55 cc/mol. In addition to the determination of Al2O3 contents of enstatite, the univariant reaction pyrope+forsterite=enstatite+spinel was reversibly located in the range 1,100–1,400°C. A “best-fit” line passes through 22, 22.5 and 25 kbar at 1,040, 1,255 and 1,415°C, respectively. Our results for the univariant reaction are in agreement with previous studies of MacGregor (1974) and Haselton (1979). However, comparison of the experimentally determined curve with thermochemical calculations suggests that there may be a small error in the tabulated ΔH f(970,1) 0 value for enstatite. A value of−8.32 rather than−8.81 kcal/mole (Charlu et al. 1975) is consistent with the present data. Application of garnet-enstatite-spinel-forsterite equilibria to natural materials is fraught with difficulties. The effects of nonternary components are poorly understood, and the low solubilities of Al2O3 in enstatite under most geologically reasonable conditions make barometric or thermometric calculations highly sensitive. More detailed studies, including reversed determinations in low-friction assemblies, are sorely needed before the effects of important diluents such as Fe, Ca and Cr can be fully understood.

Amphibole-plagioclase equilibria: An empirical model for the relation albite+tremolite=edenite+4 quartz

Abstract: Equilibria between plagioclase, calcic amphibole and quartz can be described, in part, by the relation among mineral components: NaAlSi3O8+□Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2; this relation governs the partitioning of Na between plagioclase and the A-site of coexisting amphibole Data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade The ideal portion of the equilibrium constant (K id = X Na, A/X□, A · X Ab) derived from natural samples is sensitive to bulk composition, inasmuch as both plagioclase and amphibole are highly non-ideal Samples from a single outcrop have values ranging from 05 (X Ab=074) to 41 (X Ab=010) The continuous reaction, NaAlSi3O8+□Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2, proceeds to the right with increasing grade of metamorphism and for a given bulk composition, K id increases with increasing temperature Two related discontinuous reactions, actinolite+albite=hornblende+oligoclase+quartz and actinolite+oligoclase=hornblende+anorthite+quartz, also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of Na between plagioclase and amphibole A-site A Schreinemakers' net is presented that is consistent with natural occurrences The results of this study should aid in the delineation of metamorphic facies within amphibolites

Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals, and stable isotope exchange in metamorphic rocks

Abstract: Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.

The probable occurrence of interstitial Al in hydrous, F-bearing and F-free aluminosilicate melts

Abstract: Experiments carried out on the system SiO2-NaAlSi3O8-KAlSi3O8(Qz-Ab-Or) at 1 kbar in the presence of H2O and F show that the quartz-alkali feldspar field boundary is progressively displaced towards the feldspar join as F contents increase from 0 to 4 wt. %F. Increasing $$P_{H_2 O} $$ , in the absence of F, has already been shown to have a similar effect (Tuttle and Bowen 1958; Luth, Jahns, and Tuttle 1964). The increased size of the quartz field in the F-bearing system compared to the hydrous system is believed to be caused by progressive removal of Al from the tetrahedral network of the melt by complexing with F. The residual network in the melt is thus enriched in Si and this stabilizes precipitation of quartz rather than feldspar for certain bulk compositions. The common presence of quench cryolite (Na3AlF6) in certain experiments carried out with 4 wt.% F supports this interpretation and indicates that some Al in the melt may be present in six-fold coordination with F−. The effect of H2O in the absence of F may be similar, with Al being progressively removed from four-fold coordination as more H2O is dissolved in the melt. Although a proportion of Al in hydrous melts may occur in six-fold coordination, dry melts predominantly contain Al in four-fold coordination. This major difference in Al complexing may be one of the main causes for differences in the high-pressure phase relations of wet and dry Albearing silicate systems.

Origin of coexisting minette and ultramafic breccia, Navajo volcanic field

Abstract: Trace element evidence indicates that at the Buell Park diatreme, Navajo volcanic field, the felsic minette can be best explained by crystal fractionation from a potassic magma similar in composition to the mafic minettes. Compatible trace element (Cr, Ni, Sc) abundances decrease while concentrations of most incompatible elements (Ce, Yb, Rb, Ba, Sr) remain constant or increase from mafic to felsic minette. In particular, the nearly constant Ce/Yb ratio of the minettes combined with the decrease in Cr, Ni, and Sc abundances from mafic to felsic minette is inconsistent with a model of varying amounts of partial melting as the process to explain minette compositions. The uniformity of rare earth element (REE) abundances in all the minettes requires that an accessory mineral, apatite, dominated the geochemistry of the REE during fractionation. A decrease in P2O5 from mafic to felsic minette and the presence of apatite in cognate inclusions are also consistent with apatite fractionation. Higher initial87Sr/86Sr ratios in the felsic minettes relative to the proposed parental mafic minettes, however, is inconsistent with a simple fractionation model. Also, a separated phlogopite has a higher initial87Sr/86Sr ratio than host minette. These anomalous isotopic features probably reflect interaction of minette magma with crust. The associated ultramafic breccia at Buell Park is one of the Navajo kimberlites, but REE concentrations of the matrix do not support the kimberlite classification. Although the matrix of the breccia is enriched in the light REE relative to chondrites, and has high La, Rb, Ba, and Sr concentrations relative to peridotites, the concentrations of these elements are significantly lower than in South African kimberlites. A high initial87Sr/86Sr ratio combined with petrographic evidence of ubiquitous crustal xenoliths in the Navajo kimberlites suggests that the relatively high incompatible element concentrations are due to a crustal component. Apparently, Navajo kimberlites are most likely a mixture of comminuted mantle wall rock and crustal material; there is no evidence for an incompatible element-rich magma which is characteristic of South African kimberlites. If the mafic minettes are primary magmas derived from a garnet peridotite source with chondritic REE abundances, then REE geochemistry requires very small (less than 1%) degrees of melting to explain the minettes. Alternatively, the minettes could have formed by a larger degree of melting of a metasomatized, relatively light REE-enriched garnet peridotite. The important role of phlogopite and apatite in the differentiation of the minettes supports this latter hypothesis.

Green clinopyroxenes and associated phases in a potassium-rich lava from the Leucite Hills, Wyoming

Abstract: Green, salitic pyroxenes occur as megacrysts and as cores in diopsidic pyroxene phenocrysts and microphenocrysts in a wyomingite lava from Hatcher Mesa, Leucite Hills, Wyoming. Al-rich phlogopite (16–21% Al2O3), apatite, Fe-Ti-oxide, Mg-rich olivine (Fo93) and orthopyroxene (En61) also occur as megacrysts or as inclusions in diopside phenocrysts. All of these phases are found in ultramafic xenoliths in the host lava, and petrographic and chemical evidence is presented that the megacrysts originate by the disaggregation of the xenoliths. It is concluded that the latter are accidental fragments of the wall rocks traversed by the wyomingite magma and it is suggested that the clinopyroxene-rich xenoliths, from which the green pyroxenes are derived, formed in the upper mantle as a result of local metasomatism or by crystallization from magmas of unknown composition during an earlier igneous event. The precise role of the clinopyroxene-rich xenoliths (which also contain apatite, Fe-Ti-oxide and amphibole) in the genesis of the Leucite Hills magmas cannot be elucidated on the basis of the available data, but it is unlikely that they represent the source material from which these magmas are derived.

On curved laminae, graded layers, convection currents and dynamic crystal sorting in the Ploumanac'h (brittany) subalkaline granite

Abstract: Curved, layered objects in a subalkaline, potassic, intrusive granite are analyzed: biotite laminae and cyclic reverse size and modal layers are encountered in an annular zone; both cyclic and intermittent layers are observed in rhythmic arrangement. Truncation, the bifurcation curvature of layers with ladle shape, represents the action of magmatic currents, as would convection cells or fluid-dynamic fingers. Emphasis is put on the Bagnold effect related to such currents, which would explain the reverse size-grading of layers; this kind of size-sorting (hydrodynamic) occurs in highly viscous granitic magma. Simple convection seems more likely than diffusive convection.

Cordierite-garnet-H 2 O equilibrium: A geological thermometer, barometer and water fugacity indicator

Abstract: The high-grade assemblage Cd-Ga-Si-Qz can be thermodynamically modelled from available calorimetric data on the metastable reaction: (I) $$3 MgCd \rightleftarrows 2 Py + 4 Si + 5 Qz$$ naturalK D Fe-Mg between garnet and cordierite and experimental results on cordierite hydration. In the system SiO2-Al2O3-MgO-H2O, reaction (I) becomes (II) $$3 MgCd \cdot nH_2 O \rightleftarrows 2 Py + 4 Si + 5 Qz + 3 nH_2 O$$ . However, hydrous cordierite is neither a hydrate nor a solid solution between water and anhydrous cordierite and when nH2O (number of moles of H2O in Cd) is plotted against $$P_{H_2 O} $$ , the resulting isotherms are similar to adsorption isotherms characteristic of zeolitic minerals. Reaction (II) can thus be considered as a combination of reaction (I) with a physical equilibrium of the type nH2O (in Cd)⇆nH2O (in vapor phase). Starting from a point on equilibrium (I), introduction of H2O into anhydrous Mg-cordierite lowers the chemical potential of MgCd and hence stabilizes this mineral to higher pressure relative to the right-hand assemblage in reaction (I). The pressure increment of stabilization,ΔP, above the pressure limit of anhydrous cordierite stability at constantT, has been calculated using the standard thermodynamics of adsorption isotherms. Cordierite is regarded as a mixture of Mg2Al4Si5O18 and H2O. The activity of H2O in the cordierite is evaluated relative to an hypothetical standard state extrapolated from infinite H2O dilution, by using measured hydration data. The activity of Mg2Al4Si5O18 in the cordierite is then obtained by integration of the Gibbs-Duhem equation, and the pressure stabilization increment,ΔP, computed by means of the relation: $$\Delta V_s \Delta P \cong - RT\ln a_{MgCd}^{MgCd \cdot nH2O} \left( {\Delta V indepentdent of P and T} \right)$$ . Thus, one may contour theP-T plane in isopleths of nH2O=constant within the area of Mg-cordierite stability allowed by the hydration data for $$P_{H_2 O} = P_{total} $$ . The present model indicates greater stabilization of cordierite by H2O than the model of Newton and Wood (1979) and the calculated curve for metastable breakdown of hydrous MgCd is consistent with experimental data on cordierite breakdown at $$P_{H_2 O} = P_{total} $$ . Mg/(Mg+Fe) isopleths have been derived for cordierites of varying nH2O in the SiO2-Al2O3-MgO-FeO-H2O system using the additional assumptions that (Fe, Mg) cordierite and (Fe, Mg) garnet behave as ideal solutions, and that typical values of the distribution coefficient of Fe and Mg between coexisting garnet and cordierite observed in natural parageneses can be applied to the calculations. The calculated stable breakdown curve of Fe-cordierite under conditions of $$P_{H_2 O} = P_{total} $$ to almandine, sillimanite, quartz and vapor has a positive slope (dP/dT) apparently in contrast to the experimental negative slope. This may be explained by hydration kinetics, which could have allowed systematic breakdown of cordierites of metastable hydration states in the experiments. The bivariant Cd-Ga fields calibrated from the present model are, potentially, good petrogenetic indicators, as, given the iron-magnesium ratio of garnet and cordierite and the hydration number of cordierite, the temperature, pressure and water fugacity are uniquely determined. As this geothermobarometer is in part based on the water content of cordierite, it can be used only if there is some assurance that the actual hydration is inherited from high-grade metamorphic conditions. Such conditions could be realised if the slope of unloading-cooling retrograde metamorphism is more or less parallel to a cordierite isohydron.